official launch of the environmental engineering research ... · official launch of the...

CSE Research Bulletin

ISSN 0219-0370 January 2000 NO. 13

Official Launch of the Environmental Engineering Research Centre (EERC)

Reorganisation of School In April 1999, the School of Civil & Structural Engineering was re-organised into four divisions, namely, the Divisions of Environmental & Water Resources Engineering, Geotechnical & Transportation Engineering, Construction Technology & Management, and Structures & Mechanics. Two positions of Vice Deans were also created, the Vice-Dean (Academic), who oversees academic matters including teaching workloads and MSc programmes, and the Vice-Dean (Research), who takes charge of postgraduate programmes by research and other research matters. The new organisational structure is to enable the School to better focus on niche areas of teaching and research. Under the new structure, the Head of Construction Technology & Management will concurrently serve as Director of the Centre for Advanced Construction Studies ([email protected]), the Head of Geotechnical & Transportation Engineering also assumes the post of Director of the Geotechnical Research Centre ([email protected]), while the Head of Environmental and Water Resources Engineering is also Director of the Environmental Engineering Research Centre ([email protected]). The other two research centres within the School are the Centre for Transportation Studies ([email protected]) and the Protective Technology Research Centre ([email protected]). In the coming years, the five research centres are expected to evolve into the focal points for research activities within the School. The School aims to become a Civil Engineering School of choice, and to rank among the top civil engineering schools in Asia in the not too distant future, with niche areas of academic and research excellence. The School will work towards attracting a steady flow of prominent overseas visitors, establishing more research collaboration with other institutions, and pursuing greater cross-disciplinary teaching and research. A new undergraduate curriculum has been designed and implemented to better prepare the students for the emergence of a knowledge-based economy. In the coming years, more MSc programmes are likely to be introduced, and the number of well qualified postgraduate research students will increase.

The Environmental Engineering Research Centre (EERC), which has been jointly set up by NTU and the Ministry of the Environment (ENV), was officially launched on 6 September 1999 at the ENV Penthouse. Dr Cham Tao Soon, President of NTU,

Dr. Cham Tao Soon President, NTU (right) and Mr. Tan Gee Paw, Permanent Secretary, ENV, viewing exhibits at the offical launch of EERC delivered the opening address at the launch ceremony, which was witnessed by Mr Tan Gee Paw, Permanent Secretary, ENV. Other distinguished guests present at the launch ceremony included Mr Lee Suan Hiang, Chief Executive, Productivity and Standards Board (PSB) and Mr Tang Kin Fei, President, SembCorp Engineering Private. In conjunction with the launch, NTU signed three separate Memoranda of Understanding (MOUs) for research collaboration with the ENV, PSB and SUT (SembCorp Utilities & Terminals Pte Ltd). Later in the same afternoon, six more MOUs for research collaboration were signed at the School between NTU and Cooperative Research Centre (CRC) for Waste Management and Pollution Control Ltd, Australia, the Regional Institute of Environmental Technology (RIET), ST Assembly Test Services Ltd, NatSteel Ltd, SH Soil Works Pte Ltd and JPL Industries Pte Ltd. The afternoon signing ceremony was graced by his Excellency, Mr Murray McLean, the Australian High Commissioner. The signing of the MOUs highlighted the already excellent working relationship between NTU and the various environmental partners, and signalled the mutual desire to further expand the scope and depth of the ongoing collaboration.

The new Centre is a multi-disciplinary research centre whose mission is to serve as a focal point for

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mailto: [email protected]


http://www.ntu.edu.sg/grc/enter.htm



http://www.ntu.edu.sg/Centre/wwweerc/eerc.htm



http://www.ntu.edu.sg/cts/



http://www.ntu.edu.sg/ptrc/





http://www.env.gov.sg/

CSE Research Bulletin

For more information about the latest developments within the School, you are invited to take a cyber tour of the School at the following web site: http://www.ntu.edu.sg/cse/

upstream research and development in environmental engineering, particularly in the development of niche home-grown technologies in the area of environmental management to meet local and regional needs. The centre has identified four main functions :Research & Development, with the focus on wastewater technology, management of water quality, solid waste, air pollution, and industrial and hazardous wastes; waste recycling and reuse; sea water desalination; clean technology and development of environment friendly materials, etc.

Education and Training, which will involve organising and conducting seminars, workshops and specialised training courses for the continuing education of local and regional professionals and government officials.

Professor Chen Charng Ning, Deputy President, NTU delivering the welcome address at the MOU signing ceremony between NTu and six industry partners

Information and Resource Centre, to monitor regional developments in the areas of government policies, legislation and regulations for environmental management.

Technology Transfer and Industrial Service, to facilitate information exchange through seminars, workshops and short courses, to be conducted by invited prominent professionals or researchers on state-of-the-art developments relevant to the region. Staff from the centre will also offer specialised consultancy services to regional and international organisations.

One key role of the EERC is to serve as an R&D support infrastructure for the local Environmental Technology (ET) companies. By tapping the technical capabilities and state-of-the-art facilities of the centre, it is hoped that the local ET companies will be able to carry out their R & D activities more cost effectively, and hence be better able to sustain their competitive edge to meet the regional challenges.

The EERC values opportunities to work with other organisations in areas of mutual interest. If you have any proposals for possible research collaboration, do email the Centre Director at [email protected]. More information about the Centre can be found at the following web site : http://www.ntu.edu.sg/centre/wwweerc/eerc.htm

Please read the DISCLAIMER before you begin exploring the CSE Research Bulletin.

Copyright 2000 School of Civil & Structural Engineering, NTU. All rights reserved.

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http://www.ntu.edu.sg/centre/wwweerc/eerc.htm


CSE Research Bulletin No. 13. January 2000: In Focus

ISSN 0219-0370 January 2000 NO. 13

Official Launch of the Environmental Engineering Research Centre (EERC)

Reorganisation of School In April 1999, the School of Civil & Structural Engineering was re-organised into four divisions, namely, the Divisions of Environmental & Water Resources Engineering, Geotechnical & Transportation Engineering, Construction Technology & Management, and Structures & Mechanics. Two positions of Vice Deans were also created, the Vice-Dean (Academic), who oversees academic matters including teaching workloads and MSc programmes, and the Vice-Dean (Research), who takes charge of postgraduate programmes by research and other research matters. The new organisational structure is to enable the School to better focus on niche areas of teaching and research. Under the new structure, the Head of Construction Technology & Management will concurrently serve as Director of the Centre for Advanced Construction Studies ([email protected]), the Head of Geotechnical & Transportation Engineering also assumes the post of Director of the Geotechnical Research Centre ([email protected]), while the Head of Environmental and Water Resources Engineering is also Director of the Environmental Engineering Research Centre ([email protected]). The other two research centres within the School are the Centre for Transportation Studies ([email protected]) and the Protective Technology Research Centre ([email protected]). In the coming years, the five research centres are expected to evolve into the focal points for research activities within the School. The School aims to become a Civil Engineering School of choice, and to rank among the top civil engineering

The Environmental Engineering Research Centre (EERC), which has been jointly set up by NTU and the Ministry of the Environment (ENV), was officially launched on 6 September 1999 at the ENV Penthouse. Dr Cham Tao Soon, President of NTU,

Dr. Cham Tao Soon President, NTU (right) and Mr. Tan Gee Paw, Permanent Secretary, ENV, viewing exhibits at the offical launch of EERC delivered the opening address at the launch ceremony, which was witnessed by Mr Tan Gee Paw, Permanent Secretary, ENV. Other distinguished guests present at the launch ceremony included Mr Lee Suan Hiang, Chief Executive, Productivity and Standards Board (PSB) and Mr Tang Kin Fei, President, SembCorp Engineering Private. In conjunction with the launch, NTU signed three separate Memoranda of Understanding (MOUs) for research collaboration with the ENV, PSB and SUT (SembCorp Utilities & Terminals Pte Ltd). Later in the same afternoon, six more MOUs for research collaboration were signed at the School between NTU and Cooperative Research Centre (CRC) for Waste Management and Pollution Control Ltd, Australia, the Regional

file:///D|/CEE_old/RESEARCH/Bulletin/1999_2000/HTM/infocus.htm (1 of 3)9/27/2003 10:47:17 AM



















http://www.env.gov.sg/


schools in Asia in the not too distant future, with niche areas of academic and research excellence. The School will work towards attracting a steady flow of prominent overseas visitors, establishing more research collaboration with other institutions, and pursuing greater cross-disciplinary teaching and research. A new undergraduate curriculum has been designed and implemented to better prepare the students for the emergence of a knowledge-based economy. In the coming years, more MSc programmes are likely to be introduced, and the number of well qualified postgraduate research students will increase.

For more information about the latest developments within the School, you are invited to take a cyber tour of the School at the following web site: http://www.ntu.edu.sg/cse/

Institute of Environmental Technology (RIET), ST Assembly Test Services Ltd, NatSteel Ltd, SH Soil Works Pte Ltd and JPL Industries Pte Ltd. The afternoon signing ceremony was graced by his Excellency, Mr Murray McLean, the Australian High Commissioner. The signing of the MOUs highlighted the already excellent working relationship between NTU and the various environmental partners, and signalled the mutual desire to further expand the scope and depth of the ongoing collaboration.

The new Centre is a multi-disciplinary research centre whose mission is to serve as a focal point for upstream research and development in environmental engineering, particularly in the development of niche home-grown technologies in the area of environmental management to meet local and regional needs. The centre has identified four main functions :Research & Development, with the focus on wastewater technology, management of water quality, solid waste, air pollution, and industrial and hazardous wastes; waste recycling and reuse; sea water desalination; clean technology and development of environment friendly materials, etc.

Education and Training, which will involve organising and conducting seminars, workshops and specialised training courses for the continuing education of local and regional professionals and government officials.

Professor Chen Charng Ning, Deputy President, NTU delivering the welcome address at the MOU signing ceremony between NTu and six industry partners

Information and Resource Centre, to monitor regional developments in the areas of government policies, legislation and regulations for environmental management.





Technology Transfer and Industrial Service, to facilitate information exchange through seminars, workshops and short courses, to be conducted by invited prominent professionals or researchers on state-of-the-art developments relevant to the region. Staff from the centre will also offer specialised consultancy services to regional and international organisations.

One key role of the EERC is to serve as an R&D support infrastructure for the local Environmental Technology (ET) companies. By tapping the technical capabilities and state-of-the-art facilities of the centre, it is hoped that the local ET companies will be able to carry out their R & D activities more cost effectively, and hence be better able to sustain their competitive edge to meet the regional challenges.

The EERC values opportunities to work with other organisations in areas of mutual interest. If you have any proposals for possible research collaboration, do email the Centre Director at [email protected]. More information about the Centre can be found at the following web site : http://www.ntu.edu.sg/centre/wwweerc/eerc.htm

Please read the DISCLAIMER before you begin exploring the CSE Research Bulletin.

Copyright 2000 School of Civil & Structural Engineering, NTU. All rights reserved.





CSE Research Bulletin No. 13. January 2000: Research Projects

ISSN 0219-0370 January 2000 NO. 13

Quality Improvement of Recycled Rainwater and Innovative Use of the Waste Generated during its Treatment

Under this Innovation Development Scheme (IDS) project, NTU will conduct joint research with JPL Industries Pte Ltd and Productivity and Standards Board on converting sludge generated during wastewater treatment into useful products for the construction industry, such as high strength construction aggregates, bricks and pavement. The studies will determine the characteristics of the sludge and other potential wastes, and investigate the feasibility of converting them into useful construction products, and commercialising those products. Specifications for the products, and for setting up the testing laboratory and manufacturing plant, will also be tested and established.

Project co-ordinator: Show K Y ([email protected]).

Recycling Marine Clay and Industrial Solid Wastes into Value-added Products

Large amounts of marine clay are often generated in major construction projects that involve deep excavations, as in some of the MRT projects. This project will study the feasibility of recycling the marine clay and other industrial solid wastes into value-added products, and the minimisation and treatment of the marine clay and other industrial solid wastes. The total project research funding is $7.9 million with support from NSTB at $1.7 million over three years.

Project co-ordinator : Darren Sun ([email protected])

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Re-Utilisation of By-products From Steel-Making Into value-added products

Steel manufacturing generates a large amount of a waste by-product, ladle slag, which has been classified as industrial solid waste by ENV, and is thus not suitable for direct land disposal. This joint project between NatSteel and EERC aims to develop commercially useful products from the wastes, which if viable, could potentially form a new business for NatSteel which is in line with the Group’s policy of being environmentally friendly. The total project funding is $6 million with $1.1 million support from NSTB over three years.


Development of Enhanced Photo Catalytic Oxidation Technology

The project will focus on developing the EPCO (Enhanced Photo Catalytic Oxidation) technology, and the commercial application of EPCO membrane reactors, for purification of treated wastewater. The study will characterise and derive the reaction mechanism for the destruction of biological and organic species; investigate the mass transfer between solid and liquid phase in EPCO membrane reactors; investigate the membrane efficiency for separation catalyst in EPCO membrane reactors; and evaluate the feasibility of commercial application for turning treated wastewater into high quality industrial/potable water. The total research funding is $0.5 million for three years.







Steel Fibre Reinforced (SFR) Concrete Material And Fibremat Hardening System For Structures To Resist Blast Effects

The PTRC has carried out extensive work on the development of static constitutive models to define the tensile characteristics of steel fibre reinforced concrete (SFRC). However, analytical and small-scale statical studies cannot provide the solution to problems in a dynamic environment. For this reason, the PTRC has been involved with Land and Estates Organisation (MINDEF) on explosive tests of SFRC panels. The experience has provided a better understanding of the dynamic behaviour of the material. The effects of high strain rate on material behaviour in a dynamic environment will be quantified as part of this investigation.

Project co-ordinator : Lok T S ([email protected])

Research on Flat-Plate Structures

The flat-plate structural system is increasingly being used in modern high-rise residential buildings, due to its buildability and overall cost effectiveness. In many such buildings, the columns tend to be elongated in cross section and their layout irregular. As a result, the design methods recommended for typical flat-plate buildings in the International Building Codes are not applicable. This joint Building and Construction Authority-NTU research project aims to produce a reliable design procedure for irregular flat-plate structures, within the shortest possible time. The research programme includes testing of individual as well as large multiple-panel flat-plate floor specimens. The experimental and theoretical studies will cover the important factors that determine the behaviour of flat-plate structures, from the lower level of the serviceability limit state up to the ultimate limit states and beyond.

Project co-ordinator : S Teng ([email protected])





Other recently approved research projects are summarised below. You are welcome to email the investigators concerned for more information on their projects. PROJECT TITLES PRINCIPAL INVESTIGATORThree Dimensional Finite Element Modelling of Reinforced Concrete Structures. (Collaboration with Imperial College, UK)

A/P Fan S C ([email protected]) Prof M Pavlovic (Imp Col) Dr Lee C K

Strength Improvement of Peaty Soils. (Collaboration with University of Kyoto, Japan)

A/P Leong E C ([email protected]) Prof M Kamon A/P H Rahardjo A/P Teh C I Prof K Kogure Dr S Noto

Development of a Freight Research Strategy. A/P Luk Y K ([email protected])

Shear Strength of RC Columns Under Multi-Directional Loading. (Collaboration with Univ. of Tokyo, Japan)

Ast/P P Irawan ([email protected]) Prof K Maekawa (Univ of Tokyo) A/P S Teng

Photo-degradation of Organic Pollutants by UV/Oxidation Process.

Prof Tay J H ([email protected]) A/P Goh Ngoh Khang Dr Chia L S (NIE)

Utilisation of Marine Clay for Innovative Building and Construction Materials. (Collaboration with ENV)

Prof Tay J H ([email protected]) Dr Chu J Dr Show K Y

Behaviour of Concrete Flat-Plate Slabs Under General Loading. (Collaboration with Cambridge University, UK)

A/P S Teng ([email protected]) Prof Cheong H K A/P Lee S C Dr P Irawan Dr C T Morley (Cambridge Univ)

Instability and Liquefaction of Reclaimed Land under General Stress Conditions. (Collaboration with Imperial College, UK)

Asst Prof Chu J ([email protected]) Prof David Hight (Imperial Coll) Asst Prof K Adalier



















Permeability and Pore Structure of Blended Cement Concrete.Asst Prof J H Cahyadi ([email protected]) A/P Chan K S

Use of Biotechnology in Treating Oil-Polluted Seawater. Asst Prof Stephen Tay ([email protected]) Prof Tay J H A/P F Wilson Dr Lee Krumholz (Univ of Oklahoma)

Wind Characteristics of Thunderstorms and Their Effects on Wind Loading Design in Singapore.

A/P Edmund Choi ([email protected]) A/P J W Brownjohn A/P Chan Weng Kong (MPE)

The Effect of External Flow on Interior Air Quality. (NTU-CUED collaboration)

A/P Adrian Law ([email protected]) A/P Edmund Choi Dr Rex Britter (Univ of Cambridge)

The Effect of Principal Stress Rotation on the Strength and Deformation Behaviour of Singapore Soils. (Collaboration with Imperial College)

Dr Chu Jian ([email protected]) A/P Leong E C Dr R Jardine (Imp Col) Prof V Choa

Development of Enhanced Photo Catalytic Oxidation. (Inter-school research centre project)

Ast/P Darren Sun ([email protected]) Prof Tay J H

Effects of Impact and Penetration on Conventional Construction. (Inter-school research centre project)

A/P Lok T S ([email protected]) A/P Fan SC Ast/P Kevin Tsung

Punching Shear Strength of Flat-Plate Structures. (NTU/CIDB)

A/P S Teng ([email protected]) Prof Cheong H K A/P Lee S C Dr P Irawan Mr Steven Cheong G T (CIDB)

Development of an Integrated Multiple Base Station Infrastructure to Support Concurrent High Precision Differential GPS Applications. (NTU/NSTB)

A/P Goh Pong Chai ([email protected])

Impact of Rainwater from Buildings without Roof Gutters. (NTU/ENV Collaboration)

A/P Tommy Wong ([email protected]) A/P Shuy E B



















Steel Fibre Reinforced (SFR) Concrete Material and Fibremat Hardening System for Structures to Resist Blast Effects.

A/P Lok T S ([email protected]) A/P Zhao J

Damage Assessment of Lightly Reinforced Concrete Beam-Column Joints under Reverse Cyclic Loading.

A/P Pan T C ([email protected])

Dynamic Response of Rock Tunnels Subjected to Blast Load. A/P Hao Hong ([email protected])

Instrumentation and Monitoring of Rock Caverns During Construction and Operation.

A/P Zhao Jian ([email protected]) A/P Tor Y K Dr J Sharma

Elastic Constants of Aluminium Truss-Core Sandwich Panel. A/P Lok T S ([email protected])

CIDB-NTU Collaborative Research Initiatives on Water tightness and Evaluation of Building Envelope.

A/P Edmund Choi ([email protected]) Mr Allen AL Ng (CIDB)

Management of Quantity and Quality of Storm Runoff Using on-Site Infiltration Pits (NTU-ENV Collaboration )

A/P Shuy E B ([email protected]) A/P Tommy Wong A/P Chui P C

Analysis of frame structures in fire (Collaboration with Imperial College, UK)

AP Tan K H ([email protected]) A/P Fung T C Prof G. England (Imp Col)

















CSE Research Bulletin No. 13. January 2000: Research Activities

ISSN 0219-0370 January 2000 NO. 13

Inside this issue, there are thirty-four technical articles highlighting the School’s research activities in the areas of Construction, Environment, Geotechnics, Structures, Surveying, Transportation and Water. You are welcome to email the respective authors to find out more about their work.

All the research articles are available in the form of PDF (portable document file) format. If you do not

have PDF, please click to download and install the Acrobat Reader for Windows 95, Windows 3.1, Window-NT, DOS, Macintosh, OS2, LINUX, etc. To view and print the articles, run Acrobat Reader and click the links below. Construction

Analysis of Premature Failures in CFRP-Strengthened RC Beams

Design & Build Practice in the Singapore Public Sector

Construction Automation in Improving Construction Productivity in Singapore

A Simple Approach to Strength and Stability of Steel Columns in Fire

A Rankine Approach for Fire Resistance of Steel Frames

Effect of Stress-Path on the Failure of Concrete Under Tri-Axial Stress State

Constitutive Model of Concrete Under Multi-Axial Stresses

Hybrid Precast Concrete Beam

Environment

Removal of Color and TOC Using a TiO2/

Fe2O3 Suspended Bubble Photo Catalytic

Oxidation Reactor Optimizing the Preparation of Activated

Carbon from Digested Sewage Sludge and Coconut Husk

Reuse of Treated Municipal Wastewater in Urban Agriculture: A Hydroponic Study

Biomass Growth and Retention in Anaerobic Filters

Long Term Predictions of Phosphorus Dynamics in Kranji Reservoir

Use of Industrial Sludge for Concrete Aggregates

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Geotechnics

Behaviour of Cracked Diaphragm Walls Dynamic Testing of The Bukit Timah

Granite Material Using the Split Hopkinson Bar

Effect of Soil Movements on Piles Interpretation of Initial Horizontal Stresses

in Rock Using the Hydraulic Fracturing Test Measurements

A Brief Geotechnical Analysis of an Earthquake: Cukurova-1998

Surveying

Near-Real-Time Measuring of Sea Level Using GPS

Structures

3D Crack Simulation Using Boundary Element Method

Shear Strength and Behaviour of Beam-Column Joints Under Cyclic Loading

Elastic Moduli of Concrete - A Micromechanics Approach

Shaking Table Tests of RC Model Structures Under Simulated Ground Shocks

Dynamic Properties and Serviceability of Floor Slabs with Human-Structure Interaction

Fuzzy Probabilistic Damage Analysis of Rock Mass to Explosive Loads

Steel Fibre Reinforced Concrete Panels Exposed to Air-Blast Loading

Transportation

Comparison of the HCM and Singapore Models of Arterial Capacity

Driver Stopping Response Latency at Signalised Junctions

Mobility Model for Urban Public Transportation

Water

Taylor Dispersion by Surface Waves GIS as a Tool for Management of Oil-spill

incidents in the Coastal Environment Performance of a Membrane Wave Barrier of

Finite Draft Riprap Failure Around Bridge Piers Under Live-

Bed Condition



The Second GRC (Geotechnical Research Centre) Lecture on Anisotropy In Soils - Its Measurement And Practical Implications was delivered by Professor David Hight on 27 November 1998.

The GRC organised a 3-day Seminar on Characterisation Of Soil For Design And Analysis by David Hight, Nick Shirlaw, and Chu Jian from 1-3 December 1998.

The Centre For Transportation Studies (CTS) organised the Conference On Transportation Into The Next Millennium in September 1998 at the Mandarin Hotel Singapore, which coincided with the implementation of the Electronic Road Pricing (ERP) system. The conference was supported by the Land Transport Authority as well as transport research centres at the University of California, Berkeley; Leeds University; Monash University; and University of Sydney. About 150 delegates from 20 countries participated in the Conference, which was opened by the Minister for Communications, Mr Mah Bow Tan. The ASEAN Transport Ministers who were in Singapore for an ASEAN meeting also attended the Opening as guests of the Minister. In addition to the technical sessions, the delegates also had the opportunity to visit both the ERP system and the CityCab GPS Dispatch Centre.

At the request of the Ministry of Foreign Affairs (MFA), CTS conducted two courses for officials from developing countries under the Singapore Technical Assistance Programme for Sustainable Development. One was on Urban Traffic Management and Control which was held from 1 to 11 December 1998 and attended by 12 government officials from Bhutan, China, Fiji, Maldives, Mauritius, Mongolia, Seychelles, Thailand, Tonga and Vietnam. The other was on Transport Planning and Design conducted from 11 to 21 January 1999, which was attended by 11 participants.

In April 1999, CTS staff organised a customised workshop on the use of the Sea-Space Capacity Model developed by CTS for officers of MPA.

CTS conducted a short course on Traffic Planning for Industrial Land Uses for officers of JTC during the months of March and April 1999.

The Centre for Advanced Construction Studies (CACS) conducted a short course attended by 65 participants on Construction Claims & How to Resolve Them by Assoc Prof Goh Phai Cheng & Prof Dennis Ballou from 3-24 August 1998

The CACS conducted a seminar on Management of Large Scale Construction by Prof Dennis Ballou & A/P Lim Ewe Chye on 20 August 1998, attended by 102 participants

CACS conducted a short course on Construction Management Training Programme for Bhutan Engineers & Managers from 24 August to 4 September 1998, which was attended by 5 participants

CACS conducted a








seminar on Concrete Floating Bridges in the U.S.A by Myint Lwin, Department of Washington, USA on 15 March 1999, which was attended by 90 participants

CACS organised the Dr Tan Swan Beng Memorial Symposium at the Mandarin Singapore from 18 to 19 March 1999. It was attended by 185 participants

CACS conducted a seminar on Risk Management by Dr Takayuki Minato, Asian Institute of Technology, Thailand, on 4 April 1999.It was attended by 50 participants

A/P Edmund Choi has successfully developed a Testing Device for the Field Testing of Water Tightness of Building Envelope for the Building and Construction Authority (BCA), under their Innovation Development Scheme. A patent has been applied, and is now under the patent pending status. The patent will be jointly owned by A/P Choi and his industrial partner.

A/P Edmund Choi has developed a Testing Method with Defined Testing Parameters for on-site water tightness testing of curtain walls and windows for BCA. The method and the parameters have been adopted by BCA for their CONQUAS Scheme for building quality assessment.

As the chairman drafting the “Wind Loading Code” for Singapore, A/P Edmund Choi represented Singapore at the APEC network group meeting on “Code alignment in the APEC region”. He is also a member of the “International Codification Forum” sponsored by the International Association for Wind Engineering.

Prof A Appan was invited to deliver a paper titled ” Water Pollution and health in some megacities in South East Asia” at the Conference on Water and Security in South East Asia & Oceania. The conference was sponsored by the Global Environmental Change and Human Security Project of the IHDP and HDP, Canberra, Australia, from 16 to 19 November 1998

Prof A Appan was invited to deliver a paper titled “Economic and Water Quality aspects of Rainwater



Catchment Systems”, at the International Symposium on Efficient Water use in Urban Areas, Innovative ways of finding water for Cities, in Kobe, Japan, from 8-10 June 1999


CSE Research Bulletin No. 13. January 2000: Research Reports

ISSN 0219-0370 January 2000 NO. 13

Abstracts of all the research reports are available in the form of PDF (portable document file) format. If

you do not have PDF, please click to download and install the Acrobat Reader for Windows 95, Windows 3.1, Window-NT, DOS, Macintosh, OS2, LINUX, etc. To view and print the article, run Acrobat Reader and click the link below.

Abstracts of research reports

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CSE research Bulletin No. 13 January 2000

1

RESEARCH REPORTS

Interpretation of In-Situ Tests in Clay with Particular Reference to Reclaimed Sites

Candidate: Cao Laifa

Report No: CSE/PhD/1998/15

A solution to the undrained expansion and subsequent contraction of both spherical and

cylindrical cavities in modified Cam clay material has been developed. The solution was

applied to the interpretation of in-situ tests including the piezocone test, the self-boring

pressuremeter test and the dilatometer test. Soil parameters, especially the

overconsolidation ratio estimated from these in-situ tests, are in good agreement with

results obtained by other investigation methods. An intensive test program comprising in-

situ and laboratory tests was conducted at four recently reclaimed sites in Singapore. The

proposed methods of interpretation have been modified for application in clays which are

still undergoing consolidation. The degree of consolidation predicted from in-situ tests has

been found to agree well with that obtained from laboratory tests and field observation.

Soil parameters derived from in-situ tests have also been found to be reasonable.


2

Nonlinear Finite Element Analysis of RC Plates and Shells Using the Unified

Strength Theory

Candidate: Wang Fang

Report No: CSE/PhD/16

A new finite element model of RC plates and shells has been successfully developed by

using the present plastic-damage material model in combination with the proposed

strength criterion for concrete. The new plastic-damage model requires only one

parameter in derivation. It is capable of predicting with reasonable accuracy the overall

responses of not only the ultimate load-carrying capacity, but also the progressive failure

modes and failure crack pattern for all the numerical examples given in the thesis. In

addition, it is able to reveal the weak spots so as to make optimised structural design

possible.


3

Riprap Protection and Failure Mechanisms at Bridge Pier

Candidate: Lim Foo Hoat

Report No: CSE/1999/PhD/ 17

Experiments conducted under live-bed conditions show that a riprap layer at a cylindrical

bridge pier will fail in either one of the following two modes: total disintegration or

embedment. The study proposes a criterion to demarcate the limiting condition between

the two types of failure. It also confirms that embedment failure is a more common failure

mode of riprap layer under live-bed conditions. The causes of embedment failure are two-

fold: (1) bed feature destabilization; and (2) differential mobility. Bed level fluctuations

caused by the propagating bed features result in bed feature destabilization whereas

differential mobility is due to the different response of the riprap stones and bed sediments

to the flow field. Experimental results also show that the riprap layer can degrade to an

equilibrium level for a given flow condition. Finally, the study proposes a semi-empirical

equation to compute the maximum depth of riprap degradation, which occurs at the upper

end of dune regime.


4

Fluid Transients Induced Vibration in a Pipeline System

Candidate: Wang Zhongmin


The main objectives of the study were to establish a computational model and numerical

methods for the coupled phenomena, and hence to evaluate the performance of a long

water supply pipeline subjected to a strong earthquake load.

The governing equations for the fluid transients and the induced structural responses of a

fluid-filled pipe system were derived. The effects of fluid transient pressure on pipe

vibration in the longitudinal and lateral directions, and the torsion about the longitudinal

axis are reflected in the structural vibration equations. The effect of pipe movement on the

fluid, through Poisson’s coefficient and other coupling terms, is in turn included in the

extended fluid transient equations. A solution procedure combining the method of

characteristics (MOC) and the finite element method (FEM) was used to solve the coupled

equations. A feedback-loop-iteration approach was adopted in the procedure.

Application of the FSI model to a typical water-supply main and an artificial seismic loading

on the pipe main indicates that the magnitude of the induced pressure is discernible and

could be considerable in some cases, such as during concurrent dynamic structural

loading (earthquake) and fluid flow interruption (pump failure).


5

Experimental and Numerical Study of the Behaviour of Piles Subjected to Lateral

Soil Movement

Candidate: Pan Jieliang

Report N0: CSE/PhD/1999/19

Experimental and numerical studies were carried out to study the behaviour of piles in soft

clay when subjected to lateral forces induced by horizontal soil movements from nearby

construction activities such as deep excavations, tunnelling and approach embankments.

In the experimental study, a uniform soil movement profile was applied on model piles

embedded in soft clay. Both single piles and coupled pile tests with different pile flexibility

and pile fixity conditions were carried out. Three-dimensional finite element analyses were

also carried out to back analyse the model pile tests. The results showed fairly reasonable

agreement with the model test results. The computed distributions of the maximum soil

pressures acting along the pile shaft were similar to the corresponding model pile test

results. Based on the experimental data and numerical analyses, some simplified limiting

soil pressure distributions have been proposed for preliminary design of these piles.


6

Effects of construction methods and concrete cracking on the performance of

propped diaphragm walls

Candidate: Poh Teoh Yaw


Diaphragm walls are increasingly being used in Singapore for supporting propped

excavations. In this research, three case histories of propped excavations were back

analysed and extensive parametric studies were conducted to study the effects of concrete

cracking on their performance. Also, case histories of wall panel construction were

evaluated to provide a better understanding of the effects of the wall construction on

adjacent ground deformations and changes in lateral earth pressures. In addition, the

performance of a well-instrumented jet grout trial and the production grouting at a site in

Singapore were evaluated to investigate the effects of jet grouting on the diaphragm walls,

adjacent ground, and nearby structures.


7

Discrete Element Modelling of Jointed Rock Masses under Dynamic Loading

Candidate: Chen Shou Gen


This study investigated the discrete element code, UDEC, in modelling the behaviour of

jointed rock masses under dynamic loading. The project was done in three phases.

Firstly, the UDEC was verified on wave attenuation in rock material and at joints. It was

found that the UDEC is able to model wave propagation in jointed rock masses. However,

some improvements had to be made for modelling explosion problems. The UDEC was

then improved and a coupled method of UDEC and AUTODYN was proposed to model the

detonation process and the wave propagation in jointed rock masses. The dynamic

equation of state of rock material was incorporated into the UDEC to model the behaviour

of the rock material under high strain rate. Finally, the improved UDEC was utilised to

model a field explosion test. The modelling results are significantly better than the

predictions of empirical formulae.


8

Dynamic Buckling of Columns and Plates under Intermediate Velocity Impact

Candidate: Cui Shijie


It was realized a few decades ago that dynamic buckling of structures depends on both the

amplitude and duration of impact load. Most previous studies of dynamic buckling of

structures were, however, focused on either high velocity or low velocity impact, neglecting

the loading duration effect. The present project investigated the dynamic buckling

characteristics of columns and plates under intermediate velocity impact with consideration

of both the amplitude and duration effect. The project involved analyzing the previously

obtained test data, deriving close form solutions for columns by using the nonlinear

dynamics theory, and numerically calculating the dynamic response and buckling of plates.

The dynamic buckling, yielding and collapsing criteria of columns and plates were defined.

Their relationships with the structural properties such as slenderness ratio, aspect ratio,

boundary conditions and material parameters, as well as the amplitude and duration of

impact load were derived and explicitly presented. Buckling and post-buckling

characteristics are also discussed with respect to structural and loading properties.


9

Coastal Zone Environmental and Resource Management

Principal Investigator: Tan Soon Keat

Report No: CSE/1998/59

This report presents the findings of an attempt, firstly, to illustrate that sandy sediment

accumulates in some natural sediment sinks in Singapore waters, secondly, to locate

them, and, finally, to estimate the probable quantity and rate of sediment accumulation

there.

The evidence gathered so far shows that natural sinks for sediment deposit are located at

the confluence of tidal streams. Examples of locations are the Raffles Shoal, Sultan Shoal

and Sister Shoal. Sediment at these areas has been dredged and used as landfill

materials. The investigation at Sister showed that a sandpit of 120 m by 70 m trapped a

total of 4684 m3 of sediment over a period of 6 months, making an average sediment

deposit of 1.1 m over a period of 1 year.


10

Flow and Entrainment Characteristics of Density Stratified Liquids In A Slotted

Compartment Induced by A Submerged Flow Stream

Principal Investigator: Chen Charng Ning

Report No : CSE/98/60

This project investigates the entrainment characteristics and feedback mechanisms in

mixing layer oscillations induced by a flow beneath a rectangular compartment. An

approximate unified equation was derived for estimating the mixing rates across a density

interface in rectangular and circular tanks with partially open bottoms, induced by a

submerged flow stream beneath the tanks. The dynamics of a free shear layer, formed as

the flow passed underneath a submerged rectangular compartment, were studied under

both non-impinging and impinging conditions. Under non-impinging conditions, the flow

resembled a wall jet. The presence of large vortical structures in the mixing layer above

the potential core was observed and quantified. The passage frequency of these

structures was well predicted by the linear stability theory. Fluid-dynamic and fluid-

resonance feedback mechanisms were studied for the flow under impinging conditions.

Two resonating conditions in fluid-resonance feedback were studied: liquid column mass

oscillations and internal wave oscillations. The results show that, when fluid-dynamic and

fluid-resonance conditions co-exist, the two feedback mechanisms may compete for

control of the mixing layer oscillation frequency.


11

Evaluation of Risks in BOT Projects

Principal Investigator: Robert Tiong


The project involved an international survey on risk management of BOT projects in

developing countries with emphasis on power projects in China. The objectives of the

survey were:

• To identify the unique or critical risks associated with China's BOT projects;

• To evaluate the effectiveness of measures to mitigate these risks;

• To examine the adequacy of key contract clauses used in the first state approved BOT

project in China (Laibin B) that are related to these risks and to propose possible

improvements to these contract clauses;

• To provide a risk management framework for investing in future BOT projects in China.

Ten unique or critical risks associated with China's BOT power projects, in descending

order of criticality, were identified: Tariff Adjustment, the Chinese Parties' Reliability and

Creditworthiness, Dispatch Constraint, Change in Law, Exchange Rate and Convertibility,

Force Majeure, Financial Closing, Delay in Approval, Expropriation and Corruption. Most

measures for mitigating each of these risks were regarded as effective or very effective

while a few were only fairly effective. The key contract clauses related to these risks in

Laibin B contracts were regarded as generally adequate. However, improvements are

needed in the area of foreign exchange approvals and tariff adjustments. A risk

management framework for investing in China's BOT projects was also recommended.


12

Development of the Asymptotic State Concept For Soils and Its Application to Slope

Stability Analysis

Principal Investigator: Chu Jian


A new state concept, the so-called asymptotic state, has been proposed based on

experimental studies on sand and clay. Further experimental study was carried out on a

laboratory compacted residual soil to verify whether the asymptotic state could also be

established for residual soils. Compaction, consolidation, triaxial drained, triaxial

undrained, stress/strain path, and ring shear tests were conducted to establish the stress-

strain behaviour of the tested soil. The stress-dilatancy relation at the asymptotic state

was established. The effect of water infiltration on the strain hardening and softening

behaviour of the soil was also studied through stain path tests.

The anisotropic strength behaviour of an undisturbed residual soil was studied. The failure

envelopes determined for both horizontal and vertical cut specimens were curved and

went through the origin. Both drained and undrained strengths in the horizontal direction

were stronger than that in the vertical direction. The instability behaviour of an undisturbed,

unsaturated residual soil was also investigated.

The residual strength of two different residual soils was studied. The residual friction angle

for both residual soils at unsaturated state was in the order of 20 to 300. The residual

strength measured was relatively high. However, this was expected for soil with a clay

fraction of 20%. There was no residual strength when the soil was soaked in water.


13

Strength and Deformation Characteristics of Granitic Rocks In Singapore

Principal Investigator: Zhao Jian

Project No: CSE/1999/63

The project studied the strength and deformation characteristics of the Bukit Timah granite

of Singapore under static and dynamic compression. Tests, including static uniaxial

compression and triaxial compression at different confining stresses, dynamic uniaxial

compression and triaxial compression at different loading rates and confining stresses,

were conducted.

The studies show that the Bukit Timah granite exhibits typical class II post-peak behaviour

under uniaxial compression and at high confining stresses, it exhibits class I post-peak

behaviour, under static compression loads. The strength characteristics can be described

by the Hoek-Brown criterion.

The compressive strengths (both uniaxial and triaxial) of the granite increase with

increasing loading rate. The failure mode becomes more brittle with increasing loading

rate.

Modulus of elasticity and the Poisson's ratio, however, are not significantly effected by the

confining stress and loading rate.

The study confirmed and quantified the change of strength of the granite with change of

loading rate, and also examined the mechanism of the change in strength. It provides a

better understanding on the behaviour of the rock under dynamic loading.


14

Large Prestressed High Strength Concrete Deep Beams

Principal Investigator: Tan Kang Hai

Project No: Cse/1999/64

This research comprises two sections. Part A focuses on the general behaviour, analysis

and design in the shear strength of large reinforced and post-tensioned concrete deep

beams. The influence of main steel ratio, concrete strength, shear span-to-overall-height

ratio, overall height and prestress force on deep beams were investigated.

A total of 46 beams were tested in Part A. A simple strut-and-tie model based on the

Canadian Code was used for the predictions of 46 reinforced and post-tensioned deep

beams, together with 233 specimens collected from literature.

Studies in Part B focus on the strut-and-tie approach to shear strength predictions of deep

beams. Both non-prestressed and prestressed deep beams as well as deep beams with

web openings were considered. A different strut-and-tie model was used that

encompasses non-prestressed deep beams, deep beams with web openings and

prestressed deep beams. All the predictions were verified by experimental results and

code predictions. It was shown that the proposed strut-and-tie model in Part B agrees well

with all the different case studies.


15

The Effect Of Rainfall On The Slope Stability Of Residual Soil In Singapore

Principal Investigator: Rahardjo Harianto


This research project was undertaken to characterise the soil properties of the residual

soils of Singapore and to study the mechanism of rainfall-induced slope failures.

The residual soils of the sedimentary Jurong Formation were found to have an effective

cohesion, c’ of approximately 30 kPa, an effective angle of internal friction, φ’ of 26

degrees and a φb angle of 26 degrees. Field instrumentation revealed that maximum

changes in matric suction occur near the ground and the decrease in matric suction during

rainfall events is highly dependent on the surface conditions of the slope. Field

reconnaissance showed that rainfall-induced slips are relatively shallow (one to two

metres). The daily and antecedent rainfalls are important triggering factors for landslides in

the residual soils of the sedimentary Jurong Formation.

Hysteresis was found to have a profound effect on the shear strength behaviour of

unsaturated soils. Soils following a wetting curve were found to have lower strengths than

soils following a drying curve. The stress path followed during shearing was found to affect

the strength of the soil. The shear strength obtained from constant suction shearing tests

was lower than that measured in the infiltration shearing tests.


16

Flux Boundary Conditions For Geotechnical Problems

Principal Investigator: Rahardjo Harianto


The effect of typical flux boundary conditions, such as infiltration and evaporation, on pore-

water pressures in a residual soil slope was investigated through the use of field

instrumentation. The depth of infiltration was found to be approximately 3.0 metres and the

infiltration rates varied from 20% to 30% of the rainfall rate. The evaporation rate was

determined to be approximately 5 mm/day. Through field instrumentation, laboratory tests

and numerical analyses, the majority of infiltration was found to occur at the crest of a

slope. The factor of safety of a slope was found to increase by 34% during evaporation

and to decrease by 27% during infiltration under a typical rainfall event.

Horizontal drains that were installed in the face of a slope were found to be ineffective for

intercepting infiltration and were unable to maintain matric suctions in the slope.

Horizontal drains located at the toe of the slope were found to be beneficial for slope

stability since they prevent rises in the groundwater level.

Soils at wetting were found to be more brittle and had a lower Young’s Modulus than soils

at drying. It was also found that rapid deformation occurs at failure during infiltration. This

may explain the possible catastrophic nature of rainfall-induced landslides.


17

Floating Tied System for Offshore Airport

Principal Investigator: Wibisono Hartono


This research investigated the possible implementation of a new system of floating airport.

Unlike the other systems that are also currently under research, it does not move and

oscillate under the action of waves and wind, however it still makes use of the buoyancy of

seawater to resist the gravity forces. The floating tied system is a box like concrete

structure fixed to the seabed by a fibre composite cable system. The cable system forms a

diagonal bracing at each side of the box so that the structure will be fixed in both vertical

and horizontal direction. The research studied the structural behaviour of the floating tied

system under static and dynamic loads (seismic excitation) using the Finite Element

method. A comparison of the structural dynamic responses with and without incorporating

the mass of surrounding water was made.


18

Biological Treatment Of High Strength Wastewaters Using Attached Biomass Films

Principal Investigator: Francis Wilson


The purpose of the study was to investigate aspects of the use of the Rotating Biological

Contactor (RBC) to treat high strength wastewater in a tropical climate. Total organic

carbon (TOC), BOD and COD were used as analytical parameters to determine the

efficiency of treatment of the wastewater. The experiment used high influent

concentrations (400 - 6500 mg COD/L) and high loading rates, applied to a laboratory

scale rotating disc system operating under ambient temperatures in Singapore. The results

indicated that the RBC system, under these temperature conditions, could produce high

treatment efficiencies, with effluent qualities meeting the local sewer discharge standard of

215 mg/L for loads up to 21 g TOC/m2.d. Correlation studies of the three analytical

parameters of TOC, BOD and COD produced high correlation coefficients, and it was

therefore concluded that the quicker TOC test would be satisfactory for use as the main

analytical parameter.


19

Investigation and Studies on the Design of Reinforced Concrete Composite

Structures

Principal Investigator: Chiew Sing Ping


The research work described in this report can be broadly divided into two parts. Part A of

the report touches on the general aspects of finite element modeling and analysis, and

experimental comparison and verification of important structural elements such as

concrete-filled composite columns. This includes numerical simulation, study of the

influence of boundary conditions on the simulation of failure mode of steel circular hollow

section (CHS) columns and simulations of concrete-filled steel CHS composite columns.

Part B of the report details the development of a damage model of 3-D interface element

which can be incorporated into an existing finite element code to model the interface bond

behavior between the concrete and steel surfaces which exists in all concrete-steel

composite elements.


20

Wind and Wind-driven Rain Characteristics in Singapore

Principal Investigator: Edmund C.C. Choi.


This project is a study on the wind and wind-driven rain (WDR) characteristics in

Singapore. Buildings and their performance are affected by wind-driven rain in many

ways. Water leakage into the interior of a building can cause damage to building contents,

deterioration of interior finishing and disruption of daily life of the occupants. Water

penetration and absorption also leads to the decay and deterioration of building material.

A good understanding of the wind-driven rain phenomenon will help to prevent and

minimize these problems.

This project investigates the unobstructed wind-driven rain over an open field. It looks into

three important areas of WDR, intensity-duration relationship, driving rain intensity

coefficient, Cdr(Ih), and raindrop size distribution (DSD). The intensity-duration relationship

studies the intensity of vertical rainfall and driving rain at short duration. The study of the

driving rain coefficient investigates the relationship of WDR with wind and rain.

Another factor which affects wind-driven rain is the wind itself. Its characteristics,

especially the characteristics of wind during thunderstorms, are investigated. Wind

properties such as gust factors, velocity profile and also the co-occurrence of wind and

rain are also being studied.


21

Biotechnological Application on Wastewater Treatment

Principal Investigator: Tay Joo Hwa


The objective of the project is to investigate biogranulation in various biological processes

in wastewater treatment such as upflow anaerobic sludge blanket reactors, anaerobic

filters, hybrid reactors and others.

One major accomplishment of the project is the development of a biological selection

pressure, termed “Microbial Load Index” (MLI), to represent relative substrate sufficiency

to which the biomass is subjected during startup of reactors. The MLI provides a useful

tool to quantitatively measure and expedite startup and biogranulation processes. Another

important finding of the study is that media porosity plays a more important role than

media surface area in the performance of anaerobic filters. It has been established that

support media of larger porosity promote growth of suspended granules with less

likelihood of clogging. A neural fuzzy technique was developed to model the complex

systems instead of conventional kinetic methods. The model, combining the merits of both

neural networks and fuzzy systems, has successfully simulated the anaerobic systems

and presented a good prediction of system upset one hour in advance. A molecular

mechanism model of sludge granulation based on proton translocation-dehydration theory

was proposed in this project. The experimental results showed that proton translocating

activity on bacterial surfaces was a crucial factor in sludge granulation.

CSE Research Bulletin No. 13. January 2000: Publications

ISSN 0219-0370 January 2000 NO. 13

The CSE publications list is available in the form of PDF (portable document file) format. If you do not

have PDF, please click to download and install the Acrobat Reader for Windows 95, Windows 3.1, Window-NT, DOS, Macintosh, OS2, LINUX, etc. To view and print the article, run Acrobat Reader and click the link below.

CSE publications list

file:///D|/CEE_old/RESEARCH/Bulletin/1999_2000/HTM/Publications.htm9/27/2003 10:49:58 AM



1

PUBLICATIONS

Anandrajah, A.* and Chu, J., 1998. The strength behaviour of an undisturbed residual soil.Proceedings of the 13th Southeast Asian Geotechnical Conference, 6-20 November,Taipei, pp. 7-12.

Appan, A. and Rahman, A.*,1998. Recycling of wastewater – an inevitable option forcities: The case of Singapore. Proceedings of the 8th Water Symposium, StockholmInternational Water Institute, Stockholm, Sweden, 10-13 August.

Appan, A., 1998. Water pollution and health in some megacities in South East Asia.(Invited Paper) Proceedings of the Workshop on Water and Security in South East Asia &Oceania, sponsored by the Global Environmental Change and Human Security Project ofthe IHDP and HDP, Canberra, Australia, 16-19 November.

Appan, A., 1999. Economic and water quality aspects of rainwater catchment systems.(Invited paper) International Symposium on Efficient Water use in Urban Areas: Innovativeways of finding water for cities, 8-10 June, Kobe, Japan.

Appan, A., 1999. Trends in water demands and the role of rainwater catchment systemsin the next millennium. (Keynote address) Proceedings of the 9th International Conferenceon Rainwater Catchment Systems, Petrolina , Brazil, 6-9 July.

Appan, A., 1999. Water supply and sanitation: trends and developments. (Invited paper)Proceedings of the 9th Water Symposium, Stockholm International Water Institute,Stockholm, Sweden, 9-12 August.

Bo Myint Win*, Cao, L.F., Chu, J. and Choa, V.*, 1998. One dimensional consolidationtests on Singapore marine clay at Changi. Proceedings of the 13th Southeast AsianGeotechnical Conference, 6-20 November, Taipei, pp. 199-206.

Brownjohn, J.M.W. and Xia, P., 1999. Finite element updating of a damaged structure.Proceedings of the IMAC 17, Florida, USA, pp. 457-462.

Brownjohn, J.M.W., 1998. Dynamics of an aerial cableway system. EngineeringStructures, UK, Vol. 20, No. 9, pp. 826-836.

Brownjohn, J.M.W., Pan, T.C. and Choi, E.C., 1998. Dynamic loading and response for atall building. Proceedings of the 5th International Conference on Tall Buildings, HongKong, 9-11 December, pp. 730-735.

Cai, J., Zhao, J. and Hudson, J.A.*, 1998. Computerised rock engineering systems withneural networks and expert system. Rock Mechanics and Rock Engineering, Vol. 31, pp.135-152.

Cao, L.F., Na, Y.M.*, Bo Myint Win*, Choa, V.* and Chang, M.F., 1998. Evaluation ofsand densification by in-situ tests. Proceedings of the 2nd International Conference onGround Improvement Techniques, Singapore, October, pp. 93-100.


2

Chan, T.K., Soh, C.K. and Fung, T.C., 1998. Experimental study of a full-scale multiplanartubular XT-joint. Proceedings of the 8 th International Symposium on Tubular Structures inthe area of Static and Fatigue Behaviour of Connections, pp. 131-138.

Chang, M.F. and Zhu, H., 1998. A t-z curve with consideration of modulus degradation forpile analyses. Proceedings of the 13th Southeast Asian Geotechnical Conference, Taipei,November, pp. 461-466.

Chang, M.F., Choa, V.* and Bo Myint Win*, 1998. Use of in-situ tests in land reclamationprojects in Singapore. Proceedings of the 13th Southeast Asian Geotechnical Conference,Taipei, November, pp. 755-762.

Chang, M.F., Tsen, C.N.* and Kho C.M.*, 1998. Performance of cast-in-place pilessocketed into limestone for a bridge foundation in Singapore. Proceedings of the RegionalSymposium on Sedimentary Rock Engineering, Taipei, November, pp. 60-65.

Chen, C.N. and Shuy, E.B., 1998. Flow and entrainment characteristics of densitystratified liquids in a slotted compartment induced by a submerged flow stream. NTU-CUED Collaboration Project Report CUED-01, School of Civil and Structural Engineering.

Chen, C.N., Adrian Law, W.K. and Sun, W.D.*, 1999. Measures to mitigate the formationof sandbars at coastal drainage outlets. ENV-NTU-NUS Joint Research Seminar, 31 May.

Chen, S.G. and Zhao, J., 1998. A data processing and management method for caverndeformation monitoring. Rock Mechanics and Rock Engineering, Vol. 31, pp. 125-131.

Chen, S.G. and Zhao, J., 1998. A study of UDEC modelling for blast wave propagation injointed rock masses. International Journal of Rock Mechanics and Mining Sciences, Vol.35, pp. 93-99.

Cheng, N.S. and Chiew, Y.M., 1998. Modified logarithmic law for velocity distributionsubjected to bed seepage. Journal of Hydraulic Engineering, ASCE, Vol. 124, No. 12, pp.1235-1241.

Cheng, N.S. and Chiew, Y.M., 1998. Turbulent open-channel flow with upward seepage.Journal of Hydraulic Research, IAHR, Vol. 36, No. 3, pp. 415-431.

Cheong H.K., 1998. Engineering education in Singapore - towards a new paradigm.ASAIHL Seminar on “New trends in higher education: market mechanisms in highereducation towards the 21st century”, Jakarta, Indonesia, July.

Cheong, H.K., Hao, H. and Cui, S.J.*, 1998. Effect of damping on the intermediate impactbuckling properties of column. International Symposium on Strength Theories:Applications and Development, Xi’an, China, September.

Cheong, H.K., Hao, H. and Cui, S.J.*, 1999. Fluid-solid impacted buckling of imperfectcolumns. Proceedings of the 13th ASCE Engineering Mechanics Conference, The JohnsHopkins University, Baltimore MD, USA, 13-16 June, Session M3B, Paper no. 3.


3

Chiew, S.P., Soh, C.K. and Wu, N.W., 1999. SCF equations for the fatigue design ofmultiplanar tubular XT-joints. Proceedings of the 9th International Offshore and PolarEngineering Conference, ISOPE-99, Brest, France, 30 May to 4 June, Vol. IV, pp. 82-89.

Chiew, S.P., Soh, C.K., Fung, T.C. and Soh, A.K.*, 1999. Numerical study of multiplanartubular DX-joints subject to axial loads. Journal of Computers and Structures, UK, July,Vol. 72, No. 6, pp. 749-761.

Choi, E.C.C. and Wang, Z.H.*, 1998. Study of pressure-equalization of curtain wall.Journal of Wind Engineering and Industrial Aerodynamics, Netherland, Vol. 73, pp. 251-266.

Choi, E.C.C., 1998. Water penetration testing criteria for high-rise buildings. Proceedingsof the 5th International Conference on Tall Buildings, Hong Kong, pp. 656-661.

Choi, E.C.C., 1999. Characteristics of wind and wind-driven rain during tropicalthunderstorms. Proceedings of the 10th International Conference on Wind Engineering,Denmark, pp. 729-736.

Choi, E.C.C., 1999. Wind-driven rain on building faces and the driving rain index. Journalof Wind Engineering and Industrial Aerodynamics, Netherlands, Vol. 79, pp. 105-122.

Chu, J. and Leong, W.K., 1999. Discussion on ‘Static liquefaction of very loose sand”.Canadian Geotechnical Journal, Vol. 36, No. 3, pp. 578.

Chu, J., 1999. Discussion on “Quasi-steady state: a real behaviour?” CanadianGeotechnical Journal, Vol. 36, No.1, pp. 190-191.

Chu, J., Bo Myint Win* and Choa, V.*, 1999. Determination of undrained shear strength ofclay by direct simple shear tests. Proceedings of the 11th Asian Regional Conference onSoil Mechanics and Foundation Engineering, Seoul, 16-20 September, Vol. 1, pp. 49-52.

Chu, J., Chong, K.L*. and Wee, S.E.*, 1998. Instability behaviour of a residual soil.Proceedings of the 13th Southeast Asian Geotechnical Conference, 6-20 November,Taipei, pp. 41-46.

Chu, J., Leong, W.K., Jai, X.L.* and Lee, C.W.*, 1999. Stability of submarine granularslopes. Proceedings of the 2nd International Conference on Landslides and Stability andSafety of Infrastructures, Singapore, 27-28 July, pp. 139-147.

Chuang, P.H. and Kong, S.K.*, 1998. Strength of slender reinforced concrete columns.Journal of Structural Engineering, ASCE, Vol. 124, No. 9, pp. 992-998.

Chuang, P.H. and Li, X., 1999. Nonlinear frame analysis using flexibility approach withmathematical programming application. USNCCM99 Book of Abstracts,USACM/USNCCM99 5th US National Congress on Computational Mechanics, Universityof Colorado, Boulder, Colorado, 4-6 August, pp. 488-489.


4

Chuang, P.H., Goh, A.T.C. and Wu, X.*, 1998. Modeling the capacity of pin-ended slenderreinforced concrete columns using neural network. Journal of Structural Engineering,ASCE, Vol. 124, No. 7, pp. 830-838.

Cui, S.J.*, Cheong, H.K. and Hao, H., 1998. Theoretical analysis of dynamic buckling of acolumn under intermediate velocity impact. Proceedings of the 5th InternationalConference for Young Experts on Structural Engineering, 18-21 August, Shenyang, China,pp. 150-160.

Cui, S.J.*, Cheong, H.K. and Hao, H., 1999. Dynamic buckling of simply-supportedcolumns under axial slamming. Journal of Engineering Mechanics, ASCE, May, Vol. 125,No. 5, pp. 513-520.

Cui, S.J.*, Cheong, H.K. and Hao, H., 1999. Experimental study of dynamic buckling ofplates under fluid-solid slamming. International Journal of Impact Engineering, UK, Vol.22, pp. 675-691.

Cui, S.J.*, Hao, H., and Cheong, H.K., 1999. Dynamic buckling investigation of columnunder impact load. Proceedings of 13th ASCE Engineering Mechanics Conference, TheJohns Hopkins University, Baltimore MD, USA, 13-16 June, Session M1C, Paper no. 6.

Dong, Y.X. and Chiew, S.P., 1998. Influence of boundary conditions on the modeling ofsteel CHS tubular columns. Proceedings of the 8th International Symposium on TubularStructures, Singapore, 26-28 August, pp. 417-424.

Dong, Y.X., Zhang, C.H.*, Wang, G.L.* and Chiew, S.P., 1999. Non-overlappingconditions for crack face displacement of anisotropic bodies. Journal of Theoretical andApplied Fracture Mechanics, UK, May, Vol. 31, No. 2, pp. 99-104

Dong, Y.X., Zhang, C.H.*, Wang, G.L.* and Chiew, S.P., 1999. Strain factor ratio criterionfor crack extension in anisotropic plates. Journal of Engineering. Mechanics, ASCE,January, Vol. 125, No.1, pp. 119-122.

Findikakis, A.N.* and Law, A.W.K. 1999. Wind-mixing in temperature simulations for lakesand reservoirs. Journal of Environmental Engineering, ASCE, Vol. 125, No. 5, pp. 420-428.

Fung, T.C. and Chow, S.K.*, 1999. Responses of blast loading by complex time stepmethod. Journal of Sound and Vibration, Vol. 223, No. 1, pp. 23-48.

Fung, T.C. and Tan, K.H., 1998. Shear stiffness for Z-core sandwich panels. ASCEJournal of Structural Engineering, Vol. 124, No. 7, pp. 809-816.

Fung, T.C., 1999. Higher order accurate least squares methods for first order initial valueproblems. International Journal for Numerical Methods in Engineering, Vol. 45, pp. 77-99.

Fung, T.C., 1999. On the accuracy of Galerkin methods in time domain. Journal ofVibration and Control, Vol. 5, No. 2, pp. 155-174.


5

Fung, T.C., Fan, S.C. and Sheng, G.*, 1998. Mixed space-time finite elements forvibration response analysis. Journal of Sound and Vibration, Vol. 213, No. 3, pp. 409-428.

Fung, T.C., Soh, C.K., Gho, W.M.* and Cheung, B.L.Y.*, 1998. Current understanding ofcompletely overlap tubular joints. Proceedings of the 8th International Symposium onTubular Structures in the area of Static and Fatigue Behaviour of Connections, Singapore,26-28 August, pp. 239-247.

Gasmo, J.*, Hritzuk, K.J.*, Rahardjo, H. and Leong, E.C., 1999. Instrumentation of anunsaturated residual soil slope. ASTM Geotechnical Testing Journal, Vol. 22, No. 2, pp.134-143

Gasmo, J.M.*, Hritzuk, K.J.*, Rahardjo, H. and Leong, E.C., 1999. Instrumentation of anunsaturated residual soil slope. Geotechnical Testing Journal, GTJODJ, June, Vol. 22,No. 2, pp. 128-137.

Goh, A.T.C., 1999. Soil laboratory data interpretation using generalized regression neuralnetwork. Civil Engineering and Environmental Systems, Vol. 16, pp. 175-195.

Goh, A.T.C., Teh, C.I. and Wong, K.S., 1999. Analysis of piles subjected to embankmentinduced lateral soil movements - closure. Journal of Geotechnical and GeoenvironmentalEngineering, ASCE, Vol. 125, No. 5, pp. 427.

Guo, C.J. and Zhao, J., 1998. Engineering properties of the weathered carbonate rocksof Singapore. Regional Symposium on Sedimentary Rock Engineering, Taipei, Taiwan.

Gupta, A.* and Soh, C.K., 1999. An intelligent interactive tutoring system: our experienceat NTU. Proceedings of the 2nd UICEE Conference on Engineering Education, Auckland,New Zealand, pp. 222-224.

Hao, H. and Ang, T.C., 1998. Analytical modeling of traffic-induced ground vibrations.Journal of Engineering Mechanics, ASCE, Vol. 124, No. 8, pp. 921-928.

Hao, H. and Zhang, S.Y., 1999. Spatial ground motion effect on relative displacement ofadjacent building structures. International Journal of Earthquake Engineering andStructural Dynamics, Vol. 28, pp. 333-349.

Hao, H., 1998. Vibration in buildings caused by moving vehicles. Proceedings of theAcoustics and Vibration Asia ’98, Singapore, 11-13 November.

Hao, H., Ma, G.W. and Zhou, Y.X., 1998. Assessment of damage zone generated byunderground explosion. International Symposium on Structural Theories: Application &Developments, Xian, China, 9-11 September.

Hao, H., Ma, G.W. and Zhou, Y.X., 1998. Numerical simulation of underground explosion.International Journal for Blasting and Fragmentation, Vol. 2, pp. 383-395.

Hao, H., Ma, G.W., and Zhou, Y.X., 1998. Damage assessment of surface structuressubjected to ground shock excitation. Proceedings of the 5th International Conference forYoung Experts on Structural Engineering, 18-21 August, Shenyang, China, pp. 142-149.


6

Hao, H., Wu, Y.K., Ma, G.W. and Zhou, Y.X., 1998. Characteristics of ground motionsinduced by construction blasting in jointed rock mass. Proceedings of the Acoustics andVibration Asia ’98, 11-13 November, Singapore.

Hartono, W., 1998. A floating tied system for an offshore airport. Ocean Engineering, Vol.25, No. 7, pp. 591-596.

Jeyaseelan, S., 1999. Microbial activities in suspended growth and attached growth inacidogenous and methanogenous phases in anaerobic digestion. Proceedings of the 2nd

International Symposium on Anaerobic Digestion of Solid Waste, 15-18 June, Barcelona,Spain, Vol. II, pp. 125-128.

Jeyaseelan, S., 1999. Use of filterability constants as dewatering characteristics of oilysludge. Proceedings of the 2nd International Symposium on Anaerobic Digestion of SolidWaste, 15-18 June, Barcelona, Spain, Vol. II, pp. 164-167.

Krumholz, L.R.*, Harris, S.H.*, Tay, S.T. and Suflita, J.M.*, 1999. Characterization of twosubsurface H2-utilizing bacteria, Desulfomicrobium hypogeium sp. nov. andAcetobacterium psammolithicum sp. nov., and their ecological roles. Applied andEnvironmental Microbiology. Vol. 65, pp. 2300-2306.

Law, A.W.K. and Wang, H.W.*, 1998. Simultaneous velocity and concentrationmeasurements of buoyant jet discharges using combined DPIV and PLIF. Proceedings ofthe 2nd International Symposium on Environmental Hydraulics, Hong Kong, pp. 211-216.

Law, A.W.K., 1999. Wave-induced surface drift of an inextensible thin film. Journal ofOcean Engineering, UK, Vol. 26, Issue 11, pp. 1145-1168.

Lee, C.K. and Hobbs, R.E.*, 1998. Automatic adaptive finite element mesh generationover rational B-spline surfaces. Computer and Structures, Vol. 69, No. 6, pp. 577-608.

Lee, C.K. and Hobbs, R.E.*, 1999. Automatic adaptive finite element mesh generationover arbitrary 2D domain using advancing front technique. Computer and Structures, Vol.71, No. 1, pp. 9-34.

Lee, C.K. and Lo, S.H.*, 1998. On using different procedures for the construction ofsmoothed stress in finite element method. International Journal for Numerical Methods inEngineering, Vol. 43, No. 43, pp. 1223-1252.

Lee, C.K. and Lo, S.H.*, 1998. On using different stress recovery procedures in Reissner-Mindlin finite element modelling. Proceedings of the 4 th World Congress on ComputationalMechanics. Buenos Aires, Argentina (CD-ROM Proceeding).

Lee, C.K. and Lo, S.H.*, 1999. A full 3-D finite element analysis using adaptive refinementand PCG solver with back interpolation. Computer Methods in Applied Mechanics andEngineering, Vol. 170, No. 1-2, pp. 39-64.

Lee, C.K., 1999. Automatic adaptive mesh generation using metric advancing frontapproach. Engineering Computations, Vol. 16, No. 2, pp. 230-263.


7

Lee, C.K., Sze, K.Y.* and Lo, S.H.*, 1999. On using degenerated solid shell elements inadaptive refinement analysis. International Journal for Numerical Methods in Engineering,Vol. 45, No. 6, pp. 627-659.

Leong, E.C. and Rahardjo, H. 1998. Matric suction changes in a residual soil slope. CivilEngineering Research Bulletin, Nanyang Technological University, Singapore, January,pp. 22-23.

Leong, E.C. and Rahardjo, H., 1998. A review on soil classification systems. Proceedingsof the International Symposium on Problematic Soils, Sendai, Tohoku, Japan, 28-30October, pp. 493-497.

Leong, E.C. and Rahardjo, H., 1999. Discussion on soil-water characteristic curves forcompacted clay. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol.125, No. 12, pp. 629-630.

Leong, E.C., Rahardjo, H., Chiam, S.L.* and Gan, C.C.*, 1998. Suction profiles of aresidual soil slope as affected by climatic conditions. Proceedings of the 2nd InternationalConference on Unsaturated Soils, Beijing, China, 27-30 August, Vol. 1, pp. 231-236.

Leong, E.C., Rahardjo, H., Deustcher, M.S.* and Quan, C.N.*, 1998. Instrumentation anda data acquisition system for the study of a residual soil slope at a remote site.Proceedings of the 2nd International Conference on Unsaturated Soils, Beijing, China, 27-30 August, Vol. 1, pp. 383-388.

Leong, E.C., Rahardjo, H., Tang, S.K.* and Chiam, S.L.*, 1998. Role of soil cover inmaintaining slope stability. Proceedings of the 13th Southeast Asian GeotechnicalConference, Taipei, Taiwan, 16-20 November, pp. 99-104.

Leong, W.K., Chu, J. and Teh, C.I., 1999. Instability behaviour of a granular fill.Proceedings of the Australia and New Zealand Conference on Geomechanics, 15-17February, Horbat.

Li, Q.M. and Jones, N.*, 1999. Shear and adiabatic shear failures in an impulsively loadedfully clamped beam. International Journal of Impact Engineering, Vol. 22, pp. 589-607.

Li, Q.M., 1999. Dissipative flow model based on dissipative surface and irreversiblethermodynamics. Archive of Applied Mechanics, Vol. 69, pp. 379-392.

Lie, S.T. and Lan, S.*, 1998. A boundary element analysis of misaligned load-carryingcruciform welded joints. International Journal of Fatigue, July, Vol. 20, No. 6, pp. 433-439.

Lie, S.T. and Lan, S.*, 1999. Computer prediction of misaligned welded joints. Advancesin Engineering Software, September, Vol. 30, No. 9, pp. 56-63.

Lie, S.T. and Li, Q., 1998. Analysis of tubular joints using coupled finite element andboundary element methods. Proceedings of the 8th International Symposium on TubularStructures, 26-28 August, Singapore, pp. 341-346.


8

Lie, S.T. and Xu, K., 1999. Time-domain boundary element method for transient responseof semi-infinite problems. Boundary Elements Communications, Vol. 10, No. 1/2, pp. 9-13.

Lie, S.T., Cen, Z.* and Xu, K., 1999. Analysis of 2D crack using symmetric galerkinboundary element method. Boundary Element Technology XIII, Las Vegas, USA, 8-10June, pp. 163-172.

Lie, S.T., Li, Q. and Cen, Z.*, 1998. Modelling tubular joints using a combination of finiteand boundary elements. Proceedings of the 8th China-Japan Symposium on BoundaryElement Method, Beijing, China, 11-15 May, pp. 257-266.

Lim, S.Y. and Cheng, N.S., 1998. Scouring in long contractions. Journal of Irrigation andDrainage Engineering, ASCE, Vol. 124, No. 5, pp. 258-261.

Lim, S.Y. and Chiew, Y.M., 1998. Effect of an upstream pile on pier scour. RiverSedimentation - Theory and Applications, Jayawardena, Lee and Wang (eds).Proceedings of the 7th International Symposium on River Sedimentation, Hong Kong, 16-18 December, pp. 153-158.

Lim, S.Y., 1998. Closure to "Equilibrium clear water scour around an abutment” Journal ofHydraulic Engineering, ASCE, Vol. 124, No. 10, pp. 1069-1073.

Lim, S.Y., 1999. Translation of Chapter 6: “Bed form movement” of the book “Mechanicsof Sediment Transport” by Chien, N. and Wan Z. from Chinese to English, published byASCE Press, January, pp. 193-248.

Lin, K.*, Wong, K.S. and Wong, I.H.*, 1998. FEM analysis of embankment over soft soilfoundation treated with DCM columns. Proceedings of the 2nd International Conference onGround Improvement Techniques, Singapore, October.

Liu, Q., Zhao, J., Lee, K.W. and Yang, K.S.*, 1999. Potential rock cavern development inthe Jurong sedimentary formation. Proceedings of the World Tunnel Congress '99, Oslo,Norway.

Lo, E.Y.M., 1998. A constrained dual membrane wave barrier. Proceedings of the 2nd

International Symposium On Environmental Hydraulics, 16-18 December, Hong Kong, pp.923-928.

Lo, E.Y.M., 1998. A flexible dual membrane wave barrier. Journal of Waterway, Port,Coastal, and Ocean Engineering, ASCE, Vol. 124, No. 5, pp. 264-271.

Lo, S.H.* and Lee, C.K. 1998. On using selective refinement scheme in adaptive finiteelement analysis. Proceedings of the 4th World Congress on Computational Mechanics.Buenos Aires, Argentina (CD-ROM Proceeding).

Lo, S.H.* and Lee, C.K., 1998. An optimal adaptive refinement strategy for full 3-D finiteelement analysis. Proceedings of the 4th International Conference on ComputationalStructures Technology. Edinburgh, Scotland, pp. 109-113.


9

Lo, S.H.* and Lee, C.K., 1998. Error estimation of degenerated shell elements.Proceedings of the 4th International Conference on Computational Structures Technology.Edinburgh, Scotland, pp. 95-99

Lo, S.H.* and Lee, C.K., 1998. On constructing accurate recovered stress fields for thefinite element solution of Reissner-Mindlin plate bending problems. Computer Methods inApplied Mechanics and Engineering, Vol. 160, No. 1/2, pp. 175-191.

Lo, S.H.* and Lee, C.K., 1998. On using special hybrid hexahedral element for theanalysis of thin wall structures. Proceedings of the 5th International Conference on TallBuildings. Hong Kong, China, Vol. 2, pp. 533-538.

Lo, S.H.* and Lee, C.K., 1998. Selective regional refinement procedure for adaptive finiteelement analysis. Computer and Structures, Vol. 68, No. 4, pp. 325-341.

Lok, T.S. and Cheng, Q.H., 1998. Influence of shear deformation on geometrically non-linear deflections of composite laminated plates. Proceedings of the 2nd InternationalConference on Thin-Walled Structures, Singapore, December, pp. 289-296.

Lok, T.S. and Pei, J.S.*, 1999. Blast and impact resistance of concrete reinforced withexternally-bonded glass fibremat. Proceedings of the 9th International Symposium onInteraction of the Effects of Munitions with Structures, Berlin, Germany, May, pp. 9-17.

Lok, T.S. and Xiao, J.R., 1998. A constitutive model of concrete with/without steel fibrereinforcement. Proceedings of the International Symposium on Strength Theories:Applications and Developments, Xi’an, China, Science Press, New York, September, pp.681-686.

Lok, T.S. and Xiao, J.R., 1998. Tensile behaviour and moment-curvature relation of steelfibre reinforced concrete. Magazine of Concrete Research, UK, December, Vol. 50, No. 4,pp. 359-368.

Lok, T.S. and Xiao, J.R., 1999. Effect of membrane action on the response of steel fibrereinforced concrete panels subjected to air-blast. Proceedings of the 9th InternationalSymposium on Interaction of the Effects of Munitions with Structures, Berlin, Germany,May, pp. 31-38.

Lok, T.S., Cheng, Q.H. and Heng, L.* 1999. Equivalent stiffness parameters of truss-coresandwich panel. Proceedings of the 9th International Offshore and Polar EngineeringConference, International Society for Offshore and Polar Engineering, Brest, France, June,Vol. IV, pp. 292-298.

Low, H.Y.*, Hao, H., and Ma, G.W., 1998. Numerical simulation of two types of RC slabunder blast loading. International Symposium on Structural Theories: Application &Developments, Xian, China, 9-11 September.

Luk, J.Y.K. and Ramsay, E.D.* 1998. An assessment of two driver information systems forroute diversion. Proceedings of the International Conference on Transportation into theNext Millennium. Singapore, pp. 39-49.


10

Luk, J.Y.K., 1998. Land use changes, public transport and travel demand in Australiancities. Proceedings of the 1st International Conference on Transportation and TrafficStudies, Beijing, pp. 40-49.

Ma, G.W. and Hao, H., 1999. Damage model for structural masonry. Proceedings of the13th ASCE Engineering Mechanics Conference, The Johns Hopkins University, BaltimoreMD, USA, 13-16 June, Session M3C, Paper no. 6.

Ma, G.W., Hao, H. and Iwasaki, S.*, 1999. Plastic limit analysis of a clamped circular platewith unified yield criterion. International Journal of Structural Engineering and Mechanics,Vol. 7, No. 5, pp. 513-525.

Ma, G.W., Hao, H. and Zhou, Y.X., 1998. Modeling of wave propagation induced byunderground explosion. Journal of Computers and Geotechnics, October, Vol. 22, No. 3-4, pp. 283-303.

Ma, G.W., Hao, H. and Zhou, Y.X., 1998. Spectral characteristics of ground shock waves.Proceedings of the Acoustics & Vibration Asia’98, 11-13 November, Singapore.

Ma, G.W., Hao, H., and Iwasaki, S.*, 1999. Unified plastic limit analysis of circular platesunder arbitrary load. Journal of Applied Mechanics, ASME, June, Vol. 66, No. 2, pp. 568-570.

Ma, G.W., Hao, H., Iwasaki, S.*, Miyamoto, Y.* and Deto, H.*, 1998. Plastic behavior ofcircular plate under soft impact. International Symposium on Structural Theories:Application & Developments. Xian, China, 9-11 September.

Melville, B.W.* and Chiew, Y.M., 1999. Time scale for local scour at bridge piers. Journalof Hydraulic Engineering, ASCE, Vol. 125, No. 1, pp. 59-65.

Menon, A.P.G. and Chin, K.K.*, 1998. The making of Singapore's electronic road pricingsystem. Proceedings of the International Conference on Transportation into the nextMillennium, Singapore, pp. 179-190.

Mines, R.A.W.*, Li, Q.M., Alias, A.*, Birch, R.S.* and Close, J.A.*, 1998. On themeasurement of the crush behaviour of structural foams in polymer composite sandwichconstruction. Experimental Mechanics, editor, I.M. Allison, Proceeding of the 11th

International Conference on Experimental Mechanics, Oxford, pp. 287-292.

Nguyen, M., Cheong, H.K. and Chan, T.K., 1998. Analysis of reinforced concrete beamsbonded by FRP plates. Journal of Building Sciences and Technology (Special issue)Ministry of Construction, Vietnam.

Olszewski, P. and Tan, C.S.*, 1999, Use of walking as a mode of travel in Singapore. 78th

Annual Meeting of the Transportation Research Board, Washington, D.C., USA.

Pan, T.-C. and Lu, Q.R., 1998. Response of Singapore buildings to long-distanceearthquakes. Invited Paper, Proceedings of the Asia-Pacific Workshop on Seismic Designand Retrofit of Structures, 10-12 August, Taipei, Taiwan, pp. 466–478.


11

Pan, T.-C., 1998. Estimation of peak ground accelerations of the Malay Peninsula due todistant Sumatran earthquakes. GeoForschungsZentrrum Potsdam, Germany, ScientificTechnical Report STR98/14, pp. 340–359.

Pan, T.-C., 1998. Region report – Republic of Singapore. Invited Paper, Proceedings ofthe Multi-Lateral Workshop on Development of Earthquake and Tsunami DisastersMitigation Technologies and Its Integration for the Asia-Pacific Region, 30 September - 2October, Earthquake Disaster Mitigation Research Centre, Kobe, Japan, pp. 72–76.

Pan, T.-C., 1998. Seismic loading requirements for Singapore buildings. Journal ofEarthquake Engineering, Earthquake Engineering Society of Korea, Vol. 2, No. 3, pp. 87-98.

Pan, T.-C., 1998. Seismic loading requirements for Singapore buildings. Invited Paper,Proceedings of the 1st International Symposium on New Development in EarthquakeHazard Mitigation Technology for Moderate Seismicity, Seoul National University, 3-4August, Seoul, Korea, pp. 150–162.

Park, R.*, Tanaka, H* and Li, Bing, 1998. Flexural strength and ductility of high strengthconcrete columns. ACI Special Publication - 176, "High Strength Concrete in SeismicRegions", pp. 237-258.

Rahardjo, H., Leong, E.C. and Han, K.K.*, 1998. Shear strength characteristics of residualsoil under infiltration. Proceedings of the International Symposium on Problematic Soils,Sendai, Tohoku, Japan, 28-30 October, pp. 461-464.

Rahardjo, H., Leong, E.C., Gasmo, J.M.* and Deutsher, M.S.*, 1998. Rainfall-inducedslope failures in Singapore: investigation and repairs. Proceedings of the 13th SoutheastAsian Geotechnical Conference, Taipei, Taiwan, 16-20 November, pp. 147-152.

Rahardjo, H., Leong, E.C., Gasmo, J.M.* and Tang, S.K.*, 1998. Assessment of rainfalleffects on stability of residual soil slopes. Proceedings of the 2nd International Conferenceon Unsaturated Soils, Beijing, China, 27-30 August, Vol. 1, pp. 280-285.

Robert Tiong, L.K., 1998. Financing and risk management of infrastructure projects.Economic Bulletin, A Journal of Singapore International Chamber of Commerce, October,pp. 5-8.

Shang, J.L.*, Shen, L.T.*, Zhao, Y.H. and Zhao, J., 1999. Dynamic constitutive equationof the Bukit Timah granite. Chinese Journal of Rock Mechanics and Engineering, (inChinese), Vol. 17, pp. 634-641.

Sharma, J.S., 1998. Effect of mechanical properties of surcharge on the behaviour of softground stabilized by stone columns. Proceedings of the 13th South East AsianGeotechnical Conference held in Taipei, Taiwan, November.

Show, K.Y. and Tay, J.H., 1999. Influence of support media on biomass growth andretention in anaerobic filters. Journal of Water Research, Vol. 33, No. 6, pp. 1471-1481.


12

Shuy, E.B. and Chua, H.C.*, 1999. Fluid-dynamic feedback in shear layer oscillationbelow a submerged sluice gate. IAHR Journal, Vol. 37, No. 1, pp. 107-120.

Shuy, E.B., Chui, P.C., Chua, H.C.* and Chen, C.N., 1998. Entrainment across a densityinterface inside a cylindrical tank with a concentric base opening. Journal of HydraulicsResearch, Vol. 36, No. 2.

Soh, C.K., Chiew, S.P. and Dong, Y.X., 1999. Damage model for concrete-steel interface.ASCE Journal of Engineering Mechanics, Vol. 125, No. 8, pp. 979-983.

Soh, C.K., Fung, T.C., Chan, T.K. and Nakacho, K.*, 1998. SCF of T tubular jointsreinforced with doubler plates. Transactions of JWRI, Japan, Vol. 27, No. 1, pp. 105-108.

Tan, K.H., Kong, F.K.* and Weng, L.W.*, 1999. Discussion with authors’ closure on “Highstrength reinforced concrete deep and short beams: shear design equations in NorthAmerican and UK Practice. ACI Structural Journal, March/ April, Vol. 96, No. 2, pp. 314 -315.

Tan, T.H. and Cheong, H.K., 1998. An apparatus for testing concrete under active andpassive confining stress. International Symposium on Strength Theories: Applications andDevelopment, Xi’an, China, September.

Tao, N.F.* and Brownjohn, J.M.W., 1998. Estimation of ground motion acceleration andbuilding response to a long distance earthquake. Journal of Earthquake Engineering, Vol.2, No. 3, pp. 477-485.

Tay, J.H. and Show, K.Y., 1998. Efficient granular/low sludge processes in anaerobic wastetreatment. Invited Paper for A Workshop on Wastewater Works Upgrades for OptimalPerformance & Sludge Minimisation, Environmental Technology Institute, Singapore, 23-24November, pp. 1-57.

Tay, J.H. and Show, K.Y., 1998. Media-induced hydraulic behavior and performance ofupflow biofilters. Journal of Environmental Engineering, ASCE, Vol. 124, No. 8, pp. 720-729.

Tay, J.H. and Show, K.Y., 1999. Reclamation of wastewater sludge as innovative building &construction materials. 4th World Congress on R’99 - Recovery, Recycling, Re-integration,Geneva, Switzerland, 2-5 February, Vol. 5, pp. 252-257.

Tay, J.H., Anthony Goh, T.C. and Show, K.Y., 1999. Use of Flyash in Jet Grouting. InvitedPaper for the ENV-NTU-NUS Joint Research Seminar, Ministry of the Environment,Singapore, 31 May.

Tay, S. T.-L., Hemond, H.F.*, Polz, M.F.*, Cavanaugh, C.M.* and Krumholz, L.R.*, 1999.Importance of Xanthobacter autotrophicus in toluene biodegradation within a contaminatedstream. Systematic and Applied Microbiology, Vol. 22, pp. 113-118.

Tay, S. T.-L., Hemond, H.F.*, Polz, M.F.*, Cavanaugh, C.M.*, DeJesus, I.* and Krumholz,L.R.*, 1998. Two new Mycobacterium strains and their role in toluene degradation in acontaminated stream. Applied and Environmental Microbiology. Vol. 64, pp. 1715-1720.


13

Ting, S.K. and Zhou, J.*, 1998. Mobile factory for improving construction productivity.Proceedings of the 1st Workshop on Asean Infrastructure Planning and Management,Bangkok, Thailand, 29 September – 1 October, pp. 101-108.

Ting, S.K. and Zhu, Y.*, 1998. Procurement methods used in the construction industry.Proceedings of the 8th KKNN Seminar on Civil Engineering, National University ofSingapore, Singapore, 30 November – 1 December, pp. 344-349.

Ting, S.K. and Zhu, Y.*, 1999. The development of design and build In Singapore. Invitedpaper, Proceedings of The Dr Tan Swan Beng Memorial Symposium on Excellence inInfrastructure Engineering, Singapore, 18-19 March, pp. 229-235.

Tiong, L.K. and Shami, M.*, 1998. Construction process re-engineering. Proceedings ofthe Conference on Asean Infrastructure Planning and Management in Bangkok, 29September – 1 October, organised by AIT & CACS.

Tiong, L.K., Ting, S.K., Wang, S.Q.* and Ashley, D.*, 1999. Evaluation and managementof foreign exchange and revenue risks in BOT projects. Invited Paper, Proceedings of theDr Tan Swan Beng Memorial Symposium – Excellence in Infrastructure Engineering,Singapore, 18-19 March, pp. 213-228.

Trayford, R.S.*, Karl Jr, C.A.* and Luk, J.Y.K., 1998. Social equity in transport, congestionpricing and the role of car pooling. Proceedings of the International Conference onTransportation into the Next Millennium. Singapore, pp. 371-379.

Wang, S.Q.*, Tiong, R.L.K., Ting, S.K. and Ashley, D.*, 1999. Political risks: analysis ofkey contract clauses in China’s BOT project. ASCE Journal of Construction Engineeringand Management, May/June, Vol. 125, No. 3, pp. 190-197.

Wang, S.Q.*, Tiong, R.L.K., Ting, S.K. and Ashley, D.*, 1999. Risk managementframework for BOT power projects in China. Journal of Project Finance, InstitutionalInvestors, USA, January, Vol. 4, No. 4, pp. 56-67.

Wang, S.Q.*, Tiong, R.L.K., Ting, S.K., Chew, D. and Ashley, D.*, 1998. Evaluation andcompetitive tendering of BOT power plant project in China. ASCE Journal of ConstructionEngineering and Management, July/August, Vol. 124, No. 4, pp. 333-341.

Wilson, F., Yu, H.Q.*, Tay, J.H. and Gu, G.W., 1998. An empirical model for predicting theoutput organic quality of anaerobic filters. Journal of Water Environmental Research,USA, Vol. 70, No. 3, pp. 299-305.

Wong, K.S., Goh, A.T.C., Jaritgnam, S .* and Chang, J.D.*, 1998. Optimisation of jet groutconfiguration for braced excavation in soft clay. Specialty Symposium of Jet Grouting,Singapore.

Wong, S.C.*, Lee, C.K. and Tong, C.O.*, 1998. Finite element solution for the continuumtraffic equilibrium problems. International Journal for Numerical Methods in Engineering,Vol. 43, No. 7, pp. 1253-1273.


14

Wu, Y.K., Hao, H., Zhou, Y.X. and Chong, K.*, 1998. Comparison study of couplingeffects of explosive charge on ground shock and vibration. International Journal forBlasting and Fragmentation, Vol. 2, No. 1, pp. 25-38.

Wu, Y.K., Hao, H., Zhou, Y.X. and Chong, K.*, 1998. The orientation effects of rock jointson blast-induced ground shocks. International Journal of Soil Dynamics and EarthquakeEngineering, Vol. 17, pp. 407-412.

Xiao, J.R., Lok, T.S. and Xu, S.Z.*, 1999. Boundary element-linear complementaryequation solution to polygonial unilaterally simply supported plates. Journal of EngineeringAnalysis with Boundary Elements, UK, April, Vol. 23, No. 4, pp. 297-305.

Yang S.Q.* and Lim, S.Y., 1998. Boundary shear stress distributions in smoothrectangular open channel flows. Proceedings of the Institution of Civil Engineers - Water,Maritime & Energy, London, UK, September, Vol. 130, pp. 163-173.

Yang S.Q.* and Lim, S.Y., 1999. Closure to “Mechanism of energy transportation andturbulent flow in a 3D channel”. Journal of Hydraulic Engineering, ASCE, Vol. 125, No. 3,pp. 319-320.

Yang, D.Q.*, Alonso, E.E.* and Rahardjo, H., 1998. Modelling of the volumetric behaviourof an unsaturated expansive soil. Proceedings of the 2nd International Conference onUnsaturated Soils, Beijing, China, 27-30 August, Vol. 2, pp. 249–254.

Yang, D.Q.*, Rahardjo, H., Leong, E.C. and Choa, V.*, 1998. A coupled model for heat,moisture, air flow and deformation in unsaturated soils. Journal of Engineering Mechanics,ASCE, Vol. 124, No. 12, pp. 1331-1338.

Yang, D.Q., Rahardjo, H., Leong, E.C. and Choa, V.*, 1998. A coupled model for heat,moisture, air flow and deformation problems in unsaturated soils. ASCE Journal ofEngineering Mechanics, December, Vol. 124, No. 12, pp. 1331-1338.

Young, B. and Hancock, G.J.*, 1998. Web crippling behaviour of cold-formed unlippedchannels. Proceedings of the 14th International Specialty Conference on Cold-formedSteel Structures, St. Louis, University of Missouri-Rolla, Mo., USA, pp. 127-150.

Young, B. and Hancock, G.J.*, 1999. Compression tests of thin-walled channels withsloping edge stiffeners. Proceedings of the 4th International Conference on Steel andAluminium Structures, Espoo, Finland, Elsevier Science, pp. 45-52.

Young, B. and Rasmussen, K.J.R.*, 1998. Behaviour of locally buckled singly symmetriccolumns. Proceedings of the 14th International Specialty Conference on Cold-formed SteelStructures, St. Louis, University of Missouri-Rolla, Mo., USA, pp. 219-238.

Young, B. and Rasmussen, K.J.R.*, 1998. Inelastic bifurcation analysis of locally buckledchannel columns. Proceedings of the 2nd International Conference on Thin-walledStructures, Singapore, Elsevier Science, pp. 409-416.


15

Young, B. and Rasmussen, K.J.R.*, 1998. Shift of the effective centroid of channelcolumns. Proceedings of the 14th International Specialty Conference on Cold-formed SteelStructures, St. Louis, University of Missouri-Rolla, Mo., USA, pp. 265-287.

Young, B. and Rasmussen, K.J.R.*, 1998. Tests of cold-formed channel columns.Proceedings of the 14th International Specialty Conference on Cold-formed SteelStructures, St. Louis, University of Missouri-Rolla, Mo., USA, pp. 239-264.

Young, B. and Rasmussen, K.J.R.*, 1999. Behaviour of cold-formed singly symmetriccolumns. Thin-walled Structures, Elsevier Science, UK, Vol. 33, No. 2, pp. 83-102.

Young, B. and Rasmussen, K.J.R.*, 1999. Local, distortional, flexural and flexural-torsional buckling of thin-walled columns. Proceedings of the 4th International Conferenceon Steel and Aluminium Structures, Espoo, Finland, Elsevier Science, pp. 27-34.

Young, B. and Rasmussen, K.J.R.*, 1999. Shift of effective centroid of channel columns.Journal of Structural Engineering, ASCE, Vol. 125, No. 5, pp. 524-531.

Yu, H.Q.*, Wilson, F. and Tay, J.H., 1998. Kinetic analysis of an anaerobic filter treatingsoybean wastewater. Journal of Water Research, UK, Vol. 32, No. 11, pp. 3341-3352.

Zhang, M.*, Tay, J.H., Qian, Y. and Gu, X.S., 1998. Coke plant wastewater treatment byfixed biofilm system for COD and NH3-N removal. Journal of Water Research, UK, Vol.32, No. 2, pp. 519-527.

Zhang, W.R.*, Wang, S.Q.*, Tiong, R.L.K., Ting, S.K. and Ashley, D.*, 1998. Riskmanagement of Shanghai's Privately financed tunnels. Journal of EngineeringConstruction and Architectural Management, UK, December, Vol. 5, No. 4, pp. 399-409.

Zhao, J., 1998. Rock mass permeability of the Bukit Timah granite in Singapore.Engineering Geology, Vol. 50, pp. 211-216.

Zhao, J., 1999. A new JRC-JMC shear strength criterion for rock Joint. Chinese Journalof Rock Mechanics and Engineering, (in Chinese), Vol. 17, pp. 349-357.

Zhao, J., 1999. Flow-rock heat transfer interaction in rock fractures. Chinese Journal ofGeotechnical Engineering, (in Chinese), Vol. 18, pp. 119-123.

Zhao, J., Li, H.B., Wu, M.B.* and Li, T.J.*, 1999. Dynamic uniaxial compression tests of agranite. International Journal of Rock Mechanics and Mining Sciences, Vol. 36, pp. 273-277.

Zhao, Y.H. and Zhao, J., 1998. Compressive strength of rock materials at different strainrate. Proceedings of the International Symposium on Strength Theories Applications andDevelopments, Xi'an, China, pp. 497-505.

Zhao, Z.Y. and Lan, S.R.*, 1999. Boundary stress calculation - a comparison study.Journal of Computers and Structures, UK, Vol. 71, No. 1, pp. 77-85.


16

Zhao, Z.Y., 1998. A simple error indicator for adaptive boundary element method.Journal of Computers and Structures, UK, Vol. 68, No. 5, pp. 433-443.

Zhao, Z.Y., 1998. Error estimation in adaptive BEM by post-processing interpolation.Journal of Communications in Numerical Methods in Engineering, UK, Vol. 14, pp. 633-645.

Zhou, Y.X., Hao, H., and Ma, G.W., 1998. Ground shock damage criteria for inhabitedbuildings. 28th DOD Explosive Safety Seminar, 18-20 August, Orlando, Florida, USA.

CSE Research Bulletin No. 13. January 2000: Editoral Board

ISSN 0219-0370 January 2000 NO. 13

Shuy Eng Ban - Chairman Robyn Collins (Language editor) Carmel Heah (Language editor) Chin Chen Onn Fung Tat Ching Low Bak Kong James Luk Niall MacAlevey Tor Yam Khoon Wang Jin-Yuan Lee Chi King (Online edition)

ADDITIONAL COPIES AND ENQUIRIES

For general enquiries about this publication, and to request for hard copies, please write to :

The Dean School of Civil and Structural Engineering Nanyang Technological University Nanyang Avenue Singapore 639798 Tel : 7905264 Fax : 7910676 Email : [email protected]

Published by Dean School of Civil and Structural Engineering, Nanyang Technological University

file:///D|/CEE_old/RESEARCH/Bulletin/1999_2000/HTM/editoral_board.htm9/27/2003 10:50:12 AM



1

Analysis of Premature Failures in CFRP-Strengthened RC Beams

Reinforced concrete (RC) beams strengthened with steel or fibre reinforced plastic (FRP) platescommonly fail due to either debonding of the plate or concrete ripping-off, premature shear failure ora combination of these because of the plate-end termination effect. The tension force carried by theplate is transferred to the beam by the adhesive layer and the concrete cover. At the free ends of theplate, discontinuity of the beam stiffness occurs, and stress-concentration in the adhesive layer or inthe interface between concrete and the tension reinforcing steel at the plate-end region is an importantcause of debonding or concrete ripping-off failures of the strengthened beam. This article presents ananalysis of premature failures of RC beams strengthened with FRP plates with emphasis on thedebonding and ripping-off failure modes. The work was based on the results of tests on a series beamsstrengthened with carbon fibre reinforced plastic (CFRP) plates.

Debonding of the plate

Debonding of the plate is a delamination between the plate-adhesive-concrete at the plate-end regionof the strengthened beam. This failure may be limited by the maximum stresses in the adhesive beingnot greater than the bonding strength between plate-adhesive-concrete at the plate-end region. A keyproblem in the debonding analysis is the determination of the stress-concentrations. Maximumstresses in the adhesive layer can be predicted based on the interaction behaviour between the plate-adhesive-concrete while the bond strengths may be determined from the single or double-lap bondtests between the applied FRP plate, adhesive and concrete.

Using a number of assumptions in addition to conventional plane-section beam theory, the stress-concentration in the adhesive layer is predicted. The strain in the concrete tension surface can becalculated using the beam theory for the unplated beam under external loading and the additionalforce carried by the FRP plate (Figure 1a). The relationship between the strains in the concrete and theplate is expressed as a function of the shear deformation in the adhesive layer. The maximum shearstress τ0 in the adhesive layer can then be predicted by the following equation:

α−α+

α

β=τ

421 p

p

002

2

ap0

lq

lM

Vbl

(1)

where α and β are coefficients depending on the properties of the FRP plate, adhesive used and thestiffness of RC beam, lp is the length of the plate, ba is the width of the adhesive layer, q is theuniformly distributed load, and V0 and M0 are the shear force and bending moment at the plate-termination position of the beam respectively.

Shear stress-concentration in the adhesive layer obtained using equation (1) compared well with theresults obtained from the finite element analysis of the RC beam strengthened with glass FRP platewhich was tested by Saadatmanesh and Ehsani (1991) and failed in debonding of the plate (see Figure2).

The developed formulation is in closed form and is easily applied for practical analysis and design.However, its limitation is that the maximum shear stress may not theoretically be close to thetermination point of the plate because full effectiveness of the adhesive may be at a certain distancefrom this point. Calibration of this analysis with experiments is required for a given FRP andadhesive.


2

Ripping of concrete

As high-modulus epoxies have been developed and used in strengthening works, the ripping of theconcrete layer between the longitudinal reinforcing steel and the plate has been indicated as thecommon premature failure mode of CFRP beams. Concrete ripping-off initiates from the shear crackstarted at the plate end due to the high-stress-concentration in the adhesive layer. The shear crack thenresults in the eccentricity between the plate and the steel bar layer. Hence, unlike debonding of theplate, stress concentration occurs between the concrete and steel bar layers.

Based on the test results of CFRP strengthened beams, the distribution of the strains along the plate atultimate load of the beams can be divided into 3 distinct zones (see Figure 1-b), namely, the “de-stressed” zone, the “bond development” zone and the composite behaviour zone. The “de-stressed”zone is the first 50mm (of the order of the concrete cover thickness) of the plate, where the strains inthe plate are negligible because of the shear cracks formed from the end of the plate. The compositezone is where the plated beam is considered to have composite action up to failure. The “bond-development” zone is the transition between the “de-stressed” and composite zones. Theoretical andexperimental research show that the length of the “bond-development” zone ldev is a function of theconcrete cover, the beam effective depth and a parameter, λ, which depends on the properties of theplate, adhesive and concrete cover. The value of ldev should be greater than the effective bond lengthbetween plate-adhesive-concrete.

Based on this composite model, two approaches to predicting the concrete ripping-off failure load canbe used. In the first approach, ripping-off failure can be limited by the stresses at the interfacebetween the reinforcing bar and concrete; these should not exceed the tensile and shear strengths ofconcrete. The interface stresses can be determined as the “concrete block” cantilever subjected to theshear stress acting in the adhesive layer. The shear stress in the adhesive layer is calculated on theassumption that the tension force in the plate is linearly changing in the “bond-development” zone.Table 1 shows the predicted and the experimental failure loads of the CFRP beams tested in theSchool. The limitation of this approach is that the cracking intervals in the plated beam are not wellunderstood because of the presence of the plate in the concrete surface. Therefore, using the formulato determine the cracking intervals of the conventional RC beam in this analysis requires moreresearch and adjustment.

In the second approach, concrete ripping-off or other premature failures may be prevented by limitingthe plate strain at the end of the “bond-development” zone to a certain value, depending on the bondproperties of a given CFRP plate, adhesive and concrete, as well as the shear span of the beam. This iscalled the plate-end strain limit method, and is being developed in the present work.

Conclusion

Theoretical and experimental work on CFRP-strengthened beams is in progress. Whilemodel beams have been tested so far, the next phase involves tests on full-size beams. Thecurrent theoretical work is focused on the plate-end strain limit approach to analyze the concreteripping-off, premature shear failure and the combination of these failure modes of CFRP strengthenedbeams.

Nguyen D Minh, ([email protected])Chan Toong Khuan ([email protected])Cheong Hee Kiat ([email protected])


3

Figure 1. a) Debonding of the plate; b) Concrete ripping-off

0

0.1

0.2

0.3

0.4

0.5

0.6

0 40 80 120 160 200Distance from the end of the plate (mm)

Shea

r st

ress

in a

dhes

ive

(MPa

)

Finite element solution [3]Present solut ion

Figure 2. Shear-stress distribution in the adhesive layer of GFRP beam that failed in debonding of theplate, as tested by Saadatmanesh and Ehsani (1991)

Table 1. Predicted and experimental failure loads of CFRP strengthened beamsBeam fcu (ft)

(MPa)lp (mm) ∆p

(mm)Tp

(kN)Flexuralcap. (kN)

Shear cap.(kN)

Pu (kN)theo.

Pu (kN)exper.

error(%)

Failure mode

B1150 33 (3) 1150 340 32.40 82 114 68.61 58.9 16 ripping.-offB1100 33 (3) 1100 365 32.40 82 114 63.91 57.3 12 ripping.-offB950 33 (3) 950 440 32.40 82 114 53.0 56.2 -6 ripping.-off

P/2

debonding of the plate with failure in plate-adhesive-concrete interface

strain-distributionalong theplate“bond

development“zone

compositebehavior zone

“de-stressed”zone

compositetheoreticalvalue of εp

the plate-endstrain εpe εp

ldev

d

CFRP plate

concrete beamcracks

CL

neutral axisx

d p

concreteripping-off

b)

dreinforcing bar

CFRP plate

concrete beamcracks

P/2 CL

neutral axis

x

d p

adhesive layer

uniformly distributed load q

Tp

MN

a)


1

Design and Build Practice in the Singapore Public Sector

Introduction

The Design & Build (D&B) procurement method is an agreement between the owner and a singleentity to perform both design and construction under a single contract. Portions or all of the design andconstruction may be performed by the entity or subcontracted to other companies. It first evolved orrather re-evolved in the 1950’s and early 1960’s due to the increased competitiveness in the constructionindustry and the drawbacks in the “Conventional” method. It was perceived to have the advantages ofsingle point responsibility, virtually guaranteed fixed cost and completion date, fitness for purpose,buildability of design concept and fewer claims. During the 30 to 40 years that followed, D&Btransformed from a procurement method that was mostly used to procure utilitarian facilities into a formthat was suitable for various types of project. Numerous variations have also developed from it: PackageDeal; Design, Manage and Construct; Develop and Design; Novation Design & Build; and Build,Operate and Transfer (BOT).

Background information

The Singapore government has recognised the importance of the construction industry and its role inoverall socio-economic development and has taken measures to improve it. A number of majorgovernment agencies were set up for developing and monitoring specific areas in the constructionindustry. They are the Housing Development Board (HDB), the Jurong Town Corporation (JTC), theLands & Estates Organization (LEO), the Land & Transportation Authority (LTA) and the PublicWorks Department (PWD). Among the various measures taken was the choosing of D&B as one ofthe fronts to enhance buildable designs in order to improve productivity and achieve constructiondevelopment which is the ultimate goal.

The development of D&B in Singapore

In Singapore, the initial use of D&B was not in the private sector as in many other countries. Instead, itwas first implemented in the public sector. During the 1970’s and 1980’s, nearly all the public sectorbodies responsible for procuring construction services in Singapore tried to award their contracts on aD&B basis, though on a rather limited scale. Over the following 7 years, other agencies also took part in

the D&B market (Table 1). By 1998, there were 133 D&B projects adding up to S$8.8 billion worth ofcontracts awarded on a D&B basis. Of these, 87 projects, or 69% of the total value were awarded bythe public sector. The value of public D&B building projects has increased steadily over the years

$0$200$400

$600$800

$1,000$1,200

1992 1993 1994 1995 1996 1997 1998

Chart 1. D&B projects awarded by public and private sector (by value of contracts S$ millions)

Public Sector

Private Sector


2

(Chart 1). The number of contracts awarded increased dramatically in 1995, almost tripling the numberin 1994. The figure has remained steady at over S$1,000 million ever since. The value of private sectorD&B building projects also started to increase from 1995. However, the economic crisis in the region in1998 has reduced the contract values in both sectors, with the private sector being more affected.

Table 1. D&B practised in public agencies

HDB JTC LEO LTA PWDYear for fullimplementation

1991 1995 1992 1984 1995

Project description residential Factory &dormitory

Camp MRT line Schools

No. of D&B projectsawarded since 1991

25 7 6 8 41

Size (million S$) $100-$200 $25-$200 $40-$200 $200-$300 $10-$25Type of D&B D&B D&B D&B D&B Develop and

Construct

The practice of D&B in Singapore and its characteristics

When adapting to the D&B approach, the organisation structure and some aspects of the method weremodified by the agencies so as to make it suitable for their own particular practice. This resulted in aunique D&B practice in the Singapore construction industry. There are basically two types of D&Bapproaches practised in the public sector: traditional D&B and its alternative - Develop and Construct.Only the PWD uses the latter approach while the former is more widely practised (Table 1). Due to thefact that most of the consulting firms are small, no designer-led D&B teams were formed. Contractorslead all the D&B teams. The consultants are sub-contracted under the contractor. The other uniquecharacteristic of the organisation structure is the formation of a supervision team and its relationship tothe contractor and clients during the construction. The quality of the construction has been observed tosuffer from the D&B approach. Yet no standard or effective methods have been found to avoid this.The industry is aware of such issues and each agency has made its own adjustments to the arrangementof the supervision team. The formation of the supervision team can take place in three ways and thecontractual arrangement can also vary.

The forming of supervision teamsThree ways to form the supervision team were defined in public agencies:a) The supervision team is directly appointed by the consultants under the design-builder's team.b) The supervision team is jointly appointed by the consultants and the client.c) The supervision team is solely appointed by the client and takes charge during construction.

Contractual arrangement between the supervision team, contractor and clientThere are two types of contractual relationship for supervision teams:a) The client signs only one contract with the contractor for the whole project. The site supervision

is included in this lump-sum contract.b) The client signs a direct supervision contract with the supervision team.

Through interviews and statistics gathered from the CIDB, it can be concluded that most issues in theoutcome of D&B projects do conform to the perceived advantages to varying degrees.


3

a) The number of claims from contractors and variation orders from the clients were reducedsignificantly-by up to at least 70%.

b) The number of in-house staff involved in D&B project construction is much less. Usually only1 to 3 in-house staff members are needed for a single project.

c) The construction time for D&B was 3 to 4 months less than similar traditional projects.However, some agencies have pointed out that such time saving is very insignificant compared tothe 4 to 5 years total project span.

d) The common scenario is that the project cost is higher in the first few projects, then becomesmore competitive with traditional projects.

e) The Agencies agree that new innovations do occur from time to time. Yet the frequency is notyet impressive.

Conclusion

For public D&B projects in Singapore, the following are recommended in future practice:a) Two stage selection tendering is a good way to achieve a better understanding of the briefing for

consultants and proposal for client.b) The agencies' control over the construction should be reinforced so as to ensure the quality of

construction. Such control could be adjusted according to the demand of finishes andworkmanship required.

c) A more specific briefing is needed so as to minimise the contractor's opportunity to compromisequality for profit.

d) An environment of mutual trust and understanding of professions in the construction industry isneeded to ensure the success of D&B projects.

D&B in Singapore is at a relatively early stage of development. Various adjustments were made whenadopting this approach to suit the Singapore construction industry. New adjustments to the tenderingstrategy and construction quality control are still ongoing. The results of these adjustments should befurther studied after the completion of the projects. However, up to now, the D&B approach has beenaccepted reasonably well by the construction industry in Singapore. Though problems still exist, once theindustry is more experienced with it, the outcome can only be better.

Reference

Ting, S.K. and Zhu, Y., "Design And Build Practice In The Singapore Public Sector", 7th East Asia-Pacific Conference on Structural Engineering and Construction (EASEC-7), Kochi, Japan, August 27-29, 1999, pp. 1709-1714.

Ting Seng Kiong ([email protected])


1

Construction Automation in Improving Construction Productivity inSingapore

Nature of the construction industry

Construction operations bring sophisticated resources and technologies together and convert them intothe physical components of a project. The whole process of construction for most types of buildinggenerally consists of 20~30 different skilled trades and these operations are performed in the field withsome uncontrollable external factors such as inclement weather conditions and equipment breakdowns.The nature of construction, namely the large scale, open job site, and extensive fragmentation in theconstruction process gives rise to the chaotic dynamic features of the production process in constructionwhich are not faced by factory based mass production industries. The effects of technological advancesare not as obvious in construction as in manufacturing. This situation ultimately contributes to theproductivity lag in the construction sector compared to the developing status of productivity in otherindustries. In Singapore, the construction sector contributes 6% of the country’s GDP. Innovations toimprove productivity in construction are thus not only necessary but also imperative.

A process oriented solution

The widely acknowledged difficulties and complications in construction resulting from the chaotic natureof the production process determine that the effective solution for improving productivity should be aprocess oriented approach. Past technical innovation experiences have proved that single-task orientedtechnology improvement did not have a major impact on the overall construction productivityperformance and often resulted in problems of poor process and resource interaction or “island oftechnology”. Using information technology in construction makes it possible to set up a process orientedreengineering model and to shape it in reality with the help of current available mechanisationtechnology. Major changes are taking place with respect to construction processes, products,organisations, resources, markets, methods, and technological strategies. Many of the efforts are beingcarried forth in the name of Computer Integrated Construction (CIC). Faced with the chaotic nature ofconstruction production, increasing the ability of data processing and data storage alone cannot beenough to bring out a process oriented solution to improve construction productivity. Standardisationmust be added into this model to improve productivity. Reflecting on past experience of productivityimprovement in manufacturing, productivity can be increased only if every task involved in theproduction process has a clear, simple, rigid definition. This productivity improvement model is built up atthe cost of losing flexibility in the manufacturing process. This new approach for improving constructionproductivity involves building up a construction process friendly environment by way of CIC and astandardised production in the aspects of material, process and final product. Based on this approach, amodel of “a mobile or moving factory” was set up [1]. In the model, a well defined process friendlyworking environment must be built up for virtual information sharing between different stages ofconstruction where CIC technology is involved. Meanwhile this environment provides a comfortableconstruction job-site working area that is immune to the random influence of outside factors like adverseweather. It simplifies and standardises the work in situ by using a large quantity of prefabricationmembers in design and construction. “The mobile factory” is set up on the basis of standardisation andthe application of CIC technology.


2

The development of construction automation system

A well integrated and standardised construction process is expected to be established to assistconstruction firms in response to the difficult environment. In Japan, more and more constructioncompanies recognise it to be their most strategic option when facing the challenges of increasingcompetition. In their approach, the development of a Construction Automation System is a leading partof this whole innovation strategy. Under this strategy, several types of Construction Automation Systemswere developed such as Shimizu’s SMART system, Obayashi’s BIG Canopy system, Taisei’s T-Upsystem, Takenaka’s Roof Push-Up Construction Method and Kajima’s AMURAD system. Thesesystems have different approaches but the same objective. An indoor factory-like working environmentis established so that the process of site construction is immune to the random bad weather whichaccounts for 20% of the construction delay in the conventional method. The wide use of heavytransportation technology and prefabricated members standardise the production process in constructionto avoid a lot of unnecessary repetition in operation and to diminish the unskilled work in construction. Itis reported that an Automated Construction System will cut the man-hours required in construction by30% and reduce the volume of industrial waste by 50%. With the introduction of CIC, the whole processof construction, including design, procurement and site construction, is integrated. Management for thewhole process is computerised and automated construction is realised.

The construction industry in Singapore mainly follows the traditional procurement method, with a maincontractor responsible for undertaking the work. Projects are normally divided into two packages:substructure and superstructure, which may not necessarily be undertaken by the same company. Maincontractors in Singapore use nominated or domestic specialist “supply and fix” subcontractors for suchwork as structural steel, mechanical and electrical engineering installations and landscaping. To improveproductivity in construction, Singapore has set up its own development guideline based on theinvestigation of its construction productivity status in the 1992 CIDB Construction Productivity TaskForce Report. In the report the construction sector in Singapore was perceived as a low productivitysector due to its low-tech image and visible employment of a large number of foreign workers. Theindicator of square meters of completed construction per man-day revealed that Japan’s constructionproductivity was 30% higher than Singapore’s and Finland’s highly prefabricated industry was 60%higher than Singapore’s. As a long-term strategy, the features of a high productivity project were welldefined and outlined as follows: 1) High degree of standardisation in grid layout and sizes of components;2) Building systems and installation details which are simple, easy to construct and repetitive; 3) Designsthat use or facilitate the use of a high degree of prefabrication or pre-assembled forming systems;4) Architectural details applied in design to minimise the use of wet trades, replacing them with drycomponents or semi-dry trades; 5) Well managed construction with proper planning and working detailssupplied; 6) High level of mechanisation and good housekeeping; 7) Skilled workers in the relevanttrades, entering and leaving the project on schedule. The task force concluded that Singapore must beprepared for a quantum leap in the level of prefabrication with an accompanying high level ofstandardisation.

From the late 1970s, the construction industry in Singapore faced similar problems to Japan, such aslabour force shortage, the aging of skilled workers, labour protection and heavy workload. Theseproblems have led to the difficulty the local construction industry has in attracting new workers. In 1996,the Singapore construction industry had a shortfall of about 16% in technical personnel. Only 10% of siteworkers in Singapore were certified as skilled in 1995. This is actually a very low rate compared to thatin Japan and Finland whose skilled worker rate reaches 65%. In construction, structural work is themost labour-intensive site job so it has the highest potential for productivity improvement. Usingprefabricated structural members on site will significantly reduce labour power input. The use of


3

prefabrication and standardisation in the construction process will simplify the operation on site andsubstantially reduce the dependence on the skill of site workers. Greater usage of prefabricatedcomponents is recommended in the design of buildings. BCA aims to increase usage of prefabricatedconcrete components from the current level of 7% to 15% by 2005.

A Comparison: Projects in Japan versus the China Square Project in Singapore [2]Obayashi Corporation successfully completed two projects in Japan using the Big Canopy System.Presently, there is one more project in Kobe using the System. The China Square project in Singaporewas the first commercial building project on which this system was used, though previously it was usedon a housing project construction (Table 1). A comparison of the work unit ratio (man-day/squaremeter) of the projects in Japan and the project in Singapore reveals the difference in productivityperformance in the different construction environments (Table 1).

Table 1. Profiles of the different Big Canopy projects

Project Name Nomura Yachiyo(Japan)

Nexus Kashii(Japan)

Hinode-cho N(Japan)

China Square(Singapore)

No. of Unit 254 114 301 CommercialNo. of Floor 26 20 33 28

Total Area (m2) 33,307 16,081 38,664 42,653Building Height (m) 90.5 69.1 114.3 138.3

No. of Pre-cast Member 8376 3,919 8,781 8,200Ave. Time per Floor (day) 8.0 6.79 6.65 6.0

No. of Worker 29.5 26.9 17.6 50Work Unit Ratio

(man-day per square meter)0.19 0.22 0.10 0.196

Unlike in the conventional method, there are no different trade categories in this system. All workers inconstruction are labelled as “multi-skilled” worker which means the whole process of site constructiondoes not rely much on worker’s skill as a large quantity of well finished prefabricated components areused. The effect of labourer skill is diminished in this construction method more than in any otherconventional one. For instance, in Obayashi’s past projects, only 30 workers were needed in theconstruction of skeletal structure instead of 120 workers in the conventional method. Large structuremembers are assembled before installation and almost no wet work can be seen in situ. Therefore thewhole working area is cleaner than usual. The process of construction scarcely produces constructionrubbish. A comfortable working area is created. In such an automation system, workers do not have toclimb up and down and safety is improved.

Conclusion

For a long time, large-scale, open site and extensive fragmentation of the construction process havebeen the main difficulties in achieving better construction productivity performance. Past task orientedtechnical innovation did not have a major impact on the overall construction productivity performance. Aprocess-oriented approach is carried out under CIC and there is standardisation of the wholeconstruction process. The application of CIC in construction builds up a friendly information sharingvirtual environment. Through standardisation, the construction production is simplified for processmanagement. The development and application of a Construction Automation System brings about aconstruction friendly environment that eliminates the effect of weather. A factory like workingenvironment is set up at the job site where the project plan is turned into the building structure tocomplete the innovation chain of productivity improvement in construction.


4

References

[1] Ting Seng Kiong & Zhou Ji, “Mobile Factory For Improving Construction Productivity”,Workshop Proceedings on ASEAN Infrastructure Planning & Management, Bangkok, Thailand, 1998,pp. 101-108.

[2] Ting, S.K. and Zhou, J., “Construction Automated System in Singapore”, 7th East Asia-PacificConference on Structural Engineering and Construction (EASEC-7), Kochi, Japan, August 27-29, 1999,pp. 1621-1626.

Ting Seng Kiong ([email protected])


1

A Simple Approach to Strength and Stability of

Steel Columns in Fire

Introduction

A simple and direct analytical formula has been derived to determine the fire resistance of steelcolumns. Based on the Rankine principle, the proposed formula allows the strength and thestability aspects of a steel column at a particular temperature to be separately determined. Here,the evaluation of column strength is based on the rigid-plastic theory, while the stabilityconsideration is based on the elastic buckling theory. By considering the two factors in thismanner, the proposed approach allows for their interaction. The proposed formula can be usedfor both axially and eccentrically loaded columns, and no calibration against test results isneeded. The proposed formula has been verified against a set of published test results andpredictions using EC3-1.2 design formulae[1]. The comparison study shows consistently goodagreement with both the test results and the EC3 formulae considered.

Proposed Rankine formula

At each particular temperature T, the proposed Rankine formula allows the interaction of therigid-plastic collapse load Pp and the elastic buckling load Pe to determine the critical load Pc asfollows:

)(1

)(1

)(1

TPTPTP epc

+= (1)

The Pp of an axially loaded column is simply the yield load Py. But for an eccentrically loadedcolumn or a column with end moments, Pp is reduced by the bending stresses and is less than Py.To take the combined action of compressive force and bending moments into account, thefollowing simplified strength criterion is adopted to determine the reduced Pp.

1)(

)()(

)(=+

TMTM

TP

TP

py

p(2)

where Py(T) = fy(T)A is the yield load at T in the absence of M(T); fy(T) = k y(T) fy is the yieldstrength of material at elevated temperatures; k y(T) is the reduction factor of yield strength fy;M(T) = Pp(T)e is the maximum bending moment along the column length in which e is theequivalent eccentricity; Mp(T) = fy(T)Zp is the sectional plastic moment of resistance at T in theabsence of Pp(T); and Zp is the plastic-section modulus.

For axially loaded columns and eccentrically loaded columns bending into symmetric singlecurvature, the maximum Pe(T) is given as follows:

2

2 )()(

e

el

ITETP

π= (3)

where E(T) = kE(T)E is the elastic modulus of material at elevated temperatures; kE(T) is the


2

reduction factor of elastic modulus E; I is the moment of inertia; and le is the effective length.

The Rankine approach makes the following assumptions:a. Temperature distribution within a member is uniform;b. Member is perfectly straight;c. Flexural buckling about the major or minor principle axis is determined separately;d. Local and torsional buckling are not considered;e. Effect of axial force on Mp is ignored.

The variations of E and fy with respect to elevated temperatures are assumed to comply withEC3-1.2. The reduction factors for the slope of linear elastic range kE(T) and the effective yieldstrength k y(T) for the non-linear stress-strain relationships in the code are applied to E and fy,respectively.

Verification study

In this study, the proposed formula is verified against a set of column test results and the EC-1.2design formulae. A total number of 102 full-scale standard fire test results on hot-rolled I-section steel columns have been collected from the literature[2, 3] to form the database. Thesetest data comprise a wide range of columns of different slenderness ratios, end conditions andloading levels. All the columns were subjected to uniform heating, with failure temperatureabove 400 °C, and were free to expand.

Both the proposed formula and the EC3 design formulae are applied to determine the failuretemperatures of all columns subjected to given external loads. The predictions of bothapproaches are then compared to the test results. Table 1 summarises the regression analysesfor both the approaches, showing that the proposed formula yields a mean of 1.052 for thefailure temperature ratios (the ratio of predicted to test failure temperatures) of the 102 columns,with a coefficient of variation (COV) of 6.64 %. A large proportion of the failure temperatureratios lie between 1.0 and 1.1, and the variation of the values is small, changing from 0.87 to1.29. For axially loaded columns, the proposed formula gives extremely accurate predictions onthe failure temperature while for eccentrically loaded columns, it tends to give an average of 7% overestimate.

On the other hand, the EC3 design formula yields a conservative prediction of failuretemperatures for axially loaded columns with a mean of 0.963. However, it yieldsunconservative predictions for eccentrically loaded columns, with a mean of 1.093. These leadto an average overestimate of 4.9 % on the failure temperatures when all the 102 columns areconsidered, with a COV of 10.53 %. This discrepancy is relatively greater than that of theproposed formula.

Table 1. Comparisons of predictions of failure temperature ratioColumn type

Overall Axially loaded Eccentrically loadedProposedRankineFormula

Col. No.Mean

COV(%)

1021.0526.64

341.0195.54

681.0696.56

EC3-1.2Design

Formulae

Col. No.Mean

COV(%)

1021.04910.53

340.9636.97

681.0939.36


3

Conclusions

The proposed Rankine formula is an empirical approach which does not take initialimperfections and residual stresses into account. But through the comparison with test resultsand predictions by EC3, it has been established that the proposed formula based on the Rankineprinciple is simple, reliable and consistent. It can be applied as a quick tool to assess the fireresistance of steel columns.

References

[1] Eurocode 3: Design of Steel Structures: Part 1.2. General Rules. Structural Fire Design.(EC3-1.2). (1995). Draft ENV 1993-1-2, Commission of European Communities, Brussels,Belgium.

[2] Talamona, D. (1995). Buckling Curves in Case of Fire - ECSC 7210 SA316/515/931/618 : Fire Resistance of Steel Columns With Eccentric Load, CTICM, ReportNo. INC-96/450-DT/VG Part 1, Saint-Remy-les-Chevreuse, Paris.

[3] Schleich, J.B., and Cajot, L.G. (1996). Buckling Curves in Case of Fire: Draft Final Report,Part I (Main text), CEC Agreement 7210-SA/316/515/618/931, ProfilARBED-Recherches,Luxembourg.

Toh Wee Siang ([email protected])Tan Kang Hai ([email protected])Fung Tat Ching ([email protected])


1

A Rankine Approach for Fire Resistance of Steel Frames

Introduction

A novel and simple analytical approach has been derived to determine the fire resistance ofsteel frames. Based on the Rankine principle, the proposed approach allows the strengthand the stability aspects of a steel frame at a particular temperature to be separatelydetermined. Here, the evaluation of frame strength is based on the rigid-plastic theory,while the stability consideration is based on the elastic buckling theory. By considering thetwo factors in this manner, the proposed approach allows for their interaction. The proposedapproach has been verified against the test results on six portal frames and the predictionsof a finite element model (FEM). Apart from one prediction, the comparison study showsconsistently good agreement with both the test results and the FEM predictions.

Proposed Merchant-Rankine approach

At each particular temperature T, the proposed Rankine approach allows the interaction ofthe rigid-plastic collapse load factor λp and the elastic buckling load factor λe to determinethe critical load factor λc as follows:

)(1

)(1

)(1

TTT epc λλλ+= (1)

Alternatively, if the critical temperature Tc is of interest while the applied working loadsremain constant, i.e. λc(Tc) = 1.0, a trial-and-error procedure is necessary to determine Tc

such that the sum of 1/λp(Tc) and 1/λe(Tc) is equal to unity.

For a steel frame at T, the λp(T) is evaluated according to the plastic theorems incorporatingtemperature effects[1]. For simple rigid frames, all possible mechanisms are easilyvisualised. The direct method of combined mechanisms is the most convenient manualway of deriving λp(T). For more complex problems, the first-order elastic-plastic hingemethod[2] can be used. On the other hand, the λe(T) can be evaluated by using the proposedsecond-order elastic-plastic hinge method with appropriate modifications[3].

The proposed approach makes the following assumptions:a. Temperature distribution within a member is uniform.b. Members are perfectly straight, isotropic and prismatic.c. Members buckle in the plane of frame only.d. Local and lateral torsional buckling are not considered.e. Applied working loads are concentrated and proportional.

The elastic modulus E and the yield strength fy are two dominant parameters in theproposed Rankine approach. In the rigid-plastic analysis to determine λp, the steel stress-strain relationship is assumed to be perfectly plastic; in the elastic-buckling analysis todetermine λe, the steel stress-strain relationship is assumed to be linearly elastic. Thevariations of E and fy with respect to elevated temperatures are assumed to comply withEC3-Part 1.2[4].


2

Verification study

In this study, the proposed Rankine approach is verified against the test results of six simpleportal frames[5], as well as the predictions of the FEM developed by Toh[3]. The details ofthe frame tests are given in Figure 1, and the analytical results of both the Rankineapproach and the FEM are then compared to the test results in Table 1. Apart from oneframe, the predictions of the Rankine approach are within the accuracy of ± 10 % error,compared to the test results. The maximum error of 23 % corresponds to Frame 6; thisparticular test result may not be representative due to experiment error. Compared to theFEM results, the Rankine predictions seem to be more consistent and accurate. Themaximum errors in the predictions merely exceed 5 % of the FEM values.

Frame l h fy F1 F2

(cm) (cm) (N/mm2) (kN) (kN)

1 119 117 395 56 14

2 124 117 395 84 21

3 124 117 382 112 28

4 125 150 389 20 5

5 125 150 389 24 6

F1

h

l/2 l/2

F2

6 125 150 389 27 6.7 All sections: IPE 80

Figure 1. Details of frames.

Table 1. Analytical results of frames by Rankine approach.

Rankine approach Test FEM % error bet.

At predicted Tc Tc (°C) Tc (°C) Tc (°C) (1) & (2) (1) & (3)Frame

λp λe λc (1) (2) (3)

1 1.105 10.567 1.000 625.4 600 620.5 4.24 0.80

2 1.100 11.041 1.000 544.3 530 544.8 2.70 -0.09

3 1.119 9.425 1.000 451.4 475 458.2 -4.97 -1.48

4 1.463 3.161 1.000 521.7 562 530.7 -7.16 -1.68

5 1.527 2.899 1.000 457.9 460 478.6 -0.45 -4.32

6 1.605 2.651 1.000 402.7 523 425.3 -23.01 -5.32

Conclusions

The proposed Rankine approach serves as a simple tool for assessing the fire resistance ofsteel frames, from the simple interaction of stability and strength aspects. The verificationstudy shows that this approach yields consistently good agreement with both the test resultsand the FEM predictions.


3

References

[1] Fung, T.C., Tan, K.H., and Toh W.S. (1999). ‘Rigid plastic analysis of frames underfire conditions’, submitted to J. Structural Engineering & Mechanics.

[2] Tan, K.H., Fung, T.C., and Toh, W.S. (1999). ‘A first-order elastic-plastic analysis ofsteel frames in fire’, Conference Proceedings of EASEC-7, Japan.

[3] Toh W.S. (1999). Stability and strength of steel structures under thermal effects. PhDthesis submitted to Nanyang Technological University.

[4] Eurocode 3: Design of Steel Structures: Part 1.2. General Rules. Structural Fire Design.(EC3-1.2). (1995). Draft ENV 1993-1-2, Commission of European Communities,Brussels, Belgium.

[5] Rubert, A. & Schaumann, P. (1986). ‘Structural steel and plane frame assembliesunder fire action’, Fire Safety Journal, Vol. 10, pp. 173-184.

Toh Wee Siang ([email protected])Tan Kang Hai ([email protected])Fung Tat Ching ([email protected])


1

Effect of Stress-Path on the Failure of Concrete Under

Tri-Axial Stress State

To improve the ductility of concrete, transverse reinforcement is often used to create a state of triaxial

stress in the concrete brought about by the lateral expansion of the concrete. The mobilisation of

passive confining stress as a reaction of the reinforcement to the expanding concrete is different from

the active confining stress condition often employed in triaxial test chambers. In other words, the

stress paths are different under these two conditions. Calibration of constitutive formulation based on

active confining stress may not be applicable to that under passive confining stress. This study focuses

on the subtle differences resulting from the two different confining conditions.

Specimens from three groups of concrete with different unconfined compressive strengths were tested

under different confining conditions for the investigation. The dimensions of the concrete cylinders

were 100 x 300 mm. The specimens were subjected to a constant confining pressure for active

condition whereas for the passive condition, confining pressure corresponding to a predetermined

lateral stiffness Kst was used. Experimental results indicated that the failure envelope under passive

confinement is different from that of active confinement (see Figure 1). The failure envelope of

passive confinement was observed to be more non-linear than that of active confinement.

From the tests results, it was also found that for the same grade of concrete under passive

confinement, the higher the stiffness of lateral confinement, the closer the failure envelope to that of

Confining Stress, σ2 (MPa)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Axi

al s

tress

, σ1 (M

Pa)

0

10

20

30

40

Compressive strength: 23.74 MPa

σ2=0.50 MPa σ2=1.20 MPa

σ2=1.90 MPa

σ2=2.76 MPa

Extrapolated failure envelope of passive confinement

Failure envelope of activeconfinement

Figure 1. Stress-path under passive confinement


2

active confinement. The above observation suggests that the stress-path can influence the shape of the

failure envelope and different stress-paths will result in different failure envelopes.

However, the differences between them were quite insignificant. For engineering purposes, it is

reasonable to consider that the effect of stress-path on failure envelope is negligible and the failure

envelope can be assumed to be stress-path independent. Using the “five-parameter” multiaxial failure

criteria, the compressive meridian of the failure envelope (see Figure 2) can be expressed as follows:

−++−= '

'

'

''' 2577.91713.1713.0

co

l

co

lcocc f

fff

ff

where 'ccf = peak strength of confined concrete; '

lf = lateral confining pressure on concrete;

'cof = unconfined compressive strength of concrete

It is important to note that for passive confinement, provided the maximum mobilisable confining

stress is high enough that the specimen reaches its peak strength before attaining this confining stress.

For a particular stiffness of lateral confinement, the concrete may reach its peak strength at a certain

confining stress, termed the critical confining stress (σ2p), which can be lower than the maximum

mobilisable confining stress. Although the effect of stress-path on failure envelope appeared

Normalized, σ2 / fc (MPa)

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Nor

mal

ized

, σ1 /

f c (M

Pa)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

activeKst=750 MPaKst=1500 MPaKst=3000 MPa

Proposed failure envelope

Figure 2. Experimental results compared with proposed failure envelope


3

insignificant, the stress level of the critical confining stress was found to be a function of stiffness of

lateral confinement as well as the uniaxial compressive strength of concrete. (see Figure.3)

For the same grade of concrete, the higher the value of Kst, the higher the stress level of σ2p, which

means higher peak compressive strength can be achieved. In practice, for the case of concrete

confined by lateral reinforcement, if the confining stress provided by the yield strength of

reinforcement is less than the value of σ2p, the peak strength of concrete is similar to that of active

confinement. Otherwise, concrete will attain its peak strength at σ2p before the reinforcement reaches

its yield strength and any increase in yield strength of reinforcement cannot further enhance the

compressive strength of confined concrete. Therefore, the determination of critical confining stress is

important in practice.

Tan Teng Hooi ([email protected])Paulus Irawan ([email protected])

Normalized, Kst

(MPa)

0 1000 2000 3000 4000 5000

Nor

mal

ized

, σ2p

(MP

a)

0

2

4

6

8

10

12

G25G50G80

Best fit of G80

Best fit of G50

Best fit of G25

Figure 3. Relationships between critical confining stress and stiffness of lateral confinement


1

Constitutive Model of Concrete Under Multi-Axial Stresses

Concrete is a heterogeneous material with highly non-linear behaviour. To analyse the behaviour of concrete

structures realistically, a reliable constitutive model that is able to simulate this non-linearity is needed. Several

constitutive models of concrete under multi-axial stresses have been proposed in the past. However, most of

these models can only predict the behaviour of concrete under monotonic loading. Few existing constitutive

models can adequately simulate the behaviour of concrete under cyclic loading.

The source of concrete non-linearity generally can be traced to continuum fracture and plasticity. Maekawa and

Okamura (1983) developed a combined elasto-plasticity and fracture model for two-dimensional problems of

concrete, which was expanded to three-dimensional model by Maekawa et al. in 1993. The model is called the

Triaxial Elasto-Plastic Fracture Model (TEPF), in which concrete is regarded as a set of infinitesimal elasto-

plastic components. The parallel springs represent concrete elasticity, and dashpots represent concrete

plasticity. Broken springs are used to indicate damage in concrete. The summation of the contribution made

by the internal stress in each non-damaged element of the constituents is considered as the total stress in

concrete. So the elastic strain is proportional to the internal stress intensity applied to non-damaged

components.

The TEPF model is an effective constitutive model for concrete which has been was proven to cover the plastic

and fracturing characteristics of concrete under monotonic loading. This model has clear physical meaning for

every component, and has a simple and reliable mathematical formula to be applied in Finite Element Method

(FEM). However, the TEPF model cannot simulate the hysteresis behaviour of concrete, although the

degradation of stiffness due to the damage in the concrete can be represented.

This research aims to develop a path-dependent constitutive model for concrete under hysteresis loading using

the TEPF approach. The model named the Triaxial Elasto-Plastic Viscoelastic Fracture model (TEPVF) can be

obtained by using an extra element to manage the energy consumption during the unloading and reloading. The

energy consumption of concrete during unloading and reloading can be simulated by the viscous dissipation of

energy as represented by the additional dashpot in parallel with the Hooke's spring, as shown in Figure. 1.

The hysteresis characteristic of concrete in the new TEPVF model will be investigated in four parts, namely,

the volumetric elasticity and fracture, deviatoric elasticity and fracture, shear plasticity and volumetric plasticity

(dilatancy). The hysteresis behaviour of concrete in these four parts is the main subject of the study. The

constitutive equation of concrete under multi-dimensional cyclic loading can be expressed in the incremental

form as:

[ ] [ ] [ ]( ) [ ] eDEeVEI edCdCdLIMdee

ηηεσ +++= −

1

1 (1)


2

where σ and ε are total stress and total strain respectively, M and L are continuum fracture and plasticity

matrix respectively, VEη and DEη are damping functions to represent the hysteresis behaviour of concrete in

volumetric and deviatoric elasticity respectively, ee is the elastic deviatoric strain tensor and eIC

1 and

eeC are

functions to account for the various influencing factors. A parametric investigation, employing mostly

experimental tests of concrete in a tri-axial chamber, is currently being done to determine the functions that

affect the hysteresis behaviour of concrete under multi-axial stresses.

Figure 1. Schematic Outline of Triaxial Elasto-Plastic Viscoelastic Fracture system

Reference

[1] Maekawa, K. and Okamura, H. (1983), "The Deformational Behaviour and Constitutive Equations forConcrete Using Elasto-Plastic and Fracture Model", Journal of the Faculty of Engineering, TheUniversity of Tokyo (B), Vol. XXXVII, No. 2, pp.253-328

[2] Koichi Maekawa, Jun-ichi Takemura, Paulus Irawan, and Masa-aki Irie (1993), "Triaxial Elasto-Plasticand Continuum Fracture Model for Concrete", Concrete Library of JSCE, Vol. 22, pp.131-161.

Cheong Hee Kiat ([email protected]),Tan Teng Hooi ([email protected])Paulus Irawan ([email protected])

Total Stress

Internal Stress

Total Stress

Dam

aged

Plas

ticity

Elas

ticity

Visc

oela

stic

ity


1

Hybrid Precast Concrete Beam

Precast concrete technology is widely used in today’s construction industry. Its core advantage is thatit is a fast track construction methodology that enables developers and contractors to economise onconstruction cost and time. In recent years, there have been rising demands for improved quality ofprecast concrete technology by the Singapore construction industry. New types of precast memberswhich simplify the connection of precast segments are always of interest to the industry. A new typeof hybrid precast concrete beam is currently being studied.

Figure 1. Details of hybrid precast beam

The typical details of a hybrid precast beam are shown in Figure 1. The precast concrete beam isdesigned as a reinforced concrete beam with normal high tensile steel bars. Mild steel stirrups areused for vertical shear link reinforcement. All the beam reinforcements are provided in accordancewith the requirements of BS 8110 without considering the contribution from the structural steel I-section. At each end of the beam, a Grade 43 structural steel I-section is partly embedded inside theprecast concrete beam. The protruding parts of the embedded structural steel I-sections are used toconnect the beam to the supporting columns. There are two layers of 22-mm diameter mainreinforcing bars at the bottom of the specimen. One layer is placed on the top of the bottom flange ofthe embedded structural I-steel section, while the other layer is placed beneath. All beams have thesame dimensions of 250 mm width by 450 mm height by 4000 mm span. The length of the protrudingstructural steel I-section at each end is 250 mm. The embedded length of the structural steel I-sectionis varied from one effective depth, d, of the concrete segment to 2.5d, in increments of 0.5d. Thebeams are divided into two groups. The first group of 4 beams are reinforced with stirrups comprising8 mm mild steel stirrups spaced at 350 mm, whilst the second group of 4 beams are reinforced withstirrups spaced at 200 mm. Grade 40 concrete is used. The design compressive cube strength of theconcrete is 40 MPa. However, the achieved 28-day compressive cube strength was 50.6 MPa and the28-day cylinder strength was 40.0 Mpa. The dimensions of the Grade 43 structural steel I section are317 mm Χ 165 mm.

Test results of the beam specimens are summarised in Table 1. V is the ultimate failure loadrecorded from each actuator.


2

Table 1. Test results of beam specimens

BeamMark

D(mm)

d(mm)

a(mm) d

a le(mm)

he

(mm)As

(mm2)ρ

(%)V

(kN)

B-350-1.0B-350-1.5B-350-2.0B-350-2.5B-200-1.0B-200-1.5B-200-2.0B-200-2.5

450450450450450450450450

425425425425425425425425

15001500150015001500150015001500

3.533.533.533.533.533.533.533.53

42563885010634256388501063

317317317317317317317317

19001900190019001900190019001900

1.681.681.681.681.681.681.681.68

160173178176196206217206

The effects of stirrup spacing and different embedded lengths of the steel I-section on the failure loadof the hybrid beams are shown in Figure 2. For each value of embedded length of the steel I-section,the specimen with nominal spacing of 350 mm consistently failed at a lower failure load than thatreinforced at spacing of 200 mm.

The embedded length is a governing factor in the specimen's ultimate load capacity. For beams havingstirrup spacing of 350 mm, the failure load increased marginally with the increase in the embeddedlength. For each embedded length, the failure load of beams with stirrup spacing of 200 mm is higher.The increase in failure load is higher with the increase in the embedded length. However, the apparentoptimum failure load is observed for an embedded length of 2d.

The typical failure pattern of all beams was a diagonal shear crack that developed within thereinforced concrete portion of the hybrid beam. For each of the beams having stirrup spacing of 200mm, the diagonal crack eventually extended approximately along the soffit of the embedded steel I-sections. The horizontal crack predominantly governs the beam’s ultimate failure. Very few inclined

Figure 2. Shear force versus embedment length

0

0.5

1

1.5

2

2.5

0 500 1000 1500

Embedment length (mm)

Ve/(b

d') (M

Pa)

R8@350R8@200


3

shear cracks were observed in the composite steel/concrete segments at both ends of the hybrid beamat ultimate failure.

From the typical crack development patterns, two types of failure modes are noted. The diagonalshear failure is a basic failure mechanism. This basic failure mechanism is characteristic of the beamseries with nominal stirrup spacing of 350 mm. The major diagonal shear crack near to the endsupport region of the reinforced concrete portion of the hybrid beam marks the full shear capacity ofan individual beam in this series. In the beam series with designed shear stirrup spacing of 200 mm, inaddition to the basic failure mechanism, an individual beam finally fails when the horizontal crackdevelops and fractures along the soffit of the steel I-section. The propagation of this horizontal crackmarks a bond failure of the lower layer of the tensile reinforcement.

Yip Woon Kwong ([email protected])Susanto Teng ([email protected])


1

Removal of Colour and TOC Using a TiO2/Fe2O3 Suspended Bubble PhotoCatalytic Oxidation Reactor

Introduction

Traditional methods for recycling and reuse of biologically treated wastewater into high quality industrialwater or potable water consist of various combinations of physical and chemical processes. These processesmainly transfer the contaminated substances from biologically treated wastewater to solid waste, whichneeds further treatment for safe disposal. Photo catalytic oxidation (PCO) is a clean reaction process. It justrequires oxygen, photo catalyst and UV light. The development of novel PCO technology for destroying ordegrading bacteria, both organic and inorganic species, could be applied to considerable advantage.However, the quite low mineralisation rates in a PCO system provide plenty of scope for improvement,especially in the catalyst and the reactor. This study investigated the possibility of improving the PCOefficiency by coating titanium dioxide (TiO2) onto haematite particles for wastewater purification.

Experiment and methodology

Figure 1. Diagram of bubble PCO reactor.

The PCO reactor: The annular PCO reactor shownin Figure 1 consisted of an outer cylindrical Pyrexcasing (i.d. of 10 cm and height of 35 cm) fittedwith an air diffuser and an UV lamp (NEC). Thereactor volume was 2.8 l. A low pressure mercurylamp (18 W) with major emission at 253.7 nmwas suspended vertically in the middle of thereactor as an UV source. A masterflexmicrocessor pump was used for recirculation at acontrolled flow rate. The solution containingphotocatalyst particles was placed between theannulus of the UV lamp and the Pyrex cylinder.Air was bubbled upwards through the air diffuser.

Preparation of new photo catalyst: There are manydifferent ways of producing photocatalyticparticles. This study focused primarily on theproduction and use of TiO2 catalyst coated ontohaematite particles. The resulting new photocatalyst contained 12% TiO2 and had a BETsurface area of 168 m2/g, an average size of 10µm and mainly anatase in crystal form. All theother chemicals were reagent grade.

Results and discussion

Figures 2 and 3 show the results of the removal of humic acid (colour substances) for total organic carbon(TOC) and color400 respectively. It was clear that increasing the illumination time resulted in higher TOC andcolor400 removal. It was found that color400 removal was up to 81 % at 180 minutes UV illumination time.However, the TOC removal was only 54% at the same UV illumination time.

Results in Figures 2 and 3 also revealed that the use of TiO2 as catalyst to remove TOC and color400 in abubble photo catalytic reactor had a lower efficiency than the use of TiO2/Fe2O3 as catalyst. Comparing theresults obtained for photo catalytic oxidation with TiO2, the TOC and color400, the removal rates wereenhanced by approximately 1.8 times and 1.3 times respectively at 180 minutes illumination using


2

TiO2/Fe2O3 as catalyst. It is clear that the TOC and color400 removal rates were substantially increased due tothe Fe2O3 support.Many investigators have reported the mechanism of photo catalytic oxidation using TiO2 catalyst for waterpurification. It is clear that photo generated holes (h+) and electrons (e-) may migrate to the irradiated TiO2

surface to act as adsorption sites or receptors for the organic compound and molecular oxygen in an airbubble reactor. More adsorption sites (receptors) may enrich the molecular oxygen and the humic acid onthe TiO2 surface. Hydroxy or peroxy radicals formed from the protonation of the superoxide (O2

-) anion(arising from adsorption of molecular oxygen on electron-rich sites) are highly reactive and may readilyattack the chemisorbed humic acid ion to release aqueous organic anions, a surface carboxylate radical andCO2.

Figure 2. Influence of catalyst on total organic carbon removal in PCO reactor. UV wavelength was253.7 nm at 18 watts. The initial concentration of humic acid was 32.13 mg/L TOC. The pHwas 7.0. Reaction temperature was 26oC.

Figure 3. Influence of catalyst on color400 removal in PCO reactor. UV wavelength was 253.7 nm at 18watts. The initial concentration of humic acid was 32.13 mg/L TOC. The pH was 7.0.

Reactiontemperature was 26oC.

Figure 4 shows the SEM surface morphology of TiO2/Fe2O3 catalysts prepared in this study. It can be seenthat the TiO2 layer was uniformly coated on the surface of Fe2O3 to form a new porous catalyst. This porousTiO2/Fe2O3 catalyst had a BET surface area of 168 m2/g, which is three times higher than that of TiO2 (50m2/g). In contrast to the results in Figures 2 and 3, clearly, the large surface areas of Fe2O3 enriched themolecular oxygen and humic acids on the TiO2 surface and enhanced the electron migration to TiO2 surface,which acts as an adsorption site. At the same time, Fe2O3 as porous material might be acting as a “sink” for

0

10

20

30

40

50

60

0 50 100 150 200Illumination time (min)

Tot

al o

rgan

ic c

arbo

n re

mov

al (

%)

0.04 g/L TiO20.04 g/L TiO2 on Fe2O30.08 g/L TiO2 on Fe2O30.12 g/L TiO2 on Fe2O30.2 g/L TiO2 of Fe2O3

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200Illumination time (min)

Col

our r

emov

al (

%)

0.04 g/L TiO20.04 g/L TiO2 on Fe2O30.08 g/L TiO2 on Fe2O30.12 g/L TiO2 on Fe2O30.2 g/L TiO2 on Fe2O3


3

electrons released when a photon with sufficient energy strikes the TiO2 catalyst, then reducing theelectron/hole recombination. Thus, depending on the adsorption/desorption equilibrium of humic acid, thePCO of an adsorbed humic acid using TiO2/Fe2O3 catalyst might be of some importance for the overallefficiency of its oxidative degradation (reaction 1). In addition, the TiO2 layer on the surface of Fe2O3 mayprevent the dissolution of iron during the usage in aqueous suspensions (bubble photo reactor). However,more testing on the prevention of dissolution of iron needs to be carried out to verify this.

(1) )()()( .2.2 adsads XRTiORXhTiO ⋅+→+ ++

The percentage removal rates of TOC and color400 were also affected by the catalyst loading. Figure 5 showsthe TiO2/Fe2O3 catalyst loading as a function of percentage removal rates in the bubble photo reactor. It canbe seen that increasing the catalyst loading from 0.04 g/l to 0.2 g/l did not significantly affect the percentagecolour removal. However, as the percentage catalyst loading was increased, the percentage TOC removalinitially rose and then fell. It would appear that when catalyst loading is greater than 0.08 g/l, there is UVlight scattering and shielding as the number of particles in the solution increased. Furthermore, thepercentage TOC removal rate may drop as a result of loss in surface area available for UV light-harvestingoccasioned by agglomeration (particle-particle interaction) at high solid concentration.

Figure 4. SEM micrograph surface of photo TiO2/Fe2O3 catalyst.

Figure 5. Influence of catalyst loading on color400 and TOC removal. UV wavelength was 253.7 nm at18 watts. The initial concentration of humic acid was 32.13 mg/L TOC. The pH was 7.0.Reaction temperature was 26oC.

20

30

40

50

60

70

80

90

0 0.05 0.1 0.15 0.2 0.25

TiO2/Fe2O3 catalyst loading (g/L)

Rem

oval

(%)

TOCColor


4

Conclusions

An UV bubble photo reactor for advanced PCO process using a new TiO2 coated haematite catalyst has beendeveloped in this work. SEM results revealed that TiO2 was uniformly coated on the surface of Fe2O3 as acatalytic layer to form a bulk photo catalyst TiO2/Fe2O3. The porous TiO2/Fe2O3 catalyst had a BET surfacearea of 168 m2/g, which is three times higher than that of commercial TiO2. The experimental resultsindicated that the suspended TiO2/Fe2O3 catalysts in a bubble photo reactor was effective in removing TOCat 54% and color400 at 81% at 180 minutes illumination time. Comparing the results obtained for photocatalytic oxidation in a bubble photo reactor with TiO2 as catalyst, TOC and color400 removal rates wereenhanced by approximately 1.8 times and 1.3 times respectively with TiO2/Fe2O3 as catalyst. It is believedthat increasing the BET surface area of new TiO2/Fe2O3 catalyst (1) enriched the molecular oxygen andhumic acids on TiO2/Fe2O3 surface area, (2) enhanced the electron migration to TiO2 surface to act asactivated sites and (3) provided a better electron transfer by reducing electron/hole recombination.

Darren Delai Sun ([email protected])J LiJ H Tay


1

Optimising the Preparation of Activated Carbon from Digested Sewage Sludgeand Coconut Husk

Introduction

Due to the ever increasing amount of sewage sludge and the more stringent environmental quality standards,the treatment and disposal of sewage sludge has become an expensive and environmentally sensitiveproblem worldwide. Furthermore, with some traditional disposal routes coming under pressure and othersbeing phased out, it is necessary to seek a cost-effective and innovative solution to the problem caused bysewage sludge disposal. Sewage sludge is rich in carbonaceous organic material, therefore, it has thepotential to be converted into activated carbons if pyrolysed under controlled conditions or with somechemical treatment. This sludge conversion will offer the combined benefits of reducing the volume ofsludge to be disposed of and producing a valuable adsorbent with a lower cost than commercial activatedcarbons. In this study, the effect of preparation condition parameters such as the concentration of activatingagent (ZnCl2), heating temperature and the mixing ratio of additive coconut husk to sewage sludge wereexamined.

Experimental and methodology

The anaerobically digested and dewatered sewage sludge was collected from the Kranji Sewage TreatmentWorks, Singapore. The sludge was pre-dried at 103 °C, crushed and sieved into a uniform size of 0.5-2.0mmbefore it was mixed with the dried and crushed coconut husk. Ten gm of the mixture was then impregnatedinto 25ml of ZnCl2 solution for 24 hours at room temperature. After removing the supernatant liquid, themixture was dried at 103°C for 24 hours and subsequently pyrolysed in a stationary vertical furnace (Lenton,Eurotherm 91e) with purified nitrogen as purge gas. The heating rate, dwell time and nitrogen gas flow weremaintained at 15°C/min, 2h and 400 cm3/min, respectively. Following pyrolysis, the product was washedwith 3M HCl solution, rinsed with deionised water and then dried at 103 °C for 24 hours. Subsequently theproduct was subjected to a surface area and porosity analysis and phenol adsorption test for characterisation.


Effect of heating temperature and concentration of activating agent

In this test, the mixing ratio of coconut husk to sludge was maintained at 1:2. The heating temperature andthe ZnCl2 concentration were varied from 400 to 700°C and 3M to 5M respectively. Figure 1 shows theeffects of heating temperature on the BET surface area of the final products with the activation of variousconcentrations of ZnCl2 solution. It was found that low heating temperature could not sufficiently producecarbons with well developed porosity. The elevated heating temperature improved the development ofporosity of the products. However, when the temperature exceeded 600°C, the porosity decreasedsignificantly. This probably resulted from a sintering effect, which destroyed the pore walls between adjacentpores and widened the micropores to meso or macropores. The activation of 3M ZnCl2 solution appeared toperform not as well as 5M and 7M ZnCl2 solutions in terms of total BET surface area, possibly due to aninsufficient activation. However, it is favoured to produce a highly microporous activated carbon (Figure 3).Figure 2 indicates the variation of phenol removal efficiency of the products with the heating temperature. Itwas found that the heating temperature of 600°C could yield activated carbons with the highest phenolremoval efficiency, which did not agree with the variation with BET surface area and heating temperature.

Effect of mixing ratio of coconut husk to sewage sludge

Sewage sludge has a relatively low carbon content and high ash content compared to other commercialprecursors for activated carbon production. The purpose of addition of coconut husk is to improve the


2

properties of the precursors, especially to increase the carbon content and decrease the ash content. However,in order to make adequate use of sewage sludge, a reasonable mixing ratio of coconut husk to sludge isnecessary to make sewage sludge the predominant precursor in the mixture. In this section, the mixing ratiowas varied from 1:4 to 1:1, while the heating temperature and ZnCl2 concentration were maintained at 500°Cand 5M respectively, where the products with best BET surface area were prepared. Two samples made frompure coconut husk and pure sewage sludge respectively were adopted as a comparison.

The results indicated that the mixing ratio did not make a significant difference on the BET surface area ofthe products, whereas it affected the microporosity considerably (Figure 4). Nevertheless, as most of theproduced carbons were in fact the microporous materials, the BET surface area could not depict their actualtotal surface area. For comprehensive consideration of the surface area of the products and maximisation ofsludge utilisation, the mixing ratio of 1:4 appeared to be a favourable cost-effective ratio. As dewateredsewage sludge prior to drying normally has around 80% of water content, this ratio was actually 1:20 interms of undried sludge, which is reasonably low for practical use.

30

35

40

45

50

55

60

65

70

400 500 600 700

Temperature (°C)

Phen

ol re

mov

al e

ffic

ienc

y (%

) 3M ZnCl25M ZnCl27M ZnCl2

400

450

500

550

600

650

700

750

800

400 500 600 700

Temperature (°C)

BET

sur

face

are

a (m

2 /g)

3M ZnCl25M ZnCl27M ZnCl2

Figure 1. Variation of BET surface areaof the

products with heating temperature

Figure 2. Variation of phenol removal efficiency of the products with heating temperature

0

20

40

60

80

100

400 500 600 650 700

Heating temperature (°C)

Perc

enta

ge o

f mic

ropo

res

surf

ace

area

(%)

3M ZnCl25M ZnCl27M ZnCl2

Figure 3. Variation of percentage of micropores area of the products with heating temperature


3

0100200300400500600700800900

1000

Sludge 1:4 1:3 1:2 1:1.5 1:1 CoconutHusk

Mixing ratio of coconut husk to sludge based on dried weight

m2 /g

0102030405060708090100

%

BET surface area

Percentage ofmicropores area

Figure 4. Variation of BET surface area and percentage of micropores areawith mixing ratio of coconut husk to sludge

Conclusion

Activated carbons were produced from digested sewage sludge with additive coconut husk. It was found that5M ZnCl2 activation and 500°C of heating temperature could produce activated carbon with the highest BETsurface area. The mixing ratio of coconut husk to the sludge did not have a significant effect on the BETsurface area of the products whereas it affected the microporosity considerably.

S. Jeyaseelan ([email protected])

X.G. Chen ([email protected])


1

Reuse of Treated Municipal Wastewater in Urban Agriculture: A HydroponicStudy

Introduction

The rapid development in Singapore has caused a big increase in demand for potable water. With limitednatural water resources, developing non-traditional water resources is essential. Seawater desalinationprovides an alternate reliable water resource. It is however expensive when compared to other conventionalwater resources. Reuse of treated municipal wastewater offers another promising source of water supply andneeds to be systematically studied. Currently, some 3% of secondary effluent is reclaimed for industrial usein general washing, cooling and cleaning. The use of treated municipal wastewater for industrial purposes isencouraged and expected to increase in the years to come. Reusing municipal wastewater in urbanagriculture is another attempt to create non-traditional water resources. The purpose of this study is toexamine the feasibility of growing hydroponic vegetables using municipal wastewater effluents and toidentify the potential factors that may affect the plant growth and development.

Materials and methods

A 32-day experimental trial was conducted at the Ngee Ann Polytechnic’s rooftop greenhouse. Butterheadlettuce (Lactuca sativa L. cv. Baby butter) and chinese cabbage (Brasicca chinensis) were grown withprimary and secondary effluents in two independent deep flow hydroponic systems (DFT). A controltreatment containing a balanced nutrient solution (Cooper solution) was operated in parallel with the twotypes of effluents. Twenty four plants, twelve each of lettuce and cabbage, were transplanted to each DFTper treatment. Six plants from each of the three treatments were harvested 16 and 32 days after transplanting(DAT). Effluent quality including pH, EC, COD, suspended solids (SS), N (free ammonia, TKN, nitrite andnitrate), total P, Cl, Na, K, Ca, Mg, Fe, Cu, Mn, Zn, Cd and Pb were determined. The fresh weight andheight of the plant shoots/roots of each harvest were measured to compare their growth and development.Finally, plant tissues were oven-dried, burned and digested for elemental analyses.


Control butterhead lettuce and chinese cabbage plants grew well though minor chlorosis on the chinesecabbage was observed. The chlorosis may be due to lack of nitrogen as only half of the normal nutrientstrength was utilized for the control solution. Compared to the control plants, butterhead lettuce grown onwastewater effluents grew surprisingly well. The secondary butterhead lettuce were larger than those grownon primary effluent though the primary plants appeared to be greener. Chinese cabbage might be sensitive tothe wastewater effluents as only four stunted chinese cabbage plants survived in the primary effluenttreatment.

Effluent quality analysisTable 1 shows the quality results of the wastewater effluents and control solution. The pH and electricconductivity (EC) were maintained at 6.0 and 1.0 respectively throughout the experimental trial. The resultsindicate that after 32 days of planting, 87% and 68% of COD were removed from the primary effluent andsecondary effluent respectively. The continuous solution recirculation throughout the experiment facilitatedthe oxidation of organic matters. Similarly, 98% and 96% of SS were removed from the primary effluentand secondary effluent respectively. The results suggest that the hydroponic systems purified thewastewater effluents.

No clear trend was observed for nutrient removal. Although the constant solution recirculation provided anaerobic environment to enhance nitrate production, no clear trend of nitrogen removal was observed. Thiscan be explained by the fact that, within the hydroponic system, the organic matter and biofilms create


2

complex partitioned micro-environments that promote nitrogen cycling. The nature of the effluent dependson the balance between the rates of the nitrification and denitrification activities on the surface of therhizomes. The complicated nitrogen transformation will be left for future study. The increase of total Pmight have been affected by the addition of phosphoric acid when adjusting the pH.

Table 1 also demonstrates that plants have taken up necessary macro- and trace-elements from thewastewater effluents though the trend of metal ion removal is not clear. The fluctuations of metal ionconcentrations might have been caused by volume changes due to active evaporation. In fact, nutrientuptake is highly dependent on the plant species and the growing stage.

Table 1. Effluent quality results

Before Transplanting 1st Harvest (16 DAT) 2nd Harvest (32 DAT)Parameter(mg/L) P S C P S C P S C

pH 7.78 7.77 7.88 6.03 6.28 6.32 6.22 6.02 6.40EC (mS/cm) 1.0 0.9 1.0 1.0 0.9 1.0 1.0 1.0 1.0

Total P 4.18 2.82 6.93 6.36 6.31 6.44 6.56 6.86 6.76Free Ammonia 21.24 9.15 2.41 10.43 0.61 1.12 15.97 1.11 2.19

TKN 55.60 20.38 28.15 25.31 11.28 6.87 42.76 48.14 49.50Nitrite 0.04 0.13 ND ND ND ND ND ND NDNitrate 1.70 16.00 124.55 27.83 12.93 115.70 30.7 13.05 103.87COD 438.00 36.33 29.33 54.67 28.67 23.00 58.25 12.00 30.67

SS 146.00 16.27 NA 7.92 0.66 NA 2.52 0.65 NAChloride 234.10 333.23 19.95 234.67 221.70 8.47 238.50 330.90 14.23

Na 126.24 120.26 14.08 118.00 123.86 14.32 159.18 136.53 14.02K 12.85 11.71 95.21 16.91 19.51 113.29 10.91 10.15 50.33Ca 66.75 67.45 114.49 29.18 33.43 92.13 86.17 72.98 203.34Mg 8.78 8.56 33.50 9.48 10.43 26.84 11.61 10.18 25.84Fe 0.24 0.11 7.15 0.16 0.20 5.68 0.18 0.25 5.85Cu 0.13 0.12 0.22 0.10 0.12 0.67 0.24 0.11 0.69Mn 0.16 0.12 0.12 0.26 0.06 0.34 0.17 0.07 0.15Zn 0.18 0.11 0.03 0.10 0.11 0.19 0.24 0.15 0.24Cd 0.05 0.07 0.03 0.03 0.02 0.02 0.03 0.02 0.02Pb ND ND ND ND ND ND ND ND ND

ND - Non Detectable; NA - Not Available; P – Primary; S – Secondary; C - Control.

Plant growth and developmentTable 2 shows the root and shoot growth characteristics of butterhead lettuce. The results indicate that thesecondary lettuce had a better root development than the primary lettuce. Until the first harvest, thesecondary lettuce developed 17% of its total root mass, which is comparable to the control lettucedevelopment, while the primary lettuce developed less than 8% of the total root mass. The root mass of thesecondary lettuce was about 95% of the control mass, which is much higher than that of the primary lettuce(70%). The slower and smaller root development might have been caused by the competition for rootsurface of the biofilms that are used to remove the high COD and SS in the primary effluent. This can beexplained by our observations that all the primary plants had dark coloured roots. It is believed thatsignificant microbial activities had taken place around the root zone for COD and SS removal.

For the shoot growth, the results show that butterhead lettuce of the primary and secondary treatments were59% and 72% the weight of the control lettuce respectively. Several factors may contribute to the shootdevelopment retardation. For instance, nutrient deficiency, especially nitrate and the macro elements (e.g.,


3

K, Ca and Mg), could hinder the shoot development. However, there were no deficiency symptoms recordedon the butterhead lettuce plants on all treatments. Root surface competition may also contribute to theretardation. Nutrient transport could become inefficient if nutrient needs to penetrate biofilms to reach theroot surface for use. In addition, high ionic concentration might affect the function of root membranes.Further study is needed to understand the exact impacting factors.

Table 2. Root and shoot growth characteristic of butterhead lettuce grown on different wastewater effluentsand control (non-effluent) treatments

Primary Secondary ControlEffluent \Plant Weight (g) Length (cm) Weight (g) Length (cm) Weight (g) Length (cm)Root @ H #1 0.30 ± 0.37 6.03 0.88 ± 0.59 13.50 0.96 ± 0.42 22.20Root @ H #2 3.92 ± 1.23 13.96 5.19 ± 1.66 28.12 5.49 ± 1.55 47.68Shoot @ H #1 5.96 ± 3.55 10.73 ± 1.74 7.36 ± 3.20 11.67 ± 0.88 12.52 ± 0.69 12.73 ± 0.47Shoot @ H #2 47.23 ± 6.02 14.26 ± 1.62 57.79 ±17.67 14.38 ± 0.82 80.15 ±31.40 15.00 ±0.50Six plants each were measured; “H” stands for harvest.

Elemental analysisElemental analysis results reveal that plant grown on the wastewater treatments accumulated higher levels ofK, Ca, Mg and Fe in the shoots than in the roots. This implies that plants can get essential nutrients fromwastewater effluents. The uptake pattern was somewhat different for the trace elements. It appears that theroots contained higher concentrations of Cu, Mn and Zn than did the shoots. This suggests that the rootscould accumulate higher levels of trace elements than could the shoots. Therefore, by careful selection,plants could take up necessary nutrients from wastewater effluents for shoot development, whileaccumulating heavy metals in the roots to purify wastewater effluents.

Overall, butterhead lettuce grown on primary and secondary effluents appeared to be healthy, while chinesecabbage did not survive as it might be sensitive to wastewater effluents. The results indicate that plantscultivated in hydroponic systems can absorb nutrients from wastewater effluents for their growth whileaccumulating heavy metals in the roots.

Conclusions

The study has demonstrated that it is possible to grow hydroponic butterhead lettuce with treated municipalwastewater, while efficiently purifying the wastewater effluent for direct discharging or further reuse. Thepurified water could be reused for gardening, flowers and other horticultural crops. To grow market-gradeagro-products, additional nutrients may have to be added to the wastewater effluents. Although thewastewater-grown butterhead lettuce appeared healthy, the plants may have been stunted by potentialgrowth-affecting factors, e.g., root surface competition, nutrient deficiency and toxicity, pH and temperature.These factors may adversely affect the plant growth of some species such as chinese cabbage. Furtherresearch is needed to determine the correlation of these potential affecting factors and to conduct a humanhealth risk analysis for the consumption of the wastewater-grown crops.

J Y Wang ([email protected])J H TayK K Chow


1

Biomass Growth and Retention in Anaerobic Filters

Introduction

It has been established from previous studies that the ability of support media in anaerobic filters (AFs) toretain high concentrations of biomass is an important factor for satisfactory system performance. This studywas initiated to examine the influence of support media on biomass growth and retention, either as suspendedgrowth trapped within the interstitial void spaces, or as attached biofilm adhered to the media surfaces inthree laboratory-scale anaerobic filters treating synthetic protein-carbohydrate wastewater. The significanceof the suspended biomass and the attached biofilm on wastewater treatment was evaluated quantitatively.

Methods and materials

Three 15l plexiglas columns packed with Raschig rings were used as the anaerobic filters (AFs). The ringsthat packed into the reactor designated AF-1 were of open-pored surface texture glass material (with specificsurface of 187 m2/m3 and porosity of 75%), while the AF-2 and AF-3 reactors were packed with smoothsurface polyvinyl chloride (PVC) Raschig rings (with specific surface/porosity of 132/90 and 187/75,respectively). A synthetic waste was used to provide a consistent organic substrate. After startup, thereactors were operated simultaneously at the same hydraulic and organic loading rates. The reactors wereoperated until a steady-state performance was reached as indicated by constant gas production and effluentCOD concentration. The steady-state conditions were maintained to enable collection of data for analysis andperformance evaluation. The organic loading to all reactors was then increased simultaneously, and thereactors were operated until the next steady-state conditions were reached. A testing protocol based on asimulated methane gas production by the biomass using a batch serum bottle technique was used to estimatequantitative COD removal by the biomass during steady-state conditions. The method involved measurementsof hourly methane production and corresponding total organic carbon (TOC) concentration from the biomasssample.

At the end of the experiment, all the reactors were dismantled. The reactor liquid and Rashig rings weretaken out to determine the concentrations and distributions of suspended and attached biomass within thereactors. The rings in each AF were washed with distilled water to slough off the attached biofilm. While theouter film could be removed readily from the rings, a considerable amount of inner film was still attachedfirmly on the media surfaces. The rings were scrubbed individually with a brush to transfer completely allbiomass to a container for biomass growth determinations.

Analysis of results

The effects of media porosity and surface texture on reactor performance were evaluated. As shown inFigure 1, the COD removal efficiencies indicated that all reactors seemed to exhibit somewhat equalperformance at the loadings of 2 and 4 g COD/l/d, with overall COD removal efficiency in excess of 90%.When the loading was increased to 8 g COD/l/d and subsequently to 12 and 16 g COD/l/d, removal at thesestages began to show the relative superiority of reactors AF-1 and AF-2 over AF-3. The COD removalefficiencies were about 90% in both AF-1 and AF-2 and was 75% in AF-3 at 8 g COD/l/d. At higherloadings of 12 and 16 g COD/l/d, both AF-1 and AF-2 showed similar COD removal of about 77% and 73%compared with the markedly reduced removal efficiencies of 67% and 57% in AF-3, respectively. The resultsindicated that both media surface texture and porosity have a significant impact on the performance of AFs.The higher removal efficiencies of AF-1 associated with its open-pored media texture is likely attributed tohigher growth of attached biofilm. As for AF-2, its larger media porosity may promote higher growth ofsuspended biomass leading to higher removal efficiencies. The fact that both AF-1 and AF-2 exhibited muchlower concentration of effluent suspended solids, as will be discussed, may also justify their higher retentioncharacteristics.


2

Results from the serum bottle tests at loading rate of 16 g COD/l/day are tabulated in Table 1. The resultsindicated that the suspended biomass extracted from the lower quarter (25 cm) of the reactor height in allreactors showed the highest methane production at both loading rates. The production of methane at thisreactor zone in general constituted about 30% of the total methane produced by the suspended biomass. Thiscould be due to higher substrate concentration near the influent inlet area and higher solids accumulation inthe lower zone. Above the lower 25 cm zone, distribution of the methane production is somewhat consistent.In general, reactor AF-2 exhibited the highest methane production at the respective reactor height, followedby AF-1 and AF-3. The results indicated that the attached biofilm in AF-1 produced a higher proportion ofmethane (56%) than the suspended biomass (44%), while the suspended biomass in AF-2 demonstratedhigher methane production (58%) than the attached biofilm (42%). No clear distinction of the two biomasswas observed in AF-3. The fact that the suspended biomass in AF-2 showed the highest methane productionis likely due to its higher media porosity and pore size. This may reduce the possibility of clogging or `deadspace' formation, which in turn allows more contact between biomass and substrate leading to better organicremoval. Likewise, the higher methane yield exhibited by the attached biofilm in AF-1 could be accounted forby its open-pored media surface texture. This corresponds well to their superiority in COD removal efficiencyas compared with AF-3, discussed previously.

An examination of effluent suspended solids (SS) may indicate the extent of biomass washout. Effluent SSmeasurements during the steady-state period operating at the COD loading of 16 g/l/d indicated that muchhigher SS values, in the order of 11,000 to 12,000 mg/l, were observed in AF-3 than in AF-1 and AF-2, thelatter with SS of below 1600 mg/l. The observation of higher effluent SS in AF-3 indicated that significantbiomass washout had taken place leading to a much lower COD removal efficiency in AF-3. This could bedue to the poor biomass retention capability of the smooth surface media being packed in AF-3. The effluentSS results may justify the hypothesis that AF failure at high loadings is likely due to biomass washout ratherthan short circuiting. This suggests that the contribution of short circuiting to AF failure may not be assignificant as normally thought.

Figure 2(a) shows the distributions of suspended solids (SS) in the reactor liquid. The bulk of suspended solidswere in the lower 25 cm of all reactors. The distribution patterns were likely due to the accumulation of solidsat the reactor bottom along with the higher waste concentration at the lower zone. As expected from thebatch serum bottle test results previously mentioned, the SS concentrations in AF-2 were consistently thehighest at all reactor heights, likely due to its larger media porosity. The solids volatility was in the range of82-90% in all reactors and was consistently maintained at this level along the reactor height.


3

Distributions of the attached biofilm in all reactors are illustrated in Figure 2(b). The results indicated similardistribution to that shown by the SS; the biofilm concentration was higher at the lower reactor heights than itwas on the upper sections. The concentrations of biofilm in AF-1 associated with its open-pored mediasurface texture were significantly higher than the other two units at all reactor heights. This indicated that theunique feature of the double-pore structure of the media is able to provide better biomass adhesion than thesmooth surface media. Although biomass washout was more in AF-3 than in AF-2, as discussed previously,the attached biofilm was greater in AF-3 than in AF-2. This is simply attributed to a much higher mediaspecific surface in AF-3 than in AF-2, which enables greater growth of biofilm.

The amount of biomass retained by each AF was also determined. Table 2 indicates that AF-1 retained themaximum amount of 340.0 g VSS [column (6)], while AF-2 retained a slightly lower biomass of 336.9 g VSSand AF-3 the lowest mass of 315.0 g VSS. It is noted that the open-pore media structure of AF-1 resulted inthe maximum retention of 52% biofilm in the reactor. On the other hand, AF-2 showed the maximumretention of about 72% SS owing to its larger media porosity. It can be seen from Table 5 that the biomass ineach AF is present in two physical forms. The first form is in suspension [column (2)] being held loosely inthe interstitial void spaces within the bed. The other form is the attached biofilm [column (5)] consisting of“outer film” [column (3)] which was attached loosely and those on the “inner film” [column (4)] which werefirmly adhering to the media surfaces. The former could be readily detached by washing with distilled water.The latter, which formed into a slimy crust, could only be removed by scrubbing. As the biomass growing onthe loosely-held outer layers can be easily sloughed off subject to hydraulic shear or biogas lifting, the biomasscontributed only about 15-17% of the total attached biofilm.

A comparison of the results in Table 2 with Table 1 (discussed previously), indicates that the amount ofmethane produced by the respective biomass corresponded well with the biomass distribution at the loading of16 g COD/l/d. The measured 52% and 45% of total biofilm in AF-1 and AF-3 respectively [Table 2 column(5)] fit closely with the estimated 56% and 49% [Table 1 column (9)] of methane production by the serumbottle technique. Reactor AF-2 indicated somewhat different results where the methane production of 42%


4

[Table 1 column (9)] by the attached biofilm seemed to be over-estimated considering the measured lower28% of biofilm [Table 2 column (5)]. Likewise, the methane production of 58% by the suspended biomass[Table 1 column (8)] seemed to be under-estimated considering the measured 72% of biomass [Table 2column (2)]. One possible explanation for this discrepancy is that, of the measured 72% of suspendedbiomass, not all of them were active enough to convert the waste into methane.

Conclusions

The results of COD removal efficiency indicated that media surface texture and porosity have a significantimpact on anaerobic filter performance. The fact that the suspended biomass in the reactor produces thehighest proportion of methane is likely due to its largest media pore size and porosity. A higher methane yieldexhibited by the attached biofilm in the other unit could be accounted for by its open-pored media surfaces.To optimise the retention of biofilm attached on the media surfaces and the suspended biomass trapped withinthe interstitial void spaces, support media of open-pored surfaces and high porosity should be used.

J H Tay ([email protected])K Y Show

Table 1. Estimation of CH4 produced by suspended biomass and attached biofilm

Reactor CH4 Produced throughReactor Height (L/d)

Total CH4 Producedby Suspended Biomass

MeasuredTotal CH4

Production

% CH4 Produced

25 cm 50 cm 75 cm 100 cm (L/d) (L/d) Suspendedbiomass

Attachedbiofilm

(1) (2) (3) (4) (5) (6) (7) (8) (9)

AF-1 7.9 8.3 5.4 6.3 27.9 62.8 44 56

AF-2 8.9 6.0 7.7 7.3 29.9 51.4 58 42

AF-3 6.0 5.3 5.4 5.6 22.3 43.6 51 49

Table 2. Distribution of biomass (expressed as g VSS)

Reactor Total suspended biomass Outer biofilm Inner biofilm Total biofilm Total biomass in the reactor(1) (2) (3) (4) (3) + (4) = (5) (2) + (5) = (6)

AF-1 162.9 (47.9%)a 30.8 (9.1%) 146.3 (43.0%) 177.1 (52.1%) 340.0 (100%)

AF-2 241.4 (71.7%) 14.7 (4.4%) 80.8 (23.9%) 95.5 (28.3%) 336.9 (100%)

AF-3 174.3 (55.3%) 20.4 (6.5%) 120.3 (38.2%) 140.7 (44.7%) 315.0 (100%) aValues in brackets are expressed as a percent of total biomass in each reactor


1

Long-term Predictions of Phosphorus Dynamics in Kranji Reservoir

Introduction

Kranji reservoir, which is located in the northeast of Singapore, has been subjected to differenttrophic states since it was built. The reservoir was rapidly undergoing eutrophication during the1960's and 1970's due to large volumes of water pollutants emanating from predominantly farming,industrial, urban and semi-urban areas. Since 1976, industrial and domestic discharges have beencontrolled by legislation, a vast network of sewers and wastewater treatment. The water quality wassubstantially improved. However, the results are not as expected. The eutrophication is stillsustained and trophic status in Kranji reservoir is between eutrophic and mesotrophic. The reason isthat the phosphorus, which is stored in sediments during the period of heavy pollution, iscontinuously released to support the eutrophication process in the reservoir.

It is not clear how long the eutrophication will sustain and how much phosphorus is released fromthe sediment to overlying water during that period. Optimal control and management ofeutrophication in the reservoir require more comprehensive studies from an integrated point of viewto provide symbolic logic which is capable of expressing the functions and relationships ofphosphorus dynamics associated with environmental processes and predicting the release flux. Theutilisation of mathematical models is an attempt to realise these purposes as they can quantitativedescribe the sediment-water interaction system and predict the impacts of elemental changes onwater ecosystems.

Mathematical models of sediment phosphorus and long-term predictions

A set of mathematical models has been developed to simulate the phosphorus dynamics in aquaticsediments. The models, consisting of six interacting differential equations, have the followinggeneral matrix form:

[ ]∂

∂ ty y t yi i= [ ' ( , ) ]

where i = 1 to 6 corresponding to dynamic variables of dissolved phosphorus (DP), exchangeableparticulate phosphorus (EP) and organic phosphorus (OP) respectively in the aerobic and anaerobiclayers.

The sediment phosphorus models were applied to the Kranji reservoir to understand the dynamiccharacteristics of phosphorus in aquatic sediments and to predict internal loadings of phosphorusover a long-term period up to 30 years. The results will be greatly helpful for the management ofreservoir eutrophication over a long-term period.

Determinations of model parameters

The parameters to be used in the proposed sediment phosphorus models include dynamic variablesand state constants. The former are the external variables including the organic loading andenvironmental factors and the latter characterise the properties of sediments and the physical,chemical and biological mechanisms regulating phosphorus dynamics. The dynamic variables inKranji reservoir were obtained by field surveys. The state parameters with respect to thecharacteristics of sediments, sorption and biological decomposition were determined by laboratory


2

work. Because no data were available to estimate the transport parameters of diffusion, mixing andburial processes, they were adopted from the literature. The obtained average parameters and valuesof external variables are listed in Tables 1 and 2.

Table 1. State parameters used for the long-term prediction

Parameters Aerobic layer Anaerobic layerSorption:Kinetic rate constant 2.90 (day-1) 2.63 (day-1)Langmuir isotherm constant 5.97 (l/mg) 4.61 (l/mg)Sorption minimum 6.65 (mg/g) 3.72 (mg/g)Decomposition:Rate constant of labile fraction 0.0378 (day-1) 0.0253 (day-1)Labile fraction 1.91 (%) 1.59 (%)Rate constant of refractory fraction 0.0049 (day-1) 0.0037 (day-1)Refractory fraction 7.16 (%) 7.50 (%)

Table 2. Annually averaged values of external variables

External variables ValuesOrganic matter 26.31 (mg/g)Dissolved oxygen 2.7 (mg/l)Dissolved phosphorus 3.7 (µg/l)Temperature 29.49 (°C)Calibrated depositional flux 0.51 (g/m2.day)

Because information on the recovery processes of Kranji reservoir is not available, fourhypothetical scenarios are used for predictions: organic loading remaining at the present level, beingdecreased by 5%, 10%, 25% and 50% annually. Similarly, during the simulation period, thedissolved oxygen, phosphorus and temperature are kept constant whereas the organic matterremains constant in the first five years and then annually decreases by 5%, 10%, 25% and 50%respectively.

Prediction results and discussion

The predicted results of phosphorus internal loadings for the long period of 30 years are shown inFigure1. If current conditions of organic loading remain unchanged, the resulting internal loadingof phosphorus released to the overlying water will remain almost without variation and be in asteady state level. However, if organic loadings reduce yearly, the internal loadings will declinegradually at a very slow rate. The quickest response is observed when the reductions of organicloading are larger than 25%.

The periods to reach 50% decrease of internal loading are approximately 25, 15, 8 and 5 yearswhen organic loadings are reduced by 5%, 10%, 25% and 50% respectively. The response speedsof predicted internal loading declinations are dependent on the reduction rates of organic loading.The larger the reductions of organic loadings, the higher are the declinations of the phosphorus


3

internal loading. However, when the reductions of organic loading are larger than 25%, thisfunction will be weakened. The calculated initial rates of yearly decreases of internal loadings forthree cases are respectively 2.5%, 4.3%, 9.8% and 19.0%(Table 3). These rates decrease graduallyin response to the time and tend to be the same. The internal loadings do not show a significantdeclination due to the fact that the stored sediment phosphorus will sustain the release processwhen external organic loads are reduced.

0

2

4

6

8

Time (year)

P In

tern

al lo

adin

g (m

g/m

.day

) Constant 10% reduction

25% reduction 50% reduction

0 5 10 15 20 25 30

Figure 1. Long-term predictions of the internal loading of phosphorusfor 30 years in Kranji reservoir

Table 3. The indexes characterizing the recovery of internal loading

Organic Annual decl. rate (%) RecoveryLoading Initial Final (%)

5% reduction 2.5% 3.7% 63.4%10% reduction 4.3% 4.4% 78.3%25% reduction 9.8% 4.6% 85.5%50% reduction 19.0% 4.5% 87.0%

During the declination of internal loadings when the organic loading is reduced, the storedexchangeable particulate phosphorus is correspondingly consumed as shown in Figure 2. Unlike thedecrease of internal loadings, the exchangeable particulate phosphorus decreases at a much slowerrate to compensate for the reductions in the regeneration flux from the decomposition of organicphosphorus. This mechanism can be clearly seen from the simulated variations of adsorption-desorption fluxes occurring in the anaerobic layer, as shown in Figure 3. The sediments act as thesource when the organic loading remains at current conditions. However, when the organic loadingsare reduced yearly, the adsorption fluxes of phosphorus in the sediments gradually reduce to zeroand finally desorption takes place. The sediments become the source to sustain the phosphorusrelease. The reduction speeds are associated with the reduction rates of organic loadings.


4

ConclusionThe results of long-term predictions of Phosphorus dynamics in Kranji reservoir indicate thatinternal loadings of phosphorus will remain approximately constant if the organic loading intosediments is kept unchanged. However, if this organic loading reduces yearly, the internal loadingsof phosphorus will decline gradually at a very slow rate. The response speeds of this declination aredependent on reduction rates of organic loadings. The larger the reductions of organic loadings, thehigher the declinations of phosphorus internal loadings.

0.0

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g)

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Figure 2. Simulated variations of exchangeable particulate phosphorusin sediments of Kranji reservoir for 30 years

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ay)

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Adsorption zone

Desorption zone

Figure 3. Simulated adsorption-desorption fluxes in sediments of Kranji reservoir for 30years

The calculated initial rates of annual decreases of internal loadings are 2.5%, 4.3%, 9.8% and 19.0%respectively when organic loadings are reduced by 5%, 10%, 25 % and 50%. These rates decreasegradually in response to time and tend to be equal finally. The internal loadings predicted do notshow a significant and immediate declination as the sediment phosphorus stored in sediments willsustain the release process when the external organic loading is reduced.


5

The exchangeable particulate phosphorus stored in sediments is predicted to decrease at a muchslower rate. The sediments act as the source when the organic loading remains unchanged. Howeverwhen organic loadings are reduced yearly, the adsorption fluxes of phosphorus in sedimentsgradually reduce and finally desorption takes place. The sediments in this case will become thesource to sustain the phosphorus release.

A Appan ([email protected])Wang Hong


1

Behaviour of Cracked Diaphragm Walls

Introduction

To date there have been limited reports in the literature of the need to incorporateconcrete cracking in the analysis and design of diaphragm walls supporting proppedexcavations. A back-analysis of a case study using finite element analysis is presentedto highlight the differences in the behaviour of a diaphragm wall when the wall isassumed to behave linear elastically, and when the effects of wall cracking areincorporated.

Case study

In this case study, the excavation was carried out using the top-down method ofconstruction. The excavation was 10 m deep. It was retained by a 1.0 m thick and 35.5m long diaphragm wall, with three levels of slabs as props. The ground surface levelwas at RL 102.5 m. The ground conditions at the site generally consisted of a 5 m thicklayer of loose fill, underlain by a thick layer of marine clay. Beneath the marine claywas a medium dense to very dense clayey sand layer.

Finite element program

A finite element program EXCAV97 (Wong and Goh 1997) was used in the back-analysis. The program can simulate excavations in clay and sand under plane strainconditions. Four-node isoparametric elements were used to model the soil. Theprogram uses the hyperbolic model to simulate the nonlinear, stress dependent, andinelastic behaviour of the soils. It can also simulate the sequential nature of theexcavation and strut installation

In the analysis, the walls and slabs were modelled as beam elements. The analysistakes into account the effects of the cracking of the diaphragm wall by using a non-linear moment-curvature relationship based on Branson's (1977) formula. The moment-curvature relationships calculated based on Branson’s formula, and that used in thefinite element analysis are shown in Figure 1. Figure 1 shows that the concretecracking reduces the bending moments significantly. This figure also shows that aftercracking is initiated, the continued use of an uncracked section in the analysis wouldover-predict the wall-bending moments, especially when the magnitude of the wallcurvature is large. However, the use of a fully cracked section in the analysis wouldunder-predict the wall-bending moments.

Results

For comparison, finite element analyses were also carried out assuming linear elasticwall properties. The measured and computed wall lateral displacements for excavationto RL 92 m are shown in Figure 2. The maximum lateral wall deflection for the analysisincorporating the effects of cracking was 65 mm, compared with 57 mm for the linearelastic analysis. The corresponding bending moments deduced from the wallinclinometer and strain gauge readings together with the computed values are shownin Figure 3. The computed wall bending moments agreed well with those deduced fromthe wall inclinometer and strain gauge readings, in both the shape and magnitude ofthe bending moments. The results also indicate that the computed wall bendingmoments for the analysis assuming that the wall behaves linear elastically are much


2

larger than that for the analysis that incorporated the effects of concrete cracking. Thecomputed maximum bending moment for the analysis based on the linear elastic wallproperties is approximately 60% larger than that obtained from the analysis thatincorporated the effects of concrete cracking. The maximum bending moment for theanalysis incorporating the effects of cracking was 1290 kNm/m, compared with 2030kNm/m for the linear elastic analysis.

0

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-30 0 30 60 90

Lateral wall deflection (mm)

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th (m

)

RL 92 m : MeasuredRL 92 m: FEM (Cracked)RL 92 m: FEM (Uncracked)

Soil Profile

Fill

Softmarine clay

Soft to medium stiff silty clay

Medium denseto very denseclayey sand

Medium stiffmarine clay

Medium densesilty sand

Soft organic clay

RL of ground surface: 102.5 m

Figure 2. Effects of cracking on lateral wall deflections.


3

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-1500 -1000 -500 0 500 1000 1500 2000 2500

Bending moment (kNm/m)

Dep

th (m

)

Strain gaugesWall inclinometerCracking momentFEM: CrackedFEM: Uncracked

Excavation level: RL 92 m

Figure 3. Effects of cracking on wall bending moments.

References

[1] Branson, D. E. (1977). Deformation of concrete structures. McGraw-Hill, New York,N.Y.[2] Wong, K. S. and Goh, A. T. C. (1997). EXCAV97 - A computer program for analysisof stresses and movements in excavations. NTU-PWD Geotechnical Research Centre,Geotechnical Research Report NTU/GT/97-1

Anthony T C Goh ([email protected])K S Wong ([email protected])T Y PohI H Wong


1

Dynamic Testing of the Bukit Timah Granite Using the Split Hopkinson Bar

Introduction

The mechanical properties of most materials vary with strain-rate. The split Hopkinson pressurebar (SHPB) is usually used to measure dynamic properties of material within the strain-rate rangeof 102/s to 104/s. It is difficult, however, to get exact testing results for brittle materials because ofthe wave dispersion. Much research has been conducted to minimise this problem, and the FastFourier Transform technique (FFT) has been used to process the testing data. But this process israther complex. In this article, a new experimental method which is much simpler and moreeffective than the FFT technique is introduced. With this method, dynamic tests of the Bukit Timahgranite are performed at a strain-rate ranging from 101/s to 103/s.

Dynamic testing method

The setup of SHPB is shown in Figure 1. Two strain gauges are fastened to the middle of theincident and transmitter steel bars. The specimen is usually clipped tightly between the two longbars. When the striker bar strikes the incident bar, a loading wave is generated and propagatesthrough the bar and the specimen. With the data recorded by the strain gauges, the dynamicstress-strain curves of the specimen are derived. But the original rectangular loading wave will bedispersive according to the one dimensional wave theory. As a result, the initial part of the loadingwave is oscillatory. Because the brittle rock material is always damaged when subjected to thisinitial part of the loading wave, this wave dispersion will result in a great number of errors in theresults. If a gap is made between the incident bar and the specimen, then the oscillatory part of theloading wave will be reflected back totally in the incident bar instead of acting on the specimen.Thus the loading wave is closer to a rectangular shape, as shown in Figure 2. This pre-gapped testmethod has been adopted for testing the Bukit Timah granite.

Dynamic testing results

The cylindrical specimen of the granite is 35 mm in diameter and 20 mm in length. Five series oftests were conducted at the compressive strain-rates of 46/s ,101/s,186/s,335/s and 874/s . Fromthe dynamic stress-strain data in Figure 3, it was found that the modulus and strength increasewith the increasing strain-rate. But beyond a very high strain-rate, the modulus decreases againstthe increasing strain-rate. This phenomenon implies that the strain-rate hardening effect anddamage softening effect should be taken into consideration to examine the dynamic properties ofrock materials.

Incident bar

Transmitter bar

Damper

Absorber bar

Strain gauge

Parallelilluminant

Striker bar Specimen

Figure 1 . The set-up of Hopkinson pressure bar


2

t ( µs )

ε i , ε

r (

µε )

ε t (

µε

)

εi

εr

εt

ε i : The incident waveεr : The reflected wave

ε t : The transmitted waveFigure 2 . The actual pressure waves on specimen

Figure 3 . The dynamic stress-strain curves

Liu Jianfei ( [email protected])Zhao Jian ([email protected])


1

Effect of Soil Movements on Piles

Introduction

Soil movements can induce large bending moments in piles supporting existing super-structures. These movements can be caused by various construction activities such asbasement excavation, tunneling for railway and underground utilities, construction of roadembankments and land reclamation. Failure can occur when the induced moment exceedsthe yield moment of the piles. It is important to recognise this potential problem and be ableto analyse it reliably and accurately.

A computer program (BCPILE) has been developed in NTU to analyse the effect of verticaland horizontal soil movements on vertical as well as batter piles. It can handle complexconditions such as layered soil deposit, variable pile stiffness and pile-head fixity. The soilmovement profile is part of the required input. The program computes the pile displacement,shear and bending moment along the pile and the variations of soil reaction with depth. Adetailed description of the methodology can be found in the papers by Teh and Wong (1995)and Goh et al. (1997).

Case study 1

In the reclamation of the Jurong Islands, it was necessary to fill over some of the existingbridges that linked the islands. Pile failure was reported at one of the bridges. One or morepiles supporting the trestle buckled as shown in Figure 1. It was postulated that the failurewas due to the impact of displaced mud created from the reclamation work in the vicinity asthere was no evidence of direct impact from the sand fill.

According to a post-failure site investigation at the location of the failed piles, there was 5 mof very soft displaced mud overlying the original seabed. The undrained shear strengthvaried between 4 to 11 kPa. Analyses were conducted to study the effect of displaced mudon the stability of the pile. Results showed that the pile was strong enough to resist soilmovements in the transverse direction i.e. perpendicular to the bridge (Figure 2a) but not inthe longitudinal direction i.e. parallel to the bridge (Figure 2b). This was in agreement withfield observation. The pile actually buckled in the longitudinal direction.

Case study 2

This case study reports the movements of two large diameter (1.2 m) bored pilesconstructed at a building boundary prior to the basement excavation. Inclinometers wereinstalled in these piles to monitor the pile movements. When the excavation reached theformation level of about 10 m depth, the piles deflected about 150 to 170 mm.

The soil profile consisted of 5 m of sand overlying 3 m of upper marine clay, 4 m ofintermediate stiff clay, 19 m of lower marine clay, and underlain by the Old Alluvium. Thetotal pile length was about 40 m.

An analysis was conducted to estimate the magnitude and profile of the free-field soilmovements due to excavation using the computer program EXCAV97. The computeddisplacements were calibrated against the measured displacements from a nearby in-soilinclinometer. The data were then used as input to the program BCPILE to compute the piledisplacements and bending moments. Results are shown in Figure 3. The soildisplacements are larger than those of the pile which indicate that the soil actually moved


2

past the pile. The computed displacements are in reasonable agreement with the measureddeflections of the two piles.

ConclusionsSoil movements can cause existing piles to deflect and induce large bending moments. Thecomputer program BCPILE provides a valuable tool to practicing engineers to analyse thisclass of problems. The two case studies served as a validation to the program.

Figure 1. Case Study 1 – pile failure due to displaced mud during land reclamation.


3

References

[1] Teh, C. I. and Wong, K. S., Analysis of Downdrag of Pile Group, Geotechnique, 45 (2),1995, 191-207.[2] Goh, A.T.C., Teh, C. I. and Wong, K.S., Analysis of Piles Subjected to EmbankmentInduced Lateral Soil Movements, Journal of Geotechnical and Environmental Engineering,Vol. 123, No. 9, September 1997, 792-801.

Wong Kai Sin ([email protected])Teh Cee Ing ([email protected])Anthony T C Goh ([email protected])


1

Interpretation of Initial Horizontal Stresses in Rock From HydraulicFracturing Test Measurements

Introduction

For the design of underground structures in rocks, the initial stresses must be known. One of themost reliable methods of evaluating the initial state of stress in rock at great depth is thehydraulic fracturing test. The test is performed by inducing a fracture hydraulically in a sealedsection in the test borehole (Figure 1a). After the test is performed, the inclination and thedirection of the induced hydrofractures at the borehole wall are determined using an impressionpacker and a magnetic orienting instrument (Figure 1b).


2

When the fracture induced by the test is vertical, the conventional method of interpretationproduces reliable results (Haimson and Fairhurst 1969). In cases where the fracture induced ishorizontal or mixed-mode (horizontal and vertical), the arbitrary application of the conventionalmethod, which was originally developed for vertical fractures, will yield wrong results regardingthe magnitude of initial horizontal stresses which may lead to unsafe design. In these cases, themodified stress path (MSP) method developed by Hefny and Lo (1992) should be used. Thismethod does not include any injudicious assumptions, and it takes into account the anisotropicbehaviour of rocks where appropriate. The following section briefly illustrates a case of theimportance of interpretation.

Initial stresses at the AECL Underground Research Laboratory

Atomic Energy of Canada Limited (AECL) has been conducting geomechanical research at itsUnderground Research Laboratory (URL) in Manitoba to assess the feasibility of nuclear fuelwaste disposal deep in a plutonic rock mass (Martin 1990). The geological structure at the URLof AECL is shown in Figure 2. Figure 3 shows the maximum initial horizontal stress at the siteas interpreted from various methods of stress measurements. Above fracture zone 2, the inducedfractures by the hydraulic fracturing tests were vertical and the application of the conventionalmethod gave horizontal stress values that are in agreement with those determined by othermethods. Below fracture zone 2, the induced fractures were horizontal and the initialinterpretation of maximum horizontal stress, as reported by Martin (1990), is given in Figure 3.As can be seen from the figure, the interpreted stresses from the hydraulic fracturing tests are toohigh and do not agree with those calculated by convergence measurements.

If the results are re-interpreted by the MSP method (Hefny and Lo 1992, 1995), the results agreewell not only with results of convergence measurements, but also with performance of the shaftand performance of experimental tunnels at the same elevations (Figure 4). For details ofcomparison with field performance and for results of stresses measured in other boreholes atgreater depths at the site, refer to Hefny and Lo (1995). For the analysis of several other casehistories using the MSP method, refer to Hefny and Lo (1992) and Lo and Hefny (1993).

It is clear, therefore, that the initial horizontal stresses in rocks can be reliably interpreted fromthe hydraulic fracturing test measurements for any mode of fracturing and without the use of anyarbitrary assumptions.


3

References

Haimson, B.C. and Fairhurst. C. 1969. In-situ stress determination at great depth by means ofhydraulic fracturing. Proceeding, 11th U.S. Symposium on Rock Mechanics, Berkeley, pp. 559-584.

Hefny, A.M., and Lo, K.Y. 1992. The interpretation of horizontal and mixed-mode fractures inhydraulic fracturing tests in rock. Canadian Geotechnical Journal, Vol. 29, No. 6, pp. 902-917.

Hefny, A.M., and Lo, K.Y. 1995. Interpretation of initial stresses from hydraulic fracturing testsat AECL’s underground research laboratory, Manitoba. Canadian Tunnelling Journal, pp. 123-134.


4

Lo, K.Y., and Hefny A.M. 1993. The evaluation of in situ stresses by hydraulic fracturing testsin Anisotropic rocks with mixed-mode fractures. Canadian Tunnelling Journal, pp. 59-73.

Martin, C.D. 1990. Characterizing in situ stress domains at the AECL Underground ResearchLaboratory. Canadian Geotechnical Journal, Vol 27, pp. 631-646.

A M Hefny ([email protected])


1

A Brief Geotechnical Analysis of an Earthquake:Cukurova (Turkey)-1998

Introduction

On June 27,1998 a moderate earthquake measuring 5.9 on the Richter scale struck the Cukurovaregion of Turkey, causing 145 deaths and injuries to approximately 1000 people, and damage to about30,000 buildings. The coincidence of the earthquake epicenter and the fault with a very vulnerablegeological surface formation (geologically young, thick alluvial deposits of loose fine silty sandsoverlaid by soft silty-clay) led to widespread geological deformations (mainly due to liquefaction).This provided a unique opportunity to assess the performance of various types of structures underconditions of loss of bearing capacity and lateral support, and subjected to lateral spreading andsubsidence. The geological deformations included lateral spreading, flow failures, ground fissuresand extensional cracking, sand boils, ground subsidence and slope failures.

This article presents the post-earthquake reconnaissance observations of the author and themain geotechnical lessons learned or relearned from this earthquake. The two most significantgeotechnical aspects of this earthquake were the widespread soil liquefaction and associated groundfailures and ground motion amplification at “soft” soil sites. These two geotechnical phenomenasignificantly influenced the damage patterns.

Main observations and lessons:

• Local site amplifications, or site effects, can result in significant differences in structural damagewithin the same general area. At several sites it was observed that whilst one building washeavily damaged, a building of similar construction a block or two away was completelyunaffected.

• Damage patterns at several sites suggested topographic amplifications even on minor hills.

• Young sedimentary cohesionless soil deposits with a low water table are exceptionally vulnerableto liquefaction. When this liquefaction happens in sloping ground (or ground with initial staticshear stress), it is usually accompanied by lateral spreading, which was found to be the mostdamaging liquefaction induced phenomena during this earthquake as well as in most of the otherprevious big earthquakes (Alaska and Niigata 1964; Northridge 1994; and Kobe 1995).

• In areas away from incised river channels (or any other free face) and any slope where theprincipal effect of liquefaction was vertical ground settlements, surprisingly little damage to man-made structures was observed. However, in areas not far from river channels or sloping ground,where liquefaction generated lateral ground displacements, foundation performance was typicallypoor. Likewise, ground failure rather than ground shaking appeared to be the dominant cause ofdamage to buried utilities such as pipelines and irrigation canals.

• Liquefaction of loose sand layers was usually accompanied by the formation of sand boils due tothe low permeability and cohesion of an overlying upper silty-clay layer. In general, the greaterthe thickness of the overlying layer, the fewer but larger were the sand boils. Sand boiling byitself caused minor damage only in a few cases when it led to localized differential settlements infoundation soils.

• Buildings with strong continuous mat foundations or thick strong base slab performed noticeablybetter in the areas subjected to minor to moderate effects of liquefaction, such as lateral spreading,sand boils, and ground subsidence. In these instances, the foundation behaved as a diaphragm,preventing differential ground displacements from propagating upward into the superstructure.


2

• As in many other seismic disasters such as in Mexico City, the San Francisco Bay area, Kobe-Japan, and Izmit-Turkey, significant damage in the Cukurova-1998 event was clearly related tosoft ground conditions. Microzonation for ground deformations and for amplifications at suchsites are imperative. Mitigation of ground deformation effects on built environment may involveground improvement or improved foundation systems. Most importantly, for amplificationeffects, design on soft ground must be based on the suppression of resonance by either groundimprovement or structural improvement by damping.

Figure 1. Ground motion-acceleration time histories recorded at Ceyhan Station.

Figure 2. Ground shaking triggered liquefaction in a subsurface loose silty sand layer, leading to differentiallateral and vertical movement in an overlying unliquefied silty clay layer, which moved towards the incisedCeyhan River channel. This mode of ground failure, termed “lateral spreading” was the most striking anddamaging adverse effect of liquefaction caused by the June 27, 1998 Cukurova Earthquake.

Korhan Adalier ([email protected])

Acc

eler

atio

n (g

)

-0.3-0.2-0.10.00.10.20.3

+T EASTMax: 0.273g

East-West

Acc

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n (g

)

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+L North

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Time (sec)

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)

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Vertical


1

Near-Real-Time Measuring of SeaLevel Using GPS

Introduction

A prototype vertical GPS array, comprising twoGPS antennas, was successfully tested inmonitoring the changes in the sea level vis-à-vis the results obtained using a conventionaltide gauge. This finding indicates that it isfeasible to determine the profile of the sea bedusing sea levels derived from GPS and depthsof sea water measured using a conventionalecho sounder in a hydrographic survey withoutreference to a tide gauge.

Geoidal corrections to GPS heights

The heights as determined using GPS refer toa mathematical surface - the WGS84 (WorldGeodetic System 1984) ellipsoid, whereasheights of points on the earth's surface refer toa surface of equal geopotential. A simpleplumbline intersects perpendicularly with thissurface which is known as the Geoid. Due tothe non-homogeneity of the earth mass, thissurface is very irregular. The sea surface is thebest approximation of the geoid. The heightsabove the geoid are called the orthometricheights, or the reduced level, when the meansea level is taken as the approximation to thegeoid, which strictly needs to be determinedusing a gravimeter.

Figure 1 illustrates the relationship betweenellipsoidal height and orthometric height. Thedifference between the two heights is known asthe geoidal height or geoid undulation, which isthe correction to be applied to the ellipsoidalheight.

Figure 1. Relationship between the ellipsoidaland orthometric heights

Local EGM96 Geoid Model

A global geoid model EGM96 (EarthGravitational Model 1996) was jointlydeveloped by NASA Goddard Space FlightCenter (GSFC) and the National Imagery andMapping Agency (NIMA) with the assistance ofthe Ohio State University (OSU) and the U.SNaval Surface Center.

A constant correction of +2.024 m was addedto the geoid heights as derived from theEGM96 to derive the Local EGM96 model forSingapore. The correction was foundnecessary after examining the geoidal heights(ellipsoidal minus orthometric heights) of 212GPS control points well distributed withinSingapore. Figure 2 shows a gradual change ingeoidal heights of about 9 m from the westernto 11 m in the eastern part of Singapore.

Figure 2. Local EGM96 Geoid Model

Vertical GPS array

A prototype GPS array, separated vertically by1.121 m, was devised to provide checkmeasurements in the observed ellipsoidalheights. It was mounted on the side of asounding boat and was approximately abovethe transducer of the echo sounder (Figure 3).A base-plate for the lower antenna was foundto be essential to minimize the multipath effect.The height of the lower antenna above thewater line (draught of the boat) was noted toreduce the ellipsoidal heights of the twoantennas to that of the sea level.

A site bounded by the port limit at the south-westerly part of Singapore, Tuas and the


2

Jurong Islands was chosen to test theperformance of the vertical GPS array. Thistest site is denoted as the Tuas Test Area inthis paper.

Figure 3. Prototype vertical antenna array

Figure 4 shows the ellipsoidal heights of bothantennas observed around Point I which is oneof the nine test points (A, B, C, D, E, F, G, Hand I as shown in Figure 6). In all the testpoints, the patterns of both heights are almostidentical. This proves empirically that the lowerantenna is not adversely affected by the upperantenna in signal reception or multipathinterference.

12.412.612.813.013.213.413.613.814.014.214.414.614.8

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Antenna 1

Antenna 2

Figure 4. Ellipsoidal heights of vertical GPSarray

Comparison with tide gauge

The ellipsoidal heights of the sea level werededuced from that of the two antennas and theLocal EGM96 model was used to convert themto the orthometric heights (heights above the

known mean sea level). The averages of thesea levels obtained are very ‘noisy’, mostprobably due to the influence of the sea waves.A regression line was fitted to the whole dataset to produce a smoothed sea level. Figure 5shows the differences in the sea level betweenthe regressed data and the measured dataobtained from the tide gauge. The goodcorrelation between the smoothed andmeasured (tide gauge) sea level demonstratedthat it is feasible to monitor the changes in thesea level using GPS in a near-real-time modeusing simple regression technique.

2.1

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Smoothed water level derived from GPS Antennas 1&2

Smoothed water level derived from Tuas View tide gauge

Figure 5. Difference between the regressedand the measured (tide gauge) sea level

Comparisons of the differences in sea levels atthe nine test points show an increase in thedifferences from 0 m to 0.35 m in a south-easterly direction as illustrated in Figure 6.

Figure 6. Differences in GPS-derived and TideGauge sea level at Tuas Test Area


3

Local Tuas View geoid

The increasing differences in the sea levelsfurther away from the coastline are notsurprising as the Local EGM96 geoid for theoffshore area is extrapolated. A new geoidcalled Local Tuas View Geoid was developedfor the Tuas Test Area. This geoid wasdeveloped from five control points in Tuas andthe nine test points. The geoid was evaluatedusing seven independent points and a RMS of± 0.031m was achieved. An independentmeasurement using hydrostatic levellingbetween Tuas View and Sultan Shoal(S.Shoal) also compared favorably with theresults obtained using the Local Tuas ViewGeoid. The difference is only 0.015m.

Conclusion

The finding shows that it is feasible to use GPSinstalled on a boat (or other floating platform)and a local geoid model to measure thechanges in sea level with a RMS of ± 0.031m.

E L Tan ([email protected])Y K Tor ([email protected])

Acknowledgement

The authors would like to express theirappreciation to the Survey Department ofJurong Town Corporation (JTC) for theirassistance in this study.


1

Comparison of the HCM and Singapore Models of Arterial Capacity

Arterial roads form the backbone of any urban road system. Information on arterial road capacity andperformance under different traffic conditions is vital for transportation planners and traffic engineers.The capacity of an arterial depends mainly on its width (number of lanes) and on characteristics oftraffic control at signalised intersections. The US Highway Capacity Manual (HCM) provides a well-known method of analysing arterial performance. The HCM model assumes that travel time along anarterial increases with rising traffic volume due to increased delay incurred at intersections. Thus, themethod involves detailed calculations of vehicle delay at each signalised intersection, based on trafficvolume and signal timing parameters.

A study of arterial road capacity in Singapore conducted at the Centre for Transportation Studies (CTS)used a different approach. A direct relationship was established between travel time and traffic density.After calibration using data from five arterials, this relationship was transformed into a speed-flowmodel. The model derived in this way had only two parameters: intersection spacing and the minimumintersection delay. The last parameter was defined as the average stopped delay incurred by vehicles atsignalised intersections under very light traffic conditions. The CTS model is useful for planningapplications as it does not require the input of detailed signal timing data. The estimated capacity ofarterials ranged from 840 to 1000 pcu/h/lane. A major finding of the study was that arterial runningspeed is strongly dependent on traffic volume – a relationship which is ignored in the HighwayCapacity Manual method.

It is interesting to compare the speed-flow relationships obtained from the HCM 97 and CTS modelsfor typical Singapore arterials. Figure 1 shows the comparison of curves calculated with the followingparameter values: intersection spacing of 400 m; cycle time of 120 s and green split ratio of 0.4. Twocases are presented: (a) favourable signal progression (signals coordinated for a ‘green wave’ effect),which results in a low minimum delay value of 9.5 s, and (b) no progression, resulting in a minimumdelay of 17.1 s. For any level of flow, the HCM model generally predicts lower travel speed.According to the CTS model, arterial speed is much more sensitive to flow, especially underuncongested conditions. This is not surprising as in to the HCM method flow only affects the speedindirectly, through the value of vehicle delay at intersections. In the case of no progression, thedifferences between the two models are also large when flow is near capacity. While the HCM modelallows for oversaturation (i.e. flows temporarily greater than capacity), the CTS model predicts reducedflows associated with very low speeds (severely congested conditions). Interestingly, the CTS modelpredicts that good signal progression leads to an increase in overall arterial capacity by about 100pcu/h/lane. However, the CTS model is not sensitive to signal timing parameters such as the greensplit ratio.

A practical problem in using the CTS model is the choice of value for the minimum intersection delayparameter. Its magnitude depends on signal timing characteristics and the quality of progression. Arecent survey of 5 intersections revealed that the minimum delay varied between 5 and 30 seconds,depending on the duration of green and red periods as well as the proportion of vehicles arriving duringgreen. The value of minimum delay can be estimated from the standard delay formula using a lowvalue of degree of saturation (say, 10%). Figure 2 shows the comparison of predicted and observed


2

minimum delay values. The HCM-97 delay formula gave predictions which were very close toobservations (root-mean-square error of 1.1s or 8%).

0

10

20

30

40

50

0 200 400 600 800 1000 1200Mid-block flow (pcu/h/lane)

Trav

el s

peed

(km

/h)

HCM-97

CTS

0

10

20

30

40

50

0 200 400 600 800 1000 1200Mid-block flow (pcu/h/lane)

Trav

el s

peed

(km

/h)

HCM-97

CTS

Figure 1. Speed-flow curves based on the HCM and CTS models

Future research is needed to explore possible ways of improving both models. The main problem withthe HCM method is the lack of linkage between the magnitude of traffic flow and arterial segmentrunning time. Such a relationship should be calibrated and incorporated into the HCM method. TheCTS model could be improved by inclusion of a parameter reflecting the average green split ratio atsignals along the arterial.


3

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Observed delay (sec)

Pre

dict

ed d

elay

(sec

)

Figure 2. Observed and predicted minimum intersection delay

In order to improve model accuracy, it is necessary to use more precise definitions of arterial flow andcapacity. A practical problem arises in reconciling measurements of flow and number of lanes at anintersection approach and between intersections. Most arterial intersection approaches are flared, i.e.additional turning and through lanes are added at signals. Similarly, flow measured betweenintersections (mid-block flow) includes turning movements whereas intersection analysis normallyinvolves the through movement only. It may be useful to introduce a ‘through factor’ parameter torelate the mid-block flow to the through flow at an intersection approach.

Piotr Olszewski ([email protected])


Figure 2. Setup as seen on the pedestrianbridge

Transitionzone

Stop-Line

Figure 1. View from the camera with thesuperimposed transition zone

Driver Stopping Response Latency at Signalised JunctionsIntroduction

Signalisation of traffic junctions is widely used as a means of allowing conflicting traffic movements to besafely discharged through time-separated movements. However, as junctions are accident-prone areas,understanding driver behavior at junctions is important in the design of the traffic signal operations.

The knowledge of perception-response time (PRT) at traffic junctions is an important parameter for trafficsignal design and is also applicable for legislative purposes whereby a traffic engineer may be called upon toanswer questions on the limits of driver performance. Past research has tended to use values based onobservations taken over a wide range of distances from the stop-line. The authors feel that values so obtainedmay be overly conservative, as vehicles that are far away from the junction stop-line have a range of choiceas to where to apply the braking action.

Stopping response latency and the transition zone

This study uses the stopping response latency as a practical measure of PRT. It is the time interval from theonset of the amber signal indication to the onset of the vehicle brake lights. The "transition" zone is theregion where a vehicle would have a certain probability of stopping (Figure 1). It is derived from the conceptof the dilemma zone but encompasses a greater range.

Values within the transition zone are used as the practical measure of the limits of driver response. This isbecause when a driver operates within the transition zone, he has to make his decision within a very smalltime window. This window is the most crucial period for designers, as the decision to run the red or to stop ismade within this period. Beyond the transition zone, the choice is taken away from the driver.

By studying the stopping response latency for the transition zone, a more realistic representation of the actualstopping latency exhibited by drivers is obtained.

Methodology

The study made use of high definition digital cameras (Panasonic AG-EZ30E) to capture data under daytimedry-weather conditions. The cameras were mounted on tripods and placed on pedestrian bridges (Figure 2)along selected sites. Distance on the ground was measured using the road markings as benchmarks. The


Table 1. Results obtained using GLM analysis (For all data points)Variable Classes P Value Pr > F Conclusion

Speed(km/h) <50, 50-60, 60-70,70+ 6.66 0.0003 SignificantDistance(m) 30~95 36.12 0.0001 SignificantVehicle Type Motorcycle, car, truck,

Heavy vehicles0.43 0.7322 Insignificant

Camera Yes, No 2.18 0.1419 Insignificant

distances of road markings from the stop line were measured on-site using a road meter. The data collectedon site were extracted using a video editor and a projector. The lower-most portion of the rear wheel wasused to track the position of target vehicles and the data were recorded on a spreadsheet. Only first stoppingand last crossing vehicles were taken as valid data; the movement of the target vehicles must not beconstrained by other vehicles within the traffic stream.

Overall findings

A plot of distance at the instant of amber verses time is shown in Figure 3. This encompasses all the datapoints. Two segments are obtained which show stopping vehicles (red area) and crossing vehicles (greenarea). The overall plot reveals that vehicles very distant from the stop-line always stop, while vehicles near tothe stop-line always proceed, with the transition zone having both stopping and clearing vehicles. Theoverall mean stopping response latency was 1.04s, with a standard deviation of 0.43 and an 85th

percentile value of 1.3s.

Statistical analysis was undertaken using SAS's GLM procedure. The results are shown in Table 1.

The broad results reveal that distance and speed had significant effect, while vehicle type and the presence ofsurveillance cameras did not show significant influence on stopping response latency.

The transition zone: FindingsAs explained earlier, the values as determined over the total observational distance are not suitable for designpurposes. The PRT values should be analyzed for drivers moving within the transition zone during the onsetof amber. Consequently, the transition zone was determined by considering the observed speed and distancevalues. The values thus obtained would give a more suitable PRT to be used for signalised junctions.

What can be concluded from the results is that the transition zone was found further from the stop line asvehicle speed increased. Stopping vehicles had stopping response latencies clustered around a fairly constantmean and 85th percentile value, with a mean of 0.8~0.9s and an 85th percentile of 0.94~1.12s for allapproach speeds. The results are summarized in Table 2.

Concluding remarks

Of the variables studied, stopping response latency was significantly influenced by the distance of a vehiclefrom the stop-line and its approach speed at the onset of amber, while vehicle type and the presence of red-light surveillance cameras apparently had no significant effect on stopping response latency.

Empirical results showed that within the transition zone, the stopping response latency remained fairlyconstant. The mean stopping response latency within the zone is less than PRT times proposed by theInstitute of Transportation Engineers Handbook and some other studies. Additional work is in progress tofurther substantiate the effect of junction layout and time of day on driver stopping response latency.

P K Goh ([email protected])Y D Wong ([email protected])


Figure 3. Corresponding data points to the transition zone for crossing andstopping vehicles

Stop-line

StoppingResponseLatency

Speed at onset ofamber

Crossing Vehicles

Stopping Vehicles

Time to CrossStop-line

0

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5 3 3.5Time(s)

Dis

tanc

e fr

om s

top-

line

at th

e on

set o

f am

ber(

m)

20 to 30km/h

30 to 40km/h

40 to 50km/h

50 to 60km/h

60 to 70km/h

70 to 80km/h

80 to 90km/h

20 to 30km/h

30 to 40km/h

40 to 50km/h

50 to 60km/h

60 to 70km/h

70 to 80km/h

80 to 90km/h

Table 2. Results obtained for data points within the transition zoneSpeed at amber

(km/h)Transition zone

(m)Mean

latency (s)StandardDeviation

85 th percentilevalue (s)

50- 22.6 to 55.4 0.85 0.25 1.1250 to 60 37.0 to 65.1 0.86 0.27 1.0760 to 70 56.6 to 74.9 0.87 0.22 1.05

70+ 61.7 to 84.6 0.82 0.16 0.94


1

Mobility Model for Urban Public Transportation

Urban public transportation is an important part of the transportation systems in many urban areasin Asia. This is especially the case for an island state like Singapore. The scarce land supply anddense population have made public transport the major mode of transport, which accounted forabout two thirds of the total motorised trips made in a day, according to the latest statistics.

Accessibility is a term often used in transport and land-use planning. Although there is not a fixeddefinition of accessibility, it is often defined as the opportunity that an individual at a given locationpossesses to take part in a particular activity. Mobility is considered as the ability of that individualto move about, and therefore accessibility can be considered as a proxy of mobility. There havebeen numerous studies on accessibility, most of which are concerned with factors such as cost oftravel and opportunities. Measures like geographical contours, defined indices, revealed value andtime-space geography have been proposed by various researchers to represent accessibility.

In this paper, an approach to evaluate the mobility provided by urban public transportation ispresented. Accessibility indices (AI) were developed to represent the relative ease of accessingvarious locations by public transportation, using Singapore as an example. As buses and the MassRapid Transit (MRT) constitute the backbone of public transportation in Singapore, these are theonly two transit modes considered in this research.

The accessibility indices developed are the function of a series of factors, which include frequencyof services (ff), hours of operation (fh), directness of journey (fd), level of service (fn), and numberof direct connections available by services (fc). The product of these factors is used to representhow accessible a particular transit service is at a certain point and is called the Serviceability Indexof that service. If there are n number of transit services at any geographical location, theAccessibility Index (AI) is the summation of service indices for all services available within a 400m radius, taking into account any physical and topological barriers on site. The AI is adimensionless number and should only be meaningful when its relative magnitude is considered.

Each of the factors constituting AI is considered as the performance ratio of an existing service tothe ideal service. In this study, the ideal public transportation service is defined as one that offersservices throughout the day, high frequency of services, high speed (80 kph) and one which alwaysruns on the shortest paths between two points. The performance of this ideal service can beconsidered as the benchmark value to gauge the corresponding performance of existing services.Table 1 shows the values used in this study. To obtain an accessibility index for a region (or aparcel of land), the summation of individual AI of all the m access points to public transportservices can be obtained.

∑=

=n

iicndhf fffffAI

1

)(


2

Table 1. Comparison of performance indicators of an ideal service and a bus service j

Factor For bus service j For an ideal serviceFrequency Mean hourly volume for service j 360 buses/hrHour of operation Hours of operation for service j 24 hrsSpeed Average speed of service j 80 kphDirectness of journey Distance taken by road from

points A to B by service jShortest distance by roadfrom points A to B byservice j

Number of directconnections

Number of connections offeredby service j

250

Data obtained in the western part of Singapore was used to develop a set of accessibility indices forexamination of the model. The area bounded by the Pan Island Expressway (PIE), Jalan Bahar,Clementi Road and the coastline was reviewed. Contours of the accessibility index were drawn onthe street network maps as shown in Figures 1 and 2. The range of accessibility values isrepresented by a gradation of colours, with red representing the highest value on the accessibilityindex and yellow the lowest. The results show that areas near the MRT stations received thehighest value on the accessibility index, while those areas only served by a few bus services had thelowest value. When comparing the peak and off-peak indices, there is a noticeable decrease in theindices, especially in the areas surrounding the MRT stations, suggesting that in off-peak hours thepublic transport services may not be as good as they are in the peak hours.

In conclusion, the mobility model based on AI has potential to be an effective and objective tool toevaluate the performance of public transport services in any region. As a result, the accessibilityindex can be useful as an objective indicator that guides transit services planning and policymaking.

Lam Soi Hoi ([email protected])


3

Figure 1. Accessibility Indices at Peak Hours

Figure 2. Accessibility Indices at Off-peak Hours


1

3D Crack Simulation Using Boundary Element Method

Introduction

Tests have shown that a non-load-carrying fillet welded attachment, such as that shown inFigure 1, has always failed at the weld toe when it is subjected to fatigue loading. The sharpchange in the section geometry causes fatigue cracks to initiate from this severe stressconcentration site containing crack-like surface flaws due to fusion welds.

The 3D fatigue crack development in any welded joints can be monitored by taking AlternatingCurrent Potential Drop (ACPD) crack depth measurement at fixed points along each expectedcrack location, and the experimental stress intensity factor range can be obtained at the fixedpositions.

In order to check the validity of the experimental results, a kind of subdomain boundary elementscheme is adopted to estimate the stress intensity factor of the semi-elliptical crack. The quarter-point elements and transition elements are used in the neighbourhood of the actual crack front tocalculate the stress intensity factors.

Numerical modelling of the cracked welded joint

Typical BEM mesh is presented in Figure 2. For different crack depths, only the mesh on thecrack face (see Figure 3) is modified in the discretization and the following assumptions andsteps are used:• the shape of the two crack corners are assumed to be two elliptical surfaces as shown in

Figure 3. Each of these is expressed by 12222 =+ byax , where a and b can bedetermined with two experimental points adjacent to each corner.

• the linear interpolation method is used to obtain the intersecting line L of the crack face andthe free surface, using experimental points on that line.

• the intersecting points of the line L and the two elliptical surfaces for estimating the positionof the two crack corner points are calculated.

• a reasonable number of nodes are placed on two corner parts.• spline is used to interpolate the middle part of the crack front, and the tangential direction of

the points at the two ends of the spline can be obtained from the elliptical parts.• the crack face with the experimental points and the calculated interpolation points on the

crack front and the line L , are modelled using ordinary 8 nodes serendipity elements (seeFigure 3).

• quarter-point elements are generated along the two sides of the crack front (see Figure 3).The local coordinates of each corner node on the crack front are calculated at the sametime.

• transition elements are generated just behind the two rows of quarter-point elements.• the rest of the elements are generated, in the process ensuring that they are not too

distorted.

Some 7 or 5 nodes transitional elements are used at unimportant places to reduce the totalnumber of nodes. In a typical mesh, there are 2214 and 1974 degrees of freedom for subdomainI and subdomain II respectively.


2

Effective stress intensity factors

Stress intensity factors are calculated after the displacement field of the specimen is obtainedusing the sub-domain BESD3D program. However, the experimental SIFs are extracted fromthe generalized Paris’ equation:

( ) mKCNa eff dd ∆=

(1)

where effK∆ is the effective stress intensity factor, which is influenced by IK∆ , IIK∆ and

IIIK∆ , i.e. ( )IIIIIIeff ,, KKKfK ∆∆∆=∆ .

As the crack propagates, the percentage of the contribution from Mode II and Mode III to thecrack growth energy decreases compared to that from Mode I; and the total energy needed tomake the crack grow is influenced by the ratio of IK to III-IIK . Thus, the following formulationis used in this study:

( )( )CCreAKK Br +−+=α∆α=∆ 1 ; Ieff

(2)

where r is the energy rate of mode II-III to Mode I,

2I

2III

2II 1(( KKKr ∆ν))−∆+∆=

(3)

If 0>A , 0<B and BBrC 10 += , then BBCrr )1(0 −== , and

( )0max )1( 1 0 rBeA Br +−+=α . Letting 4.1max =α (an empirical value), then the value of

))1((4.0 00 rBeA Br +−= .

There are eight probes along the weld toe used in the experimental tests (see Figure 1). Typicaleffective stress intensity factors at probes 3 and 4 are plotted against the number of cycles asshown in Figure 4. The numerical results generally agree with the experimental results.

Conclusions

In this study, a comparison between experimental and numerical stress intensity factors of 3Dsurface semi-elliptical cracks found at the weld toe of a non-load-carrying cruciform weldedjoint, has been carried out successfully. The subdomain boundary element method, incorporatingthe transition and quarter-point elements along the actual crack front, is used to estimate thestress intensity factors. It has been found that the numerical results generally agree with theexperimental results.


3

S T Lie ([email protected])S Yan ([email protected])


1

Shear Strength and Behaviour of RC Beam-Column JointsUnder Cyclic Loading

IntroductionBeam-column joints are important elements of reinforced concrete structures. Well-designed beam-column joints are essential to guarantee the seismic resistance of RCstructures. Currently, three design methods for beam-column joints, namely, the ACI-ASCE method (developed in 1976, revised in 1985), the New Zealand method (developedin 1982, revised in 1995) and the AIJ method (developed in 1988), are among the most wellknown in the world. However, those three design methods differ significantly from eachother in two respects.

First, the joint shear strength vu according to the ACI-ASCE method depends on thestrength of the concrete, that is, cfγ where γ is a coefficient that depends on theconfinement provided by the framing beams. According to the AIJ method, vu is alsodetermined by the concrete strength ckf where k=0.3 for interior joints and 0.18 for exteriorjoints. In the New Zealand method, the joint shear strength depends on the amount oftransverse reinforcement provided in the joint.

Second, the amount of joint transverse reinforcement required by the New Zealand methodcan be 2 or 3 times the amount required by the ACI-ASCE method, while the ACI-ASCEmethod in turn requires 3 to 5 times the amount required by the AIJ method. Further, someaspects of beam-column joint behaviour under cyclic loading, such as the effect of theeccentricity between the beam axis and the column axis, are not addressed in the threedesign methods.

This research project was initiated with two objectives – to investigate the shear strength ofeccentric beam-column joints and to narrow the current philosophical differences in thethree design methods for beam-column joints. The first objective can be achieved by testingeccentric beam-column joint specimens and the second by analysing all availableexperimental data of beam-column joints around the world on a unified basis.

Test programmeSix interior beam-column joint specimens have been designed, cast, and tested. Theyconsist of two types of specimens, designated as Type 1 and Type 2 respectively. Type 1has a column aspect ratio of ¾, while Type 2 has an aspect ratio of ½. Each type includesone concentric specimen, one medium-eccentricity specimen and one maximum-eccentricity specimen. The specimens were intended to represent a typical RC frame with6m beam span and 3.6m story height with a reduced scale of 2/3 (see Figure 1). Thespecimens were reinforced according to the ACI-ACSE design method except that the jointshear stress at both beam yielding was higher than the nominal joint shear strength for eachspecimen, so that the joints could fail in shear during loading. Extensive instrumentation


2

was arranged for each specimen for recording steel strains, joint shear deformation,displacements of specimens, imposed beam end loads, and column axial load. The cyclicloading was imposed by displacing the two beam-ends equally in opposite directions withtwo 100T actuators, while keeping the column in vertical position. The loading wascontrolled by storey drift ranging from 0.5% to 5%.

Summary of resultsThe storey drift versus storey force is shown in Figure 2 for the Type-2 maximum-eccentricity specimen (designated as S6). Although the eccentricity was originally expectedto dramatically reduce the shear strength of the specimen due to its very narrow columnsection, the specimen still yielded during loading and the maximum storey force of 68.0KNexceeded the theoretical yielding storey force of 59.7KN by about 14 %. This indicated thatthe effect of eccentricity on the maximum joint shear stress that could be reached duringloading was not significant. It was the shear stiffness of the joint and, consequently, thelateral stiffness of the specimen that were affected by the eccentricity more significantly.

As for the behaviour of concentric beam-column joints, a major finding is that themaximum joint shear stress during loading mainly depends upon the joint shear inputdetermined by the moment capacity of the beam. However, once the joint shear stressreaches a certain value, joint shear deformation increases rapidly. Excessive joint sheardeformation is irreversible after unloading and would cause permanent drift of the buildingafter the earthquake. This should not be allowed for earthquake resistant RC structures. So,the joint shear strength should not be determined simply by the maximum joint shear stresswhich can be resisted by the joint. Instead, it should be determined by limiting the jointshear deformation to a reasonable level.

Zhou Hua ([email protected])Susanto Teng ([email protected])

Figure 1. Test set-up


3

Figure 2. Storey-drift versus storey-force for specimen S6Storey Drift versus Storey Force for S6

-100

-50

0

50

100

-6 -4 -2 0 2 4 6

Storey Drift(%)

Storey

Force(

KN)

Figure 2. Storey-drift versus storey-force for specimen S6


1

Elastic Moduli of Concrete - A Micromechanics Approach

Introduction

The primary phases in concrete are the aggregate, the cement-sand mortar (matrix), and the interface

between the aggregate and the mortar. The interface between the mortar and the coarse aggregate (Figure 1)

is not necessarily perfect; in fact, it has many defects. Nilsen and Monteiro (1993) have shown that the

matrix in the interface zone, just surrounding the aggregate, has quite a different stiffness property from that

away from the aggregate.

In this study, concrete is considered as a composite material in which the coarse aggregates (also termed

inhomogeneities or inclusions) are embedded in a matrix of mortar. The interfaces between the mortar and

the coarse aggregates are considered to be imperfect. Based on Mori-Tanaka's procedure, the Eshelby's

classical solution for a perfectly bonded inclusion will be combined with Volterra's solution for an equivalent

Somigliana dislocation field - which models the imperfect interface - to obtain a new constitutive model for

concrete. Currently, this work is limited to elastic concrete properties.

Three-phase model with imperfect interface

Consider an infinitely extended mortar medium D containing many spherical inclusions (coarse aggregates)

with an imperfectly bonded interface. The elastic stiffness tensors of the mortar phase (subscript "o") and the

coarse aggregate phase (subscript "1") are respectively 0L and 1L . There is a homogeneous boundary

condition in the external surface S of D:

( ) nn0σσσσ =S (1)

where 0σσ is a constant stress tensor, and n denotes the exterior normal to the external surface S .

Suppose the homogeneous boundary condition represented by Eq. (1) is applied to the composite. If the

solid composite did not contain any inclusion, the strain field would be:

010

0 σσεε −= L (2)

Due to the presence of inclusions, the average stress in the mortar phase of a composite with inclusion

becomes:( ) ( )εεεεσσσσσσ ~~ 0

000 +=+= L (3)

According to Eshelby's equivalent principle, the average stress in the aggregate is:( )

( )( )*0

0

01

01

~

~

~

εεεεεεεεεεεεεε

σσσσσσσσ

−++=++=

++=

pt

pt

pt

L

L (4)


2

where ε* represents the eigenstrain and the superscript " pt " represents perturbation components of strain and

stress tensors. For an imperfectly bonded interface, the interfacial traction remains continuous, while both

the normal and the tangential displacements experience a jump across the interface (Zhong and Meguid

1996). Thus, the interface conditions can be written as:

[ ] 0=jij nσ (5)

[ ]( ) kTkiiki Tnnu αδ =− (6)

[ ] kNkii Nnnu α= (7)

where Tα and Nα denote the compliance in the tangential and the normal directions of the interface

respectively, [.] =(out)-(in), in is the outward unit normal on the interface, and ( )kiikjkji nnnT −= δσ and

ijkkji nnnN σ= represent the shear and the normal tractions at the interface respectively. Tα and Nα should

be positive and they describe the interfacial behavior.

For an inclusion with imperfect interface, following Eshelby's equivalent principle, the relationship between

the eigenstrain and perturbation strain in the inclusion can be written as:*

klijklpt

ij S εε = (8)

where:Sijkl

Eijklijkl SSS += (9)

Here EijklS is Eshelby's solution (1957) for a uniform eigenstrain problem of inclusion with a perfectly

bonded interface and SijklS is the contribution of the imperfect interface. To simulate the effect of an

imperfect interface, Somigliana's dislocation theory can be used. If σσ denotes the overall average stress

tensor and ci (i=0, 1) represent the volume fractions of mortar and coarse aggregate, separately, we can write:( ) ( )1

10

0 σσσσσσ cc += (10)

The body average of the strain over the body volume V can be defined by an 'outside observer' as:

( ) ( ) [ ] [ ]( )∫ +++=S ijjiijijij dSnunu

Vcc

211

10

0 εεε (11)

If we consider that both the mortar and the coarse aggregate are isotropic, the effective bulk and shear

moduli, κκ and µµ , of the concrete can be obtained based on the above equations as:

( )( )( )( )010010

01010

0 κκκκααλλκκκκκκαακκ

κκκκ

−++−++

=cc

cc(12)

( )( )( )( )010010

01010

0 µµµµββµµµµµµββµµ

µµµµ

−++−++

=cAc

cc(13)

where:


3

( )10

100 13 k

kβ

βλ

+= ( )

( ) ( )4320040320

43200200 1 kkkkkk

kkkkA+++++

++=βαβα

βαα ( )( ) ( )4320040320

2000 1 kkkkkk

kB

+++++−

=βαβα

αβ

Here 0ν is Poisson's ratio of the mortar and

,, 00

00 aa

NT αµβαµα == ( )( )

( )( )

( )( )

( )( )0

04

0

03

0

02

0

01 1105

11354,

11051974

,115

572,

1314

νν

νν

νν

νν

−+

=−

+=

−−

=−+

= kkkk

with:SESE ββββββαααααα +=+= , (14)

( ) ( )( )( )0

0000

0

0

1752557

,1

212ν

νβλ

νν

α−

+−=

−−

=BASS

Thus, the elastic moduli of concrete with different interface conditions can now be computed.

Experimental verification

The elastic moduli of concrete computed from the proposed micromechanical model described above have

been compared with the experimental data for elastic moduli of concrete reported by Hirsch (1962) and

others. Table 1 shows some of the results. It can be seen that the proposed micromechanical model can be

used successfully for modelling the elastic behaviour of concrete. This research is the basis of subsequent

research on concrete behaviour under multi-axial stress states.

References

[1]Hirsch, T.J. (1962) "Modulus Elasticity of Concrete Affected by Elastic Moduli of Cement Paste Matrix

and Aggregate," ACI Journal, V. 59, pp. 427-451.

[2]Nilsen, A.U. and Monteiro, P.J.M. (1993) "Concrete: a Three Phase Material," Concrete and Cement

Research, V. 23, pp. 147-151.

[3]Zhong, Z. and Meguid, S.A. (1996) "On the Eigenstrain Problem of a Spherical Inclusion with an

Imperfectly Bonded Interface," Journal of Applied Mechanics, ASME, V. 63, pp. 877-883.

Li Qingliu ([email protected])

Susanto Teng ([email protected])


4

Batch Volume Ratio ofCoarse Aggregate

E c(GPa)(Hirsch)

Ep(GPa)(proposed)

(E p-E c)/E perror (%)

GR-6 0.57 36.1 35.14 -2.7GR-5 0.50 32.5 32.52 0.1GR-4 0.40 29.7 29.17 1.8GR-3 0.30 27.4 26.46 -3.6GR-2 0.20 23.0 23.76 3.3LI-6 0.57 23.4 25.09 6.7LI-5 0.50 23.9 24.27 1.5LI-4 0.40 22.3 23.16 3.7LI-3 0.30 20.6 22.10 6.8LI-2 0.20 20.2 21.08 4.2

Table 1. Comparison of results

Figure 1. Microstructural model of concrete

Aggregate

MortarInterface Cracks


1

Shaking Table Tests of RC Model Structures Under Simulated GroundShocks

As an integral part of a broad research project on the damage criteria of surface structuressubjected to underground explosion induced ground excitations, shaking table tests of RC modelstructures are being conducted to investigate the structural response characteristics and damageprocess under high-frequency shock excitations, and to validate the corresponding numericalpredictions.

Existing regulatory guides on blasting vibration limits have developed from empiricalcorrelations of observed damage and recorded peak particle velocity of the ground motions,without a clear identification of the structural response and damage mechanisms. This haslimited the applicability of the relevant guides and left ranges of blasting vibration problemsuncovered. Obviously, the most rational way of arriving at reliable shock damage assessment isthrough structural dynamic response studies, and in this process, testing of model structuresunder controlled vibration conditions is a powerful and yet economical means to provide thebadly needed experimental data in sufficient detail.

Shaking table tests have been extensively used in the past to study the complex behaviour ofstructures under earthquake ground motions. However, the use of shaking table testing for shockresponse simulation is seldom reported. The main difficulties are associated with the twoinherent features of the shock excitations, namely high frequency and enormous peakacceleration. Both parameters can be far beyond the reach of the conventional “earthquakesimulators”. In order to tackle these problems, in this investigation use is made of anelectromagnetic shaker which is capable of producing high frequency and high accelerationmotions. To cope with the relatively small force power of such a shaker, small-scale RC modelstructures are adopted. The similitude requirements concerning the model materials and modelfabrication are strictly followed to ensure the reliability of the models. The model structureconfigurations (see Figure 1) include a single-storey bare frame; a single-storey frame withmasonry infill walls; a multiple-storey frame (3 storeys and 2 bays); and two 2-storey frames,one of which is subjected to a vertical vibration test (not shown).

In the test implementation, the established similitude law on artificial mass is extended to thelocal elemental level so that the crucial local-mode vibration effects could be reproduced in themodel response (Figures 2 and 3). The test instrumentation covers accelerations, displacementsand reinforcement/concrete strains at various locations. The shaking table is driven via avibration control system, while response signals are acquired, pre-processed and stored throughan automated data acquisition system.

The tests are performed typically in three phases covering high, medium and low frequencyranges of input motions. Such a test scheme allows for an evaluation of the variation ofstructural response with the principal frequency of excitations. It also allows for a progressivedamage observation on the model structure during the course of the tests.

The test results reveal distinctive response features under high-frequency shock excitations ascompared to low-frequency seismic type ground motions. In principal, the shock response maybe characterised by significant local-mode vibration induced high shear stress while the overallresponse (displacements and drifts) remains low. This observation is significant as it is incontrast to the features of typical seismic response from which the conventional deformation-based damage criteria have been derived. The limitation of the PPV-based blasting vibrationguides is also clearly demonstrated from the experimental and the associated analyticalobservations. As such, the need for more rational shock response criteria is obvious. In view of


2

the observed shear-dominated shock response, it is deemed necessary to emphasise theimportance of enhancing the stress and strain capacities of the concrete material in thecorresponding protective design. The test program is still going on, and more comprehensiveresults are expected in due course.

Figure 1. Photograph of test model structures

- 10

0

10

Acc

eler

atio

n (g

)

T im e (s )0.6

-2

0

2

She

ar (kN

) Column shear-Time history

0.6

Time (s)

0 100 200 300 400

Frequency (Hz)

0.00

0.02

0.04

Am

plitu

de

Column shear-FFT

Figure 2. Typical test set-up and input motion Figure 3. Typical measured response

Lu Yong ([email protected])Hao Hong ([email protected])Ma Guowei ([email protected])Zhou Yingxin ([email protected])


1

Dynamic Properties and Serviceability of Floor Slabs with

Human-Structure Interaction

Lightweight pre-cast floor systems using post-tensioning to increase strength are increasingly popular and dynamicserviceability issues become increasingly important. In particular, human sensitivity to response caused by walking orjumping may be the governing factor for serviceability. Meanwhile, stationary human observers can also participate inthe response and can alter it in helpful ways; a human behaves as a highly effective damper even when seated.Experiments and simple lumped mass models have been used to study the effect.

Background

While the effect of a moving (walking or jumping) person on a floor or footbridge is well modelled as a movingdynamic load through Fourier series, the effect of a stationary human body is less well understood. Complex models ofhuman dynamics exist to study behaviour of limbs and body parts, but it appears that humans participating in verticalmotion of floors behave as heavily damped one-degree of freedom systems and not simple added masses.

Figure 1 shows the complex (ISO) representation of the human body and a simplified system that may be morerepresentative for study of human-structure dynamics. Since damping is a crucial issue, the study focuses on the energytransfer and balance on a vibrating floor and how the human acts as a passive or active energy absorber.

Figure 1. ISO and simple 2DOF models Figure 2. Human structure interaction

Laboratory study and mathematical models

Figure 2 shows a laboratory set up to examine the human-structure dynamics.

A 7m by 1m by 75mm simply supported pre-stressed concrete plank is used, driven to vibrate by a ‘chirp’ (fast sinesweep) signal generated by an electro-dynamic shaker. Subjects stand on a metal (force) plate used to measure contactforces between plank and human. The shaker and force plate signals as well as acceleration signals for the plank areused to measure the energy flow from shaker to plank, and to the human at the point of contact.

A set of experiments was done using a subject with different postures, also with an equivalent dead mass. Figure 3shows the frequency response function (FRF) data in these cases overlaid.


2

Figure 3. FRFs for different loads

For most cases the frequency decreased as if effective mass decreased, while the damping ratio increased massively,depending on posture. The reduction in frequency was greater than when using a dead weight and is consistent with thedescription of a human as a single degree of freedom (SDOF) system. For one situation, with knees very bent thefundamental frequency reverted to the value for the bare plank ± a few % but with a very high damping ratio. While alittle artificial, the situation is not so easily explained using simple SDOF models with constant parameters. In mostpractical situations the frequency reduction effect of humans not acting as load generators will not be so significant asthe dampening effect.

The multi-degree of freedom (MDOF) human body and plank model (Figure 1) and its simplified 2DOF equivalent,using known plank dynamics i.e.

fplank= 3.16Hz , ξplank=0.8%

was used to predict the natural frequencies and damping ratios for human and human-plank systems. For the human themodel yields a solution:

fhuman= 4.88Hz, ξhuman=37%.

For the 2DOF model, the values are:

fplank= 2.93Hz, ξplank=2.2%.fhuman= 5.32Hz, ξhuman=36%.

where ‘plank’ and ‘human’ refer to components where the response is most significant. These values are consistent withexperimental data.

The model was used to study the energy transfer mechanism in the vibrations and checked against experimental data. Itwas found that one human standing passively and naturally on a 1 tonne slab can more than double the energydissipation capacity, consistent with the above models.

Full-scale studies

The enhanced damping capacity of a floor occupied by humans has been verified by full-scale studies of a long spanfloor. In the experiment involving a crowd of over several hundred people, some jumping, some standing and somesitting, it was found that the expected resonance of the floor due to the jumping was mitigated by the presence of thestationary occupants. Figure 4 shows floor response to jumping when occupied by only three (jumping) people, (thincurve) and response to 400 jumping and stationary occupants (thick line). The peaks below 6Hz are not resonances butare the fundamental and harmonics of jumping frequencies. The curves are normalised to the same response at thefundamental and there is only very weak response at the floor resonant frequency (between 6Hz and 8Hz) with thecrowd present.

2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 410

-3

10-2

10-1

frequency/Hz

FR

F

(in

ert

an

ce)

mo

-1

f=3.16Hz, ζ=0.8% bare plankf=2.95Hz, ζ=1.1% 80kg weights

f=2.87Hz, ζ=2.0% standing erectf=2.86Hz, ζ=6.0% knees bent

f=3.10Hz, ζ=9.2% knees v. bentf=2.82Hz, ζ=2.8% seated


3

Further refinements on the experimental set up, studies on light floors with frequencies above the human bodyfundamental, on the nature of the human-structure contact and on crowd dynamics are in progress.

James M W Brownjohn (cjames @ntu.edu.sg)

0 2 4 6 80

1

2

3

4

5

6

7

f / H z

root

(PSD

)/m

m.s

ec-2

Figure 4. Floor response to jumping:heavy line =standing & jumping

thin line =jumping only


1

Fuzzy Probabilistic Damage Analysis of Rock Mass to Explosive Loads

There exist a lot of uncertainties in the analysis of rock responses to explosive loads. Coping with theseuncertainties is an important task in rock engineering. Uncertainties involved in material strength andloads have been treated with probability theory; but some uncertainties, such as failure criterion, whichare not random in nature, may play important roles in the safety assessment of a rock mass. When rockmass is subjected to explosive loads, the failure of an element, according to the theory of fuzzy sets,can be treated as a fuzzy event. Based on the fuzzy-random probabilistic theory, a unified model for thefailure analysis of rock mass under explosive loads considering the effect of both randomness andfuzziness is proposed. Several types of membership functions are suggested to represent the fuzzinessof material failure.

Based on the hypothesis of strain equivalence, initial damage is defined as:

00

~1

EE

D −= (1)

where E~

is the elastic modulus of the damaged material and 0E is that of undamaged material.

Measurement of the speed or propagation time of plane waves in boreholes of the rock mass leads tothe determination of its elastic modulus E

~, expressed in terms of the velocities of longitudinal wave

Lv~ and transverse wave Tv~ as:

22

222

~~

~4~3~~

TL

TLLE

υυυυ

υρ−−

= (2)

where ρ is mass density of the rock mass. By using the method of statistical analysis, a probabilitydistribution law for the initial damage of the rock mass under consideration was found to have the betadistribution with the probability density function:

)1(),(

1)( 1

01

00−− −= ba DD

baBDf (3)

where a is the parameter of size; b is the parameter of shape; ),( baB is the beta function.The damage variable of the rock mass per volume

0V under explosive loads consisting of boththe initial damage and cumulative damage can be written as: ])(exp[)1(1),,,( 000 VDDD crcr τεεατεε β−−−−= (4)

where ε is the equivalent tensile strain; and crε is critical tensile strain.

Based on continuum damage mechanics, the failure criterion of a rock mass to explosive loads isdefined by a minimum normal criterion cDD ≥ , where Dc is the critical damage value. The criticaldamage value, which depends on many factors, is essentially a fuzzy set in the domain of discourse ofreal number. Thus, the fuzzy failure probability can be defined as:

dD)D(f)D()DD(PP~D

c~~~f ∫ µ=≥= (5)

where )D(f~D is the probability density function of the damage variable D, which is assumed having

the beta distribution as discussed above; and )( Dµ is membership function.


2

The construction of membership function is a difficult task and a debatable one in fuzzy set theory. Inmost engineering applications, it is established on the basis of experts’ experience due to lack of data.Normally, a linear ascending type of membership function is used in engineering:

where UD and LD are an upper limit and a lower limit of the fuzzy region.

The constitutive law and fuzzy-random damage model described in the previous sections have beenimplemented to simulate a series of field blasting tests. The tests were carried out at a granite site. Itwas found that the best fitted curves for the peak particle accelerations (PPA) and peak particlevelocities (PPV) in free field are respectively: 4531.13/1 )/(2.1928 −= QRPPA (g) (7) 1455.13/1 )/(396.0 −= QRPPV (m/s) (8)

where R is distance in meters measured from the charge centre; Q is the equivalent TNT charge weightin kilograms.

Figure 1 shows the calculated peak particle velocities (PPV) in the rock mass at different scaleddistances, obtained by using the mean initial damage and the mean critical tensile strain in thecalculation. The corresponding best fitted curves of the field measured data of these waves are alsoplotted in the figure. As can be seen, numerical results agree well with the test results.

Figure 2 shows the recorded and simulated acceleration time histories at 2.5m from the charge holewith a charge weight of 50kg.

Figure 3 shows the calculated fuzzy failure probability distributions around the charge hole under themembership function.

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

0 0.001 0.002 0.003 0.004 0.005 0.006

t(s)

acce

lera

tion

g

Numerical

Test results

Figure 1. Attenuation of PPV

Figure 2. Comparison of horizontal acceleration timehistories obtained at 2.5m from the charge hole (chargeweight 50kg )

Scaled range m/kg1/3

0.01

0.1

1

0.1 1 10 100

Scaled range m/kg

Peak

par

ticle

vel

ocity

m/s

NumericalTest results

Scaled range m/kg1/3

>

≤<−

−≤

=

U

ULLU

L

L

DD

DDDDD

DDDD

D

1

0

)(µ (6)


3

Hao Hong ([email protected])Wu Chengqing ([email protected])

0.8m

5m

)( Dµ

Figure 3. Fuzzy failure probabilities of the rock mass around the charge hole under membershipfunction )( Dµ (charge weight 50kg)


1

Steel Fibre Reinforced Concrete Panels Exposed to Air-Blast Loading

Despite the recent development of advanced building materials and protective methods, conventional concretecontinues to be preferred in the construction of important surface structures, sub-surface and otherinfrastructure facilities where the threat of air-blast, shock or ballistic loading is likely to cause substantialinjury to personnel and damage to equipment inside the building. However, the residual strength of concretebecomes questionable once damage has occurred. This could result from a first wave attack on the facility.Resistance of the damaged concrete, thereafter, is indeterminate. Therefore, increasing the survivability ofthe material is a critical factor to avoid further degradation from potential repeated blast or impact loading.

Improving survivability of conventional concrete by incorporating discrete steel fibres into the mix has beenstudied. The investigation demonstrates the effectiveness of steel fibre reinforced concrete (SFRC) as apotential material for use in infrastructure development and construction. SFRC reduces projectile penetration,frontal spallation and rear face scabbing. In tests, SFRC was subjected to single and multiple impact fromsmall and large calibre projectiles.

Recent air-blast tests conducted jointly with LEO(MINDEF) to investigate the survivability of steel fibrereinforced concrete panel structures and other retrofit protection materials have demonstrated theperformance of SFRC. The SFRC structures were subjected to various magnitudes of air-blast pressures.Similar panels reinforced with conventional weldmesh reinforcement and epoxy fibremat were also tested tocompare damage and response. A number of panels with different boundary conditions were considered inthe test programme. Results of a test on encastre slabs are shown in Table 1.

Conclusion

The performance and resistance of steel fibre reinforced concrete model panels subjected to air-blastpressures have been investigated. Further studies are planned to quantify the dynamic material characteristicsand structural resistance/response of the steel fibre reinforced concrete panels subjected to air-blast loading.

Acknowledgement

The work reported here is part of an Explosive Testing of Structural Components programme, and is madepossible with financial support from the Lands and Estates Organisation (MINDEF). The permission of theDirector, LEO (MINDEF), to publish this note is gratefully acknowledged.

T S Lok ([email protected])


Table 1. Response of encastre panels (as an open box) to air-blast pressure at 4.0 metre stand-off distance

Steel fibre reinforced panels (0.5% fibre volume concentration)Plain

concreteWeldmeshreinforced

corrugated fibres short enlarged-endfibres

Hooked-end fibres

panel (6mmφ@200 crs)panel

L/d = 49.3 L/d = 33.4 Length = 60 mmDiameter = 0.8 mm

L/d = 75

Length = 30 mmDiameter = 0.5 mm

L/d = 60

Length = 60 mmDiameter = 0.8 mm

L/d = 60FailurePattern:

FrontView

FailurePattern:

BackView

Residualdisplacement

profilePanel destroyed

B-SR

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plac

emen

t (m

m)

B-XA

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-8

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Dis

plac

emen

t (m

m)

B-EA

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-8

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-4

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0

2

Dis

plac

emen

t (m

m)

B-DA

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-8

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0

2

Dis

plac

emen

t (m

m)

B-DC

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-8

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0

2

Dis

plac

emen

t (m

m)

B-DE

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2

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plac

emen

t (m

m)


Taylor Dispersion by Surface Waves

Introduction

Pollutants, once discharged and beyond the initial active entrainment phase, will be passively advected and dispersed inthe water body. The dispersal mechanisms are mainly through turbulent diffusion and longitudinal dispersion with tidaland wind-induced shear currents. This study addresses the longitudinal dispersive effect due to mass transport inducedby progressive surface waves.

Ever since the classic work of Taylor (1953), there has been extensive research on the longitudinal dispersion generatedby turbulent and laminar shear flows and the resulting variation in velocity profile, in various chemical and hydraulicengineering applications. Thus far, nearly all investigations on Taylor dispersion involve cross-sectional velocityvariations induced by turbulent or laminar shear stresses. It is therefore not surprising that this dispersion mode is oftenreferred to as “shear dispersion”, as evidenced in the classic text of Fischer et al. (1979). The term implicitly suggeststhat the presence of shear is necessary for the mechanism to take place. In reality, other than the background diffusion,the only requirement is a cross-sectional variation of the flow profile in steady currents. Such variation can also exist inwater motion that is primarily irrotational. A clear example is surface gravity waves that are commonly considered to beirrotational in the leading order except near boundaries. For surface waves, it is well known that Lagrangian drift existswith a varying profile along the water depth. The cross-sectional variations of the steady drift velocity should providethe necessary agents for Taylor dispersion to take place. Therefore the presence of progressive surface waves shouldincrease the longitudinal mixing of a contaminant cloud.

The following section quantifies the dispersive effect for the cases of a Stoke drift profile, a drift profile with viscouseffect and also a profile under the cover of a contaminated inextensible surface cover. Discussion on the implication ofthe results is provided in the last section.

Longitudinal dispersion

In a progressive wave environment, as the pollutant particles move passively in phase and magnitude with the waterparticles, they will undergo the second order period-averaged drift displacement. Since the motion of a pollutant particleis followed, the drift displacement should obey the Lagrangian pattern. Assuming a velocity scale of du , the

longitudinal dispersion coefficient can be computed as:

( )[ ] ∫ ′∫ ′′′−′′′= ′10

2y0 m2

22d ydyduyuD

duDK (1)

where D is the laminar or eddy diffusivity that for this analysis is assumed to be isotropic in all directions and constantthroughout the water depth, du the velocity scale, mu ′ the non-dimensional depth-averaged velocity, u ′ the non-

dimensional drift, and d the water depth.

In the absence of viscosity, the drift profile will follow the pattern of the second-order Stoke drift. Stoke drift is inducedbecause in the movement of a particle under the wave motion, the particle stays longer below the crest than the trough,and the particle velocity is slightly higher in the top part of their orbit than the bottom part. The depth-averaged driftvelocity, mu , and the corresponding non-dimensional drift, u ′ , without a pressure gradient, is known to be:

)tanh(1

Td4H

u2

m επ= (2a)

εε′ε+′ε

ε=′)cosh()sinh(

)y(sinh)y(coshu

22(2b)

where H is the wave height, ε = kd and k the wave number. Substituting (2) into (1) yields:

( )εΩ= s4f

D

K(3a)


2

( )

( )( )

( )

+εεε

−ε

+

−εεε−ε

ε

π=

3

1

2sinh

)2cosh(

2

1

1)4cosh(4

44sinh

tanh16f

2

2

2

s(3b)

where Ω DT/H= represents the ratio of the wave height and the diffusive movement within one wave period.Figure 1 shows the relationship between fs and ε. It is obvious that fs increases monotonically with ε suggesting that thedispersive effect is more prominent in transitional water. In the figure, the range of ε is limited to be less than π which isthe commonly adopted deep water limiting condition. For larger ε, the surface wave motion will not be able to mobilizethe bottom water such that the bottom layer will behave similarly to the dead zone in the river case.

0

0.02

0.04

0.06

0.08

0.1

0.12

0 1 2 3εε

fs

Figure 1. Variation of fs versus ε .

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3εε

fv

Figure 2. Variation of fv versus ε .

The finite viscosity in water introduces vorticity, particularly in the boundary layers at the surface and at the seabed.The resulting drift profile with the viscous effect for the case of zero-mass flux under a pressure gradient, as obtained byLonguet-Higgin (1953), is:

( )

( )( ) ( )

( ) ( )

′+′εε+

′−′

+

εε

−

′ε+

ε=′

y2y2sinh

y2y32sinh

2

3

y2cosh23

sinh4

1u

2

(4)

where u’ is non-dimensionalized with a velocity scale of (ka)2C where a is the amplitude and C the phase celerity.Substituting (4) into (1) yields:

( )εΩ= v4f

D

K(5a)

( ) ( ) ( )

( ) ( )( )ε+ε−ε

+ε+ε−ε+ε

−εε+ε

ε−ε+

ε−ε−+ε

−ε−

ε+επ

=424313440

31537806182248

421026302100

3961204

315222

648

4

2468

3

3

57

4

68

2

coshcosh

sinhsinhcosh

fv(5b)

The relationship between fv and ε is illustrated in Figure 2. Unlike the monotonic increase with ε, as in the case of Stokedrift, the dispersive effect is more substantial in shallow water as well as near the deep-water limit. The minimum fv of0.0085 occurs at a transitional water depth of ε = 1.04. With the stronger drift variation induced by the viscous effect, themagnitude of the dispersion coefficient is generally an order of magnitude larger than the Stoke drift alone.


3

When an inextensible slick such as thick oil or grease contaminates the water surface, the drift velocity increasessignificantly. Craik (1982) derived the non-dimensional drift profile under the slick with zero-mass flux due to a pressuregradient as:

( )

−′′

σν

εε=′ 1y2

3y

8

cothg2u

23 (6)

where σ is the wave angular frequency. Again the velocity scale is (ka)2C. Substituting (6) into (1) yields :

( )εΩ= c4f

D

K(7a)

( )

3

2242

c53760

cothgf

σν

εεπ= (7b)

The relationship between fc and ε is illustrated in Figure 3 assuming a kinematic viscosity of ν = 10-6 m2/s and σ = 0.78 (T= 8 seconds). The resulting K/D is several order of magnitudes larger than the previous two cases, which is expectedwith the much stronger drift current. The monotonic increase with ε is again observed showing the growing importanceof the dispersive effect in deeper water.

0

1

2

3

4

0 1 2 3εε

fc

(x106

)

Figure 3. Variation of fc versus ε .Discussion

The results from the current study show that with small background turbulent diffusivity and persistent wave height

being much larger than DT , the longitudinal dispersion of a contaminant cloud could be significantly enhanced by thesurface wave motion. The dispersion is much more prominent with the presence of viscosity, and particularly under thecover of a contaminated water surface. The actual importance of this dispersive effect with the non-broken waves in aparticular environment should be evaluated by comparing with other existing mechanisms such as due to wind or tidal-induced shear flows.

References:

[1] Craik, A.D.D., 1982. “The drift velocity of water waves.” J. Fluid Mech., Vol.116, pp187-205.[2] Fischer, H.B., List, E.J., Koh, R.C.Y., Imberger, J., and Brooks, N.H., 1979. “Mixing of inland and coastal waters.”Academic Press, 483 pages.[3] Longuet-Higgins, M.S., 1953. “Mass transport in water waves.” Phil. Trans. A, No.245, pp.535-581.[4] Taylor, G.I., 1953. “Dispersion of soluble matter in solvent flowing slowly through a tube.” Proc. R. Soc. London Ser.A, 219, pp186-203.

Adrian W K Law ([email protected])


1

GIS as a Tool for the Management of Oil-Spill Incidents in the CoastalEnvironment

Background

Given Singapore’s position as one of the world’s largest oil refining centres and leading bunker-serviceports, and as an established tanker repair and dry-dock facility, its ports are popular among maritimetraders (Singapore Maritime Index, 1992). Although the port waters cater to the needs of a widespectrum of users, the largest demand arises from the vessel population. In 1988, about 36,000 vesselspassed through Singapore (Lim, 1991) and this figure increased to 104,014 in 1995 (PSA, 1996).

Potential for oil-spill incidents and hazard

As the traffic volumes continue to rise in the busy sea-lanes, the port waters of Singapore and the regionhave become more vulnerable to oil-spills. Oil-spills can be caused by vessel collisions and/or sludgedumping. Examples of recent oil-spill incidents in Singapore are: (a) the Showa Maru oil-spill in the watersoutside Singapore’s territorial waters in 1975;(b) the oil-spill resulting from a fire on a barge which led tosevere oil pollution on the shore in May 1985; (c) the illegal discharge of slop into the sea by Song Sanwhich caused the beaches at Sentosa Island to be polluted by oil slicks in August 1996, and (d) thecollision of Global Orapin and Evoikas in October 1997.

Oil-spills can have a serious economic impact on coastal activities and users. In most cases, such impact istemporary and is caused primarily by the physical properties of oil creating nuisance and hazardousconditions. There will be lost revenue due to interruption of activities. The impact on marine life iscomplicated by the diversity and variability of biological systems and their sensitivity to oil pollution. As aresult, modern coastal resource managers have begun to adopt an “integrated-management” approachwhich considers all related activities and resources, and the inter-related complexities of the competing uses- economic and social issues, and above all, management of oil-spill and other incidents, responsestrategies and damage analysis. The tool used to manipulate these cascades of information is theGeographical Information System (GIS).

Figure 1. The sector of the coastal waters south of Jurong as at 1995.


2

Management of oil-spill - a methodology

Land-based GIS has been applied widely and the results are promising. However, the application of GISin marine resources is in its infancy. The drawback of the GIS is the need for the user to thoroughly knowand understand the operation of the system.

The Study Area

The study focuses on the sector of the coastal waters south of Jurong, from Sentosa and the St. John groupof islands in the east, to the Raffles Shoal in the west, and Sinki Fairway in the south, as shown in Figure 1.This area is dotted with petroleum installations and chemical plants. In addition, the area is heavily used byfreighters and ocean liners, and is therefore very vulnerable to oil-spills.GIS Database

GIS databases are made up of the (a) physical, (b) geographical and, (c) transient information which isalready in the GIS database. The geographical and physical data are information that do not change rapidlywith time. The transient information includes data on chemical stockpiles, location of rescue vessels andresources. This last category of information requires frequent updating as the stockpiles may have beenused or the rescue vessels may have been deployed to other locations. The spread of the oil slick, anothertype of transient information, can be estimated by either using the GIS tools such as the “Disperse” functionor through hydrodynamic simulation. These 3 categories of information form the static database uponwhich scenario testing and decision support can be made.

The analyses of damages and cost are more involved. The GIS tools of “overlay” and “classification” canbe used to establish the affected environment and activities, and the cost in lost-revenue and cleaning-up.Obviously the decision to contain the oil slicks or to carry out the clean-up operation would need to bemade in conjunction with the cost factors derived from the damage analysis. The decision making is aniterative process. Figure 2 shows an example output of the area affected by the oil-spill and theenvironmental resources involved.


3

Figure 2. An example output of the GIS data set showing the oil slick affected area and thedumping ground, coral areas and nature reserve (as indicated).

References

[1] Lim, K.W., 1991. Management and utilisation of Singapore’s port waters: new directions, Urban coastal area management: theexperience of Singapore, ICLARM conf. Proc 25. L.S.Chia and L.M.Chou (eds.), 128p.[2] PSA, 1996. Singapore Port Statistics: Sep, 1996. Strategic Planning Department, Port of Singapore Authority, Singapore.Singapore Maritime Index ’92, 1992. FETP Business Publications Pte. Ltd.

S K Tan ([email protected])

Oil slickaffected area

Coral / Natureconservation

dumpingground


1

Performance of a Membrane Wave Barrier of Finite Draft

Introduction

Studies have been made on the interaction of water waves with rigid, vertical barriers. Theseincluded barriers which descend from the water surface to some distance below, non-surfacepiercing underwater barriers, and barriers with a gap in between. Both single and doubleparallel barriers, and slotted barriers have also been studied. Recently, other investigationsconsidered the use of vertically tensioned membranes which spanned the entire water depth.These included single and dual membrane arrangements.

As a wave barrier, there are certain inherent advantages of a barrier which descends from thesurface to only a fraction of the water depth, i.e. of finite draft. This is because the wave energywhich is concentrated near the surface is intercepted by the barrier. At the same time, watercirculation is maintained in the gap below, and the flow of sediment and marine life near theseabed is unimpeded. Here we report the results of the analytical work used to model theperformance of a flexible membrane wave barrier of finite draft.

Eigenfunction solution

The membrane was modelled as a thin, verticallytensioned flexible sheet fixed at both ends, asshown in Figure 1. Linear wave theory andsmall membrane motion were assumed. Noovertopping was also assumed. Formonochromatic incident waves, the wavevelocity potentials in the incident andtransmission regions were expanded in terms ofpropagating and evanescent wave modes.

The membrane vibrates in response to the pressure from the waves. Its displacement ξ isdetermined by the membrane equation, and is also expanded using membrane modes satisfyingthe fixed boundary conditions. The solution proceeds by recasting the membrane equation as adynamic boundary condition on the wave potentials.

++= ξ

ω∂

ξ∂ωρ

φφT

mz

iT 2

2

2

21(1)

The usual notations of ω, ρ, and i being the angular frequency, water density and √(-1),respectively, apply. In addition, m and T are the mass per unit area and tension per unit widthof the membrane. Over the gap, the dynamic condition of the continuity of the velocitypotential applies. Together with the kinematic condition of the velocity being continuous, amatrix equation for the amplitudes of the wave and membrane modes is obtained and solvednumerically.


The wave energy transmission coefficient Ct of a membrane at varying draft d/h and differentmembrane tension T′, given by T/(ρgh2), is shown in Figure 2 for two values of kh of 1.06(intermediate depth) and 4.272 (deep water), and at normal incidence. The rigid barrier results

Incident wave z

x

Membrane d

1 2 z=-h

xFigure 1. Definition sketch


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using the BIEM method by Liu et al. (1982), and results obtained through experiments byWiegel (1960) are also shown. A good comparison is obtained for the rigid membrane case.

2(a). kh = 1.06 2(b). kh = 4.272

Figure 2. Transmission coefficient versus draft d/h at tension T′ indicated and normal incidence.

The effect of membrane tension is insignificant at very small d/h. The transmission coefficientCt begins to decrease from unity at about kd of 0.4, i.e. when the membrane begins to span anon-negligible fraction of the wavelength. Thereafter, the effect of tension becomes noticeable,with Ct showing a local minimum of zero wave transmission at specific values of d/h dependingon kh and membrane tension. This minimum shifts to larger d/h as tension increases and khdecreases. This minimum arises because of motion from the membrane. There is cancellationbetween the propagating wave from the membrane motion and that from diffraction in the gapbelow, resulting in a small transmitted wave. This is in contrast to a rigid barrier where thetransmitted wave arises solely from diffraction across the gap.

The trend in wave transmission approaches that of a rigid barrier when the membrane tensionreaches about 0.4. At the larger tensions of 0.2 to 0.4, the membrane offers significantreductions in wave transmission over specific regions of d/h. This reduction can occur at asmaller value of d/h as compared to a rigid barrier (Figure 2a).

The trend of Ct with wave frequency kh at normal incidence and different membrane tension isshown in Figure 3 for the membrane draft of 0.5. The experimental results of Isaacson et al(1998) for a rigid barrier atd/h = 0.5 are also shown,indicating good agreement.The wave transmissionapproaches unity at vanishingkh (i.e. in long wave limit)for d/h of 0.5, and is incontrast to the case of d/h = 1(not shown) where itapproaches zero. This isbecause the wave energy,while unable to couple to themembrane modes atvanishing kh, is still able todiffract through the gapbelow.

Figure 3. Transmission versus kh at T′ indicated


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The transmission decreases from unity as kh increases from zero and develops a local minimum.This minimum occurs because of wave cancellation between the waves generated by themembrane motion and diffraction in the gap. Above kh of 2, there is a series of transmissionpeaks for the flexible case when d/h is close to unity. These peaks are shifted to increasing khwith decreasing d/h and increasing tension. Thus, as shown in Figure 3, having a d/h<1 enablesthese transmission peaks to be shifted to larger kh values at the same membrane tension.

Only small effects were seen at oblique incidence. There were minor deviations in Ct from thevalues at normal incidence up to about 40o off normal. Thereafter, Ct rose monotonically tounity at 90o oblique incidence.

Conclusion

The transmission of water waves with a vertically tensioned membrane of varying draft hasbeen studied. The membrane draft and tension significantly govern the wave transmission, withthe transmitted wave being generated from diffraction across the finite gap and the membranemotion.

References

[1] Isaacson, M., Premasiri, S., Yang, G., (1998), “Wave interactions with vertical slottedbarrier,” J. Wtrwy, Port, Coast., and Oc. Eng., ASCE, 124(3), pp118-126.[2] Liu, P.L.F., and Abbaspour, M., (1982), “Wave scattering by a rigid thin barrier,” J. Wtrwy.,Port, Coast., and Oc. Div., ASCE, 108(WW4), pp479-491.[3] Wiegel, R.L., (1960), “Transmission of wave past a rigid vertical thin barrier,” J. of Wtrwyand Harbors Div., ASCE, 86(WW1), pp1-12.

Edmond Y M Lo ([email protected])


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Riprap Failure Around Bridge Piers Under Live-Bed Conditions

Using riprap stones to deal with pier scour problems is very common in civil engineeringpractice. However, riprap layers often fail to protect bridges against excessive scour duringsevere flood conditions. One of the main causes of such failures is the general movement ofsediments during severe flood conditions. During floods, a live-bed condition with the presenceof bed features, e.g. ripples and dunes, is very likely to occur, especially in rivers consisting offine bed sediments. The movement of bed sediments along the channel and the propagation ofbed features cause the immobile riprap stones to lose their stability, eventually embedding intothe bed. This phenomenon enhances failure of the riprap layer.

Failure behavior of riprap layer around bridge piers

Experimental observations have revealed that a riprap layer can undergo two distinct modes offailure depending on the flow condition, the characteristics of the riprap stones, and the bedsediments. These failure modes are (1) total disintegration failure; and (2) embedment failure.Figures 1 and 2 show two photographs illustrating these two modes of failure. The former refersto the break-up of the entire riprap layer, where most, if not all the riprap stones have beenwashed away by the flow field generated around the pier. Total disintegration failure is morelikely to form under a clear-water condition and when the size of the riprap stones dR50 is similarto the size of the bed sediment d50. Because of these requirements, riprap failure observed inmost published laboratory studies conducted under clear-water conditions and with a relativelysmall dR50/d50 ratio will tend to fall under this type of failure.

The present study examines riprap failure around a bridge pier under live-bed conditions and arelatively large dR50/d50. Here the dominant failure behavior is very different from totaldisintegration failure. The riprap stones tend to embed beneath the sediment bed rather than bewashed away by the flow (see Figure 2). The reason for the embedment is that with a largedR50/d50, the riprap stones are inherently more stable than their smaller counterparts. Under agiven flow condition, the riprap layer will remain intact even when the bed sediments havestarted to move. This induces a differential mobility between the riprap stones and the bedsediment. When the fine sediment particles upon which the riprap stones rest begin to move, thelatter will lose their footing and stability, eventually sinking into the bed by virtue of theirweight. This phenomenon is similar to the embedment of a large exposed grain resting on a bedconsisting of fine sediment particles. The embedment process is further enhanced whenbedforms such as dunes, propagate past the riprap layer. When the trough of a dune arrives at thebridge site, the riprap layer will inevitably sink to the trough level, to be buried by the incomingsediment when the dune crest arrives. The two phenomena described above combine to causeembedment of the riprap stones, and eventually failure of the riprap layer.


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Although embedment failure is the dominant failure mode for a riprap layer at bridge piers underlive-bed conditions, little is known about its causes and characteristics. The main reason is thatthe influence of bed features on riprap stability is not investigated even though it is the maincause of embedment failure. Another reason is that most studies were conducted with arelatively small dR50/d50 ratio. Interactions between the riprap layer and migrating bed featuresand how they cause degradation of the riprap layer are highlighted in the article.

Bed-feature destabilization

Bed-feature destabilization is prompted by the fluctuation of the bed level due to the propagationof bed features (ripples and dunes) past the pier. When the dune trough arrives at the ripraplayer, a high level of turbulence is generated at the reattached zone of the flow over the dunecrest. This combines with the flow field that formed at the pier to pluck and erode additionalriprap stones from the degraded riprap layer. Once these stones are eroded, the degraded layerbecomes thinner and is further loosened, which gives an impetus for winnowing, resulting in theembedment of the riprap layer.

A unique feature of bed-feature destabilization is the periodic fluctuations of scour depth causedby the propagation of the crest and trough of bed features past the pier. Figures 3(a) and 3(b)show the temporal variations of the pier scour depth for two different tests. Both the tests wereconducted with the same riprap layer dimension: diameter, D = 70 mm; d50 = 0.26 mm; dR50 =9.12 mm; riprap thickness, t = 36 mm (= 4dR50); and riprap cover, c = 280 mm (= 4D). The flowcondition used for testing was also identical: undisturbed approach flow depth, yo = 210 mm andvelocity ratio, U/Uc(d50) = 4.5 where U = mean velocity and Uc(d50) = mean velocity for bedsediment entrainment. In both cases the bed features that formed on the approaching bed weretransition flat bed. The only difference was that in the former, the flow velocity was increased asquickly as practicable so that transition flat bed was formed immediately after the onset of thetest. This was to avoid the formation of ripples and dunes on the approaching bed. Because ofthis, bed feature destabilization cannot effectively influence the failure modes of the riprap layer.In the latter, the velocity was increased incrementally to allow ripples and dunes to form prior tothe final test with U/Uc = 4.5.

Superimposed in both Figures 3(a) and 3(b) is a horizontal line that depicts the maximum depthof riprap degradation, drp, which is measured from the undisturbed mean bed level. Under thegiven flow condition, drp = 133 mm and 176 mm for tests conducted in Figures 3(a) and 3(b),respectively. The data in both figures show the expected scour depth fluctuations related to pierscouring with transition flat bed. Figure 3(a) shows that the maximum depth of scour coincideswith drp. This implies that the riprap layer has sunk to the lowest level under the given conditionand further degradation of the scour hole is not present. On the other hand, Figure 3(b) shows adeeper drp, which is well below the fluctuations of the scour hole. The deeper drp associated withthis test run is due to the formation of dunes and ripples before the final test conducted with U/Uc

= 4.5. In those tests, ripples (U/Uc = 1.91), and different size dunes (U/Uc = 2.40 and 3.19) wereobserved on the approaching bed. A sufficient duration, which ranged from 19 to 32 hours, wasallowed for all these tests so that equilibrium scour hole was reached for every instance. Figure 4shows the temporal variations of the scour depth for U/Uc = 3.19 in which dunes were formed onthe approaching bed. The migrating dunes effected bed feature destabilization, causing the ripraplayer to degrade to 176 mm. With the commencement of the test with U/Uc = 4.5, which isassociated with the formation of transition flat bed, no dunes were present on the approachingbed to expose the embedded riprap layer. Since the degraded riprap layer remains buried beneaththe scour hole, drp remains the same for both the tests conducted in Figures. 3(b) and 4.

The difference of drp between the two tests (Figures 3a and 3b) denotes an additional degradationdepth that can only be attributed to the action of bed feature destabilization. The results showthat riprap layer degradation will increase not only with flow velocity but also with the height ofapproaching bed features.


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Figure 3. Temporal variations of scour depth in transition flat bed regime with differentantecedent bed features: (a) immobile flat bed (b) immobile flat bed, ripples, and dunes.

Figure 4. Temporal variations of scour depth in upper end of dune regime


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Summary

The study shows that bed feature destabilization presents an additional dimension to riprapfailure. The stability of the riprap layer is highly sensitive to the bed features propagating pastthe bridge site.

Y. M. Chiew ([email protected])

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