SAG Refresher Course No. 12203

A

PROJECT REPORT

on

DEMOLITION OF STRUCTURES

by

A. ACHUTA RAO, CGE/UBL

Guide: SK BANSAL, SP/Projects

CONTENTS

1. INTRODUCTION 2. PLANNING 3. PRECAUTIONARY MEASURES 4. PRINCIPLES OF STRUCTURAL DEMOLITION 5. METHODS OF DEMOLITION 6. SPECIAL STRUCTURES 7. COMPLETION OF WORKS

CASE STUDY – DEMOLITION BY IMPLOSION

1. INTRODUCTION Many of the structures on Indian Railways have already passed their design life and need to be reconstructed for safety and operational requirements. For this purpose, the old structures need to be demolished for replacement by new structures. Small structures can be demolished by manual methods but machinery and advanced techniques are required for demolition of bigger structures. Advanced techniques are also required for faster demolition and demolition in confined areas. Demolition means dismantling, razing, destroying or wrecking any building or structure or any part thereof by pre-planned and controlled methods. Demolition methods can vary depending on the area where it will be held on, time available, the building material, the purpose of the demolition and the way that debris is going to be disposed. Time saving methods are more expensive than the slower ones. If noise, dust, and vibrations are to be restricted, it will add to the cost of demolition. The growing importance of recycling and practicing environmental sustainability in demolition has even given rise to new industry buzzwords such as deconstruction. Deconstruction is the selective dismantlement of building components, specifically for re-use, recycling, and waste management. It differs from demolition where a site is cleared of its building by the most expedient means. Various demolition methods, such as the use of the mechanical equipment and the employment of explosives to ‘implode’ buildings, have evolved. Both were used in conjunction with more traditional methods such as hand tools. Use of any of these methods requires a proper understanding of the structure and the use of appropriate methods at minimising the risks of causing damage to persons, properties and the neighbourhood environment. This paper covers the methods commonly used in demolition of structures giving attention to the planning and execution of the demolition work, ensuring safety at the work place.

2. PLANNING Before beginning the actual work of demolition a structure, a careful study shall be made of the structure which is to be pulled down and also of all its surroundings. This shall, in particular, include study of the manner in which the various parts of the structure to be demolished are supported and how far the stage by stage demolition will affect the safety of the adjoining structures. A definite plan of procedure for the demolition work, depending upon the manner in which the loads of the various structural parts are supported, shall be prepared and approved by the engineer-in-charge and this shall be followed as closely as possible, in actual execution of the demolition work. Before the commencement of each stage of demolition, the engineer shall brief the workmen in detail regarding the safety aspects to be kept in view. It should be ensured that the demolition operations do not, at any stage, endanger the safety of the adjoining structures. Moreover, the nuisance effect of the demolishing work on the surrounding structures should be kept to the minimum.

2.1 Building Appraisal and Demolition Plan The study of the structure, before demolition, shall be carried out by means of surveys which shall include a General Survey and a Structural Survey, with photographs or videos taken for future reference. Based on the findings of these surveys, a demolition plan shall then be prepared and got approved by the competent Authority. The demolition plan must also be accompanied by a report together with structural calculations assessing the stability of the building to be demolished and all affected buildings, structures, streets, land and services. 2.1.1 General Survey The General Survey shall cover the following:

a) construction materials b) existing use and, if possible, the past uses of the structure prior to

demolition c) presence of wastewater, hazardous materials, asbestos, flammable or

explosive materials, radioactive materials etc. and possible presence of materials which can contribute to air pollution and soil contamination

d) adjoining properties, site conditions such as the existence of slopes and retaining walls, illegal structures, bridges, overhead railway structures, overhead cables, and other utility service connections

e) drainage conditions and possible problems on water pollution especially on sloping sites and water receiving bodies

f) shared facilities with adjoining building, including common staircases, party walls, and possible effect on them, such as self-enclosed walls to the adjoining buildings, during demolition

g) sidewalk requirements h) adjoining pedestrian and vehicular traffic conditions i) available headroom, clear spaces and distance of structure from boundary

which may affect the loading and transportation of debris during demolition j) sensitivity of neighbourhood with respect to noise, dust, vibration and traffic

impact k) removal of any hazardous materials on the site like explosives, chemicals,

petroleum etc. l) available site area to allow on-site sorting of building debris and m) street features such as parking space, street light, and hawkers’ stalls which

could be affected by the demolition project 2.1.2 Structural Survey Prior to the Structural Survey, the existing records like layout, structural plans and structural details shall be studied. A Structural Engineer shall check these records for presence of any unusual detailing that may cause abnormal structural behaviour during demolition, e.g., upward anchor of tensile reinforcement in cantilevered structures.

The Structural Survey shall cover the following: a) structural materials used b) structural system employed in the design c) method of construction d) any dilapidation and degree of deterioration on any structural elements e) structural conditions of adjoining structures and its shoring which may be

affected by the proposed demolition work f) presence of continuous structures that may be truncated by the demolition g) nature of walls, whether it is masonry walls, reinforced concrete walls, load

bearing walls or partition walls h) cantilevered structures such as canopies, balconies, or other forms of

architectural features i) possibilities of structural modification for demolition and j) any limitation on shoring and other temporary supports.

In the case when no structural details are available, the Structural Survey shall include on site measurement and retrieve any structural details, as much as practicable, by performing tests and exposing some key structural elements to facilitate checking on existing structure. This will allow the development of procedures that ensure the stability of the building at all stages during demolition. 2.1.3 Demolition Plan and Stability Report including Calculations (A) Demolition Plan A Demolition Plan shall include the following :

(1) A plan showing: a) the location of the building to be demolished b) a detailed topography of the site and its surrounds together with ground

level contours and sections of the slopes and ground supported by the building where appropriate

c) details of ground removal and/or backfilling and d) the distances from the building to be demolished to its adjacent buildings,

streets, and structures. (2) A layout plan of all floors of the building to be demolished, with adequate

sections, showing: a) the occupancy usage of the floors b) the structural support systems c) principal materials of construction d) the condition of the building e.g. the degree of deterioration and e) the relationship of the building to be demolished with neighbouring

properties affected by the demolition, which include all adjoining

buildings and unauthorized structures, shared staircases, party walls, truncating continuous frames, slopes, retaining wall, overhead cables, and underground utility services.

(3) A plan showing the structural arrangement and construction of all unconventional structural elements, such as prestressed concrete structures, precast concrete members, steel framed structures, trusses, long span beams, arches, earth retaining structures, buildings which also act as earth-retaining structures supporting adjacent ground and large cantilevered structures. (4) A plan showing the procedure for the demolition of the building, detailed sequence of demolishing particular structural members and the method of demolition to be adopted including the restrictions on the use of any particular type of equipment. (5) In the case when powered mechanical plants and equipment are used, a plan showing the route of movement of powered mechanical plants and equipment including the method of lifting mechanical plant, where necessary, onto the top floors of the structure; any structural alterations required to suit the demolition, e.g. temporary strengthening to suit early removal of any ground floor structure to facilitate vehicular movement at ground floor, or strengthening of deteriorated key structural members and any shoring, temporary supports and/or floor propping required. (6) A plan showing all precautionary measures for the protection of the public including sidewalks, scaffolding, protective screens and safety nets. (7) A plan showing the proposed shoring and precautionary measures for all affected adjacent buildings, slopes, retaining structures and services at each stage of the demolition works. (8) A plan showing the proposed shoring and temporary support to be provided to the building to be demolished (9) A plan or descriptive notes on the proposed methods for handling and disposal of debris including :

(a) the permissible temporary accumulation of building debris at upper floors and at ground floor

(b) method of handling demolished building debris (c) the routing and movement of debris from each floor to ground holding

area prior to leaving the site (d) means of transportation of debris off the site (e) time and frequency of debris disposal off site

(f) record scheme on the tonnage of each truck load, truck licence plate, driver’s name and location of dump site.

(B) Stability Report including calculations The Stability Report shall include the following parts: (1) a report on the stability of the building to be demolished during all stages of demolition (2) in the case when powered mechanical plants or equipment are used, a

report on the stability of the building with supporting calculations to demonstrate that the use of the plants and equipment will not cause damage to any building, structure, street, land and services (3) in the case when powered mechanical plants or equipment are used, structural calculations for all temporary supports and bracings (4) a report on the stability of neighbouring buildings, adjoining properties, party walls, streets, land and services which may be affected by the demolition work (5) in the case when temporary or permanent supports are required to these neighbouring buildings, adjoining properties, and party walls, structural calculations for these temporary and permanent supports and (6) a report with calculations demonstrating that the demolition work will not render inadequate the margin of safety of, or cause damage to any building, structure, street, land and services.

2.2 Utilities The common utilities encountered in building demolition generally include Electricity, Water, Gas, Telecommunication, Drainage and its accessories, and Overhead and Underground Cables. Prior to actual demolition, concerned Authority shall be consulted so as to cause all utilities to be terminated. The demolition plan shall ensure that during the course of demolition, no existing utilities in the vicinity of the demolition sites are affected by the demolition operation. During demolition, basic utilities like water for water spraying to abate dust, telecommunication link and electricity for lighting etc. shall be required to provide a safe and healthy working environment.

2.3 Hazardous Material If hazardous materials, such as asbestos containing materials, petroleum contamination and radioactive contamination, exist in the building, further investigation and removal of such hazardous material or contamination by specialist shall be considered.

3. PRECAUTIONARY MEASURES 3.1 General Site safety features shall emphasise protection of the public, particularly, the pedestrian and vehicular traffic and the adjacent properties. Proper safety features shall be designed to make sure that the demolition can be carried out safely and the site personnel are protected. The demolition works including precautionary measures shall be carried out in accordance with the approved plans and other related documents, with continuous supervision to the works. On every demolition job, danger signs shall be conspicuously posted all around the structure and all doors and openings giving access to the structure shall be kept barricaded or manned except during the actual passage of workmen or equipment. However, provision shall be made for at least two independent exits for escape of workmen during any emergency. During nights, red lights shall be placed on or about all the barricades.

Where in any work of demolition it is imperative, because of danger existing, to ensure that no unauthorized person shall enter the site of demolition outside working hours, a watchman should be employed. In addition to watching the site, he shall also be responsible for maintaining all notices, lights and barricades. All the necessary safety appliances shall be issued to the workers and their use explained. It shall be ensured that the workers are using all the safety appliances while at work. Goggles preferably made of celluloid lens shall be worn at the time of demolition of walls, floors, tearing of plaster, etc, especially when instruments like jack hammers are employed in demolition work, to protect the eyes from injuries from flying pieces, dirt, dust, etc, that may be blown up by the wind. Leather or rubber gloves should be worn by the workers while demolishing RCC work or removing steel work, etc, where the hands of the workers are likely to be injured. Water may be used to reduce dust while demolishing. If this is impracticable, workmen shall cover the face and nose with piece of muslin or alternatively respirators. No unnecessary work shall go on below when demolition is in progress above. When some work is to be done at the lower level, adequate protection shall be provided for all the workmen so engaged. Safety belts shall be used by labourers while working at higher level to prevent falling from the structure. First-aid equipment shall be got available at all demolition works of any magnitude. Also, by prior arrangement, a qualified doctor shall be available at call. When there is a possibility of fire breaking out, appropriate portable first-aid fire appliances shall be kept at hand. All utilities like water supply, electricity and other service connections shall be disconnected temporarily as discussed in 2.2. If a structure to be demolished has been partially wrecked by fire, explosion or other catastrophe, the walls and damaged roofs shall be shored or braced suitably. 3.2 Walkways and sidewalks Walkways shall be provided for the use of the workmen who shall be instructed to use them and all such walkways shall be kept adequately lighted, free from debris and other materials. Before any demolition work is started, every sidewalk or road adjacent to the work likely to be affected shall be closed or protected. Children and members of the public shall be kept out of the building and the adjoining yard. If the structure to be demolished is more than two storeyed or 7.5 m high, measured from the sidewalk or street which cannot be closed or safely diverted, and the horizontal distance from the inside of the sidewalk to the structure is 4.5 m or less, a substantial sidewalk shed shall be constructed over the entire length of the sidewalk adjacent to the structure of sufficient width with a view to accommodating the pedestrian traffic without causing congestion. The sidewalk shed shall be lighted sufficiently to ensure safety at all times. A toe board of at least 1m high above the roof of shed shall be provided on the outside edge and ends of the sidewalk shed. Such boards may be vertical or inclined outward at not more than 45 degrees.

Except where the roof of a sidewalk shed solidly abuts the structure, the face of the sidewalk shed towards the building shall be completely closed by providing sheeting planking to prevent falling material from penetrating into the shed. As per IS-4130, the roof of sidewalk sheds shall be capable of sustaining a load of 73 N/mm2(very high – may be 7.3 KN/m2). Only in exceptional cases, say due to lack of other space, the storing of material on a sidewalk shed may be permitted in which case the shed shall be designed for a load of 146 N/mm2(very high – may be 14.6 KN/m2). Roof of sidewalk shed shall be designed taking into account the impact of the falling debris. The height of sidewalk shed shall be such as to give a minimum clearance of 2.4m. The deck flooring of the sidewalk shed shall consist of plank of not less than 20mm in thickness, closely laid and deck made watertight. All members of the shed shall be adequately braced and connected to resist displacement of members or distortion of framework. When the horizontal distance from the inside of the sidewalk to the structure is more than 4.5 m and less than 7.5 m, a sidewalk shed or fence may be built or in place of such a shed or fence a substantial railing shall be constructed on the inside of the sidewalk or roadway along the entire length of the demolition side of the property with movable bars as may be necessary for the proper execution of the work. Where workers’ entrances to the building being demolished are not completely protected by sidewalk sheds, all such entrances shall be protected by canopies extending from the face of the building to a point not less than 2.5 m from it. In such case, such overhead protection shall be at least 0.6 m wider than the building entrance or opening and every canopy shall be as strong as the sidewalk shed. 3.3 Catch platforms In demolition of exterior wall of multi-storeyed structure, catch platform of heavy planking shall be provided to prevent injuries to the worker working below and to the public, when the external walls are more than20 m in height. Such catch platform shall be constructed and maintained not more than 3 storeys below the storey from which exterior wall is being demolished. When demolition has progressed to within 3 storeys of ground level, catch platform will not be considered necessary. Catch platforms shall not be less than 1.5m in width and shall consist of outriggers and planks laid tight together. Catch platform shall be provided with a continuous solid parapet along its outer edge of at least 1 m height. Materials shall not be dumped on catch platform nor shall such catch platform be used for the storage of materials.

3.4 Protective screens Protective screen covers shall be placed, where necessary, to prevent flying pieces from injuring the fellow workmen. Bamboo scaffolds or metal scaffolds shall be used for providing protective screens to completely enclose the building structure for retaining dust and small debris. Tarpaulin and heavy duty nets shall be used to cover the exterior face of the scaffold. The protective screens shall be secured to the scaffoldings at intervals in both horizontal and vertical directions.

3.5 Temporary Supports Temporary supports are required to cater for the loads due the machinery used in demolition, debris accumulated, impact from fallen debris and lateral loads due to the fallen debris and wind force etc. A suitable factor of safety shall be considered. They are also provided when any part of the structure or any element being demolished is not self-supporting or when the temporary stability of the structure or its elements could be impaired as a result of the demolition activities. Temporary supports shall not be removed until its supporting loads are completely removed. On the other hand, temporary supports shall be removed as much as possible and practicable after demolition. In the case when temporary supports have to remain, routine inspection and maintenance of such temporary works shall be done until they are completely removed. Temporary supports shall also be provided to adjacent properties including buildings, public or private utilities, slopes, retaining walls or land when the removal of the building or any part of the building being demolished could affect the stability of such properties. Common features, such as truncated continuous beams, exposed party walls and common staircases, shall be protected and stabilised. When a demolition project is shut down for a prolonged period before its completion, the remaining structure, if any, shall be stabilised by temporary support and/or bracing systems. The temporary supports used for demolition shall be built with structural steel, heavy timber, or other material which is considered to be appropriate for the purpose. Pre-manufactured components such as tubular shores, telescope steel props, framed towers, etc., may be used as temporary supports provided their design capacity and their erection and maintenance requirements are followed in strict accordance with manufacturer's recommendations. All temporary support systems shall be supported on adequate foundations or floors. In the case when the immediate floor below the floor under demolition is not adequate to carry the imposed loading from the demolition activities, shoring shall be carried down to the lower floors until adequate support is achieved. 3.6 Protection of Properties 3.6.1 Party Walls and External Walls Party walls that separate the adjoining building and the demolition project shall remain and be protected during and after the demolition project. Redundant party wall shall be removed as far as possible. The exposed party walls or unprotected external wall may be temporarily supported and shall be maintained until the application of the permanent treatment which may be incorporated in the construction of the new building. Demolition of structural elements adjacent to the party wall or the external wall of adjoining building shall be performed by manual method with extreme care to prevent any damage to the party wall or the external wall. The party wall or external wall shall be protected against infiltration and water seepage, by cement mortar treatments, when it is exposed to the weather. All loose bricks or fill materials shall be removed. All openings and voids shall be filled up.

3.6.2 Foundation Support A thorough evaluation shall be conducted for demolition involving any structure that may affect the foundation of the adjoining properties. Appropriate shoring, underpinning or other protective measures shall be installed if necessary. 3.7 Protection of Traffic Any closure of roads and walkways may seriously impact the traffic/pedestrian circulation and, as far as practicable, shall be avoided. If unavoidable, prior permission/ arrangement of the Transport Department and the Police shall be obtained. Temporary closure of a traffic lane may be considered for night work. Proper headroom, segregation, loading/unloading location, illumination etc. shall be provided for the protection of vehicular and pedestrian traffic from the ingress and egress of construction vehicles. 3.8 Special Safety Considerations 3.8.1 Training and Communication Demolition workers, including plant or equipment operators, shall go through proper job safety training and be informed of the potential hazards by attending training sessions as well as on-the-job training. They shall be trained regarding working at heights, working in confined spaces, working with lifting appliances, use of personal protective equipment, handling of chemicals, health hazards in demolition works and safe operating zones. Site safety and project understanding shall be promoted through an induction meeting at the beginning of the project, where information related to the project such as the proposed method and procedures, potential danger during the operation, safety measures and project specifics can be disseminated to all on site personnel. The safety concept can be maintained by regular safety meetings throughout the project period. 3.8.2 Equipment Maintenance All equipment shall be tested and examined before use. They shall be properly stored and maintained. The equipment shall be inspected daily and results of the inspection shall be recorded accordingly. A detailed safety instruction shall be provided to cater for specific situations of the project, if necessary. 3.8.3 Electrical Safety A properly connected power source from a local electric utility supplier or a mobile electricity generator shall be utilised in demolition sites. The safety requirements given in the Electricity Regulations shall be adhered to. 3.8.4 Fire All flammable goods shall be removed from site unless they are necessary for the works involved. Any remaining flammable goods shall be stored in proper storage facilities. All furniture, timber, doors, etc. shall be removed before any welding work is performed. Fire fighting appliances shall be provided and maintained in working conditions. Emergency access to site shall be provided. 3.8.5 Occupational Health The health of workers on site shall be properly protected in accordance with the relevant

subsidiary regulations of the Factories Act with particular attention to Exposure to Dust, Chemical Exposure, Ventilation, Noise Exposure, Medical and First Aid Facilities, Sanitation and Occupational Diseases. 3.8.6 Vibration Demolition work will cause vibration to neighbouring buildings or structures to various extent, depending on the method of demolition, which should be controlled by suitable monitoring. The most serious vibration is caused by implosion. 3.9 Environmental Precautions The general requirements to minimise environmental impacts from construction sites can also be applied to demolition processes. The following sections contain some of the procedures to be adopted: 3.9.1 Air Pollution Concrete breaking, handling of debris and hauling process are main sources of dust from building demolition. Dust mitigation measures, such as water spray, shall be adopted to minimise dust emissions. Burning of waste shall not be allowed. Diesel fumes generated by mechanical plant or equipment shall be controlled.

3.9.2 Noise Noise pollution arising from the demolition works due the use of powered mechanical equipment such as pneumatic breakers, excavators and generators, loading and transportation of debris, etc. affects the workers, and the sensitive receivers in the vicinity of the demolition site. Silent type equipment shall be used to reduce noise impact as much as practicable. Demolition activity shall not be performed within the restricted hours established.

3.9.3 Water The discharge of wastewater from demolition sites shall be controlled by a licence. Effluent shall be treated to the standards as stipulated in the licence before discharge. The Demolition Contractor shall maintain proper control of temporary water supply and an effective temporary drainage system. 3.9.4 Hazardous Materials A suitable plan shall be made for removal of any asbestos containing material. Other materials such as LPG cylinders in domestic flats, toxic and corrosive chemicals for industrial undertakings, and any other hazardous materials have to be identified and properly handled and removed, as per Regulations, prior to the commencement of the demolition of the building. 3.10 Debris and Waste Handling 3.10.1 Chutes Debris, waste and other materials shall not be thrown, tipped or shot down from a height where they are liable to cause injury to any person on or near the site. Existing lift shaft, light well and openings on floor may be used to convey debris down the building floors. Areas adjacent to the openings of these features used as a chute shall be barricaded when they are not in use. Warning signs shall be posted to prevent workers from entering the area. As an option, plastic chutes may be used inside the floor

openings and lift wells to minimise noise and confine the falling debris. a) Lift Shaft: Lift shaft may be used to convey debris inside the building. The

openings to the elevator shall be adequately enclosed to prevent spilling out of debris.

b) Light Well: All the glass windows in the light well shall be taken out or protected before using the light well for conveyance of debris in order to minimise any dangerous situation.

c) Opening on Floor: Openings on the floor may be used to convey debris. If openings are created on the floor, the total openings shall be less than 25% of the total aggregate floor area. The size of opening shall be substantiated with structural justifications with regard to the safety of the remaining structure and minimizing the possible risks arising from the impact force induced. Openings shall not cut through structural support elements that may affect the stability of any structural components.

d) Exterior Chutes: No demolition materials shall be allowed to fall freely outside the building unless it is confined within a chute. If exterior chutes are used, adequate clear spaces shall be provided for their operation. The chutes shall not cause any obstruction to the public. A dust barrier shall be provided if the chute outlet is near public access. The chute shall be designed and constructed with adequate strength and support to allow safe conveyance of debris.

3.10.2 Debris Recycling Better site management and practice would not only prevent the mixing of the inert portion together with the non-inert portion of construction and demolition waste, but could also facilitate and allow on site sorting, and separation at source of construction. The method of ‘selective demolition’ should be adopted as far as practicable. It involves demolition and removal of wastes of the same category one at a time. The goal is to facilitate recycling of wastes for beneficial reuse, thus minimizing the burden on municipal landfills and public filling areas. In general, domestic wastes such as furniture, household appliances, etc., metal components such as window frames, pipes, etc., timber components such as doors, wooden floors, etc., other wastes such as tiles, asphaltic materials, ceramic products should be removed first. Most of these materials may be recycled. The building demolition shall begin after all the above non-structural materials have been stripped and removed. The sequence of demolition shall be planned to allow the separation and sorting of building materials. Concrete and/or brick debris shall be broken down into smaller sizes and separated from reinforced steel for disposal. Concrete debris may be pulverised into aggregate size and used for road base, temporary haul roads, fill materials or aggregates for concrete. Old bricks may be salvaged for reuse as architectural features or other uses. Broken concrete may be disposed of at construction and demolition (C&D) materials recycling facilities for processing into recycled products and aggregates for beneficial reuse. In the event that broken concrete is mixed with some other wastes, broken

concrete should be sorted out at site from the mixture of wastes, before disposal at a C&D materials recycling facilities.

3.10.3 Dust Minimization To prevent dust generation during the debris hauling, water spraying shall be applied during the hauling process. However, the contractor shall ensure proper control of water supply and floor drainage system in order to avoid flooding which is a nuisance and may cause overloading of floors. 3.10.4 Debris Accumulation In general, the debris accumulation on the floors is not allowed unless the debris accumulation is justified by engineering calculations. Debris shall not accumulate against the hoarding or external wall. Excessive accumulation of debris may cause overloading condition and may induce lateral loading on the walls and shall be avoided. The propping design shall include the debris loading.

3.10.5 Debris Disposal and Management System To avoid accumulation of debris and to make sure that they are disposed of promptly, a debris disposal and management system should be made out clearly laying down the following details:

a) method of handling demolished building debris b) movement of debris from each floor to holding area prior to leaving the site c) means of transportation of debris off site d) time and frequency of debris disposal off site e) record scheme on the tonnage of each truck load, truck licence plate, driver’s

name and location of dump site and f) site supervisory personnel responsible for the debris management system.

3.10.6 Waste Management On-site sorting of surplus construction and demolition (C&D) material is strongly recommended so that inert material can be disposed of at public filling areas as far as practicable, and the remaining C&D waste disposed of at landfills. All construction and demolition materials arising from or in connection with demolition work shall be sorted on-site and be separated into different groups for disposal at landfills, public filling areas, filling areas provided by the Contractor, or recycling as appropriate. All public fills to be disposed of at public filling areas shall be sorted and broken down according to the licence conditions for dumping. 3.11 Post-Demolition Precautions Once the demolition is completed, the site shall be reinstated to eliminate any potential hazard to the public. The site shall be levelled and cleared of any debris and adequate drainage shall be provided. If the new development after demolition is not immediately commenced, the site shall be completely enclosed to prevent public trespassing. Supports to adjacent building structures, weather-proofing and stabilisation of exposed party walls shall be completed. Any excavation shall be braced and stabilised.

4 PRINCIPLES OF STRUCTURAL DEMOLITION 4.1 General When selecting the most suitable method of demolition, the knowledge of the site should be applied and the interdependency of elements and the nature of the structure should be taken into account. Each of the basic principles of structural demolition, i.e. progressive, deliberate collapse or deliberate removal should be considered. The strategic use of inherent forces should be considered as an aid to efficient demolition. These same principles should be applied whether there is full or partial demolition, or where structural alterations are to be carried out. 4.2 Types of structural demolition:

(a) Progressive demolition Progressive demolition is the controlled removal of sections of the structure, whilst retaining the stability of the remainder and avoiding collapse of the whole or part of the building to be demolished. Where progressive demolition is adopted, it is essential that the key structural members on which the integrity of the structure relies should, together with their sequence of removal, be clearly indicated in the method statement and also on site. Progressive demolition should be considered for the majority of sites and is particularly useful in confined and restricted areas.

(b) Deliberate collapse mechanisms Demolition by deliberate collapse is the removal of key structural members to cause complete collapse of the whole or part of the building or structure. Where deliberate collapse demolition is adopted, the key structural members to be removed should be clearly indicated, together with the sequence of removal, in the method statement and also on site. This method should be employed only on detached, isolated, reasonably level sites where the whole structure is to be demolished. There should be sufficient space to enable equipment and personnel to be removed to a safe distance. Where parts of a structure are to be demolished by deliberate collapse in separate operations, there should be no potential instability of the remaining structure to cause a hazard to personnel on the site and to other people near the structure being demolished.

(c) Deliberate removal of elements The deliberate removal of elements should be considered to be the removal of selected parts of the structure by dismantling or deconstruction. The elements to be removed should be identified and the effects of removal on the remaining structure fully understood and included in the method statement, with the elements to be removed marked on site. Sections of the structure should not be removed if instability of any of the remainder could result in a possible risk to personnel on the site and to other people nearby. Expert advice should be sought.

4.3 Load-bearing structural elements (a) Roof trusses Where a pitched roof is to be removed progressively, the roof structure should be removed upto bearing plate level. Enough purlins and bracing should be retained to ensure stability of the remaining roof trusses while each individual truss is removed progressively. Temporary bracing should be added where necessary to

maintain stability. The end frame opposite to the end where dismantling is commenced, or a convenient intermediate frame, should be independently and securely guyed in both directions before work starts. Generally, the bottom ties of trusses should not be cut in situ. (b) Cantilevers Cantilevers such as cantilevered retaining walls, propped cantilevers, balanced bridge construction, canopies, cornices, staircases and balconies rely on superimposed loads or balancing restraining loads for their stability. They should be either demolished or supported before the counterbalancing loads are removed. (c) Columns The engineered removal of columns can aid efficient demolition but the degree and type of fixity of the connections at the top and bottom should be determined as these can be crucial to the predictability of collapse patterns and thus the safety of the demolition. (d) Reinforced concrete slabs Before demolition is commenced the load bearing characteristics of the slab should be confirmed by first establishing the reinforcement details including the direction of the main reinforcement by, e.g. electronic sensing or by making small trial holes. The strength and quality of the concrete should also be determined. The method of demolition should take into account the pattern of reinforcement, e.g. whether it is one-way or two-way spanning. Additionally, account should be taken of any stress reversal, which can occur in slabs or beams propped at, e.g. mid span, because the main steel reinforcement will not be situated in the appropriate position to form part of a continuous structure. (e) Beams When removing beams by cutting or dismantling, the method adopted should consider any weakening of the structure by, say, increasing the effective length of columns, effects of stress reversal if continuous beams are to be reduced to shorter lengths, and effect of the demolished loads on the structure. (f) Jack arches Where tie rods are present, they should not be cut until the horizontal thrusts have been removed by, e.g. removal of the arch or series of arches in the floor. The floor should be demolished in strips parallel to the span of the arch rings (i.e. at right angles to the beams supporting the arches). (g) Portal frames A single portal frame should be considered a combination of two columns and a beam (which can be continuous or pin-jointed) forming a frame in a vertical plane, where the joint between each column and beam is designed to be moment-resisting. The type of rigidity at the base can differ as can the rigidity of the connection where the lengths of beam are joined and should therefore be established. (h) Arches Arches carry load by exerting horizontal thrusts outwards at their spring points over the length of the construction. To ensure that the stability of the arch is

maintained, the horizontal thrusts should be resisted. Methods to maintain stability during demolition in the vicinity of arches and of the arches themselves should ensure the balance of the forces by shoring to transmit horizontal thrusts to the ground, providing temporary tie bars, reducing vertical imposed loads and demolition of the arch in whole strips. (i) Walls The verticality of walls should be ascertained, load-bearing walls identified and the security of bonding at the ends of cross walls should be established. The structural condition of walls which are to remain, including the walls of any adjoining property, should be established. The method decided upon for demolishing cantilevered retaining walls should take account of their dependency on loading from above, or the propping action from floors or other structural members which can affect their stability. (j) Composite structures (slabs and beams) Some basic structural elements have little inherent strength and rely for their stiffness upon the composite nature of the complementary members. The nature of any dependency can be broken by early demolition activities and this should be identified as it might be necessary for temporary supports to be used. Any demolition sequence involving composite structures should take account of the factors like lateral restraints to the compression flange provided by friction between the slab and the top flange or otherwise.

4.4 Structural materials Steel behaves in a ductile way under load, and will deflect before failure occurs in a member; however, care should be taken to avoid catastrophic failure which can occur at the connections without warning. Concrete can be subject to instantaneous failure and collapse. Concrete can be weaker than might be assessed from external inspection because of, say,. internal deterioration, and the potential for this should be assessed and taken into account. In case of Cast iron and wrought iron columns and beams, the type of iron should be identified because sudden collapse can occur, e.g. due to brittle failure, in cast iron. Explosives generally should not be employed in the demolition of this type of construction unless it is proposed to demolish the building as a whole. In case of Prestressed material, effective measures should be taken, during the demolition of any structure or elements, to ensure the control of inherent stored energy. In selecting a demolition technique, the potential consequences of sudden release of stress within the immediate working area and beyond should be considered. It is to be considered if the member is Pretensioned or post-tensioned and if it’s grouted.

4.5 Avoidance of unplanned structural collapses The potential for structural collapses should be foreseeable and avoided by planning suitable methods and sequences of demolition. Unplanned collapse, which includes the collapse of an entire structure or, more likely, parts of a structure or building, generally occur prematurely (i.e. unintentional at that time) because of inadequate residual

structural integrity, e.g. when pre-weakened or unintentionally (i.e. not intended for demolition) because of inadequate stability resulting in part of the structure or building to collapse or fall. To predict the possibility of premature and unintentional collapse, an assessment of the structure should be undertaken and the effects on the structure of the proposed methods and sequences of work determined before work commences. Floors should not be overloaded with machinery and/or debris resulting from the demolition process. Some floor slabs are weaker in one direction and are therefore sensitive to the orientation of loading from machines and thus more susceptible to the potential for unplanned premature collapse. One method which should be considered for avoiding premature collapse is the provision of temporary structural support with proper assessment for any changing load patterns, including any induced reversal of stresses in the structure as demolition progresses. When the structure is weakened in the demolition process (e.g. by the removal of internal members) and work cannot be completed during the working day, the structure should be left in a condition capable of withstanding appropriate wind loads, if local conditions dictate. 4.6 Designing safe deliberate collapse mechanisms As creating instability in a structure to initiate collapse or partial collapse is a fundamental principle of demolition, deliberate pre-weakening or pre-strengthening should be competently designed following assessment of the risks, including structural assessment. As certain types of structure are inherently strong and can require weakening to ensure that they collapse successfully, consideration should be given to the removal of load-bearing members or walls, as necessary, to ensure a successful full or partial demolition. Any operation that requires the disturbance or removal of key elements should be fully assessed, with calculations carried out by a competent engineer to obviate the danger of unexpected or premature collapse. Consideration should also be given to carrying out structural strengthening before demolition work begins, in order to enhance temporary stability. When partial demolition of a structure takes place, the structural integrity of the remaining parts of the structure and any adjacent structures should be maintained. A detailed assessment of all imposed loads acting on the remaining part of the structure should be made. Environmental loadings, e.g. wind, should be assessed. Vibration and impact from traffic or adjacent demolition activities should also be considered. Foundation details of load-bearing and retaining walls should be established. The demolition method should avoid adversely affecting adjoining buildings and structures that can become unstable where common elements such as party walls, combined foundations, continuous beams and basements are affected.

5. METHODS OF DEMOLITION 5.1 General There are multiple types of demolition procedures used to take down structures. The choice of demolition method depends on the project conditions, site constraints, and sensitivity of the neighbourhood and availability of equipment. Several methods of demolition can be used in combination or at different parts of the demolition site. In general, the choice of technique should enable the re-use and/or the recycling of materials arising from the demolition. Irrespective of which demolition method is adopted, its choice should be based on minimizing the risk to personnel. Simple manual demolition is indicated for smaller buildings such as single story homes. Manual demolition usually involves pulling the structure down. Closely related is mechanical demolition. Quite often smaller buildings use a combination of manual and mechanical methods. Mechanical demolition involves both the pulling down and the knocking down of the structure by machines. Mechanical demolition uses such machinery as bulldozers, cranes, and excavators. Larger structures involve wrecking balls. Most of these methods are top down methods where the demolition starts from the top floor and proceeds towards the ground. For structural projections, such as balconies, canopies and verandahs extending beyond the building lines, demolition by hand held tools or the cut and lift process may be a safe solution. The most dramatic type of demolition is implosion. This involves a controlled series of explosions that causes a collapse of the structure. The explosive charges are placed and fired in a controlled manner that causes the building to come down into its own "footprint", minimizing damage to surrounding structures. Implosion is only used as a last resort because of the high safety risk involved. This type of demolition takes a long time to set up and a long time to clean up, but the actual implosion process takes mere seconds to unfold. A new type of demolition that is growing in popularity is called "deconstruction." This is the most environmentally sound method of demolition and it is sometimes called "Green demolition" for this reason. It involves a slow and careful process that is almost the reverse of the construction method. The building is slowly taken apart and as much material as possible is salvaged for reuse elsewhere. This lightens the load on the landfills that usually receive the debris of a building destroyed by conventional demolition procedures. 5.2 Manual Method 5.2.1 General Manual methods are carried out top down, proceeding, in general, from the roof to ground. The particular sequence of demolition may vary, depending on site conditions and structural elements to be demolished. For reinforced concrete buildings, jack hammers are commonly used to break down the concrete. Oxy-acetylene torch could be used to cut the reinforcements. The reinforcements shall remain until all the concrete connecting to or supported by the reinforcement is broken away or when its support is no longer required. Cantilevered

canopies, balconies and exterior walls are critical elements in building demolition. In congested areas, these features could have critical impact on the safety of the public. Demolition of these features shall be performed with extreme caution. If rope or tie wires are used to pull down the structural elements, the pulling wire must be at least 4 times stronger than the anticipated pulling force. In addition, workers shall be shielded from the rope or tie wires. The rope or tie wire shall be checked at least twice per day.

5.2.2 Demolition Sequence Demolition sequence shall be determined according to actual site conditions, restraints, the building layout, the structural layout and its construction. In general, the following sequence shall apply:

a) All cantilevered structures, canopies, verandahs and features attached to the external walls shall first be demolished prior to demolition of main building and its internal structures on each floor

b) When demolishing the roof structure, all lift machine rooms and water tanks at high level shall be demolished in “top down” sequence to the main roof level

c) Demolition of the floor slabs shall begin at mid span and work towards the supporting beams

d) Floor beams shall be demolished in the order of cantilevered beams, secondary beams and then main beams

e) Non-load bearing walls shall be removed prior to demolition of load bearing walls

f) Columns and load bearing walls shall be demolished after removal of beams on top and

g) If site conditions permit, the first floor slab directly above the ground floor may be demolished by machine standing on ground.

5.2.3 Cantilevered Structures and Balconies Cantilevered structures, balconies and canopies may project out of the building over the pedestrian footpath or in some cases over a portion of the traffic lane. Temporary supporting structures shall be placed directly underneath them as precautionary measures. The general sequence of dismantling cantilevered slabs and beams is described in the following:

a) The exterior wall shall be demolished first b) Any structure or dead load supported by the cantilevered system shall be

removed prior to demolishing the cantilevered slabs and beams c) The concrete shall be broken down gradually starting from the exterior

edge of the cantilevered floor, working inwards and toward its supporting beams. Figure 5.1 illustrates the demolition of cantilevered slab

d) The cantilevered beam shall be demolished after the demolition of the connecting floor slab. Demolition of the cantilevered beam shall not advance further than the floor slab so that the support for the slab is always maintained. Figure 5.2 illustrates the demolition of cantilevered beam with the slab and

e) Saw cut and lift may be used to dismantle the cantilevered features. The slab shall be cut into a manageable size and lifted away. The cantilevered beams shall be cut and removed after the removal of the slab load and any load supported by them. The cut and lift applications are discussed in 5.7.3.

Fig 5.1 Demolition of cantilevered RCC slab (Manual method)

Fig 5.2 Demolition of cantilevered RCC slab and Beam(Manual method)

5.2.4 Exterior Walls, Beams and Columns (a) Brick in-fill Wall:

To avoid any potential hazard of bricks falling out of the building, all the

brick in-fill shall be removed by pushing inward, before dismantling the reinforced concrete framing. Working platforms outside the building shall be used for removal of the walls. Brick removal shall begin from the top layer downwards. The works shall be carried out layer by layer.

(b) Exterior Beam: The exterior beam may be demolished by gradually breaking away the concrete or by dismantling the entire beam section. Demolition of the exterior beams is illustrated in Figure 5.3 and described in the following: i) Wire and winch or other systems shall be used to secure the cross

beam to other structural members ii) The concrete is first broken away at both ends near its column

supports to expose the reinforcement iii) Reinforcement shall be cut at one end to allow the beam to partially

drop. The wire shall safely winch the beam down to the building floor in a controlled manner and

iv) The dismantling would be completed by cutting the reinforcement at the remaining end, and the beam will then be lowered completely in a controlled manner.

(c) Exterior Column Exterior column may be demolished by the following procedure and as illustrated in Figure 5.4. i) The top of the column shall first be secured to a structural member by

wire and winch ii) Pre-weakening shall be performed at the bottom of the column to

reduce the pulling force and to ensure that the break occurs at the desired location. The concrete cover of the reinforcement shall first be removed. Reinforcement at the interior face shall remain. Reinforcement at the exterior face shall be cut immediately before the pulling of the column and

iii) After pre-weakening, the column shall be pulled down by the wire and winch towards the interior in a controlled manner.

Fig 5.3 Demolition of External Beam (Manual Method)

Fig 5.4 Pre-weakening and dismantling of Column (Manual Method)

(d) Exterior Reinforced Concrete Frame The exterior reinforced concrete frame may be demolished in sections. The demolition procedures are generally described in the following: i) The optimum section of the frame to be demolished shall be a bay

between the two adjacent columns ii) The frame section shall be secured to other structural members with

wire and winch before disconnecting the framing from the remaining structure

iii) Pre-weakening shall be performed at the bottom of the two columns and

iv) The reinforcing bars connecting the beams shall be cut off after pre-weakening. The framing shall be pulled down by exerting force through winch and pulley system.

(e) Reinforced Concrete Wall (1) Load Bearing Wall

Reinforced concrete walls may be demolished by cutting down the wall into manageable sections. The width of the wall shall not be wider than 2 m. Demolition of the reinforced concrete wall sections is illustrated in Figure 5.5 and described in the following: i) Before demolition begins, wire and winch systems shall be used to

secure the wall section; ii) Pre-weakening at the bottom of the wall shall be performed,

particularly if the wall section contains columns. The concrete along

the cut line of the interior face of the wall section shall be broken away by hand held tools. Pre-weakening shall follow the similar details as for columns and

iii) After the concrete along the cut line is removed, the reinforcing bars along the vertical cut line shall be separated. Force shall be exerted through the wire and winch systems to pull the wall down into the building.

(2) Non-Load Bearing Wall For non-load bearing walls or walls with heavy cross beams, the dismantling procedures are similar to that of the load bearing wall except that the cross beams are dismantled separately from the building walls.

Fig 5.5 Dismantling of RCC wall

5.2.5 Floor Slabs Reinforced concrete floor slab shall be demolished by gradually breaking away the concrete. The reinforcement shall remain and be cut off after the concrete is broken away. The sequence for demolition of typical floor slabs is discussed in the following:

(a) Two Way Slab The two way slab is supported by beams on all four sides. Demolition of the slab shall begin in the middle of the slab and advance towards the sides in all 4 directions. Figure 5.6 illustrates the demolition of two way slab.

Fig 5.6 Demolition of Two Way slab

(b) One Way Slab The breaking of concrete shall begin at the unsupported end and proceed in strips perpendicular to the supporting beam or structural member. The strips shall be demolished from centre to the supports in both directions.

5.2.6 Interior Beams and Columns The dismantling of interior beams and columns is similar to that of exterior beams and columns, except that the interior beams, normally, support slabs on both sides and the supporting beam shall not be removed until all other dead loads imposed on the beam are removed, including the slabs supported by the beam.

5.3 Top Down — By Machines 5.3.1 General The sequence of demolition by machine is typically the same as the top down manual method, except that most of the demolition is done by mechanical plant. The demolition begins with the lifting of the mechanical plant on to the building top floor. When rope or tie wire is used for pulling, the workers shall be protected or stay away from the area within reach of the rope or tie wire. Adequate propping shall be installed at floor levels below the working floor to safely support the operation of the mechanical plant. The movement of the mechanical plant shall only be within the propped area. The propped areas shall be suitably marked. The movement of the mechanical plant shall be prohibited within 2 m of the building edge, within 1 m of any floor openings or any cantilevered structures. The mechanical plant shall be lifted onto the roof of the building by the use of mobile crane or other appropriate means as approved. The machine shall descend down to the next floor by means of a ramp. The ramp may be a temporary structure or other appropriate design. The slope of the ramp shall be no steeper that 1.75 to 1 or as recommended by the machine manufacturer. As an alternative, the machine may also be lowered to the next floor by the use of mobile crane. Demolition sequence shall i̧n general, be as for manual methods. In general, the following sequence shall apply:

a) prior to demolition of internal floors, all cantilevered slabs and beams, canopies, and verandahs shall first be demolished

b) the structural elements, in general, shall be demolished in the sequence of slab, secondary beams and then main beams

c) mechanical plant shall descend from the floors with temporary access ramp, or be lowered to the next floor by lifting machinery or by other appropriate means

d) when a mechanical plant has just descended from the floor above, the slabs and beams, in two consecutive floors may be demolished by the mechanical plant simultaneously. The mechanical plant may work on structural elements on the same floor and breaking up the slabs on the floor above

e) the beams and columns shall be demolished by gradually breaking down the concrete or by pulling them down in a controlled manner.

Figure 5.7 illustrates the sequence of top down method with mechanical equipment.

5.3.2 Cantilevered Canopies and Balconies In general, cantilevered structures shall be demolished prior to the demolition of the main structure of the building for each floor and before the removal of their supports or holding down loads. In the case when this cannot be satisfied, cantilevered structures shall be properly shored until they are completely demolished, considering the effects of removal of ant loads. A cantilever counterbalanced by top load on beams will collapse if top load is removed. When one side of balanced cantilever is removed, the remaining cantilever will topple.

Demolition of the cantilevered structures is described in the following(Fig 5.8): a) The exterior wall linking the cantilevered structure shall be removed first.

The floor slab and cantilevered beam may be demolished in sections. b) The machine arm with wire passing through the slab section shall be used to

stabilise the structure while the cutting is performed; c) Cuttings may be performed by jack hammer or pneumatic hammer for the

concrete and oxy-acetylene flame cutter for the reinforcements. The concrete shall be broken away first before the cutting of reinforcement. Alternatively the reinforced concrete slab may be cut by saw cutting; and

d) The slab shall be lifted into the building by a derrick arm.

1. Demolish slab & beam 2. Continue demolishing slab & beam

2. Access ramp to lower floor 4. Demolition of interior column

5. Cut external wall & column 6. Pull down wall section

Fig 5.7 Typical sequence of demolition by top down method with Machines

1.M/C mounted on suspended floor

2. M/C mounted on ground

3. Cutting of cantilevered slab

4. Lifting of cantilever slab

Fig 5.8 Demolition of cantilever slab by Machine

5.3.3 Exterior Walls, Beams and Columns (a) Brick in-fill wall

Demolition of the brick-in-fill wall is generally described in the following: i) The in-fill bricks shall first be manually removed. The brick shall be

removed from the top layer down by pushing in from outside work and

ii) then, the reinforced concrete frame may be demolished by dismantling the framing sections.

(b) Exterior Column i) The excavator arm with wire or hydraulic crusher attachment shall be

used to brace the column ii) Pre-weakening shall be performed at the bottom of the columns iii) After pre-weakening, the column shall be pulled down in a controlled

motion into the building by the excavator arm; then iv) Demolition inside the building by the excavator arm.

(c) Exterior Reinforced Concrete Frame i) The concrete along the proposed cut-line shall be broken first. The

reinforcing bars shall be kept to stabilise the structure. The excavator arm shall secure the reinforced concrete framing

ii) Pre-weakening may be performed at the bottom of the columns. The excavator arm shall continue to stabilise the frame while cutting the reinforcing steel at the disconnecting points and

iii) The excavator arm shall pull and guide the frame safely onto the floor. (d) Reinforced Concrete Wall

The process of demolishing a reinforced concrete wall section is similar to that of a reinforced concrete frame. Demolition of a reinforced concrete wall section is illustrated in Figure 5.9 and is described in the following:

i) The reinforced concrete wall shall be vertically separated from the remaining wall by breaking away the concrete. The width of the wall section shall be determined by the Structural Engineer. The reinforcing bars shall remain to provide support to the wall section

ii) If the wall section contains columns, pre-weakening shall be performed at the level where the wall is to be separated.

iii) The machine arm shall be used to secure the wall section during the cutting of the reinforcements along both sides of the wall section and

iv) After the reinforcing bars are severed, the machine arm shall steadily guide and pull down the wall section into the building for further break down.

1. Breaking away concrete vertically to separate wall section

2. Excavator arm with wire to brace wall, while pre-weakening column bottoms

3. Excavator pulls down wall, after cutting reinforcement

Fig 5.9 Demolition of RCC wall by Excavator 5.3.4 Floor Slabs Floor slabs may be dismantled by breaking down the concrete gradually with machine mounted attachments. Reinforcing bars shall be cut afterwards. The slab may be demolished by machine with breaker, hydraulic crusher or other appropriate attachments. 5.3.5 Interior Beams and Columns Interior Beam and Columns may be demolished by using the same procedures as for the exterior beam and column.

5.4 Mechanical Method by Hydraulic Crusher with Long Boom Arm The crusher attachment breaks the concrete and the reinforcement by the hydraulic

thrust through the long boom arm system. The hydraulic crusher can be operated from the ground outside the building. This method is also suitable for dangerous buildings, silos and other industrial facilities. Figure 5.10 illustrates the typical operation of hydraulic crusher with long boom arm. For environmental reason, it should be used wherever practicable because of its quietness. The operation shall have a minimum clear space of ½ the building height as a safety zone for the falling debris. The excavator shall operate on firm ground that can support the machine during the crusher operation. Each section of the structure shall be demolished in a top down sequence to ensure stability of the structure. Debris may be used to build up a platform for the excavator to extend the range of reach, by dense compacting, upto 3m. The width in both directions of the platform shall be at least one and one-half the length of the machine to allow safe manoeuvre during the demolition operation. To minimise the dust impact, the structure shall be pre-soaked with water before demolition. Water shall be continuously sprayed during the crushing operation. Debris may fall out of the building during the demolition. The site shall be completely fenced off and guarded.

Fig 5.10 Demolition by Hydraulic Crusher with long boom

5.5 Wrecking Ball The wrecking ball application consists of a crane equipped with a steel ball. The destruction of the building is by the impact energy of the steel ball suspended from the crawler crane. The wrecking ball operates outside the building. This method is suitable for dilapidated buildings, silos and other industrial facilities. However, the operation requires substantial clear space. The application also demands high level skill operators and well-maintained equipment. Figure 5.11 illustrates the operation of Wrecking Ball. The balling of each section of the structure shall proceed from top to bottom. Recommended techniques for the wrecking ball operations include (1) Vertical Drop -

free falling of the wrecking ball onto the structure and (2) Swing in line - swinging of the ball in-line with the jib. A second dragline will normally connect to the ball horizontally to control the ball motion. The ball shall be swung into the building and shall strike at the top of the member so as to avoid the member from falling outside the building. The jib or boom shall be operated with no less than 3 m above the portion of the structure being demolished. Clear space for operation between the crane and the structure being demolished shall be ½ of the height of structure, with additional distance between crane and boundary wall for manoeuvrability. High strength wire shall be used to allow pullout of the wrecking ball from potential traps. To minimise the dust impact on the surrounding area, the structure to be demolished shall be pre-soaked with water before demolition. Water spraying shall continue on the structure during demolition. The operation shall not be performed adjacent to overhead power lines. The site shall be entirely fenced off to forbid public access.

Fig 5.11 Operation of Wrecking Ball

5.6 Implosion Implosion is the strategic placing of explosive material and timing of its detonation so that a structure collapses on itself in a matter of seconds, minimizing the physical damage to its immediate surroundings. The technique weakens or removes critical supports so that the building can no longer withstand the force of gravity and falls under its own weight. The explosives are just the trigger for the demolition. It's gravity that brings the building down. Explosives are loaded and progressively detonated on several different levels of the building so that the building structure falls down on itself at multiple points. When everything is planned and executed correctly, the total damage of the explosives and falling building material is sufficient to collapse the structure entirely, so cleanup crews are left with only a pile of rubble.

The Reading Grain Facility, Philadelphia, blasted in 1999

In order to demolish a building safely, each element of the implosion must be studied ahead of time. This is done by a blasting expert. The first step is to examine architectural blueprints of the building to determine how the building is put together. Next, the building is surveyed to study about the support structure on each floor. Based on this data and drawing from past experiences with similar buildings, the expert decides what explosives to use, where to position them in the building and how to time their detonations. The main challenge in bringing a building down is controlling which way it falls. Ideally, a blasting crew will be able to tumble the building over on one side, into a parking lot or other open area. This sort of blast is the easiest to execute, and it is generally the safest way to go. Tipping a building over is something like felling a tree. To topple the building to the north, the blasters detonate explosives on the north side of the building first, in the same way a tree would be chopped into from the north side if it is to fall in that direction. Blasters may also secure steel cables to support columns in the building, so that they are pulled a certain way as they crumble. Sometimes, a building is surrounded by structures that must be preserved. In this case, the blasters proceed with a true implosion, demolishing the building so that it collapses straight down into its own footprint (the total area at the base of the building). The basic idea in implosion is to think of the building as a collection of separate towers. The blasters set the explosives so that each "tower" falls toward the centre of the building. When they are detonated in the right order, the toppling towers crash against each other and all of the rubble collects at the centre of the building. Another option is to detonate the columns at the centre of the building before the other columns so that the building's sides fall inward. Generally speaking, blasters will explode the major support columns on the lower floors first and then a few upper stories. In a 20-story building, for example, the blasters might blow the columns on the first and second floor, as well as the 12th and 15th floors. In most cases, blowing the support structures on the lower floors is sufficient for collapsing the building, but loading columns on upper floors helps break the building material into smaller pieces as it falls. This makes for easier cleanup following the blast. Once the blasters have a clear idea of how the structure should fall, any debris is cleared out of the building and non-load-bearing walls within the building are taken out. If these walls were left intact, they would stiffen the building, hindering its collapse. Some materials, such as glass, that can form deadly projectiles, and insulation that can scatter over a wide area must be removed. Destruction crews may also weaken the supporting columns with sledge hammers or steel-cutters, so that they give way more easily.

Next, blasters can start loading the columns with explosives. Blasters use different explosives for different materials, and determine the amount of explosives needed based on the thickness of the material. For concrete columns, blasters use traditional dynamite or a similar explosive material. Dynamite is just absorbent stuffing soaked in a highly combustible chemical or mixture of chemicals. When the chemical is ignited, it burns quickly, producing a large volume of hot gas in a short amount of time. This gas expands rapidly, applying immense outward pressure on whatever is around it. Blasters cram this explosive material into narrow bore holes drilled in the concrete columns. When the explosives are ignited, the sudden outward pressure sends a powerful shock wave busting through the column at supersonic speed, shattering the concrete into tiny chunks. For buildings with a steel support structure, blasters typically use the specialized explosive material cyclotrimethylenetrinitramine, called RDX for short. RDX-based explosive compounds expand at a very high rate of speed, 8,230 meters per second. Instead of disintegrating the entire column, the concentrated, high-velocity pressure slices right through the steel, splitting it in half. Additionally, blasters may ignite dynamite on one side of the column to push it over in a particular direction.

Concrete columns (on the left) are blown apart with Dynamite. Steel columns (on the right) are sliced in half using RDX.

To ignite both RDX and dynamite, a severe shock needs to be applied. In building demolition, this is accomplished with a blasting cap, a small amount of explosive material (called the primer charge) connected to some sort of fuse. The traditional fuse design is a long cord with explosive material inside. When one end of the cord is ignited, the explosive material inside it burns at a steady pace, and the flame travels down the cord to the detonator on the other end. When it reaches this point, it sets off the primary charge.

Blasting caps are used as a catalyst to set off the explosives loaded in support columns. These days, an electrical detonator is used instead of a traditional fuse. An electrical detonator fuse, called a lead line, is a long length of electrical wire. At the detonator

end, the wire is surrounded by a layer of explosive material. This detonator is attached directly to the primer charge affixed to the main explosives. When current is sent through the wire (by hooking it up to a battery, for example), electrical resistance causes the wire to heat up. This heat ignites the flammable substance on the detonator end, which in turn sets off the primer charge, which triggers the main explosives. To control the explosion sequence, blasters configure the blast caps with simple delay mechanisms, sections of slow-burning material positioned between the fuse and the primer charge. By using a longer or shorter length of delay material, the blasters can adjust how long it takes each explosive to go off. The length of the fuse itself is also a factor, since it will take much longer for the charge to move down a longer fuse than a shorter one. Using these timing devices, the blasters precisely dictate the order of the explosions.

Columns are fully loaded with explosives and hooked up to blasting caps and fuses.

Blasters determine how much explosive material to use based largely on their own experience and the information provided by the architects and engineers, who originally built the building. To make sure they don't overload or under-load the support structure, the blasters perform a test blast on a few of the columns, which they wrap in a shield for safety. The blasters try out varying degrees of explosive material, and based on the effectiveness of each explosion, they determine the minimum explosive charge needed to demolish the columns. By using only the necessary amount of explosive material, the blasters minimize flying debris, reducing the likelihood of damaging nearby structures. To further reduce flying debris, blasters may wrap chain-link fencing and geotextile fabric around each column. The fence keeps the large chunks of concrete from flying out, and the fabric catches most of the smaller bits. Blasters may also wrap fabric around the outside of each floor that is rigged with explosives. This acts as an extra net to contain any exploding concrete that tears through the material around each individual column. Structures surrounding the building may also be covered to protect them from flying debris and the pressure of the explosions. In addition to these measures, the blasters must prepare the people in the area for the blast, assuring local authorities and neighbouring businesses that the demolition won't seriously damage nearby structures. If there are slopes and earth retaining walls or features, a geotechnical assessment shall be conducted to ensure that the blasting will not affect the stability of these features. The surrounding structures are inspected prior to the blast ̧ so that any damage claims can be assessed. What level of vibration a particular implosion may cause can be predicted ahead of time using data collected from previous blasts. Portable field seismographs are used to measure ground vibrations during an implosion. Once the structure has been pre-weakened and all the explosives have been loaded, it

should be made sure that the building and the area surrounding it are completely clear. With the level of destruction involved, it is imperative that all spectators be a good distance away. Blasters calculate this safety perimeter based on the size of the building and the amount of explosives used. The impacts of noise and dust generated during the blasting shall be considered. Radius of the typical exclusion zone shall not be less than 2.5 times the building height. Blasters might overestimate the amount of explosive power needed to break up the structure, and so produce a more powerful blast than is necessary. If they underestimate what explosive power is needed, or some of the explosives fail to ignite, the structure may not be completely demolished. In this case, the demolition crew brings in excavators and wrecking balls to finish the job. Safety is the blaster's number-one concern, and, for the most part, they can predict very well what will happen in an implosion. The entire site shall be under 24-hour security from the installation of explosive until final blasting. All personnel must be evacuated from the site before and during blasting. For the purpose of crowd control, blasting should be carried out in the early morning of a Sunday or public holiday. An emergency plan shall be prepared to handle emergency situations such as premature explosion, misfire or interruption due to bad weather including thunder and lightning. Once the area is clear, the blasters retreat to the detonator controls and begin the countdown, after which the detonators are activated to affect the blast. The blasters may sound a siren at the 10-minute, five-minute and one-minute mark, to let everyone know when the building will be coming down.

Two types of blasting machines, a traditional rack-bar and a modern electronic

control box Typically, the actual implosion only takes a few seconds. Following the blast, a cloud of dust billows out around the wreckage enveloping nearby spectators.

The Scudder Homes in Newark, N.J., blasted in 1996

After the cloud has cleared, the blasters survey the scene and confirm that all of the explosives were detonated and remove any explosives that did not go off. Most of the time, experienced blasters bring buildings down exactly as planned. Damage to nearby structures, even ones immediately adjacent to the blast site, is usually limited to a few broken windows. Any error can be disastrous, however, and some demolitions have failed, severely damaging neighbouring structures. The greatest danger is from flying debris which, when improperly prepared for, can kill onlookers. Even more dangerous is the partial failure of an attempted implosion. When a building fails to collapse completely the structure may be unstable, tilting at a dangerous angle, and filled with un-detonated but still primed explosives, making it difficult for workers to approach safely. A further danger comes from air overpressure that occurs during the implosion. If the sky is clear, the shock wave, a wave of energy and sound, travels upwards and disperses, but if cloud coverage is low, the shock wave can travel outwards, breaking windows or causing other damage to surrounding buildings.

5.7 Other Methods and equipment 5.7.1 Non Explosive Demolition Agent Non Explosive Demolition Agent (NEDA) is a static demolition agent, commercially available. These are also referred to as expansive grouts. When the reaction takes place in a confined drill hole, the NEDA generates an expansive pressure to crack and break concrete and stone. The NEDA is a suitable application in a restrictive environment where noise, flying debris and vibration are less tolerated. A drilling pattern shall first be designed. For large projects, test breaking shall be performed. The NEDA shall be mixed with water to form slurry and immediately placed into the pre-drilled holes. The loading intensity and water content shall be controlled to optimise the expansive pressure and prevent blow-out of the NEDA. The breaking effect of NEDA is relatively small comparing to explosives. Secondary efforts are required to further break down and remove the debris by mechanical means.

NEDA may be used on foundations works, pile caps or structures that are fully supported. When used in rock, NEDA should be contained within strong, flexible, impermeable bags to prevent uncontrolled entry into rock joints. 5.7.2 Saw Cutting Saw cutting is suitable for alteration and additional works where accuracy in the cutting is important and the tolerance to noise and vibration is very limited. It can be used to

cut concrete slabs and wall elements into segments. An entire building may be dismantled by saw cutting. Saw cutting generally includes conventional disc saw and chain saw, diamond core stitch drilling and wire saw. Wire saw cutting comprises a special steel wire often impregnated with diamond beads to increase its cutting ability. The wire saw method is a suitable application for projects that require precision and total control of demolition work. A hole shall first be pre-drilled for the passage of the diamond wire, and then the wire cutting operation follows. Because of its flexibility, it may be used for “hard to reach” areas. A diamond wire saw may also be applied in cutting off piling of marine structures and bridges. Its flexibility and range of application are depicted in Figure 5.12.

Fig 5.12 Wire Saw technique

Disc saw (Fig 5.13) is a toothed circular metallic disc used for cutting concrete. Circular saw blades especially with diamonds would be preferred for the task of concrete cutting. Unlike other type of cutting tools, circular saw doesn’t get struck into the material if set too deep. The blades y provide cutting in a straight line.

Fig. 5.13 Disk saw

Diamond core stitch drilling (Fig 5.14) may be adopted to cut concrete elements by continuously coring a set of holes to carve up the concrete structure. The thickness of the concrete to be cut depends on the depth of the drilling or coring equipment. Diamond core stitch drilling is particularly suitable in the removal of existing pile cap for construction of large diameter bored pile foundation.

Fig. 5.14 Diamond core stitch drilling The sawing and drilling operations require large amounts of water to cool down the blade which cuts through the concrete at high speed. Provision shall be made to provide a water source for the operation and for the disposal of the cooling water. 5.7.3 Cutting and Lifting Cutting and lifting involve the initial cutting of the structure into individual pieces or segments, and then lifting the pieces or assembly by crane onto the ground for further demolition or hauling away. Slabs can be cut into segments and then lifted off for further cutting into smaller pieces before disposal. Precast concrete structures can be cut into pieces and then lifted off as a reversal of the construction sequence when the precast elements are fabricated from pieces into an assembly of structure. Cutting and lifting may be applied to safely remove projections such as canopies, architectural features, balconies and bay windows. The typical procedures for cutting and lifting are summarised in the following:

a) Prior to cutting, the stability of the remaining structure shall be checked. b) The structural element to be removed shall be secured, either by temporary

supports or by tie wires connected to lifting appliances. The lifting appliances must have adequate capacity to support the weight of the structural section and

c) After cutting with disc saw, chain saw or diamond wire saw, the structural element shall be lowered to the designated area in a controlled manner. Free falling shall be avoided.

5.7.4 Mechanical Demolition from outside Mechanical demolition from outside involves the use of large machinery with attachment to dismantle the building. The common mechanical methods include the use of a pusher arm, wire rope and clam shell. These methods shall only be applied to isolated buildings on relatively flat ground. The concerns and good practices of the mechanical demolition generally included the following:

a) The machine shall be operated on smooth and firm ground. It shall also have adequate counter-weight to prevent overturning during the operation.

b) A minimum safety distance, depending upon on the height of the building element being demolished, shall be maintained between the machine and the building being demolished.

c) The impact of the collapsed structural sections on the floor or ground shall be checked to prevent the potential overloading of the suspended floor, vibration and disturbance to adjacent properties and damage to underground utilities.

d) The site shall have full time security to prevent unauthorized personnel

entering the site. No person shall stay within the working area of the machine and the building while the machine is operating.

e) Sufficient water spray or other anti-dust precautions shall be provided to minimise air pollution by dust.

f) The machine cab shall be equipped with impact proofed glass and its shall be robust enough to protect the operator from flying debris.

The common mechanical methods used are Pusher Arm, Clam Shell and Wire Rope method. Other methods include demolition by Impact Hammer, Hydraulic Shear, Pulverizer etc. High reach machines fitted with suitable attachments and tower cranes may also be considered.

(i) Demolition by Pusher Arm involves the the progressive demolition of a structure using machines equipped with a pusher arm attachment for applying horizontal thrust to progressively demolish the structural element, either by pushing into the structure or pulling out of the structure. The height of the structure should be reduced progressively by pushing over small sections. The point where the pusher arm is applied to the wall being demolished should be at an appropriate distance below the top of the wall.

(ii) Demolition by Wire Rope pulling generally involves the use of an earth mover machine or mechanical winch device equipped with heavy steel wire for pulling down structural members. Wire Rope pulling should not be used on brick or masonry structures. The method should only be used on structures where it is possible to safely attach the ropes. Restraining ropes shall be provided to prevent premature collapse before the pull commences, from before pre-weakening until it has been completed.

(iii) Demolition by Clam Shell typically involves the use of a crane equipped with a Clam Shell attachment which progressively bites away the structure.

Pusher Arm – pushing in Pusher Arm – pulling out

Clam Shell Wire Rope pulling

Fig 5.15 Mechanical Methods

(iv) Demolition by Impact Hammer involves the progressive demolition of masonry and concrete structures by applying heavy blows to a point in contact with the material, and may be pneumatically or hydraulically operated. When impact hammers are working on upper floors, regular inspections of the supporting floors, the areas around these and any temporary works should be made from places of safety to ensure that there is no deterioration in stability due to vibration from its operation. Access to floors beneath, other than for those inspections, should be prohibited during these activities. If it is necessary to cut steel reinforcement, remote means should be considered. The reinforcement should be cut so that it does not spring and injure operators during the operation. Impact hammers should not be used to demolish tall vertical features such as walls or columns from the side, where there is the possibility of debris falling onto the machine or the operatives.

(v) Demolition by Hydraulic Shears involves cold cutting of metal and reinforced concrete sections by cutting and severing material using shear jaws. Shears attachments can be rigidly mounted to the machine or be able to rotate to provide increased working versatility for cutting. Machines fitted with hydraulic shears should be considered for use where a wide range of materials, including metal sections and reinforced concrete are to be removed by cold cutting methods, and where materials are to be cut in situ. This method should also be considered for the processing of materials at ground level.

(vi) Mechanical demolition by a machine mounted Pulverizer is the progressive demolition of reinforced concrete or brick structures by crushing the material with a powerful jaw action by closing the moving jaw(s) against the material. The pulverizer attachment should be considered for crushing beams, columns, floor slabs and panels either in-situ or as a subsequent processing operation, when reinforcing bars can also be separated. It should also be considered as an option for the lifting and loading of steel and concrete beams and other solid materials.

Hydraulic Shears Pulverizer Grapple

(vii) A grapple is designed for use in primary demolition and rehandling applications for, e.g. steel and concrete beams, columns, walls and floor sections, and roof joists progressively to ground level. The jaws interlock to enable partial loads to be safely secured. The parallel-jaw closing action ensures that material is drawn into alignment during the dismantling, lifting and loading cycle as appropriate.

5.7.5 Thermal Lance A Thermal Lance is a steel pipe packed with mixed metal wires. Pure oxygen gas is passed through the pipe from an oxygen cylinder and regulator. The end of the pipe is lit with a high temperature source, e.g. an oxy-acetylene torch. The iron in the steel burns in the oxygen coming down the pipe to produce enormous heat and a liquid slag of iron oxides and other materials, which dribbles and splashes out. The temperature reached in the centre of the combustion zone is approx. 4000 degrees Celsius, greater than the melting point of any substance on earth. Cutting of reinforced concrete by thermal lance involves very high temperature up to 2,000 - 4,000°C. The extremely high heat requires special precautionary measures and care. The use of a thermal lance in cutting reinforced concrete shall not be used unless the project demonstrated that there is no other viable alternative. Adequate protective measures shall be provided to isolate the operation and to prevent any potential fire spreading out, and to prevent the injury of the workers, and any third party by flame and the molten concrete.

Cutting of concrete by thermal lance

5.7.6 Water Jet Water jetting involves the use of a water jet stream pumped at high pressure to erode the cement matrix and wash out the aggregates. This is also known as Hydrodemolition. Abrasive compounds may be added for cutting reinforcing steel. Provision shall be made to dispose the water used in the operation, and to recycle the water for continuous operation through local filtration and sedimentation. The area behind the structural member to be cut shall be shielded to avoid damage to persons and properties during the cutting. In the case when abrasive water jets are used, further precautionary measures shall be provided in accordance with manufacturer recommendations to confine the rebound of the abrasive compounds. All site personnel shall wear adequate safety cover and clothing.

Water Jet cutting 5.7.7 Micro-blasting In micro-blasting, the same concept as in dynamite is used, but in a smaller, more

controlled manner. Small diameter holes are drilled on the surface to be blasted and cleaned out with air to remove all dust that interferes with the firing operation. Cartridges of a smokeless powder are placed in the holes and the blast is initiated with a firing mechanism. The blast is large enough to break thick concrete, but results in little flying debris, which can be controlled by placing carpet over the blast area.

Micro-blasting

6. SPECIAL STRUCTURES 6.1 Precast Concrete Structures Precast concrete structures are constructed of precast concrete elements joined together. The continuity of the structure depends on the treatment of joints. The joint details shall be studied. In case of doubt, open up inspection at critical positions may be required. 6.1.2 Simple Precast Construction In simple construction, the joints do not normally provide continuity. The stability of this type of structure relies on other elements such as stairs, lift shafts, shear walls or other framed structures. Each precast element shall be removed in the reverse order of construction and broken on the ground or an adequately supported floor. Elements providing lateral stability shall not be demolished prior to the removal of the precast elements or prior to the installation of the temporary bracing. Temporary supports shall be adequately braced or tied to laterally stable elements. The re-use of the existing lifting points or accessories to lift the precast elements shall not be allowed unless the erection plans showing the function of the existing lifting points are checked and verified to be adequate for current use. Special consideration shall be given to long span precast elements with narrow compression flanges during lifting. Spreader beams shall be used to reduce the spacing of the lifting points. The use of spreader beam is illustrated in the figure below.

Spreader beam to reduce span

6.1.3 Continuous Precast Construction In this type of structure, the precast elements have continuity at their joints and the lateral stability is provided by the precast elements themselves. The continuous precast elements may be in the form of shear walls or moment resisting frames. It is possible that a combination of the simple construction and continuous construction exist in a single structure. The demolition of this type of structure may be performed in a way similar to that of a cast-in-place concrete construction provided that the continuous joints are cut in such a way that the lateral stability is maintained. If the precast elements are intended to be removed in a piece by piece manner in their reverse order of construction, the continuous joint shall be cut by appropriate pre-approved method such as saw cutting. The precast elements shall then be lifted off their support and lowered to the ground or to an adequately supported floor for demolition. Temporary bracing during lifting may be required. The position of reinforcement needs to be considered in fixing the points of lift. 6.2 Prestressed Concrete Structures The prestressed concrete structures are constructed of either precast or cast-in-place concrete in which prestressing is introduced to the concrete by tensioning the steel reinforcement, or tendon, to counteract a desired degree of stress resulting from a given external loading. Depending on the method of stress transfer, the structures are classified as pre-tensioned or post-tensioned, and whether it is grouted. Due to the high energy stored in the prestressed members, the demolition of such members must be proceeded with in a planned sequence and well controlled manner. During detensioning of the tendons, a protective screen made of sand bags or similar material such as a backed plywood screen shall be placed at the anchor ends. The prestressed concrete floor system shall be properly shored prior to detensioning to prevent the collapse of the system. The release of energy during the demolition of prestressed concrete could be extremely hazardous. All workers on site must be informed of the presence of prestressing in the structure and the hazardous result on deviating from the prescribed procedures. A pre-determined safety plan shall be in place. For a structure with bonded construction, the conditions of the grout shall be checked. If the tendons are not fully grouted, additional grout shall be applied to fully fill the ungrouted voids. After grouting, the prestressed structure may be demolished similar to that of a bonded construction. The following procedures for each class and category of prestressed concrete shall only be used as a guideline. Detailed procedures shall be independently developed for each structure by an engineer experienced in prestressed construction based on the design, layout of the tendons, sequence of the stressing and construction. (a) Precast Pre-Tensioned Structures

Precast pre-tensioned structures are typically single span elements and must generally be demolished in the reverse order of construction. The precast pre-tensioned elements can be lifted off and turned on their sides, after the connections at the supports are removed. The lifting points shall be located

near the ends of the units and shall be adequately designed to ensure safe lifting of the precast elements with these elements turned on to their side. The above process will generally fracture the structure, causing a sudden release of energy. After the energy is released, the elements can then be cut or pulverised into pieces before they are hauled away. If turning the elements on their sides does not release the energy, a sand bag or other suitable screen shall be provided around the ends. The prestressed energy can be released by cutting away the concrete in front of the anchorage until the anchorage has been loosened.

(b) Precast Pre-Tensioned/Post-Tensioned Structures Sometimes two or more pieces of precast prestressed elements are continuously connected together at the supports by post-tensioning. This post-tensioning shall be detensioned according to the recommendations for demolition of post-tensioned structures. After the post-tension energy is released the remaining precast prestressed elements may be demolished in accordance with the procedures for pre-cast pre-tensioned elements.

(c) Precast Post-Tensioned Structures These precast elements shall be lifted off from their support and placed on their side if the prestressed tendons are of grouted construction. If the conduits are not fully grouted, the elements shall be placed level on the ground and the post-tensioning forces shall be released in accordance with the procedures for post-tensioned ungrouted members. Adequate protection must be provided at the ends of the elements in case the tendons shoot out at the ends. In general, cutting of unbonded tendons at mid-span will dampen the shoot off effect.

(d) Cast-in-situ Post-Tensioned Grouted These elements shall be demolished as precast elements. For a single span slab, the slab may be saw cut into segments and lifted off similarly to precast elements. For continuous spans, the prestress over the support shall be released prior to cutting the slab into segments. It must be noted that prestressing may be provided in two directions and the detailed procedures shall take this into account. For beam and slab, caution shall be exercised to avoid upward failure of the beam when the slab is removed. When detensioning of tendons is involved, all slab and beam spans shall be temporarily supported to prevent unintentional collapse of the structure.

(e) Cast-in-situ Post-Tensioned Ungrouted The demolition of these elements shall generally proceed by shoring up all slab and beam spans for which detensioning of tendons is required and removing all superimposed dead load. The prestressing forces can be released by cutting away the concrete in front of the anchorage until the anchorage has been loosened. Alternatively, the forces may be released by saw cutting at appropriate locations along the tendons. During the detensioning, the ends of the tendons shall be protected from shooting off. Then, the structure can be demolished as normal reinforced concrete.

(f) Cast-in-situ Post-Tensioned in Stages and Fully Grouted Care shall be exercised to avoid premature failure of the elements when the dead load superimposed on the elements is reduced as demolition progresses. The load which is carried by the element must be supported by temporary structures extended above the element. After the temporary supports are constructed, the elements may then be demolished by the following steps: i) Locate the profile of the tendons and mark it on both faces of the member; ii) Expose the exterior tendons on each face of the member midway between

the centrelines of all intermediate columns supporting the member; iii) Cut the exposed tendons at each location starting from the centre of the

member on alternating faces and proceed to the ends of the member; then iv) Repeat steps (ii) and (iii) until all the tendons have been completely severed. Demolition using the above procedures shall be exercised with caution to prevent the tendons from pulling together the columns at the ends of the elements due to the elastic shortening at the exposed tendons.

(g) Cast-in-situ Post-Tensioned in Stages but Ungrouted Care shall be exercised to avoid premature failure of the elements when the dead load superimposed on the elements is reduced as demolition progresses. Temporary structures shall be provided to shore up the elements as needed. The prestressed force shall be de-tensioned sequentially in reverse order of stressing in accordance with the amount of dead load removed. The sequence of the detensioning shall preferably be in the reverse order of the tensioning when the element was constructed. When all supporting dead load and tendons are removed, the element can be demolished in the same manner as for normal reinforced concrete.

(h) Segmental construction The segmental post-tensioned structures involve the construction of the main structural elements in segments. Their final integrity is achieved through post-tensioning of tendons which pass through and tie the segments together.

The demolition of segmental construction shall proceed in the reverse order of the segmental erection. Temporary supports shall be provided as required before the post-tensioning forces are released. Where the segmental units are pre-tensioned, demolition shall proceed as for precast pre-tensioned/post-tensioned construction. Where the units are not pre-tensioned, demolition shall proceed in the same manner as for post-tensioned construction. The demolition of segmental construction is relatively complex and it must be demolished under the guidance of a professional engineer experienced in this type of construction.

6.3 Steel Structures and trusses Steel structures are, in most cases, designed as “simple design” or “semi-rigid design” according to earlier structural steel design codes. Under such design assumptions, the detailing of the beam column joints is, in most cases, not rigid joints and the structure may become statically determinate during demolition or substantial alteration. Failure of any member in a statically determinate structure leads to a sudden collapse and hence

should be properly supported. Similar to conventional buildings, steel structures may be demolished by top down method, cut and lift or other methods that are adequate for the site condition. Structural steel members are generally designed as slender members subject to bending and/or compression. Except for concrete encased steel members, the Structural Engineer shall check the load resisting capacity of the slender structural members when lateral restraints are removed during demolition. Proper shoring shall be installed, if required. All structural steel members shall be lowered from the structure and shall not be allowed to drop. Trusses shall preferably be removed by lifting and lowering to ground level prior to demolition. In the case when the truss has to be dismantled on the spot, the stability of every partially dismantled configuration shall be checked. When dismantling roof trusses, enough purlins and bracing should be retained to ensure stability of the remaining roof trusses while each individual truss is removed progressively. Temporary bracing should be added where necessary to maintain stability. The end frame opposite to the end, where dismantling is commenced, or a convenient intermediate frame, should be independently and securely guyed in both directions before work starts. On no account should the bottom tie of roof trusses be cut until the principal rafters are prevented from making outward movement. 6.4 Masonry and brick arches Expert advice should be obtained and, at all stages of the demolition, the closest supervision should be given by persons fully experienced and conversant in the type of work to ensure that the structure is stable at all times. As much dead load as possible may be removed provided it does not interfere with the stability of the main arch rings. but it should be noted that the load-carrying capacity, of many old arches relies on the filling between the spandrels. On no account should the restraining influence of the abutments be removed before the dead load of the spandrel fill and the arch rings are removed. A single span arch can be demolished by hand by cutting narrow segments progressively from each springing parallel to the span of the arch, until the width of the arch has been reduced to a minimum which can then be collapsed. Where it is impossible to allow debris to fall to the ground below, centering designed to carry the load should be erected and the arch demolished progressively. The design of the centering should make appropriate allowance for impact. Where deliberate collapse is feasible the crown may be broken by the demolition ball and working progressively from edges to the centre. Collapse of the structure can also be effected in one action by the use of explosives. Charges should be inserted into boreholes drilled in both arch and abutments. This method is the most effective for demolition of tall viaducts. In multi-span arches before individual spans are removed, lateral restraint should be provided at the springing level. Demolition may then proceed as for a single span, care being taken to demolish the spandrels down to the springing line as the work proceeds. If one span is demolished, without proper restraint, large, unbalanced horizontal force comes on the pier leading to collapse of the pier followed by successive spans.

The normal sequence of demolition is as shown in the Figure below, namely: a) Divide the depth of soil in to 2 parts – part A from top of soil to level of

crown part B from level of crown to abutment level. b) Divide the width of arch into odd number of equal parts ( 7 in the figure). c) 4 parties shall simultaneously remove soil from outside. In first phase soil

shall be removed from A1,A2, A3 and A4. d) After completing A1,A2, A3 and A4, soil shall be removed from a5, A6, A7

and A8, then from A9, A10, A11 and A12, and then A13 and A14. e) Then, 4 parties will similarly remove the spandrel wall. f) Then, the arch portion is broken similarly upto mid-section section 13 and

14, which are broken by pulling down. g) Afterwards the piers are pulled down.

Elevation

Plan Demolition of Arch Bridge

Where explosives are used it is preferable to ensure the collapse of the whole structure in one operation to obviate the chance of leaving unstable portions standing.

6.5 Bridges Bridges should be demolished either by deliberate collapse methods or in the reverse order of construction using, e.g. cutting and lifting (or removing the entire deck as one unit). After removal or deliberate collapse, those parts of the structure should generally be broken up before removal from site, if required. Precautions should be taken to ensure that the collapsed structure does not present a hazard when being cut into smaller pieces. Any relevant additional temporary supports to help carry the dead load of the

structure and demolition loadings, including plant and equipment, should be in place before work commences, together with any support to aid lateral and longitudinal stability. Where any part of a bridge is post-tensioned, the recommendations of 6.2 should be applied. Engineering advice should be obtained for all stages of the demolition of bridges to ensure that the stability of the structure is maintained. The demolition should be closely supervised by persons fully experienced in, and conversant with, the type of work being undertaken. Work should be programmed in consultation with, and to the satisfaction of, the authorities responsible for the bridge and for the land below it. Temporary works should be designed to carry the required loads and temporary bracing should be incorporated into the bridge structure, where necessary, in order to maintain stability under the severest conditions. Before transverse members are removed, temporary supports should be fixed to the main beams or girders, if appropriate. Where the horizontal thrusts from the abutments are designed to be taken by the deck, these should be dealt with by, e.g. inserting temporary struts prior to removal of the deck. Deliberate collapse methods should be designed such that appropriate initial preparation work is carried out to aid efficient demolition. Explosives or mechanical methods of pre-weakening should be considered for initiating collapse of the structure. Where bridges have been constructed using a counterbalanced cantilever design, the demolition technique needs to ensure stability by taking into account considerations such as out-of-balance loadings and lack of fixity at supports by using, e.g. temporary supports. Where counterbalancing is used, the balancing materials should be securely and safely fixed. The bridge or elements of it should be transversely braced, if necessary. Where a bridge is continuous over one or more supports, consideration should be given to either cutting the deck above the piers to form a simply supported structure, or cutting the deck at mid-span to form statically determinate cantilevers, but taking into account the ability of the structure to maintain stability in these forms. For arch bridges, the provisions in 6.4 may be applied. Where tie rods are present, they should not be cut until the horizontal thrusts have been removed. 6.6 Structures Supporting Ground or Sitting on Slopes Demolition of buildings or structures supporting land or slopes, or buildings or structures sitting on slopes or retaining walls may affect the stability of adjacent buildings, structures and land and may even create regional slope instability due to removal of toe weight. Maintaining adequate ground support by backfilling or structural support during demolition work is important. The demolition plan should be properly engineered by a competent and experienced geotechnical engineer. Top down method is suitable for demolition of hillside slope structures. Other methods may be applicable depending on the actual site conditions. The following guidelines are generally applicable: (a) Buttress/Shoring for Building Supporting the ground

If part of the building structure serves as a retaining wall system, the height of the building that is required to be left in order to safely support the retaining structure shall be determined. Adequate shoring and/or buttress shall be provided prior to

the demolition of the remaining structure. A demolition plan shall be provided to the foundation contractor so that the shoring work installed during demolition is considered and protected during the foundation work.

(b) Retaining Wall System Prior to demolition of the retaining wall, the slope or land supported by the retaining wall system must first be stabilised. Stabilisation may be achieved by excavation of the soil behind the retaining wall to a free standing stable slope or by installing temporary or permanent support such as sheet piling or other appropriate methods.

(c) Surcharge on Slope and Retaining Wall No storage of debris or surcharge shall be imposed on the area behind or on the top of the retaining wall and/or slope. Surcharge on the top of the retaining wall and/or slope may affect its stability.

(d) Drainage The water table may affect the stability of the slope. Drainage from surface runoff, off-site drainage and infiltration shall be considered and managed. Existing subsoil slope drainage system should also be maintained.

7. COMPLETION OF WORKS On completion of the demolition work, the site should be cleared of all debris and be left in a safe and secure condition. Handling of waste and debris has already been dealt in paragraph 3.10. All pits, trenches, sumps or voids should be left filled, securely covered and marked, or guarded in a safe condition. The site drainage system should be thoroughly cleaned and tested to ensure that it continues to operate. Any walls that are to remain, in particular to act as the boundary to the site, should be checked to ensure that they remain stable under anticipated loadings. All contaminants should be removed or left in a condition such that they present no hazard to health or to the environment. Records of all contaminants, where they remain on site or whether they have been removed, should be identified. Where they remain, all details should be notified to those who will have the responsibility for the site so that they can, for instance, control entry to these areas. A report may be prepared on the precautions to be taken in the sight and handed over to the persons responsible for further managing the site. In addition, all service disconnections, drainage terminations, etc. done for the demolition work shall be restored. CASE STUDY – DEMOLITION BY IMPLOSION

In Western Railway, Mumbai division, there was a G+3 storied building, numbered 25/T, constructed in 1924 by BB & CI Railway as a first cement concrete residential structure in Mumbai. It was having 80 Nos. (4×20) type I quarters with a total land area about 430sqm. The total height of the building was 13.2m above rail level. The structure completed its designed life and the existing condition of the building was so deteriorated that there was no option, but to demolish the same for safety of running

trains and adjoining residents.

There were four running lines just adjacent to structure on east side, the face of building being 4.55m away from track centre. There was a OHE mast supporting the portal for five OHE live conductors at a distance of 5.80m from face of building towards north side. There was a stone masonry boundary wall at a distance of 2.80m from the face of building.

In addition to above mentioned railway assets, there was a 40 storied structure about 100m away from the building to be demolished and a diamond factory on the east side having an exterior glazed building.

Since the structure was very near to the running track, which requires continuous track protection / traffic block, resulting in long disruption to running traffic, the manual method of demolition was not considered suitable. The mechanical method was not suitable, as there was no space to bring heavy machinery near the structure to work at a height of 13.5m in view of the above, it was considered ‘implosion’, which brings the building down on its footprint in a very less time was considered suitable.

Holes for charging the explosive were drilled in the columns. 20 holes of 25mm diameter and 30cm in depth were provided in central column. It was reduced to 4 holes of 25mm diameter and 20cm depth for columns away from centre.

The quantity of explosive to be placed in the holes depends upon the size of column and extent to which it is to be destroyed. Dismantling of the central columns would bring the structure inwards. 250gm of explosive in central 24 columns and 125gm in outer 16 columns was placed. This would enable to destroy the central column completely with surrounding columns with lesser charges collapsing inwards under gravity. The detonators were placed to have a time delay between successive blasts in columns from centre towards the ends. All holes in the columns were covered with gunny bags and iron net after charging with explosives, to prevent debris from flying off.

The explosive used was of brand name ‘Power Gel‘, which is an ammonium nitrate based explosive which expands at very high speed and applies a very high pressure upon blasting. Electronic detonators were used to ignite the explosive.

Before carrying out the actual blasting, a test blast was conducted to ascertain the efficiency of explosive and detonators.

Before test blast After test blast

The implosion followed as planned and the structure came down on its footprint with no disturbance to the adjoining properties.

References: 1. Code of Practice for Demolition of Buildings, 2004, Buildings Department,

Hongkong 2. BS 6187:2000 – Code of Practice for Demolition 3. IS- 4130: 1991 – Demolition of Buildings – Code of Safety