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LITERATURE REVIEW CHAPTER 2

2.1 GENERAL

The geometrical properties of reinforced concrete members vary many a times. This

variability is a consequence of inaccuracies in construction. In some cases the

variability is of a more systematic type but most frequently it is random. These

variations must be considered when dealing with structural safety aspects because

they could present major uncertainties in a structure. The geometrical variations of

reinforced concrete members can also greatly influence the cost of construction. In

this chapter an extensive review of the literature connected with several aspects, such

as construction errors, tolerances, deterioration of structures, structural safety and

reliability aspects, is presented.

2.2 SURVEY OF CONSTRUCTION ERRORS

Codes of practice are formulated to provide guidelines on various aspects of analysis

and design and to set minimum standards of safety that are consistent with economy.

The earlier studies on the compliance of code specifications in construction practice

revealed considerable deviation from the specified or intended practices.

Rao and others (48) in their investigations conducted an extensive survey on

several aspects of reinforced concrete construction. Their survey pertains mainly to

the construction practices and to its comparison with the IS code specifications. The

results presented pertain to two storeyed hostel block, a housing colony, and the

structures of an institution. The results of their survey indicate a wide chasm between

codal specification and actual practices. They suggested that some of the

specifications of the codes be revised and tolerances are included for some of the

parameters. The limited data presented in this article suggested that site supervision

leaves much to desired at least at the most common sites.

Morgan and others (38) investigated into pre pour placement of reinforcement

in rectangular slabs and its compliance with relevant code requirements in Australia.

They observed that the current tolerances on fixing reinforcement were not achieved

on any of the sites investigated. The variations in bar placement were found to have

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no relation to slab panel size or thickness, or to bar size and layout, for the range of

slab type and layout encountered. It was mentioned that within a site, consistent

workmanship can be expected although the investigation did reveal in-homogeneity

of bar placement data within two sites. There were good and bad sites in each state

investigated. The variations in bar placement were found to be normally distributed.

There was a relation between the height of bar chairs and the mean bar position.

Spacing of bar chairs for all sites was observed to be in accordance with the

recommended one meter spacing. The results for the bottom cover of pre-cast panels

were not significantly better than those for the other sites with in-situ panels.

Robert (59) studied the distribution of bar placement errors and illustrated the

influence of this error on column strength. Studies were made on bar locations in

232 columns from 12 existing buildings and it was shown that placement errors

often exceed specified tolerances. In no case were bars found to be missing but

placement error greater than 125mm were encountered. Strength calculations based

on actual locations of reinforcement showed individual reductions of strength

greater than 15 percent and for all columns a mean reduction of nearly 5 percent for

high effective eccentricities. Probability models to describe the distribution of

reinforcing placement error were derived to facilitate reliability analysis.

Pfrang (46) studied the effect on column strength of varying the elements of

the column cross section. He found that an increase in cover ratio, from 0.05 to 0.15

led to a decrease in strength about 10 percent for balanced load conditions where 4

percent reinforcing was concentrated in exterior layers.

Roger Hauser (60) made a review of an investigation on about 800 European

failures which focuses the efforts of code writing bodies to the most efficient way to

maintain a given level of structural safety. It was found that the structure, itself

initiates most failures due to unfavorable influences of the natural environment and

incorrect introduced factors either in the planning or the construction phase. Error in

the planning phase occurs mainly in conceptual work or during structural analysis.

The errors in construction stage are due to insufficient knowledge or ignorance of the

site personnel. It was mentioned that only very few errors are unavoidable and in

majority of the cases a little additional checking helps considerable.

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Ashraf M. EI-Shahhat et.al (3) investigated the safety of multistory buildings

during construction. The safety of the structure, in these early days of its life is greatly

influenced by a large no of factors including the loads, the geometry, and the material

properties of the building and the method of construction. The probability of failure

of the building during its relatively short period of construction is greater than that of

its service life. A parametric study was performed to investigate the effect of concrete

strength, spacing of forms and the construction cycle length on the construction

safety. From the results, the probability of failure of multistory concrete buildings

with the assumed system of construction using two levels of shores and one level of

reshores ranges from about 4.3 to 13.4 percent. It was mentioned that the effect of

human factors and errors during construction on the limit state probabilities should be

the focus of future studies.

Mirza et.al (64) studied the variability of strength and stiffness of normal

weight structural concrete and suggested representative distributions for use in

estimating the effect of these variations on the strength of reinforced concrete

elements. This paper was based on data obtained from a number of published and

unpublished sources and involves no additional laboratory tests.

Mirja and MacGregor (62) have reviewed the data on dimensional variations

of in-situ and precast concrete members and suggested normal distribution for various

parameters for use in estimating the effect of variations on the strength of members.

Their work is based on data obtained from a number of published sources

Ranganathan and Joshi (53) presented the collection of field data on variations

in dimensions of RCC members and statistical analysis of the same. In order to get

the field data on effective depth of slabs and beams, a few slabs and beams were

cast near the laboratory in the field simulated conditions and measurement were taken

after chipping the concrete at randomly selected points. It was observed from the

collected date that the mean deviations of (i) effective depth of slab (ii) width and

effective depth of beam (iii) width and depth of column section and (iv) length of

footing in plan lie within the limits of tolerance specified by the code. However, in

the case of depth of beam, breadth of footing and cover of reinforcing bars in column,

it was found that on average, specified tolerance was not satisfied.

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It was also found that all variables of geometrical parameters follow normal

distribution. The coefficient of variation of dimension of RC member increases with

decrease in nominal size and it is equal to 4.9/nominal size of the member in mm. The

reported data is based on field simulated conditions and hence its value is limited.

Narasimha Rao D.L. et al (42) conducted tests on straight reinforced concrete

columns with initial curvature in the main reinforcement. It was observed that the

reduction of the ultimate load of a column due to dislocation of the main

reinforcement is directly proportional to the amount of dislocation. The percentage

reduction in the ultimate load for a maximum dislocation of 12.5 mm in a 180 mm

square column in about 3.2 percent and for a 50mm dislocation it was 12 percent. In

most field work the dislocation of the main reinforcement depends upon the length

and size of the column, the diameter of the reinforcement bars, the type of form work

and supports, the workability of concrete, and the methods of placing of concrete. In

ordinary circumstances the maximum dislocation may be of the order of one to two

percent of the height of the column. In such cases the percentage reduction will be

less than six percent. For long columns the effect may be worse.

Broms and Leroy (9) investigated the effects of arrangement of reinforcement

on crack width and spacing of reinforced concrete members. The spacing and width of

surface and internal cracks were investigated for long and short tensile members

reinforced with one and several reinforcing bars. The average crack spacing Save was

found to increase approximately linearly with increasing distance from reinforcement

as predicted by the equation Save = 2 tc where tc is an effective cover thickness.

Measurements of internal and surface crack widths indicate that the average width

Wave can be predicted by the equation; Wave = 2tc Es where Es is the average steel

strain.

The test data indicated that the average crack width depends primarily on the

distance from the reinforcement(cover) and on the average steel strain, and the

average crack width is independent of the steel percentage, and therefore of the size

of the concrete member.

Investigation of the internal crack pattern indicates that the number of cracks

decreases with increasing distance from the reinforcement and that the crack width is

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small close to the reinforcement. Measurements of the end deformations of short

tension members confirm that the width of the main primary cracks, those which

penetrate to the surface of the member increases with increasing distance from the

reinforcement.

Ranganathan and Dayaratnam (51) in their article presented the statistical

analysis of typical office building floor loads. The results of the statistical analysis

of the loads, and strength of concrete and steel were used in probabilistic analysis.

The mean value of the floor load was observed small compared to the minimum

values specified by IS code. But the coefficient of variation of load was observed high

and it affects the probability of failure of the beam at different limit states. The

frequency distribution of floor load in office rooms was found to be lognormal

with 5 percent significance level. It was suggested that an extensive survey on floor

loads in similar building may be carried out to arrive at rational values with certain

confidence level for characteristic loads.

Drysdale (21) in their investigation pulse velocities were measured through

1145 concrete columns in 15 structures located in Hamilton Toronto area of Canada

to indicate the variability of concrete strength. The information on variability of

concrete strength is intended to assist in the evaluation of the strength of existing

buildings.

It was concluded that poor-quality concrete can be located easily by use of

ultra sonic pulse-velocity measurements where low readings simply indicate low

strength. An approximate evaluation of concrete strength variation in a building

can be obtained from pulse-velocity readings without performing a laboratory

calibration for the concrete in question. Accurate calculation of concrete strength

from pulse-velocity measurements must be based on calibration curves obtained

from combined pulse-velocity and strength testing of representative specimen. The

coefficient of variability of concrete strength for the columns in a building usually

falls in the range from 0.10 to 0.20 but has been found to be as high as 0.30. These

values agree with previous estimate of strength variability.

In general it was suggested that pulse-velocity testing during or immediately

after construction could be an efficient and effective method of inspection. As an

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alternative to the standard strength tests, this procedure is far more likely to detect low

strength portions of the structure and thus ensure greater structural safety and

economy through immediate correction. It may be appropriate that a random sample

which will provide information on quality control can be used to define the extent of

testing required.

John Fraczek (33) presented the details of a survey conducted by ACI

Committee 348 in which 277 cases of errors in concrete structures were reported. The

survey indicated that about three-quarters of the errors were actually detected by

the structure with 39 cases of collapse and 172 cases of distress, cracking, spalling,

leakage, settlement, deflection or rotation reported. About one-half of the errors

originated in the design and the other half occurred during construction, with each

phase responsible for about the same number of collapses. Of the errors due to

faulty construction nearly three quarters were detected during construction and

over one-half resulted in failure or distress. Most design errors were detected

during occupancy and most resulted in serviceability problems. The survey only

reported 11 errors detected prior to construction, with about 60 percent detected

during construction and the remaining 40 percent detected during occupancy. It was

recommended that a new survey be conducted to collect further information on the

cause and prevention of structural failures.

Tso and Zelman (74) in their paper reported on the investigation of variation

of concrete strength in 403 columns in 10 buildings. A statistical analysis was done

on the field data and a correlation was attempted between the measured pulse

velocities and the cylinder strengths obtained from concrete samples taken when the

buildings were built. The ultrasonic testing method was used. The conclusions

indicated that statistical analysis of the pulse velocity data serves as a useful

indication of the construction workmanship. In conjunction with some standards it is

possible to assess the degree of consistency of workmanship at site condition.

Constructional tolerance analysis indicated that more care should be paid to the

dimensions of the form work at the top of the columns. There is a definite tendency

for pulse velocities at the top of the column being less than at the bottom, which

implies that the concrete at the top of the column is weaker than in the bottom.

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2.3 CONSTRUTION TOLERANCES

Dimensions of RCC members may differ from the specified values. There may be

deviations from the specified values of the cross section shape and dimensions which

may be due to size and shape and quality of form work and concreting and vibrating

operations. Variations also occur in the effective depth of members. The actual

effective depth available may be different from the specified values because of

improper placement of reinforcing steel bars, not providing proper cover blocks and

change in values when needle vibrator are used during casting of members. Amount

of variation in dimensions varies from place to place and structure to structure

depending on quality of construction techniques and the training of the site personnel.

Tolerances are provided in code specifications for the variations in the actual

construction. Earlier studies on construction practices indicate that there is lot of

deviations from codal specifications.

Rao in his studies (47) observed the inconsistencies between the codes of

different countries. Some of the mundane aspects of detailing such as diameter of

hooks, anchorage lengths, concrete cover and corner reinforcement in slabs are

discussed. It was pointed out that there is a need to bring consistency and uniformity

between the codes of practice of various countries on one hand, and rationalise the

specifications to make them practicable on the other. Further it was mentioned that the

rational design specifications on temperature effects are still lacking despite the

evidence of distress to structures when these aspects are ignored. The need for proper

cover specifications and tolerances and for their implementation is emphasised. There

is a need to formulate specifications for bar supports as well, in order to ensure the

required concrete cover. There is further need to study the influence of several design

parameters on structural performance.

Morgan and others (39) in their study reviewed the provisions of the

interacting Australian codes controlling concrete slab tolerances. A simplified

model was presented to indicate the possible variability of top and bottom bar

locations. The interacting factors contributing to the physical tolerances of RC slabs,

such as manufacturing tolerances, fabrication tolerances and construction tolerances

were combined to produce an estimate of possible overall variability. It was seen that

Australian practice cannot achieve the present concrete cover to reinforcement and

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effective depth requirements. Revised proposals were formulated for code changes

permitting a better correlation between specification, codes and actual practices. It

was observed reinforcement chairs play an important part in the pre-pour and post-

pour position of bars in RC elements. It was recommended that the Standard

Association give consideration to the publication of a code for reinforcing bar chairs.

Scanlon (61) in his article discussed a number of parameters relating to

serviceability failures, particularly those involving excessive slab deflections.

Problems associated with excessive slab deflections, causes of excessive deflections

and building code provisions were reviewed. Interpretation of building code

requirements was also discussed in considerable detail. Excessive deflections can be

attributed to a combination of several contributing factors which include design,

construction, materials, environmental conditions, and change in occupancy use. It

was mentioned that construction errors affecting deflections under service load

include, slab thickness smaller than specified, improper placement of

reinforcement, particularly top bars placed too low. Improper concrete placement

including inadequate vibration, lack of heat in freezing temperatures, and inadequate

protection during hot weather also affects the deflection. Suggestions were made for

evaluation procedures that can be used for investigating slab systems that exhibit

deflection problems.

Connolly and Brown (32) was made a pilot study to develop the technique

for non-destructive measurement of variations in depth, width and location of

reinforcing steel in existing reinforced beams and joists. The results of their study

revealed that current as built structures normally exceed the design assumed

tolerances. They concluded that relatively simple, inexpensive, and non-destructive

methods exists for determining both over all dimension and bar placements in as

built reinforced concrete structures.

Also beam dimensions and bar locations can conveniently be described by

well-known probability models which reflect the inherent random nature of the

parameters used in design of concrete structures. For both top bars and bottom bars

observed depth of cover tends to be normally distributed with a mean value equal to

that specified by design.

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Too much cover and too little cover appear to be equally likely, with small

deviations from design cover being more likely than large ones there is a 50

percent chance that cover will be less than that specified. Based on the probability

models presented, the combined effect of variability in bar placement and beam

dimensions was to reduce the ultimate moment by about 4 percent. They

recommended that advancement of the state of the art in reinforced concrete design

with respect to the question of dimensional tolerances requires that this variability

be taken in to account as codes specification and design practice seldom

explicitly recognize the inherent random nature of various design parameters.

2.4 DETERIORATION OF CONCRETE STRUCTURES

Concrete being inherently durable the structure made of it usually requires minimal,

but definite maintenance and repair during its life span. Corrosion of steel bars

leading to concrete cracking and spalling is the most recurrent and damaging cause of

concrete deterioration. Remedial treatment of a concrete structure damaged by steel

corrosion may be effected in various ways from casual patching up to long lasting

preventive repair. A repair strategy is to be formed which satisfies the protection

requirements of the structure and sites the limited resources of the owner.

Chung (12) in his article discussed the factors affecting the repair strategy,

particular life of the repaired structure and the “trouble free period" of the repair

option adopted. A systematic approach was suggested for deriving an appropriate

strategy for a repair project.

Hadipriono (27) in his study analysed the events in recent structural failures.

A study of nearly 150 recent major collapses and distresses of structures around the

world discloses the external events and deficiencies in the areas of construction and

design to be the principal sources of failures. More than one-third of the surveyed

structures were bridges and the remaining were low-rise, multistory, plant industrial,

and long-span buildings. The events causing these failures were categorized as

deficiencies in six areas, structural design; design detailing, construction, maintenance

of the structure, material and construction of external events. Almost a third of the

total number of low-rise building failures was caused by construction deficiencies,

arising from false work and concreting problems. In addition, quality control during

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concreting processes was not sufficiently enforced, concrete mixing was often

conducted by means of shovels, and concrete was inadequately cured. As a result,

poor quality concrete was frequently produced.

Beeby (6) attempted to produce a general survey of the factors which

influence corrosion of reinforcement, in concrete. The three main factors that

influence corrosion were crack width, cover and mix proportion. It has been shown

that crack widths have little influence on corrosion, and major parameters

controlling corrosion are cover and concrete quality. It was pointed out that many

design recommendations require unnecessary detailed calculations for crack control

as a corrosion control measure. It has also suggested a possible approach to a more

logical method of design for corrosion than the current arbitrary prescriptions. These

observations were made by reference to published date from exposure tests carried out

in many countries.

Baweja et al (4) examined the long term performance of plain and blended

cement concretes for ten individual structures. They observed that the major factor

influencing concrete performance adversely was the low initial specified strengths.

They found that chloride ion concentration profiled with in the first 50mm of the

surfaces of the wharf structures and the slab on grade structures considered showed

these to peak at around 20mm below the concrete surface. This could have

implications on reinforcement corrosion if bars were located in these regions.

Beeby (5) reviewed the evidence arising from exposure tests on reinforced

concrete members relating to the influence of cracking on corrosion. This evidence

gives no reason to conclude that any relationship exists between crack width and

corrosion. This result was confirmed by considerations of the chemistry of corrosion

of steel in concrete and the physical nature of cracks. This study was limited to the

consideration of the corrosion of reinforcing bars in structures exposed to the

atmosphere. Pre-stressed concrete and submerged structures are not considered.

A structure, however well it is designed and constructed requires periodic

maintenance. The maintenance and repair of concrete surface may be

necessitated due to any one of several causes, such as the effects of normal wear and

tear, stresses induced by abnormal differential temperatures, inadvertent errors in

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design, detailing and construction, exposure to aggressive environments like fire

and earthquake, etc.

Datta and Aggrwal (15) in their paper outlines the agencies and

processes which cause the deterioration of concrete surfaces and describes

methods of maintenance and repairs including removal of stains. Development of

cracks in buildings results in loss of strength and stability, causes rain penetration,

decreases sound insulation and affects aesthetics and overall efficiency.

Suresh Chand (72) in his paper mentioned the task of selecting causes of

cracks and suggested remedial measures to combat the situation. Methods of repair

and precautions to be taken while repairing the cracks have also been described.

Sirivivatnanon (66) in his paper presented a review of durability problems in

reinforced concrete structures caused by lack of sufficient concrete cover and a

statistical concept to analyses and to quantify in situ concrete cover in buildings.

Cover data of large number of buildings in Australia and Japan were analysed. It was

found that the level of confidence (LOC) for achieving minimum concrete cover for

durability were poor, with less than 50 percent of the structures achieving a 90%

LOC. It was suggested that an LOC of 90% could be achieved with improvements in

design detailing, selection of suitable spaces and good installation practice. The

correct choice of the concrete type, cover thickness and good concreting practice,

could prove to be the most economical way of achieving the design service life of

concrete structures.

Subramanian (71) in his article described about the causes of failure of the

Congress Hall, Berlin. It was observed that the collapse of the 'pregnant oyster'

was mainly due to the mistakes in the planning and execution of the roof structure,

which lead to the corrosion and finally to the failure of tensioning elements. The

failure of this building demonstrates that the long term effects, if ignored will lead to

the failure of the structure. The lessons learnt from this failure are of immense value

to structural engineers.

Suresh Chand (73) identified the factors responsible for the development of

cracks in different type of buildings and suggested suitable preventive measures

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for cracks from planning and designing stages to actual construction sites with a view

to minimizing development of serious cracking.

Blockley (8) assessed the various parameters for 23 major structural

accidents and one existing structure and were analysed using a simple numerical

interpretation. He observes that human errors of one form or another were the

dominant reasons for the failures considered. Parameters for 23 major structural

accidents showed that failures were due to a variety of causes and combination of

circumstances. However, human error, in using existing technology was the

predominant overall factor in the accidents considered. Insufficient research and

development in formation and the resulting uncertainty surrounding design and

construction decisions was also a major factor in the failures considered.

2.5 STRUCTURAL SAFETY

Leonardt (36) presented simple design rules to control cracking in concrete structures.

Causes of checking and its effect in serviceability and durability are discussed. The

paper was primarily applicable to large structures such as bridges. However general

concepts presented are applicable to any concrete structure.

Ang and Cornell (2) stressed that concept and methods of probability are the

proper bases for the evaluation of structural safety, performance and the development

of design criteria. Then only the effects of different sources of uncertainties can be

combined and analysed systematically in a manner suitable for quantitative

assessment of safety and performance. They identify the minimum information

required for the evaluation of safety as mean value and coefficient of variation of each

design variable. The authors emphasised that the lack of (statistical) data is no ground

for rejecting the probabilistic basis of design.

Costello and Chu (14) in their study demonstrated that when statistical data are

available, probability theory can be rigorously applied to problems in the safety of

reinforced concrete structures. It was also shown that if material strengths were

considered to be random variables, their variation should be between a non zero

lower limit and a finite upper limit. The failure probabilities derived in their article

are really conditional since sources of uncertainty other than concrete and steel

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strengths have not been considered. Future investigation into the variability of

resistance, including the effects of uncertainties in overall dimensions and steel

location would be useful and are contemplated.

Stewart (70) in his study reported the effects of human error on a typical

structural engineering design task. This was implemented by the development of a

mathematical model based on Probabilistic Risk Analysis (PRA) techniques. The

effects of human error are measured in terms of structural reliability. Mathematical

models for two realistic quality management procedures are outlined, and the optimal

procedure presented. The selection of the optimal procedure indicated that preventive

measures may be more cost effective than control measures.

A progressive collapse is a chain reaction of failures following damage to a

relatively small portion of a structure. The resulting structural damage

characteristically is out of proportion to the damage which initiated the collapse.

Since progressive collapse constitutes an unacceptable hazard in many buildings,

methods for its control should be incorporated in building standards. Design strategies

for reducing the risk of progressive collapse was described by Ellingwood and

Leyendecker (25) based on reducing the risk of initial failure and controlling the

damage when localized failure occurs.

2.6 RELIABILITY

Reliability analysis forms the basis of the current engineering standards for safety

evaluation. There is a universal need to balance safety, economy and serviceability in

the design of a structure. Among these, safety is of paramount importance to the user.

However some degree of unsafe or undesirable performance has to be accepted as

absolute safety requires enormous amount of resources. The probabilistic approach to

structural safety in civil engineering has been the subject of extensive studies in recent

years. Methods of reliability analysis and design are well developed. Some of the

significant developments in these areas are covered in this literature review briefly.

The basic concept of structural reliability analysis was first outlined by

Freudenthal(30). He analysed the safety factor in engineering structures in order to

establish its magnitude. The safety factor could refer to design only as an entity and

not to different features of it. He observed that individual safety factors for different

19

influences, such as for dead load or live load separately being inconsistent with the

basic concept, could have no real meaning.

Cornell (13) derived the bounds on the reliability of structural systems. His

upper bound calculation represents the case when there is perfect dependence among

the various load and resistance variables. Similarly the lower bound corresponds to

conditions of perfect independence among model resistances and successive loads.

A Monte Carlo technique was described by Warner and Kabila (75) which

can be used to find the cumulative distribution functions of stochastic variables such

as ultimate strength and factor of safety. Using this technique the variability in

strength of an axially loaded short reinforced concrete column was investigated and

the results were compared with closed form solution.

A level 2 reliability study was conducted by Iyengar and Srikanth (31) to

estimate the variations of strength of a reinforced concrete member in flexure and

shear and to check the partial safety factors for loads recommended in IS:456-1978.

The paper summarised the results obtained by Monte Carlo method.

Desayi and Balaji Rao (18) have analysed simply supported rectangular

doubly reinforced concrete beams, designed as per IS: 456-1978. Beams were

examined for limit state of strength in flexure.

Desayi and Balaji Rao (19) performed probabilistic analysis of cracking

moment from twenty two simply supported reinforced concrete beams. For the

assumed distributions authors found the cracking moment to follow a normal

distribution. An expression for characteristic cracking moment of reinforced concrete

beams for limit state of cracking was presented.

Ranganathan and Deshpande (54) developed a method to verify and analyse

the dominant mechanisms in the case of reinforced concrete frames. Under the

assumption that full moment redistribution might not takes place and any critical

regions might fail because of insufficient rotation capacity before a collapse

mechanism is formed. A reliability model for the rotation failure prior to mechanism

collapse was proposed. The safety margin equations in relation to and consistent with

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the mechanism were generated on the basis of the partial utilisation of moment

distribution at failure. The structural reliability was reassessed by combining

rotational failure modes with other possible mechanism modes.

The level of probability of a prestressed concrete section was evaluated by

Ranganathan and Dayaratnam (52) using the results of the statistical analysis of

strength of materials, geometric properties of section and load distributions. Monte

Carlo method was used to generate random strength of a section subject to certain

specifications. It was observed that normal and Type III extremal (smallest)

distributions satisfy the generated data for the resisting moment of the section for

under-reinforced and over-reinforced cases respectively at one percent level of

significance. Probability of failure of prestressed concrete beam sections designed by

Indian Standard specifications is in the order of 10-7for deterministic load and it

varies from 10-10 to 10-9for probability load.

Ellingwood and Galambos (24) presented probability based loading and

resistance criteria that are suitable for safety checking in design. The criteria were

based on a comprehensive analysis of statistical data on structural loads and

resistances and an examination of levels of reliability implied by the use of current

design standards and specifications. The criteria were intended to be used in

specifications that are oriented towards limit state design.

David Arul Rai & Ranganathan (16) in their study investigated the level of

reliability in stabs designed as per the code IS.456.1978: limit state approach.

The theoretical models for the resistance of slabs have been developed using yield

line theory. It was concluded that the grade of steel and effective depth are the two

main variables which affect the statistics of strength of RCC slabs significantly. The

slabs having steel grade Fe 250 for reinforcing bars have higher reliability compared

to slabs having Fe 415 bars. However this conclusion was based on the limit state of

collapse in flexure. This may have to be verified considering limit state of bond

strength. The partial safety factor for dead load is almost constant under different

design situations and a value of 1.2 was suggested.

Frangopal (29) presented an overview of the most common concepts and

methods used in reliability base structural optimization in order to provide some

21

clarity and insight in to these aspects. It is interesting to note that the results

obtained by sensitivity analysis can be very useful inputs for decision making in

reliability modeling and control of human errors.

Ellingwood et.al (22) used the data obtained from a comprehensive analysis of

statistical and probabilistic information on various types of structural loads and

capacities have been used to calculate reliability indices associated with existing

design practice. This provides guidance for selecting target reliabilities for

probabilistic criteria development. Practical probability based criteria have been

developed which retain the relatively simple characteristics of existing criteria

and yet have a well established documented rationale. A method has been

presented where by specification writing groups can determine resistance factors that

are consistent with the load factors recommended.

Moses and Stevenson (40) presented methods for incorporating reliability

analysis into optimum design procedures. The approach adopted was to design for a

specified probability of failure, in which the failure probability was evaluated from a

sequence of numerical integrations. The subject of sensitivity of statistical

parameters was considered, with the presentation of results for the reliability based

design of rigid frames, using different frequency distributions and parameters.

Moses (41) highlighted the need for extending the simplified second moment

code format and the extended reliability format to the structural systems problem. He

also stressed the need for further research on the reliability of structural systems

in connection with dynamic loadings and non linear behavior. Moses presented

and discussed some simplified analysis approximations including the

determination of appropriate partial safety factors for elements in systems.

Ranganathan and Deshpande (56) presented a technique for reliability analysis

of frames using stiffness matrix method of linear elastic and piece wise linear

elasto-plastic structural analysis and the first order second moment reliability

method. The method involved generation of stochastically dominant mechanisms and

their safety margin equations. Bounds on the system reliability were calculated.

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2.7 MODELS FOR HUMEN ERRORS

Douna and others (20) presented some thoughts and guidelines on avoidance of gross

errors, the major cause of failure of structures. Except for those cases of failure which

were caused by natural phenomena, failures can often be attributed to "gross errors" in

concept and planning, design, drawing, or construction practices. It was pointed out

that thorough checking in the concept, planning, design and detailing stages is

essential in order to avoid gross errors.

Stewart and Melchers (67) developed two models to reduce the incidence of

human errors in design task. The variability in design output was obtained using

Monte Carlo simulation and the effect of checking observed. It was found that

checking efficiencies between 0.6 and 0.9 appear to be most effective in increasing

structural reliability. More realistic checking procedures were outlined and their effect

on structural reliability presented. It was found that no more than three, and often

only two design checks are sufficient to reduce the nominal reliability of the error-

included design to that of an error-free design.

Dayaratnam and Ranganathan (17) presented the statistical analysis of data on

strengths of concrete used in different projects. They found that the strength of the

concrete varies as a random phenomenon subjected to normal or lognormal

distribution. The least-square fit of the field data of several groups of concrete

indicates that the probability of failure of the concrete is of the order of 5.5%. It was

stated that the strength of the concrete follows a normal distribution with at least one

percent significance level if minimum control on the quality of the concrete is

ensured. They indicated that a well designed concrete mix will follow the normal

distribution with 5% level of significance.

The reliability analysis of large and complex structural systems requires

approximate techniques in order to reduce computational efforts to an

acceptable level. Bucher and Bourgund (10) in their study a new adaptive

interpolation scheme is suggested which enables fast and accurate representation of

the system behavior by a response surface. This response surface approach utilizes

elementary statistical information on the basic variables ( mean values and standard

deviations ) to increase the efficiency and accuracy. Subsequently the response

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surface was utilized in conjunction with advanced Monte Carlo simulation technique

to obtain the desired reliability estimates. The proposed method was shown to be

superior both in efficiency and accuracy to existing approximate methods, i.e. the first

order reliability methods. The suggested adaptive interpolation procedure was shown

to be a very efficient scheme with respect to both computational effort and accuracy.

Ellingwood (23) reviewed the status of design and construction errors in

structural safety studies. It was ascertained that a majority of structural failures and

associated damage costs were due to errors in planning, design, construction and

maintenance rather than stochastic variability in construction material strengths

and structural loads. A review of published failure data indicated that only about

10% of failures were traceable to stochastic variability in loads and capacities, the

remaining 90% were due to other causes, including design and construction errors. A

similar review of European failure data (60) indicated that 22% of failures were

caused by stochastic variability but did not indicate what part of the remaining 78%

was caused by errors.

Lind (37) in his paper considered various models of systems with random

capacity to withstand a random demand. Several empirically known aspects of the

influence of human error on the probability of failure of such systems, particularly

structures, were reflected in these models. Simple discrete error models of the

multiplicative type showed a moderate and gradual increase in failure probability with

error probability.

Nowak and Robert (45) made a simple classification system for errors

identified two major categories of uncertainty which cause failure is variations within

accepted practice and departure from accepted practice. The second category was

called as human error. Material properties dimensions, service loads and round off

accuracies are recognized as uncertain within accepted practice without error. Safety

factors provide protection against these random variations whose limits are called

tolerances. Code provisions specifications, engineering manuals, design and detailing

aids, fabrication and construction practices and checks lead towards an accepted

probability of overdesign to avoid failure from variations which fall within design and

construction tolerances. Ignorance, negligence and fraud are major causes of human

error. Their many types include omitting, misplacing, misreading, misunderstanding

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and miscalculating. Errors will be assumed to occur randomly but with frequencies

which vary among professionals, organizations, materials, methods and structures.

In practice the major difficulty is caused by the great variety of possible gross

errors and events in number and magnitude. Nowak (44) in his study made an attempt

to estimate the influence of the possibility of gross errors on structural safety. Four

basic means of control in the occurrence rate and magnitude of the abnormal cases

were discussed; these are inspection and checking, proof loading, adjustment of

code safety factors and proof design. It was found that due to the variety of gross

errors in number and magnitude the most efficient approach on an individual basis

one error at a time. A numerical example was also provided to illustrate the influence

of some possible errors on structural reliability.

Neesim (43) proposed a probabilistic model for the occurrence, detection and

consequences of human errors in structures. The models are developed in an overall

frame work of decision theory applied to the problem of allocating control efforts

that would narrow the gap between the estimated and actual rates of structural

failure. The probability models presented are intended for application to decision

regarding the optimal efficiency of control measure by providing a link between the

efficiency of a given error control measure and reliability of the structure.

Reliability based analysis permits a more consistent approach to structural

safety by including the statistical variability of loads and strengths in the safety factor

evaluation (34). Evaluation of reliability allows one to formulate a rational design

and optimization procedure. It is furthermore necessary to include types of failures

other than collapse in the reliability analysis to obtain an overall optimum design

Kapse and Bela Rani (35) briefly summarize in their paper the various

investigations that have been directed towards understanding the corrosion and

protection of steel reinforcement in concrete.

Sibly and Walker (65) in their paper presents reviews of the histories of four

large metal bridges which failed either during construction or shortly after being

brought into service. From this material the Authors conclude that the accidents had

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certain causes in common and that their histories hold lessons for present day

engineering practice.

Stewart (69) conducted a survey to estimate the percentage of construction

sites that exhibit poor, fair, or good levels of performance for each construction

task. An "overall probabilistic model" of concrete compressive strength was

developed from these survey data. The overall probabilistic model represents the

variation expected between buildings. It was found that poor curing is most

detrimental to concrete compressive strength, and that the proposed probabilistic

models are best represented by the lognormal distribution.

Udoeyo and Ugbem (28) undertaken an investigation on the variations in

dimensions of reinforced concrete member resulting from construction activity in

Nigeria. Poor inspection enforcement during construction, among other factors, was

identified as being responsible for the geometrical imperfections of the studied

members. Based on this study, normal distributions are recommended to represent the

probability models of all the imperfections. This was established by the goodness-of-

fit test conducted on the data obtained. The recommended mean deviation from

nominal dimensions and the corresponding standard deviation for slabs, beams and

column are given.

Human errors, particularly in design have been found to be the dominate cause

of structural failure. Stewart and Melchers (68) developed an “economic decision”

model for evaluating the optimal level of design checking using expected utility at the

criterion for decision making. The model was developed for a simple structural

member using previously reported data on the process of member design and a design

checking. It was found that the use of thorough self checking and overview checking

only was the optimal strategy, unless the consequences of failure are expected to be

catastrophic, in which case one independent design check is also necessary. In

principle, the model should be valid for any structural engineering decision making

application.

A large proportion of structural failures are due to human error in the design

stage of a structural engineering project and many of these failures could have been

averted if there had been adequate design checking. Stewart and Melcher (67)

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reported survey data and mathematical models for three design-checking processes.

The processes appropriate to design checking are self checking, independent detailed

design checking, and overview checking. It was shown that relatively simple models

may be developed for component of these design checking processes.

The variability of the short time ultimate strength of rectangular cast-in-place

slender tied reinforced column bent in single curvature was studied by Mirza and

MacGregor (63). The variation in material strengths and geometric imperfections on

the ultimate strength were determined. The results indicated that the longitudinal

steel ratio, the slenderness ratio, and the end eccentricity ratio had significant

influence on the probability distribution properties of the slender column strength.

The necessity of treating the variation of live loads probabilistically is well

recognized by many researchers in the safety analysis of structures. Although there

are accurate techniques available to assess structural behavior under given loads,

yet the loads themselves remain an estimate to be computed based upon field

measurements, professional logic and experience. With this need Chalk and Corotis

(11) developed a probabilistic format for the determination of building floor live loads

upon examination of live load data and the behavior of live load process.

Ellingwood (26) developed probability distributions and statistical parameters

for wind and snow loads using the available data. He found that in majority of

reliability studies the Cumulative Distribution Function (CDF) of load was not

described and attempts were only made to match the CDF of the load in its centre

range. On the other hand Ellingwood's procedure fits the CDF of the load in the

region of safety checking point.

Ranganathan and Chikkodi (55) derived the partial safety factors, using

advanced level 2 approach, for reinforced concrete design as given in IS: 456-1978.

The paper explained the methodology, as well as the assumptions made in the

derivation of partial safety factors. Curves for the selection of partial safety factors

corresponding to a given reliability were presented.

Ranganathan (57) analysed wind speed and wind load statistics for

probabilistic design based on data obtained from different geographical stations in

27

India. He assumed that the established statistics of wind speed and wind load can

form a basis for the calibration of Indian Standard code on reliability theory.

Benjamin (7) described some of the advantages of rational probabilistic

analysis and design concepts compared to deterministic procedures. He observed that

deterministic procedures are inferior to probabilistic concepts in informational

content, modeling of reality, refinement of analysis and design and the resulting

structures.

Raj and Ranganathan (50) investigated the reliability level in slabs designed as

per IS: 456-1978 using limit state approach. The reliability index and partial safety

factors for loads and resistance were determined using level 2 approach. Curves were

drawn for optimal safety factor selection for different design situations.

2.8 SUMMARY

A review of statistical analysis of structural failures reveals that many a structural

failure and associated damage costs are due to errors in planning, design, construction

and maintenance. Probabilistic concepts are used extensively in current design

methods and in reliability analysis. However these methods do not take into

consideration field errors in design and construction. The reliability of a structure can

be substantially enhanced by controlling field errors.

2.9 CONCLUDING REMARKS

Errors occur in all phases of building process, planning, design, construction and

maintenance. A review of statistical survey of structural failures reveals that many a

structural failure is due to errors rather than variability in construction material

strengths and structural loads. These findings have raised considerable interest in

studying the role of field errors in structural performances. The major deviations in

the specifications and practices adopted in the construction need further study. In this

regard an attempt has been made to collect data regarding the cover to reinforcement

in slabs, beams and columns of different types of buildings where in different levels

of quality executions are exercised. The data obtained is statistically arranged for

further generations to use it in a scientific way.