Structural Failures 4p6n6r

A Seminar on

STRUCTURAL FAILURES Presented by

ANKIT KUMAR USN No.-4JC08CT008

CONTENTS 1. INTRODUCTION 2. CLASSIFICATION OF FAILURES 3. CAUSES OF FAILURES    

Construction errors Design errors Earthquake failure Foundation movements

4. MULTISTOREYED BUILDING FAILURES   

Shape Failures Detailing Failures Soft storey collapse

5. STEEL STRUCTURE FAILURES  

Buckling of Thin Plate Fracture of steel column

6. CORROSION CANCER IN STRUCTURES  

Factors influencing corrosion Damage due to corrosion

7. REMEDIAL MEASURES 8. CONCLUSIONS 9. REFERENCES

INTRODUCTION  The failure of structure is a very disturbing event for a civil engineer.  It was almost non-existent in the past. Structures were known for the

durability, soundness, and stability.  Modern materials are said to be of better quality and of substantially

increase strength.  There is also today an improved understanding of structural analysis

backed by sophisticated design technology.  There have been rapid advancements in almost every technological

field. Civil engineering is no exception.  This has been backed by computerized in planning, design

and construction management.  These advances in technology should have produced better and

more durable structures. Unfortunately it is not so.

CLASSIFICATION OF FAILURES  These are usually observed as cracks and deformation in structures.  Cracks could be broadly classified as structural or non- structural.  Structural cracks-

Ex: Extensive cracking of an RCC beam  Non structural cracks-

Ex: Vertical cracks in a long compound wall due to shrinkage

CAUSES OF FAILURES 

Sometimes there is a single simple explanation for a failure. But usually it is a combination of condition, mistakes, oversights, misunderstandings, ignorance, even dishonest performance, but not a single item by itself that can be picked as the sole and only cause of failure.

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The dimension between success and failure in any construction operation is that of the thinnest hair. This is why the failure problem becomes interesting though uncertainty prevails.

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Reasons for failures are either man made or natural.

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They can be briefly listed as follows:1. 2. 3. 4.

Construction errors Design errors Earthquake failure Foundation movements

1. Construction errors    

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They range from defective temporary work to wrong construction procedures. Most of the construction failures can be attributed to acceptability standards, which are mostly left to individuals. There is usually vast variation in these standards depending upon an individual’s grasp of the situation. Projects are sublet to sub contractors and their sub agencies. Each of these agencies look at the project to the extent they are involved without any regard to other agencies. Lack of co-ordination among such agencies is quite often because of failure.

2. Design errors 

Deficiency in the basic design of a structure such as the amount of reinforcement at points of maximum moment, or incorrect dimension of concrete section to provide sufficient resistance for assumed loading, are extremely rare

 Another reason for design deficiency is lack of design checks and revisions

after preparation of drawings  Detailing is one of the most important parts of design and drafting. Lack of proper detailing like position of laps and splices, end anchorages, position of construction ts, lack of knowledge about length of bars are reasons for numerous failures.

3. Earthquake Failure  Although codes specify the wind and earthquake analysis and design

requirements, in some cases it may so happen that these loads may be more than those assumed by the designer.  The engineering approach to earthquake resistant design must recognize the limitations of ground conditions, available materials And labour.

4. Foundation Movements Can be classified in two categories:  Lateral spreading  Settlement

MULTISTOREYED BUILDING FAILURES Multistoreyed building failures may be due to following: 1. 2. 3.

Shape failures Detailing Failures Soft storey collapse

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Shape failure

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Conceptual design is particularly important when deg for dynamic conditions.

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The principal behind conceptual design is to keep the design as simple as possible and limit the points were stress concentrations can appear.

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To prevent this type of failure it is important to keep the designs symmetrical and simple. The figure is shown below.

Fig. SHAPE FAILURE

2. Detailing failures  With a lot of building failures it is not always easy to see exactly why a structure

has failed. This is especially true for detailing, as bad detailing is often one of a number of factors, which can weaken a structure.  Bad detailing is the cause of large proportion older concrete buildings, which

were designed using older codes.  In an earthquake a concrete beam/column will vibrate. This will cause the

concrete to crack if it is not restrained.  Figure shows a basic design of a column

3. Soft storey collapse  Soft storey collapse is often resulting of bad conceptual design, however

this is not the only reason it occurs.  A soft storey collapse is named because it occurs when a building has one floor that is weaker than the others.  The photo shown is a good example of this and has occurred because the restaurant owner required a glass front.

Fig. Soft storey collapse

STEEL STRUCTURE FAILURES

 Steel is largely governed by ductile failure.  Steel in a dynamic environment like seismic, also suffer

ductility.

Buckling of Thin Plate  The earthquake has caused the structure to twist move in some way

and this in turn has resulted in the plate steel loosing its stiffness and rigidity and so failing by crushing.  This photo shows the failure of plate steel when loaded vertically.

Fig. Buckling of thin plate

Fracture of steel column  The photo shows a fractured cast steel column of an unknown age.  The column has failed brittle, the most probable reason for this kind of

failure though is that there was either a weld or material defect that weakened the steel and allowed a crack.

Fig. Fracture of steel column

CORROSION CANCER IN STRUCTURES  Reinforced concrete is a very versatile and highly durable material

suiting any type of structure.  Corrosion of reinforcement in RC structures is one of the phases of the

durability problems.

Factors influencing corrosion  The factors, which generally influences of reinforcement in RC

Structures are: moisture, oxygen, carbonation, chlorides, temperature and relative humidity, quality of construction materials, quality of concrete, cover to the reinforcement, initial curing conditions, and formation of cracks.

Damage due to corrosion The process of corrosion once set off, results in deterioration and distress of the RC member. The various stages of destruction are as follows:       

Formation of white patches. Brown patches along reinforcement. Occurrence of cracks. Formation of multiple cracks. Spalling of cover concrete. Snapping of bars. Buckling of bars and bulging of concrete.

REMEDIAL MEASURES The remedial measures that should be taken to avoid structural failures are discussed briefly below

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Proper detailing should be done at design office to show the details of reinforcements etc. at the junctions, where there is a congestion of reinforcements. Formwork should be designed not only for alignment but also for settlements etc. At the site proper shuttering the reinforcement should be aligned with properly designed constant covers by using bar spacers. Water/cement ratio must be strictly adhered to. The mix should be a designed mix with optimum quantity of cement bearing in mind that cement quantity is required not only for strength but also for durability. Proper curing is a must. Curing with membrane forming curing compounds protects the concrete even in the initial stages. Steel should be protected by properly coating the steel with suitable coatings. Occasional inspection and immediate repairs should be carried out.

CONCLUSIONS 

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It is evident from the foregoing discussion that precautions are better than cure. Proper supervision at construction sites by qualified engineers is essential. Facilities and structures must be constructed with due and deserving care, attention and involvement Proper care should be taken for concrete placing, compaction and curing while fixing steel, proper covers to steel should be maintained New and higher strength materials and advanced planning provisions may have limitations not covered by current codes and practices. Details of connections, that is, load transfer mechanism and junctions are as, or even more important than sophisticated computer aided designs. In the process of analysis, human factors are as important as physical. It is therefore, necessary to consider these factors so as to advance the art of engineering, rather than to get compensation and keep the failures operating.

 Perhaps every technical and trade organization should award annual

prizes for complete descriptions and analysis of failures, so that every one can learn from that experience and avoid similar problems in his own practice.  Structural Failures continue to occur until good effort is put by all agencies involved in the task of achieving the functional requirements of structures.  Well-specified procedures and clear documents are very important in civil engineering. There is no use in trying to shift the blame on to others as long as a lesson is not learnt from mistakes. Finally it is worthwhile mentioning that a minute failure wipes out years of success.

REFERENCES 

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Jagadish, R. (1985),”Investigation of structural defects and the proposed rectification scheme of the partially constructed building, report “submitted to the corporation of the city of Bangalore. Ravi, S (1989) “Case studies of corrosion damage reinforced concrete over head water tank” , a thesis submitted to the award of M.Tech degree, Karnataka regional engineering college, Surathkal. Raikar, R. N. (1987) “Learning from Failures” –R & D Centre, structural designers and consultants (P) Ltd. Raikar Bhavan, Mumbai. Jacob (1968) Construction Failures –John Wiley and sons. Vishwanatha, C.S. (1989)”Stages of corrosion damages in RC ”, torsteel bulletin. Vishwanatha, C.S. (1989) “Report on the restoration measures for over head water tank”. Submitted to the executive engineer, Zilla Parishad Engineering Division, Gadag. www.structural failures.com www.cen.bris.ac.uk www. [email protected]