What is Structural Failures of Concrete Structures?
Fasi Ur Rahman
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Structural failures in concrete structures, a part of human failures, occur as long as man and structure exists. In the past man’s comprehension of the structural response was simple and straight; but in today’ complexity of the man and his structure, the structural failure phenomenon is of a multi-dimensional and multi-disciplinary character.
Failure is often stated as the stepping stone to success, but there is a high price to pay in terms of energy, time and money. Nobody wants a failure but yet they occur. Lessons from failures are everlasting, revealing and often shocking.
We define failure as the absence of a derived function, goal or objective, mission, task or purpose; failure is the opposite to success and there is no easy way to define each of them.
What is Structural Failures of Concrete Structures?
Structural failures refer to the absence of its desired / designed / intended performance, behavior, response under all expected environmental conditions (loads, forces, etc.).
There are tension, compression, shear, flexure and torsion failures, occurring singly or in a combined state.
The classical notions of factors of safety have undergone tremendous changes giving rise to partial safety factors and limit state factors. There is undoubtedly a great rationale in the stipulation of these factors to design but fabrication, erection and assembly factors to application are left entirely to the field conditions.
Material failures in structures are viewed as ductile or brittle failures, or sometime as transition ductile brittle failures. Soil and concrete media have their own unique failure mechanisms. Steel is largely governed by ductile failure.
Structural failures of concrete structures often imply large and unwanted deformations, severe honeycombing and cracking with spalling, relative displacement of supports and ultimate collapse.
In a damaged structure, the vexing issues that arise are:
What is the extent of damage and how to quantify the same as required in the strengthening calculations?
What has been the rate of decay of the material properties and what realistic values should be assessed for strength assessment at that point of time?
What is the mode of treatment to be adopted and what is the life span of such treatment?
What is the cost benefit ratio of salvaging a damaged system?
What should be the criterion for demolition and how to accomplish the same?
Buildings (dominant in civil engineering) do not normally fail; but they are always in a damaged state over a period, with faults, defects, cracking, decay, spalling, ground settlement etc., very pronounced with consequent changes in the structural soundness and psychological human perceptions to comfort and safety.
Structural control and testing are intimately interlinked. Testing need not be destructive (DT) while today Non-Destructive Tests (NDT) has become the routine rather than the fashion. Structural testing calls for a philosophy, technology and methodology and an ability to interpret what is observed and infer what is invisible.
Structural failures in Indian and Western environments have some striking differences; our perception and reaction to failure is highly subjective. They learn from past mistakes, while we repeat them.
We shudder to admit our faults, discrepancies and deficiencies and we push them under the carpet. Sometimes we pass the buck on to a weaker neighbor. For us failure means end of everything in life.
While we know that failure is a better teacher than success, in practice we are not prepared to pay the price to learn the art of success. The administrative, financial and legal overtones coupled with enormous delays in post mortem, have made us shudder to think of failure.
Money and time are in opposite senses and are a great premium to us, and we always prefer to “play safe”. Structural soundness and cost effectiveness rarely go together. This is in short, our story to structural failure.
Issues pertaining to failed structures, such as collection and compilation of data and evaluation of the most probable cause of the collapse, emerged as a special branch of civil engineering. The science of material chemistry, material testing and in situ strength assessment — all this put together gave rise to a very absorbing technique known as “forensic engineering”, which is now about three decades old.
Interestingly old structures are still more or less performing satisfactorily. The problem is observed with structures recently constructed in the early fifties or later. This paradox of better performance of old structures vis-a-vis recent structures, offers an interesting insight into the quality of old structures and why and where we are now lacking.
At this stage, it is necessary to analyze our activity, to identify grey areas, which we have inadvertently permitted to continue in our set up. Startlingly, in spite of advances, in technology, we have not changed our basic set up. This could be one of the major contributors to our handicap.
There have been rapid advancements in almost every technological field. Civil engineering is no exception. There has been remarkable progress in the use of new materials, new design techniques, improved provisions, codal awareness, construction methods and user needs etc.
This has been backed by computerized support in planning, design and construction management. However, despite all this back up of development and advancement, the construction industry’s performance has not been satisfactory.
There is a kind of awareness to maintain the interior of a structure but not the common exterior or the material as a whole. We are more interested in beautifying the interiors like fixing marble slabs and resorting to costly painting but are totally oblivious and least interested about the basic quality of the material.
We spent liberally on the interior decoration but are hesitant to spend more on the quality of concrete and reinforcement. Our basic attitude needs to be revised.
Malfunctioning of structure performance, is on the increase. Failures have become more common than we would like to accept. They are somehow ill-reported and not discussed due to involved complexity, and a sort of aversion by those involved.
It is important that failures are discussed threadbare. This will create necessary awareness about the extent of malfunctioning, leading to timely remedial action.