Collapse of Buildings

Understanding the Probable Causes

C. Daniel, Senior Engineer

Collapse of a building generally refers to "loss of the load-carrying capacity of a component or member within a structure or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformations. In a well-designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. Ultimate failure strength is one of the limit states that must be accounted for in structural engineering and structural design" as per Wikipedia definition. Buildings, like all structures, are designed to support certain loads without deforming excessively. The loads are the weights of people and objects, the weight of rain and snow and the pressure of wind-called live loads - and the dead load of the building itself.

With buildings of a few floors, strength generally accompanies sufficient rigidity, and the design is mainly that of a roof that will keep the weather out while spanning large open spaces. With tall buildings of many floors, the roof is a minor matter, and the support of the weight of the building itself is the main consideration. Like long bridges, tall buildings are subject to catastrophic collapse.

A number of reasons have been given to explain why these buildings are collapsing and these include reasons like the concrete mix ratio was not right, there was not enough planking and strutting in place to uphold excavations in place, the column spacing was too wide, the reinforcement was not adequate, the slenderness ratio was too high, the contractor was cost cutting by changing recommended concrete mix or reducing the amount of reinforcement recommended, addition of illegal structures to approved plans and a multitude of other reasons are amongst are what is fronted for these collapses.

All the above reasons do nothing to endear the construction industry to the public. They only spell of negligence, lack of supervision, not adhering to recommended and approved plans and specifications. This ultimately reeks of a general lack of seriousness in the industry for first of all allowing such practices to occur in the first place and then, for continuing to let them happen.

From engineering perspective, buildings, like all structures, are designed to support certain loads without deforming excessively. The loads are the weights of people, wind, snow, rain and other objects occupying the structure referred to as live loads, and the weight of the building itself , also referred to as dead loads. Structural engineers are responsible for engineering design, analysis of the loads and the integrity of the entire structural system of the building.

However, buildings are susceptible to catastrophic collapse due to many causes as summarized below:

Poor Structural design

The structural engineer may make errors in computation, and fail to take into account the loads the structure will be called upon to carry. He may also adopt erroneous theories and rely on inaccurate data, ignore the effects of repeated or impulsive stresses, and make improper choice of materials to be used during construction. The engineer is responsible for these failures, which are created at the drawing board.

Faulty construction

This is usually the most important cause of structural failure. The engineer or the supervisors on site are also at fault here, if inspection has been lax. This includes the use of poor quality of sand and aggregates to make concrete, non adherence to the mix design, lack of quality control of the work, substitution of inferior steel for that specified, inadequate curing of concrete and other practices well known to the construction worker.

Foundation Failure

Even an excellently designed and constructed structure will not stand on a bad foundation. Although the structure will carry its loads, the earth beneath it may not. The Leaning Tower of Pisa is a famous example of bad foundations. Detailed geotechnical investigation should be undertaken by the Engineer to determine the design parameters the structure’s foundation.

Extraordinary loads

These are often natural, such as repeated heavy snowfalls, or the shaking of an earthquake, or the winds of a hurricane. A building that is intended to stand for some years should be designed to withstand these challenges. A flimsy flexible structure may avoid destruction in an earthquake, while a solid masonry building would be destroyed. Earthquakes may cause foundation problems when moist filled land liquefies.

Unexpected modes of failure

This is yet another reason why buildings do fail. This is the hardest category to deal with as it is indeed unexpected. For any structure constructed, one there are properties that are unique and affect it in its own way. Until all these properties are assessed and understood they remain unexpected and fall under the unexpected failure modes if they do lead to the failure of the structure. Finally a building could collapse due to more than one of the above reasons hence a Combination of causes.