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Do Structures Really Fail?

By Eric R. Kizak, P.E.

Often when investigating a building with structural deterioration I am asked “If it is so bad, why hasn’t it fallen down?”  This question highlights a general misunderstanding of how structures work and fail.  Modern design theory accounts for the probabilities of the construction material having less capacity than intended (understrength) and higher than expected demand (overload).  This technique is similar to earlier methods based upon allowable stress or safety factors but is more quantifiable and has a more rigorous underpinning. 

Now back to our topic:  If it is so bad why hasn’t it fallen down?  The design is based upon the simultaneous occurrence of both moderate understrength and overload condition.  Code design loads are based upon the maximum expected load but buildings rarely experience this level of demand.  For example, consider a roof design for code snow loads.  The full code loads are expected to develop for about 2 months over a period of 20 years, or less than 1% of the time.  The other 99% of the time there is additional reserve for material understrength and deterioration.  It is this additional reserve as a result of the more common underload condition that preserves a building’s structural integrity.

Now you may say if 99% of the time there is additional reserve capacity, why not design for lighter loads?  This is a good question.  If the code loads are reduced a less costly structure could be built, right?  Yes, but at a cost to everyone’s safety.  Though infrequent, the code loads are based upon decades of observations and measurements of countless engineers and scientists; they will occur.

Additionally, if a structure deteriorates it loses strength, so more of the reserve capacity is “used” to maintain structural integrity until one day the demand on the structure exceeds the reserve capacity and the structure fails.  For example:

 

So next time you observe significant structural deterioration, consider the consequences:  if left unmaintained it will fall down, eventually.