Is My Building Safe?

If two of the tallest buildings in the world can collapse catastrophically in about one hour, can= any building be considered safe? The collapse of the World Trade Center was the resu= lt of circumstances of exceptional magnitude. Although the towers were designed to survive being struck by an aircraft and in fact did so, the resulting = fire weakened the structures and led to their collapse. It might be unrealistic = to expect any building to survive that combination of circumstances.

However, other structur= es fail under more “normal” conditions. Failure is almost always caused by one or more of the following factors:

v Design error: the engineer makes a mistake.

v Construction error: the builder does not follow the plans.

v Material failure: inferior materials are used.

v Deterioration: proper inspection and maintenance schedules are not followed.

All of these factors ca= n be controlled. The first three can be controlled by adequate supervision = in the design and construction process. The fourth factor, deterioration, may = be the most common factor in the failure of structures and can be prevented through a program of regular inspections by a qualified engineer.

After the size and gene= ral configuration of a building have been determined, the engineer must make a determination of the type and magnitude of forces that he or she expects the building to sustain. These forces are commonly called “loads.” = Most people are familiar with such terms as “dead loads” and “= live loads.”

Other types of loads that a building may = be subjected to include snow loads and rain loads on roofs, &= nbsp; wind loads on the entire structure, and in some parts of the country, seismic lo= ads. In some circumsta= nces structures may also be designed to withstand extraordinary events of low &= nbsp; probability such as explosions or vehicular impact. Load predictions are based on statistical probability: there is a specific probability that a specific lo= ad will be exceeded in the lifetime of a structure.

Once the materials have been selected, the engineer calculates the sizes of the different structural components needed to resist the expected loads that will be placed on them. Since expected loads are based on probability, there is always s= ome probability they will be exceeded. The predicted breaking strength of mater= ials is also based on statistical probability. To allow for the possibility of a relatively high load being placed on a relatively weak material, the engine= er applies a factor of safety.

A factor of safety is sometimes jokingly referred to as a “factor of ignorance.” It i= s a measure of uncertainty. If our predictions were 100 percent accurate, = we wouldn’t need a factor of safety. A factor of safety is determin= ed by considering the predictability of loads, the predictability of the stren= gth of different materials, and the consequence of failure.

Certain types of loads = are predictable with a greater degree of certainty than others. Dead load, the weight of the structure itself, can be determined with a great degree of certainty. Dead loads are assigned a factor of safety of 1.4, or 40 percent over. Live loads, such as wind loads, are much harder to predi= ct and therefore are assigned a factor of safety of 1.6. The probability = that both dead loads and live loads will each be exceeded by their maximum at the same time is less likely, so the combined factors of safety may be reduced = by a load combination factor.

The performance of any material is never 100 percent consistent. Even in a material such as steel, which is produced under very controlled circumstances, the yield strength of one piece of steel may differ from that of another. To allow for this variation in the properties of materials, a factor of safety is applie= d to the material strength. The allowable load typically varies between 70 perce= nt and 90 percent of the design load.

The consequences of fai= lure of certain structures are greater than that for others, and different facto= rs of safety are applied to buildings designed for different uses. This particular application of the factor of safety is called the “importa= nce factor.”

Most buildings have an importance factor of 1.00. In other words, no additional factor of saf= ety is applied to most buildings.

However, a lower factor= of safety may be applied to certain structures such as some storage facilities= or agricultural buildings. A higher factor of safety may be applied to structures such as hospitals, the survival of which is essential after any disaster, and nuclear power plants, at which failure could have catastrophic consequences.

In essence, at least tw= o, and sometimes three, factors of safety may be applied to a given structure. These factors of safety are applied in addition to the careful calculations performed by the engineer and are not intended to account for errors on the part of the engineer.

Safety is a proactive condition. At the root of most failures is the desire to save money. There is nothing wrong with the desire to build less costly buildings, but the false economies of saving money on design, construction = supervision, and maintenance can have disastrous consequences.

Criterium-Jans= en Engineers are licensed Professional Engineers with over 15 years of experie= nce. We have offices in Toronto, Mississauga and Orangeville and clients in and around the greater Toronto area (GTA) including Oakville, Burlington, Kitchener, Guelph, Georgetown, Milton = and Caledon. Our commercial building services i= nclude property condition assessments, phase one environmental assessments, and st= ructural inspections.