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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 result of circumstances of exceptional magnitude. Although the towers were designed to survive being struck byby an aircraft and in fact did so, the resulting fire weakened the structures and lead to their collapse. It might be unrealistic to expect any building to survive that combination of circumstances.

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

  • Design error:; the engineer makes a mistake.
  • Construction error:; the builder does not follow the plans.
  • Material failure:; inferior materials are used.
  • Deterioration:; proper inspection and maintenance schedules are not followed.

All of these factors can 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.

The engineer designs a building to withstand the forces the building will likely encounter during its lifetime.

After the size and general 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, wind loads on the entire[rpn1] structure, and, in some parts of the country, seismic loads. In some circumstances structures may also be designed to withstand extraordinary events of low probability such as explosions or vehicular impact.

Load predictions are based on statistical probability; : there is only a certain specific probability that a certain specific load will be exceeded in the lifetime of a structure.

Once the materials selection hasve been madeselected, 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 some probability they will be exceeded. Also tThe predicted breaking strength of materials is also based on statistical probabilitiesprobability. To allow for the possibility of a statistically relatively high load meeting being placed on a statistically relatively weak material, we add a factor of safety.

The Engineer engineer applies a factor of safety.

A factor of safety is sometimes jokingly referred to as a “factor of ignorance.” It is a measure of uncertainty. If ourt predictions were 100 percent percent accurate, we wouldn’t need a factor of safety. A factor of safety is determined by considering the predictability of loads, the predictability of the strength 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 percent over. Live loads, such as wind loads, are much harder to predict 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 percent consistent. Even in a material like 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 applied to the material strength. The allowable load typically varies between 70 percent andto 90 percent percent of the design load.

The consequences of failure of certain structures is are greater than that for others, and different factors of safety are applied to buildings designed for different uses. This particular application of the factor of safety is called the “importance factor.” Most buildings have an importance factor of 1.00. In other words, no additional factor of safety 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 such structures such as hospitals, whose the survival of which is essential after any disaster, and nuclear power plants, where at which failure could have catastrophic consequences.

In essence, at least two, 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 begins at the start of the project.

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.
 

Volume 12, Number 4

December 2001

Copyright © 2001

The Engineering Advisor is intended to enhance your knowledge of technical issues relating to buildings.  For additional information on any subject, please feel free to call us.  Our commitment is to provide you with timely, accurate information.