Setting realistic limits for building performance
The horrible events of September 11, 2001, have raised many questions in the minds of the public, the media, political leaders and design professionals. The primary question is how safety can be increased in buildings in the event of another terrorist attack.
Lots of ideas have been suggested. Many of the suggestions, unfortunately, do not improve realsafety and could cost taxpayers and developers hundreds of millions of dollars annually.
The misguided suggestions are the result of a lack of understanding of two fundamental facts: (1) There is no practical way to design commercial buildings to take the impact of a large passenger aircraft without causing a collapse, and (2) the disaster at the World Trade Center (WTC) was not a fire, but a military-style attack that had fire as only one of the damage components.
The physics of stopping a 767 aircraft weighing 275,000 pounds and traveling 500 mph at the face of a building would require a deceleration force of several hundred million pounds. A typical 50-story building has a lateral capacity of about 4 million to 5 million pounds.
The WTC fire was not a test of fire-protection systems or fire codes because the event was not a "fire." At least six fire-protection systems that would have been available in a normal fire were destroyed in the attack.
High-rise buildings have an excellent fire-safety record. In the entire decade of the 1990s, there was only one fire-related death in the U.S. in an office building taller than seven stories.
Different kinds of sprinkler systems, tougher structural-steel fireproofing or wider stairwells with increased separation in order to increase the survivability of building occupants are among the ideas that have been suggested. But none of these measures will provide increased safety because they would all be immediately and totally destroyed, along with the whole building, by the initial crash energy.
Another proposed change is to require that buildings be designed with special progressive-collapse prevention systems. Under this proposal, any column could be removed without causing global or local collapse. Depending on the specific building, this can be extremely expensive. The normal approach would be to have strong ties in the horizontal framing system to transfer loads to adjacent, undamaged columns. Yet, if this is being done to protect against terrorism, it may be exactly the wrong approach. Experience has shown that when a building with strong horizontal ties is subjected to a large initiating event (i.e., the removal of several columns), it is more likely to completely collapse than is a building with weakened horizontal planes.
Structural techniques that can increase the capacity of buildings to resist certain types of attacks are available. However, there is no reliable way to design for the impact of a large airliner. It is not possible to try to "out-build" the terrorists. If you design for one truck bomb, they will attack with two. We must approach any changes to building methods in a rational, effective way. Every building professional must carefully think through these new sets of problems in order to get to the right questions so that the right solutions can be found for society and real increases in safety can be achieved.