I read the article "Engineers Caution Against Overreaction," and as a former structural engineer, I couldnt agree more with the dissenting voices (ENR 1/25 p. 10). The reasons for the collapse of the trade center towers and a portion of the Pentagon were many faceted. Not only did the buildings sustain huge loads, via impact of the airplanes, the fuel caused fire to erupt. This is a deadly combination. The 1906 earthquake in San Francisco, albeit a natural disaster, unleashed a similar terror from fires from ruptured gas lines.
We cannot base building design on extreme events, though such events cause deaths. Where does one draw the line?
Let us remember our motto, "The safety of the public shall be paramount." How can we be most efficient in heeding that command? I do not believe that we can make any building completely secure. We must design buildings in a way that allows occupants enough time to escape. The only way to do this is to ensure that buildings are protected by sprinklers and that the structural skeleton is fire rated, in a way not easily dislodged.
I suggest that use of chemical fire suppressants be explored in high-rises, as water service is more prone to failure. Architectural layout must be such that it allows structural redundancy. Finally, as the article suggests, buildings ought to be assessed for risk, prior to design. In Canada, essential disaster response sites are designed to resist lateral loads 50% higher than ordinary buildings.
We cannot do the impossible. In any extreme event lives will be lost, and we, as a society, have to live with that. Our duty is to save as many as is possible. The only way to do that is to keep a building erect long enough to let people to exit safely. For that, only fire suppression will work.
I wont insult the intelligence of my esteemed colleagues who disagreed with me on the latest design methodology of high-rise buildings, i.e. strong cores rather than strong perimeters. What I would like to point out, though, is that a perimeter column becomes a beam when impacted by a concentrated horizontal force, regardless of the source. Joists or other lightweight floor members are excellent column braces but poor beam supports and not very fire-resistant.
I am not suggesting any errors in the structural engineering of the WTC buildings that were designed long before terrorism became a frightening reality.
From my perspective as a demolition contractor who worked at the World Trade Center and at catastrophic fire and earthquake sites worldwide, dealing with failed yet standing structural systems, I also have to lean toward a conservative approach. No matter what we do from a design-construction standpoint, a terrorist will find a way around what we do.
This is one case where I agree with U.S. policytaking the war to the terrorists before they bring it to us. Better we spend money in that regard than on over-design and over-construction that will never, ever, be enough.
Toward Better Design
February 28, 2005