A Q&A with Professor Henry PetroskiDecember 1, 2001
Henry Petroski, Aleksandar S. Vesic Professor of Civil Engineering and professor of history, is an expert in the implications of failure for engineering. In his book, To Engineer Is Human: The Role of Failure in Successful Design (1985), Petroski explored how engineers learned from engineering failures. In a recent interview with Dialogue, Petroski discusses how the collapse of the World Trade Center towers has changed engineering thinking.
Q. In the immediate aftermath of the World Trade Center attacks, you said you expected this would be the end of tall high-rises, and indeed the new plans for the WTC area don't include any building higher than surrounding ones. Do you expect this to continue? Are there any good economic or engineering reasons to drive buildings any taller?
Petroski: I do not expect that there will be any supertall buildings built in the United States for the foreseeable future. (That is not to say that such buildings will not be built in parts of the world where terrorism is perceived to be less of a threat to the infrastructure.) There never were good economic or engineering reasons to build as tall as the Twin Towers of the World Trade Center or the Sears Tower, and since those structures were completed in the early 1970s no taller skyscraper was constructed in America. In fact, there are economic disincentives to build as tall as the Twin Towers and the Sears Tower. As buildings rise higher, more space inside the structure must be devoted to elevators to move people up and down, and the more space devoted to elevators the less there is to rent and recoup the investment in the building. Supertall buildings have been built not so much for economic or engineering reasons as for reasons of civic or corporate symbolism.
Q. The collapse of the buildings has received a lot of study. Have any of the results of these studies been of particular interest? What has been surprising?
Petroski: Among the most interesting results of engineering failure analyses of the collapsed towers has been the incontrovertible evidence that fire and the heat that accompanies it can trigger the collapse of a structure the way they did in New York. There had been fires in skyscrapers before, but none had collapsed, because the fire and the attendant structural damage was confined to a floor or two and thereby localized in their effect and the structural damage they caused. In the case of the World Trade Center, the massive structural damage due to the impact of the hijacked airplanes, in combination with the intense heat of the resulting fires, produced a theretofore incredible combination of forces on the buildings. Such combinations of forces are, obviously, no longer incredible.
Q. Has engineering changed because of what we learned from the buildings' collapse?
Petroski: The collapse of the World Trade Center Twin Towers will have an enormous and long-reaching effect on structural engineering as it relates not only to tall buildings but to any structure susceptible to terrorism. There have already been calls for changing what building codes require in terms of fire protection, evacuation routes, and the ability for a structure to withstand the massive damage that can result from a terrorist attack. It is likely to take some time before these changes are incorporated into formal building codes, but in the meantime engineers will no doubt design more terrorist-resistant and more escapable structures.
Q. What, if anything, is different about how engineering has adapted to the WTC collapse compared to other structural disasters, such as the collapse of the Hyatt skywalk?
Petroski: Engineering adapts in pretty much the same way after any catastrophic failure. There is typically a moratorium on designing and building anything that resembles the structure that has collapsed, not only because it would be unwise to do so until the causes of the failure were fully studied and understood but also because of the psychological reason that people would be disinclined to want to use a structure that so reminded them of one that collapsed. The Hyatt Regency skywalks were not rebuilt as elevated walkways hanging by slender steel rods from the ceiling of the hotel. Rather, a single elevated walkway supported from below by massive concrete columns was constructed over the lobby. It conveyed a sense of strength and stability that was reassuring in the hotel lobby that had been the scene of such a tragedy.