A Q&A with Professor Henry Petroski

December 1, 2001

Henry Petroski, Aleksandar S. Vesic Professor of Civil Engineering andprofessor of history, is an expert in the implications of failure forengineering. In his book, To Engineer Is Human: The Role of Failure inSuccessful Design (1985), Petroski explored how engineers learned fromengineering failures. In a recent interview with Dialogue, Petroskidiscusses how the collapse of the World Trade Center towers haschanged engineering thinking.

Q. In the immediate aftermath of the World Trade Center attacks, you saidyou expected this would be the end of tall high-rises, and indeed the newplans for the WTC area don't include any building higher than surroundingones. Do you expect this to continue? Are there any good economic orengineering reasons to drive buildings any taller?

Petroski: I do not expect that there will be any supertall buildings built inthe United States for the foreseeable future. (That is not to say that suchbuildings will not be built in parts of the world where terrorism isperceived to be less of a threat to the infrastructure.) There never weregood economic or engineering reasons to build as tall as the TwinTowers of the World Trade Center or the Sears Tower, and since thosestructures were completed in the early 1970s no taller skyscraper wasconstructed in America. In fact, there are economic disincentives to buildas tall as the Twin Towers and the Sears Tower. As buildings rise higher,more space inside the structure must be devoted to elevators to movepeople up and down, and the more space devoted to elevators the lessthere is to rent and recoup the investment in the building. Supertallbuildings have been built not so much for economic or engineeringreasons as for reasons of civic or corporate symbolism.

Q. The collapse of the buildings has received a lot of study. Have any ofthe results of these studies been of particular interest? What has beensurprising?

Petroski: Among the most interesting results of engineering failureanalyses of the collapsed towers has been the incontrovertible evidencethat fire and the heat that accompanies it can trigger the collapse of astructure the way they did in New York. There had been fires inskyscrapers before, but none had collapsed, because the fire and theattendant structural damage was confined to a floor or two and therebylocalized in their effect and the structural damage they caused. In the caseof the World Trade Center, the massive structural damage due to theimpact of the hijacked airplanes, in combination with the intense heat ofthe resulting fires, produced a theretofore incredible combination of forceson the buildings. Such combinations of forces are, obviously, no longerincredible.

Q. Has engineering changed because of what we learned from thebuildings' collapse?

Petroski: The collapse of the World Trade Center Twin Towers will havean enormous and long-reaching effect on structural engineering as itrelates not only to tall buildings but to any structure susceptible toterrorism. There have already been calls for changing what buildingcodes require in terms of fire protection, evacuation routes, and the abilityfor a structure to withstand the massive damage that can result from aterrorist attack. It is likely to take some time before these changes areincorporated into formal building codes, but in the meantime engineerswill no doubt design more terrorist-resistant and more escapablestructures.

Q. What, if anything, is different about how engineering has adapted tothe WTC collapse compared to other structural disasters, such as thecollapse of the Hyatt skywalk?

Petroski: Engineering adapts in pretty much the same way after anycatastrophic failure. There is typically a moratorium on designing andbuilding anything that resembles the structure that has collapsed, not onlybecause it would be unwise to do so until the causes of the failure werefully studied and understood but also because of the psychologicalreason that people would be disinclined to want to use a structure that soreminded them of one that collapsed. The Hyatt Regency skywalks werenot rebuilt as elevated walkways hanging by slender steel rods from theceiling of the hotel. Rather, a single elevated walkway supported frombelow by massive concrete columns was constructed over the lobby. Itconveyed a sense of strength and stability that was reassuring in the hotellobby that had been the scene of such a tragedy.