A Q&A with Professor Henry Petroski

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

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

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

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.