When the engineers of New York's World Trade Center wanted to find out how much their building could sway in the wind before residents of the uppermost floors would notice, they called on psychologist Paul Hoffman, PhD, of the Oregon Research Institute (ORI).

It was 1965, and ORI, which Hoffman had founded five years before, was the only independent research organization in the United States that conducted basic behavioral research. So although Hoffman and his staff had no particular expertise in studying perceptual thresholds for horizontal acceleration--the psychological phenomenon that lay at the root of the engineers' question--they were still a natural choice for Les Robertson, the World Trade Center's (WTC's) chief engineer. Together with Paul Eskildsen, PhD, then a graduate student in psychology at the University of Oregon, and a team of engineers, Hoffman built a full-size office and reception area inside a downtown Eugene, Ore., office building and rigged the rooms so they could be swayed gently from side to side.

Hoffman and his team then invited unsuspecting participants into the office for an eye exam--Eskildsen was a licensed optometrist--and slowly ramped up the speed at which it swayed, watching carefully to see when participants first noticed that something unusual was happening. Hoffman and Eskildsen found that the threshold for noticing horizontal acceleration was ten times lower than the engineers had assumed it would be. Partly as a result of this study, Robertson decided to strengthen the exterior columns of the towers and make other changes to reduce the motion of the uppermost floors.

In an analysis conducted after Sept. 11, 2001, a team of engineers concluded that the "robustness and redundancy of the steel framing system"--in part a result of Robertson's concerns about the effects of wind-induced motion on the towers' occupants--helped the towers remain standing after the initial impact of the hijacked airliners.

Q:The office in which you set up the study was labeled the "Oregon Research Institute Visual Research Center" and decorated to look like an optometrist's office. What was the reason behind the disguise?

A:I knew that we'd have to place individuals in a room and that the room would have to move so that we could measure acceleration, but I wanted to try to find a way to get that motion in a way that didn't signal to the subjects the expectation that the room might begin to move. I assumed those expectations would influence our estimate of horizontal acceleratory threshold. As a matter of fact, as our study showed, it did influence it. So I began to think: How would you invite a person to come down to an office and sit in a room, for some purpose or other, and be able to start that room in motion?

Q:How did the WTC's designers react when you told them that people were much more sensitive to horizontal acceleration than they had thought?

A:When we sent our report to them and let them know what our findings were, they immediately sent out a large portion of their design team, including world-famous architect Minoru Yamasaki, members of the Port of New York Authority Board of Directors and Les Robertson. They wanted to see exactly what we had done to come up with this number. We had a very engaging session with them all; they wanted to take a ride in the device, so we obliged them.

After they left, I wasn't really sure what they were going to decide to do, but shortly after that they made the decision to replicate the study in the ventilation shaft of the Lincoln Tunnel in New York. Paul Eskildsen went back to New York and helped them set up the apparatus. Looking at it from the outside, it looked like a big packing crate suspended by cables, but the interior was very much like the one we had created at ORI.

Q: Did the replication support your findings?

A:Yes, the results were surprisingly similar. It was on that basis that Les Robertson began to rethink and redesign the structural elements of the building.

Q:The study was kept secret for more than 30 years, at the request of the Port Authority. You've said that you also were happy about the study's secrecy. Why was that?

A:My reasons were different from theirs. They never explicitly stated their reasons, but I gathered that it was a matter of considerable importance that the occupants of the building be as comfortable as possible. I was definitely aware of the possibility that people in the uppermost floors, where the displacement would have been greatest and where the price of the space would have been the highest, might experience acceleration at greater levels than others, and that would not be good for the Port Authority because they had to recover the cost of their buildings. From my perspective, I was happy about secrecy because Eugene is not that large a town. We wanted to be able to attract subjects who had no notion of the purpose of the project.

Q:Did you ever think about the safety implications of your research before Sept. 11?

A:I never thought about the safety implications. I think Les had mentioned to me at one time that he had considered what would happen if an airplane had crashed into the building, but he wasn't thinking about today's huge commercial jets, loaded with fuel. But I didn't think much about that. Our focus was on, first of all, whether people could perceive the acceleration in the twin towers due to their sizeable horizontal displacement in the expected Manhattan breezes and storms. Secondly, we were interested in whether occupants were likely to be made uncomfortable as a result of the acceleration.

Q:The New York Times journalists James Glanz and Eric Lipton have suggested that the design changes prompted by your study might have saved lives on Sept. 11. The results of the World Trade Center Performance Study seem to support that idea. What do you think?

A:Glanz felt that this might very well have saved a lot of lives, because, if you recall, the floors at the point of impact were completely demolished and most of the people there unfortunately perished very quickly, but the people who were residing in the lower floors had considerable opportunity to evacuate the building because it didn't all collapse at once. Not being an engineer, I really don't know, but it seemed to me that Glanz had the thought that if they had not strengthened the building the way they did, it might have come tumbling down much more quickly than it did. If that's the case, I'd be truly grateful for our contribution to the savings of thousands of lives.

Further Reading

Hoffman describes his study in more detail in the November issue of Psychological Science Agenda (www.apa.org/science). Glanz and Lipton's article in The New York Times Magazine is available from the World Trade Centers Association (iserve.wtca.org/news/RecentPress/wtcanewscenter.htm), and the World Trade Center Performance Study is available from the Federal Emergency Management Administration (www.fema.gov/library/wtcstudy.shtm).