For experience with assessing how structures perform in the wake of a terrorist attack. W. Gene Corley is hard to beat. He is senior vice president of Construction Technology Laboratories, Skokie, Ill., and was principal investigator and leader for a study sponsored by the American Society of Civil Engineers (ASCE) of the 1995 Murrah Federal Building bombing in Oklahoma City.
Now he has been called upon again by the ASCE, this time to lead its investigations into the 11 September collapse of buildings at the World Trade Center (WTC), in New York City, and the destruction of part of the Pentagon, in Washington, D.C., on the same day. As might have been expected, he has reassembled a good part of his Oklahoma team.
Actually, two teams are being put together by Corley and the ASCE’s Structural Engineering Institute. Each consists of seven to 10 experts, including structural engineers as well as experts in blast and fire protection. All are working pro bono for the time being, though the ASCE is paying out-of-pocket costs. The cost in terms of salary-related expenses will be upwards of US $500 000. The cost of data acquisition and computer simulations will add substantially to that figure.
Last week, Corley spoke with IEEE Spectrum’s Harry Goldstein about the investigation, how the Towers 1 and 2 collapsed, and what the future holds for high-rise construction.
When did the investigation start?
The day after the attack.
What does the data acquisition involve?
First, we will be looking for photos showing the condition of the building from the time it was first hit and later. Still shots will be the most useful because we need their quality to identify the impact damage to the steel columns on the exterior of the building. We also will be looking at the videos–those should show us some of the damage that was not picked up by the still shots–and at Tower 7, which also collapsed, and at buildings that were badly damaged but stood up.
We will also be looking for information about the aircraft and estimates of how fast they were going when they hit, how much fuel they had, that sort of thing, in order to analyze their impact on the buildings. Then we’ll determine where the fire spread to immediately and how it spread after that, and make calculations to model what happened to each tower and the other buildings, to see if we can predict the way and the time at which they collapsed.
Based upon that, we will examine some structural variables to see if any reasonable things could be done in the future to give more time before collapse. The ideal would be to have time enough to put the fire out. Once the fire’s out, the buildings could be stabilized.
If the fire could have been stopped, would the buildings still have collapsed?
The general belief is that the impact of the aircraft did not bring the buildings down, it was the fire that followed. So if the fire could have been put out, it is reasonable to believe that the buildings would not have collapsed.
And that’s because the steel columns became heated?
Yes. As steel heats up, it loses strength. Depending on the type of steel you have at a given location, you can determine what the loss of strength is for a given temperature. When the planes hit, they took out some of the structure, but the building redistributed the loads and carried its the weight. The surviving structural members had to carry more load, so the fire didn’t have to heat them up as much as it would have normally for them to collapse.
At what temperature does steel fail?
You have to look at it for each member in the building because there are different kinds of steel in different members. A rule of thumb is that if you heat it up to around 600 C, you’ll lose approximately half the strength of the member and eventually, you can no longer to carry the loads. That’s not necessarily what happened, 600 C isn’t necessarily the temperature that was reached. That’s something we will be determining.
What kind of computer simulations will you be running?
I don’t have the name of the computer program yet, but it’s been described to me as one that can model impact, fire, and structural performance. If we can’t get that program or it can’t do all that, we’ll just have to go back and forth among separate programs. We definitely have access to programs that can model a fire, and we can model the impact in another and the structure in a third. We can use output from one to feed the other and iterate until we get an answer. We haven’t gotten far enough to decide all of these things yet.
What’s the timetable for data acquisition and the modeling?
The data acquisition started virtually the same day we started forming the committee. We had a press conference the Thursday after the attack. We were really starting to collect information then. We asked people for photos and video clips to analyze the exterior damage. Then we have had Bill Baker, a structural engineer with Skidmore, Owings and Merrill, Chicago, on the site; he was helping the search and rescue, but also was making mental and some written notes on what he saw as they unscrambled the debris. The next step will be to get our team on site to collect the needed information, and also get them access to plans and specifications and other things that we need to model the structure.