Having spent a good part of my career investigating structural failures, I've seen spectacular damage on building sites. It's the scale of the destruction, however, that distinguishes the attack on the World Trade Center. It was the biggest single building collapse in history.
On a personal level, like millions of others, I was horrified by the tragedy and loss of life. However, it was impressive and heartening to witness the heroism of people at the scene when the events occurred, and the heroism of the rescue workers and construction workers who searched for the living and then cleared the site.
One of the most significant findings of the investigation from a structural engineering standpoint was that the plane's impact on each building was not sufficient to cause the collapse. The collapse required a second major event: fire.
Each of the Trade Center towers survived for a relatively long time. Based on studies by others, it appears that, except for those who stayed behind to help with the rescue or who otherwise chose not to escape, almost everyone was able to get out. The long survival time was made possible by the large number of exterior columns that formed the towers' structural skin and the extra strength of these columns. Even after two-thirds of them were knocked out on one side of each building, the remaining columns had enough capacity to carry the load. It was only after the fire heated the steel enough to significantly reduce its strength that the collapse occurred.
Although the fuel in each aircraft caused several floors to catch fire immediately, we calculated that the aircraft fuel was burned up in approximately nine minutes. Therefore, most of the fire was fueled by the buildings' contents and by flammable materials brought in by the aircraft.
The aircraft impact did cause serious damage, of course. It destroyed all of the escape stairs in the North Tower and two of the three escape stairs in the South Tower. As a result, nobody escaped from Tower One above where the aircraft hit, and only a few are known to have escaped from above where the plane hit Tower Two. We also noted that the aircrafts' impact dislodged fireproofing, thereby shortening the time it took to bring the buildings down.
Damage in other nearby buildings also was significant. Two buildings near the towers caught fire and collapsed — the first examples of fire-rated buildings collapsing as a result of fire.
In one of the two, the collapse was triggered by the failure of connections. In the other building, we believe that the fire weakened a transfer truss, causing the building to collapse. This suggests that, for buildings that might be terrorist targets, more attention needs to be paid to fireproofing of connections and transfer trusses.