Engineering firm investigates WTC collapse.

A Silverstein Properties-sponsored analyses of the World Trade Center attack determined that the buildings' collapse was unavoidable. On April 23, Weidlinger Associates, the engineering firm charged with the investigation of the event, cited damage to core support columns and extensive fuel fires as causes for the towers' destruction.

According to Weidlinger, the hijacked planes impacted the central core of the North Tower and the Southeastern exterior corner and central core of the South Tower, severing support columns, destroying fire-proof coating, and spilling burning fuel over multiple floors. The rising temperatures then weakened the buildings steel framing, eventually over whelming its load capacity.

"What happens to steel as a result of fire is that it loses its strength," explained Matthys Levy, a founding partner of Weidlinger Associates. "By the time the temperature inside the buildings reached 400 degrees, the steel would have lost approximately 50% of its strength. Eventually, gravity took over and the towers began to fall."

Levy was careful to point out, however, that even after the trauma of the initial explosions, the buildings remained standing, testifying to the durability of their design.

"Although more than 1/3 of the exterior support columns were destroyed in the initial impact, the buildings still stood for about an hour," he said. "Our analyses determined that the towers were extremely well built. They withstood the original impact and provided enough time for most of the occupants to escape."

Weidlinger also determined that the collapse of the South Tower, which fell first, had no significant impact on the collapse of the North Tower and did minimal damage to the buildings outside the immediate World Trade Center zone.

"The collapse of the towers was really a minor event in terms of impact on the adjacent buildings," Levy said. "And although the South Tower, when it fell, caused some damage to the exterior of the North Tower, no serious damage was done. And the reason for that was that the two towers were not facing each other."

According to Levy, little could be done to prevent the buildings' destruction, considering the damage the planes had caused. However, he feels that a more effective rescue-plan could have saved more people.

"A critical aspect of the event was the location of the fire stairs," he noted. "They were clustered around the centers of the buildings and were made unusable when the explosions occurred. That is why almost everyone on the lower floors managed to get out and almost no one from the upper floors escaped."

This destruction of the fire escapes and the fact that it took firefighters almost an hour to reach the upper floors accounted for most of the lives lost. Though the New York Fire Department was using the most sophisticated technology available, Weidlinger determined that a more efficient system for putting out high-rise fires needs to be developed.

"The buildings had tremendous reserve capacity and that was reflected in all of the elements we analyzed," Levy said. "In fact, because there were so much excess capacity, the columns even in the impact floors did not buckle immediately, but failed as the result of the fire. There is nothing that could have been done to change that result. But one of the issues we have to address is what to do about fires in high-rise buildings. The primary issue was the fire."

Weidlinger's study was done on behalf of Larry Silverstein for the purpose of his insurance claim. In its investigation the firm used advanced computer modeling, photo and video analyses, and original engineering plans for the World Trade Center.