Engineers see measure of success in performance of WTC design

By Michelle Morgante Associated Press Writer
Friday September 28, 2001

CORONADO — As he watched the World Trade Center burn, Bob Hendershot knew the clock was ticking. In the safety of their San Diego home, his wife stared at the televised images and asked “What’s gonna happen?” 

The structural engineer knew that fire would most surely cause the buildings to collapse. At best, the thousands of people inside had three, maybe four hours to get out. 

In the end, the twin towers collapsed less than two hours after being rocked by jetliners. But Hendershot and his colleagues at a meeting in Coronado said Thursday they do not see the Sept. 11 disaster as a building failure. 

“From the point of view of those who escaped, this was a tremendously successful design,” he said. 

What lessons can be learned from the collapse of the World Trade Center towers is being discussed at the annual gathering of the Structural Engineers Association of California. 

Much of that will be determined after a team of “forensic engineers” goes to New York next week to begin reassembling the building debris. 

About a dozen structural engineers, metallurgists, fire protection engineers and other specialists will lay out pieces of the wreckage on a field in Staten Island, said Ron Hamburger, a member of the team. 

“Piece marks” recorded on the steel will be used as guides to enable them to put the debris together “like a giant jigsaw puzzle,” Hamburger said. 

The project will help explain what exactly caused the buildings to collapse. The engineers meeting at a Coronado resort have some preliminary ideas. 

The fires caused by fuel from the jetliners reached temperatures upward of 3,000 degrees, according to reports. Typical ceramic or asbestos fireproofing, such as that used in the twin towers, is intended to withstand fires as hot as 1,400 degrees for about 15 minutes. Meanwhile, steel is significantly weakened at 1,100 degrees. 

The damage from the jet crashes likely breached the towers’ fireproofing. That was why as he watched the buildings burn, Hendershot expected them to collapse. 

“Over a period of time, the steel would start to sag,” Hendershot said, recalling the time he watched the disaster unfold as emotionally traumatic. 

What likely happened, Hamburger said, was that in the intense heat, the connections which held the horizontal floor-framing members failed first. The floors then dropped off their horizontal support columns. 

“So as the floors started to fall off the columns as those connections failed, the columns would then begin to buckle,” he said. 

The weight of the dropping floors then caused a succession of buckling and collapse. 

But, Hendershot said, “these buildings withstood at least an hour, allowing thousands of occupants to exit — thousands of occupants. I think this is a testimony to quality engineering and consideration of life-safety issues.” 

Melvyn Green, who will replace Hendershot this week as the president of the California association, is joining a team of engineers who will consider whether the WTC disaster should lead to changes in building codes. 

One consideration Green has is the need to design stairways that accommodate firefighters entering buildings even as civilians are evacuating them. Workers who left the World Trade Center said they jostled to get past firefighters going up narrow stairwells. 

Designing “redundancy” in fire-protection mechanisms is another matter, he said. Green intends to stress the importance of alarms, practiced exit plans and perhaps fire wardens spaced throughout high-rise buildings. 

He also noted that the water sprinklers at the WTC were not effective against flammable liquid such as jet fuel and likely only served to spread the fire. 

The terrorist attack “was a horribly, carefully calculated procedure to do maximum damage,” he said. 

But planning for such unforeseeable catastrophes isn’t practical, due to the costs of using materials capable of withstanding intense heat, such as those used for space craft exteriors. 

“The fact is that when we design buildings, structures, bridges, we consider events that are likely to affect them, not any event that could credibly occur,” Hamburger said. The World Trade Center disaster “is not one that could credibly be anticipated.” 

Moreover, he said, the basic intent of designers is to allow people to safely evacuate a structure, not prevent structural damage. 

“Given that 90 percent or more of the occupants of the center were able to escape,” he said, “I think you would have to call it a success. A complete success? No. But a success.”