Strange as it sounds, the World Trade Center towers lived up to engineers' hopes in the moments after two hijacked jets slammed into their upper floors Tuesday morning.
The north tower remained standing for more than 1 1/2 hours after it was hit by American Airlines Flight 11. The south tower held on for almost an hour after it suffered more serious damage from the impact of United Airlines Flight 175.
The buildings withstood the impact of 400,000-pound Boeing 767s crashing into them at perhaps more than 300 mph.
"They rocked back and forth, and then stopped," says Masoud Sanayei, professor of civil and environmental engineering at Tufts University in Boston. "If there was no fire, we might have demolished the buildings a year or so down the road but people would have had an opportunity to evacuate."
But there were fires, fed by the estimated 60,000 pounds of jet fuel that each jet was likely carrying. These infernos may have hit 2,000 degrees.
That was just too much for the steel and concrete structures. Steel loses more than half its strength at 1500 degrees.
What followed was a building designer's nightmare. Some of the upper floors probably sagged two feet or more before finally breaking loose from the steel outer frames and inner cores that supported the buildings.
That dumped tons of concrete, fixtures and furniture on lower floors. A single floor could weigh as much as 3,000 tons. The falling debris started a top-down domino effect engineers call pancaking.
"The momentum was too much to handle," says William Faschan, a partner at Leslie E. Robertson Associates, the towers' structural engineers. "A given floor normally could support the weight of three floors. But that assumes the weight is imposed in a gradual manner."
There was nothing gradual about the process once each tower began to collapse. It took just moments for the growing weight and velocity of the wreckage to rip through lower floors as though they were tissue.
The buildings imploded just as they would if demolition experts had been hired to do the job. It was a pitiable ending for the 1,362-foot buildings that were engineering marvels and models of stability in 1973, when construction was complete.
The 110-story towers were assembled from prefab grills, shipped from as far away as Seattle and Los Angeles and hoisted into place by cranes specially built in Australia for the project. When bolted together, they created more than 130 vertical steel support columns on the exterior, spaced 3 feet 4 inches apart. Horizontal steel beams
from this outer tube helped to support the 4-inch concrete floors. They also connected to an inner tube - where the elevators were - which was supported by another set of steel columns.
"It was like a doughnut with this core element providing vertical support," says Tod Rittenhouse, principal of Weidlinger Associates. "This shell had tremendous strength."
Indeed, architect Minoru Yamasaki designed the towers to be far sturdier than state building codes required. The biggest threat to a building that tall is wind. New York insisted that buildings stand in the face of 90 mph storms.
But the towers were "built to withstand the largest hurricane that could be envisioned," Faschan says. The simplicity of the design helped. "It was such a box," says Mysore Ravindra, president of LeMessurier Consultants, the firm that engineered New York's Citicorp building.
Even so, after the collapse of the WTC towers, many will try to come up with ways to avoid similar calamities. Some, however, say that there's no realistic way to prevent a building from succumbing to such overwhelming attacks. USA Today
Published in CNN.