Wednesday, September 12, 2001

The two airplanes that smashed into the World Trade Center’s twin towers may have destabilized them, but a deadly combination of fire, energy shifts and gravity brought down the skyscrapers, one of the nation’s leading experts in demolition said yesterday.
“When we take down structures with explosives implosion, if you will the explosives don’t bring down a building gravity does,” said Mark Loizeaux, president of Controlled Demolition Inc. in Phoenix, Md., “and that’s exactly what happened today. The airplanes did not bring down the structures. Gravity did.”
The massive, 110-story World Trade Center towers were especially sturdy, with load-bearing, re-enforced steel-mesh walls, said Mr. Loizeaux, who has 35 years of demolition experience and whose company brought down the terrorist-damaged Alfred P. Murrah Federal Building in Oklahoma City, the Seattle Kingdome and several other stadiums across the country.
When the jet aircraft both Boeing 767s struck the towers yesterday, they damaged sections of the skyscrapers’ outer metal columns and their internal connections to floor slabs.
Together, these structures carry much of the buildings’ weight, and once they were damaged, the weight shifted, “immediately overloading other sections of the buildings,” Mr. Loizeaux said.
The impacts from the planes probably also “knocked a lot of the fireproofing off the remaining columns,” he speculated.
As the fires from the exploding planes burned out of control, they heated up the metal in the buildings, he said. “The hotter metal gets, the softer it gets. Eventually it yields.”
“When I saw [on television] that a large airplane had collided with the first building, I knew it had pretty much destroyed [the buildings] support structure,” said Austin, Texas, structural engineer Ron Koenig of Koenig Consulting Engineers. “The combination of the impact and the heat from the fire on the steel columns probably began it. Once it started collapsing, the upper floors would overload each floor beneath continuously all the way to the ground.”
The Empire State Building survived an airplane crash on its 79th floor in 1945 because the Army Air Corps B-25 twin-engine bomber that got lost in a fog was a much smaller plane, and the building’s structure was more rigid, Mr. Loizeaux said.
The slender, carefully built Empire State Building had the “structural integrity” to shift and maintain the loads of weight and energy, he said.
The World Trade Center towers, he added, were built to withstand many kinds of forces, including wind and earthquake, but not large airplanes moving at 300 mph. “No one designs for that,” he said. “That’s like designing a structure to be hit by a meteor. What are the odds? The odds are changing, aren’t they?”

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