FROM CONCEPTION to destruction: The World Trade Center's twin towers took years to complete and less than two hours to disappear. But it's those two hours that have engineers marveling today.
These buildings' strength under pressure allowed thousands to escape.
"Absolutely!" says John Hooper of Skilling Ward Magnusson Berkshire, the Seattle engineering firm that led structural design of the famed towers. "It's not that they collapsed, but how long they stayed up after being hit."
Thousands of lives were lost in the assault by two Boeing 767 terrorist-controlled passenger jets, hijacked and flown 18 minutes apart into the 110-story towers—which, besides being engineered by Seattleites, were modeled after a Seattle building, designed by a Seattle-born and -schooled architect, built with Seattle-fabricated steel, and struck and felled by Seattle airplanes.
Set afire by jet fuel that melted steel at 1,600 degrees, the cores of the mammoth skyscrapers imploded and collapsed in a horrifying slow-motion free fall Tuesday morning, seconds sure to last America's lifetime. Yet tens of thousands of people had time to escape from the buildings, engineered 35 years ago chiefly by Seattle's John Skilling and Les Robertson.
"Ninety-nine percent of buildings would have been leveled on impact," says Hooper. The World Trade towers "stayed up for an hour, hour and a half, allowing thousands and thousands to evacuate. That's amazing under the circumstances."
A key element of the buildings' strength was the towers' redundant tube-and-donut construction—encircling walls of steel exterior bracing, stacked skyward in sections. That displaced the gravity load from the building's core, giving added structural integrity—a feat engineered by Skilling and conceived by the WTC chief designer, Seattle-born Minoru Yamasaki.
"The exterior walls had very closely spaced columns," says Skilling CEO Jon Magnusson, "welded to horizontal steel beams at each floor level to make an incredibly stiff grid." The grid "could carry the weight of the building above, even with a large section of the grid destroyed by the impact of the airplanes."
But if lives were saved because of the towers' planning, the Skilling firm wasn't hearing that story. Instead, engineers were being asked if the design was faulty and contributed to the loss. "The question shouldn't have been why did [the buildings] fall down, but why did they stay up?" says Magnusson, whose firm gave five dozen interviews to worldwide media in three days last week.
Spokesman Hooper says, "Most of the finger-pointing is coming from reporters. It got really frustrating for those of us dealing with the media. They didn't seem to understand you can't harden these buildings to make them totally bombproof."
The firm nonetheless has a special feeling of loss about the WTC. In a sense, rising and falling, the Trade Center was a Seattle building. "It makes you feel real sorry," says Nick Soldano, who in the 1960s and '70s was manager of the Pacific Car & Foundry (now Paccar) steel operations in a block-long warehouse on South Holden Street, a mile from Boeing's then-headquarters.
Paccar was one of 12 companies that supplied steel to the lower Manhattan center. Paccar provided almost one third—55,150 tons of prefab framing in three-story sections.
Those sections are all that remains erect at the tower site today, a twisted steel trellis marking an ash-gray war zone of rescue workers. "That's what we built, the last thing standing," says Soldano.
Yamasaki saw the Center as "a representation of man's belief in humanity."
Today it is the disfigured image of inhumanity. But the WTC will be rebuilt, vow New York officials. And Seattle's Skilling, at least, will be back, if asked. "They haven't called us yet," says spokesman Hooper. "But of course, in a minute."