Engineers are Baffled Over the Collapse of 7 WTC
New York Times
29 November 2001
by James Glanz
Almost lost in the chaos of the collapse of the World Trade Center is a mystery that under normal circumstances would probably have captured the attention of the city and the world. That mystery is the collapse of a nearby 47-story, two-million-square-foot building seven hours after flaming debris from the towers rained down on it, igniting what became an out-of-control fire.
Engineers and other experts, who quickly came to understand how hurtling airplanes and burning jet fuel had helped bring down the main towers, were for weeks still stunned by what had happened to 7 World Trade Center. That building had housed, among other things, the mayor's emergency command bunker. It tumbled to its knees shortly after 5:20 on the ugly evening of Sept. 11.
The building had suffered mightily from the fire that raged in it, and it had been wounded by the flying beams falling off the towers. But experts said no building like it, a modern, steel-reinforced high-rise, had ever collapsed because of an uncontrolled fire, and engineers have been trying to figure out exactly what happened and whether they should be worried about other buildings like it around the country.
As engineers and scientists struggle to explain the collapse of 7 World Trade Center, they have begun considering whether a type of fuel that was inside the building all along created intensely hot fires like those in the towers: diesel fuel, thousands of gallons of it, intended to run electricity generators in a power failure.
One tank holding 6,000 gallons of fuel was in the building to provide power to the command bunker on the 23rd floor. Another set of four tanks holding as much as 36,000 gallons were just below ground on the building's southwest side for generators that served some of the other tenants.
Engineers and other experts have already uncovered evidence at the collapse site suggesting that some type of fuel played a significant role in the building's demise, but they expect to spend months piecing together the picture of what remains a disturbing puzzle.
"Even though Building 7 didn't get much attention in the media immediately, within the structural engineering community, it's considered to be much more important to understand," said William F. Baker, a partner in charge of structural engineering at the architectural firm Skidmore, Owings & Merrill. "They say, `We know what happened at 1 and 2, but why did 7 come down?' "
Engineers said that here and across the country, diesel-powered generators are used in buildings like hospitals and trading houses, where avoiding power outages is crucial. Partly for that reason, Jonathan Barnett said, a definitive answer to the question of what happened in 7 World Trade Center is perhaps the most important question facing investigators.
"It's just like when you investigate a plane crash," said Dr. Barnett, a professor of fire protection engineering at the Worcester Polytechnic Institute. "If we find a weakness in the building or a deficiency in the building that causes that collapse, we then want to find that weakness in other buildings and fix it."
In many ways, 7 World Trade Center, built and owned by Silverstein Properties, was structurally similar to its towering cousins across Vesey Street to the south. The weight of the building was supported by a relatively tight cluster of steel columns around the center of each floor and a palisade of columns around the outside, in the building's facade.
Sprayed on the steel, almost like imitation snow in holiday decorations, was a layer of fireproofing material, generally less than an inch thick. Although the fireproofing was intended to withstand ordinary fires for at least two hours, experts said buildings the size of 7 World Trade Center that are treated with such coatings have never collapsed in a fire of any duration.
Most of three other buildings in the complex, 4, 5 and 6 World Trade, stood despite suffering damage of all kinds, including fire.
Still, experts concede, in a hellish day, 7 World Trade might have sustained structural injuries never envisioned in fire codes. That day began with flaming pieces of steel and aluminum and, horribly, human bodies raining around the building.
With the collapse of both towers by 10:30 a.m., larger pieces of the twin towers had smashed parts of 7 World Trade and set whole clusters of floors ablaze. An hour later, the Fire Department was forced to abandon its last efforts to save the building as it burned like a giant torch. It fell in the late afternoon, hampering rescue efforts and hurling its beams into the ground like red-hot spears.
Within the building, the diesel tanks were surrounded by fireproofed enclosures. But some experts said that like the jet fuel in the twin towers, the diesel fuel could have played a role in the collapse of 7 World Trade.
"If the enclosures were damaged, then yes, this would be enough fuel to explain why the building collapsed," Dr. Barnett said.
Dr. Barnett and Mr. Baker are part of an assessment team organized by the American Society of Civil Engineers and the Federal Emergency Management Agency to examine the performance of several buildings during the attacks. If further studies of the debris confirm the findings of extremely high temperature, Dr. Barnett said, "the smoking gun would be the fuel."
Others experts agreed that the diesel fuel could have speeded the collapse, but said the building might have met the same fate simply because of how long it burned.
"The fuel absolutely could be a factor," said Silvian Marcus, executive vice president for the Cantor Seinuk Group and a structural engineer involved in the original design of the building, which was completed in 1987. But he added, "The tanks may have accelerated the collapse, but did not cause the collapse."
Because of those doubts, engineers hold open the possibility that the collapse had other explanations, like damage caused by falling debris or another source of heat.
The fuel tanks were not the only highly flammable materials in the building. But while some engineers have speculated that a high-pressure gas main ruptured and caught fire, there was none in the area, said David Davidowitz, vice president of gas engineering at Consolidated Edison. The building was served only by a four-inch, low-pressure line for the building's cafeteria, Mr. Davidowitz said.
The mayor's command bunker, built in 1998, included electrical generators on the seventh floor, where there was a small fuel tank, said Jerome M. Hauer, director of the mayor's Office of Emergency Management from 1996 to 2000. That tank was fed by a tank containing thousands of gallons of diesel fuel on a lower floor, he said.
Francis E. McCarton, a spokesman for the emergency management office, confirmed that assessment. "We did have a diesel tank in the facility," he said. "Yes, it was used for our generating system."
The manager of the building when it collapsed, Walter Weems, said the larger tank sat on a steel-and-concrete pedestal on the second floor and held 6,000 gallons of diesel fuel. He said an even larger cache, four tanks containing a total of 36,000 gallons of diesel fuel, sat just below ground level in the loading dock near the southwest corner of the building.
"I'm sure that with enough heat it would have burned," Mr. Hauer said of the diesel. "The question is whether the collapse caused the tank to rupture, or whether the material hitting the building caused the tank to rupture and enhance the fire."
Falling debris also caused major structural damage to the building, which soon began burning on multiple floors, said Francis X. Gribbon, a spokesman for the Fire Department. By 11:30 a.m., the fire commander in charge of that area, Assistant Chief Frank Fellini, ordered firefighters away from it for safety reasons.
A combination of an uncontrolled fire and the structural damage might have been able to bring the building down, some engineers said. But that would not explain steel members in the debris pile that appear to have been partly evaporated in extraordinarily high temperatures, Dr. Barnett said.
"Any structure anywhere in the world, if you put it in these conditions, it will not stand," Mr. Marcus said. "The buildings are not designed to be a torch."
Engineers are Baffled Over the Collapse of 7 WTC