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9/11 - First Steel-Reinforced Skyscraper
To Ever Collapse in Fire

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March 2, 2002, Saturday


A NATION CHALLENGED: GROUND ZERO; Burning Diesel Is Cited in Fall Of 3rd Tower


Massive structural beams that functioned as a sort of bridge to hold up the 47-story skyscraper known as 7 World Trade Center were compromised in a disastrous blaze fed by diesel fuel, leading to the building's collapse on Sept. 11, investigators have concluded in a preliminary report.

The tower was set on fire by debris from the twin towers and burned for about seven hours before collapsing in the late afternoon under previously unexplained circumstances. The analysis of its collapse is one of the first detailed findings by a team of engineers organized by the Federal Emergency Management Agency and the American Society of Civil Engineers to understand the fate of all the buildings around the site.

As much as 42,000 gallons of diesel fuel was stored near ground level in the tower and ran in pipes up to smaller tanks and emergency generators for Mayor Rudolph W. Giuliani's command center, the Secret Service's office and other tenants.

Investigators have determined that the burning fuel apparently undermined what is known as a transfer truss. The trusses, a series of steel beams that allowed the skyscraper to be built atop multistory electricity transformers, were critical to the structural integrity of the building and ran near the smaller diesel tanks.

A failure of the same type of structural bridge contributed to the collapse of the Alfred P. Murrah Federal Building in Oklahoma City when it was bombed in 1995. Federal guidelines for public buildings, created in 1996, warned of the dangers of such trusses in terrorist attacks.

''It's certainly right in the vicinity where the columns go into this transfer system,'' said a person knowledgeable about the investigators' draft report on the World Trade Center. ''The rest of the building is built on top of the bridge.''

While 7 World Trade Center, which stood across Vesey Street just to the north of the twin towers, was not formally a federal building, it did house crucial government offices that included the city's nerve center for emergency response.

The investigators said that their conclusions, combined with other findings about the failure and collapse of 5 World Trade Center, could prompt serious changes in the codes used in building construction.

The findings are in a draft report that has already been circulated among government agencies, and are based on videos made on Sept. 11, witnesses' reports, interviews with firefighters, evidence from the debris pile and structural analysis. Team members, who described many of the findings, cautioned that the conclusions on the collapse of 7 World Trade Center could still be modified as reviews proceed.

But Irwin Cantor, one of the building's original structural engineers, who is now a consulting engineer and member of the City Planning Commission, said the diesel-related failure of transfer trusses was a reasonable explanation for the collapse.

He said he believed that diesel tanks were not envisioned in the original design of the building. ''It ended up with tenants who had diesels,'' Mr. Cantor said. ''I know none of that was planned at the beginning.''

According to floor plans submitted to the Port Authority of New York and New Jersey, which owns the land on which 7 World Trade sat, the building complied with city fire codes, said Frank Lombardi, the authority's chief engineer. Those codes permit no more than one fuel tank with a capacity of 275 gallons or less on above-ground floors, he said.

Jerome M. Hauer, who was the director of Mayor Giuliani's Office of Emergency Management at the time the command center was opened at 7 World Trade, said several teams of engineers reviewed plans to open the office there. But no one ever mentioned any hazard associated with placing fuel tanks above ground, near a transfer truss, he said.

''There were a host of people who looked at this,'' said Mr. Hauer, who is now a managing director of the crisis and consequence group at Kroll Worldwide, a security consulting company based in New York. ''We relied on their judgment.''

Fire officials did at one point question the storage of large amounts of fuel well above the ground level, saying that one large tank for the mayor's command center, if ever compromised, might fuel a fire that would threaten the building.

The Sept. 11 draft report also has photographs and a description of debris collected from a previously undisclosed, multistory collapse within 5 World Trade Center, a nine-story office building that also burned on Sept. 11 but largely remained standing. The team has found that one specific type of bolted connection, called a column tree connection, that joined floor-support beams, failed in the heat of the fires, causing the four-story collapse in the part of 5 World Trade at the corner of Vesey and Church Streets.

Although no one died as a result of the collapses in 5 and 7 World Trade Centers, since both stood long enough to be evacuated, the team's findings are likely to lead to recommended changes in the way public and government buildings are constructed, much the way similar studies did after the Northridge earthquake near Los Angeles in 1994 and the Oklahoma City bombing.

The team is still deliberating on how tightly it can pin down the precise train of events that led to the collapse of the twin towers themselves. But until now, the collapse of 7 World Trade has stood as one of the outstanding mysteries of the Sept. 11 attack, since before then, no modern, steel-reinforced high-rise in the United States had ever collapsed in a fire.

High-rise buildings are designed to be able to survive a fire, even if the fire has to burn itself out. The strategy is to ensure that the steel support structures are strong enough or protected well enough from fire that they do not give way in the time it takes for everything inside an office building, like furniture, to burn.

In major high-rise fires elsewhere in the country, such as the 1 Meridian Plaza fire in Philadelphia in 1991 and the First Interstate Bank fire in Los Angeles in 1988, this approach has worked. The 1 Meridian fire burned for 19 hours, leaping from floor to floor and burning out as combustible materials were used up. But the fires at 7 World Trade Center raged mainly on lower floors and never burned out, and in the chaos of Sept. 11, the Fire Department eventually decided to stop fighting the blazes.

''What the hell would burn so fiercely for seven hours that the Fire Department would be afraid to fight it?'' said one member of the investigating team.

According to the Port Authority floor plans, 275-gallon diesel tanks sat on the fifth, seventh and eighth floors and were fed through pipes from the larger tanks near ground level. The team member said that while the diesel fuel remains the most likely candidate for feeding the fires, it was still unknown whether there could have been other sources of fuel in the building, kept there by tenants like the Secret Service that have disclosed little of what their spaces contained.

The huge steel transfer trusses ran mostly through the fifth, sixth and seventh floors where the fires burned. The purpose of the trusses, which included zigzagging and horizontal members and were concentrated around the building's core, was to allow 7 World Trade to be built over two Consolidated Edison substations that already existed on that spot when the building went up in the late 1980's. Together the stations held 10 transformers, each about 35 feet high and 40 feet wide.

Using the trusses to avoid having vertical structural columns pierce the transformers, the building was constructed around them like a hen sitting on a giant egg.

''We had to do design tricks to accommodate the existing Con Ed facility,'' said Mr. Cantor, the structural engineer. ''This building had an awful lot of transfers.''

Transfer trusses are a well-tested technique and are used in countless high-rise buildings, as well as in bridges around the world. Engineers say that transfer trusses, for most buildings, present no extraordinary hazard. But if there is an explosion, earthquake or long-burning fire, they can present a problem.

In Oklahoma City, during the 1995 bombing of the Federal Building, a large transfer girder on the building's third floor gave way, helping to precipitate a progressive collapse that later analysis showed was responsible for most of the 168 deaths. After this attack, federal guidelines for buildings that would hold government agencies were changed, recommending that buildings be designed so that single-point failures did not cause a catastrophic collapse.

Videos of the 5:28 p.m. collapse of 7 World Trade lend vivid support to the truss-failure theory. Roughly 30 seconds before the building goes down, a rooftop mechanical room starts to disappear, falling into the building's core. Then a second larger rooftop room sinks. The building then quickly collapses.

Both rooms were above sections of the building held up by the trusses. Other video evidence shows fire concentrated in the floors containing the trusses and the fuel tanks.

Dr. John D. Osteraas, director of civil engineering practice, Exponent Failure Analysis Associates, in Menlo Park, Calif., reviewed videos of the collapse, discussed it with other engineers and came to a similar conclusion; the fuel, the trusses and the fire brought 7 World Trade down. ''The pieces have come together,'' he said. ''Without the fuel, I think the building would have done fine.''

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First modern, steel-reinforced skyscraper to ever collapse in a fire