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March 2, 2002,
A NATION CHALLENGED: GROUND ZERO; Burning Diesel Is Cited in Fall Of
By JAMES GLANZ and ERIC LIPTON (NYT) 1715 words
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
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
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
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
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
''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
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
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.''