Massive Assault Doomed Towers
By Nadine M. Post with Sherie Winston
In the aftermath of the Sept. 11 terrorist
attacks in New York City and Washington, D.C., which brought down
the twin 110-story towers of the World Trade Center and damaged
the Pentagon, designers and contractors say they are skeptical that
signature structures can be hardened against extreme acts of barbarism.
"Only the containment building at a nuclear
powerplant" is designed to withstand such an impact and explosion,
says Robert S. Vecchio, principal of metallurgical engineer Lucius
Pitkin Inc., referring to the hijacked Boeing 767 airplanes, heavy
with fuel, that slammed into each WTC tower.
The attacks appeared to be coordinated and
in parallel. In Manhattan, at least two hijacked Boeing 767 airplanes,
one with 92 people on board and the other with 65, crashed into
the World Trade Center's twin towers, disappearing within and triggering
fire and explosions. The north tower, called One WTC, was hit at
8:45 a.m.; the south tower, Two WTC, at 9:03 a.m. Another hijacked
airliner, a Boeing 757 with 64 people on board, crashed into a section
of the Pentagon at 9:40 a.m.
Some 50,000 people work at the World Trade
Center and some 23,000 at the Pentagon. Specifics on death tolls
and damage were not available at press time. WTC rescue efforts
were put off until Sept. 12 because the area was still a "hot
zone" late in the day on Sept. 11, with falling debris from
the stumps that were once among the world's tallest buildings, and
a thick blanket of soot for blocks around. In addition, the 47-story
Seven WTC collapsed in the evening, causing more chaos. "It'll
take about a year to clean up" the remains, says Vecchio.
Sources close to the carnage indicate that
those below the 89th floor in the north tower and the 60th floor
in the south tower were most likely to have survived. Reports indicated
that at least 200 of the 400 firefighters at the scene are presumed
dead and 78 police officers were missing.
Confirmation of collateral damage to the numerous
subway lines that converge under the WTC was not available. The
owner, the Port Authority of New York and New Jersey, which just
leased the WTC complex to Silverstein Properties, New York City,
set up a command center across the Hudson River in Jersey City,
N.J., trying to locate its employees, who were on floors 63 to 85
in the north tower. Additionally, in the hours after the attack,
the port authority located temporary quarters at sites at John F.
Kennedy International Airport in Queens; at the Teleport on Staten
Island; and in Newark, N.J.
The port authority issued the following press
release: "Our hearts and our prayers go out to the families
of the countless people, including many members of the port authority
family, who were killed today in this brutal and cowardly attack.
We at the port authority are doing everything in our power to rescue
and care for the injured, and to comfort and assist the families
of the victims."
Meanwhile, Jim Rossberg, director of the Structural
Engineering Institute of the American Society of Civil Engineers,
is organizing two forensic teams, one for the WTC and the other
for the Pentagon.
The twin towers, framed in structural steel,
had exterior moment frames with 14-in. steel box columns spaced
39 in. on center. The configuration created a complete tube around
the building. The central steel core carried gravity loads only.
The exterior tube provided all the lateral resistance. Horizontal
steel trusses spanned 60 ft from the exterior wall to the core.
Concrete on metal deck completed the floor diaphragm.
Each tower contained about 100,000 tons of
steel and 4 in. of concrete topping on the 40,000-sq-ft floors,
according to Henry H. Deutch, assistant to the chief structural
engineer for construction manager Tishman Realty & Construction
Co. Inc., New York City, during the construction of the WTC and
currently head of HHD Consultants Inc., Osceola County, Fla.
Deutch says that originally, the north tower
contained asbestos in its cementitious fireproofing as did the first
30 stories of the south tower. He believes the asbestos, which had
been encapsulated, was removed after the 1993 bombing. In a press
conference, Mayor Rudolph W. Giuliani said the city's health department
had tested the air in the area and found no undue amount of chemical
The attacks were witnessed by hundreds of
people in each of the locales. Vecchio, who was part of the investigation
of the 1993 bombing of the WTC, was an eyewitness to the Manhattan
debacle from Pitkin's office about 1¼2-mile north of the
trade center. "The explosions were so tremendous," he
says. "You could smell the jet fuel in the air."
Millions across the nation also "saw"
the towers collapse, through live television news coverage. The
south tower fell at 10 a.m. and the north tower at 10:29 a.m.
Reports indicate that the impact of each plane
compromised the structural integrity of each tower, knocking out
perimeter columns and the interior structure. The explosions then
caused further damage, sweeping through several floors. "These
were airliners scheduled for long flights, full of fuel, causing
massive explosions," says Richard M. Kielar, a Tishman senior
vice president. "No structure could have sustained this kind
of assault," says Kielar.
As the fires burned, the structural steel
on the breached floors and above would have softened and warped
because of the intense heat, say sources. Fireproofed steel is only
rated to resist 1,500 to 1,600° F. As the structure warped and
weakened at the top of each tower, the frame, along with concrete
slabs, furniture, file cabinets, and other materials, became an
enormous, consolidated weight that eventually crushed the lower
portions of the frame below.
Jon D. Magnusson, chairman-CEO of Skilling
Ward Magnusson Barkshire Inc., Seattle, one of the successor firms
of Skilling Ward Christiansen Robertson, structural engineer for
the original World Trade Center, agrees: "From what I observed
on TV, it appeared that the floor diaphragm, necessary to brace
the exterior columns, had lost connection to the exterior wall."
When the stability was lost, the exterior
columns buckled outward, allowing the floors above to drop down
onto floors below, overloading and failing each one as it went down,
A big question for implosion expert Mark Loizeaux,
president of Controlled Demolition Inc., the Phoenix, Md., is why
the twin towers appeared to have collapsed in such different ways.
Observing the collapses on television news,
Loizeaux says the 1,362-ft-tall south tower, which was hit at about
the 60th floor, failed much as one wouldlike fell a tree. That is
what was expected, says Loizeaux. But the 1,368-ft-tall north tower,
similarly hit but at about the 90th floor, "telescoped,"
says Loizeaux. It failed vertically, he adds, rather than falling
over. "I don't have a clue," says Loizeaux, regarding
the cause of the telescoping.
The twin towers were part of a seven-building
complex designed by architect Minoru Yamasaki that covers eight
city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with
3-ft-thick retaining walls, is under more than half the complex,
including the twin towers and the adjacent hotel. The complex was
completed in phases beginning in 1970 (ENR 7/9/64 p. 36). The 1.8-million-sq-ft
Seven World Trade Center, constructed in the mid-80s, also had a
steel moment frame from the seventh story up (ENR 11/28/85 p. 30).
Security measures were tightened at the 12-million-sq-ft
WTC complex after a terrorist bomb on Feb. 26, 1993. That bomb blew
out one section of a north tower basement X-brace between two of
the perimeter columns. The blast ripped out sections of three structural
slabs in the basement levels between the north tower and the hotel,
threatening the structural integrity of the foundation box. It did
little damage to the north tower's structural tube, other than the
affected X-brace. Damage was extensive to the other building systems,
however, because the bomb compromised major utility lines in the
basement, and the brace compromised the central core wall, allowing
soot and smoke to shoot up the building core (ENR 3/15/93 p. 12).
In Washington, Pentagon officials were still
assessing the damage and the fire was still burning nearly seven
hours after the building was hit. At press time there were no details
about injuries and fatalities.
The impact was between the newly renovated
Wedge I and the about-to-be-renovated Wedge II, according to an
aide in the Pentagon's Renovation Office. Wedge I "did hold
up," the aide says.
Reports of damage also remain sketchy and
Pentagon officials decline to discuss specifics. However, the Pentagon
aide reports that the plane slammed into the southwest side of the
Pentagon adjacent to the heliport. The airliner reportedly hit at
the first and second floors, but later the upper three floors collapsed.
The U.S. Dept. of Defense's 6.5-million-sq-ft
headquarters, built as a temporary structure 58 years ago, was not
constructed with fire-resistant or bomb-resistant materials. The
overdue, multiyear renovation includes technologically advanced
materials designed to ward off severe damage from such attacks (ENR
9/4/00 p. 58).
Fire was leaping out the windows, primarily
on the upper floors of the unrenovated section. The adjacent renovated
section appeared to have less damage, likely because of the reinforced
glass windows and firewalls used in the renovation that was completed
less than a year ago.
In the aftermath of the 1993 bombing, WTC
structural consultant Leslie E. Robertson, Skilling's project engineer
for the original job, was convinced that the terrorists had meant
to take down the twin towers. After the events of Sept. 11, there's
little doubt that Robertson was correct.
Survivors Saw Tragedy (9/12/01)
By Eileen Cho, Debra Rubin, and Richard Korman
On that fateful sept. 11 day, craig Trykowski,
laborer for Henegan Construction, was doing construction for Lehman
Bros. on the 34th floor of the World Trade Center's north tower
when the first plane hit.
"I was trying to clear the area of sheetrock
...and just as I was filling the dumpster, the whole building shook,
it swayed back and forth," recalls Trykowski. "I thought
at first it was an earthquake." He and 70-odd workers hit the
stairwell as water pipes broke and panic flared. As he went down,
a group of firemen were going up, never to be seen again.
From across the street, where the Port Authority
police had moved them, "I saw 15 people jump out of windows....It
was so sickening because it looked like they were dummies thrown
out the windows."
Trykowski was one worker who escaped the pair
of terrorist crashes that decimated the World Trade Center Sept.
11. Tony Sweeney, a controller for the brokerage firm Jarban Intercapital,
was also able to vacate his 55th-floor office in time. But some
employees of companies such as the Regional Alliance for Small Contractors
and the Port Authority of New York and New Jersey were unaccounted
for at press time later that day. Raytheon Infrastructure Inc. occupied
an office on the 91st Floor of Two WTC. Architect Mancini-Duffy
was also in the building. About 200 members of Local 3 of the International
Brotherhood of Electrical Workers were employed there, too.
John Pascu, an electrical engineer for Parsons
Brinckherhoff, New York City, was sending off e-mails from his 74th
floor office in WTC One when "the building shook tremendously,"
he says. "I thought I was going to die." Pascu was one
of about 20 PB staffers working there on the Port Authority's $1.5-billion
light rail link to John F. Kennedy Airport. Pascu walked down 74
flights to exit the building. "I learned that emergency lighting
Less than 20 minutes later, Greg Kelly, a
PB vice president, was on a subway with associates Yalcin Tarhan
and Lou Silano, senior structural engineers, to try to aid rescue
efforts. But upon arrival, the group was "stuck in a subway
filling with smoke," says Kelly, who saw the second plane careen
into WTC Two. When they reached the street, "there was no visibility
and the debris was an inch thick. It was very eerie and desolate,"
The catastrophes will have a monumental effect
on the construction industry, including shifted priorities regarding
major projects, says Robert Prieto, PB chairman. "The emphasis
on security at airports will go through the ceiling, and there will
be new focus on building an underground defense facility. Defense
spending will go up," he predicts. "The economy in this
region will take it on the chin. The world as we knew it has just
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