"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 agents.
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, he says.
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 would like 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.