September 13, 2001, CoStar Group, by Tim Trainor,
"The Bastards Finally Brought it Down"
Heat from fire, not structural damage from impact, called likely cause of tower collapses.
Clearly, the real estate angle in the story of the apparent terrorist attacks on New York City and the Pentagon in Arlington, VA, has no importance compared with the tragic loss of life and the terror instilled in those who experienced the horror firsthand, or watched dumbstruck as yesterday's events unfolded.
Other news organizations are far-better equipped and capable of covering the more important aspects of these events than we are.
But in keeping with our news purpose and the professional interest of our readers, we will focus on the implications for real estate/business owners affected by the destruction of more than one-tenth of the office space in Lower Manhattan.
Click here for information and data regarding the World Trade Center Complex.
Beyond their importance as an international icon of free enterprise and capitalism, the twin towers of the World Trade Center were an engineering marvel. Part of a seven-building complex designed by architect Minoru Yamasaki covering eight city blocks, more than half of the trade center, including the twin towers and the adjacent hotel, rested on an 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls.
But the real marvels were the twin towers. Framed in structural steel, the building exteriors created a rectangular tube surrounding a central steel core. The core carried gravity loads only, supporting the weight of the floors. The exterior tube provided all the lateral resistance, holding the whole structure upright. Horizontal steel trusses spanned 60 ft from the exterior wall to the core.
It will take weeks if not months before structural engineers fully assess the structural failure that led to the collapse of the twin towers that defined the Manhattan skyline for almost 30 years.
But given the catastrophic damage sustained after hijacked commercial jets crashed into each of the towers, and having watched their subsequent collapse, structural engineering experts speculated that the intense heat from burning aviation fuel, not the damage from the crashes themselves, eventually compromised the structural integrity of the 110-story towers beyond the breaking point.
"We were hopeful at first," Kevin Parfitt, an architectural engineer and instructor at Pennsylvania State University told The Washington Post. After all, the twin towers were among the strongest structures ever built, able to withstand earthquakes and hurricane-force winds. "But the longer the fire burned, the more we feared the outcome."
Experts speculated that the initial explosions likely blew away much of the fire insulation surrounding the towers' steel girders, exposing them to the jet fuel fires. Steel begins to weaken at 800°F. Fireproofed steel is designed to withstand 1,500° to 1,600° F, well above what would be found in a typical office fire. However, jet fuel burns at an exceedingly high temperature, more than 2,000° F.
The north tower, which had a large communications antenna on top, was hit by the first airplane at 8:45 a.m., just above the 70th floor. The south tower was hit at 9:03 a.m., but at a lower elevation.
Though hit second, the south tower collapsed first, at 10 a.m.; the north tower followed at 10:29 a.m. Later in the afternoon, the 47-story World Trade Center Building 7 also collapsed. Building 6, the U.S. Customs House, did not fall, but is likely damaged beyond repair.
Jon D. Magnusson, chairman and CEO of Seattle-based Skilling Ward Magnusson Barkshire, a successor firm to the structural engineer for the World Trade Center, told Engineering News-Record, an engineering and construction industry trade publication, "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."
No longer able to support the weight of the floors above them, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down in a progressive collapse, Magnusson said.
But Magnusson and other engineers said the amazing thing was that the towers remained standing so long after experiencing such a devastating blow, which they attributed as a testament to the design and the engineering skills of the designers. They also said there is no way to build a structure that could withstand the devastation created from the impact and fire of a fully fueled jetliner. Both the cost of construction and weight of the materials would be prohibitive.
"We can build them to withstand hurricanes, earthquakes, even electrical fires, but not something on the magnitude of this," said one architect.
After a terrorist bomb exploded in the World Trade Center's underground parking garage on Feb. 26, 1993, security measures were tightened at the 12-million-square-foot complex. The blast ripped out sections of three structural slabs in the basement levels between the north tower and the hotel, but fortunately did little damage to the north tower's structural tube.
The car bomb used in that attack compromised major utility lines in the basement and the central core wall, allowing soot and smoke to shoot up the building core, filling stairwells and elevator shafts. Six people died as a result of the explosion and evacuation.
A book documenting the first attack, Two Seconds Under the World, describes an exchange between an FBI agent and the man charged and eventually convicted of masterminding the bombing, a Pakistani national named Ramzi Yousef.
As the agents transported Yousef by helicopter to a federal courthouse in Manhattan for his trial, one of the agents escorting Yousef pointed to the gleaming towers and commented how his plan had failed. "See, you didn't get them after all," the book quotes the agent saying to Yousef. "Not yet," the terrorist responded quietly.
Recalling the incident for a reporter this week, one architect observed, "It looks like the bastards finally brought it down."