World Trade Center, erected as “a living symbol of man’s dedication to world peace,” (Wetxstein- 2001) was the most valuable commercial property in the history of New York City until terrorist attacks reduced it to rubble. The buildings towered over lower Manhattan for nearly thirty years; an icon of financial power.
The buildings contained more than 200,000 tons of steel, 425,000 cubic yards of concrete and 600,000 square feet of glass in 43,000 windows. Each floor, a reinforced concrete pad on a metal deck supported by steel cross beams, was about one acre and weighed about 4.8 million pounds. The building was designed to withstand the impact of a 707 commercial airliner. It was the largest commercial airliner built at the time the World Trade Center was designed. This was a factor considered due to the accident in 1945, where a B-25 crashed into the Empire State building.
The twin towers were both 110 stories tall. The towers were so tall — 1,362 and 1,368 feet — that they swayed by up to 11 inches in a strong wind. Building towers of such height posed challenges for engineers, requiring development of a new system of construction that placed major supporting elements in the outer portions of the building to increase stability. Traditionally, such elements had been placed in the building’s core around elevators and restrooms.
At the time of the towers’ construction, this “tubular skyscraper” scheme was hailed as the key that would push the world’s buildings to elevations undreamed-of by previous generations. (Duin-2001)
In most buildings, structural steel supports are 20 to 25 feet apart. In the World Trade Center, the supports are only 39 inches apart. Each tower was a great rectangular tube: The inner core of steel columns contained the elevator shafts and the mechanical/electrical systems and acted as the vertical support. The exterior frame supplied support against wind forces. Unlike the method of construction used in many older buildings, there were no supporting columns between the core and the exterior, leaving about an acre of space on each floor for offices.
While terrorists may have been at the helm of the aircraft that crashed into the World Trade Center and triggered the collapse of the World Trade Center’s Twin Towers, the energy was supplied by Nature’s most ubiquitous force: gravity. The force that keeps our feet on the ground and holds planets in orbit swiftly fueled a runaway momentum that created thundering impacts so great they were detected by equipment designed to monitor earthquakes. Mathematically, the momentum is a function of the buildings’ height and weight, along with the acceleration of gravity. The total energy released by the impacts, explosions and collapses was between 2 and 5% that of the Hiroshima bomb. Much of this energy went into destroying the buildings themselves and generating the cloud of debris.
Many engineers believe that the impact and explosion not only removed the fireproofing on structural elements, but completely shattered and destroyed large floor areas and many internal support columns in both buildings, leaving the rest vulnerable. In the North tower, the impact destroyed core columns on the building’s north side on three floor levels between the 94th to 99th floors. The lack of support caused the floors above the impact area to sag, and shifted some of the vertical support to the exterior frame. (Newsday-2001)
The fire, resulting from the explosion of the aircraft impacting the building, is believed to be the major cause of the failure of the building. When the Empire State Building, the Chrysler Building and other New York skyscrapers were being built, the steel columns that supported them were insulated with concrete, making them safe from a meltdown disaster. This was an expensive process, so the builders of the World Trade Center towers sought an equally effective new one. Such a process was developed — a sprayed thermal insulation of asbestos and mineral wool that could resist tremendous heat for a minimum of four hours before the girders might topple.
The twin towers were so designed, the plan being that they could withstand even the most catastrophic fire long enough to evacuate those in the higher floors by helicopter. But while the towers were being built, the enviros launched one of their hysterical campaigns against the use of asbestos, and the poltroons of the New York City Council voted to ban it — in no matter what form.
The direct result was that from the 64th floor up, the girders of the twin towers were deprived of the necessary fireproofing. At the time, Herbert Levine, an expert in the insulation of steel building columns, warned: “If fire breaks out above the 64th floor, the building will fall down.” (Toledano, Insight — 2001)
A theory advanced by some engineers that has gained prominence holds that the collapses were inevitable once the connections between the floors and the exterior frame were severed, probably from the heat of the fire, in turn leaving the columns unsupported and causing them to buckle. (Newsday 2001)
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. (Post — 2001) The collapses and lingering fires turned the towers into a “giant sand pile” – the description of Leslie Robertson, the towers’ engineer of record. The evidence that would confirm the actual sequence is buried somewhere in that pile, with little likelihood of discovery, or it was destroyed.
The size and weight of the concrete floor pads, and the way they collapsed onto one another, helps explain why so much of the mass of each building was smashed into small pieces, demolition engineers said. As each concrete floor pancaked into the one below, their combined weight grew so large that they and everything in their path broke into smaller and smaller pieces, until mostly dust and small pieces of rubble remained. (Post-2001) Once the higher floors began to collapse and fall, they gained momentum, and as each floor collapsed into the next, more weight was smashed into the floor below and the momentum built tremendously and rapidly.
When the crumbling towers thudded to the ground, they created ground tremors equal to earthquakes of magnitude 2.1 and 2.3, as measured by a seismic station in Palisades, New York, 21 miles (34 kilometers) north of Manhattan. (Rayman-2001) Even the impact of the aircraft into the building caused the seismic registrations of .5 and .7 respectively.
These would be small numbers for earthquakes, not likely to be felt by anyone, since an earthquake is typically centered miles below the surface, its energy radiating out and dissipating over great distances. But the energy from the collapse of the towers was focused in a very small area, making it remarkable that the events were detected at all outside lower Manhattan.
Structural engineers have said that if fire had not brought the buildings down, a strong wind might have eventually toppled them. “If there was no fire, in my opinion the buildings would be standing today,” Magnusson said in a telephone interview. “And they would stand until there was a significant wind storm.” (Newsday — 2001)
The location of the impact may also have played a part in the collapse. Investigating engineers believe that the South Tower — which was struck 15 minutes later than the North Tower but fell 29 minutes before its twin — collapsed more quickly because the two planes slammed into the buildings at different places. (Koenig-2001) The second crashed off-center and likely damaged more of the interior columns. It also hit lower, meaning that the weakened columns had to support the weight of 15 more floors above them.
Engineers have also suggested that the building might have had other structural damage, something that might never be determined from the rubble of the disaster. The impact of the hijacked jets could have created a vibration, or oscillation, in the buildings, said Arthur Lerner-Lam, Associate Director for geology and geophysics at Columbia University’s Lamont-Doherty Earth Observatory, which reported the seismic observations. (Britt-2001) All tall buildings can oscillate in the wind, a factor taken into account by architects. But the vibrations set in motion early in the morning on Sept. 11 would likely have been at a higher frequency than those caused by wind. It’s possible, though speculative, to say so given scant evidence, which the higher frequency might have damaged the building to the extent that it would have ultimately toppled. However, some eyewitness reports told of what appeared to be buckling in walls below the level of the fire.
The potential vibration as similar to a ringing bell. The initial ringing would have damped out quickly. But if large enough, it could have left permanent damage. In such a scenario, the steel support beams would behave like plastic: Bend them a little, they spring back. Bend them further, they stay bent. Bend them enough, they break. (Britt-2001)
Will the demise of the World Trade Center towers dampen plans for future giant skyscrapers? Engineers expect to see efforts to make the next generation of big buildings more resistant to blasts and fires. But the decision to build a superskyscraper is a political and economic question, not an engineering one.
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Post, Nadine. Behavior of Twin Towers In Collapse Mystifies Experts. 2/8/02 .
Rayman, Graham. “A Debate Over WTC Collapse .” Newsday. 12/01.
Leary, Warren . “Years to Build and Moments to Destroy: How the Twin Towers Fell.” NY Times. 9/25/2001.
The World Trade Center. 2/8/02 .
Wetzstein, Cheryl. “Twin Towers History.” Insight on the News. 10/15/01.
Kamin, Blair. “Engineers shocked by towers’ collapse.” Chicago Tribune. 9/11/01.
Duin, Julia. “Bigger better? Terrorism has ignited debate about the future of tall buildings in America. .” Insight on the News. 12/10/01.
Koenig, Robert. “WORLD TRADE CENTER: After the Fall .” Science. 11/23/01.
