Why the Towers FellBy Yasmine Abbas
On the 11th of September, the twin towers of the World Trade Center, once the tallest on earth, were toppled with horrifying efficiency. Experts continue to ask the question, could we have limited the scope of the tragedy through technological means?
“Today I found myself walking to work, about the same time, looking down 6th Ave, and there is nothing,” said Erik Nelson, who graduated from MIT in 1999 with a Master of Engineering in High Performance Structures and is now working at ground zero.
“There’s absolutely nothing down there but some smoke. Now I am starting to understand the impact and am coming to grips with my emotions,” he said.
Three weeks ago, a fully fueled 767 hit the North Tower at high speed. Moments later a second passenger jet smashed into the 110-story South Tower. By day’s end, the collisions left a pile of rubble in Lower Manhattan.
Heat topples the buildings
The possibility for tragic accidents and potential safety measures, however, are always taken into account when building urban skyscrapers. The Port Authority of New York, which had commissioned the Trade Center project, had planned for the possibility that planes could get lost in the fog and hit the towers. The largest engine the builders had taken into account, however, was smaller than a 767. Nevertheless, the towers did manage to resist the sheer impact of the hit. Instead, the complex’s Achilles heel was its inability to remain stable in the face of the intense heat resulting from the combustion of the jet fuel on board the planes.
“The heat took down the building. Structurally, the buildings performed well,” said Nelson. “For example, when 75% of the columns on the north face of Tower 1 were destroyed, the beams and frame elements above could span across the huge opening carved by the 767.”
Engineers, when making their calculations, took into consideration natural phenomena like wind and earthquakes and human factors such as height sickness and office fires. While a sprinkler system can protect against small office fires, it was no match for the conflagration which followed attack.
Debate over towers’ performance
In the case of each tower, the entire structure, termed a rigid tube structure because it is constituted of perimeter columns, is dependent on the strength of steel. Supporting columns, the inside frame, bolts, and the external walls were all made of steel, material which distorts and loses its strength when exposed to such stratospheric temperatures. The protection of the steel structure that delayed the collapsing was made using a substitute for the carcinogenic Asbestos.
Some scientists fear, however, that these substitutes, tested only at relatively normal fire temperatures, were not as efficient as the original Asbestos. They theorize that perhaps more lives could have been saved were it not for the insufficient fire protection.
Still not all experts agree. “All materials loose strength with high temperatures including concrete. Concrete columns, like in the Petronas Towers ... may have lasted longer, but there is no way of answer ... without extensive analysis” said Nelson. “I would guess that many skyscrapers would have collapsed sooner then the WTC because of its closely spaced perimeter columns and its structural redundancy.”
Engineers have put forth the idea that the twins could have resisted destruction if they were built like a nuclear power plant, which is designed to withstand attacks from incoming aircraft. Critics are quick to point out, however, that the plan would not be aesthetically pleasing.
“Maybe [they would have survived], but there would be no windows!” said Nelson. “Again, the fire is the issue, not the strength of the structures.”