Paris | Wednesday
THE hijacked planes that destroyed New York’s World Trade Center crashed into the twin towers at their most vulnerable spot, which suggests the suicide attackers may have known where to strike, an expert said Wednesday.
The jetliners smacked into the towers about three-quarters up their 110-storey height, an impact probably equal to twice the force of the biggest likely hurricane, a top British structural engineer, Gordon Masterson, said.
Structural damage from this impact, and the searing heat of blazing aviation fuel, then combined to bring down the top 20 or so floors, each weighing 2_500 tons.
The accumulating weight would have caused both buildings to collapse, floor by floor, faster and faster, he said.
“If they had struck right at the top of the tower that would have had a greater effect on the foundations, but the fire would have been more contained,” Masterson, chairman of the structural and building board at Britain’s Institution of Civil Engineers, said.
“However, they struck in both cases at an extremely vulnerable point.
“Once the fire broke out at level 90 and softened the steel supports to the upper floors, which were already damaged by the impact, then the uppermost levels would have fallen down.
“The weight of this would have been far beyond anything that this floor (at level 90) was capable of supporting, and this would have caused the whole building to go down progressively, floor by floor by floor.
“That zone is probably the vulnerable point,” Masterson said.
Asked whether this meant the hijackers knew where to hit, Masterson said, “it’s entirely possible.”
“The papers on the design and construction of the World Trade Center are readily available in the public domain and the perpetrators appear to have had access to skilled resources in all sectors, including piloting planes perhaps, so nothing would surprise me.”
A French architect, Marc Mimram, said very tall buildings were prey to a phenomenon called moment tensor inversion – a shock from a seismic wave or impact that ripples through the building but has a whiplash, leveraged effect on its higher floors.
The two towers, one 417 metres and the other 415 metres high, were designed in the mid-1960s, and were completed in 1973 after a six-year endeavour.
They basically comprise a square-sided steel tube – the external metal frame of the building – with a central core.
According to repute, the buildings were designed to withstand the impact of a Boeing 707, the predominant airliner at the time.
Masterson said he had never heard of any skyscraper being deliberately designed to take a hit from an aircraft. Such buildings were intended for civilian use and were built to withstand natural phenomena such as high winds and earthquakes as well as a certain amount of accidental damage, he said.
He presumed the Boeing 707 scenario must have been an incidental calculation, perhaps out of curiosity, to see whether a building would immediately topple over if it were hit by a big aircraft.
On that score, both towers had withstood this test, as they did not immediately fall over, for it was the resultant fire that probably inflicted the mortal blow, he said.
Whether the plane was a 707 or a somewhat bigger modern twinjet was largely irrelevant. What mattered was that a very large object impacted the building, projecting aviation fuel inside that caused an “instant inferno,” he said.
The temperature of the fire would be in the region of 800-1_000 C, easily enough to destroy the remaining integrity of the steel supports, which would soften at 600 C, he said.
Masterson said it was impossible to design skyscrapers to withstand such an event.
“What took place was way beyond the reasonable contemplation of the building’s original designers. These high-rise buildings are designed for peacetime civilisation. We choose to live in cities, not in underground nuclear bunkers.” – AFP