Thursday, September 20, 2001, updated at 16:28(GMT+8)
Many people asked why the US WTC twin steel towers in New York couldn't survive the terrorist attacks and instead of withstanding the bump of the planes they collapsed and were practically razed?
People's queries were mainly about why the twin towers fell all of a lump when they were merely hit on the upper and middle part? And why they directly fell down instead of toppled over from above?
People saw a big fire with dozens of tons of aircraft fuel at first that had done its work in softening and melting away all steels struck. Soon after collapsing of the upper stories, heat was conducted to stories below as is seen in a "domino effect" leading to the destruction of the two steel structures in their entirety. After watching videos experts say that it was not the damages inflicted but the big fire followed in destroying the two towers.
Professor Wu Huanjia from Qinghua University says in an interview that the big fire must be to blame for softening and melting away the steel, paralyzing and destruction of the towers as mere plane crash is not strong enough to topple the two large tower structures.
There were also intrinsic flaws with skyscrapers that had led to the fall of the WTC towers, for such architectural giants must be built of steel, which softens by heat and loses its strength. Of course, refractory coating must be applied when the towers were constructed but you can never expect them to endure such large conflagrations.
When constructed, experts say, the towers had been designed to survive hitting short of complete fire-fighting and anti-blast systems to be built. If the plane crash hit the towers' lower part, fire could probably be put under control and such a devastating disaster might be averted. So one can see the attacks were well knotted by the extremely intelligent terrorists.
Qian Jiaru, a civil engineering professor from Qinghua University, tells reporter that the WTC towers were supported by intense steel poles as world advanced architectural structures at that time. Why the Empire State Building, built 40 years earlier than WTC towers, didn't collapse when hit by an American military airplane in 1943? This is because that airplane was relatively small and carried less fuel that didn't cause big fires. On the other hand, the Empire State Building has steel pole supports and concrete fireproof coating that can endure bigger fire.
Besides, for an architect in designing a building he would surely consider such factors as wind, earthquake and fire though impossible for him to foresee suicidal airplane crashes, not to say airplanes loaded with some dozens tons of aviation fuel to cause bigger fires than ordinary ones.
This time's Boeing 757 belongs to a model that can carry 35 tons of fuel and Boeing 767 51 tons. They are long-distance flights from east America to the west so estimated fully loaded. Soon after take-off they changed route with only a little fuel consumed, as a result almost all top-quality fuel was poured into the twin towers.
The fully loaded 757 airplane was no less than a "super flying bomb", expert says. Calculation shows a 757 can carry a total weight of 20 tons on top of 35 tons weight of airplane itself and fuel, and a flying speed of 900 km/hour equals to a weight of 495,000 tons, altogether it gathers an equivalent force of about 20,000 kg yellow explosives.
Most high buildings would be reinforced to carry weight and resist earthquake, but not for fire and explosion. It is enough for 800 kg yellow explosives to destroy a 50-storied building. Therefore inestimable damages are caused by 20,000 explosive capacity plus high speed hits.
Second blast caused collapse according to a different view aired:
Professor Shi Yongjiu, director of civil engineering department of Qinghua University and an expert on steel structure, guesses that the lower part of the WTC twin towers may got seriously damaged.
According to steel structure's mechanical nature, the towers shouldn't collapse as late as an hour later after the planes slammed into. What's more, it should be in a way to topple over gradually instead of crashing down as seen in videotapes. It looks more like a directional blast in doing the job of destruction, so he feels that huge damages must have been done at the lower part of the towers.
As seen on TV, the big fire, climbing higher and higher, is still more than 300 meters away from the base of the towers, not big enough to destroy the steels of the lower part.
He was surprised that a 40-storied supportive building beside the towers should collapse 6 hours later, for at that time the blast force by main towers should have been lost for a long time.
Professor Shi's conjecture coincides with a running rumor in the US that terrorists had planted explosives in advance and set them off on the sly after the airplane crash, so soon collapsed the twin towers.