| The opinion of many engineers is that the architects that designed the New York World Trade Center Twin Towers did a good job in designing the buildings. The buildings were safe from the hazards envisioned (预想到的) at the time. Indeed, they were designed to withstand the force of aircraft that might crash into them. They were not however, designed to withstand the effects of the very high temperatures produced by the ignition (点燃) of aviation fuel carried by the aircraft. Steel lightly covered with concrete is the main structural part of the towers and steel loses a lot of structural strength when heated to 1100 degrees Centigrade and beyond, as it was. To protect the steel the towers beams were coated with a few centimeters of concrete. That insulation (隔离) was to provide structural state of being complete for at least 1 hour of fire. The towers met that test. The south tower lasted 62minutes. The north, 103 minutes. Tragedy followed when not all could escape before the collapse. Most experts agree that the Twin Towers were very well designed. Many also agree that the people planning the attacks were very knowledgeable and aimed the aircraft at a height that would produce the most damage. The aircraft crash and the resulting fuel fed fires were aimed at about the 90th floor. That was just high enough to insure that the upper floors would weaken with the heat and crack down, dropping into the remaining part of the building. The mass of the falling top floors created such a falling load that some experts estimate it exceeded by 5 to 10 times normal bearing loads. The effect was like a giant hammer, each floor contributing more weight to be born by the next floor down. Were the Trade Towers well designed？ Engineers say yes. But, the public holds to the belief that architects will never again design a tall building without considering the effects of terrorist attacks of all potential kinds. That not only includes the ability of the structure to withstand attack but the means to allow the buildings occupants to escape in time to survive. | 
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