[填空题]
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 {{U}} (47)
{{/U}}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 generated by the
{{U}} (48) {{/U}} of tons of aviation fuel carried by the
aircraft.
Steel {{U}} (49) {{/U}} covered with concrete
is the main structural component of the towers and steel loses a lot of
structural strength when heated to 1100 degrees Centigrade and beyond, as it
was. To {{U}} (50) {{/U}} the steel the towers beams were coated with a
few centimeters of concrete. That insulation was to provide structural
{{U}} (51) {{/U}} for at lease 1 hour of fire. The towers met that test.
The south tower lasted 62 minutes. The north; 103 m