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 (47)
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
(48) of tons of aviation fuel carried by the
aircraft. Steel (49) 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 (50) the steel the towers beams were coated with a
few centimeters of concrete. That insulation was to provide structural
(51) for at lease 1 hour of fire. The towers met that test.
The south tower lasted 62 minutes. The north; 103 minutes. Tragedy followed when
not all could escape prior to the collapse. Most experts agree
that the Twin Towers were very well designed. Many also agree that the people
planning the attacks were very (52) and aimed the aircraft
at a height that would effect 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 (53) part of the building.
The mass of the falling top floors created such a falling load that some
experts estimates it (54) by 5 to 10 times normal bearing
loads. The effect was like a giant hammer, each floor (55)
more weight to be born by the next floor down. Were the Trade
Towers well designed Engineers say yes. But the public refers to the belief
that architects will never again design a tall building without considering the
effects of terrorist attacks of all (56) 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. [A] exceeded
[F] shield
[K] lightly [B] ignition
[G] remaining
[L] knowledgeable [C] strictly
[H] envisioned
[M] replace [D] practical
[I] collision
[N] potential [E] integrity
[J] contributing
[O] happened