[nukkad] How the WTC Collapsed

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The World Trade Center's twin towers were two of America's most visible 
symbols of engineering prowess, so when they crashed to the ground in an 
avalanche of ash, many asked how it could have happened.

Experts in skyscraper design and construction say it was probably simple: 
The 110-story towers were conceived to withstand powerful impacts, but their 
steel skeletons couldn't protect them from thousands of gallons of flaming 
jet fuel.

``Steel melts, and 24,000 gallons of aviation fluid melted the steel. 
Nothing is designed or will be designed to withstand that fire,'' said Hyman 
Brown, a University of Colorado civil engineering professor and the Trade 
Center's construction manager.

``If they did it lower in the building, the fire department could have 
gotten to it sooner. In its simplicity, it was brilliant,'' he said.
Masoud Sanayei, a civil engineering professor at Tufts University, said the 
fire's heat may have disconnected one of the towers' concrete floors from 
the tubular steel columns ringing the buildings. If one or two floors 
collapsed, it would have created a pancake effect of one massive floor 
caving into the next.

``In my opinion, the fire weakened the connection between the floor system 
and the columns on the higher floors and caused a couple of the floors to 
collapse,'' Sanayei said. ``The floors are very heavy, made of reinforced 
concrete, so when one hits the next, they cause a domino effect ... and it 
can go all the way down to the first floor.''
He said no one could be expected to survive such a catastrophe.
Brown, too, noted that although the twin towers had staircases in all four 
corners for evacuation, video clips led him to believe those escape routes 
were cut off for people on higher floors long before the buildings 
collapsed.

Architect Minoru Yamasaki, who died in 1986, worked with engineers John 
Skilling and Leslie E. Robertson to design the twin towers, once the world's 
tallest buildings.

In his 2000 book ``Building Big,'' architect David MaCaulay described the 
towers' engineering as ``a series of load bearing exterior columns spaced 3 
feet apart and tied together at every floor by a deep horizontal beam, 
creating a strong lattice of square tubing around each tower.''

The core surrounding the elevators inside was much the same, with a giant 
lattice work of steel covered by poured concrete connecting the interior 
columns to the exterior ones. The design was free enough for each of the 
towers to hold 4 million square feet of space unencumbered by columns or 
load bearing walls.
Sections of exterior wall were wrapped around the outside in 24- and 36-foot 
high sections, creating a sort of patchwork so that not all the floor joints 
would meet walls at the same height, according to MaCaulay.

Minoru Yamasaki Associates issued a statement saying the firm was in contact 
with authorities and had offered assistance.
``We believe that any speculation regarding the specifics of these tragic 
events would be irresponsible,'' the statement said. ``For obvious reasons, 
MYA has no further comment at this time.''

http://dailynews.yahoo.com/h/ap/20010912/us/trade_center_construction.html




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