Right After the attack
The Structural System: Yamasaki and engineers John Skilling and Les Robertson worked closely, and the relationship between the towers’ design and structure is clear. Faced with the difficulties of building to unprecedented heights, the engineers employed an innovative structural model: a rigid "hollow tube" of closely spaced steel columns with floor trusses extending across to a central core. The columns, finished with a silver-colored aluminum alloy, were 18 3/4" wide and set only 22" apart, making the towers appear from afar to have no windows at all. Also unique to the engineering design were its core and elevator system. The twin towers were the first supertall buildings designed without any masonry. Worried that the intense air pressure created by the buildings’ high speed elevators might buckle conventional shafts, engineers designed a solution using a drywall system fixed to the reinforced steel core. For the elevators, to serve 110 stories with a traditional configuration would have required half the area of the lower stories be used for shaftways. Otis Elevators developed an express and local system, whereby passengers would change at "sky lobbies" on the 44th and 78th floors, halving the number of shaftways. (Taken from www.skyscraper.org) The structural system, deriving from the I.B.M. Building in Seattle, is impressively simple. The 208-foot wide facade is, in effect, a prefabricated steel lattice, with columns on 39-inch centers acting as wind bracing to resist all overturning forces; the central core takes only the gravity loads of the building. A very light, economical structure results by keeping the wind bracing in the most efficient place, the outside surface of the building, thus not transferring the forces through the floor membrane to the core, as in most curtain-wall structures. Office spaces will have no interior columns. In the upper floors there is as much as 40,000 square feet of office space per floor. The floor construction is of prefabricated trussed steel, only 33 inches in depth, that spans the full 60 feet to the core, and also acts as a diaphragm to stiffen the outside wall against lateral buckling forces from wind-load pressures."
First Collapse
Why Did It Collapse? Tim Wilkinson, Lecturer in Civil Engineering (This is an initial suggestion on one possible reason for failure, and should not be regarded as official advice) The structural integrity of the World Trade Center depends on the closely spaced columns around the perimeter. Lightweight steel trusses span between the central elevator core and the perimeter columns on each floor. These trusses support the concrete slab of each floor and tie the perimeter columns to the core, preventing the columns from buckling outwards. After the initial plane impacts, it appeared to most observers that the structure had been severely damaged, but not necessarily fatally. It appears likely that the impact of the plane crash destroyed a significant number of perimeter columns on several floors of the building, severely weakening the entire system. Initially this was not enough to cause collapse. However, as fire raged in the upper floors, the heat would have been gradually affecting the behaviour of the remaining material. As the planes had only recently taken off, the fire would have been initially fuelled by large volumes of jet fuel, creating potentially enormously high temperatures. The strength of the steel drops markedly with prolonged exposure to fire, while the elastic modulus of the steel reduces (stiffness drops), increasing deflections.
Second Collapse
The only evidence so far are photographs and television footage. Whether failure was initiated at the perimeter columns or the core is unknown. The extent to which the internal parts were damaged during the collision may be evident in the rubble if any forensic investigation is conducted. Since the mass of the combined towers is close to 1000000 tons, finding evidence will be an enormous task. Perimeter columns, several storeys high, and still linked together, lie amongst all the debris on the ground. This photograph shows the south tower just as it is collapsing. It is evident that the building is falling over to the left. The North Tower collapsed directly downwards, on top of itself. The same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers. However, it is possible that a storey on only one side of the South Tower initially collapsed, resulting in the "skewed" failure of the entire tower. The gigantic impact forces caused by the huge mass of the falling structure landing on the floors below travelled down the columns like a shockwave faster than the entire structure fell. The clouds of debris coming from the tower, several storeys below the huge falling mass, probably result from the sudden and almost explosive failure of each floor, caused by the "shockwave". You can actually see the top floors collapsing on this picture (Pictures taken from various news sources on the Internet)
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God Bless the USA
(by Lee Greenwood)
| If tomorrow all the things were gone I'd worked for all my life, And I had to start again with just my children and my wife, I'd thank my lucky stars to be living here today, 'Cause the flag still stands for freedom and they can't take that away. I'm proud to be an American where at least I know I'm free, And I won't forget the men who died who gave that right to me, And I gladly stand up next to you and defend her still today, 'Cause there ain't no doubt I love this land God Bless the U.S.A. |
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