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Historical Archive
September 11, 2001
World Trade Center
New York, New York
The University of Sydney - Department
of Civil Engineering General | Structural System | Why Did It Collapse | Other Links 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. (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." Taken from www.greatbuildings.com Typical Floor Plan of the World Trade Center: A perimeter of closely spaced columns, with an internal lift core. The floors were supported by a series of light trusses on rubber pads, which spanned between the outer columns and the lift core.
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)
Sydney Morning Herald graphic 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". (Pictures taken from various news sources on the Internet) American Institute of Steel Construction http://www.aisc.org Please direct any comments on this
WTC webpage to Tim
Wilkinson General enquiries on civil engineering:
office@civil.usyd.edu.au
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