World trade center - new york

World Trade Center - New York
Engineering Aspects
General Information: Height: 1,368 and 1,362 feet (417 and 415 meters)
Owners: Port Authority of New York and New Jersey
Architect: Minoru Yamasaki, Emery Roth and Sons consulting
Engineer: John Skilling and Leslie Robertson of Worthington, Skilling, Helle and Jackson
Ground Breaking: August 5, 1966
Opened: 1970-73; April 4, 1973 ribboncutting
Destroyed:  Terrorist attack, September 11, 2001 |
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 steelcolumns 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 super tall buildings designed without any masonry. Worried that theintense 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 shaft ways. 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 shaft ways.
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 gravityloads 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."
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 oflight trusses on rubber pads, which spanned between the outer columns and the lift core.
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.  Lightweightsteel 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 thatthe 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 behavior of the remaining material.  As the planes had only recently taken off, the fire would have...