The Collapse of the World Trade Centre Towers

by Prof Jerry Connor

Professor Jerry Connor from MIT gave another lecture at the Lodge on the Collapse of the Trade Centre towers on September 11th, 2002. This lecture follows a Seminar that he held in the Lodge a few months ago. New aspects of the tragedy and the subsequent reports were discussed. He also explained why the two towers behaved differently.

The impact of the plane on the South Tower destroyed some of the columns at the southeast corner. This appears to have produced a partial collapse of the floors in that area, resulting in a small tilt in the southeast direction. Failure was triggered by a collapse of the floor along the east face which led to buckling of the columns at that level, and progressive collapse (pancaking) of the floors below the impact level. The North Tower lasted approximately one hour longer after impact than the South Tower. The failure mode was similar to that of the South Tower, with one exception: there was no tilt prior to the progressive collapse of the floors. The main factor was the intense heat generated by the jet fuel fed fire.

The buildings were 210 feet by 210 feet in plan with a rectangular core of 90 feet by 140 feet. The total height was 1,400 feet and the structure was basically a tube type cantilever beam. The external surface consisted of closely spaced columns which behaved as an equivalent skin. The way in which the floor system was designed resulted in half the floor weight being transmitted to the outer skin. This tube is rigid in torsion as well as efficient for bending.

The layout and structural details of the floors consisted of a series of open web trusses spanning 60ft from the inner core to the outer skin, covered by corrugated steel decking and a 4 inch thick concrete slab. The 60 feet span floor trusses were connected to the external columns with simple L shaped angles. Figure 1 illustrates this connection detail. Although efficient from an erection perspective, this connection is too flexible and lacks redundancy.

The plane hitting the South tower near the southeast corner produced an immediate failure for the floors in that local area. After approximately one hour, additional floor collapse occurred, resulting in a rapid collapse of the total structure. This sequence is described in the writeup of the previous lecture by Jerry (see BEC, vol 13, no 2, 2001). Gravity is the main driver of the collapse which appears as a wave travelling down the building.

Different theories regarding the collapse of the North Tower have been advanced. Jerry believes that the failure of the angle (shown in figure 1), the strength and stiffness of which was reduced by the intense heat, would have produced excessive deflection and resulted in the beam sliding from its support. The weakest point of the structural design is the shear connection between floors and columns. Why the North Tower lasted longer can be explained from the location of the impact zone (mid-face and higher up in elevation). There was less internal structural damage to the floors. Also, there was less force in the columns since the total weight of the structure above this impacted region was less.

Jerry believes that there will be a good deal of further speculation, particularly now that the legal aspects are being considered. It will, however, be impossible to try to design buildings to withstand the impact of a large plane with full fuel tanks.

WIT Press is publishing a book on the collapse, edited by Professor Eduardo Kausel, also of MIT and with contributions from well-known researchers. Details of the book will be available soon from WIT Press (see website www.witpress.com).