September 11

The shocking tradgity that imprinted the date September 11 into everyone living person's memory was, as we know a cowardice act of terrorism, showing the world to what lengths people will go to fight for their beliefs. Analysing the major contributing factors that caused the collapse of the world trade centre we must first understand what this remarkable structure consisted of and the engineering elements that gave this building world recognition. We must then focus on the events that occurred on day that caused such catastrophic failure of the enormous structure. We can then interpret what the major contributing factors were that resulted in the destruction of the world trade centre.

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 centres 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.

The structural integrity of the World Trade Centre 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 disastrous. 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