  The World Trade Center towers were an unusual design, at least at the time they were built. Their support structure is called a 'bundled tube', or in engineering terms, a glass curtain wall structure.What this means is that the buildings are tubes, made rigid by a lattice of steel beams on the outside walls. These vertical columns are strengthened by horizontal beams, and this design is what helps support the building, and keep it stable in high winds. An inner concrete core houses the elevators, and provides additional vertical load support. The towers had an outer facade of aluminum and glass, and its floors were reinforced concrete. Surprisingly, the many office windows were quite small, designed that way because the architect had a fear of heights! You can see an architectural plan of the curtain wall design here. This makes the structure very light, and allows for maximum interior room. In fact, the upper floors of the World Trade Center towers had 40,000 square feet of office space, per floor; in total, 75% of the floor area was available for use, very high for a high-rise. Apparently this design, which provides support through its load-bearing walls, was one factor which prevented one of the towers from collapsing after the 1993 WTC parking garage bombing. The buildings were also well-supported. The foundations extended six floors underground, and were actually below the level of the nearby Hudson River, requiring a concrete 'dam' to prevent water seepage into the basement areas. The World Trade Center towers were designed to sway up to 2.7 metres, at the top, in a strong wind, and had moved through an arc of as much as 2 metres on occasion. Although not dangerous, this wasn't entirely pleasant for people on the upper floors. When designed, planners did take into account the possibility of an accidental collision by an aircraft, possibly from one of the nearby airports. There is precedent for this type of accident; fourteen people died when an American B-25 bomber accidentally struck the Empire State Building's north face in thick fog in 1945. The World Trade towers were designed to withstand the impact of a Boeing 707. Unfortunately, since that time, airliners have become much larger and heavier, and now carry much more fuel. As you will see below, no building design could ever hope to withstand the deliberate impact of a modern jetliner carrying a full load of fuel. 
When each airplane struck the building, it destroyed quite a few of the support beams on floors above and below the impact point. In addition, the huge supply of fuel still being carried by both planes caused a huge fire, which softened more of the beams.  This left the floors above the impact points with very little support; within a short time, these floors collapsed. Further down the building, even steel at normal temperatures gave way under the enormous weight - an estimated 100,000 tonnes from the upper floors alone.
It was as if the top of the building was acting like a huge pile-driver, crashing down on to the floors underneath.
The force of these falling levels on the rest of the building below was enough to cause all of those floors to collapse as well, and the buildings fell straight down.A tube structure building is like a garbage can, very rigid around the outside, but once the damage starts, it is very easy to crush it. The time to crush, that is, the time to achieve structural instability, was about an hour.  It is believed that if the terrorists had flown into the lowest floors of each tower, the buildings would have almost immediately fallen over like a tree. Given the size and fuel loads of modern airliners, there is probably no way to construct a building, or at least one that is to be used for offices, to withstand a direct impact, deliberate or otherwise. |