Charles C. Roberts, Jr.

No shortage of commentary existed after the attacks on the World Trade Center, which resulted in the collapse of the two towered structure. There was surprise amongst technologists and non-technologists alike as to the degree of structural failure brought on by the event. Never has such a large building collapsed with such a devastating loss of life. Likewise, who could have credibly advanced a scenario of a direct impact from a jumbo jet and ensuing petroleum fueled fire. A review of the design concept and construction of the building helps explain the failure origin and sequence.

Figure 1

Figure 1 is a simplified drawing of one of the tower structures. A central core houses elevators and supports the weight of the floors. Around the outside of the building are closely spaced (39 inch centers) perimeter columns that are designed to resist lateral wind loads and support the outer edge of each floor. Floor trusses, under a 4-inch concrete floor, are connected between the perimeter columns and the core structure. The design is efficient and attractive in that there are no columns in the work space area. The buildings were 208 feet wide, about 1360 feet tall with 110 stories. The World Trade Center was officially opened in 1973.
After the aircraft impacts, the towers remained in place for about an hour, attesting to the robust structural design. Commentators watched in disbelief as each tower collapsed, in turn, with little verbiage on how amazing it was that the structure remained intact for as long as it did. It is theorized that each aircraft impact had badly damaged perimeter and central core columns, but enough survived to take up the remaining load of the upper floors. Each tower contains about 100,000 tons of steel and concrete. Assuming the aircraft damaged the north tower structure at about 80% of the total height, then roughly about 20,000 tons would be borne by the structure that survived. The reduced structural capacity was the first ingredient in the failure. The second was the heating effect brought on by the burning jet fuel. As temperatures approach 1500 F, structural steel looses its ability to support weight and begins to deform (thermal creep). Figure 2 shows a view of typical steel beams and columns after a fire.

Figure 2

The beams are often found severely deformed and twisted in pretzel like fashion with little load carrying capacity. Several have commented that the columns in the Twin Towers melted, causing the failure. It is more likely that the remaining columns and connections failed as a result of the softening of the steel from the heat, rather than melting. Melting takes time and a lot of energy input from the fire. The steel softens first, resulting in a collapse before melting has occurred. When the columns or floor connections failed, structural loading was no longer static, but dynamic. The release of the load, by whichever floor columns failed, allowed about 20,000 tons of building material to impact on the next floor. Even if all the building structure below had been intact, the sudden impact of such a weight could not be sustained by the lower portion of the building that had been designed for static (steady, weight bearing) loads. As the upper portion of the building began to descend with considerable increase in momentum, each floor failed, in succession, terminating in a pile of building debris at the foundation area. This phenomenon is nothing new. Demolition companies use the same principle in destroying buildings by weakening the support structure, using explosives to cause critical columns to fail, and relying on the building mass to do the rest. This failure mode has also been seen in other structures, but to a lesser degree. Figure 3 shows a failed floor beam. In this case, the floor above was

Figure 3

overloaded and failed. The upper slab fell onto the floor below, causing failure of the large beam (which, in this case, had not been affected by heating), attesting to the severity of dynamic loading. The south tower failed in a similar fashion as the north tower, although not as uniformly. There was a degree of leaning of the upper structure, before collapse, as shown in Figure 4. This is most likely a result of the asymmetry of the direct structural damage from the impact and induced structural damage from heating.

Figure 4 (AP Photo)

Typical building design incorporates fire related testing such as ASTM E119-00 (Standard Test Methods for Fire Tests of Building Construction and Materials), which is a test procedure to rate the performance of steel structural components in fire environments. Standard building testing addresses conditions normally expected in typical trauma to buildings. Atypical trauma, as what was experienced in the World Trade Center attack, is likely to exceed the capacity of a modern building structure to remain intact. More detailed failure analyses are underway to glean additional information as to failure mode and sequence. The engineering community will be looking at ways to increase building survivability under extraordinary circumstances. Preliminary conclusions from the scientific community are rooted in the premise that efforts at the prevention of such attacks will be much more effective than attempting to design or retrofit buildings to survive such an attack.