Supporting WTC Design

In his letter to the editor, WTC Disagreements, Lawrence Fisher stated that the World Trade Center towers might have been more resilient if they had been configured with a strong and stiff perimeter structure (ENR 11/29/03 p. 5). In fact, they were. The structural system used in the towers was a very strong and stiff perimeter lateral and gravity system with a gravity-only core system. I strongly support the premise of the original article that there was nothing wrong with the design of the World Trade Center towers.

Occasionally you see something in print that just simply doesnt make much sense. Most of the time, I ignore it. However, there was a letter printed in ENR about the WTC attacks that demands to be answered.

The writer disagreed with the National Institute of Standards and Technology's preliminary findings that the design of the towers was not to blame for their ultimate collapse. His statement that the new methodology of strong core and relatively weak surrounding structure goes against the basic tenants of structural engineering does not even describe the system that was in the WTC. Virtually all lateral strength for the towers was in the perimeter structure, not in the core.

More importantly, there are no basic tenets that say you shouldnt use a strong core. Strong cores are used commonly in high-rise construction. The two buildings listed as the worlds tallest for several years, the Petronas Towers, use strong cores for lateral strength.

Also suggested was, if the structures had been designed in accordance with box beam philosophy, they would have better heat resistance, and relatively light airplanes would not have caused as much destruction. Again, the writer does not understand that these structures did employ a box beam philosophy, as they were designed with continuous perimeter tubes. Even though they had a box beam design, that doesnt increase heat resistance. And it will never be considered a light impact for any structure that humans have built when hit with a 275,000-lb object at a velocity of about 500 mph. Virtually all buildings would be destroyed instantly.

Every intelligent assessment of the Sept. 11 attacks must begin with an understanding of capacities of real world structural systems and the destructive force of the weapons used - airplanes. Limits of buildings to resist this kind of attack have nothing to do with increased construction cost or reduced constructibility. They are about the laws of physics. Real increases in public safety can only be achieved when this is clearly understood and mitigations are aimed in the right direction.

I agree with almost every statement in Mr. Fischers letter but strongly disagree with his apparent conclusion. There is no doubt that the towers could have been built stronger, just as almost any structure could be made stronger if strength were the only objective. The objective of structural engineering is not to build something as strong as possible, but to economically build it as strong as necessary to fulfill its function. Any structure significantly stronger than needed for its function is unlikely to be built. I doubt that we'll ever build high-rise office buildings to resist flying bombs.

Structural Engineer
Richmond, Calif.