ASEE Prism Magazine Online - December 2001
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Why They Fell

- By Christopher M. Foley

The triumph of modern structural engineering led to tragedy when people with evil inventions brought about unintended loading

The skyscraper has fascinated me since I first saw watercolor paintings of office buildings done by my father, a mechanical engineer with an artistic streak. The beauty of the tall building can be seen not only in its impact on the skyline but also in its structural efficiency to withstand lateral loads from earthquake and wind, as well as vertical gravity loading.

For this structural engineer, the tragedy of September 11 went beyond the horrific loss of human life. The following question continually flowed through my mind: How could the World Trade Center towers, two structures exhibiting engineering genius and visual majesty, suddenly become tombs for so many innocent people? The answer could give me and other individuals some understanding of at least one part of the tragedy.

Each tower contained about an acre of space per floor and rose 1,350 feet above the street. The design and construction of the towers can be appreciated when one considers that, combined, they contained 230 passenger elevators to move people throughout their 110 floors. Planning for the structures took nine years, and four years of demolition was required before they could begin to rise above New York. Construction averaged three floors on each tower every 10 days, and approximately 600 tons of structural steel arrived daily at the site.

The tower's structural efficiency can be seen when one examines the hollow pierced tube system used in construction. The exterior steel columns (14 square inches) were spaced very close together (3'3” apart), and deep spandrel elements (horizontal members) formed the top and bottom of the windows. Therefore, the exterior tube of the building was essentially a solid piece of steel with holes punched in it for windows. This tube was designed to be a Vierendeel truss to resist the lateral wind loading.

The hollow pierced tube system also incorporated a central core composed of structural steel columns that housed stairways, elevators, and other service elements. The central core and exterior tube shared responsibility for supporting the vertical gravity loading. This structural system resulted in large column-free floor plans. Floor trusses supported and acted compositely with four-inch concrete slabs. These trusses spanned from the central core to the exterior tube.

The relatively thin nature of the exterior tube (14 inches thick) can be thought of, in a simplistic sense, as a soda can. One can stand on an unopened can of soda because the liquid and pressure inside “braces” the can's sides and prevents them from buckling. However, when the liquid or pressure is removed, any small movement or slight tap on the side of the empty can cause it to “buckle” and collapse. Likewise, the exterior tubes of the WTC towers were “braced” by and eventually buckled due to the failure of the floor framing system.

To understand the collapse, one has to look at a replay of the tragic event. From the video, the aircrafts appear to be “swallowed” by the buildings. One can clearly see the flames (and most likely the wings, fuselage, etc.) passing through the interior of the building and even exiting the opposite side. Initial impact did not cause collapse of either tower thanks to a structural principle called redundancy. In simple terms, redundancy means that if a load-resisting path (in this case, the exterior tube) is compromised, other load paths are available, and therefore, collapse is prevented. When the aircraft penetrated the exterior tube, it cut some of the load paths available for the floor framing. However, the fact that the exterior tube contained many closely-spaced columns ensured that other load paths were available for floor loads to get to the foundation. Just as the exterior tube was designed as a Vierendeel truss to resist horizontal wind loading, it acted like a Vierendeel truss to span horizontally and carry the floor loading across the holes in the outer wall created by the aircraft.

As the WTC towers fought to redistribute their weight throughout the exterior tube and interior core columns down to the foundation, a very sinister element was at work—fire. The extraordinarily high-temperature fire that was burning throughout the floors adjacent to the impact zone was creating a gradual and consistent weakening of the structural framing system. It is a well-known fact that structural steel cannot survive fires for extended periods of time. Therefore, all steel building components are coated with fire-resistant materials to inhibit weakening of these elements for a period of time sufficient to allow firefighters to put out the flames. The main problem in the WTC collapse was that the fire was fed by jet fuel and not the usual sources found in office towers (e.g., paper and other combustible materials). Thus, the intensity of the fire was far greater than anything considered by
the engineers.

The structural efficiency that made the towers special may also have been one reason why they collapsed. As the fire continued to burn, the interior core, the exterior tube, and the critical connections of the floor trusses were being incrementally weakened. It is conceivable that, at some point, the floor truss connections to the exterior skin failed at one or multiple floor levels. When these connections failed, it was like letting the liquid or pressure out of the soda can. The fire-weakened exterior tube began to lose its critical bracing, which could have led to the buckling or bulging of the exterior tube. Thus, there were many floors above the compromised level that now had no vertical support at the exterior of the building. These floors then became a free falling load, which initiated a progressive collapse mechanism in the towers. The towers then collapsed in a manner similar to a controlled demolition.

The natural question one might ask is: Why didn't the bombing in 1993 take down the towers? A relatively simple answer to this question lies, once again, within the structural system. The truck-bomb parked in the garage below the tower was situated between the interior core and the exterior skin. Fortunately, the bomb was unable to compromise these critical elements.

One must understand that all structural failures of catastrophic result require many events to occur simultaneously. In the case of the WTC towers, there was the unfortunate combination of aircraft impact to the critical exterior tube with subsequent high-temperature and uncontrolled fire. The engineers who designed the WTC towers included all conceivable loadings to which the structures might be subjected. However, one can certainly understand how a jet aircraft containing passengers and filled with jet fuel flying into the upper stories of the building was not one of their design criteria.

We can always ask our engineers to design “bunker-type” building structures. The Pentagon did not suffer the same catastrophic, progressive collapse. However, would we as Americans like all our buildings to be three-story bunkers or be built exclusively underground? What would our great cities like Chicago, New York, and San Francisco look like without the mixture of short buildings, tall buildings, stadiums, and long-span bridges? Where is freedom expressed in concrete bunkers or underground tunnels? Where would we be without tall buildings? We would live in a very lifeless place, a place unworthy of being called the United States of America.


Christopher M. Foley is an assistant professor of civil and environmental engineering at Marquette University in Milwaukee, Wis. He can be reached by e-mail at