by ArchitectureWeek

When the Pentagon in Washington D.C. was hit by a hijacked plane on September 11, 2001, the damage and the loss of life were appalling. But the destruction was less severe than might have been expected from such an impact. About 20,000 people were at work in the U.S. Department of Defense headquarters, the largest office building in the world. Yet according to casualty reports, only 125 Pentagon employees were killed along with the 64 from the ill-fated airliner.

As often happens after natural disasters such as earthquakes or hurricanes, an engineering investigation was immediately launched to learn exactly how structures succomb to extraordinary forces. Such investigations may lead to further refinements in U.S. building codes.

"The Pentagon Building Performance Report" has now been released by the American Society of Civil Engineers (ASCE). After seven months of study, a team of six prominent structural, fire protection, and forensic engineers have drawn conclusions about the building's structural resistance to progressive collapse during and immediately following the airliner crash. The report may have far-reaching implications for engineering design standards.

Constructed during World War II as office space that would store unusually large quantities of files and documents, the Pentagon was designed to support heavier loads than an ordinary office building. The investigative team observed that the direct impact of the crash destroyed approximately 50 columns on the ground floor and six columns along the exterior walls of the floor above. The structural system immediately redistributed the weight of the building and its contents onto the columns left standing, thereby limiting the collapse of floors above the point of impact.

The subsequent fire ignited by aircraft fuel, and fed by the aircraft components and building contents, caused moderate damage to the reinforced concrete frame in relatively small areas on those two floors, which later collapsed.

The engineers found that the original structural design helped arrest the progression of collapse and resultant loss of life. These structural design elements include short spans between structural supports, continuity of floor reinforcement, and redundant load paths, allowing one support system to carry the load if another fails.

Many columns withstood extreme lateral loads from the impact of the aircraft, and the floor system was capable of significant load redistribution without collapse when several adjacent supporting columns were removed. Moreover, the blast-resistant windows installed as part of the Pentagon Renovation Program performed well in resisting the aircraft impact and pressure from the massive ignition of fuel.

The team recommends several design measures that could reduce the chance of structural collapse in other buildings. These measures include continuity, such as the extension of floor reinforcements through structural supports; redundancy in design and construction, such as two-directional framing of the floors; spirally steel-reinforced concrete columns that absorb energy generated from lateral loads; and design for higher-than-normal loads.

The team urges further research and development in the prevention of progressive collapse and structural response to improbable events. They specifically recommend research into the load-carrying capacity of columns and other structural elements after severe deformation, the vertical load on floor systems resulting from horizontally deformed columns, and the energy-absorbing capacity of concrete elements when subjected to impact loads that result in large deformations.

Founded in 1852, the ASCE represents 130,000 civil engineers worldwide and is the oldest national engineering society in the United States.

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Damage regions in the ground floor of the Pentagon after the terrorist attack on September 11, 2001. Image: American Society of Civil Engineers/ Pentagon Building Performance Report Some of the Pentagon wall took a half hour to collapse, enabling hundreds of workers to escape. Photo: U.S. Department of Defense Typical column in the Pentagon, showing spiral reinforcing. Image: American Society of Civil Engineers/ Pentagon Building Performance Report Typical floor slab in the Pentagon, showing bidirectional reinforcing, and continuous reinforcing over supports. Image: American Society of Civil Engineers/ Pentagon Building Performance Report Path of ejected fuel in the ground floor. Image: American Society of Civil Engineers/ Pentagon Building Performance Report   Click on thumbnail images to view full-size pictures.