Analysis by a team of six structural, fire protection, and forensic engineers concluded that the Pentagon's resilient structural system curtailed the damage done by the hijacked plane on September 11, 2001. Released today by the American Society of Civil Engineers' (ASCE), The Pentagon Building Performance Report has far-reaching implications for the engineering and building design professions — and for the general public as well.
Constructed between September 1941 and January 1943 as office space, the Pentagon was designed for post-war use as a storage facility for files and documents. It was designed to support heavier loads than ordinary office buildings. Following the crash, the structural system redistributed the weight of the building and its contents among the columns left standing, thereby limiting the collapse of floors above the point of impact.
The team concluded that the Pentagon's structural performance validates measures to reduce collapse resulting from unlikely events. These measures include:
n Continuity — such as the extension of floor reinforcements through structural supports;
n Redundancy in design and construction, such as two-directional framing of the floors;
n Spirally steel-reinforced concrete columns that absorb energy generated from a lateral load, such as high wind, or an earthquake — or as on September 11, the impact of an aircraft; and
n Design considerations that take into account excess loads upon the structure.
"We strongly urge that the team's recommended measures be considered in future building design," said the building performance study team leader, Paul F. Mlakar, a technical director with the U.S. Army Corps of Engineers, who also served on the building performance study team for the Murrah Federal Office Building bombing in Oklahoma City.
In the conclusion to the seven-month study, the engineering team observed that the direct impact of the crash destroyed approximately 50 columns on the first floor and six second-floor columns along the exterior walls. The subsequent fire ignited by the aircraft fuel and fed by the aircraft components and building contents, caused moderate damage to the reinforced concrete frame in relatively small areas on the first and second floors, which later collapsed.
Among the findings:
n 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; redundant and alternate load paths of the floor system, allowing for one support system to redistribute the load should another fail; and substantial continuity of steel floor reinforcement through the supports;
n Many columns withstood extreme lateral loads, from the impact of the plane;
n The floor system was capable of significant load redistribution without collapse when several adjacent supporting columns were removed; and
n The 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 urges further research and development in the prevention of progressive collapse and structural response to improbable events and specifically recommends research into the following:
n The load-carrying capacity of columns and other structural elements after they have been severely deformed;
n The vertical load on the floor system resulting from a horizontally deformed column; and
n The energy-absorbing capacity of concrete elements when they are subjected to impact loads that result in large deformation.
Visit www.asce.org for more on the report.
