(Image courtesy of the Pentagon Building Performance Report)

A long-awaited report on the performance of the Pentagon in the aftermath of the Sept. 11, 2001, terrorist attack contains few surprises and confirms observations made following the event. The Pentagon Building Performance Report concludes that original design features–aided by recent upgrades–of the 6.5-million-sq-ft home of the Dept. of Defense were key to limiting collapse after a hijacked airplane slammed into the building.

"The Pentagon's structural performance during and immediately following the Sept. 11 crash has validated measures to reduce collapse from severely abnormal loads," states the report released Jan. 23 by the American Society of Civil Engineers, Reston, Va.

The six-person study team of experts in structural, fire and forensic engineering asserts that if a building is required to resist progressive collapse, it should be designed to include structural continuity and redundancy. And the building's frame should also have energy-absorbing capacity and reserve strength.

(Image courtesy of the Pentagon Building Performance Report)

In the Pentagon, continuity refers to the use of the extension of bottom beam reinforcement through girders and bottom girder reinforcement through columns. Redundancy refers to the structure's two-way beam and girder system. Spirally reinforced columns provided energy-absorbing capacity. Reserve strength was provided by the original design for live load in excess of service.

The first major renovation of the five-story facility, completed in 1943, was under way at the time of the attack. The first phase of the work in Wedge One–the point of impact–included blast-resistant windows and was near completion on Sept. 11. "The structural upgrades of the exterior wall performed reasonably well, considering they were not specifically designed for aircraft impact," the report states. "It really is remarkable that it wasn't worse," says team leader Paul F. Mlakar, a technical director with the Army Corps of Engineers.

The study recommends additional research into progressive collapse mitigation and deformation capacity of spirally reinforced columns subjected to lateral loads over their height. The study's authors say they are in no way implying that buildings should be expected to survive such catastrophic events.