Chances are good the Pentagon will be in the news this month for action related to Sept. 11, 2001. To construction and engineering professionals, one of the more obscure reports involving the world's largest reinforced concrete building should come as no surprise: The Pentagon's resilient structural system curtailed damage inflicted by the crash of the hijacked American Airlines Flight 77 shortly after 9:30 a.m. on Sept. 11. Its design helped arrest progression of collapse and resultant loss of life.

Those are among observations of The Pentagon Building Performance Report, prepared by a six-member team of structural, fire protection, and forensic engineers and released in late January by the American Society of Civil Engineers. They noted that the structural performance of the Pentagon — a five-story fortress whose 1941-1943 construction saw placement of more than 400,000 yd. of concrete — validates measures to reduce collapse resulting from unlikely events, including 1) continuity, such as the extension of floor reinforcements through structural supports; 2) redundancy in design and construction, such as two-directional framing of the floors; and, 3) spirally steel-reinforced concrete columns that absorb energy generated from a lateral load, such as high wind, earthquake … or aircraft impact.

The team figures the direct impact of the crash at the Pentagon's southwest corner destroyed approximately 50 first floor and six second floor exterior wall columns. The subsequent fire ignited by the aircraft fuel and fed by the aircraft components and building contents caused moderate damage to the frame in small areas of the first and second floors, which later collapsed. The engineers also found that the structural design elements that helped arrest progressive collapse include short spans between structural supports; redundant and alternate load paths of the floor system; and, substantial continuity of steel floor reinforcement.

The Pentagon Report team assembled shortly after Sept. 11 under the guidance of ASCE's Structural Engineering Institute, whose objectives include investigation of building failures to identify conditions or shortcomings that might be corrected through changes to current building codes. While the Pentagon document cites a need for some additional research into reinforced concrete assemblies, it does not appear to call for fundamental changes in practice, model codes or ACI 318 Building Code Requirements for Structural Concrete.

The Pentagon document followed The World Trade Center Building Performance Study (BPS; separately noted in NCMA Chairman's Report, page 22), which ASCE and the Federal Emergency Management Agency released in May 2002. Among many aspects of the WTC twin towers' engineering, it questioned the limits of a) spray-on fireproofing material for structural steel columns and floor trusses; and b) ASTM E 119, a widely subscribed fire-rating test method for construction materials or assemblies that enabled the towers' substitution of concrete or masonry with drywall in vertical conditions.

ASCE's most recent report tied to Sept. 11 raises few questions on the subject building's response to fire exposure. The same cannot be said for the BPS, which is the springboard for a two-year, National Institute of Standards and Technology investigation into the twin towers' construction and failure. As the investigation plan was in the works, NIST Building and Fire Research Laboratory Director Dr. Jack Snell suggested: “Perhaps the most important potential change to [construction] practice that should result from the [WTC] disaster is to bring modern fire science and protection engineering together with structural engineering into a new set of tools for fire safety design.”