Following the September 11 crash at the Pentagon of an airliner commandeered by terrorists, the American Society of Civil Engineers established a building performance study team to examine the damaged structure and make recommendations for the future.

The members of the team reviewed available information on the structure and the crash loading and drew on focused assessments by others. In addition to analyzing the essential features of column response to impact, they investigated the residual frame capacity and the structural response to the fire. Plausible mechanisms for the response of the structure to the crash were established. Recommendations are offered for future design and construction along with suggestions for research and development.

The material presented in this publication has been prepared in accordance with generally recognized engineering principles and practices, and is for general information only. This information should not be used without first securing competent advice with respect to its suitability for any general or specific application.

THE PENTAGON BUILDING PERFORMANCE REPORT AUTHORS:

• Paul F. Mlakar, Ph.D., P.E., Lead; Technical Director; U.S.Army Corps of Engineers
• Donald O. Dusenberry, P.E.; Principal; Simpson Gumpertz & Heger, Inc.
• James R. Harris, Ph.D., P.E.; Principal; J.R. Harris & Company
• Gerald Haynes, P.E.; Fire Protection Engineer; Bureau of Alcohol, Tobacco, and Firearms
• Long T. Phan, Ph.D., P.E.; Research Structural Engineer; National Institute of Standards and Technology
• Mete A. Sozen, Ph.D., S.E.; Kettelhut Distinguished Professor of Structural Engineering; Purdue University

Download The Pentagon Building Performance Report

Pdf format, 2.28MB.

Published by the American Society of Civil Engineers, 2003

EXECUTIVE SUMMARY:

The Pentagon, the headquarters of the United States Department of Defense, was constructed between September 1941 and January 1943. A major renovation of the entire 6.6 million sq ft facility began in 1999 and is scheduled for completion in 2010. On September 11, 2001, a hijacked commercial airliner was intentionally crashed into the building in an act of terrorism. One hundred eighty-nine persons were killed and a portion of the building was damaged by the associated impact, deflagration, and fire.That same day the American Society of Civil Engineers established a building performance study (BPS) team to examine the damaged structure and make recommendations for the future. Team members possess expertise in structural, fire, and forensic engineering.

The BPS team’s analysis of the Pentagon and the damage resulting from the attack was conducted between September 2001 and April 2002.The members of the BPS team inspected the site as soon as was possible without interfering with the rescue and recovery operations.They reviewed the original plans, the renovation plans, and available information on the material properties of the structure.They scrutinized aircraft data, eyewitness information, and fatality records; consulted with the urban search and rescue engineers, the chief renovation engineer, and the engineer in charge of the crash site reconstruction; and examined the quick, focused assessments of the disaster conducted by the United States Army Corps of Engineers and Pentagon Renovation Program staff.

On the basis of this information the BPS team analyzed the essential aspects of the response of the structural system of the Pentagon to the crash. Impact analyses revealed that the spirally reinforced columns could withstand substantial dynamic lateral loads and deflections. Static analyses indicated that the floor system was capable of significant load redistribution without collapse when several adjacent supporting columns were removed or severely damaged by an extreme action.Thermal analyses showed that the ensuing fire could have sufficiently weakened some damaged frame members to result in collapse within an hour of initiation.

The BPS team concluded that the impact of the aircraft destroyed or significantly impaired approximately 50 structural columns.The ensuing fire weakened a number of other structural elements. However, only a very small segment of the affected structure collapsed, approximately 20 minutes after impact.The collapse, fatalities, and damage were mitigated by the Pentagon’s resilient structural system.Very few upgraded windows installed during the
renovation broke during the impact and deflagration of aircraft fuel.

The BPS team recommends that the features of the Pentagon’s design that contributed to its resiliency in the crash—that is, continuity, redundancy, and energy-absorbing capacity—be incorporated in the future into the designs of buildings and other structures in which resistance to progressive collapse is deemed important.The team further advocates that additional research and development be conducted in the practical implementation of measures to mitigate progressive collapse and in the deformation capacity of spirally reinforced columns subjected to lateral loads applied over the height of the column.

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