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On September 11, we all watched with sadness and disbelief as hijacked airliners slammed into, then caused collapse of the twin World Trade Center towers in New York and the Pentagon in Virginia. Just as in the aftermath of a major earthquake, we were struck both by the tragic loss of life and the vulnerability of our built environment to extreme events. In the United States we had been fortunate. Though other nations have experienced earthquakes that were as destructive to property and life as the terrorist attacks of September 11, we have not had any earthquake in the United States that approached these staggering tolls. In fact, with the exception of a hurricane that struck the City of Galveston, in 1900, no single disaster in the United States has been this costly.

There are many parallels between these tragic events and the destruction and losses caused by earthquakes. In addition to the scale of the losses that occurred, other parallels include the following. The same Urban Search and Rescue teams used to search for victims following earthquakes were pressed into service in New York. The same ATC-20 procedures used by structural engineers in California and Washington to perform post-earthquake assessments of building damage and safety were used by structural engineers in New York to assess the condition of buildings surrounding the World Trade Center. Just as fire is often a major hazard and cause of loss following earthquakes, fire was a major cause of damage and loss following the aircraft impacts. Just as structural redundancy, continuity and toughness are key to the performance of buildings in earthquakes, these same properties were key to the performance of buildings in New York and Virginia and indeed, were key to the avoidance of even more staggering losses. Portions of the Pentagon had recently been retrofitted, permitting the effectiveness of the particular retrofit techniques to be judged. Just as the earthquake community learns important lessons on how to mitigate the effects of extreme events following each earthquake, we can learn important lessons from the events of September 11.

On September 12, just one day after the attacks, the Strutcural Engineers Institute of the American Society of Civil Engineers (SEI/ASCE) began to form two Building Performance Assessment Teams (BPATs) to act in a very similar role to that of EERI’s post-earthquake reconnaissance teams. One team focused on the World Trade Center and surrounding buildings while the other team focused on the Pentagon. These teams were charged with collecting perishable data on the performance of affected buildings, evaluating how each of the buildings performed, identifying potential vulnerabilities in our design and construction methods that could be practically mitigated, recommending detailed studies and investigations, where appropriate to confirm these observations, and preparing a report to document each teams’ findings and recommendations.

Each of the BPATs was formed in cooperation with other organizations. In addition to ASCE/SEI, participating organizations included the National Council of Structural Engineers Associations (NCSEA), the American Institute of Steel Construction (AISC), the American Concrete Institute (ACI), NFPA, the International Code Council (ICC), the Society of Fire Protection Engineers (SFPE), the National Research Council of Canada (NRCC), the National Institute of Standards and Technology (NIST), the Council on Tall Buildings in the Urban Habitat (CTBUH), the Masonry Society (TMS) and a number of Universities, private firms and individuals. The World Trade Center Team, headed by EERI member, Dr. Gene Corley, in cooperation and with support from the Federal Emergency Management Agency. The extensive support and close cooperation of the New York City Department of Design and Construction and the Structural Engineers Association of New York is particularly worthy of mention. The Pentagon team, which is somewhat smaller, was headed by Dr. Paul Mlakar of the Army Corps of Engineers.

Each of the teams conducted field reconnaissance in early October 2001. Investigations included review of construction drawings for the affected buildings, photographs and television footage of the events, and recordings of 911 calls received by emergency services. Selected samples of construction materials were removed from the affected buildings and subjected to laboratory testing. In addition, detailed analytical modeling of the aircraft impact events, the resulting blasts, fire spread and heat generation, and the structural behavior of affected buildings were undertaken.

Specific details of the findings of the investigations are available at http://utc.hist.gov/