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Study of Building Performance in the WTC Disaster
By: By James Milke, Ph.D., P.E.

The impacts of aircraft into the twin towers of the World Trade Center set off a chain of events seen via television coverage by many of the world's population on September 11, 2001. These aircraft damaged portions of the structure of the twin towers and also initiated fires on several floors. By the end of the day, four buildings collapsed, three buildings were severely damaged by fire, and seven buildings sustained significant damage, while numerous others suffered minor damage.

In late September, the Building Performance Study (BPS) team was formed to study the response of the affected buildings to impacts and fires. The principal support for the study was provided by the Federal Emergency Management Agency (FEMA) and the Structural Engineering Institute of the American Society of Civil Engineers. Also supporting the effort was a coalition of organizations including the Society of Fire Protection Engineers and National Fire Protection Association (NFPA).

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Team members included structural engineers and fire protection engineers. Some of the structural engineers specialized in the response of buildings to static loads, the effects of dynamic loadings on buildings, or metallurgy. Fire protection engineers included on the team had backgrounds in fire behavior and response of structures to fires. Individuals selected for the team with fire protection expertise included:

Jonathan Barnett, Ph.D., P.E., Worcester Polytechnic Institute Robert Duval, NFPA Richard Gewain, P.E., Hughes Associates Venkatesh Kodur, Ph.D., National Research Council of Canada Chris Marrion, P.E., ArupFire James Milke, Ph.D., P.E., University of Maryland Harold "Bud" Nelson, P.E., Hughes Associates

The team met in New York in early October 2001, visiting Ground Zero and also the recycling yards. During the seven-month period of the study, the team met with members of the design teams for the World Trade Center buildings and spoke with eyewitnesses and emergency responders. Information reviewed by the team included videotapes from television networks and private individuals, photographs of the incident, audio tapes from New York's Emergency Operations Center, plans, engineering documents, and aircraft data. 1 In addition, the team returned to the recycling yards many times. The team conducted some elementary numerical analyses and also reviewed the results of elementary analyses conducted by others. In addition, a limited number of metallurgical laboratory analyses were conducted on steel samples recovered from the recycling centers.

The principal purpose of the BPS was to establish the basic facts involving the performance of the affected buildings. In addition, the study team sought to provide preliminary thoughts about the probable collapse mechanisms and identify areas for future study.

While much of the attention of the BPS was directed to the twin towers, the performance of a total of 16 affected buildings was addressed in the study. The buildings included in the study and indicated in Figure 1 are:

  • WTC 1, 2, 3, 4, 5, 6, and 7
  • Banker's Trust
  • The Winter Garden and WFC 3 (The American Express Building) of the World Financial Center
  • Verizon Building
  • 30 West Broadway
  • 130 Cedar Street
  • 90 West Street
  • 45 Park Place
  • One Liberty Plaza
The emphasis of the Building Performance Assessment Team (BPAT) report was to describe the structural performance of the affected buildings. A brief description of the evacuation of WTC 1 and 2 was also included in the report based principally on media accounts. The focus of this paper will be on the structural performance of WTC 1, 2, 5, and 7. A description of the performance of the other 12 buildings is included in the BPAT report. 1 An analysis of the evacuation behavior of the occupants of WTC 1 and 2 is ongoing.

WTC 1, 2
Each building was 110 stories tall, with seven subgrade levels. The floor plate for each building was 63.1 m square, with chamfered 2 m corners. The area per floor was approximately 3,980 m 2 . The buildings were steel frame buildings. Because of their tall height, weight was a design constraint, with lightweight alternatives used where possible.

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FPE Spring 2003
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