New York, NY

Global Analysis of World Trade Center Towers


The collapse of the World Trade Center (WTC) towers on 11 September 2001 was an unprecedented event in U.S. history. NIST retained SGH and ARA to develop computer simulations of the structural response of WTC 1 and 2 to aircraft impact and subsequent fires, and of WTC 7 to damage from impact of falling debris from WTC 1 and 2 and the subsequent fires at various points during the collapse. ARA performed the aircraft impact analyses and SGH determined the structural response of the damaged towers to subsequent fires. We compared the model results with observations of the collapse process captured in photographs and videos and used the information learned from the study to better understand the behavior of tall steel buildings.

SGH developed finite element models of components, connections, and subsystems and studied the structural response to fire-induced temperature time histories. Using the results of these studies, SGH developed computationally-efficient global models to analyze the WTC towers from initial impact through the collapse of each tower.

Our study showed that the tower collapses were caused by the combined effects of aircraft impact damage to the structure and fireproofing, and the subsequent fires. SGH concluded that without damage to the fireproofing and subsequent weakening of the structure, the likelihood of collapse was small. Lessons learned have formed the basis of revisions to building and fire codes and standards.

In several separate assignments, SGH was hired by insurance companies to determine whether the terrorist attacks constituted one or two “occurrences” for the purpose of insurance coverage. SGH also conducted assessments of neighboring buildings at 130 Liberty Street, 90 West Street, and 130 Cedar Street for New York City Department of Design and Construction. These assessments were intended to determine the immediate safety of these structures considering damage that resulted from the adjacent collapses.

Project Summary



Completion Date


Failure Investigation
Structures | Advanced Analysis
National Institute of Standards & Technology (NIST)
Specialized Capabilities
Failure Analysis | Risk & Resilience Assessment

Key team members

Mehdi Zarghamee
Mehdi Zarghamee
Senior Principal