A Forensic Engineering Investigation of the WTC Twin Towers' Collapse by Dr. Najib Abboud, Weidlinger Associates Inc.
September 11, 2001 was the most tragic day in New York’s history and, perhaps, the history of our nation. On that date, 19 militants associated with the Islamic extremist group al-Qaeda hijacked four airliners and carried out suicide attacks against targets in the United States. Two of the planes were flown into the towers of the World Trade Center in New York City, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed in a field in Pennsylvania. Over 3,000 people were killed during the attacks in New York City and Washington, D.C., including more than 400 police officers and firefighters.
Two of the four hijacked airliners crashed into the Twin Towers, one into the North Tower (1 World Trade Center) and the other into the South Tower (2 World Trade Center). The collapse of the twin towers destroyed the rest of the complex, and debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures.
On January 28, 2015, Stevens Institute of Technology, School of Engineering & Science, hosted Dr. Najib Abboud for the Dean’s Seminar Series. In his talk, Dr. Abboud shared the results of the forensic engineering study that investigated the WTC collapses, explaining why the Twin Towers stood for as long as they did, and why they ultimately collapsed, analyzing the structural effects of the high-speed impacts caused by the Boeing 767 airplanes. Additionally, Dr. Abboud discussed the series of analyses undertaken to quantify and study the response of and damage to each of the Twin Towers related to the multi-year dispute over whether the attacks had constituted one event or two under the terms of the insurance policy, which provided for a maximum of $3.55 billion coverage per event.
“Dr. Abboud is an international leader in structural dynamics and the forensic investigation of structural failures,” says Michael Bruno, Feiler Chair Professor and Dean, School of Engineering and Science. His seminar was at once informative, technical, and emotional. I know that the audience of students, faculty, staff, alumni and guests shared that view, and we were all impressed by Dr. Abboud's ability to describe highly complex dynamics and analysis techniques in a way that everyone could understand. We all learned a great deal today.”
In his talk, Dr. Abboud explained how the collapse of each tower resulted from the combined damage due to each airplane impact and the subsequent structural capacity degradation of fireproofing-stripped members exposed to the fires in the impact zone.
According to Dr. Abboud, the computational analyses demonstrated that the impacts created an environment of heavy and energetic flying debris in the interior of the impact zones, one that clearly no fireproofing material in current practice is designed to resist. The robustness of the tubular perimeter wall system and the redundancy afforded by the hat truss proved substantial, as the analyses demonstrated, allowing the towers to withstand the immediate impact damage and the subsequent fire damage long enough to allow the safe evacuation of tenants below the impact zones.
“Ultimately though,” says Dr. Abboud, “the thermal loads overwhelmed the remaining structural system capacity and collapse became inevitable. It is clear to us that any more standard office tower in any American metropolis would likely have fared worse if confronted with such attacks. However tragic, collapses and more generally structural failures are critical junctions in the evolution of engineering knowledge and its practice.”
“These incidents sometimes lead to advances in our understanding of the behavior of materials and structural systems, or to developments of more accurate predictive analytical approaches that better capture the complex behavior of structures subjected to extreme and/or unforeseen loads, or to the re-examination of various practices involved in the design and construction of our built environment,” he says.
Finally, Dr. Abboud emphasized, “in some cases, dramatic failures and collapses also compel us to consider broader questions in the realm of public policy such as our tolerance for risk and our capacity to mitigate it.”
Dr. Abboud leads the New York and District of Columbia offices of the Applied Science and Investigation division of Weidlinger Associates Inc. He is responsible for the computational mechanics research and development efforts in support of the various United States (U.S.) Navy projects. He also manages Weidlinger’s forensic engineering practice in the eastern U.S. which led major collapse investigations over the years, including the WTC towers’ collapse and the Tropicana Casino and Resort Garage collapse. Dr. Abboud also manages the infrastructure protection and resilience activities at Weidlinger. He focuses on physics-based, high-fidelity Threat, Vulnerability and Risk Assessments (TVRA) of critical infrastructure and the protective design of mitigation measures to enhance their resilience against terrorist and other threats.