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World Trade Center 7 Collapse Investigation

On September 11, 2001, World Trade Center 7 was showered by debris from the collapse of WTC 1, destroying portions of the structural system and igniting fires on multiple floors, which burned, unfought, throughout the day. At approximately 5:20 p.m., the building collapsed.
 
A forensic study of the failure sequence leading to the collapse was performed, combining state-of-the-art computational analysis with photos, videos, eyewitness accounts and other data, to answer three important questions: (1.) Was this a normal office fire, or an extraordinary one arising from the circumstances of 9/11? (2.) Was the building’s performance indicative of a deficiency? (3.) What does the collapse say about building codes and the modern stock of high-rise buildings?
 
Detailed nonlinear dynamic thermomechanical computational analyses captured the physics of the phenomenon, identifying the most plausible failure sequence: (1.) WTC 1 showered WTC 7 with debris, igniting fires and destroying the sprinkler system. (2.) Long-duration fires undermined the steelwork supporting the eastern portion of the 10th-floor, causing it to collapse. (3.) The eastern portion of 10th floor fell onto a fire-weakened ninth floor, causing its eastern section to collapse. (4.) A downward cascade of floor collapses ensued. (5.) As the collapse progressed, interior columns buckled. (6.) Floor framing supported by the buckling columns fell, causing failure to propagate to the top of the building, manifesting itself in the collapse of the east penthouse. (7.) Falling debris impacted beams at the edges of the intact portion of the structure, causing failure to progress across the building.
 
The study refuted the assertion that the collapse resulted from construction or design errors, and that it was the extraordinary events of 9/11, rather than any deficiency, that caused the collapse. Additional inferences can be drawn: (1.) The expectation that a modern building should withstand any unfought fire without collapsing is “aspirational,” and is not guaranteed by established codes or design procedures. (2.) The analytical tools exist to design for fires and evaluate outcomes under different fire scenarios if a risk-based performance design paradigm were to be adopted in practice.

Completion Date: 2014
Location: New York, New York
Owner or Client: Silverstein Properties
Prime Consultant(s): WAI Prime; Friedman Kaplan Seiler & Adelman LLP

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