World Trade Center 7 Collapse Investigation

Why Did World Trade Center 7 Collapse?

On September 11, 2001, World Trade Center 7 (WTC 7) was showered by debris from the collapse of WTC 1. Portions of the structural system were destroyed and fires, which burned unfought throughout the day, were ignited on multiple floors. The building collapsed at approximately 5:20 p.m.

Our Applied Science team performed a forensic study of the failure sequence leading to the collapse. We combined state-of-the-art computational analysis with photos, videos, eyewitness accounts and other data to answer three important questions:

• Was this a normal office fire, or an extraordinary one arising from the circumstances of 9/11?
• Was the building’s performance indicative of a deficiency?
• What does the collapse say about building codes and the modern stock of high-rise buildings?

Collapse Investigation Analysis

Our 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, which manifested 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 assertions that the collapse resulted from construction or design errors, and showed that the extraordinary events of 9/11, rather than any deficiency, caused the collapse.

The study also led to two inferences that could have widespread implications for building design. First, 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. Second, that the analytical tools to allow a risk-based performance design paradigm to be adopted in practice exist: engineers are now able to design for fires and evaluate outcomes under a variety of scenarios.