The collapses of the 110-story twin towers of the World Trade Center after terrorists slammed hijacked planes into them were separately initiated in the impact zone of each tower due to failure of the columns, says a recent engineering report, not the floor trusses. Two WTC, though hit by the second hijacked plane after One WTC, fell first "primarily" because the plane struck it at an off-center angle and caused damage that compromised the corner of the core of the building, concludes the report's authors, a team of engineers from several firms working for Silverstein Properties Inc., the New York City-based leaseholder of the World Trade Center.

The findings are based on analysis of original structural drawings, thousands of photos and dozens of videos, says Silverstein. The team used computer modeling, some programs developed for the Dept. of Defense, and fire evaluation techniques to simulate the condition of each tower at critical points from impact to collapse.
Click here to view diagram 1 Click here to view diagram 2

The team determined that the initial hits destroyed 33 of 59 perimeter columns in the north face of One WTC and 29 of 59 perimeter columns in the south face of Two WTC. Computer analysis showed that the impact of the planes also destroyed or disabled some 20 of 47 columns in the center of the core of One WTC and some five of 47 columns in the southeast corner of the core of Two WTC. The crashes stripped fireproofing from columns in the path of debris created by the planes penetrating the buildings, it continues.

One frame from the computer model of the initiation of the collapse of Tower 2 performed by Weidlinger Associates

The team says the towers' columns of the redundant exterior tube and core columns, connected by a steel "hat truss" at the top of the buildings, redistributed loads away from the damaged areas to remaining columns. This allowed the towers to stand as long as they did, says the report. In a release, Matthys Levy of Weidlinger Associates Inc., one of the engineers on the study team, states, "The fact that tower one stood for 103 minutes.... and tower two for 56 minutes" after the loss of so many columns, "is a testament to the strength of the buildings and the skill of Leslie Robertson and the other engineers who designed them. I believe that few, if any, other buildings could suffer that amount of damage and not collapse immediately."

The computer models, says the report, identify the failure of columns that either lost fireproofing or were destroyed on impact as the specific cause of the collapse of each tower. "No fireproofing is designed to withstand such devastating impacts," says the report.

It adds that the fireproofing on the structural elements of the towers had been inspected regularly and that the inspection program "represented a greater standard of care than is generally followed for high-rise office buildings in New York City."

The report exonerates the floor trusses for the collapses. "Failure of the floors...was shown not to have had any significant role in the initiation of the collapses," it says. Studies by Hughes Associates and ARUPFire led the team to conclude that tower floors survived the initial impact of the planes, suffering only localized damage. On the basis of a review of smoke plumes and fire spread, for each tower, the engineers concluded that the fires did not lead to the collapses of the floors affected before the towers fell. Additionally, the engineers claim that computer modeling shows that the failure of columns alone, independent of the floors explains the collapses.

Diagram illustrating the way Tower 1 redistributed loads to other columns on its north face and started off immediate collapse, as modeled by Weidlinger Associates

The findings are intended to build on the study initiated by the American Society of Civil Engineers and sponsored by the Federal Emergency Management Agency, called World Trade Center Building Performance Study. Released last May, it suggested subjecting the floor truss system to "more detailed evaluation." But the study also stated the truss systems "should not be regarded" as design deficiencies, says the Silverstein report.

The Silverstein report also concludes that fire temperatures were lower than typical "fully developed" office fires. The fires were fueled by office furniture and floor contents initially ignited by the jet fuel, which burned out quickly. Dust and debris distributed by the crashes inhibited the fires, which at the impact floors were between 750°F and 1,300°F.

The engineering team is comprised of: Weidlinger Associates Inc., led by Matthys Levy and Najib Abboud; LZA Technology/Thornton-Tomasetti Group, led by Daniel Cuoco and Gary Panariello; ARUPFire, led by Richard Custer; Hughes Associates Inc., led by Craig Beyler; SafirRosetti, led by Howard Safir; Hillman Environmental Group, led by Christopher Hillmann and John B. Glass Jr.; RWDI, led by Peter Irwin; Dr. W. Gene Corley, who led the ASCE-FEMA study; Professor Sean Ahearn; and Z-Axis Corp., led by Gary Freed and Alan Treibitz.

Silverstein commissioned the reports for its insurance claim on the World Trade Center. The organization has already given the reports to the National Institute of Standards and Technology, which is studying the collapses of the towers and Seven World Trade as part of a two-year study.