Technology Brief: WTC impact simulation validates NIST report

Five years after the real thing, researchers at Purdue University have simulated in graphic detail what likely happened inside the structure when American Airlines Flight 11 crashed into the North Tower of the World Trade Center on 9-11-01. It could help silence alternative collapse theory arguments.

The simulation provides a better understanding of which elements in the building's structural core were affected, how they responded initially to the collision and how the tower collapsed from the ensuing fire fed by thousands of gallons of jet fuel.

“The simulation enables us to ‘look' inside the building to see what happened structurally,” says Mete Sozen, a professor of structural engineering in Purdue's School of Civil Engineering.

The Purdue work comes 11 months after the National Institute of Standards and Technology released its October 2005 technical investigation of the fires and collapses of the towers. Verdicts on the nist report were split.

While most in the building design, construction, and fire and rescue fields embraced the three-year effort and began working with nist to use the report's 30 recommendations to improve building codes, standards and practices, a few claimed that factors other than those described in the nist report brought the towers down.

But NIST found no evidence suggesting that the towers were brought down by controlled demolition using explosives placed prior to 9/11, or that missiles were fired at the towers.

The Purdue simulation seems to corroborate the nist report. Christoph M. Hoffmann, a professor of computer science and co-director of Purdue's Computing Research Institute, says there's nothing in the Purdue results that supports alternative collapse theories.

"When you lose a percentage of the core support and the exterior support structure," he says, "the remaining support beams have to make up the difference, and the increased load makes them more vulnerable to the effects of the fire that followed the impact."

Sozen says the Purdue simulator and the nist report took similar approaches with similar results. “We did the same type of modeling and used similar software with similar results.”

The Purdue work also differs in some ways from the NIST report, Hoffman says. “One difference is that the NIST study not only simulated the initial crash, but also considered various models of the ensuing inferno and estimated the effect of the heat on structural integrity, whereas we considered the impact alone.”