On the morning of his interview with Cadalyst, Professor Christoph Hoffmann, director of the Rosen Center for Advanced Computing at Purdue University, received a call from someone — "an off-the-beaten-track caller," as he politely put it — accusing him of being a part of the big cover-up, of deliberately obscuring the truth behind the collapse of the World Trade Center (WTC) after the terrorist attack on September 11, 2001. He took the verbal abuse in stride.

In this article

In July, Hoffmann and his colleagues released an animation clip of the moment of impact, depicting frame by frame the effect of American Airlines Flight 11's collision with the North Tower. The Purdue team was well aware of the emotionally charged territory they were entering.

"The mood [on the job] was much more somber when we did an earlier simulation of the Pentagon attack," Hoffmann admitted. At the time, the incident was still fresh in everyone's memory. The six years that passed since have made it a little easier to concentrate on the cold hard facts, he said.

The Sources

If you have ever commanded a virtual aircraft in a computer game, you know how much effort the developers usually put in to creating the digital prototypes, some virtually undistinguishable from the real ones parked on the airstrips. Microsoft Flight Simulator, for instance, lets you strap yourself into the cockpit of an Airbus A321, a Douglas DC3, a Boeing 737-800, a Boeing 747-400, and several other recognizable models. So it's not surprising that when Hoffmann and his colleagues at Purdue wanted a Boeing 767-200 in 3D for their simulation, they purchased it from a game developer.

Figure 1. By adding structural data into the digital model of a Boeing 767-200 purchased from a computer game company, the Purdue researchers produced the FEA model needed to simulate the September 11 terrorist attack on the World Trade Center's North Tower.

"That gave us only the outside shell," explained Hoffmann. "Boeing doesn't usually provide the hard data on its planes to someone, but we can infer a lot from the existing literature on airplane construction. For instance, we could get a pretty good idea how to place the stringers and ribs in the plane's body. So there was a great deal of structural information we had to add [figure 1]."

For the ground environment, the civil engineering members of the Purdue team modeled the structural elements of the North Tower of the World Trade Center, including the concrete floor slabs and steel beams, the columns in the perimeter skin structure and the core, and the steel open-web joists supporting the floor slabs.

The Analysis

In sunny California, somewhere north of the Lawrence Livermore National Lab stands a row of red-roofed buildings. The concrete block at the gate announces the property as Livermore Software Technology Corp. Founded in 1987 by John O. Hallquist, the company develops and sells a handful of analysis software products. One of them is LS-DYNA, described as "a general-purpose transient dynamic finite-element program capable of simulating complex real-world problems."

LS-DYNA has been used to analyze heart valves, dental bridges, corrective orthopedic shoes, airbags, and train crashes. The Purdue team used this package to run the finite-element analysis (FEA) simulations of the attack on the WTC.