Resurrecting an Unforgettable Second (Tech Trends Feature)

Researchers use computer simulation to analyze World Trade Center collapse.

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 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.

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"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.

"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]."

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.

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.

"The FEA model [of the aircraft] includes structural elements, including ribs, stringers, keel beam, floor, and more," the team wrote in its report. Ayhan Irfanoglu, assistant professor of civil engineering at Purdue and another member of the team, further explained, "Smoothed particle hydrodynamics (SPH) elements were used to model the jet fuel. We focused on the physics of the aircraft-impact problem, generalized as the impact of fluid traveling at high speed on solid. We tested our theoretical work using small-scale experiments."

Of the WTC model, the report notes, "All stories were modeled, including those underground. The simulation restricted to the upper 20 floors of the building, however, [was done] with increased detail meshing near the impact region so as to achieve high accuracy of the results."

"We didn't have prior experience with meshing tools," Hoffmann said, "so there was a fair amount of time spent just putting the models together [figure 2]."

Figure 2. The FEA model for the North Tower included all stories, including those underground. However, the simulation restricted to the upper 20 floors — the impact region of the building — was done with increased detail meshing.

The Physics of Destruction

What happens when an airplane collides with a solid wall? If you're mathematically inclined, you'll be able to dissect the energy distribution and the behavior of the impact forces using the Riera curve, an algorithmic summation of such incidents (figure 3).

Figure 3. The Purdue team used the Riera curve, an algorithmic summation of the energy distribution in airplane collisions, to study (top) the moment of impact and (bottom) immediately after impact.

In one of the archived Purdue presentation documents on Riera curve, the authors (Christoph Hoffmann, Ayhan Irfanoglu, and Mete Sozen) point out that the Riera approach is especially useful in the estimation of:

• 1. mass distribution versus crushing force
• 2. velocity and force distribution during impact
• 3. impact damage without the detailed aircraft model

For hard evidence, Hoffmann said he and his colleagues also "shot beverage cans into a stiff metal plate" to obtain data to compare with their FEA-based estimates.

What It Takes to Recreate a Single Second

Still Unsettled

But don't think this simulation will answer all the questions that people have been raising about the collapse of the WTC. Even though they believe that for the aircraft and for the parts of the structure it modeled, their simulation is faithful to the physics of the problem, the team warns that "impact simulations indicate that the damage states of the WTC-I core structural elements are very sensitive to analysis parameters and as such, it is not possible to suggest the exact state of the core framing after the aircraft impact."

Hoffmann explained, "In other words, we can't say, 'Well, this number of columns were cut; therefore, the core was compromised.' In fact, in most of the simulation runs we did, after the impact, the core still had plenty of capacity to carry the load [of the tower]."

With scientific reservations, Hoffmann was prepared only to suggest, "The fire seemed to have been the culprit in weakening the structure. When you heat a steel structural column to 600 to 700 degrees Celsius, though it doesn't melt, its strength and stiffness decrease to a small fraction of their room-temperature values. In other words, it becomes extremely weakened and has less capacity to carry the load. If that happens in an element supporting loads based on its original strength (with an appropriate factor of safety), the element fails."

The Unknown

"One of the shortcomings of the simulation is that the dispersing fuel is treated by LS-DYNA as a nonvolatile liquid," the team noted. "However, [the impact] created an explosion and subsequent fire . . . This and other fire-related effects should be revisited in future work."

Hoffmann confessed that certain issues are difficult to address. "For example, the layout of the partitions, the materials of the partitions, their sturdiness, the distribution of the furniture — all those could have slowed down the impact and altered the fuel dispersal. Some [of those] may be less critical compared with others, such as how much of the [plane's] fuel dissipated outside, how much inside, and how much on the other side [of the building]."

The Cost of Fame

Since the animation was released, the Purdue team has received many phone calls from speculative filmmakers and authors who want them to confirm one theory or disprove another. Hoffmann and his colleagues welcome legitimate input, such as the eyewitness who wrote to them to say, "I was there when it happened. I don't remember the fireballs coming out on the far side like you're showing them."

At the same time, they're trying hard not to get tangled up in the works of conspiracy theorists.

Kenneth Wong is a former editor of Cadence magazine. As a freelance writer, he explores innovative usage of technology and its implications. E-mail him at Kenneth.Wong at cadalyst.com.