Simulation Shows 9/11 Plane Crash with Scientific Detail

October 7, 2004

Engineers, computer scientists and graphics technology experts at Purdue University have created the first publicly available simulation that uses scientific principles to study in detail what theoretically happened when the Boeing 757 crashed into the Pentagon last Sept. 11.

The model indicates the most critical effects were from the mass moving at high velocity.

"At that speed, the plane itself is like a sausage skin," Sozen said. "It doesn't have much strength and virtually crumbles on impact."

But the combined mass of everything inside the plane – particularly the large amount of fuel onboard – can be likened to a huge river crashing into the building.

The simulation deals specifically with steel-reinforced concrete buildings, as opposed to skyscrapers like the World Trade Center's twin towers, in which structural steel provided the required strength and stiffness. Reinforced concrete is inherently fire resistant, unlike structural steel, which is vulnerable to fire and must undergo special fireproofing.

"Because the structural skeleton of the Pentagon had a high level of toughness, it was able to absorb much of the kinetic energy from the impact," said Christoph M. Hoffmann, a professor in the Department of Computer Sciences and at Purdue's Computing Research Institute.

Sozen created a mathematical model of reinforced concrete columns. The model was then used as a starting point to produce the simulation.

Hoffmann turned Sozen's model into the simulation by representing the plane and its mass as a mesh of hundreds of thousands of "finite elements," or small squares containing specific physical characteristics.

"What we do is simulate the physics of phenomena and then we visualize what we have calculated from scientific principles as a plausible explanation of what really happened," Hoffmann said. "We hope that through such simulations we can learn from this tragic event how to protect better the lives of our citizens and the civil infrastructure of the nation."

The simulation may be the first of its kind for merging realistic-looking animation with scientifically rigorous computations.

"Most of the computer-simulated crashes you see in movies or on TV are not realistic from the point of view of physics," said Voicu Popescu, an assistant professor of computer science. "They are designed to be spectacular rather than realistic. What hasn't been done much, or, to our knowledge hasn't been done at all, is to create a visualization that looks realistic in the sense that you would recognize the Pentagon and the plane and is, at the same time, true to physics."

The mesh of finite elements in the model require that millions of calculations be solved for every second of simulation. Creating only one-tenth of a second of simulation took about 95 hours of computation time on a supercomputer. Researchers originally used a bank of computers and also worked closely with Information Technology at Purdue (ItaP) to harness IBM supercomputers at Purdue and Indiana University.

"The majority of the work had to do with producing the right models and then setting up the particular mesh so that we could work out accurately how this scenario unfolded," Hoffmann said.

In the simulation, the plane crashes into the building's concrete support columns, which were reinforced with steel bars. In this simulation the columns were assumed to be "spirally reinforced," a technique popular in the 1940s in which steel bars were wound around columns in a helical shape. The coiled steel provided added strength to the columns and probably is responsible for saving many lives, Sozen said.

The simulation might be especially useful for engineers who are trying to design reinforced concrete structures that better withstand terrorist attacks or accidents involving aircraft crashes.

"Our focus was on modeling the impact effect of the liquid fuel in the tanks of the aircraft – the amount of energy transferred to the building's structural load-carrying system, which is mainly the reinforced concrete columns, and the condition of those columns after the impact," said Sami Kilic, a civil engineering research associate who specializes in earthquake engineering.

A major challenge has been learning how to combine commercially available software with the special models needed to simulate an airliner hitting a building, Kilic said.

The Purdue team used commercial software that is normally used by auto manufacturers to simulate car crashes. But adapting the software to simulate the plane crash and then combining the realistic-looking graphics with scientific simulation has been especially difficult, Kilic said.

"Integrating these two animations is uncommon," he said. "We are discovering a new territory. We had some interaction with aeronautical engineers, and they had never heard of this kind of a simulation, with an aircraft hitting a building.

"This kind of a structure/aircraft interaction is not done commercially."

The work was funded, in part, by a grant from the National Science Foundation.

Originally posted: October 7, 2004