Simulations of 9/11 crashes could assist building designs.

Engineers, computer scientists and graphics technology experts at Purdue University, in West Lafayette, Ind., have created a simulation that uses scientific principles to study in detail what happened when a Boeing 757 crashed into the Pentagon last year.

Researchers said the simulation could be used as a tool for designing critical buildings--such as hospitals and fire stations--to withstand terrorist attacks.

The simulation merges a realistic image of the airliner approaching the building with a technical, science-based animation of the plane crashing into the structure.

"This is going to be a tremendous asset," said Mete Sozen, professor of structural engineering at Purdue. "Eventually, I hope this will be expanded into a model that we can use to help design structures to resist severe impact loads.

"Using this simulation, I can do the so-called 'what-if' study, testing hypothetical scenarios before actually building a structure."

The simulation can be recorded on a DVD and played on an ordinary personal computer.

The software tool is unusual, because it uses principles of physics to simulate how a plane's huge mass of fuel and cargo impacts a building. The plane's structure caused relatively little damage, and the explosion and fire that resulted from the crash also are not likely to have been dominant factors in the disaster, Sozen said.

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 roughness, it was able to absorb much of the kinetic energy from the impact," said Christoph M. Hoffmann, a computer sciences professor at Purdue's Computing Research Institute.

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

Hoffmann turned Sozen's model into the simulation by representing the plane and irs 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 later worked closely with Purdue's information technology staff 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.

One significant 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 researchers 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."

This article first appeared on the Purdue University News Service web site.