U.S. researchers have created what they say is the first scientifically accurate 3-D simulation of the Sept. 11, 2001, attacks on the World Trade Center in New York.

The animation, created by computer scientists at Purdue University in West Lafayette, Ind., shows the effects of a plane crashing into the World Trade Center's north tower in a bid to help engineers understand how such an incident affects the structural integrity of a building and prevent similar collapses in the future.

"The crashes and computer models you often see on television are not scientifically accurate," Voicu Popescu, an assistant professor of computer science at Purdue, said in a written statement. "This provides an alternative that is useful to the non-expert but is also scientifically accurate, so it provides a more realistic picture of the event."

Fuel caused collapse

The simulation suggests that the weight of the 39,000 litres of fuel in the airplane more than anything else was responsible for the tower's destruction.

"It is the weight —the kinetic energy of the fuel — that causes much of the damage in these events," said Christoph Hoffmann, a professor of computer science and director of Purdue's Rosen Center for Advanced Computing. "If it weren't for the subsequent fire, the structural damage might be almost the same if the planes had been filled with water instead of fuel."

The five-minute animation depicts the plane ripping through several floors of the World Trade Center's north tower within half a second, peeling back the aircraft's skin and sending the titanium engines rocketing through the structure as the vessel disintegrated.

The depiction also shows the way in which the fuel became a fiery flash flood, knocking out structural support columns and stripping away fireproof insulation.

The simulation took years to complete and used weeks of supercomputer time to develop a reliable model of what the researchers believe happened during the crash.

"We calibrated our calculations using data from experiments we had conducted to evaluate the energy imparted from fluid moving at high speed to solid targets," said Mete Sozen, Purdue's Kettlehut distinguished professor of structural engineering and a principal investigator on the simulation.

"We concluded that the damage map we calculated for our numerical model of the building would correspond closely to the actual extent of the damage."

Tower was doomed

Sozen said the simulation shows that once the fireproofing insulation was torn from the structure there was no chance the building could have remained standing.

"The aircraft moved through the building as if it were a hot and fast lava flow," Sozen said. "Even if all of the columns and girders had survived the impact — an unlikely event — the structure would fail as the result of a buckling of the columns. The heat from an ordinary office fire would suffice to soften and weaken the unprotected steel."

The researchers kept unnecessary details to a minimum to avoid any appearance the simulation was exploiting the attack. For example, the airplane has no windows or airline insignia, and although flames and smoke are shown, the fire's effects are not depicted.

The simulation was completed in September 2006 but needed to be tested before it was released. The final test used more than 80 hours of high-performance computing resources.

The video can be viewed on Purdue's website.

The research was partly funded by the U.S. National Science Foundation.