Simulating September 11

Researchers have created a simulation that uses scientific principles to study what happened when a commercial airliner crashed into the World Trade Center’s North Tower on September 11, 2001.

Researchers at Purdue University have created a simulation that uses scientific principles to study in detail what likely happened when a commercial airliner crashed into the World Trade Center’s North Tower on September 11, 2001.

The simulation could be used to better understand which elements in the building’s structural core were affected, how they responded to the initial shock of the aircraft collision, and how the tower later collapsed from the ensuing fire fed by an estimated 10,000 gallons of jet fuel, said Mete Sozen, the Kettelhut Distinguished Professor of Structural Engineering in Purdue’s School of Civil Engineering.

It took about 80 hours using a high-performance computer containing 16 processors to produce the first simulation, which depicts how the plane tore through several stories of the structure within a half-second, said Christoph M. Hoffmann, a professor of computer science and co-director of the Computing Research Institute at Purdue.

‘This required a tremendous amount of detailed work,’ Hoffmann said. ‘We have finished the first part of the simulation showing what happened to the structure during the initial impact. In the coming months, we will explore how the structure reacted to the extreme heat from the blaze that led to the building’s collapse, and we will refine the visual presentations of the simulation.’

The researchers are analysing how many columns were destroyed initially in the building’s core, a spine of 47 heavy steel I-beams extending through the centre of the structure, Sozen said.

‘Current findings from the simulation have identified the destruction of 11 columns on the 94th floor, 10 columns on the 95th floor and nine columns on the 96th floor,’ he said. ‘This is a major insight. When you lose close to 25 percent of your columns at a given level, the building is significantly weakened and vulnerable to collapse.’

The simulation research, funded by the National Science Foundation, was carried out by a team that includes Hoffmann; Sozen; Ayhan Irfanoglu, an assistant professor of civil engineering; Voicu Popescu, an assistant professor of computer science; computer science doctoral student Paul Rosen; and civil engineering doctoral students Oscar Ardila and Ingo Brachmann.

Mathematical models are used to represent the Boeing 767 and the building.

‘The simulation is enabling us to “look” inside the building to see what really happened structurally,’ Sozen said. ‘This is not the first simulation, but I would say it’s the most scientifically realistic one. We have spent a great deal of time on details of the mechanical properties of the columns and of the airplane, and we have benefited from the results of previous efforts at simulation.’


A team consisting of many of the same Purdue researchers in 2002 created a similar simulation of the September 11 attack on the Pentagon.

Experience gained during the Pentagon-related research and laboratory experiments have helped the Purdue civil engineers and computer scientists develop the new model, Sozen said.

‘As a result of the Pentagon research, we have a better understanding of what happens when a tremendous mass of fluid such as fuel hits a solid object at high velocity,’ Sozen said. ‘We believe most of the structural damage from such aircraft collisions is caused by the mass of the fluid on the craft, which includes the fuel.

The simulation represents the plane and its mass as a mesh of hundreds of thousands of “finite elements,” or small squares containing specific physical characteristics. Like the previous Pentagon simulation, the software tool uses principles of physics to simulate how a plane’s huge mass of fuel and cargo impacts a building.

This image was taken from a simulation created by researchers at PurdueUniversity in connection with NSF-funded work to study what happened structurally to the WorldTradeCenter‘s NorthTower when a commercial airliner crashed into it on Sept. 11, 2001. (Purdue University Department of Computer Science photo)

Details about the NSF modeling project are available here.