Simulated attack

An animated visualisation created by researchers at Purdue University aims to help structural engineers understand exactly what happened during the attacks on the World Trade Center in 2001.

Christoph Hoffmann, a professor of computer science and director of Purdue's Rosen Center for Advanced Computing, said that the animation reveals more information than could be conveyed through a scientific simulation alone.

‘Scientific simulations restrict us to showing the things that are absolutely essential to the engineer,’ Hoffmann said. ‘This gives us a simulation that doesn't deliver much visual information to a layperson. Our animation takes that scientific model and adds back the visual information required to make it a more effective communication tool.’

The scientific simulation, completed last September, required several test runs before the researchers were satisfied; the final test run required more than 80 hours of high-performance computing.

The simulation depicts how a plane tore through several stories of the World Trade Center north tower within a half-second, and found that the weight of the fuel acted like a flash flood of flaming liquid, knocking out essential structural columns within the building and removing fireproofing insulation from other support structures. The simulation used lines and dots to show the aircraft and building during the event.

To develop the new animated visualisation, Voicu Popescu, an assistant professor of computer science, developed a translator application that creates a link between computer simulations and computer visualisation systems to automatically translate simulation data into a 3D animation scene.

‘This translator is scalable and can be used in other simulations,’ Popescu said.

In the animation, elements that were not part of the scientific simulation, such as flames and smoke, are clearly rendered, although the visualisation does not show the subsequent effects of the fire.

Even though details were added in this animation, Popescu says the visualisation was intentionally kept "non-descript" so that they would not be exploitive of the horrific attack. ‘For example, on the airplane there are no airline insignia or windows,’ Popescu said.

Still, Popescu said that the visualisation has a realism never seen before.

‘The crashes and computer models you often see on television are not scientifically accurate,’ he said. ‘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.’

Purdue researchers have developed a scientifically based video animation of the 9/11 attacks on the World Trade Center. Their research has found that it was the weight of the fuel combined with the fire, and not the aircraft itself, that caused the most damage to the buildings

The visualisation begins with a Google Earth map of lower Manhattan as it appeared on September 11, 2001. The video then shows the damage caused by the aircraft as it hit the north tower, follows the disintegrating plane through the interior, and then shows the airplane metal, ignited fuel, dust and smoke exiting the building on the opposite side.

The simulation found that the airplane's metal skin peeled away shortly after impact and shows how the titanium jet engine shafts flew through the building like bullets.

As with an earlier simulation developed by this team that examined the 9/11 attack on the Pentagon, the World Trade Center simulation showed that it was the weight of the 10,000 gallons of fuel more than anything else that caused the damage.

‘It is the weight, the kinetic energy of the fuel that causes much of the damage in these events,’ Hoffmann said. ‘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.’

Mete Sozen, Purdue's Kettlehut Distinguished Professor of Structural Engineering and a principal investigator on the simulation project, says the researchers worked for years and used the best computing resources available to recreate the event.

‘To estimate the serious damage to the World Trade Center core columns, we assembled a detailed numerical model of the impacting aircraft as well as a detailed numerical model of the top 20 stories of the building,’ Sozen said. ‘We then used weeks of supercomputer time over a number of years to simulate the event in many credible angles of impact of the aircraft.’

Sozen said that the actual damage to the building's facade that was observed was identical to the damage shown by the numerical simulation.

‘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,’ he said. ‘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.’

The simulation represented the plane and its mass as a mesh of hundreds of thousands of "finite elements," or small squares containing specific physical characteristics. In the visualisation, these scientific data points are used to show how airplane components swept through the building and out through the other side as the fuel ignited.

‘The aircraft moved through the building as if it were a hot and fast lava flow,’ Sozen said. ‘Consequently, much of the fireproofing insulation was ripped off the structure. 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. Evaluation of the effects of the fire on the core column structure, with the insulation removed by the impact, showed that collapse would follow whatever the number of columns cut at the time of the impact.’

The animation is the latest in a series of projects by the Purdue team that arose after 9/11 to determine the structural damage that occurs when an airplane collides with a building. Although one goal was to develop structures that can withstand a terrorist attack, the team also has used this research to investigate other scenarios, such as an airplane inadvertently crashing into a building located near an airport.