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 toughness, 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 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 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.”—by
Emil Venere, Purdue University.
This article first appeared on the Purdue University News Service
web site.