Just days after a two-week scientific reconnaissance mission to the site of the collapsed World Trade Center, UC Berkeley structural engineer Abolhassan Astaneh-Asl was back on campus in his hard hat testing a new technique to prevent future high-rise disasters. During the October 18 experiment in the campus' civil engineering test bay, a massive hydraulic jack - the largest in the country - pushed on a 20-foot by nine-foot concrete and steel-plated wall while Astaneh and graduate student Qiuhong Zhao took careful notes.
After several minutes under the 600,000 pounds of pressure, the equivalent of subjecting the wall to a 9.0 magnitude earthquake, the concrete cracked but the metal barely budged. As Astaneh had hoped, the experiment showed that adding six inches of cement to standard steel shear walls would help protect a building from the tremendous trauma of an earthquake or, perhaps, an airplane collision.
"The concept is not just the materials," Astaneh says, "It's how you combine the materials that make this work."
Outfitting skyscrapers with steel shear plates to prevent columns and beams from bending or breaking during a bomb blast or earthquake is a recent improvement over traditional concrete shear walls. Astaneh's innovation - studied under a National Science Foundation grant - was to combine the two techniques into an innovative Composite Shear Wall to help prevent a structure from collapsing under lateral loads.
The double-strength system consists of a six-inch slab of inexpensive pre-cast concrete bolted to a 3/8-inch steel plate. The concrete prevents the steel from buckling under compression while the steel supports the concrete under tension.
According to Astaneh, in the event of an airplane collision this combination of steel and concrete would act as a deflector shield helping to keep the plane, and its explosive fuel, from penetrating deep inside the structure as easily as in the September 11 crashes. Indeed, immediately after the terrorist attacks, Astaneh correctly speculated to a reporter that it was the blaze not the impact of the airplanes that tragically brought down the 110-story buildings. Designed in the late 1960s with then state-of-the-art engineering to withstand the accidental impact of a Boeing 707, the largest plane flying at the time, the towers stood their ground for nearly an hour after the airliners crashed into them. As evidenced by the battered beams and columns Astaneh is studying at the recycling yard in New Jersey where the debris from the World Trade Center was hauled, the buildings only fell when the 1,000-degree Fahrenheit inferno began to sap the strength from the steel supports.
As part of a collaboration with Lawrence Livermore National Laboratory, the data Astaneh collects from Ground Zero will be used to construct a high-resolution computer model of the catastrophic collapse. Then, the possible benefits of new systems like the concrete and steel composite can be studied through computer simulation.
Courtesy
Abolhassan Astaneh-Asl
In
1978, Hassan Astaneh, with his wife and one-year-old son,
emmigrated from Iran to the United States to begin his graduate
studies. One of his first memories after landing in New
York City, he says, was the sense of awe he felt upon seeing
the enormous World Trade Center. Now, the data he collects
from studying the charred and twisted steel remains of the
towers will be used to protect buildings from future terrorist
attacks.
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In June, under a grant from the General Services Administration, Astaneh tested another technique to make large buildings more resistant to ground-level car bombs, like the one that toppled the Oklahoma City Federal Building in 1995. This time, the innovation, originally proposed in 1996 by UC Berkeley emeritus professor of civil engineering Joseph Penzien, was to construct large buildings more like suspension bridges. The design calls for inexpensive 1.25-inch diameter steel cables to be embedded inside the perimeter of a typical building's concrete floors and anchored to corner support columns or braced spans.
"With this mechanism, if a supporting column goes, the floor hangs from the cables just like the cables in a suspension bridge hold up the road," Astaneh explains.
For the experiment on a thousand square foot sample floor in the test bay, Astaneh yanked out a critical support column and mechanically pushed down on the floor with 190,000 pounds of pressure. The floor dropped about three feet but, thanks to the cable, didn't collapse and actually sprung up nearly 18 inches when the pressure was released.
The new mechanism, called the Cable Catenary System, is already being integrated into the construction of new buildings. The next step in this research is a National Science Foundation-supported viability study of retrofitting existing buildings with suspension cables, Astaneh says.
"We're really going beyond seismic now," says Astaneh, commenting on how his focus has shifted in recent years toward protecting public buildings from terrorist attacks. "I could be doing this research for the rest of my career."
Abolhassan Astaneh-Asl's
home page
"Report
from Ground Zero: Engineer studies World Trade Center collapse
for clues to failure" by Robert Sanders
