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The cable system designed by UC Berkeley researchers stopped the above floor from collapsing after it dropped two feet.   The cable system designed by UC Berkeley researchers stopped the floor, shown here, from collapsing after it dropped two feet. Abolhassan Astaneh-Asl photo

Cables hold promise in protecting existing buildings from bombs, researchers find

– Securing steel cables around the floors of existing buildings may be an effective way to prevent a catastrophic collapse caused by a terrorist bomb, according to test results released by researchers at the University of California, Berkeley.

Abolhassan Astaneh-Asl, professor of civil engineering at UC Berkeley's College of Engineering, and four of his civil engineering graduate students have successfully tested a system that would shift the gravity load of a collapsing floor to supporting cables if a column were destroyed by a natural disaster, such as an earthquake, or a terrorist bomb.

"Cable is the best structural element we have to reduce impact," said Astaneh. "By nature, it is very flexible, so it bends rather than breaks under a lateral blast load, yet it can carry large tension forces similar to the way cables work on suspension bridges."

Astaneh presented his findings at the annual meeting of the American Association for the Advancement of Science (AAAS), held Feb. 13-18 in Denver. He co-organized a symposium on security technologies, which included presentations from researchers in the field of biometrics, airport security and mail safety. The presentation of the cable design was part of a larger talk by Astaneh about protecting buildings from terrorist attacks and on lessons learned from the collapse of the World Trade Center towers.

The cable system was designed to address the vulnerability of a building's outer support beams, which are closest to public sidewalks and city streets.

"It is almost impossible to always keep terrorists from getting close to those columns," said Astaneh. "In Oklahoma City, a terrorist was able to park a rental truck about 10 feet away from an outer support column (of the Alfred P. Murrah Federal Building). The explosives took out just one column, but that caused the upper floors to bear down in a chain of events that brought down half of the building, killing 168 people."

To test the cable system, Astaneh and civil engineering students Roger Jung, Erik Madsen, Brant Jones and Samuel Tan constructed a full-scale side of a floor with five support columns at a test bay in UC Berkeley's Davis Hall. They conducted three trials, one without cable support, and two with cables. The cables were made up of 30 strands of intertwining steel wires. The researchers passed the cable through 1-inch holes cut in the columns and fastened it to the opposite ends of the concrete floor.

In the first two trials, a vital support column was knocked out, and more than 120,000 pounds of downward force were applied to the weakened floor to simulate the weight of the floor - full of people and office equipment - above the missing column. In the third trial, twice as much weight was applied to stretch the limits of the cable support.

In the trial without the cable, one of the connections between the floor girder and the column broke completely. When tested with the cable, the floor began to cave in at the point where the column had been removed. But despite the audible cracks of stressed concrete and steel and a few busted bolts, the cables were able to halt the impending collapse after the floor dropped two feet, and the connections held.

When twice as much weight was used for the third trial, the floor dropped three feet at the location of the column, but the cable still kept the floor connected to the adjacent columns that carried the load of the floor.

"The cable prevents a progressive and catastrophic collapse," said Astaneh. "The floor may slope down two to three feet, but people can still escape to safety from the building. Not only can the system potentially save lives, it can limit the damage to something that can be repaired easily."

Because it would have been too dangerous to set off an explosive during the test, Astaneh worked with computer models and scientists from the Lawrence Livermore National Laboratory to simulate an explosive attack and calculate the effects of dynamic force on the system. "We found that if you apply the force gradually, as we did in our test bay," he said, "the cable could take 40 percent more load than if it were an explosive force."

Astaneh said the test data can be used to design a cable system that would handle explosions. "For instance, if the floor weight is 100,000 pounds, we can design the system to support 140,000 pounds to take the 40 percent dynamic effect into account," he said.

Astaneh has done prior research using cables embedded in concrete flooring for new building applications, but this is the first time he has shown that the cables could work if affixed to the floor of an existing building.

"The process of taking the building fašade off and installing cables is relatively easy compared with what you gain," said Astaneh. "You can install the cable from the outside without forcing the occupants to move."

He said the cost of the system is minimal compared to the total construction cost of the average, modern office building. "Installing cables on existing buildings would only add $2 per square foot, or about one percent of the current cost for building construction," he said.

Admittedly, however, the cable system would only protect the columns on the perimeter of a building. "The approach to protecting interior columns is to increase security so that questionable vehicles cannot enter the parking structures or structurally vulnerable areas of high-risk buildings," said Astaneh.

This research is supported by a grant from the National Science Foundation.