Engineering expertise spies similarly-built local structures
Concrete cracked, buckled and popped like a crumbling sugar cookie under more than 600,000 pounds of pressure at UC Berkeley today, but the steel frame it supported held firm.
Dr. Abolhassan Astaneh-Asl, UC Berkeley professor of civil and environmental engineering, performed the experiment to demonstrate what he said could be a major breakthrough in disaster-safe building design.
Many modern buildings are reinforced with steel plate shear walls – steel sheets bolted to columns and beams to increase stiffness and limit lateral motion.
Steel, however, buckles when compressed, and concrete cracks under tension.
Astaneh’s new shear wall is a marriage of the best of both materials – a six-inch slab of reinforced concrete bolted to the 3/8-inch steel plate. The concrete supports the steel wall and keeps it from buckling under stress.
A 20-foot high section of steel and concrete, the half-scale prototype wall lay on its side in the hangar-like test lab.
The concrete flaked and broke as a huge hydraulic pressed on its upper left corner, displacing it nine inches back and forth, but the steel never cracked.
The wall prototype performed even better than expected, surviving an earthquake of more than magnitude nine without catastrophic failure.
“We are very happy,” Astaneh said. “Without the concrete, a steel wall would be buckled by now.”
Astaneh originally set out to design buildings that would survive severe earthquakes. Later, he realized the same designs would resist damage from car bombs and rocket attacks.
Astaneh said the wall prototype is made out of ordinary materials, and would probably increase the cost of new buildings by no more than 2 percent.
Older buildings could be retrofitted with the concrete slabs, which could be replaced after an earthquake.
Astaneh recently returned from New York City, where he spent weeks sifting through the rubble of the World Trade Center, looking for answers.
He stood before a table displaying pieces of the WTC and held up small pieces of twisted steel, lumps of concrete, and plastic baggies of powdered drywall and fireproofing. He also held up bits of aluminum, one printed with a serial number, which had been found stuck to steel columns from the twin towers. They were pieces of the airplanes.
“I haven’t seen anything like this,” he said, holding up a foot-long twisted shard of steel. “It’s like a piece of bread, but it was high-strength steel.”
From his research, Astaneh showed that the towers’ supporting columns withstood the original impact of the planes.
They remained structurally sound until the heat of flaming jet fuel reached 1,000 degrees Celsius and began to melt the steel. The softened columns could no longer support the floors above, and the entire structure began to collapse.
One of Astaneh’s main concerns was the collapse of Building Seven of the WTC, because there are several hundred similar buildings throughout the United States, including a few in San Francisco.
“Why did Building Seven collapse?” he asked. “What made it burn for eight hours?”
There have been some reports that there was fuel stored in the upper floors of the building, possibly for a small electrical power plant, which intensified the heat and duration of the fire. Astaneh refused to speculate on how well the building would have fared if the fuel had not been present.
Astaneh spent some of his time in New York training iron and steel workers at the recycling yards to scan the 300,000 tons of steel wreckage for pieces of metal that may contain valuable clues to the structural collapse.
Designed to withstand the impact of a Boeing 707, Astaneh wanted to assure people that the towers were well built. The buildings admirably withstood being hit by 767s.
“That building in my opinion was really the best-designed building I’ve seen,” he said. “Our tall buildings are some of the best-designed structures in the world.”
Astaneh also spoke about his experience sorting through the rubble in New York City.
“You’re standing there and looking at something like this, and under it are 5,000 people like ourselves. It was terrible. It was part of us, it was like family,” he said. “Suddenly 5,000 of your people are no longer here.”
He never went inside the ruins themselves.
“I couldn’t go inside,” he said, “I wouldn’t walk on that.”