Simulated earthquake tests bridge’s strength
On Dec. 11, 2008, researchers engineered a magnitude-8 earthquake in Nevada with the
intent to destroy. The target? A 32-meter-long model bridge. Luckily, the bridge survived
the 10-second seismic beating, giving researchers new insight on how to build more
earthquake-resistant bridges.
Earthquake damage to bridges can have severe consequences because decommissioned
bridges cripple the highway network. So an engineering team at the University of
Nevada at Reno decided to build a better bridge. “The objective was to see if we could
use unconventional materials in bridges so they remain serviceable even after a strong
earthquake,” says M. Saiid Saiidi, the project director of the University of Nevada
research team. “We used materials that have not been used in bridges.”
Saiidi’s team crafted their large-scale model bridge using a combination of three
advanced materials and designs. First, they used nickel-titanium alloy to make the
bridge’s bars used inside concrete columns. Unlike the typical steel bars, nickel-titanium
bars “deform under seismic waves and absorb the energy, but come back to their original
formation,” Saiidi says. The team also mixed polyvinyl fibers into the bridge’s concrete
columns to prevent the concrete from rupturing when the columns that support the road
deform during an earthquake. Finally, the team installed rubber padding in the columns to
absorb the most critical stresses an earthquake causes. This idea was originally developed
(but not applied) in Japan with only partial success. The Nevada team strengthened the
rubber with steel plates to make them feasible.
When Saiidi’s team built the six-column bridge, each set of columns received a different
deformation-prevention method. Two of the columns had the nickel-titanium alloy, two
had the rubber with steel plates, and two had a post-tension system that used central rod
pressing the columns to prevent them from permanently tilting.
The results of the experiment were mixed: “[The post-tension system] was successful in
preventing permanent tilt, but [the system itself] was severely damaged. The other two
performed very well,” Saiidi says, as did the mixture of polyvinyl fibers in the concrete.
Saiidi and his doctoral students still have to finish analyzing all the data, but he is hopeful
that the new materials he tested will be incorporated into new bridge design soon,
especially in California where earthquake damage is a severe threat. Still, Saiidi is aware
that it might take more than one experiment to convince others. “These materials are new
and different. Considering the big liability every time we build a new bridge, [bridge
designers] would be very cautious,” he says. “They know what materials have worked
before and they have lots of confidence in the materials.”
But engineers at California’s Department of Transportation are keeping an open mind.
“The tests showed this is very promising technology that we may be able to put into
practice to improve earthquake bridge performance,” says Mike Keever, chief of
earthquake engineering at the California Department of Transportation in Sacramento.
The initial costs of building a bridge with the materials that Saiidi tested are about 5
percent more expensive than building a traditional bridge, but Saiidi says it’ll pay off in
the long run. “With the lifecycle costs of a bridge, you’re saving lots of money [with the
new materials]. They don’t have to be replaced and they require few repairs,”—especially
after an earthquake, he says. Saiidi points out that, after a natural disaster such as an
earthquake, usable roads and bridges are most necessary for the transport of relief
supplies.
Keever is equally optimistic about the economic benefits of better bridges. “We’re
talking about much bigger dollars when you have to close a bridge because of an
earthquake. The economic cost to a region is much bigger [than the repair costs],” he
says. “That’s where we would see the payoff.”
Alexandra Ossola