Berkeley Engineers Test New Bridge Design
A 30-ft, scale-model steel bridge was rocked and shaken on an earthquake simulator at the University of California, Berkeley last week to demonstrate a new bridge design that Berkeley engineers say can better resist significant damage and improve transportation safety in the event of large earthquakes.
In a series of simulated quakes, which ranged from moderate to severe, the bridge trembled, shook and rocked violently— but the deck stayed intact and settled back on its supports after each event.
“What we’re doing today is looking at some new technology that we think will be very good for California,” says Steve Mahin, director of Berkeley’s Pacific Earthquake Engineering Research Center (PEER). PEER is a consortium of nine West Coast universities, affiliated with other campuses abroad and industry partners, that was set up in 1996 to research seismic safety for buildings and structures.
Research in the new bridge design is supported by PEER, the National Science Foundation and Earthquake Protection Systems, a Vallejo-based seismic isolation bearing manufacturer.
The demonstration, and a second one held later in the day, attracted some 100 engineers and bridge-safety officials, some from as far away as Japan and Canada, plus members of the media.
What they came to see was how three pieces of technology, working together, solved a problem that has kept bridges from being able take advantage of seismic-safety breakthroughs used in buildings for the last 20 years.
The model bridge was built in segments, like many of the major bridges in the Bay Area as well as those supporting elevated freeways and the ones envisioned for a new high-speed rail system proposed for California.
Joints where the segments meet have been a challenge for engineers, says Mahin. In a quake, each segment shakes and rolls independently, severely stressing a roadway or rail line sitting on top.
The three devices tested — two types of isolators plus a new “lockup guide” — were designed to keep the segments moving together during a quake, while also allowing enough movement to keep the bridge from shaking apart.
Isolators have long been used to allow a building to move separately from its foundation, but they’ve been difficult to adapt to bridges, Mahin explains. While they're effective in separating the motion of a bridge’s columns from its deck, the deck remains in jeopardy because its segments move in different directions.
The lockup guide, which constrains motion between segments, is the newest piece of the puzzle, and its use with the isolators was what was on display. The devices were developed by PEER engineers working with Earthquake Protection Systems, a manufacturer of seismic isolation equipment in Vallejo.
The trials mimicked the ground motion of four modern quakes that are considered historic: California’s 1989 Loma Prieta, with a magnitude of 6.9, and 1994 Northridge (6.7); Japan’s 1995 Kobe quake (6.9); and Chile’s 1985 temblor (7.8).
Eleven times, a big, noisy hydraulic system set the bridge shaking according to various sets of computerized commands governing the size, direction and acceleration of the “quake.” Each time, the roadway and tracks on the model bridge's deck moved as a piece.
For some of the tests, a rail car was lowered onto the tracks. When the shake table was set in motion, the rail car pitched and rolled but stayed but on the tracks.
Mahin points out that bridges are essential pieces of the Bay Area’s infrastructure.
“After an earthquake, they’re even more vital to speed response and recovery,” he says. “You don’t want them to be part of the problem.”
But engineers have been slow to adopt modern earthquake technology to bridges, he said.
Mahin says that the new methods put through their paces last week not only have the potential to keep people safer and bridges intact, but they’re less expensive and more environmentally sustainable than the old ones.Without solving the isolator problem, bridges have been made stronger by building them bigger — which uses more materials, he adds, and preventing damage to bridges saves the cost involved in post-quake repairs.