Road Science Tutorial
“We have designed a new system for bridges to go through an earthquake in a safe manner, yet remain open and functioning for the public following a very large earthquake,” says Stephen Mahin, director, Pacific Earthquake Engineering Research Center at Berkeley.
“We normally divide a bridge [superstructure] into segments,” Mahin says. “Each of the segments are like people in a line; if you have 10 people in a line, each person will be moving sideways or out of phase. We are trying to keep everybody in a line, so that white line down the road will be continuous, and they stay in line.”
That’s done with the different types of newly-designed seismic bearings or appurtenances, all manufactured by Earthquake Protection Systems, Inc., Vallejo, Calif.
Triple pendulum isolators are placed at the top of the bridge columns or pier caps, and control the maximum displacement of the bridge superstructures. They have three different pendulum mechanisms that sequentially engage as shaking increases.
“The triple pendulum isolator has a spherical bowl [negative concavity] inside, which allows the device to move back and forth, and roll like a pendulum,” Mahin says. “But the surface inside is coated with Teflon, so instead of simply rolling, it moves with a bit of friction.” It’s topped with a matching concave half that permits the “pendulum” within to rotate, but also move sideways.
The adjacent bridge superstructure segments also must move in unison, and “lock-up guides” allow the bridge to move while guided in longitudinal and transverse displacements. The connection is rigid in both the vertical and transverse directions, but can rotate around its vertical axis, thus the guides keep the bridge deck and centerline continuously aligned during and after earthquake shaking.
Finally, linear isolators are used at bridge abutments. They allow unidirectional sliding in the longitudinal direction along a Teflon-lined surface, says UC-Berkeley.
They are allowed to rotate 360 degrees around the vertical axis, 12 degrees about the x and y axes, and have no tension capability.
“Our tests exceeded our expectations, and we look forward to doing some actual analysis of the bridge so we can be more confident in our findings, and come up with some recommendations for future bridge designs,” says Mahin.
High-Performance Deck Sealants
When it comes to the wearing surface of the superstructure, high-performance deck sealants protect decks and ensure longer bridge lives. And the improved technology of value-added products is making that work easier and more environmentally acceptable.
For example, in summer 2010, the 33,000-square-foot Trout Creek Bridge in Wawarsing, Ulster County, N.Y, was sealed in just two days.
Because of watershed concerns at Rondout Creek Reservoir on County Road 77, the New York Department of Environmental Protection (NYDEP), New York City Water Authority and Ulster County Department of Public Works wanted a product that would eliminate all possible leakage into the public water supply.
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