Researchers say they have a new design for a bridge framework that will improve earthquake resistance and reduce damage.
The design also boasts faster on-site construction and uses common construction materials.
"The design of reinforced concrete bridges in seismic regions has changed little since the mid-1970s," John Stanton, a professor at the University of Washington, said in a press release from Purdue University.
During California's Loma Prieta Earthquake of 1989, the upper level of the double-deck Nimitz Freeway collapsed onto the lower deck, killing dozens of people.
The concept for the design was developed by Stanton, who teaches in UW's Department of Civil and Environmental Engineering.
The team also included professor Marc O. Eberhard and graduate research assistants Travis Thonstad and Olafur Haraldsson from the University of Washington; and professor David Sanders and graduate research assistant Islam Mantawy from the University of Nevada, Reno.
Speeding Up Construction
Faster construction is achieved by pre-fabricating the columns and beams, or "bents," off-site so they can be quickly erected and connected once shipped to the job location.
Bridge bents are currently made using cast-in-place concrete; therefore, the next piece can't be added until the one before it gains strength, researchers explained.
Pre-fabrication eliminates this waiting time, speeding on-site construction and reducing traffic delays.
"However, pre-fabricating means the pieces need to be connected on-site, and therein lies a major difficulty," Stanton noted.
"It is hard enough to design connections that can survive earthquake shaking, or to design them so that they can be easily assembled, but to do both at once is a real challenge."
The Rubber Band Effect
An important feature is that the columns are pre-tensioned.
Stanton described the idea by comparing it to a toy set of wooden building blocks with a hole through each one.
"Stack them on top of one another, put a rubber band through the central hole, stretch it tight and anchor it at each end. The rubber band keeps the blocks squeezed together.
"Now stand the assembly of blocks up on its end and you have a pre-tensioned column. If the bottom of the column is attached to a foundation block, you can push the top sideways, as would an earthquake, but the rubber band just snaps the column back upright when you let go," Stanton explained.
Since real bridge columns don't have rubber bands, very high-strength steel cables can be used to achieve the same effect, Stanton said. The cables, as well as some conventional rebar, are installed during the pre-fabrication process to help reduce site operations.
University of Washington, Seattle / NEES photo
The design includes the ability of bridge columns to re-center after an earthquake.
The "re-centering" capability is of utmost importance to ensure that bridge columns are vertical and not leaning at an angle after an earthquake.
During an earthquake, the rocking columns experience high local stresses at the points of contact, making the concrete vulnerable to being crushed. The researchers took special measures to mitigate this possibility by protecting the ends of the columns with short steel tubes, or "jackets," to confine the concrete.
Testing the System
The researchers said they have conducted successful cyclic testing on individual connections to test their design's durability.
"Cyclic tests of the critical connections have demonstrated that the system can deform during strong earthquakes and then bounce back to vertical with minimal damage," Stanton said.
This month, the team plans to test a complete bridge built with the system. The test will be done at 25 percent of full-scale on the earthquake-shaking tables at a Network for Earthquake Engineering Simulation (NEES) facility at the University of Nevada, Reno.
NEES is a shared laboratory network based at Purdue University.
Thonstad led the component design and building for the upcoming test. The column and cap beam components were then shipped to the testing facility, where Mantawy will lead the bridge construction.
The team will process data from the project, archive it, and make it publicly available through the NEES Project Warehouse data repository.
The research will be presented at Quake Summit 2014, which takes place July 21-25 in Anchorage, AK, and is part of the 10th U.S. National Conference on Earthquake Engineering.