The 730 ft tall Foresthill Bridge, the tallest bridge in California and the fourth-tallest in the country, recently underwent a seismic retrofit that included replacing many bracing members and gusset plates and installing large buckling-restrained braces near the abutments. J. Chou
Engineers use what may be the nation’s largest buckling-restrained braces as part of the seismic retrofit of Foresthill Bridge, in Placer County, California.
June 10, 2014—The 730 ft tall Foresthill Bridge is the tallest bridge in California and the fourth-tallest bridge in the United States. The iconic deck-truss structure has appeared in several automobile commercials and movies. Given the bridge’s high profile, local officials were surprised when they recently discovered that the state had inadvertently omitted the bridge from its seismic retrofit program and acted quickly to get it incorporated into the effort. Now the community is celebrating the bridge’s recently completed restoration and innovative seismic retrofit, which included the use of enormous buckling-restrained braces.
The U.S. Bureau of Reclamation constructed Foresthill Bridge across the North Fork Canyon and over the American River in the early 1970s, connecting the town of Auburn with the community of Foresthill. The Bureau made the bridge so tall because it anticipated damming the river near Auburn, the reservoir rising to the top of the bridge’s 430 to 450 ft tall piers. But after numerous active faults were discovered in the area, the Bureau canceled the dam project and transferred ownership of the bridge to Placer County. Since then, the 2,428 ft long crossing, which is sometimes referred to as the Auburn-Foresthill Bridge, has carried Foresthill Road and provided the main access to Foresthill. It serves local traffic as well as recreational and logging traffic to and from the nearby national forest area.
As the bridge neared 40 years of age, Placer County considered applying for federal funding to repaint the bridge’s weathered truss. During that discussion, questions arose about the bridge’s seismic stability, and that’s when county officials discovered that the bridge had been accidentally left out of the California Department of Transportation’s (Caltrans) program for retrofitting hundreds of bridges statewide to prevent collapse during an earthquake. Eager to include the bridge in the program, the county hired Quincy Engineering, Inc., an engineering firm specializing in transportation projects headquartered in Sacramento, California, to conduct a seismic analysis of the structure.
Before it even executed the contract, Quincy Engineering determined that the analysis was unnecessary because it was apparent that the bridge should have been included in the state’s program. Instead of mobilizing the contract, the engineers and Placer County officials explained the situation in correspondences to Caltrans, which agreed to add the bridge to the program without the analysis. “The bridge so obviously should have been included in the seismic retrofit program that we didn’t even have to spend any money on the contract to get it inserted into the program,” says John Quincy, P.E., M.ASCE, the president of Quincy Engineering.
An analysis revealed that the bracing members near the
abutments carried much of the seismic load. Engineers designed
large buckling-restrained braces, capable of handling up to 2,000
kips, to absorb some of that energy and reduce deformation at
those locations. J. Chou
Once funding for the repainting and seismic retrofit project was secured through the Federal Highway Administration’s (FHWA) Highway Bridge Program and the state, the county advertised a request for proposals for the project. As a result of that process, it selected the Quincy Engineering team to assess the bridge and develop a retrofit solution for the structure. The firm had experience in the design of seismic retrofits for several other bridges throughout the state—including the San Francisco-Oakland Bay, Benicia-Martinez, and Coronado bridges—and drew upon that experience to develop a retrofit design that will allow Foresthill Bridge to withstand a 6.5 magnitude earthquake.
Engineers began by using the bridge’s original design and shop drawings to develop a three-dimensional structural analysis model to assess how the existing bridge—designed for just 10 percent g—would perform during seismic simulations. Taking a multilevel, strain-based approach, they analyzed everything from the bridge’s foundations and piers to its gusset plates and flanges. “We actually looked at the bridge’s individual elements to determine whether each element was yielding or buckling,” Quincy says. “For every single gusset plate, for every single member, and for every single portion of a member, we knew what was going on throughout the simulated seismic event in the model.”
The bridge’s massive hollow concrete piers were expected to be problematic but the analysis revealed that they will sustain only minor cracking during an earthquake, Quincy says. The truss also performed well during simulations except near the abutments, where the lower chord deformed under the loads. But the primary vulnerability, he says, involved the truss bracing members, which yielded and buckled relatively early in the event because they were designed for much lower loads than are considered in a modern analysis. “One of the things you don’t want to have happen with a truss bridge is for the bolted connections and gusset plates to fail because as soon as that happens, those members are essentially useless and the bridge loses stability really quickly,” Quincy explains. “Those gusset plates and those connections must remain intact no matter what happens to the bracing member.” Once they identified the vulnerabilities with the bridge, engineers presented their findings to Caltrans and FHWA, both of which agreed with the assessment.
The engineers then developed a strategy for resolving the issues. They added steel to some bracing members, replaced other bracing members, and replaced many of the gusset plates for increased strength. They then developed a plan to reconstruct the bracing members near the abutments, which carried much of the seismic load, and initially considered incorporating dampers at those locations to absorb some of the energy and reduce deformation. But some dampers have developed leaks in the past and can cause maintenance issues over time, Quincy notes. So they decided to use buckling-restrained braces (BRBs) instead. A peer-review panel comprising seismic design experts reviewed the entire strategy and Caltrans and FHWA also approved the plans.
Engineers detailed each step of the seismic retrofit so that the
bridge could remain open to traffic during construction.
There was just one problem: no one makes BRBs substantial enough for a bridge of this size. “The amount of deformation and energy that we needed to absorb meant that the BRBs had to be extremely large,” Quincy says. “I don’t think anyone had ever done a BRB that big before.” As a result, engineers designed what may be the largest BRBs in the United States—capable of handling up to 2,000 kips (2 million pounds) of load. They sent a conceptual set of the BRBs to the University of California, San Diego, which has one of the few earthquake testing facilities large enough to test BRBs of such size. Once the BRBs passed the testing, Nippon Steel & Sumikin Engineering Company, a steel production firm headquartered in Tokyo, in conjunction with Yajima USA, a structural and specialty steel manufacturer based in Reno, Nevada, manufactured two sets of the BRBs—one for each abutment.
Another area of concern for the bridge was the connection between the superstructure and piers. The superstructure is on roller bearings at the tops of the piers, allowing it to move as the result of temperature changes. But an analysis showed that the bridge could exceed the capacity of the rollers during a seismic event, causing the rollers to tip and allowing the bridge to jolt vertically. To prevent that from happening, engineers incorporated a sliding system that will allow the bridge to slide beyond the rollers during a seismic event and back onto the rollers once the movement stops. “Any time the pier and the truss get out of phase, this combination of the rollers and Teflon sliding surface will allow those two elements to displace relative to each other without buckling any members,” Quincy explains. “We actually allow the bridge to move the way it wants to move relative to the piers and just control the displacements.”
Once they had the strategy in place, engineers detailed the construction sequencing so the bridge could remain open to traffic throughout the project. “One of the challenges of the job was replacing these bracing members while you’ve got public traffic and logging trucks on the bridge,” Quincy says. “We went through a very detailed construction sequence where we actually told the contractor, ‘Take off this gusset plate and this gusset plate and replace them. Now, take this member out and this member out and replace them. That allows you to take this other member over here out and replace it.
Golden State Bridge, a construction engineering and contracting firm headquartered in Benicia, California, was the prime contractor on the project. It performed the work from a cable-supported temporary work platform that extended the full length of the bridge, allowing crews to work on any part of the structure at any given time. F.D. Thomas, a coating and specialty contracting firm headquartered in Central Point, Oregon, and the painting contractor on the project, also used the platform, enclosing it in tarps to contain the lead-based paint that was blasted from the structure. Engineers analyzed the bridge for the loads of the construction equipment and temporary platforms as well as the increased wind loads caused by the tarps. “It took a lot of detailed analysis and coordination with the contractor to make sure we didn’t do something that would cause an unstable situation,” Quincy says, adding that all coordination and communication on the project was conducted with Placer County’s participation and managed by the construction management firm, The Hanna Group, located in Sacramento, California.
Construction of the $74.4-million project commenced in January of 2011, and the seismic retrofit was completed in January 2014. Crews finished painting the bridge a couple of months later, and in April the county hosted a ceremony to commemorate the bridge’s renewal. Quincy says he and his team are thrilled that they were able to bring the famous bridge into compliance with modern seismic design standards without incurring a great deal of unexpected cost. Change orders accounted for only about 4 percent of the total cost, much less than the typical 10 to 15 percent, he says. “We had a good contractor, good construction management, and a good, solid set of plans and specifications, so it didn’t require a whole bunch of change orders in order to get the bridge retrofitted in the right way,” he says. “We’re very proud of that.”