On May 23, a truck carrying an oversized load struck the sway-frame members of the Interstate 5 Skagit River Bridge, in Burlington, Washington, causing the bridge’s northernmost span to collapse. WSDOT
Replacing a portion of a Washington bridge that collapsed in May will involve sliding a new permanent span into place. The bridge’s truss will also be raised to avoid future collapses.
September 3, 2013—On May 23, a truck carrying an oversized load attempted to cross the Interstate 5 Skagit River Bridge, in Burlington, Washington. But as the truck began to traverse the nearly 60-year-old through-truss bridge, its load struck several of the structure’s sway-frame bracing members, sending the northernmost span plunging into the river. While the truck made it across the river, two passenger vehicles weren’t so fortunate, and boats were deployed to rescue three trapped passengers. Less than a month later, two temporary spans were erected in place of the collapsed span, allowing the critical crossing to reopen. Now plans are advancing to construct a permanent replacement span and reconfigure the remaining bridge truss to increase the clearance over the deck.
Constructed in 1955, the Skagit River Bridge is a 1,112 ft long crossing that connects the cities of Mount Vernon and Burlington approximately 60 mi north of Seattle. The bridge has four truss spans and eight cast-in-place concrete approach spans, carrying four lanes of traffic—two in each direction. Although it was deemed to be in good condition prior to the collapse, the bridge—like many others across the United States—is listed as fracture critical because it was designed so that each member is essential to its overall structural stability. The National Transportation Safety Board (NTSB) is investigating the incident and will have the final say regarding the contributing factors of the collapse, said Dave Chesson, a spokesperson for the Washington State Department of Transportation (WSDOT), in response to written questions posed by Civil Engineering online.
Less than a month after the Skagit River Bridge collapse, the
Washington State Department of Transportation installed two
prefabricated temporary bridge spans, allowing the critical crossing
to reopen. WSDOT
In June the NTSB released a preliminary report about the collapse. In the document, the board said the driver of the truck carrying the oversized load felt “crowded” by a passing vehicle, so he maneuvered his truck to the right. That’s when the load, which the driver estimated to be 15 ft 9 in. high, collided with several of the bridge’s sway brace members, which extend in an arched manner as low as 14 ft 8 in. over the outside lanes, according to the report. “The impacts caused significant damage to the load-bearing members of the bridge’s superstructure, resulting in the failure and subsequent collapse of the northernmost bridge span,” the report states.
Following the collapse, the WSDOT worked quickly to remove the debris from the failed span and erect a temporary span to reopen the crossing to the approximately 71,000 vehicles that traverse the bridge each day. During that time, the department also repaired an adjacent span that had also been damaged by the oversized load. It completed the work in less than a month by forming new pier caps on the existing bridge piers and then installing two 160 ft long prefabricated modular spans from Acrow Bridge, an engineering and design firm that specializes in steel bridge solutions for temporary and emergency use, based in Parsippany, New Jersey.
Additionally, the WSDOT made repairs to several remaining steel truss members that were damaged in the collapse. While the bridge has since been open to traffic traveling at a reduced speed, vehicles with loads over the posted height and weight limits must continue to use a detour route through Burlington—crossing another bridge just upstream of the Skagit River Bridge—until the permanent span is constructed and the sway-frame truss is reconfigured, said Patrick Fuller, P.E., a WSDOT project engineer for the I-5 Skagit River Bridge permanent replacement project, who wrote in response to questions posed by Civil Engineering online.
The permanent replacement span is being constructed on
temporary piles adjacent to the temporary spans. WSDOT
The WSDOT put the permanent span replacement project out to bid as a design/build (D/B) project, requesting proposals that would enable the work to be carried out in a manner that wouldn’t require closing the bridge for a long period of time. It did not specify what type of structure the new span had to be. The WSDOT awarded the $8.5-million D/B contract to Max J. Kuney Construction, a firm based in Spokane, Washington, which proposed constructing a concrete girder span adjacent to the temporary spans and then moving the new permanent structure into place. “Constructing the new permanent span next to the bridge will facilitate the plan to move the new span into place in the shortest amount of time possible,” Fuller said. “The contractor’s plan is to close the freeway for 12 hours to slide the temporary span out and the new span into place.”
Crews have already installed temporary piles alongside the temporary spans to support construction of the new permanent span. Eight prestressed concrete girders were formed off-site at a facility in Tacoma, Washington, and then shipped to the site. The girders have since been set on the temporary piles and crews are in the process of putting the finishing touches on the new superstructure, Chesson said. New pier caps to support the new permanent span will also be formed on the existing piers while the temporary spans are in use. Construction of the new pier caps and permanent span are expected to be completed soon after Labor Day, at which time a rail system will be used to raise the temporary spans and move them upstream and then slide the new permanent span into place. “Once the roadway is opened, crews will lower the temporary spans onto a barge for disassembly,” Fuller explained.
A rail system will be used to slide temporary spans out of the way
and the permanent replacement span into place. WSDOT
The WSDOT has also put out for bid a contract to reconfigure the bridge’s sway-frame truss members. As it stands now, the truss is high enough to accommodate overheight trucks of the size that struck the bridge, but only if those vehicles are traveling in the inside lanes; the arch-shaped truss is lower above the outside lanes. On the day of the collapse, the truck carrying the oversized load appears to have been in the southbound outside lane, Fuller said. Beginning at about the same time that the new permanent span is installed, the truss work will begin. This will involve raising the curved sway-frame members over the outer sides of the bridge so that the truss height is uniform across all lanes of the bridge. This will be accomplished by cutting away portions of each sway-frame member and replacing them with new members. Reinforcing members will also be installed around the truss portals. “By increasing the clearance, we can hopefully reduce the risk of a similar collision happening again on this bridge,” Fuller said. The truss work is expected to be completed by Thanksgiving.
The WSDOT looks forward to the coming months when all of the bridge work is completed and travelers can resume using the Skagit River Bridge without restriction. “The loss of the bridge has had a negative effect on the local economy as well as on the daily drivers and freight haulers,” Fuller said. “This corridor is vital to imports and exports for the state and the nation. We hope that [by completing this work] the bridge will not be struck by an oversized load again.”