With the immediate future of the closed West Seattle High-Rise Bridge still in doubt, the City of Seattle has tapped an HNTB-led team to design a long-term replacement for the 2,607-ft long structure.
The city made the selection even as it continues to weigh options for repair or replacement of the 36-year-old bridge, which was shut down March 23 following the discovery of accelerated cracking in its support structure. Analysis by the city’s structural engineering consultant WSP and an independent technical advisory panel have determined that repairs may be feasible, with options ranging from shoring to partial or full superstructure replacement.
In announcing the selection of the HNTB team, the Seattle Dept. of Transportation (SDOT) explained that even if the cost-benefit analysis identifies an economically feasible repair alternative, full replacement of the bridge is inevitable.
By starting the design process now, SDOT said, “we’re ensuring that all of the pieces are in place to quickly pivot if it becomes clear that repairing the bridge is not advisable, and that we’ll not lose valuable time hiring a design team to begin the rebuilding process.”
HNTB’s core team includes Parametrix and Bohlinger Consulting, plus more than 20 other planning, engineering, infrastructure funding and project delivery consultants. The team’s initial task is to build on an SDOT repair/replacement cost-benefit analysis already underway to fine-tune the type, size and location of replacement options, including both above-ground and tunnel alternatives.
“The study will determine the functional and physical characteristics of the bridge or tunnel…how it would be constructed, and whether it would be built in the same footprint of the current bridge or along a different path,” SDOT added.
Meanwhile, Kramer North America has begun efforts to stabilize the existing bridge by wrapping cracked and vulnerable girders with carbon fiber-reinforced polymer. This fall, Kramer will install external post-tensioning tendons inside the bridge girders and replace bulging lateral bearings on Pier 18, one of four main piers supporting the bridge’s three 140-ft high spans above the Duwamish River.