Most bridges are designed to handle cars, and trucks and maybe the occasional pedestrian, but for this project, engineers had to design a span durable enough to handle commercial jets.
The three-span, 245-ft-long, prefabricated concrete bridge is located at Boeing’s 737 factory in Renton, Wash., where more than 11,600 commercial airplanes flying today were built. The bridge would allow aircraft to be towed from the factory to an airfield for final inspections before being turned over to their owners. “With imminent plans for fabrication of a new series of aircraft, the existing seismically deficient bridge needed to be replaced in quick order,” says Greg Banks, project manager for BergerABAM, the engineer for the project.
The site also faced a number of environmental constraints, including working adjacent to an active factory and airport, wet winter weather conditions, noise limitations, and “fish windows” that reduced the team’s access to the waterway during spawning season.
Banks’ team determined that an accelerated bridge construction approach using prefabricated concrete columns, crossbeams, and deck panel, would enable them to meet the tight schedule and add the necessary float to accommodate spawning season. “Accelerated bridge construction techniques were sought on this project not just to minimize impact to the traveling public, but also to meet the factory's planned production schedule for a new line of aircrafts,” he says.
Working around all the site constraints led to a uniquely complicated schedule, with construction occurring over three in-water work windows, Banks explains. The prefabricated elements, included full-depth precast concrete deck panels with ultra-high-performance concrete (UHPC) closure joints, and seismic resisting precast concrete crossbeam-and-column supports. The panels utilized a hidden pocket that was later filled with UHPC to form composite action with the girders, and the columns were cast into cast-in-place drilled shafts.
Because the bridge is located in a high seismic region, the design used a displaced based approach, in accordance with the American Association of State Highway and Transportation Officials’ AASHTO Guide Specifications for LRFD Seismic Bridge Design and features a Type 1 earthquake resisting system.
“The precast elements were detailed as if they were being constructed by cast-in-place methods even though the precast elements were fabricated in a controlled environment by a PCI-certified precaster,” Banks explains. “This, in conjunction with the flexural precast elements being designed for “zero” tension under service conditions should enhance the durability of the bridge.”
Banks estimates that the use of precast concrete columns and crossbeams cut 20 days within the fish window, and the full-depth precast concrete deck panels shaved roughly 6 weeks from the construction schedule, all while delivering a durable, low-maintenance structure that will meet the needs of the Boeing plant for decades to come.