Completed in December 2022, the $7.9-million Union Street Pedestrian Bridge provides an accessible connection between Seattle’s downtown core and the waterfront.

Work had to be completed within just 150 days. The roughly 150-ft by 50-ft site contained three Seattle City Light (SCL) vaults, including one 115-kV oil transmission vault that supplies downtown Seattle that had to remain functional throughout construction.

Crews installed five 45-ft-deep, 5-ft-dia permanently cased drilled shafts and set four architectural columns on top. Due to the heavy load from the drill rig and oscillator, the team engineered a temporary support that allowed shaft construction to be done at minimal cost during the project’s first weeks. By shoring two cranes simultaneously with lumber, the contractor was able to both protect the SCL transmission vaults and maintain access to them.

Two steel tower elevator frames were built off site and set during a single night shift, which required precise anchor bolt placement for the 66 bolts in the concrete foundation with a tolerance of less than 1/8-inch. A crane placed each of the tower structures in perfect line with the anchor bolts, saving an entire month off the critical path schedule.

Instead of lumber, the contractor collaborated with a subcontractor to create foam infills before concrete placement for stylized arched bridge soffits. This resulted in a higher quality product that required less dry finish, no near-perfect seams and saved 21 days of onsite fabrication and about $50,000 in construction and dry finish costs.