A case study on the repair of West Seattle Bridge

*Image of the work carried out in phase 1 of the project (image from www.seattle.gov)*

The West Seattle Bridge is a cantilevered segmental bridge with a maximum clearance of 140 ft (43 m) and a total length of 2,607 ft (795 m), that connects West Seattle to the city’s greater metropolitan area.

Its construction started in 1981 and was completed in 1984, when the bridge was put into service.

However, during inspections in 2013, cracks were first noticed on the structure. Some remediation works were performed, mainly in the form of epoxy resin injections, monitoring devices were installed, and frequent inspections were conducted.

Despite that, cracking continued to intensify until 2020, when an engineering analysis prompted the Seattle Department of Transportation (SDOT) to close the bridge in March of the same year.

After evaluating and planning accordingly, it was decided to act in two phases. The first phase would include stabilization of the bridge, while the second phase would either involve replacement or rehabilitation of the structure.

It is worth noting that cracks can compromise the structural integrity of the structure in different ways, one of which is reinforcement corrosion, something that a bridge over water surely is susceptible to.

The first phase included pressurized injection of epoxy in the cracks to remediate the concrete’s bonding strength, as well as to create a watertight seal and protect the reinforcement. It also included placement of post-tensioning within the bridge’s hollow girders, as well as the installation of carbon fiber reinforced polymer (CFRP) on the inside and outside of the girders, in selected locations, both of which would help reinforce and strengthen the structure.

Phase 1 mainly consisted of work taking place on the bridge’s central span, while it took place in 2020.

Phase 2 was planned and designed during 2020 and 2021, while actual construction lasted from 2021 to 2022.

Image of the work carried out in phase 2 of the project (image from www.seattle.gov)

For the second phase, it was decided to move forward with the same techniques used in phase 1. However, this time, strengthening work would also take place on the bridge’s side spans, with additional work also taking place on the central span.

The rehabilitation process included epoxy injections in a 3-mile total linear crack length, while approximately 10 miles of post-tensioning cabled had to be installed.

The whole process lasted two and a half years, and came at a cost of $47 million, but the West Seattle Bridge opened to traffic once again on September 17 of 2022.