Spanning the Thea Foss Waterway in Tacoma, Washington, the Murray Morgan vertical-lift bridge is a lifeline between downtown Tacoma and the Port of Tacoma. The people of Tacoma celebrated the bridge’s 100th birthday in February 2013—but the centennial celebration wasn’t always a foregone conclusion.
A member of the PCL project team crosses over the
Thea Foss Waterway on the top catwalk of the
Murray Morgan Bridge.
The 100-year-old bridge is not just an important
piece of Tacoma history; it serves to reduce
response times for emergency vehicles and provides
an egress by which people can efficiently exit the
tide flats in the event of a crisis.
Washington State officials barred vehicles from crossing the bridge in 2007. With a sufficiency rating of 2 out of a possible 100, this was the safe choice. Shortly after, the bridge was marked for demolition—it was an unsafe relic that had suffered years of delayed maintenance and repairs. After a vigorous campaign to save the Murray Morgan Bridge (led by local preservationists, elected officials, and civic boosters), the City of Tacoma obtained the bridge from the State of Washington and set out to rehabilitate the landmark.
A Challenging Proposal
The City of Tacoma had a very tight budget for this project—$57 million (a third of which was raised by the City) to rehabilitate the bridge from a sufficiency rating of 2 to 80. The project used the design-build delivery method, meaning that the proposal needed to present, within budget, firm solutions for a rehabilitation where many variables were yet unknown. With only a surveillance report, list of performance criteria, and confined budget to work from, this was a challenging proposal for the team to get right.
The proposal drew upon the experiences and lessons learned by both PCL and the designer, Hardesty & Hanover, on previous rehabilitation projects. In addition, they drafted the proposal after meticulous examination of the surveillance report the City provided, taking into account the sections in poor condition, and factoring in ways to use the sound areas to support the project and ultimately reduce costs. Finally, the proposal included a complete conceptual design. Though not a requirement, the conceptual design ensured that even minute details of the design concept were accurately communicated to the client at the outset.
To Truss or Not to Truss
Numerous parts of the Murray Morgan Bridge were in complete disrepair (hence, the sufficiency rating of only 2). Planning where to place equipment on the bridge in order to perform the rehabilitation work was a challenge owing to the poor condition of the structure. The bottom chords of the bridge trusses were especially compromised, and had become heavily corroded over the 100-year life of the bridge.
Generally, a project team will repair a bridge truss by temporarily supporting the load within the bottom chord while removing and replacing the deteriorated plies that comprise the truss chord. This is a very costly and time-consuming method of repair, but is usually considered the only option in cases of severe debilitation. To mitigate these costs, sound sections of the truss chord were used to support large-diameter, high-strength rods that serve as a new load path around the weaker portions of the truss chord. This negated the need to repair the damaged plies of the trusses and effectively doubled the capacity of the bottom chords. This work was completed with virtually no aesthetic impact and at a reduced cost to the client.
“Due to the poor condition of the bottom chord and the way the bridge was constructed in 1912 using an existing bridge as a temporary support, replacing the bottom chord of the lift span was a major issue. The use of high-strength steel rods by the project team was an innovative approach that saved both time and money, ” said Tom Rutherford, PE—project manager, Public Works Engineering of the City of Tacoma.