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Friday, Nov 08, 2019

Cost-effective deck rehabilitation and seismic retrofit extends the lifespan of the Alexandra Bridge in BC’s Fraser Canyon

Alexandra Bridge

The iconic 481 metre long Alexandra Bridge is the longest bridge in the Fraser Canyon. Located near Spuzzum, BC, on the Trans-Canada Highway, the Alexandra Bridge was originally built in the early 1960s. The structure stretches across the Fraser River via a 257 metre riveted steel arch main span and multiple steel girder approach spans with concrete decks. 

In 2009, the BC Ministry of Transportation and Infrastructure retained Associated Engineering to carry out a detailed assessment of the bridge’s condition and provide renewal options. Our assessment concluded the bridge required rehabilitation and did not meet current seismic standards. However, the topography would make replacing the bridge extremely challenging. 

Shane Cook, Assessment Lead, says, “The objective of the project was to extend the bridge’s service life, and improve its functionality and seismic resilience by carrying out a major rehabilitation and a seismic retrofit of the existing structure.”

Alexandra Bridge

The scope of the renewal project consisted of a seismic evaluation, arch stability analysis, load rating, as well as the detailed design and engineering and contract administration (Ministry Representative) services during construction. The main scope of the construction work consisted of concrete deck rehabilitation and widening; expansion joint replacements and re-articulation of the structure through installation of link decks; isolated steel girder strengthening work; creation of jacking locations and replacement of steel bearings with laminated elastomeric bearings; abutment modifications and approach works; deck drainage reconfiguration; partial painting; and concrete substructure repairs.

Katrin Habel, current Project Manager and Engineer-of-Record, tells us, “The existing structure had substandard railings and the narrow sidewalks were under-used. During the renewal work, the deck cross-section was widened to create wider shoulders. Railings were installed to current standards, allowing safer passage of vehicles across the structure.”

Alexandra Bridge

The design took advantage of synergies to incorporate cost-effective solutions. The seismic upgrade was achieved by replacing the existing deteriorated steel bearings with laminated elastomeric bearings and installing link decks. Alfred Kao, Project Manager for the initial phases of the project and Quality Assurance Lead, says, “The installation of link decks not only created a robust seismic load path, but increased durability by eliminating several deck joints. The deck cantilevers and parapets improved roadway safety and significantly enhanced the protection of the steel structure from salt spray, moisture, and solar radiation.” 

Use of laminated elastomeric bearings and the installation of the link decks helped improve seismic resiliency

Jason Dowling, Bridge and Seismic Engineer, explains, “This approach avoided any costly retrofitting of the lightly reinforced substructure with its poor seismic detailing.”

Alexandra Bridge

The rehabilitation work also needed to accommodate several constraints, notably traffic management during construction, active railway below the approach spans, and limited space for access and laydown areas. We designed the rehabilitation work to accommodate two-stage construction of the overlay to allow for single lane alternating traffic at all times.

During construction, areas of the structure were found to be corroded to such a degree that additional assessment and strengthening repairs were required. The extent of the required additional work merited a second phase of the project, which is currently under construction.