Seal Island Bridge Emergency Repair
Location: Cape Breton, NS
Client: NS Department of Transportation and Infrastructure Renewal
Duration: 2016
Approximate Value: $100K
Key Staff: Wade Pottie, Robbie Fraser
Project Description
The Seal Island Bridge Structure, officially known as the Great Bras d’Or Crossing, was constructed in 1961 in Victoria County, Nova Scotia. The Seal Island Bridge is the third longest bridge span in Nova Scotia and is a crucial link in the Trans-Canada Highway between Sydney and Baddeck, carrying more than 7500 vehicles per day during peak periods over the Great Bras d’Or channel of the Bras d’Or Lake.
The bridge structure consists of 8 steel box truss spans with three 250 ft. approach spans, two 250 ft. splay spans, and a three span continuous main span composed of two 350 ft. side spans and a 500 ft. center arch span. The steel superstructure is supported on tall reinforced concrete piers. In November 2016, the Seal Island Bridge Structure sustained a fatigue failure in one of the center arch span truss diagonals resulting in the center arch span to displace vertically 38 mm. NSTIR consulted Harbourside to provide structural engineering services for the structural assessment of the three span continuous main span and rehabilitation of the damaged truss diagonal located within the 500 ft. center arch span.
Harbourside’s Role
Harbourside completed an emergency condition assessment of the bridge structure to ensure there were no additional connections presenting evidence of fatigue failure. In parallel with the condition assessment, HEC developed a 3D finite element model of the three span continuous main span structure. A structural analysis was completed to determine whether the center arch span truss structure was adequate during in-service conditions after sustaining a 38 mm global displacement from the failure of the truss diagonal.
Understanding that the Seal Island Bridge is a critical structure, Harbourside was able to maintain a single lane of alternating traffic on the opposite traffic lane relative to the failed truss diagonal, thus preventing overstresses in the arch chords. HEC developed an innovative structural repair with detailed phasing drawings which permitted the replacement of the damaged truss diagonal while reinstating the geometry of the truss chords by jacking the nodes on the top and bottom chords of the truss together along the axis of the failed diagonal member. Thus, additional stresses that were developed in the truss chords from the global displacement of the truss were relieved and the axial load was reinstated back into the replacement diagonal.
Unique Challenges
The urgency of this project posed unique timing restraints. Working within these, Harbourside was able to repair the structure while reducing the expected impact to traffic flow. Given the nature of the project, Harbourside’s analysis was thorough, ensuring comprehensive rehabilitation of the structure.
Social Impact
As a crucial link along the TransCanada Highway, the Seal Island Bridge is one of the only connections between Cape Breton and Newfoundland via the ferry terminal. Its closure would disconnect the two provinces, and impact nearly 7500 daily travellers. The swift analysis and repairs to the bridge is a monumental boon for the region, ensuring continued connection between Newfoundland, Cape Breton, and Nova Scotia at large.