Shubenacadie River Bridges Assesment And Rehabilitation, Shubenacadie,NS


Project Description

The existing Shubenacadie River Bridges are twin 4 span structures, each with a total span of approximately 195 meters constructed in 1975. The structures are twin trapezoidal, steel box girders bridges with a composite, cast in place concrete deck supported on substructure elements, including 3 in-water piers, founded on end bearing steel piles. The two boxes within an individual bridge are linked by external cross-frames that do not line up with the internal cross frames of the boxes. The abutments feature a traditional expansion joint. The alignment of Highway 102 is skewed 20 degrees to the Shubenacadie River.

Failure of the expansion joints at each end of each bridge allowed salt laden water to spill onto the steel superstructure below resulting in excessive corrosion of the ends of the steel box girders. Also identified were cracks in the webs of the box girders at each connection point of an external cross frame. The load carried by an external cross frame resulted in out of plane loading of the thin web plate resulting in a through thickness crack of the web plate. The cracks are a classic example of a distortion induced fatigue problem known as the “web-gab” detail. Further, HEC identified poor workmanship in the fabrication of the girders as well as overall poor detailing to address fatigue issues.
Phase 1 of the project included a full assessment of the existing structures including on site inspections and steel thickness testing as well as a full structural analysis of the girders to determine the existing capacity of the bridge. The Phase 1 report included three options to address the problems with the structures complete with full capital cost estimates and preliminary construction phasing for each. Ultimately Option 3, full superstructure replacement was recommended b HEC and agreed to by NSTIR.
HEC are currently finalizing the design of the replacement structures. The scope of work includes the detailed design of the replacement structures incorporating the existing
sub-structure elements, design of a temporary detour structure, a fully detailed demolition plan for the existing structure, a fully detailed erection plan for the new structures, highway alignment for the detour structures and full construction phasing.

The two span temporary detour structure is a total of 120 meters long and has been designed to go between the two existing bridges. HEC has developed an innovative central support that spans between the existing central piers of the two highway bridges and eliminates the requirement for an expensive temporary pier that would be subjected to large ice forces.

Client: NS Department of Transportation and Infrastructure Renewal

Value: $2.4 M (Approximate)

Year: 2009 to Present

Role: Bridge Assessment / Rehabilitation / Replacement Designers, Project Lead