The planning, design, and construction of the iconic St. Patrick’s Bridge in Calgary, Alberta, Canada had been underway for more than four years when, only three months away from expected completion, a record flood threatened to wipe out the bridge in June 2013.

The bridge passes over the Bow River which is fed by glaciers and snowpack from the Rocky Mountains. Rapidly melting snow and ice combined with heavy rainfall upriver from Calgary and sent torrents of water past the bridge and flooded the city and the surrounding region, causing nearly CAD$5 billion in damage throughout the province of Alberta, the most expensive natural disaster in Canadian history. The bridge designers, RFR (Paris, France), in partnership with Parsons Brinckerhoff-Halsall (now part of WSP) had anticipated and designed for this and other hazards. However, the design of the temporary works did not account for such a hazard, and the flood came at a vulnerable phase in the construction process.

Bridge Design and Construction

St. Patrick’s Bridge consists of a 182 meter (597 foot) continuous three-span network tied-arch, with a main span length of 99 meters (325 feet). The network arch form combines the arch ribs and deck in a truss-like structure, where the ensemble is stronger than the parts (see Figure 1). The arch ribs are each composed of twin steel tubes joined by welded top and bottom plates forming oblong cross-sections, and they run continuous between the sliding bearings on the two abutments.