St. Patrick’s Bridge was initiated by an international design competition launched in August 2009. The owner, Calgary Municipal Land Corporation (CMLC), recognized early on that the wild Bow River posed a risk to the future bridge and so a robust set of design criteria was established from the outset. The design criteria, communicated as part of the design competition Request for Submissions, included specific seismic, hydraulic, geotechnical, and geometric criteria to ensure resiliency against natural hazards. These criteria included the following:
- Seismic design per the Canadian Highway Bridge Design Code;
- All piers and abutments to be founded on driven piles or drilled caissons;
- Eliminate or minimize the number of bridge piers located in the river;
- Design for a 1:100 flood event with a water velocity of 2.8 meters/second and mandatory calculation of pier scour;
- Bridge deck soffit to be a minimum of 1 meter (3 feet) above the 1:100 year flood elevation;
- Maximum ice level of elevation 1042 meters, slightly higher than the 1:100 flood elevation, with an ice crushing strength of 700 kiloPascals (100 psi);
- Minimum bridge freeboard of 1.5 meters (5 feet) over maximum ice level; and
- Mandatory involvement of the client’s river hydrology specialist.
After CMLC selected the concept put forth by the RFR/WSP team, the detailed design began in the summer of 2010. As the design progressed, the river ice was studied in more detail by the client’s hydrology specialist and the design loads were clarified.
Because the Bow River is a fast moving river, ice buildup is typically composed of frazil ice, which is an accumulation of floating ice crystals created by super-cooled moving water. The frazil ice buildup can be up to 3 meters (10 feet) thick when it forms at the bridge site, especially where the water overtops land such as the island, but with little to no internal strength. At the time of the seasonal spring breakup when the river starts moving again, the ice in the river channel is typically about 0.5 meters (1.6 feet) thick and is substantially disintegrated, so the design ice crushing strength was reduced to the corresponding code value of 400 kiloPascals (60 psi). This ice pressure and thickness was applied to the arch and strut members present at the maximum ice elevation to maximize its effects on the design. All of the criteria to protect against natural hazards were incorporated in the design without any exceptions.
One of the goals of the bridge project was to link St. Patrick’s Island, located in the middle of the Bow River, with both river banks. However, the island surface is below the 1:100 year flood elevation, so the criteria did not permit the bridge deck to touch down on the island. Instead, a secondary ramp links the island span of the main bridge with the ground surface of the island. A slender elevated ramp was used as opposed to an earthen embankment to avoid adding fill to the floodplain.
Despite the strict design criteria for natural hazards, the designers succeeded in creating a graceful structure that will be a Calgary landmark for years to come. This was made possible by design choices for the structure type, member cross-sections, and material selections, all of which contributed to the slender but strong design. And, thanks to the early test put forth by Mother Nature, the owner and the designers know without a doubt that the bridge is resilient.