Resiliency Put to the Test: St. Patrick’s Bridge

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The new Bridge was Designed to Withstand Both High Flood waters and High Ice Floes, But a 1-in-100-year Flood Came at a Vulnerable Phase in the Construction Process.

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 | Parsons Brinckerhoff) 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.

st. patrick's bridge profileFigure 1 — Profile of St. Patrick’s Bridge
st. patrick's bridge conceptual arch cross section Figure 2 — Conceptual isometric cross-section of arch ribs and deck (image courtesy of RFR )resilience saint patricks bridge archFigure 3 – Aerial image of bridge construction just prior to flood (note gravel berms partially blocking river channels)saint patricks bridge floodFigure 4 – High water during flood

The arch rib cross-sections, which are only 400 millimeters (16 inches) tall but vary in width from 1130 millimeters (45 inches) to 1550 millimeters (60 inches), give the arches significant out-of-plane strength to resist lateral loads from wind, water, ice, and earthquakes, and eliminate the need for bracing between the arches except for one bracing point at the peak of the main span, all while maintaining a slender profile.

The slender 320 millimeter (12 inches) thick cast-in-place concrete deck, with longitudinal post-tensioning, acts as a tension tie to hold the ends of the arches together, while minimizing the grade with its minimal profile. Over the majority of the two river channels, the only connections between the arches and the deck are the hangers by which the deck is suspended from the arches (see Figure 2). Slender steel struts support the deck from below over the island span.

In order to construct the deck, rebar was placed and concrete was poured on forms that were set on a series of closely spaced falsework towers (temporary support towers). The towers were supported by temporary gravel berms that partially blocked the river and on temporary steel girders that spanned the openings in the berms (see Figure 3).

The Flood

When the flood waters rose in June 2013 (see Figure 4), the deck had only recently been completed and was not yet suspended from the arches, in part because the prefabricated steel arch sections were still in the process of being assembled and welded together. The floodwater and the debris that it carried overtopped the berms, washing them away and knocking out significant portions of the falsework (see Figure 5). This forced the deck to span distances for which it was never designed. A post-flood investigation revealed that the deck was severely cracked with permanent yielding of the steel reinforcing bars and steel post-tensioning tendons inside the deck. Consequently, the new deck had to be demolished and reconstructed, setting back the project completion date by a year.

Some good news came from the flood event; before the bridge was even completed, it had experienced a 1-in-100 year flood and survived with no serious consequences to the permanently fixed structure, which includes the piers, abutments, and the steel arches, whose lower sections were in place and were inundated by the flood waters. This demonstrated the benefit of having designed the bridge to be resilient from the beginning. Furthermore, the water never overtopped the deck, validating the design freeboard requirement.

Design for Resilience

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/Parsons Brinckerhoff-Halsall (now part of WSP | Parsons Brinckerhoff) 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.

saint patricks bridge floodFigure 5 – After the flood, berm and falsework were washed away

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.

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