Extensions mean a wider roadway
WINNER OF STEEL EDGE AWARD
Owner: City of Edmonton
Engineer: CH2M HILL Canada Limited
CISC Steel Fabricator/Erector: Waiward Steel Fabricators Ltd.
CISC Detailer: M&D Drafting Ltd.
The Quesnell Bridge is on Edmonton's busiest traffic corridor, Whitemud Drive, with volumes of more than 120,000 vehicles per day. When this section needed widening, rerouting that traffic onto a detour was not an option. This presented a particular challenge with respect to widening the bridge.
"We needed to design a system that would be cost effective, feasible and involve minimum construction time while allowing traffic to continue to flow," says Gary Kriviak, principal project consultant with CH2M HILL Canada Limited.
Early analysis determined that there was some reserve capacity for additional weight on the existing piers and foundations, indicating that pier cap extensions were a feasible approach for supporting a widened bridge deck. A more conventional pier-widening scheme would require construction from the foundation level up.
Extending the pier caps was not without technical design and installation challenges. Selecting structural steel as a primary component of the extensions turned out to be a key factor in addressing the challenges, Kriviak says.
Ten steel cap extensions were designed, two for each of the five piers. Each extension consisted of a compartmentalized pier cap top element, an inclined strut and a pier shaft bracket connection-all fabricated from steel to minimize weight while meeting geometric, stiffness and strength requirements.
One of the challenges of using this extension system was its connection to the existing concrete piers. Connection of the struts to the pier shafts was accomplished using steel strut end brackets anchored to the existing concrete with a group of steel tie rods grouted into cored holes. External steel post-tensioning rods were used to connect the top cap element to the existing concrete pier cap to hold it in place and transfer load back to the concrete.
When completed, each pier cap extension was a composite structure, with a hollow central compartment and peripheral compartments filled with concrete to enhance strength. An interface layer of new concrete between the steel extensions and the existing concrete piers was also used.