June 4, 2014
Category: Box Girders, Launched Bridges - Published by: Canam-Bridges
In recent years, Canam-Bridges has elaborated an innovative bridge erection technique: a launched-girder erection system using counterweights. This method was put to use during the construction of the North Channel Bridge, which connects the City of Cornwall and Cornwall Island in Ontario, Canada and the State of New York in the U.S. Spanning the St. Lawrence Seaway, over 120,000 commercial vehicles and 2,300,000 passenger vehicles use this vital corridor every year.
The Federal Bridge Corporation Limited, which owns the bridge, awarded the contract to replace the aging structure to general contractor Aecon Group Inc. who in turn selected Canam-Bridges, a division of Canam Group Inc., to fabricate and erect the steel superstructure. The bridge design was entrusted to engineering firm McCormick Rankin, a member of MMM Group.
The new 1,100 ft. (335.2 m) bridge, which replaced the existing structure that was inaugurated in 1962, features four spans composed of 20 box girders, each measuring 13.75 ft. (4.2 m) wide, 10.85 ft. (3.3 m) high and weighing a maximum of 89.1 metric tons.
Canam-Bridges’ mandate included the fabrication, delivery and assembly of the structure at the erection site, the incremental launching and the leveling of the bridge on its bearings after the launch was completed as well as the design and fabrication of 20 Goodco Z-Tech structural bearings composed of twelve PF-Series fixed pot bearings installed at piers 1, 2 and 3, and eight PMG-series unidirectional pot bearings for the abutments. Canam-Bridges also performed all the engineering required to execute the launches, including the verification of structural efforts at each phase and the engineering of all launching system elements, and prepared the procedure for the final assembly and leveling of the bridge on its permanent supports.
The 1,800 tons of steel components that form the superstructure were fabricated at the Canam Group plant in Quebec City, Quebec. Canam-Bridges’ expertise was brought to bear in all aspects of this project, from the complex fabrication and handling of the girders, to the transportation of the oversized components to the erection site located some 248 miles (400 km) away. In view of the unique conditions at the site, the erection of the structure also held major challenges for the team in charge of providing engineering and construction services.
Conventional bridge building methods typically involve the construction of jetties in the water or the use of barges to support high-capacity cranes. In the case of the North Channel Bridge replacement project, the launched-girder erection technique was chosen in order to minimize the environmental risks associated with conducting operations in the channel’s powerful currents. With this method, the assembly of bridge components can be completed behind an abutment and the assembled sections are then pulled over the water until the structure reaches the opposite shoreline. As a result, no transitional work was performed in the river itself during the erection process. In addition, the assembly of the structure was carried out at ground level on the northern shore, thus reducing the risk of accidents and optimizing productivity at the site.
One of the particular features of the new structure is that it was erected below the existing bridge, thus restricting the amount of space available to assemble the sections prior to the launches. A temporary pier was built near the northern shoreline, at the edge of the assembly area, to be able to achieve the minimum required assembly length. To minimize structural efforts during the launches, Canam-Bridges designed a special launching nose weighing less than the sections themselves that would be erected in front of the structure. This lightweight launching nose featured a sloped bottom chord to allow the girders to land on the piers without the use of lifting equipment.
Given the limited assembly area, only two box girder segments could be assembled ahead of each launch. Also, due to the sloped bottom flange of the launching nose and the horizontal bottom flange of the box girders, it was impossible to roll on the girders and launching nose simultaneously during the first two phases of the launch. To solve this problem, Canam-Bridges elaborated an exclusive technique that consisted in rolling the structure and then tipping the launching nose to complete the first 2 increments of the launch.
This technique relied on the use of counterweights installed at the back of the assembled structure in order to undertake the first few meters of the pull. Just before the tipping point was reached (equivalent weight both behind and in front of the main support), the counterweights were removed in order to allow the assembly to touch down gently on the launching nose girders.
For the second launch, the box girder and launching nose assembly measured over 380 ft. (116 m) and the tipping operation was performed over the water, leaving very little room to maneuver. It was therefore crucial to properly guide the structure so that once the assembly had touched down, the extremities of the two launching nose girders were perfectly centered on the supports.
In addition to the development of this exclusive technique, the simultaneous launch of two box girders required Canam-Bridges to gather data on an ongoing basis in order to validate the anticipated stress in the structure throughout the launching operations. Critical considerations, such as the reaction of supports, vertical deflection and the lateral displacement of structural elements were continuously analyzed onsite by the engineering team. Specialized systems composed of rollers and guide rails were designed and installed on each pier to limit the structure’s lateral movements. The roller system was also equipped with a leveling mechanism in order to properly distribute the reactions under each box girder and ensure the adequate transfer of loads to the bridge foundations.
Discover this innovative launching technique using counterweights by viewing a video of the erection of the North Channel Bridge.
General contractor: Aecon Group Inc.
Engineering firm: McCormick Rankin
Owner: The Federal Bridge Corporation Limited
Structural engineering analysis: Roche Ltd, Consulting Group
Manpower at the erection site: St.Lawrence Erectors
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