July 13, 2012
Structal-Bridges began delivering box girders in the scope of the North Channel Bridge Replacement Project in Cornwall, Ontario, in mid-May. The erection of the girders, which was also entrusted to Structal-Bridges, is being executed using the launched-girder erection technique that requires the use of a steel launching nose: a lightweight girder section with variable inertia that is mounted at the front of the structure in order to minimize structural loads during launching.
The first two erection phases, which have since been completed, required the use of a particular technique that consisted in rolling the box girder and launching nose assembly into place. The bottom flange of the launching nose features a vertical slope, which is essential for the subsequent stages of the launch. Since the bottom flanges of the box girders are horizontal while those of the launching nose are sloped, this prevented the erection team from rolling on both the girders and the launching nose simultaneously during the initial phases. The following drawing illustrates the overall technique.
Dominique Blouin, Technical and Construction Manager, explained: “This technique was made necessary because of space restrictions at the erection site that simply didn’t allow for the assembly of several girders at once. The procedure consisted in installing counterweights at the rear of the assembled structure in order to execute the first few meters of the launch. Just before reaching the balance point (an equivalent weight both behind and in front of the main support), the counterweights were withdrawn, allowing the assembly to slowly roll onto the girders of the launching nose. This technique, which was devised by our team, has never been executed before. In the case of the second launch, which involved a girder/launching nose assembly of over 381 ft. (116 m), the operation was performed over water with extremely limited maneuvering room. It was therefore very important to properly guide the structure so that once the operation was completed, the extremities of both launching nose girders were perfectly centered on their supports.”
Structal-Bridges was mandated to fabricate and erect 22 box girders as well as supply 40 Goodco-Z-Tech structural bearings for this 1,100-ft. (335 m) bridge. The final girder is expected to be erected in late August.