In my previous video post (right), I asked a question of bridge structural engineers: how they approach a case when a construction stage governs the design. Would they consider adding permanent material to strengthen their structure as a good practice? The other options being reconfiguring the construction sequence (if possible) or the use of temporary works to strengthen or reduce the demands. In this article, I will discuss how a couple of such situations were addressed with contrasting approaches.
The first example is of the Deh Cho Bridge across the Mackenzie River in the Northwest Territories of Canada. The bridge is a 1045m long crossing and forms an all-weather connection of Yellowknife to the southern highway system of the country. The superstructure structural system is a constant depth underdeck steel truss with a cable-supported main span of 190m and typical spans of 112.5m. The erection scheme assumed during the design of the truss was incremental launching. To accommodate this erection methodology, we need to verify that the truss had adequate capacity to resist demands from the launch sequence. We found that the point reaction from a typical roller support overstressed a portion of the bottom chord between panel points. Strengthening the entire bottom chord meant adding a significant amount of structural steel given the long length of the bridge. This extra steel would, of course, be redundant during the entire service life of the bridge resulting in non-sustainable use of construction materials. Instead, we choose to specify a minimum length over which the reaction of the temporary roller support should be distributed to reduce the local demand on the bottom chord to below its capacity. This meant that the contractor needed more sophistication in their roller assembly by mounting multiple roller units on a pivot beam.
The second example is the Sombrio Bridge on Vancouver Island in British Columbia, Canada. The bridge is a two-span crossing of a dramatic ravine requiring an asymmetrical span arrangement of 40m – 82m with the longer span crossing the deepest portion of the valley. The designer of the bridge assumed an incremental launch methodology for the erection of the steel girders due to extremely difficult access conditions in the ravine. Saqib Khan, Principal at Spannovation was involved in the design of the bridge during his employment with MMM (now WSP). The designer duly checked the girders for launching demands and detailed them to accommodate the launching equipment. On the schematic erection drawings produced by the designer, a launch nose was depicted to control the cantilever demands and raise the leading edge above the roller supports. Subsequent to the construction contract award, the winning contractor engaged my team to perform erection engineering services for the incremental launch. The first exercise we undertook was to investigate opportunities for saving temporary works to reduce the cost of the launch. We determined that the girders had enough strength to free cantilever for about 80m. Next, we calculated that with the addition of a mere 6 tons of additional steel in the bottom flange (less than 1% of the total steel tonnage), the girders would have adequate strength to cantilever the full length of 82m (a crane positioned at the abutment was used to lift the tip onto the bearings). The cost of additional material was significantly lesser than the price to design, procure, fabricate and handle a custom launch nose. We were able to gain owner and designer approval of the re-designed bottom flange prior to the fabrication of the girders. The contractor paid for the cost of additional structural steel added.
To summarize, in the case of the Deh Cho Bridge the erection equipment was modified to reduce the launching demands. In contrast, on the Sombrio Bridge, additional material was added to the permanent structure to reduce temporary works. So which approach would constitute a better practice for a designer?
The approach we recommend for designers is to avoid adding permanent material for a construction stage; primarily because this practice is not sustainable as the additional material would sit idle for the entire life of the structure. This excludes minor strengthening elements such as cantilevering tendons in a segmental bridge and top flange wind bracing on steel girder systems. However, designers should be careful in identifying governing construction stages and flag any associated temporary works, strengthening, supports or conditions assumed or required to maintain the strength and stability of the partially erected structure. It is ultimately the contractor’s prerogative to determine the means and methods of construction which may include the addition of permanent material to the structure with the owner’s blessing as in the case of the Sombrio Bridge.