Creating a New Motor City Gateway
Interstate 94 Arched Bridge Erection
Bridge Erection Engineering
Wayne County, Michigan

Ruby+Associates performed construction engineering for these twin tied-arch bridges-each supporting a 240-ft-long deck span designed to carry four lanes of traffic. The design and erection had to minimize disruptions to traffic and meet a deadline that could not be extended. Ruby proposed an alternate construction method that eliminated the need for massive and expensive shoring.

Challenges:

As originally designed, temporary shoring towers were to support not only the bridge deck steel, but also the formwork, rebar, screed machines and deck concrete. The towers would have had to support 120,000 pounds while maintaining an installation tolerance of plus-or-minus 1/8 inch for the deck elevation. The available window below the deck steel for the spandrel system over heavily traveled Telegraph Road was approximately 30 inches. Creating a spandrel system that was stiff enough to resist 240,000 lb of vertical load over the road while limiting the deflection to 1/8 inches was not cost effective. The complex geometry of the arch sections required specially designed hitch fixtures to securely grasp the sections and also tilt the pieces inward once in place. This was complicated by strict design requirements that did not allow any penetrations or welds to the arches.

Solutions:

An alternate plan reduced the weight on each shore to just 32,000 lb and minimized tight tolerances during installation: only the bridge steel was supported by the shoring towers while the remaining bulk of the span's weight was shifted to the bridge's arches through the tensioning of the cable strands. By using the arch to support the dead load during construction, crews used a less expensive, rented shoring system instead of larger, specially fabricated shoring towers. The smaller shores required only timber mats at the base. Because they were easily removed once the load was transferred, they could be immediately reused for construction of the second bridge. Additionally, the impact on Telegraph Road was minimized. Three hitches were designed to lift the sections and to rotate the arch. These never directly touched the steel. Half-inch strips of polyurethane material were placed between the hitches and the arch sections to provide padding and friction resistance so that no bolts or welds were required to be attached to the arch segments.