The Detour Bridge
Understanding the "Shoofly" Detour Bridge Approach
Larry Gescher narrates the shoofly process. (3 min 18 sec)
Multnomah County is using a detour bridge in order to save time and money during the construction of the new Sellwood Bridge. Referred to as a "Shoofly" in the construction trade, this approach was suggested to the county by the firms bidding on the design and construction contracts in early 2011.
The original bridge truss was shifted on January 19, 2013 and the Shoofly opened to traffic on January 23. Visit The Big Move page for more information on the moving process and a time-lapse video of the move itself.
Advantages over original plan
Originally, the county intended to build the new bridge in halves by using the existing bridge for traffic while half of the new structure was built to the south. Once complete, traffic then would have shifted to the new half while the existing bridge was demolished and the other half of the new bridge was built to the north. This approach would have required extensive staging to keep the bridge open to traffic. It also would have cost more, due to longer construction times and redundant structural features; each half of the bridge had to be freestanding on its own, requiring a total of four rib arches rather than two.
Under the detour bridge "Shoofly" method, staged construction is not required. Instead, temporary piers were erected to the north of the existing bridge piers. In January 2013, the entire 1100-foot long steel deck truss will be slid over on rails using hydraulic jacks. Temporary approach spans at the east and west ends of the truss will link the structure to SE Tacoma Street and Highway 43.
The temporary detour bridge will remain in place until the new bridge opens in 2016, freeing up the existing alignment for the work crews and removing traffic from the bridge construction zone. The construction firm being hired to do this work – Slayden/Sundt Joint Venture – has successfully used this detour bridge method on other bridge projects.
The detour bridge offers several advantages over staged construction:
- The temporary detour structure will be as strong as, if not stronger, than the existing Sellwood Bridge (including seismically). The weakest part of the old bridge (the west approach) will not be part of the detour bridge.
- The new bridge can be built entirely in one phase.
- Cost savings are estimated to be in the range of $5 to $10 million.
- This approach saves up to a year in construction time.
- There is no need for redundant structural features (allows for a sleeker design).
- There is a safety benefit in separating construction workers from drivers.
- This approach requires fewer temporary work bridges and less in-water impacts.
The bridge will need to be closed for one week to set up the detour bridge. The bridge will also be closed late in the project to set up road connections at each end. The detour bridge will not change the number of days the bridge will need to be closed (no more than 30 days during the project).
At the east end of the bridge, the old approach from SE Tacoma St. will continue to be in use as part of the detour bridge. An additional construction phase will be needed at the end of the project to construct the north side of the east approach.