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Detour bridge option – the Shoo Fly explained

Multnomah County and its consultant team are exploring a construction option for the new Sellwood Bridge which could reduce the project's cost and construction time. Other potential benefits include increased safety for bridge users and construction workers and improved bridge design. The "shoo fly" option involves moving the main spans of the existing bridge north to create a detour bridge. Shifting the location of the detour bridge would create an open space where the contractor can build the new bridge in a single, shorter construction phase.

Potential benefits and risks of the shoo fly are being evaluated and compared to the original construction plan.

Original Construction Plan

The original plan called for the new bridge to be built in two phases.

  • In the first phase, half of the new bridge would be built along the south side of the existing bridge, while traffic continues to use the old bridge.
  • After the first half of the bridge is completed, traffic would shift to the new bridge. The old bridge would be removed and the contractor would build the second half of the new bridge where the old bridge stood. The completed bridge would be a single structure.

Disadvantages of Phased Construction

The original plan fulfills a commitment to keep the bridge open to traffic as much as possible during construction. But the plan has several disadvantages:

  • Time: Building the new bridge in halves takes nearly as much time as building two bridges. It would take about four to five years to build the new bridge this way.
  • Money: Added construction time translates into millions of dollars more for labor and equipment.
  • Safety: The contractor needs to build the second half of the new bridge and connect it to the first half while traffic uses the new bridge. Combining traffic and construction in such a confined space increases safety risks for the public and construction workers.
  • Design: Building the bridge in halves requires redundant structural features that are not needed if the bridge is built in one phase. For example, the first half of the bridge would require two main arch ribs to be stable while the second half is built, for a total of four arch ribs. If the bridge is built in a single phase, it would need only two arch ribs.

Shoo Fly Concept





How the shoo-fly process works.

The shoo fly uses the old bridge as a detour in a location that creates space to build the new bridge in a single phase. Construction could be phased several ways. The project team is considering a phasing plan that would require these steps:

  • Build temporary foundations and columns for detour bridge on north side of old bridge.
  • Build temporary approach spans on west and east side.
  • Disconnect main steel river spans of old bridge from top of supporting columns and place on rails connected to temporary columns to the north. Use jacks to slide truss spans roughly 40 feet north and attach to tops of temporary columns.
  • Open temporary detour bridge to traffic.
  • Construct main span of new bridge and approach spans.
  • Move traffic to new bridge and remove old bridge and temporary shoo fly sections.

Advantages of Shoo Fly

  • Time: Construction could be shortened by up to a year. Depending on permit approvals, construction material (steel) availability and regulatory agency approvals, construction in the Willamette River might start in Winter, 2011.
  • Money: Construction costs could be reduced by millions of dollars. Cost savings could help close the project's $20 million funding shortfall.
  • Safety: Greater separation will improve safety for construction crews and bridge users.
  • Design: Building the bridge in a single phase will reduce redundant structural elements and improve the appearance.

Shoo Fly Risks and Issues

Similar bridge shifts have been done in Oregon and other states in recent years. The sections of the Sellwood Bridge that have cracked girders will not be moved. As with any construction activity, the shoo fly involves risks. Bridge spans would need to be repositioned in a single, unified movement to not damage the bridge. If the shoo fly is built, the project team will need to identify and mitigate risks.

  • Residential Impacts: The shoo fly would shift the temporary bridge alignment closer to several residential units on the north side of the bridge, into space that would otherwise be occupied by heavy construction equipment for an extended period of time. The occupants of one unit would need to relocate for a short period of time during construction of the shoo fly. The temporary alignment may lead to a slight increase in traffic noise experienced by several residents north of the bridge. However, the shoo fly would eliminate up to one year of construction time for the new bridge and associated noise and inconvenience for bridge neighbors.
  • Environmental Impacts: The shoo fly would require additional in-water work to construct temporary foundations. But it would eliminate the need for in-water work later by combining two construction phases into one. The cumulative environmental impact of the shoo fly would be less than if the bridge were built in two halves.

Decision Process

Multnomah County has selected the firms that will design and build the new Sellwood Bridge. The construction firm is providing input during design to help reduce the project cost and construction schedule. The Slayden/Sundt construction team have proposed the shoo fly method because of its perceived benefits. Permits will be required in 2011 from several regulatory agencies before the shoo fly can be constructed. The project team is consulting with regulators as the shoo fly is studied. The process of applying for permits has begun.

The Multnomah County Board of Commissioners will need to approve an agreement for use of additional property needed to build the shoo fly. The board is expected to consider the issue this spring. The County board and regulatory agencies need to approve the shoo fly before it can be built. If the shoo fly is approved, construction would likely begin in December 2011.

Project team members are meeting with stakeholders such as property owners and neighborhood representatives to brief them on the shoo fly concept and identify related concerns and issues.

Shoo Fly Questions and Answers

Is it safe?

The sections of the old bridge that will be moved are the steel spans that are in fair condition. The west approach concrete spans that have the most serious cracks will not be used. A critical task is to reposition the bridge spans in a unified move without damaging them. The project team will need to identify and plan for all risks, as with any construction activity.

Has this been done before?

Shoofly in Elkton, OR

Click to Watch Video
Shoo fly in Elkton, OR.

Shoo flies have been used in Oregon and elsewhere recently. Click the image at right to watch a video about a similar approach to moving a bridge in Elkton, OR. The same contractor and bridge designer that replaced the Elkton bridge are working on the new Sellwood Bridge. Having an experienced team is vital.

How long will the bridge be closed to traffic to build the shoo fly?

The contractor estimates no more than a one-week closure would be needed to implement the detour bridge. The goal remains to have no more than 30 days when the bridge is closed during the entire multi-year project.

When was the shoo fly proposed and who suggested it?

Three of the five teams that bid to construct the bridge this year proposed the shoo fly concept. The concept has also been used in the past by the firm that is leading the bridge design.

Why not build the new bridge in one phase on the south side of the old bridge?

Many people live close to the east end of the Sellwood Bridge on either side. Many additional housing units would need to be acquired to build a new bridge in one phase on the south side of old bridge (if no detour bridge is used).

Don't the temporary features needed for the shoo fly make it a more costly option?

No, the overall cost savings from building the new bridge in one phase far exceed extra costs for the shoo fly.

Would the vehicle weight limit on the shoo fly be the same as on the existing bridge?

The exact structural capacity of the shoo fly will be determined as the design is developed. Because the weakest parts of the bridge will not be used in the detour, it may be possible to raise the current 10 ton vehicle weight limit.