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SLIDE IN BRIDGE 8-19-15 LMS

Greg Gifford, Project Manager for the Federal Highways-Western Federal Lands Highway Division, introduced the Salmon City Council to a unique way of building bridges known as the Slide In Bridge.

He spoke to the council at its August 19 meeting and updated the city on plans for Shoup Bridge installation. The structure is also known as the Williams Creek Bridge since that’s one area where it eventually leads. The fact it leads and provides access to federal lands is what ultimately provided the funding.

Gifford recapped the history of the project that has taken many years to finalize. He said the present one lane bridge is known as an accident waiting to happen. An application was made to a Forest Highway Program four years ago and was granted. Then Congress replaced that program with the Federal Lands Access Program (FLAP) and the application process had to be repeated. Gifford said the application was approved but came with the requirement of a local agency match. Through a combined effort between the Forest Service and Lemhi County the match was met.

The logistics of the bridge, according to Gifford, are making the project challenging since to interrupt current traffic flows would require a 105 mile detour. He said choosing a location for the replacement bridge was also interesting. If it were to be located upstream it would mean moving the Bureau of Land Management Campground which was not a welcomed idea. Downstream to the north involved a piece of private property and also a large riparian area so that was not popular either. It came down to putting the bridge in the same place as the old one. Gifford said the big question then became how do you replace a bridge with the existing bridge still in place?

The resolution is to place a temporary pier upstream of the existing bridge and building a work bridge on top of it. Eventually traffic will be diverted to the new crossing while the old bridge is demolished, new abutments are built and the two supporting piers now in the river are removed. Gifford said removal of those piers is a priority with river users as well as with environmental agencies.

The new bridge, the entire structure, will then be slid along rails on each side of the river and deposited in the empty space left by the removal of the old bridge. The plan is to schedule the slide-in over a weekend and for the work to be done within a time frame of 48 hours. Total coordination with emergency services is another part of the 48 hour closure plan. Gifford said whether the closure will take place on a Friday/Saturday or a Saturday/Sunday will depend on the public comments received at a meeting that was taking place that evening.

Other constraints that had to be faced and which led to the decisions he described included the bridge’s proximity to the highway and the length of span. Gifford said the grade on both ends of the bridge is flat. To span the 225 to 230 foot length would require a girder from ten to eleven feet deep. He said since the present bridge is a three span the girders are only four to five feet deep. They again conferred with National Marine Fisheries and the U.S. Fish and Wildlife Service and asked how important it was to not have a support in the river and were told, “…pretty important.” Reasons given included; no pier would open the channel, there are about four listed species of fish in the river and it would also be important for boaters.

The solution to the support problem will be a lattice arch bridge. Gifford said it will be a steel arch about 30 feet high that will span the river. The deck will be suspended by cables attached to the arch. Gifford said that way a very low profile can be kept and the bottom of the new bridge will be the same height as the present one so there will be no reduction of clearance under the bridge.

Stacy Thomas, Public Information Specialist, said there have been two public meetings. She said about 40 people attended the first meeting which took place a year ago and around 30 people attended a public meeting last February. She said she was sure the issue of having emergency services people available on the west side of the river during the closure was going to be one of the main topics of discussion during the third public meeting which was taking place that evening.

The design of the bridge is now 70 percent complete. Completion is expected by the Spring of 2016 at which time the process of contracting the work will begin. Actual construction will start next Summer and last for approximately two years.


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