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BRIDGE STYLE SELECTED 4-1-15 LMS

The Salmon City Council has voted to give the new Island Park Bridge the same look as the old one. After hearing the list of bridge style options presented by Dan Sharp of Northwest Engineering Services of Salmon the council agreed a Girder Bridge best suits the need.

The other options researched and presented included a Truss Bridge and a Suspended Bridge. Sharp said the Truss Bridge is the most familiar in this part of the country and represented locally by the old Rattlesnake Bridge on US Highway 93 South. He said that while those bridges are the lightest weight and therefore least expensive, the assembly costs and maintenance costs are quite high. As evidenced by the Rattlesnake Bridge, truss bridges with their maze of bolts and joints are prone to erosion and weak spots and are generally falling out of favor due to maintenance costs.

Sharp said the most famous of all suspension bridges is the Golden Gate. He said there are some new design concepts coming out for suspension bridges that force bridge members into a compressive effort as a way of holding the bridge together. Tension members from the top arc of the bridge to the structure’s deck complete the picture. Sharp said they are very attractive bridges and they are also very expensive to build due to critical tolerances required from all the components.

He said based on all of the above, Northwest Engineering Services’ recommendation is to go with the same style as the one chosen 55 years ago which is when the present Island Park Girder Bridge was built.

A girder bridge goes mostly unnoticed due to the fact there is no above deck structure however, according to the engineer it is very efficient for short span bridges and is frequently used on farms and ranches.

Sharp said a girder bridge will be the simplest and the most economical to construct. He said, “Material costs will probably be a little higher than what you’d find with a truss style bridge but, the end result is that you can swing this bridge into place in a day versus having a crew out assembling a bridge over time.”

In answer to questions from the community about finding and using an old bridge he said he is open to the possibility but hesitant that it is the right approach. He said getting a bridge built 10-20-30-40 years ago approved for today’s load requirements can be challenging. In addition, a required analysis on a rivet or bolt style bridge would be very costly. His recommendation was to select a girder style bridge and work out the economic differences between steel or concrete girders.

The next step will be to identify the normal high water level which has to be correlated with data already on hand. He said there is an environmental regulatory advantage to staying outside the high water marks. Once that is known, the needed length of span will be known.

The present bridge is approximately 48 feet long. The narrowing of the river at the bridge abutments caused water velocities to increase and resulted in the abutments being scoured and undercut.

The council vote for Northwest Engineering Services to proceed towards a girder type structure was unanimous.

Northwest will now begin contacting steel and concrete bridge manufacturers to get updated information of what each company has to offer.




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