|LESSONS IN BRIDGE BUILDING|
NEXT STEP TAKEN FOR NEW BRIDGE 2-23-15 LMS
The bidding process on the crane work needed to install Rattlesnake Bridge beams from one side of the Salmon River to the other begins this week. At their February 23 meeting, the Lemhi County Commissioners set a bid opening date of March 16 at 2PM. The beams will span a distance of 190 feet and support a 300,000 pound $500,654 bridge structure to be built by TrueNorth Steel. That structure is scheduled to be delivered to the new bridge site, located just north of the old one and approximately 16 miles south of Salmon, on May 22, 2015.
The commissioner’s decision came after getting an update from project head Dan Sharp who is with Northwest Engineering of Salmon. He said specifications that will be listed in the bid will include the crane having a reach of 95 feet. The cost will consist of a mobilization fee which is the most expensive component, a rate per day on-site and an hourly rate for the crane operator. He said another specification is that the company be fully bonded and insured. All told, the crane contract is expected to equal approximately half of the overall $1.2 to $1.5 million estimated cost.
The bidding process on building the bridge abutments will begin shortly and is of prime importance since those abutments have to support a crane weighing close to 600,000 pounds. Sharp said the pressure exerted on the abutment pads by the crane’s weight will be far greater than anything they will ever experience again.
The technique selected for building the abutments involves a series of heavy duty wire mesh baskets in which fill material is compacted. He said compaction is a key element and for that reason is tested many times during the process. That part of the project is contracted. The fill material will be hauled by the county. Sharp recommended that Road and Bridge crews as well as the commissioners observe and learn the abutment building process for use on future county bridges.
Removal and disposal of the old Rattlesnake Bridge will be offered as an alternate bid and as a chance for the selected contractor to make more money while the crane is on-site. The bridge was condemned last year leaving no way for vehicles to get from one side of the river to the other.
Sharp said possible removal options would include the large crane being walked down the highway to the old site where it would simply lift the structure off its abutments and swing it to where it can be dismantled. State Highway Department permission and proper pavement protection would be required. Another option is to place smaller cranes on each side of the river, using the new bridge as access, and then chop off sections as it is moved across the river.
The size crane required will be hauled to the site by 27 trucks. Each of the cranes massive tracks comes on a separate truck and its body comes in several pieces all to be assembled on-site. When the crane is in operation traffic control will be necessary on Highway 93 south sine part of the crane will be blocking the roadway. Sharp is hoping the crane will only be needed for a few days since once here the daily and hourly $ meter will be running.
The bridge beams will arrive on two trucks in 100 foot and 50 foot sections. The center beam will be assembled and put in place first. It will have cross pieces which will be connected to two side beams thus protecting the girders from tipping in strong winds. That process is expected to take a couple of days and is very critical in terms of worker safety and equipment performance.
Sometime hopefully well before a May 1st high water deadline, a loader and excavator will be walked across the river to where they will be building the west side abutments. A permit has been acquired for 300 yards of material from a Bureau of Land Management pit. The agreement is that the material be used for the abutment construction only. Materials to build the bridge’s side ramps will be supplied by the county and can be hauled across the new bridge rails and deck which will be in place by that time.
Sharp said the 100 year flood level has been identified and that the bottom of the new bridge would be three feet above that level. In a normal high water season the bridge will have eight and a half feet of clearance and during normal water level seasons there will be a 12 ½’ clearance. Plans call for a three percent slope to the bridge from the east side to the west side.
Sharp said a final set of Northwest Engineering drawings will be completed by the end of this week. He said all elements, the abutments, the crane and the bridge, are sitting on the cusp of coming together.
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