Butler Engineering provides bridge temporary works engineering for heavy construction clients. Sometimes a project comes along that challenges our ingenuity and motivates the desire to produce something unique. We need to think outside the box and consider all possibilities.
The Del Rio Bridge provides access to St. Anthony, a small town in Idaho, which has a sawmill and agricultural services for large farms. The bridge had become functionally obsolete. There was advanced deterioration of concrete on the approach spans, piers, bridge deck, and curbs. Other concerns were environmental issues raised by birding and sportsman’s groups, as well as potential lead-based paint contamination from the old steel truss.
The Idaho Transportation Department (ITD) completed replacement plans and made them available for bid during February and March 2014. DL Beck Construction, a general contractor that builds large structures, put together its team of partners, including Butler Engineering for bridge temporary works, Teton Prestress for girder construction and delivery, and Wagstaff Crane.
After studying the project carefully, a bid was prepared that totaled $2.3 million, including bridge and highway approach reconstruction. DL Beck Construction was awarded the project.
During the bidding phase, the contractor’s team visited the site and discussed the evident difficulties and worked out the methods for construction of the bridge. Lots of considerations were included, especially the fact that the river was relatively swift and a high-quality trout stream prized by anglers in the area. The construction window was late April to early November.
We considered how quickly we could get to the business of actually building the new bridge. That meant a swift and early removal of the existing structure since the new bridge was on the same alignment. Eastern Idaho has high-water snow melt runoff in May and June. It was necessary to get in-river work done and coffer dams in place to protect against the high water and the swift conditions at the site before runoff began.
The main span of the new bridge consisted of prestressed concrete bulb tee girders 153 feet long. Large cranes were required, which were not locally available, entailing high costs for mobilization. On previous projects we had used specially designed small launching trusses that operate on rail systems attached to the piers to place large precast concrete girders across the river. While we were talking about the launching truss option, some of the men said, “Why can’t we use this existing truss somehow?” My response was that we ought to investigate it.
Considerations of the capacity of this 1930s truss, materials and equipment needed to relocate it, and the modifications needed to launch these large girders across the river were discussed. The big question from the general contractor was how much will it cost?
ITD provided copies of original plans for the truss, and in cooperation with the state’s contract administration section, we worked out the possibility of repurposing the truss to assist in erection of the new bridge. We developed a STAAD.Pro model of the old truss and did field inspections to check the joints and members to see what condition they were in.
We quickly learned that the truss could be used with some cantilever beams projecting outward from the truss below the major panel points. We developed cost estimates based on crane costs versus preparing the truss to use in launching the precast concrete girders. Using the launching system could save approximately $40,000.
Our conceptual plans were shared with and allowed by the ITD. The main points were that we would remove the approach spans and lighten up the truss by removing the asphalt and concrete deck materials and cleaning the truss down to the bare structural frame. Temporary foundations would be placed in the river and a jacking system developed to lift the bridge and roll it over to the temporary piers.
We sent a survey crew out after developing our conceptual plans in the office to get information on the river bed at the location of each temporary pier footing. The reports from the field indicated the river bed was mostly basaltic rock with small deposits of sand and gravel in crevices and potholes. The general contractor ordered the additional steel needed to construct the temporary piers and transfer girder system. Precast concrete blocks were used for footings and each one was custom built to the correct height. Temporary pier steel and the concrete blocks were fabricated at the contractor’s shop while work was beginning on deck removal.
Work began on the bridge site in late April. As soon as the deck was removed, a temporary work platform was constructed for personnel access only.
The precast blocks were placed inside a super-sack with some fluid concrete and then placed in the river bed. A pneumatic rock drill was used to anchor them to the foundation rock with steel rods. The fluid concrete in the bottom of the super-sack conformed to the river bed underneath each block. Since nothing is perfect, we also allowed for grout pads to be placed between the temporary pier columns and the top of the concrete.
A jacking system using four hydraulic jacks was devised that would have the capability of raising the structure at least 7 feet. The transfer beam needed to be threaded through our jacking system and set on top of the old pier. The transfer beam had a channel rail system on top. Hillman rollers were set underneath the bridge bearing points and jacks were lowered to place the weight on the Hillman rollers and rail system. A hand-powered come along was used to move the bridge.
The bridge was jacked up and set on the transfer beams ready to move. Pipe bracing was provided and anchored back into rock to forestall any lateral movements of the jacking and transfer frame.
Once the old truss was on temporary piers, we began immediately with demolition of the old permanent bridge piers and building the new piers. Concurrently with the construction of these new piers we repurposed the old truss to prepare it to launch the girders. Plans were developed to modify the truss and use it as a launching platform to run precast concrete girders across the river. Cantilever beams were attached at panel points across the bridge, projecting out on the side with the girder transport rail. A rail the full length of the steel truss was installed on the exterior of the bridge.
The center span of the new bridge had long slender girders that were sensitive to weak axis bending. Problems with handling were experienced. Unknown to the engineer, additional steel beams were added on top of the precast girders to remain in place until temporary cross braces were securely in place after the girders were on the piers. The final transport width was 8 feet and the weight was 140 kips, of which 21 kips was temporary stiffening steel. We are pleased that we had a safety factor.
A skate was developed to carry the girder across the bridge which used six sets of Hillman rollers, four for vertical loads and two for horizontal positioning. A laser level on the skate was used to develop the grade on the delivery road that brought the girders close to the bridge for crane pickup. A horizontal rail attached to the truss frame stabilized the girder against any lateral movements.
HDPE tanks were used for ballast to balance the bridge against the concentrated cantilever load on the exterior of the frame. These plastic tanks were pumped full of water out of the river just prior to launching the girders. After the girders were all in place, spigots were opened and the water ran back into the river.
After the girders were in place, we built the new superstructure, except for railing and decorative columns. The old truss was then jacked up an additional height on the temporary piers, rolled over to the new bridge deck, and set on trucks to be hauled off. The new bridge was completed and ready for service in early November 2014.
Robert Butler, P.E., a self-employed consulting civil engineer for the last 23 years, organized with a group of engineers, draftsmen, and survey crews and established a reputation as bridge engineers, building large highway and bridge projects in Idaho, Montana, Wyoming, Utah, California, and Arizona. In 2015, he merged Butler Engineering with Harper-Leavitt Engineering. The Del Rio bridge project was recognized as a finalist in the Innovation in Structures category in Bentley 2015 Be Inspired Awards.