ASPIRE Spring 11 - Aspire - The Concrete Bridge Magazine

STATEAlong with many other states, Idahofaces challenges in managing itsbridge assets by cost effectively extendingtheir service life or replacing them. Forbridges with spans of less than 130 ft, theneed for economical, durable, and quicklyconstructed bridges usually makes prestressedconcrete the first choice. Innovativeoptions, especially with accelerated bridgeconstruction (ABC) techniques, also haveproven to work well with precast concrete.Precast, prestressed concrete bridges havebecome Idaho Transportation Department’s(ITD) first choice for a variety of reasons,particularly because they usually provide thelowest first-cost solution. But they also offerlow, long-term maintenance costs due totheir high durability. They provide flexibility,offering many cross sections to choose fromand providing a range of ways they can beused.Precast concrete bridge components arereadily available in the state, and severalfabricators regularly supply products. In somecases, for bridges with spans under 30 ft, ITDuses cast-in-place or precast concrete boxculverts or three-sided structures.Generally, the ITD undertakes 50 to 60bridge projects each year, but they vary widelyin their scope. Only about 10 of those projectsSpanning thePast and FutureIdaho Looks to Preserve Existing Bridges whileExpanding Capabilities for New Structuresby Matthew Farrar, Idaho Transportation Departmentare complete replacements for existingstructures. The rest are rehabilitations,widening, and upgrades of all types. Keepingthe existing bridges accessible and functionalis a key part of the work.Rainbow Bridge RestoredAn example of rehabilitation work is theState Highway 55 Bridge over the north fork ofthe Payette River. This structure, known as theRainbow Bridge, is a 411-ft-long, open-spandrel,cast-in-place concrete arch bridge. Built in 1936,it is now on the National Register of HistoricPlaces and has become well recognized in thestate. It’s definitely considered a keeper, andgreat efforts have been taken to preserve itsstructural integrity. In 2007, it was completelyrehabilitated. Techniques included chlorideextraction and the installation of materials tomitigate corrosion in the future.A few components were replaced during thework, such as the end portion of some stringers.Also, the existing railing had deteriorated,so it was demolished and a precast concreterailing replaced it. As this design had to perfectlyreplicate the original, it was fabricated in aprecast concrete plant and shipped in segmentsto the site to provide better control over itscharacter and quality. All of the work was donein stages, to allow vehicle access to the bridgeThe I-84 Ten Mile Interchange in Meridian, Idaho,features a 189-ft single-span, cast-in-place concretebox girder. A single-point urban interchange (SPUI),the post-tensioned design provided cost effectivenessand constructability at the complex site.throughout the construction. The local officeof CH2M-Hill assisted ITD with the restorationproject.Snake River Bridge CompletedAt the other end of the spectrum on bridgeconstruction is the new U.S. 95 Spur over SnakeRiver near Weiser, Idaho, which was completedin 2010. The $10-million project replaced abridge originally built in 1950, providing widerlanes (a total of 46 ft 4 in. versus 30 ft 6 in.) andcapable of carrying greater traffic volumes.The 876-ft-long bridge features six spans ofprecast, prestressed concrete girders comprising144 ft-long, 84-in.-deep, modified bulb-teeRestored in 2007, the Rainbow Bridge on StateHighway 55 over the north fork of the Payette River ison the National Register of Historic Places.40 | ASPIRE, Spring 2011