ASPIRE Spring 11 - Aspire - The Concrete Bridge Magazine

The U.S. 95 Spur Bridge over the Snake River features six spans of precast, prestressed concrete modified bulb-tee girders 144 ft long and 84 in. deep.girders. The 144-ft girders are the longest yet tobe used in Idaho, and they were selected whileworking closely with city officials to replicatethe look of the original bridge. A cast-in-placeconcrete railing was provided, along withvintage light standards to create the proper tone.As the 144-ft-long girders indicate, ITD hasbeen pushing to achieve longer spans withprecast concrete girders. They eliminate the needfor piers in the water, which has become a muchlarger concern. Handling and transportingsuch long spans can create its own challenges,however, so a balance must be maintainedbetween effective span lengths and efficienthandling methods.Another complex bridge to be completedthis year is the I-84 Ten Mile Interchange inMeridian, Idaho. The project was singled outby the ITD and the Community PlanningAssociation of Southwest Idaho as the highestpriority for a new interchange in the 2001I-84 Corridor Study and it is finally nearingcompletion. The project will result in a newinterchange and improvements to thesurrounding roadways.The 189-ft-long, single-span bridge consistsof post-tensioned, cast-in-place concrete boxgirders designed as a single-point urbaninterchange (SPUI). The post-tensioneddesign was chosen for its cost effectivenessand constructability at this complex site.The concrete boxes were necessary to providethe span length needed to complete theSPUI geometry and to ensure the design wasaesthetically pleasing. HDR Engineering assistedITD with this project.Segmental Design UsedWhen long-span bridges are necessary, ITDfound that concrete segmental construction iscost effective. One of the largest of those projectsis the Veterans Memorial Centennial Bridge overBennett Bay and Sunnyside Road near Coeurd’Alene, Idaho. It was completed in 1991.The $20-million project produced a1730-ft-long cast-in-place, post-tensionedsegmental concrete box-girder bridge. Thefour-span bridge features end spans of 345 ftand two interior spans of 520 ft. The design, byHNTB, was the most economical approach forthis unique situation, but it remains the onlysegmental cast-in-place concrete bridge on thestate highway system.As several of these projects indicate, ITDworks with outside designers on complexbridges. However, the majority of the bridgesare designed in house. Another significant statebridge designed in conjunction with an outsidedesigner was the Bryden Canyon Road Bridgeover the Snake River in Lewiston, Idaho.Designed by T.Y. Lin International in SanFrancisco, Calif., and completed in 1981, the1750-ft-long bridge is located on a local roadand spans between Idaho and Washington. Thestructure consists of cast-in-place, post-tensionedsegmental concrete box girders. The long-spandesign was used to span the backwaters of a damthat was being raised.Longevity Is Key ConcernWith all of the state’s projects, durabilityand longevity have become key focusconcerns. A primary goal is to ensurethat bridges last longer and require lessmaintenance so the state is better able tomanage its assets. To extend service life, allcast-in-place, post-tensioned concrete bridgestoday are designed for zero tension. Fly ashand other additives are used to add durabilityand reduce the cement content.ITD also has experimented with Type Kcement, a hydraulic cement for use inshrinkage-compensating concrete. It helpsmitigate cracking caused by drying shrinkage.Monofilament fibers also have been used inIdaho uses consulting engineers for complex projects.HNTB designed the I-90 Bridge over Bennett Bay,known as the Veterans Memorial Centennial Bridge.The bridge is the only cast-in-place, post-tensionedsegmental concrete box-girder bridge on the statehighway system. All photos: Idaho TransportationDepartment.ASPIRE, Spring 2011 | 41