ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/306888
36 | aspiRe , Fall 2008 Access to a new upscale development in the foothills of Utah's Wasatch Mountains just outside Salt Lake City required a grand welcoming entrance, along a path with difficult terrain. To meet the variety of needs produced by the site, especially environmental concerns and functional demands, two reinforced concrete bridges located on "switchback" curves were built. The lower bridge, discussed in this article, was an exceptionally challenging five- span, 450-ft-long bridge with a complete switchback curve along a centerline radius of only 80 ft and a 12% grade. Access to the site was limited by steep grades, limits of a conservation easement, and a requirement to provide for wildlife crossing. The project also was located less than 1000 ft from the Wasatch Fault. Typically, the most economical solution would involve building large walls and excavating steep slopes into the existing hillside. But foothill preservation and concerns from county officials, coupled with the desire to create an aesthetically pleasing entrance, led the owner to investigate nonconventional alternatives. Once the concept for the switchback curves was devised, a series of design challenges developed. The curve was too severe for tangent girders without reducing span lengths to less than 30 ft, while curved steel girders were not economical at such a small radius. Designers determined that the most feasible bridge type was a conventionally reinforced, cast-in-place concrete, box girder bridge. The girders could not be post-tensioned due to the severe curvature, which would have made it difficult to resist the bursting forces in the walls of the boxes. profile BIg CottonWood CAnyon loop roAd BrIdge / salT lake CiTY, UTaH engineer: Michael Baker Jr. inc., Midvale, Utah prime ContraCtor: Ralph l. Wadsworth Construction Co., Draper, Utah ConCrete supplier: Harper Ready Mix, salt lake City, Utah AwArds: 2008 Portland Cement Association Bridge Award by Jim Deschenes, Michael Baker Jr. Inc. Concrete girders offer only alternative for five-span bridge with severe horseshoe bend BOX BEAMS Bend BrIdge After considering various options, designers decided on a five-span configuration with integral bent caps to create a smooth, single superstructure depth. All photos: Michael Baker Jr. Inc. ASPIRE_fall08.indb 36 9/15/08 4:23:46 PM