By David Goodyear
PROJECT: Hoover Dam Bypass
OWNERS: States of Arizona and Nevada
COST: $240 million over six contracts
DELIVERY METHOD: Design-bid-build
CONSTRUCTION: January 2005 – November 2010
PROJECT MANAGEMENT TEAM:
A quarter-mile downstream of the Hoover Dam, a 1,060-foot bridge hovers 900 feet above the Colorado River. The seemingly weightless structure is North America's longest twin-rib concrete arch.
It's also the centerpiece of a much larger project to connect Nevada and Arizona on one of the Southwest's most congested stretches of highway: U.S. 93. Opened in 1935, shortly after the dam's construction, the single-lane road has been the major commercial route between Arizona, Nevada, and Utah since then, carrying 14,000 vehicles — double the volume 15 years ago — daily.
DESIGNING AROUND TERRAIN
Long-span bridges are unique, expensive assets that require innovative design approaches for the factors affecting performance: cost, aesthetics, construction, and operations. Designers of the bypass project had almost no reference to other projects with similar requirements, making the project a unique challenge.
The terrain on the Nevada rim varies, with rock outcroppings and fault lines traversing the canyon walls. The unique topography funnels wind at gusts up to 60 mph through the bridge site along the alignment of the gorge, as well as through the rugged terrain on either rim. Foundation excavation required rock-blasting of approximately 60,000 cubic yards in the vicinity of an operating roadway and operating dam facilities. Construction of the new bridge had to take place nearly 900 feet above the canyon floor and 800 feet out from the canyon rim.
The first challenge for design of the new bridge was to select the right structural system. The process was guided through two focus groups. The technical issues were presented to a structural management group (SMG) composed of the state and federal bridge engineers as well as peer review consultants. The aesthetic issues were presented to a design advisory panel (DAP) made up of state historic preservation officers, the National Park Service, Bureau of Reclamation, Native American representatives, and architectural consultants.
A comprehensive review of potential structural systems was evaluated by the SMG. The unique character of the Hoover Dam site allowed designers to focus on the two most logical options — to traverse the canyon with a suspension bridge or an arch. With Hoover Dam defining the special character of the site, both focus groups recommended a deck arch.
Although the bridge site has had little recent seismic activity, the size of the project led designers to consider site-specific criteria for seismic design. The work predated the new American Association of State Highway and Transporation Officials (AASHTO) Guide Specification for Seismic Design of Bridges, but the site-specific process completed in 2002 resulted in a project criterion very similar to the AASH-TO criteria approved in 2007. The team also used a similar approach of developing site-specific criteria for wind design.
The framing system for the arch was tailored to meet demands of wind, earthquake, and construction logistics. Twin arch ribs would allow designers the option of providing ductile links between ribs for enhanced seismic performance. Twin ribs would also lessen the size of arch elements during construction and provide greater flexibility for geometry control as the arch was cast.
Wind demands could be lessened with a chamfered box section. As the design evolved through the various stages of development, the twin-rib arch section was selected to provide the best balance of characteristics that met the needs of the site and structural demands.