Manning Crevice Bridge
Riggins, Idaho
Best Project

Owner: FHWA-WFLHD
Lead Design Firm: Atkins North America
Civil Engineer: Horrocks Engineers
General Contractor: RSCI Group
Geotechnical Engineer: Shannon & Wilson Inc.
Crane & Erection: Inland Foundation Specialties

The Manning Crevice Bridge is only the seventh structure in the world built as a single-tower asymmetrical roadway suspension bridge, and it is believed to be the first one in North America. It replaces a bridge built over the Salmon River by the Civilian Conservation Corps in 1934 that no longer met current standards. A single-lane superstructure was chosen to minimize cost and accommodate the traffic, which includes residents of the canyon and people seeking access to a river rafting launch site. By clearing the rapids with a single span, rafters can safely navigate the area without watching out for piers in the waterway.

Due to the remote location and the desire to create a rustic look, the project team chose Grade 50 weathering steel for the main tower and steel superstructure. The material requires little maintenance, since the densely rusted patina creates a protective coating on the steel. To avoid future painting, which might be harmful to the sensitive ecosystem, the remaining steel components were galvanized. The use of these elements minimized environmental impact over the life cycle of the bridge and reduced maintenance activities over the pristine Salmon River. 

Construction crews were challenged with steep canyon rock faces on both sides of the structure and were allowed no in-river access. Two crane pads were assembled above the river, requiring the widening of a shear rock cliff and precise navigation to deliver the 165-ton crane.

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