Mendocino County, California Bridges: Highway 1 Bridge across Salmon Creek

August 2017 (39.21528, -123.76806) Salmon Creek Bridge

A couple of miles north of the Navarro River is Salmon Creek and the deck truss bridge that crosses over it. The Salmon Creek Bridge is a 5-span, 685 ft long bridge with a 26 ft wide deck. The superstructure sits on two and four legged reinforced concrete towers. This bridge was built in 1950, improved in 1985, and now Caltrans is planning to replace it.

Among the reasons why Caltrans is considering replacing this bridge are:

• Functionally Obsolete - the deck geometry only provides for one foot shoulders.
• Fracture Critical - lack of redundancy in the deck truss and floor beam members, which could result in collapse.
• Structurally Deficient – the bridge deck is in poor condition.
• Low Load Rating – larger permit trucks are not allowed on the bridge.
• Deficient Rail – the existing bridge rail does not meet current design standards.
• Costly Maintenance – Painting is required every 4 to 5 years to protect the steel at a cost of $2.5 million. Spalls and delamination of concrete also need to be repaired.
• Seismically Deficient – the bridge needs to be retrofitted to prevent collapse during a large earthquake. 

Caltrans builds very few steel bridges today. Part of this is due to the high maintenance costs of steel, but it is also because cast-in-place and precast bridges have become less expensive to build. Only when there is a good reason (usually related to quickly building over an obstacle) that steel is used. Also, Caltrans ductility-based seismic design philosophy favors reinforced concrete. Perhaps with the federal push for accelerated bridge construction steel may become more popular again.

Caltrans performed a value analysis for replacing Salmon Creek (and the adjacent Albion River Bridge), held meetings with the community, and created designs for box arch (shown below), spandrel arch, and box girder bridges as options for bridge replacement.

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