Mendocino County, California Bridges: Highway 1 Bridge across Big River

September 2017 (39.3017, -123.7921)Big River Bridge

Continuing north on Highway 1 we arrived at Big River and the bridge that carries the highway across it. Walking along the river we saw people with their dogs along with joggers and hikers. The bridge is just south of the town of Mendocino and so a large community of people use the trail under the bridge to get their exercise.

The history of Big River is about entrepreneurs who built sawmills, railroads, and bridges along the river to harvest and sell the forest of redwoods that once covered this region. All of that activity has disappeared without a trace.

The Big River Bridge (10 0146) is a three span box girder structure on single column bents (built in 1962) with a 190 ft long center span. In 1994 the bridge columns were wrapped in steel casings to protect them from earthquakes. Unfortunately these casings soon began to rust due to the marine environment.

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

August 2017 (39.2264, -123.7660) Albion River Bridge

I wrote a year ago about the Albion community's reluctance to replace the aged Albion River Bridge. It's amazing all of the issues raised by a community of 200 people over this bridge replacement. Each individual has their own unique concerns about this project. How will the replacement affect the use of the harbor? Will fishing operations be affected? Will the influx of workers to build the replacement displace residents? What are the costs of repair versus replacement?

Whether the old bridge is rehabilitated or a new bridge is built, it will have a big impact on the community. I wonder how one could take this old bridge and repair/seismically retrofit it without replacing most of the members? This is an issue that is being addressed all over the world. How to preserve our past while preparing for the future? In the old days the government would just come in and do what it wanted but I think it's a good thing that the community gets a voice in these decisions.

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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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