How to earthquake-proof a 100-year-old bridge

Posted by Derek Mason

21st May 2024

Picture credit: Frank Schulenburg

Back in 1922, nobody was building Californian bridges to withstand earthquakes. But today, design standards for bridges are more stringent, leading to projects like the seismic retrofit of the 100-year-old Wohler Road Bridge, near Santa Rosa city in Sonoma County.

Connecting two main “wine country” roads and traversing the Russian River, the one-lane Wohler Road Bridge is one of the few remaining bridges from the 1900s that’s still being used in California. The state generally gets two or three earthquakes each year that are large enough to cause moderate damage to structures – magnitude 5.5 and higher.

Rather than wait for an earthquake to strike and render the bridge unusable, it makes sense to prevent a predictable problem. The fact is, there would be no way for people to cross that section of river if the bridge were to be demolished by a tremor – they’d be looking at a lengthy detour.

The $18 million (£14.2 million) project to strengthen the bridge against quakes will include adding a new lightweight concrete deck, seismic isolation bearings and joints, and steel truss reinforcements, along with foundation reinforcements. Extensive environmental, geotechnical, hydraulic, and hydrological studies had to be undertaken before work could go ahead on the landmark bridge.

It’s always interesting to see what architects, designers and engineers have to contend with in different parts of the world. It clearly makes sense to extend the life of this structure, but it still takes a lot to make this kind of project happen. From the outside, it appears to have taken years of campaigning from various neighbourhood groups, utility companies and other agencies and individuals to get the go ahead on this.

It’s a good reminder that if you want something to change locally, often you need to mount a lengthy campaign and be prepared to see it through over a protracted period of time. This seems to be the case no matter which side of the pond you’re on. The Wohler Road Bridge construction work is due to begin this summer and should be finished by the end of 2025.

How about you? Are there any projects going on that need input from a structural engineer? If you need assistance with the structural elements of an upcoming project, please do get in touch.

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