Historic Oak Grove Bridge undergoes safety evaluation


A team of structural engineers, Cal Trans inspectors, and Tulare County Resource Management Agency (RMA) road workers converged upon the Oak Grove Bridge on Tuesday, Jan. 6, to conduct a field assessment of the historic structure. The aging arch bridge, determined to be eligible for the National Register of Historic Places, is believed to be one of only a handful of the smaller type remaining in California.

“I had no idea that this 1923 bridge was even here on Mineral King Road,” said Brad Pollock, a structural engineer with Parsons Corporation. “I’ve studied the arch bridges that were retrofitted in Big Sur in the 1990s and those were built a decade later.”

Pollack said that after a cursory inspection and some initial core samples were collected, the Oak Grove Bridge, located 6.5 miles up the Mineral King Road, appears to be in pretty good shape. A report that Pollock’s office in San Francisco will help produce should be ready in a few weeks and will contain findings, alternatives, and recommendations on how the County of Tulare should proceed.

“We are well aware of the Oak Grove Bridge and its historic significance,” said Eric Coyne, Tulare County film commission and tourism director. “The purpose of the study is to make sure the structure is safe and preserve as much of the old bridge as possible.”

Surviving stone arch bridges in Europe date from Roman times, and other examples may be still found around the world and in the U.S. Pollock said the Oak Grove Bridge is unusual in California because it dates from the 1920s and has not been extensively remodeled.

Pollock said preservation of the Oak Grove Bridge is the preferred option. How much work needs to be done to ensure the safety and integrity of the structure will be contained in the upcoming report.

To facilitate the inspection of the underside of the structure, Cal Trans loaned the use of a truck with a basket crane. From a perch in the basket, an engineer with a rock hammer inspected numerous locales for any signs of deterioration or obvious stress.

Samples were taken from the platform and supports using a concrete core drill. Ideally, a core sample needs to be at least three times or twice as long as the diameter of the sample.

“If the samples are brittle or crumble easily that might suggest a loss of integrity of the material,” Pollock said.

Cornerstone Structural Engineering Group of Fresno is the lead consultant to the county on the project. Kleinfelder (geotechnical) of Fresno and the Parsons Corporation (seismic analysis) of San Francisco are Cornerstone’s principal subcontractors.

Cal Trans is the lead agency and authority on any bridge project in the public domain of the state of California.      

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