Historic Bridges of San Luis Obispo County
by Andrew Hope
Caltrans efforts to meet Federal historic preservation requirements included a survey of the bridges of the state. Of those in San Luis Obispo County, some are considered important for reasons other than their strategic locations in the daily geography of our travels. Andrew Hope examines those in our county that mark new engineering methods upon which many bridges throughout the state have been constructed. These are pioneering designs, remarkable for their origins and development from new materials by the creative engineering minds of our state.
The California Department of Transportation (Caltrans) owns and maintains more than 12,000 bridges on state highways, and has inspection responsibilities for an additional 12,000 bridges on city streets and rural roads. In compliance with state and federal historic preservation laws, Caltrans has surveyed the state’s bridges to identify those that are eligible for listing in the National Register of Historic Places. These surveys were carried out to identify the state’s most significant historic bridges in advance of projects, and to streamline compliance with historic preservation laws. This process yields careful thought into how historic bridges are rehabilitated, allowing for the preservation of features important to the communities they serve. As a result of these efforts, Caltrans has identified almost 350 bridges and tunnels that are eligible for listing in the National Register of Historic Places.
In San Luis Obispo County, there are 370 roadway bridges, of which nearly half were constructed before 1960 and were therefore included in one or both of the statewide bridge surveys. Four of the county’s bridges are eligible for National Register listing: three metal trusses and a concrete T-beam bridge.
Arroyo Grande Creek
All three of the metal truss bridges were designed by County engineer Austin F. Parsons. The Bridge Street crossing over Arroyo Grande Creek in the town of Arroyo Grande, constructed in 1908, is the oldest of the three (Figure 1). It is a steel Pratt truss with a span of 100 feet. This truss type is distinguished by lighter diagonal members that carry tension forces and larger vertical members that resist compression. Once a common bridge type in California, most of these smaller metal trusses have been replaced by concrete bridges. As an alternative to replacement, the Arroyo Grande bridge was strengthened in 1990 with the installation of a supplemental metal truss below the roadway, thus preserving the historic appearance of the structure.
The two other San Luis Obispo County metal truss bridges are Parker trusses. This truss type is a variation on the Pratt truss in which the top of the truss, rather than being horizontal, has a convex polygonal shape. The Salinas River Bridge is located east of Santa Margarita, about 500 feet southeast of the current State Route 58 alignment. It is on a former segment of the state highway that was bypassed by a new concrete bridge in 1998. Built in 1914, this bridge has a span of 160 feet.
The Las Pilitas Road bridge, also crossing the Salinas River, is located southeast of Santa Margarita (Figure 2). It was built in 1916 and has a span of 150 feet. These two bridges are among the oldest remaining Parker Truss structures in California.
Located in the city of San Luis Obispo, the Marsh Street Bridge over San Luis Obispo Creek is a concrete T-beam structure completed in 1909. It was designed by the San Francisco engineer John B. Leonard, one of the pioneers in the development of reinforced concrete bridges. Prior to the construction of the Marsh Street Bridge, almost all of the state’s reinforced concrete bridges were arch structures. Concrete T-beam and slab bridges began to be constructed in California about 1907, as engineers figured out the most effective ways to use steel reinforcement for spanning structures. The Marsh Street Bridge is therefore one of the state’s earliest examples of a reinforced concrete T-beam structure, and is the oldest surviving example of this important type south of the San Francisco Bay area.
Because they are eligible for listing on the National Register of Historic Places, these four bridges are afforded some additional consideration when planning roadway projects. As with the Arroyo Grande bridge, alternatives to replacement are sought that will provide a safe and functional crossing while retaining the bridge’s historic character and appearance. Future bridge surveys may lead to additional San Luis Obispo County bridges being determined eligible for National Register listing, as the number of surviving older bridges continues to diminish.
Andrew Hope is an Architectural Historian with Caltrans in Sacramento. He has degrees in architecture from the University of Michigan and the University of Wisconsin at Milwaukee. Andy has worked in the field of historic preservation for 19 years, including 15 with Caltrans.