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Last Updated: Thursday, January 5, 2012 11:59 AM

Historic Highway Bridge Inventory

Bridge Inventory Online

Background

In 1984-1986, Caltrans conducted a statewide inventory of highway bridges with some potential for historic significance. Many old bridges need to be rehabilitated or replaced every year, and virtually all of these projects are federally-funded and subject to federal historic preservation laws. In the 1970s, the Federal Highway Administration (FHWA) recognized that it was inefficient to continue to evaluate each historic bridge on a case-by-case basis. It encouraged the various state highway agencies to survey bridges on a comprehensive basis.

Methodology

Caltrans' master inventory of bridges included 23,000 structures. By eliminating those that were built within the past forty years, and those that were not highway bridges per se, but rather culverts, tunnels, railroad bridges etc., the number of bridges needing to be surveyed was substantially reduced. One thousand bridges were identified as having some potential for historic significance; these made up the survey population. A four-step survey process consisted of background research on bridge building in California, data gathering on each bridge in the survey, developing a quantitative evaluation system, and evaluating each bridge according to the system.

The quantitative evaluative system was derived in large part from previous surveys in other states but adapted to meet California circumstances. Developing the system involved identifying attributes that might define significance, then assigning a weighted value to each attribute. Age, for example, is commonly taken as an indicator of significance; rarity is another. A very large bridge is usually valued over a small bridge because it is a more dramatic achievement in engineering science. Bridges are also valued when they are beautiful, designed by a famous engineer, representative of innovative technologies, important to local transportation history or other local historical trends, or when they possess unusual ornamental or structural features.

Special conditions in California were also considered, such as for example, a high incidence of historic concrete bridges. We developed separate systems for the major concrete bridge types (arches and girders), in addition to that for metal truss bridges.

The final step in the inventory and evaluation process was to apply the evaluation system to the survey population. To ensure consistency is applied, Caltrans developed guidelines for applying the criteria, which were as specific as possible, particularly with respect to the more subjective categories. When the guidelines were applied to all the bridges in the survey population, a computer database program generated lists of bridges ranked numerically.

The distribution of bridges on this list was predictably bell-shaped, with a small number of excellent bridges scoring very high, a number of bridges scoring very low, and a lump of bridges in the middle. Very low-scoring bridges were eliminated from further consideration, while very high-scoring bridges were placed on into the National Register-eligible category. The middle group was re-evaluated by a team of evaluators, including representatives form Caltrans, FHWA, and the California Office of Historic Preservation. The re-evaluation relied on the point system as well as other considerations which may not have been reflected in the numerical evaluation system.

Results

This process resulted in finding 190 bridges eligible for the National Register of Historic Places. This number was added to the 90 bridges previously found eligible for the National Register. Now, when a project area includes a bridge, or when a bridge needs rehabilitation or replacement, the status of the bridge is learned merely by consulting the inventory.

Additional information about the Historic Highway Bridge Inventory can be found in the book that resulted from the survey Historic Highway Bridges of California.