Taking the Long Road: Studying Timber Bridge Service Life

Timber bridges are a much larger part of the U.S. transportation system than many people realize. In fact, one-third of the states have more than 500 timber bridges in their inventories, and wood continues to be a viable option for modern bridge construction.

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There is a significant need for more reliable data on the expected service life of highway bridges, as engineers begin to implement life-cycle costs analyses within preliminary bridge designs. Claims are made that the expected longevity of concrete and steel bridges stand at 75 years or more, while timber bridges (which engineers are less familiar with) are estimated to last only 20-30 years. Unfortunately, since little data exists, these claims are not based on actual performance data.

bridgeTo generate more quantitative and unbiased data on timber bridge durability, Forest Products Laboratory (FPL) researchers designed a nationwide study in conjunction with the Federal Highway Administration. The goal of the multi-year team effort was to assess the condition of more than 100 timber bridge superstructures around the U.S. in order to provide a better understanding of their design, performance, and durability characteristics.

Six different inspection teams with members from various organizations conducted field assessments of 132 timber bridges. Selected bridges were required to be along public roadways, be in service for at least 16 years, and have available records regarding inspection, maintenance, and repair. The in-depth inspections included visual, probing, and nondestructive evaluation techniques to characterize the condition of the primary bridge superstructure components.

Results of this study found that timber is a durable option for primary structural members in highway bridges superstructures, and that it can perform adequately for more than 70 years when properly pressure-treated with preservatives.

A comprehensive technical report (currently under review) will include more detailed information about all the timber bridges evaluated in this study, and will provide the basis for the development of life-cycle costs analyses and bridge deterioration rate modeling for timber bridges superstructures in the future.

More information on this study can be found in the Proceedings of the 2015 Structures Congress held April 23-25, 2015, in Portland, Oregon.