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Taking the Long Road: Studying Timber Bridge Service Life

Rebecca Wallace
Forest Product Laboratory
December 19, 2015

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.

Tangipahoa Parish, Louisiana.

More reliable data on the expected service life of highway bridges is a significant need, 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 stands at 75 years or more, whereas timber bridges (which engineers are less familiar with) are estimated to last only 20–30 years. Unfortunately, because little data exists, these claims are not based on actual performance data.

To 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 United States 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, have been 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 development of life-cycle cost analyses and bridge deterioration rate modeling for timber bridge superstructures in the future.