Institute for Transportation

The early American settlers used wood as the principal building material for their bridges. Although wood was slowly replaced by iron, steel, and concrete as building materials in the late 19th century and much of the 20th century, timber bridges continued to be built. However, not until the start of the Timber Bridge Initiative in 1989 has there been a renewed interest in wood as an alternative bridge material. The thrust of TBI and the ISTEA programs is to explore innovative methods for using wood for transportation structures. The early American bridges were covered bridges; however, with the development of improved wood preservatives, covering bridges is no longer necessary. Currently, there are effective preservatives that are approved by the Environmental Protection Agency, and through research, treatments are being extended for use on types of timber rarely used previously for bridge construction. Because wood is a biological material, it has many material and strength variabilities, both among and between species. Through engineered wood products, such as glued-laminated timber and structural composite lumber, many of these variables are substantially reduced. Wood is resistant to deicing chemicals and freeze/thaw cycles. If properly designed and constructed, timber bridges can carry the same loads as bridges built of any other material and will provide many years of acceptable performance. At this time, timber bridges are feasible for short- and medium-span structures, and research is being conducted to develop wood systems for longer span structures.