This paper contains a review of literature on timber bridges. It presents recent developments and evolving concepts in timber bridge technology, including aspects concerning wood material, bridge design, construction, inspection, rating, and maintenance. This review indicates that timber bridge technology has advanced with the design and construction of a prototype of a prestressed, laminated timber bridge in Ontario, Canada. In the United States, the main ef-fort of government and the timber industry is to promote timber bridge technology transfer.

Timber was probably the first type of material that humans used to construct a bridge. Although concrete and steel replaced wood as the major materials for bridge construction in the 20th century, the use of wood in short-span bridges remains as great as ever. Of United States bridges that have a span of more than 20 ft (6 m), 12.6 percent (or 71,200) are made of timber. In the Forest Service (US Department of Agriculture) alone, approximately 7,500 timber bridges are in use, and more a. re being built each year. The railroads have more than 1,500 mi of timber bridges and trestles in service. In addition, timber bridges recently have attracted considerable attention from many international organizations and foreign countries, including the United Nations, Canada, England, Japan, Kenya, and Honduras (1-3). Timber is a highly desirable raw material-b; cause it is an abundant renewable resource. It has several advantages as a material for bridge construction. Timber bridge structures present a natural and aesthetically pleasing appearance, particularly in wooded surroundings, The timber sections can be constructed in any weather, including cold and wet conditions, without experiencing detrimental effects. Timber bridges cannot be damaged by continuous freezing and thawing and are resistant to the effects of deicing agents. Because of wood's energy-absorbing ability, timber bridges are also able to sustain overloads for short periods of time. The light weight of timber allows for. easier fabrication and construction since smaller equipment is needed to lift the beams into place. A timber bridge's light weight also benefits repair and rehabilitation efforts including superstructure replacements because abutments can be reused and the available load-carrying capacity of the remaining existing structure can be increased. Initial and maintenance costs~ f timber bridges are lower than for most other alternatives and. are ce-rtainly competitive with the materials usually considered to be best (4). For example, a pres tressed, treated timber bridge costs only twothirds of its counterpart constructed with conventional steel and concrete (5). Wood does have several-shortcomings as a bridge material. First, because wood is a biological material, it is vulnerable to damage by fungi, fire, accidents, and insects. Second, the deeper beam sections may significantly reduce the hydraulic operation, reducing the flood flow capacity beneath the