The first bridges ever constructed by man were probably made of wood. However, the first large scale use of treated timber for engineered bridges was by the railroads. Railroad engineers recognized treated timber as having a number of advantages over other types of materials. This led to the construction of literally thousands of timber trestles and bridges. Attributes of timber which were recognized by the early railroad engineers are still valid today, but many practicing engineers today are not fully aware of those advantages.
The use of timber for highway bridges, in the early years of our nation, evolved for different reasons than those which motivated the early railroad engineers. The railroads received subsidies from the federal government in exchange for their push westward across the continent. They received both land and lucrative contracts for service. The companies themselves risked very little of their own capital and were therefore able to design and construct excellent rail systems without undue financial restraints. The result was a rail system with well engineered structures which were designed for strength and durability. Most of the structures lasted well over one hundred years. Many are still in service today.
Early road systems were built and financed by the early settlers themselves with no federal assistance and often without even the existence of state and local units of government. It is truly remarkable that many of the early settlers were even able to survive much less build their own system of roads and bridges. The design, or rather the lack of design and engineering was evident. Timber provided them with quick and easy crossings for the streams and rivers. Timber bridges have the ability to withstand great overloads for short periods of time. Even without the benefit of engineering effort, the early bridges served the public well. However, most of the bridges were constructed from untreated native timber. Untreated timber has a relatively short life and therefore timber bridges were not appreciated for their essential contribution to the development of our nation, but were remembered for not being durable.
The timber industry is not controlled by several large companies or represented by a large industry wide promotional organization. Steel and concrete industries have large marketing organizations to promote their respective products, who can do necessary research and publish the results. Most engineering schools use books produced by an industry organization. Educational institutions have been slow to recognize the advantages of treated wood. Research organizations such as The National Forest Products Lab and companies such as Wheeler are working to bring the advantages of treated wood to engineers, architects, and the general public.
Timber bridges have been well received in northern and coastal regions of the United States where they are more durable than other types of construction. Treated timber is unaffected by freeze/thaw cycles and more importantly, it is non-corrosive. The deicing agents which are used on the streets and highways do not harm the timber. This is witnessed by the number of treated timber structures which are used for storing the deicing chemicals (salt storage buildings).
One of the most demanding applications for any material is in a highway bridge. Bridges are placed in a hostile environment and then are subjected to numerous large impact loads. If a scientist were to be given the problem of finding a material, for bridges, which could withstand the loads, would be noncorrosive, unaffected by freeze/thaw cycles, economical, and be made from a renewable resource, he/she would invent treated timber. Treated timber is the perfect match for the construction of modern highway bridges.
|
