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Timber Stringer Bridge
Timber has been used for bridge construction in Colorado for as long as there have been bridges. The railroads used timber extensively when building through the mountains in the 19th century, constructing spindly, multiple-span trestles when stream conditions allowed for many pile bent piers, or erecting timber/iron combination trusses when longer spans were required. Bridges built for roadway use followed the same structural principals and took many of the same forms as rail-road bridges. Like the railroads, early Colorado vehicular roads made extensive use of timber stringer bridges, and for the same reasons. Early toll-road operators and county road crews typically avoided building bridges when they could, but when they could not, they built as cheaply as possible, and timber pile bridges were the cheapest and most quickly completed structures that could be built in the 19th and early 20th centuries.
During the 1910s the Colorado Highway Commission generally eschewed timber trestles in favor of concrete and steel construction, and the Commission attempted to direct the counties away from timber construction as well. During the Depression, when labor was more plentiful than materials, and during World War II, when strategic materials such as concrete and steel were embargoed by the government, timber was used extensively by the Highway Department for bridge construction. The trend continued into the 1950s, and timber pile bridges continue to be built today, but primarily at secondary locations such as forest roads.
Timber stringers consist of parallel lines of wooden beams laid over the piers and abutments in single- or multiple-span configurations. The substructure is typically timber pile bents, but stone masonry, concrete, steel pile bents, or log cribs were used as well. Like railroad bridges, timber stringer structures for wagon use rarely exceeded 30 feet in length. Those stringer bridges with longer spans were sometimes reinforced with metal tension rods attached under the beams to form what were called jack trusses. The decks and guardrails of timber trestles were almost always made up of wooden members.
The primary difference between railroad bridges and wagon bridges lay in the stoutness of their construction. Railroad bridges were engineered and built to carry heavy loads while maintaining a high degree of rigidity. Wagon bridges were required to carry far lighter loads and did not need to be as unyielding as railroad structures. So ubiquitous were timber stringer structures, that they have historically been the most common bridge type built in Colorado by a wide margin.
The timber bridges found today on Colorado’s roads typically date from the 1930s, 1940s and 1950s. The superstructural technology between the early and later bridges has remained essentially unchanged, with the only difference being the sizes of the members. The bridges of the mid-20th century tend to have more substantial substructures, however, relying more on concrete or steel substructures than their predecessors. Depression-era agencies such as the Works Progress Administration and the Civilian Conservation Corps worked extensively in timber, for reasons of both economy and aesthetics.