By Hope Good

In addition to the many Montana bridges in use today, remnants of bridges from the past can still be seen across the landscape. Others exist only in old photographs and memories.

Turn of the 20th century railroad bridges constructed in Montana were trestles, bridges that consisted of a number of short spans, supported by splayed vertical elements having all supporting members below the railway tracks. Timber trestles were extensively used in the nineteenth century in mountainous areas and to traverse floodplains adjacent to rivers as approaches to bridges. Most of these early bridges were built of wood because it was cheaper and easier to work with and less susceptible to damage than stone structures. The materials were typically constructed using peeled logs preserved with creosote as vertical elements and bolted and spiked sawn
timbers for bracing. The great British engineers who commenced and developed iron bridges also built timber bridges until the mid-nineteenth century. The first rail lines built used the same jigs and tools that the British used for building wagons in which wheel spacing gauge track used was 4’8.5” inches wide.

In 1840, William Howe patented the Truss Bridge. With it, end diagonals connect the top and bottom chords, and all wood members act in compression. Builders could use multiple panels to increase the length of the bridge, typically ranging between 100 and 150 feet. As described by historic preservationist Eric DeLony in 1994, “A truss is simply an interconnected framework of beams that holds something up. The beams are usually arranged in a repeated triangular pattern, since a triangle cannot be distorted by stress.”

In a truss bridge, two long, usually straight members known as chords form the top and bottom. They are connected by a web of vertical posts and diagonals. The bridge is supported at the ends by abutments and sometimes in the middle by piers. A properly designed and built truss will distribute stresses throughout its structure, allowing the bridge to safely support its own weight, the weight of vehicles crossing it, and wind loads. The truss does not support the roadway from above, like a suspension bridge, or from below, like an arch bridge; rather, it makes the roadway stiffer and stronger, helping it hold together against the various loads it encounters.

In theory, a truss bridge contains no redundant members. Builders considered each member or element essential to the functioning of the truss, although some were more important than others. Many of the remaining trestles we see today were made with Howe trusses. In 1895, 70% of American bridges were wooden. Wood structures were lighter than metal ones, but also had drawbacks. They were combustible and as trains became faster and heavier, they were unable to accommodate the loads. When timber beams were abandoned, lattice beams were solely made of metal.

Twentieth century construction eliminated much of the need for trestles by using far more extensive grading and tunneling. Railroads which were a major way of shipping goods and products in the history of our country for many years were almost abandoned as shipping was transferred to semi-trucks, especially following World War II.