What is a Truss Bridge?

A truss is a series of individual members, acting in tension or compression and performing together as a unit. On truss bridges, a tension member is subject to forces that pull outward at its ends. Even on a "wooden" truss bridge, these members are often individual metal pieces such as bars or rods. Compressive forces push or compress together and are heavier. The individual members form a triangular pattern.

One bridge historian describes a truss bridge in this manner: "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." (Eric DeLony, The Golden Age, Invention and Technology, 1994).

The pattern formed by the members combined with the stress distribution (tension and compression) creates a specific truss type, such as a Warren or Pratt. Most truss types bear the name of the person(s) who developed the pattern, such as the Pratt truss that is named for Caleb and Thomas Pratt who patented it in 1844. For instance, the configuration or pattern of a Pratt and Howe truss appears identical (a series of rectangles with X's), but a Howe's diagonals are in compression and the verticals in tension. In a Pratt, the reverse is true.

In theory, a truss bridge contained no redundant members. Builders considered each member or element essential to the functioning of the truss, although some were more important than others were. While most trusses could sustain considerable damage and lose the support of some members without collapsing, severe traffic damage to a member could result in the collapse of the bridge.

Tennessee's four remaining historic covered bridges utilize one of these three truss types:

• Kingpost (the Parks Covered Bridge
• Queenpost (the Harrisburg Covered Bridge and the Bible Covered Bridge
• Howe (Elizabethton Covered Bridge)