Truss Bridge Types in Bradford County, Pennsylvania

            Truss bridges are identified by their structural design. There are three classifications for the basic structure of a truss bridge related to how the road or track is laid over a span: through, pony, or deck. In addition, truss bridge types are named for the arrangement of the triangular sections, or panels that make up the sides of the bridge, and the configuration of the top members of the bridge.

Through truss bridges carry the traffic load level with the bottom outside members of the bridge, known as chords. In other words, the traffic passes between or through the trusses. A through truss has top lateral bracing (see diagram). A pony truss, also known as a half-through truss, lacks top lateral bracing (see diagram). The pony truss is approximately half as deep as a through truss, and does not require the top lateral bracing that is necessary to prevent buckling of deeper trusses.

A deck truss bridge carries its traffic load level with the top chords (see diagram). The roadway or railroad tracks of a deck truss bridge are on top of the side members.

Truss bridges are also named for the configuration of their side panels. The following truss types are present in Bradford County:

Pratt Truss: A truss composed of a series of right triangles with level top chords. The top chords and the vertical members are in compression and the lower chords and diagonals are in tension. The Pratt Truss is the most common truss type in Bradford County and Pennsylvania, and is found in spans of 25 to 150 feet (8 to 45 meters).

Parker Truss

Parker Truss: A Parker Truss is a Pratt Truss with a polygonal top chord. A Parker Truss is generally used for spans of 100 or more feet, but may be found in spans of 40 to 200 feet (12 to 60 meters).

Petit Truss

Pennsylvania (Petit) Truss: A Pennsylvania (Petit) Truss is a Parker Truss with sub-struts and/or sub-ties. The sub-members stiffen a truss that carries heavy, moving loads, such as a railroad freight train. Its name derives from its extensive use on the Pennsylvania Railroad. The Pennsylvania (Petit) Truss may be 250 to 600 feet (75 to 180 meters) long.

Warren Truss

Warren Truss: A truss composed of a series of equilateral triangles. The diagonals carry both compression and tension forces. A Warren Truss may be 50 to 400 feet (15 to 120 meters) long.

Source: Comp and Jackson 1977.