Truss types

Trusses work much like beams: they carry a combination of compression and tension forces. The main difference is that trusses are less heavy than beams. Beams use extra material in some areas; these areas don’t use the full strength available to them. Engineers and builders can determine which portions of beams can be removed. The resulting truss concentrates the forces into many smaller members and eliminates the under-stressed areas of beams.

A truss is made of many smaller parts. Once constructed of wooden timbers, and later including iron members, most truss bridges are built of metal. As for the placement of the travel surface in relation to the superstructure truss bridges are divided into king and queen post trusses. The king post consists of two angled supports leaning into a common vertical support. The queen post truss is similar to a king post truss in that the outer supports are angled towards the center of the structure.

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Covered bridges are typically wooden truss structures. The enclosing roof extended the life of the bridge. One of the most common methods used for achieving longer spans was the multiple kingpost truss. A simple, wooden, kingpost truss forms the center and panels are added symmetrically. With the use of iron in bridge construction, the Howe truss in its simplest form appears to be a type of multiple kingpost truss.

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Theodore Burr built a bridge the Hudson River in 1804. This combines an arch with a truss to form a structure both strong and rigid.

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Lattice truss type of bridge uses a number of lightweight elements, easing the task of construction. Truss elements are usually of wood, iron, or steel.

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Herman Haupt designed his truss configuration in 1839. The Haupt truss concentrates much of its compressive forces through the end panels and onto the abutments.

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The Pratt truss is a very common type, but has many variations. Originally designed by Thomas and Caleb Pratt in 1844, the Pratt truss successfully made the transition from wood designs to metal. The basic features are the diagonal web members which form a V-shape. The center section commonly has crossing diagonal members.

Charles Parker modified the Pratt truss to create a “camelback” truss having a top chord which does not stay parallel with the bottom chord. This creates a lighter structure without losing strength; there is less dead load at the ends and more strength concentrated in the center.

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A Warren truss, patented by James Warren and Willoughby Monzani in 1848, contains many triangles formed by the web members which connect the top and bottom chords. These triangles may also be further subdivided. Warren truss may also be found in covered bridge designs.

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