A truss is a rigid framework of connected beams, usually made of wood or steel, forming a series of interconnected triangles that provide support to structures. The framework of a truss bridge allows the structure to withstand the weight of the bridge deck, which is the surface area that vehicles and pedestrians use to cross the bridge. Various truss designs are used to build bridges; however; each is based on one of the basic truss types.
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Pratt trusses are the most common design used for truss bridges. They were originally made of wood, but technical advances allowed for numerous variations made of metal. Pratt truss bridges are identified by the "V" shapes in the framework, created by the diagonal members located in the centre of the structure. Additional counter braces are often utilised in variations of the Pratt truss design. Other variations consist of beams at the top of the bridge, called chords, that form a more rounded angle at each end of the structure; this lessens the bridge's weight but still maintains structural strength.
The framework of a Warren truss bridge appears as numerous equilateral triangles, each connected to the top and bottom beams of the structure. Variations of the Warren truss are built by subdividing each triangle with additional support beams. These designs create additional Warren truss types, such as subdivided, double and quadrilateral Warren trusses. Many covered bridges utilise the Warren truss design.
The Howe truss bridge design is similar to the Pratt, but each diagonal portion of the framework is angled toward the centre of the structure. This causes the truss to have an "A" shape at the centre. Howe truss bridges were common design types in the early days of railroads. While wood was used to form the main portion of a Howe truss, iron tension rods were also incorporated into the design. The Howe truss was designed as an improvement over other types of truss structures used for covered bridges.
Cantilever bridge designs often incorporate a truss bridge at the centre of the structure. A cantilever bridge is built with rigid support only at one end, such as a shoreline or the side of a ravine. A structure, sometimes utilising a truss-type design, is built outward from the rigid support and connected to a pier to provide balance. A similar structure is built on the opposite shoreline. The two cantilever structures are often connected by a truss bridge at the centre. The trusses of the cantilever structure may be constructed below or above the bridge deck. Other types of cantilever bridges are designed with truss supports both above and below the bridge deck.
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