9 02 2017
Types of trusses for bridges
A truss is a rigid framework connected pipes, usually made of wood or metal, forming a series of interlocking triangles that provide support structures. Part of a truss bridge allows the structure supporting the weight of the bridge deck, which is the surface used by vehicles and pedestrians to cross. They used different designs of trusses to build bridges; however, each is based on a type of basic truss.
The Pratt trusses are the most famous design used for bridges. They were originally made of wood, but technical advances allowed numerous variations in metal. The Pratt trusses are identified by the forms “V” in the framework created by the diagonal elements located in the center of the structure. Others consist of beams at the top of the bridge, called strings, which form an angle more rounded at each end of the structure; these reduce the weight of the bridge but retain the structural strength.
Under the Warren truss is achieved with many equilateral triangles, each connected to the top and bottom of the structure. Variations of this truss are achieved by subdividing each triangle additional support beams. These designs create additional types of Warren truss, as the split, double and Warren quadrilateral. Many covered bridges use the Warren Girder design.
The Howe truss design is similar to Pratt, but each diagonal of the frame has its angle toward the center of the structure. This causes the truss has a form of “A” in the center. Howe trusses were common in the early stages of the railways. Although wood was used to form a large part of the beam, tension cables were incorporated into the design. The Howe truss was designed as an improvement over other types of structures used to cover bridges.
The suspension bridge design often incorporates trusses in the center of the structure. A bridge of this type is built with a rigid support at one end, like a bank or the side of a ravine. This structure, which sometimes uses a truss type design, is constructed out of the rigid support and connected to a pillar to bring balance.
A similar structure is built on the opposite bank. The two hanging structures often connect with a truss in the center. The cantilevered truss structures could be constructed under or over the bridge deck. Other types of suspension bridges are designed with structural trusses above and below the platform.