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Construction of Beam and Truss Bridges

written by: Aggeliki K. • edited by: Lamar Stonecypher • updated: 3/31/2011

Bridges have long been constructed for the use of vehicular and pedestrian traffic. There are several types of beam bridges and truss bridges being used. The selection of bridges is based on the bridge loads, soil conditions, and cost. The beam and truss bridge is the most commonly used type.

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    Beam Bridges and Their Main Characteristics

    The simplest design of a beam bridge has a beam of log or board that is placed across the supports. The beam should be able to withstand the bridge loads that include the traffic load and the beam load. The beam top edge is pushed, while the beam lower edge is stretched by the application of loads on the beam bridge.

    Beam bridges with a simple deck are generally constructed of reinforced concrete or metal. The load carrying capacity of the girder, while minimizing the web depth, is increased by the addition of haunches at the ends being supported. Generally, the section at the center has a normal form with parallel projections. The curved or sloping flange ends are normally bolted or riveted by using joining plates. Due to the restrictions imposed on the transportation of huge beams to the site of construction, short beam lengths are normally connected employing splice plates Modern beam bridges are designed using finite element analysis.Concrete Beam Bridge 

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    Types of Beams

    Current beam bridges are constructed of beam girders, like box girders or I-beams. The box girders are stretched, box shaped elements that can endure the twisting loads, as on curved bridges, and are also useful for long spans. The I-beams are simple, and their fabrication is more economical. The horizontal elements of the "I" design are flanges and the vertical is the web of the construction.

    For more information on beam bridges check out the article: "What are Beam Bridges?"
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    Truss Bridges

    A truss is basically a firm and usually thin structure that is normally based on a frame with a triangular shape. Triangles in series are used in truss bridges as superstructure that transfers the deck load to the piers. The truss elements are subjected only to tensile and compressive loads, and thus are designed accordingly. Truss bridges are not intended to withstand the bending forces, and are best suitable for the straight alignments. A truss bridge is designed to endure extreme weights and also cover extended gaps. However, considerable vertical space is required to contain the truss bridge structure. The truss bridge deck can be constructed either under or over the truss structure.

    There are various types of truss bridges including the Howe Truss, Lenticular Truss, Bailey Bridge, K Truss, Bowstring Arch Truss, and Pegram Truss design.

    For more information on truss bridges see the article: "Truss Bridge Designs"Lattice Truss Bridge Willow Creek Bridge Nebraska 1913 

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    Beam vs Truss Bridges

    Estimation of the different loads acting on a bridge determines the type of build that is suitable for the environment, and the bridge materials that can endure the bridge loads.

    It has been established by experiments that the truss bridges can carry the maximum loads. The beam bridges are considered to be the most weak that can hold the minimum weight. The truss bridges are the strongest since they are able to withstand both the tensile and compressive loads, while all types of beam bridges rely mostly on the tensile loads. Therefore, keeping in view the design considerations, the beam bridges are valuable for simple applications, because of their ease in construction. As the beam bridge span is made bigger, its tendency to sag is increased. The truss bridges are generally used for the heavy loads that are determined by the truss bridge designs.

    Simple beam bridges are also easy to construct and relatively cheap. However, they cannot usually span more than 250 feet, since these bridges are supported at each end by piers and the further apart the piers, the weaker the beam becomes.

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    Prestressed Concrete for Bridges

    Concrete is characteristically weak in tension, due to which prestressed concrete has been developed that is suitable to endure these loads. Prestressed concrete is used for the manufacture of bridge beams with longer spans. Ordinary reinforced concrete may not be suitable for longer spans. Prestressing tendons consisting of high tensile rods or steel cables are employed to balance the tensile loads. Conventional reinforced concrete uses steel reinforcement bars that are placed in the concrete. Prestressed concrete contains steel strands and concrete of high strength. This arrangement produces a strong material for use in bridge girders.