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Utilized mostly to support railroads, a plate girder bridge is recognizable through the use of two or more plate girders supporting the bridge deck. A plate girder is similar in appearance to the more traditional I-section beam. The difference between the two comes from the manufacturing process. Whereas a traditional beam section is manufactured from a single rolled piece of steel, the flanges and web of a plate girder are rolled separately and welded together to form the section.
In regards to terminology, the web of a beam is the vertical section that connects two flanges, or the horizontal beam sections. In some configurations, a plate girder bridge will feature Z-section beams, where the web is connected diagonally between opposing sides of the flanges. Traditional plate girder bridges cover short spans between two abutments, usually crossing a road or watercourse.
There are three main types of plate girder bridges in common usage. These different styles are known as deck-type, half-through, and multi-span. The deck-type and multi-span bridge styles feature a similar construction, where the plate girders span between supports, and the bridge deck is fixed to the top of the girders. Bridge decks can be made from timber, steel, or concrete.
In a multi-span plate girder bridge, intermediate piers of steel, stone, or concrete construction are used. This allows the bridge to span across wider gaps than a single girder length could safely achieve. Stone and concrete are commonly only used to construct low-rise piers; steel lattice work piers are utilized for taller piers due to the lower weight of the construction material. Both deck-type and multi-span plate girder bridge designs commonly feature cross-bracing where the supporting beams are braced off of each other with diagonal steel members. This prevents the beams from buckling under the weight of train stock or other vertical loading from the bridge.
A half-through plate girder bridge is used when headroom underneath the bridge is limited and where a change in bridge level could prove problematic. Utilizing these criteria, the half-through style of bridge is most commonly found on railroads. The design of a half-through plate girder bridge differs from the deck-type and multi-span styles in that the bridge deck does not sit on top of the beams but rather is supported on the lower flanges of the beam. The result of this design is that the web and top flange of the supporting beams protrude vertically on either side of the railroad. The use of cross-bracing as support is not possible with this type of bridge, so often vertical stiffener sections are added to prevent buckling.
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