Plate girder bridges use two or more plate girders to support the bridge deck. They come in three types: deck-type, half-through, and multi-span. Cross bracing and vertical stiffening sections prevent buckling.
Primarily used to support railroads, a plate girder bridge is recognizable through the use of two or more plate girders that support the bridge deck. A plate beam is similar in appearance to the more traditional I-section beam. The difference between the two comes from the manufacturing process. While a traditional beam section is manufactured from a single piece of rolled steel, the flanges and web of a sheet metal beam are rolled separately and welded together to form the section.
As far as terminology is concerned, the web of a beam is the vertical section connecting 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 opposite sides of the wings. Traditional plate girder bridges span short spans between two abutments, usually crossing a road or stream.
There are three main types of sheet girder bridges in common use. These different styles are known as deck-type, half-through and multi-span. The multi-span and deck-type bridge styles feature a similar construction, where plate girders span between the supports and the bridge deck is attached to the top of the girders. Bridge decks can be made of wood, steel or concrete.
In a multi-span girder bridge, intermediate piers made of steel, stone or concrete are used. This allows the bridge to span wider spaces than a single girder could safely reach. Stone and concrete are commonly used only to build low-lying piers; steel lattice pillars are used for the taller pillars due to the lower weight of the building material. Both multi-span and deck plate girder bridge designs commonly feature cross bracing where the support beams are braced from each other with steel diagonal members. This prevents the girders from buckle under the weight of train material or other vertical loads from the bridge.
A half through plate girder bridge is used when headroom under the bridge is limited and where a change in deck level could prove problematic. Using these criteria, the mid-deck style is most commonly found on railroads. The design of a semi-through plate girder bridge differs from deck-type and multi-span styles in that the bridge deck does not sit above 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 support beams project vertically on either side of the rail. The use of cross braces for support is not possible with this type of bridge, so vertical stiffening sections are often added to prevent buckling.
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