A plate girder bridge is a bridge supported by two or more plate girders. The plate girders are typically I-beams made up from separate structural steel plates (rather than rolled as a single cross-section), which are welded (or occasionally bolted or riveted) together to form the vertical web and horizontal flanges of the beam. In some cases, the plate girders may be formed in a Z-shape rather than I-shape.
Plate girder bridges are suitable for short to medium spans and may support railroads, highways or other traffic.
Deck-type plate girder bridge
In the deck-type bridge, a steel or reinforced concrete
bridge deck is supported on top of two or more plate girders, and may act compositely
with them. Additional beams may span across between the main girders, for example in the form of bridge known as ladder-deck
construction. Also, further elements may be attached to provide cross-bracing and prevent the girders from buckling
Half-through plate girder bridge
In the half-through bridge, the bridge deck is supported between two plate girders, often on top of the bottom flange. The overall bridge then has a 'U'-shape in cross-section
. As cross-bracing cannot normally be added, vertical stiffeners on the girders are normally used to prevent buckling (technically described as 'U-frame behaviour'). This form of bridge is most often used on railroads
as the construction depth (distance between the underside of the vehicle, and the underside of the bridge) is much less. This allows obstacles to be cleared with less change in height.