A beam assembled from steel plates which are welded or bolted to one another, used to support the horizontal surface of a bridge. Plate girders generally have I-shaped cross sections (Fig. 1). Cross sections with two or more webs that form a U- or box-shape can also be considered plate girders, but are generally classified separately as box girders. Plate girders are commonly used in bridge structures where large transverse loads and spans are encountered. On average, the height (measured from the top of the top flange to the bottom of the bottom flange) of plate girders is one-tenth to one-twelfth the length of the span, which can vary based on the loading and architectural requirements. The dimensions of plate girders are tailored to specific applications by adjusting the width and thickness of the web and flange plates along the length of the beam. By contrast, hot-rolled beams produced at steel mills are formed to specific dimensions according to a standardized list of structural shapes, and have limitations on their size. Plate girders can be produced with much greater depths than hot-rolled beams. The depth is generally considered to be the distance between the centroids of the flanges which defines the effective moment arm in bending. When different-grade steels are used for the flanges and webs, a plate girder is classified as a hybrid plate girder.