A plate girder is distinguished from a rolled beam by the web slenderness ratio h/t w . When spans are long and loads are relatively large, it is more economical to use plate girders rather than rolled sections. With the introduction of railroads in the middle of the nineteenth century, riveted plate girders were commonly used in bridge designs spanning 50 to 150 ft. Later with the introduction of welding, riveted plate girders became less and less frequently used. Plate girders may have a constant section throughout their span or a variable section. Several plate girder sections are shown in Figure 9.1
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