What are stiffeners and why are they used? How many types of stiffeners are being used in the design of plate girder?

In a plate girder bridge, 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, in older bridges, bolted or riveted together to form the vertical web and horizontal flangesof the beam. In some cases, the plate girders may be formed in a Z-shape rather than I-shape