A plate girder bridge is a bridge often seen supporting railroad roadbeds over short spans, this bridge may be a shallow U-shape in cross section with the roadbed at the bottom of the U or the bridge may be entirely under the roadbed. Each side consists of deep steel plate shear members with a strong top and bottom chord, forming what is called an I beam. The top chord is stressed under compression, the bottom under tension. Additional light members are attached to the shear members between the top and bottom chord to add stiffness to the plate and so preventing buckling. The two members of the bridge will be connected at the bottom with additional light truss elements and at the top with decking to form a box structure. See the illustration at the right and the trestle image below.
