Structural steel members in railway superstructures are usually designed to resist only axial or transverse loads. Combined stresses in railway bridges typically arise from biaxial bending of unsymmetrical cross sections, unsymmetrical bending from transverse force eccentricities, and combined axial and bending forces caused by eccentricities, member out of straightness, self-weight, and applied lateral loads such as wind. For linear elastic materials and small deformations, superposition of combined stresses is appropriate. When a torsional moment is applied, pure torsion always exists. Pure torsion creates shearing stresses in the flanges and webs of structural shapes such as, channels and I-shaped beams. Interaction equations used in allowable stress design are determined from interaction equations developed for ultimate loads. The strength of members subjected to axial compression and biaxial bending is complex. Combined bending and shear may be significant in the webs of plate girders.