Abstract: Plate girders are used frequently in major industrial structures. In the systems supported by these types of beams, reduced deflection and decreased maximum bending moment can be considered as a retrofitting method which can be done using cross beams or braced beams as the main elements perpendiculars to the girders. In the present study, the effect of cross beams and their distance on retrofitting the girders has been studied. Differential equation governing the behavior of the new structures is a fourth grade one is determined. Later, its solution is continued using mathematical transformations and the fundamental relationships are reached at. The produced relationships have led to solution of a practical issue. The results offered have been compared with those obtained from modelling in SAP 2000, showing precise consistency. It is concluded that different values of stress, bending moments and forces in the systems may be determined by changing the stiffness and distance of the cross beams.