The buckling capacity of steel I-beams can be increased by providing lateral bracing along the length of the beams by either cross-frames or diaphragms. Metal sheeting that is often used in steel building and bridge constructions to support the fresh concrete, acts like a shear diaphragm and provides continuous bracing to steel beams. In building industry, metal deck forms are considered as a lateral support to the beams. However, due to their flexible connection detail between the girder and shear diaphragm, metal deck forms are not considered as a brace source for bridge construction industry. But with the recent studies, by improvements of the flexible connection details, metal decking can be used as a bracing system. An adequate bracing system must possess sufficient stiffness and strength. A computational study was conducted to investigate stiffness requirements of shear diaphragms used to brace stocky and slender steel I-beams. Both doubly and singly symmetric sections were studied. The computational study consists of eigenvalue buckling analyses on perfectly straight twin-girder system braced by shear diaphragms and large deformation analyses with imperfect girders with different configurations of girder sections and spans. A three dimensional computer programme was utilized to perform analytical studies. Analytical model is verified by a full-scale twin-girder system laboratory test that is carried out on a previous study. Stiffness requirements have been proposed for shear diaphragms used to brace stocky and slender steel I-beams.