The thin webs of steel members are laterally buckling under static loading and laterally vibrating under repeated dynamic loading. The consequence of that can be arising and successive development of fatigue cracks along the welded connections of the thin webs with flanges and stiffeners. Therefore, the fatigue strength and life-time of thin-walled steel members depend beside of usual material-technological and structural influences also on local stability aspects. The extensive experimental-theoretical investigation of thin-walled hybrid steel plate girders and columns with accent on their fatigue failure mechanisms and local stability aspects is presented within the paper. The outcome of obtained knowledge and results is the original empirical formula for the limitation of web slenderness that can be used for the reliable and economical design of steel members which are subjected under repeated dynamic loading.