In this paper, effects of floor slabs on ultimate earthquake resistance of steel moment frames are investigated. Earthquake response analyses of multi-story steel frames are carried out, and ultimate energy absorption is derived from the results by consulting practical ductility capacity of composite beams. In the analyses, inelastic behaviour of composite beams is considered. And ductility capacity of the beams, which defines ultimate state of the models, is defined considering the effects of floor but they are not important in earthquake resistance of the structures, except the effect on ductility capacity of beams. And the effect of floor slabs on ductility capacity has serious influence on ultimate earthquake resistance of steel moment frames, especially when its joint efficiency is poor.
