In this paper, the provisions in different design codes and some tests on the identification of the negative strength of beams including slab-bars are compared and analyzed. The results of analysis demonstrate that the effective width of slabs is a converted width for calculation. It isn't the actual working width of the slab or the yield slab width. According to the maximum storey drift, the frames can reach, an effective width of slabs can be defined as six times the slab-thickness width on each side of the web. Considering the participation of reinforcement in slab into resistance of the beam against negative flexure, the anchorage of tension bars and the transverse reinforcement placement in slab should be taken into account to ensure the effective work of longitudinal slab bars.