In this study, previous test specimens of RC column-S beam connections were analyzed by 3-D FEM to investigate the effect of connection types or reinforcing details on the ultimate shear strength of the RCS connections. From the analytical results, the effective factor of a web width is about 0.8 in case of an exterior type, and about 0.6 in case of a corner type. The effective width of outer concrete was decided by assuming that the inner concrete resists in all and the strength due to each reinforcing detail can be accumulated. The calculated results by the proposed equation gave a good agreement with the experimental results and analytical results.