Recent research has developed a family of new earthquake-resistant structural systems for steel moment frames that have the potential to eliminate structural damage and return to its original vertical position (i.e. self-center) following a major earthquake. The behavior of this self-centering moment-resisting frame (SC-MRF) is characterized by a gap opening and closing at the beam-column interface under earthquake loading. Energy dissipation is provided by supplemental elements that deform under the gap opening behavior. The gap opening behavior also causes the SC-MRF to expand, where in the deformed position the distance between the column centerlines increases relative to the original distance. This behavior imposes new design requirements on the floor diaphragm design. The objective of this study is to analytically evaluate the effects of the floor diaphragm stiffness, strength, and configuration on the seismic response of a SC-MRF. The results show that these parameters significantly affect the seismic performance of a SC-MRF and the design of the floor diaphragm must be considered when designing this system.