In this paper, strength and stiffness degradation effects on collapse capacity of steel moment frames are investigated. The aim is to determine the safety margin of steel moment frames against collapse by using FEMA-P695 methodology. For this reason, 14 special steel moment frames (SMRFs) are modeled including 4 long-period and 3 short-period structures with 5 and 8 meter bays lengths. These frames cover a range of short-period to long-period structures. First, pushover analysis is carried out and the capacity curves are compared. Target displacements are calculated and then seismic parameters including Ω0 and µT are determined. Results show that shorter structures possess higher ductility than larger ones. Second, incremental dynamic analysis (IDA) is carried out to understand the behavior of the structures from elastic to collapse state. Median IDAs are calculated and compared to obtain R factor. Also, collapse capacity of the structures is calculated by IDAs considering uncertainties in ground excitations. In the last step, margins of safety (CMR) are calculated for structural groups. Results show that short-period structures experience larger CMRs since they have more ductility capacities.