Structural over-strength is an important factor for structural collapse prevention when the structures suffer major earthquakes, however, it isn't included in our current Chinese seismic design code, and the seismic design specifications for structural safety calculations are based on structural members, with no more consideration on global carrying capacity of the whole structure. 17 RC frame buildings with different fortification intensities and storeys were designed according to the current Chinese Codes. These structures were modeled and analyzed in the platform OpenSees. By comparison of the quasi-static test data for RC frame structures with the analytical data correspondingly, the OpenSees models were verified and validated. For these 17 RC frames, structural seismic response was analyzed by nonlinear dynamic procedures (NDP), and the values of the “capacity” of structural overstrength factors and their variation rules with structural storeys and design fortification intensities were analyzed and derived. It is shown that structural ultimate load carrying capacity in collapse prevention performance level can be estimated by structural overstrength factor “capacity”. For RC frames designed in seismic fortification intensity Ⅵ, Ⅶ, Ⅷ, structural global overstrength factors “capacity” are 6, 4, 3 above respectively, and the minimum value agrees with the value 3 given in NEHRP recommended provisions.