Low-ductility steel concentrically braced frames (CBFs) are widely used in buildings as a lateral resistance system in seismic regions around the world due to their high stiffness and strength. Steel CBF buildings have been observed to have reserve capacity after brace fracture in past earthquakes. However, it is difficult for most practicing engineers to evaluate the contribution of the reserve system to the collapse prevention performance of CBF buildings because of its complex inelastic behavior. This study proposes a method to define the reserve system of multistory low-ductility CBF buildings for earthquake loads by performing a static pushover analysis. The multistory CBF building is converted to an equivalent single-degree-of-freedom (SDOF) CBF system with induced reserve parameters. A simple mathematical model describing the ductility response spectra for the reserve system in moderate seismic regions is established from the results of an extensive parametric study employing the SDOF mode to represent multistory CBF buildings. The collapse prevention capacity of the reserve system for a multistory CBF building can be evaluated by comparing the period of the multistory CBF system with a threshold value. The proposed approach is validated by application to buildings in two case studies. The results demonstrate that the reserve capacity of a multistory CBF plays an important role in collapse prevention. This simplified procedure is intended for a rapid evaluation on the multistory CBF system using an approximate seismic response.