Constant Ductility spectra are a special case of a broader class of spectra called "Constant Damage Spectra", where ductility is considered to be a simplified damage index (see section on Damage Index). Bispec provides the capability of computing "Constant Damage Spectra", by computing the required strength for a sdof system that will result in a pre-determined level of damage when subjected to a given ground motion record. Similar to "Constant Ductility" spectra, the required strength is solved for iteratively by iterating over a wide range of values and re-analyzing the sdof until the desired damage index level is achieved.