The “constant ductility” spectrum is obtained by solving for the yield strength that results in a target ductility demand under the given ground motion. The constant ductility spectrum allows a comparison between systems experiencing the same amount of yielding, as reflected by the ductility demand, which is generally considered a reflection of the damage in the system. An extension of this idea leads to “constant damage” spectra which are computed by solving for the yield strength that results in a target value for a damage index. “Constant ductility” spectra cannot be obtained directly, but need to be iteratively solved for by iterating over several different strengths, until the correct strength, that which results in the desired ductility, is found. The associated computational cost is typically many times that of other types of nonlinear spectra such as “constant strength” spectra.