An analytical procedure is developed for predicting the ductility demands in simple asymmetric-plan structures under earthquake ground motions. The procedure governs regular structures dominated by the lower vibration modes where inelastic response occurs only at the bases of first story columns and at the beam ends, in conformance with the capacity design principles.Torsional ductility spectraare generated for expressing the maximum ductility response of torsionally coupled, generic, single-story, 2-degree-of-freedom inelastic parametric systems. Five parameters characterize the parametric systems: first mode period, uncoupled frequency ratio, stiffness eccentricity, stiff-to-flexible edge strength ratio, and ductility reduction factor. A surrogate modeling approach is developed for converting the properties of the actual systems to those of the parametric system. Mean maximum ductilities of torsionally stiff, equally stiff, and torsionally flexible systems are calculated under a set of design spectrum compatible strong motions for the possible combinations of characteristic parameters. The results obtained from case studies revealed reasonable accuracy of the estimations. The results have indicated that the flexible side frames of torsionally stiff and equally stiff code conforming designs are mainly responsible for providing the intended ductility and energy dissipation capacity whereas the stiff side frames play a secondary role, particularly when the stiff edge is significantly stronger than the flexible edge. However, ductility demands in torsionally flexible systems are significantly larger at both sides compared with torsionally stiff systems.