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The study investigates the feasibility of a proposed simplified method for the estimation of the effects of general torsion on single story irregular structures under seismic action. The method (called SESA) is based on superposition of modal effects but it is extended to nonlinear behaviour of the structure by using overdamped displacement response spectra. The method can be analytically applied for the estimation of the displacement amplification due to torsion (compared to translational only behaviour) using the same steps of a regular spectral analysis. The authors applied the SESA method to torsionally flexible (twist is not restricted with structural elements placed perpendicular to seismic direction) and torsionally stiff (twist is restricted). The main parameter was the eccentricity between the Mass Center and the Stiffness Center (that coincides with the Resistance Center). The results were obtained for elastic behaviour and two intensity levels of seismic input for the inelastic behaviour. In order to validate the method the results were compared to the ones obtained by three-dimensional dynamic analysis using a set of spectrum compatible accelerograms. The results of the parametric study were compared in terms of displacement values at different points of the structure. [ABSTRACT FROM AUTHOR]