Abstract | In this paper an extended complete quadratic combination rule for quick assessment of peak floor acceleration demands (PFA) of elastic structures subjected to seismic excitation is proposed. The simplification from time history analysis to the response spectrum method is shown in detail. Based on a relative acceleration formulation combined with nonlinear optimization techniques cross correlation coefficients are determined to estimate relative and absolute PFA demands. For estimation of central tendency and dispersion of the seismic response, regression equations are derived to provide a simple implementation of the method in civil engineering design practice. Application of the proposed procedure to a 24-story moment resisting generic frame structure shows the improvement compared to common response spectrum methods.