The response of multi-degree systems to earthquake ground motion can be determined by modal analysis(1,48). Computer progtammes are prepared for generating the stiffness matrix for a frame and then reducing it to the lateral stiffness matrix. Finally an eigen value subroutine is called to obtain the normal modes of vibrations with the characteristic shapes. Three R. C. C. frames having three, eight and eleven storeys, shown in Figs. 1, 2 and 3 respectively are analysed by using actual spectrum inputs taken from the actual spectrum of Koyna earthquake average spectrum and assumed elasto-plastic spectrum. In the analysis, damping equal to 10% of critical damping is used.