The present work deals with the dynamic response of elastic multi-degree of freedom structures that are irregular in height, consisting of two parts, a lower part made of concrete and an upper part, made of steel. Irregularity is due to different dynamic response characteristics of the two parts, here expressed by their different damping ratios. The two different damping ratios make the seismic design of such structures a troublesome procedure since current codes do not provide analysis solutions for such structural systems. This work aims at concluding at a simple yet efficient method in order to evaluate the equivalent modal damping ratios of such structures.