<p>The prediction of the maximum response of an inelastic structure by means of a linear single degree of freedom system with proper lateral stiffness and viscous damping ratio is at the basis of displacement-based design procedures or assessment procedures based on non-linear-static analyses. Several closed-form expressions of the viscous damping ratio were proposed in the past, all depending on the ductility demand of the considered hysteretic systems. The use of these expressions in the case of existing buildings is not straightforward since the evaluation of the global ductility demand is required. Thus, a proper idealization criterion of the capacity curve has to be selected. Finally, the choice of the most appropriate expression of the equivalent viscous damping ratio for the structural type under examination introduces an additional element of discretion. This paper proposes a ductility-independent expression of the equivalent viscous damping ratio suitable for existing reinforced concrete framed structures. The proposed expression depends on the ratio between the elastic and the secant period of vibration of the structure and the chord rotation demand of beams and columns. These response parameters can be easily determined by non-linear static analysis without need of idealization of the capacity curve. A procedure for the application of the proposed expression to multi-storey framed RC buildings is also illustrated and applied to six case studies. The effectiveness of the proposed expression is assessed through non-linear dynamic analysis.</p>