Displacement-based design of structures or assessment procedures based on the Capacity Spectrum Method require the definition of an equivalent SDOF system with equivalent viscous damping.

Various closed-form expressions for the equivalent viscous damping ratio already exist in the literature. These formulations have been obtained based on the response of SDOF oscillators with pre-defined hysteretic behavior and are all dependent on the ductility demand of the system.

However, the applicability of such expressions for the assessment of existing structures is questionable since a few discretional choices are involved. First, an expression for the equivalent viscous damping ratio must be chosen so as to adequately represent the dissipative behavior of the structure under examination. Second, a preliminary evaluation of the global ductility demand of the structure is required, thus a proper idealization criterion of the capacity curve must be chosen.

To overcome these issues, a new ductility-independent closed-form expression for the equivalent viscous damping ratio is proposed for applications to existing RC frames. The proposed expression depends on response parameters that can be easily determined by non-linear static analysis of the structure under examination, without the need of idealization of the capacity curve.