Abstract Structural damage caused by earthquakes may be due to excessive deformations, or it may be in the form of accumulated damage sustained under repeated load reversals. The way these two components are combined is strictly dependent upon the structural material. Reinforced concrete structures, for instance, accumulate damage resulting from cyclic inelastic deformations in a way that is completely different from that for steel structures. Many attempts have been concentrated in the past on the verification of the possibility of using steel-type damage indices for reinforced concrete structures and vice versa. In spite of the good results that have been obtained in some cases, the applicability of these indices could not be generalised. The purpose of this paper is to obtain a uniform definition of damage, independently of the material used for the structure, so that practical design and assessment methods can be applied to any type of structure. It is shown that the damage in any construction material mainly results in strength deterioration. A general damage index is thus defined as the ratio between the initial and the reduced resistance capacity of a structure, evaluated by using an evolution equation for the yield strength in which the structural damageability is included. The ability of this index to model different damage situations is demonstrated.