To assess the expected seismic damage of a structure, non-linear dynamic analysis and the damage index of Park-Ang have been often used. Depending on the size of the structure and on the duration of the record, the computational effort in dynamic analyses is usually high. In this research a new damage index is proposed based on nonlinear static analysis. The damage index is a linear combination of two energy functions: 1) the strain energy associated to the stiffness variation and the ductility of the structure and 2) the energy dissipated associated to hysteretic cycles. These two energy functions are obtained from the capacity curve of the structure and from the energy balance with the spectral acceleration. To show the ability of the index to represent damage, low-rise steel buildings subjected to seismic actions expected in Mexico City are studied. The results obtained with the new method show a good agreement with those calculated by means of dynamic analyses using the Park-Ang damage index. In average, the Park-Ang damage index is well fitted by the combination of 62% of the strain energy and 38% of the energy dissipated by hysteresis. Moreover, this new damage index allows linking damage to certain characteristics of the seismic actions as, for instance, intensity and duration of the applied seismic action. Therefore, the new approach results in a practice and powerful tool for estimating the seismic damage in buildings, especially when considering probabilistic approaches, where massive computations are needed.