This study is oriented to implement the probabilistic approach for the calculation of structural damage in one case study: an eight-story RC framed building which is taken as a structure representative of the mid-rise RC buildings currently designed in Lima under the Peruvian Seismic Code. The probabilistic methodology is being developed and implemented in order to perform the performance assessment in a new generation of performance based earthquake engineering. Three of the four steps involved in this methodology are included in this study. Hazard Analysis, represented by the peak ground acceleration as intensity measure; Structural Analysis, with lateral drift as engineering demand parameter; and Damage Analysis, where the damage is measured by the probability of exceedance certain damage state. The uncertainties are involved in each step as follows: In the hazard analysis, actual records are selected and scaled to the intensity target values. These records include the actual uncertainties in parameters as duration and frequency contents. For the Structural Analysis, the uncertainty is introduced by means of a random variable for the yielding point for the capacity curve calculated for the frame. The uncertainty is produced by simulation on the material capacity. Finally, the damage uncertainty is given by the fragility functions, which relate the structural demand parameter (lateral drift) with the structural damage states for the building. The damage probabilities are summed up for all the selected records and the values of yielding strength. The final result is given in terms of probability of being in each damage state given a PGA value. The procedure is then repeated for different values of PGA and the probabilities are plotted as function of the intensity (PGA).