Estimating the vulnerability of a structure subjected to a certain hazard, that is the probability that it would not meet a given performance objective for a given hazardous event, is a key step in an overall seis- mic risk management process. Structural performance indicators, coupled to hazard analysis, directly provide decision-makers with readable information: e.g. the risk for a structure to collapse within a certain number of years. This work focuses on the vulnerability assessment of reinforced concrete moment-resisting frame struc- tures subjected to seismic hazard, based on inelastic time history structural analyses. Structural analyses entail numerous uncertainties that can compromise the reliability of its outcomes and, in turn, the assessment of the structural performance. The aim of this paper is to provide insight into the uncertainty in the structural seismic performance analysis that arises from the addition of viscous damping in the inelastic time history analyses and to compare it to the variability that comes from the ground motion. Latin hypercube samples of random variables are generated and the uncertainty is propagated to the outcomes of interest using Monte Carlo simulations.