This paper is centred in the study of the seismic behaviour of all existing steel building, located in the National University of Colombia in Manizales, zone of high seismic hazard according to the Colombian seismic codeo Two building frames are analyzed: one is a moment-resisting frame and the another one is braced fimne. The study analyses the plastification process that the building follows starting from its elastic behaviour until the collapse. Its capacity spectrum is correlated with the stress and strain states to determine the demand parameters, like the spectral displacement, used to describe the thresholds of the damage states. Once defined, these thresholds they are related to the characteristic points of the bilinear capacity spectrum. By means of statistical procedures, the fragility curves are obtained what allow calculating the damage probability matrix and the damage probabilityvector corresponding to the design seismic action. Based on this vector a class of seismic vulnerability is assigned to themoment-resisting frame and the braced frame. Finally, the vulnerability index is calculated for each case.