Due to their satisfactory seismic performances, in past years composite steel concrete structures have been enjoying a growing diffusion. Such structures, indeed, join the high deformability and ductility of steel structures with the strength and resistance to atmospheric agents typical of concrete ones. Besides, single components can be easily substituted, leading to good retrofitting properties. The characteristics listed above are, in principle, very important for a good performance under seismic excitation. However, suitability of composite structures in seismic areas has not been fully proved by experimental and numerical investigations, due to the insufficient research efforts devoted to this structural type so far. In this paper, the seismic performance of a sample structure has been investigated by conducting a fragility analysis. The assumed sample frame has been extracted from a three-dimensional frame [1] tested in the experimental program performed at the ELSA in Ispra within two European projects: the ECSC Project 7210-pr-250 (Applicability of composite structures to sway frames) and an ECOLEADER Project (Cyclic and PSD testing of a 3D steel-concrete composite frame). Therefore, this study initially focuses on the calibration of a numerical model by comparing its predictions to the actual experimental response basically consists. The geometry of the assumed sample frame is shown in Fig. 1, together with the mechanical properties of materials.