The paper illustrates the results of experimental tests performed on sub-assemblages of R/C (Reinforced Concrete) existing structures, designed only for gravity loads In order to estimate failure mechanisms and ductility properties, four internal and external R/C beam-column joints were built and tested. The specimens were built by using concrete with low strength and smooth reinforcing bars, without hoops into the panel zone. The tests were performed by increasing cyclic horizontal displacements up to the collapse. The experimental results show that seismic response of these kind of structures is mainly influenced by bond slips of longitudinal bars, and that the shear collapse regards external joints rather the internal ones. Failure mechanisms observed (column plastic hingings for internal joints, shear failure for external joints) point out the vulnerability of these structures due to the soft storey mechanism. The study is significant for better understanding of the inelastic seismic behavior of the R/C existing buildings with smooth bars, and for evaluating the effectiveness of the model classical assumptions.