1 Ultimate Shear Behaviour of Hybrid Reinforced Concrete 1 Beam-to-Steel Column Assemblages 2 D. V. Bompa and A. Y. Elghazouli 3 Department of Civil and Environmental Engineering, Imperial College London, UK 4 ABSTRACT 5 This paper examines the shear transfer mechanisms and ultimate behaviour of hybrid systems 6 consisting of reinforced concrete beams connected to structural steel columns. A series of five 7 large scale tests on structural assemblages, in which steel shear-arms are welded directly to the 8 steel columns and embedded in the reinforced concrete beams, is presented. After describing 9 the experimental arrangement and specimen details, the main results and observations obtained 10 from the tests are provided and discussed. The test results offer a direct evaluation of the 11 ultimate shear behaviour of such hybrid systems. The experimental findings also enable a 12 comparison with the strength predictions obtained from analytical models which are commonly 13 used in the design of conventional reinforced concrete members. The discussions and 14 comparative assessments presented in this paper provide an insight into the influence of various 15 shear transfer mechanisms including transverse reinforcement, compressive zones, residual 16 tensile stresses, aggregate interlock, and dowel action, in addition to the interfacial bond 17 between the steel profile and concrete. The activation and contribution of the key shear transfer 18 mechanisms are assessed in light of the experimentally-monitored crack growth, path and 19 pattern, as well as in comparison with widely-adopted analytical approaches. The results show 20 that the contribution of each transfer mechanism is a function of the crack kinematics and 21 corresponding level of applied load. Finally, modifications to existing analytical approaches for 22 conventional reinforced concrete elements are proposed in order to provide a reliable 23 evaluation of the ultimate shear capacity of such hybrid systems. The suggested expressions 24 account for the influence of the shear-arms’ characteristics on the ultimate shear strength, and 25 offer a more realistic prediction of the behaviour in comparison with conventional reinforced 26 concrete design provisions. 27 28 Keywords 29 Hybrid systems; reinforced-concrete/steel assemblages; shear transfer mechanisms; shear 30 behaviour; shear design 31