Effect of partial debonding on catenary action of reinforced concrete frames

Abstract

Progressive collapse resistance of buildings can be evaluated by introducing a middle column removal scenario to mobilize alternate load paths (ALP). Catenary action, one of ALPs, uses tensile force in beams to balance external loads applied onto structures. To this end, large deflections and rotations of beams must be achieved through continuity of reinforcement. In this paper, the development of catenary action of two reinforced concrete (RC) frames is presented in detail. Both specimens consisted of two one-bay beams, one middle joint, two side columns and two beam extensions. One frame was designed with conventional non-seismic detailing in accordance with ACI 318-05. To release the strain concentration of bars at the joint interfaces and to increase rotation capacities of beam-column connections, detailing of the second frame was designed by debonding the bottom bars in the joint regions with plastic sleeves. Test results on the specimen with conventional detailing show that catenary action could be successfully mobilized but failed to significantly increase structural resistance due to excessively premature fracture of top bars at the side joint interfaces. In contrast, the presence of partial hinges in the second frame allowed catenary action to be mobilized to significantly enhance structural resistance. Finally, the mechanism of partial debonding technique is discussed.