SEISMIC BEHAVIOR OF SUBSTANDARD RC BEAM-COLUMN JOINTS DOMINATED BY CANTILEVER ACTION

Abstract

Based on practical design of medium to high-rise of RC building frames, the larger size of column compared to beam is common. This approach uses to ensure that column can sustain larger loads both for gravity and lateral directions. The larger depth of column with a few percentage of reinforcement, together by permanent axial compression force is enough to produce the high column flexural capacity and consequently increases the column-to-beam flexural strength ratio which is considered for capacity design concept. In addition, the larger depth of column may lead high joint shear strength and substantially increases the anchorage length which is necessary for preventing deterioration of bond during cyclic excitation. However, if the relative stiffness of column compared to the beam is too large, the expected mode of failure from beam-sway mechanism may be alternated to unexpected cantilever action mode which is uncommon in practical design. In this paper a study of two subassemblages representing interior frame at the bottom story compared to typical cruciform beam-column joint under reversed cyclic loaded is presented experimentally. The dimensions of beam-column joint for both specimens were designed identically except the column of one specimen was extended from the joint and fixed at strong foundation. The stiffness of column is designed relatively larger than the stiffness of beams. Even though the beam hingings were detected for both specimens but the global behaviors were significantly different. Behavior of cruciform beam-column joint was controlled by beams flexural strength while behavior of beam-column joint with column was controlled mainly by column flexural strength. It can be understood that the behavior of specimen with larger column compared to beams was dominated by cantilever action rather than conventional beam-sway mechanism. In this paper, a simple index to check whenever the fame is dominated by cantilever action or by beam-sway action is proposed to classify the behavior beam-column joints.