Part-1 of this study presents the experimental results of beam-column joints in soft-first story buildings, in which the first-story column depth is twice that of the upper stories. In Part-2, strut-and-tie models (STMs) are developed for the specimens. The flow of internal forces shown by the developed strut-and-tie models agreed with the observed cracks. The developed models for the joints are different from those for the usual joint: the main difference is that, in the specimens large struts were extended into the wall panel. STMs are also developed for the total frame to understand the overall equilibrium. Based on the STMs, new design equations are proposed, where the joint strength is evaluated as the sum of the flexural strengths of the beam and the second-story column. In I-type joint, the effect of the wall panel and beam stirrups is also considered. In O-type joint, the effect of the hoops in the joint is also considered.

This study describes the strength of reinforced concrete (RC) beam-column joints in buildings with a soft first story. In such buildings, the sections of first-story columns are usually much larger than those of second-story columns to prevent story collapse. To investigate the strength of the beam-column joints, four specimens are constructed and the test is conducted with two types of joints: 1) the first-story column is extended toward the inside of the building (I-type joint) and 2) the first-story column is extended toward the outside of the building (O-type joint). The test parameters are the hoops in the beam-column joint, beam reinforcements, stirrups in the beam, and axial force. The beam longitudinal reinforcements are effective to prevent the yielding of the beam reinforcements in both types of joint. Stirrups in beam and hoops in joint strengthen I-and O-type joints, respectively. The force-resistant mechanisms of these joints are different from those of usual beam-column joints because sign of the bending moment in the second-story column is the same as that of the first-story column. The failure modes of these joints are also different from those of the usual beam-column joints.