EVALUATION ON ULTIMATE STRENGTH OF BEAM-COLUMN JOINT IN RC SOFT-FIRST-STORY FRAME WITH COLUMN EXTENDED OUTSIDE BUILDING: Study on joint subjected to closing load

In reinforced concrete buildings with soft-first-stories, sections of columns in the first-stories are usually enlarged from those of the second-story columns to prevent story collapse at the first stories. Test results (Ref. 4) show the strength and stiffness of this type of frames are lower than the expected values by assuming flexural failure at the top of the first-story column. These lacks are due to the damages on the beam-column joint at the top of the first-story column. In this paper, a method for calculating the strength of the beam-column joints is proposed focusing the cases that columns extended outside the building and the joints are subjected closing loads. The method is based on the failure mechanism observed in the tests and able to consider anchorage failure of beam top bars. Two failure modes named the column failure mode and the beam failure mode are considered for a beam-column joint. The column failure mode is the flexural failure of the first-story column at the beam bottom face. The moment capacity for the column failure mode is calculated by flexural analysis assuming the whole cross-section of the first-story column is effective (Fig. 6b). The beam failure mode is the flexural failure at the inclined section connecting the two reentrant corners of the beam-column joint (Fig. 7). The moment capacity for the beam failure mode is calculated from the moment equilibrium between external forces and resultant forces at the section (Eq. 5). Anchorage failure by the local crushing of the concrete should be considered in determining the forces in the beam top bars in the calculation while yielding is assumed for the column bars and the lateral reinforcement in the joint. The local failure of concrete is assumed to occur when the compressive stress of concrete inside the bend of the anchored bars (fb) reaches the compressive strength of concrete (fc). The bearing force acts inwards in the radial direction against the tensile forces of the top beam reinforcement and the first-story column reinforcement (Fig. 9). The compressive stress fb is derived by dividing the bearing force by the area determined by the product of the width of the first-story column and the width of the strut. For the case that the reinforcements are arranged in multi-layer, the sum of the compressive stress of concrete at each layer is treated as fb. The strengths calculated by the proposed method agree with the test results at a rate of 1.05 to 1.23 and evaluate the observed failure modes appropriately (Fig. 17). In addition to the method to evaluate the strength of beam-column joints, a design process to prevent the anchorage failure is also proposed. In the design process (Fig. 19), it is examined that the compressive stress of concrete inside the bend of bars at that the tensile forces of the bars reach yield strengths is lower than the compressive strength of concrete.