Control of Dynamic Collapse Mechanism for Reinforced Concrete Frames

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It is desirable to form plastic hinges at all beam ends and bottom of columns in first story for reinforced concrete (R/C) frames to dissipate hysteretic energy due to strong ground motions. The axial load developed in exterior columns in these frames is varied from compression to tension because of the change of the direction of earthquake attacks. However shear behavior and interaction between shear and flexure have been not studied for R/C columns subjected to varying column axial load. It is very important to prevent shear failure of columns at first story. This makes possible to control the collapse mechanism of R/C frames under earthquakes. Then four columns with half-scale were tested to investigate shear strength under tensile axial load. Column section was 300mm x 300 mm. The shear span to column depth ratio was 1.0. Concrete compressive strength was 222 to 247 kgf/cm^2. Yield strength of hoops was 4030 kgf/cm^2.With the increase in tensile axial load of specimens diagonal cracks developed drastically and shear strength decreased moderately ; i.e. that of the column with axial stress of 0.2 times the concrete compressive strength decayd to 93 % of that without column axial load. On the contrary, ductility of specimens was improved by the increase in tensile axial force of a column. Shear strength of the column with shear reinforcement of 0.43 % was 1.3 times larger than that with shear reinforcement of 0.21 %. Shear resistant mechanism of columns under tensile axial load, moreover, will be studied using the strain and bond stress distributions along longitudinal reinforcement, the strain in shear reinforcing bars and local deformation due to shear and flexure.