SHEARING FORCE-DEFORMATION RELATIONSHIP OF CONCRETE-FILLED SQUARE STEEL TUBE BEAM-COLUMNS UNDER MONOTONIC LATERAL LOADING

The advantages of concrete-filled steel-tube (CFT) columns include high strength and remarkable ductility, since the steel tube provides confinement to the concrete while the concrete prevents local buckling of the steel tube. CFT column composite frame systems with steel beams have been widely used in moment-resisting frame systems for mainly office buildings. The CFT columns at typical floors are short and have small ratios of buckling length to cross section depth, and those at entrances of lower floors are slender and have large ratios of buckling length to cross section depth. CFT columns from short to slender have been used in buildings. In order to verify the structural safety of CFT structures in the ultimate state against huge earthquakes, etc., a restoring force characteristic model incorporating a strength reduction after ultimate strength is required. The stress states of CFT columns applied to buildings are in a state of flexural-shear stress, and the structures are designed for the predominant flexural yielding. It is thought that accurate evaluation of a shearing force-deformation relationship under axial force leads to design of columns that have the features of CFT columns. For a shearing force-rotation angle relationship, which is the basis of the restoring force characteristic model of CFT columns from short to slender, it is important to devise a simple model incorporating a reduction of strength after ultimate strength. This paper proposes a new simplified model of a shearing force-rotation angle relationship for square CFT beam-columns under monotonic flexural-shear loading to estimate the elasto-plastic flexural-shear behavior of beam-columns from short to slender. The square CFT beam-column model incorporates a reduction in strength after ultimate strength. The proposed simplified model is a multi-linear model having a crack strength point, a yield strength point, an ultimate strength point and strength reduction points. In order to evaluate flexural-shear behavior of short to slender columns with a single model, the column member is divided into elasto-plastic parts at the column ends and a remaining elastic part. For the elasto-plastic part, the rotation angle of each point is calculated using the bending moment-curvature relation model proposed by the author and a newly proposed evaluation formula for curvature magnification. Predictions from the proposed model of the shearing force-rotation angle are found to agree approximately