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Seismic behavior and detailing of steel beam to CFT column connections

Abstract


Concrete filled steel tubular (CFT) columns have advantages in strength and ductility. Even under fire attacks, the core concrete could maintain its axial load capacity and thus the strict requirement for fire proof may be liberated. Furthermore, recycling of steel tubes is relatively easy. Typical CFT columns are in the form of square tubes or circular pipes as required by architectural restrictions. Unlike the widely-used box-section columns, the use of circular CFT columns has been limited due to the complexity of the connections to such columns.
Recently, skylines in modern cities continue to rise because of urban renewal. The columns in the lower stories in a high-rise building have to sustain high axial load and bending moment. CFT columns have advantages over conventional l and RC columns because the steel tube serves as formwork and offers superior confinement to the infilled concrete, thus improving its strength and ductility under high axial load. However, the complex design and detailing for moment connections have to be further improved, simplified, and verified with experiments.
This research proposed a beam-flange-through-type beam-column joint connection for CFT columns and tested four beam-column joint specimens to examine the effect of infilled concrete, beam flange stiffeners, and width of beam on the joint shear strength. Construction of the specimens showed that the proposed connection details are practical and easy to be implemented. Cyclic loading test results showed that the infilled concrete significantly increases the joint shear strength.
The use of beam-flange stiffeners and increasing the beam width also have significant contribution to joint shear strength. Current shear strength provisions in the SRC code can be conservatively used to estimate the shear strength of the proposed beam-column joint. However, the shear strength contribution from concrete is significant under-estimated. This research proposed a strut-and-tie joint shear strength model for concrete joint shear strength. Comparison with the test results showed that the proposed model can accurately estimate the shear strength contribution from concrete of the proposed beam-column joint. However, the shear strength contribution from concrete is significantly under-estimated. This research proposed a softened-strut joint shear strength model for concrete joint shear strength. Comparison with the test results showed that the proposed model can accurately estimate the shear strength contribution from concrete of the proposed beam-column joint. Based on experimental obervations and analytical studies, modification to the current code provision on joint shear strength contribution from concrete is proposed. Moreover, the upper and lower limits on the width of the beam flange are proposed to address the constructibility issue related to concrete infilling and shear transfer from the beam to the joint. Furhermore, design suggestions on the beam flange stiffeners are proposed.