Mechanical Behavior Analysis And Experimental Study On Connection Between Beam And L-shaped Column Composed Of Concrete-filled Steel Tubes

The research background of this paper is the frame structure system of a special-shaped column composed of concrete-filled steel tubes(S-CFST column),which could be applied to high-rise steel residential buildings.S-CFST column can be divided into L-,T-and Cross-shapes.In the high-rise steel residential buildings,the axial bearing capacity and stiffness can be met by increasing the web width and distance of mono columns of L-CFST column.The L-CFST column is used as the corner column,while the exterior column and the interior column,has more economical and reasonable cross section compared that the T-CFST column and Cross-CFST column are used directly in the exterior and interior columns,respectively.Aiming at the performance of the L-CFST column-beam joint,a comprehensive research is carried out in this paper,and the main contents include:(1)In order to study the seismic behavior of L-CFST column and steel beam connections,seven full-scale specimens were tested.The variables include the joint type,the axial compression ratio,the presence of concrete,the width-to-thickness ratio and the internal extension length of the vertical stiffeners.The test results indicated that both the corner and exterior joint specimens failed due to local buckling and crack within the beam flange adjacent to the end of the side plates.However,the failure modes of the interior joint specimens primarily included local buckling and crack at the end plates and curved corners of the beam flange.(2)In order to study the shear strength of panel zone,five full-scale specimens were tested.The variables include the joint type,the axial compression ratio,the cross-section area of the vertical stiffener and the presence of concrete.The results indicated that the cross-section area of the vertical stiffeners played a critical role in the performance of the panel zone.Two types of failure modes were observed outside the panel zone,including a fracture of the vertical stiffeners or of the beam flange connecting plate.The shear deformation of the panel zone was obvious in the exterior joint specimens,and the corresponding maximum shear deformation reached 0.05 rad.The scope of the panel zone in the corner and exterior joint specimens was also determined by the strain distribution.(3)By using the reasonable constitutive model,boundary conditions and loading conditions,the nonlinear numerical simulation of the 12 specimens was carried out,and the accuracy of the nonlinear model was verified by comparing with the experimental results.On this basis,the mechanical behavior and failure law of the joints were studied by analyzing the whole process of the failure process of the joints.(4)Four optimization methods were proposed and compared for the fracture problem of the beam flange connecting plate.Two factors were considered and analysed for the problem of the vertical stiffener fracture along the column flange.The crack propagation on the beam flange connecting plate and vertical stiffener can be reduced and even eliminated using a thickened connecting plate and a minor width-to-thickness ratio,respectively.Parametric analysis is carried out on the peak bearing capacity of widened beam flange connection with end plates.The results showed that the length of widened-beam extension,the thickness and with-to-thickness ratio of end plate have great influence on the bearing capacity.Finally,it is found from the parametric analysis that the shear strength of the panel zone is directly related to the increase of the wall thickness,the width and thickness of the short beam web,the strength of the steel tubes and the strength of the concrete.(5)Based on the superposition principle and the "steel frame yield mechanism model",the shear capacity calculation formulas of the panel zone was proposed,the ratio of the proposed formula to the test results under the yield and peak state is 0.84 and 0.94,respectively.Based on the yield line theory,the formulas for calculating the flexural capacity of widened beam flange connection with end plates,which considering the influence of the stress dispersion angle and the end plate buckling,was established.The ratio of the proposed formula to the test results under the yield and peak state is 0.82 and 0.86,respectively.(6)The seismic design methods were summarized and generalized in this paper.It included the calculation method of strong column and weak beam,the maximum torsional stress calculation method of L-CFST column around the shear center,the checking method of shear and flexural capacity of the connections.Some suggestions were also given for the anti-seismic construction measures of the connections.