Research On Sesimic Behavior And Moment-rotation Relations Of Exposed Column Base Connection With Four Bolts Subjected To Minor-axial Bending Moments

The semi-grid behavior of column base connections has great influence on the upper structure of the steel frame structure.While as so far,research on the seismic behavior and moment-rotation relations of exposed column base connections with four bolts subjected to minor-axial bending moments is very limited.And existing formulas and calculations can not be used for flexural capacity and initial rotation stiffness of column base connections.To solve the above problems,experiments,FEM method and probabilistic design based on FEM method are applied.The main work carried out in this paper are as followed.Experiments were carried out.3 specimens of exposed column base connections with four bolts were fabricated,differences of which were diameters of the bolts and thickness of the baseplate.The specimens were under minor-axial low-frequency cyclic loads with axial compression.At last,the failure mode,hysteresis curve,back-bone curve,flexural capacity,initial rotation stiffness,ductility and energy dissipation of the joints were analyzed.When the diameter of the anchor bolts are small,tensile failure occurred at the anchor bolt.When the diameter of anchor bolts are large,the weld at the joint of column flange and bottom plate of steel column was torn.The hysteretic curves of the 3 specimens show obvious "pinch" effect,and seismic performance of them is poor,while ductility of them is good.Based on the checked FEM model,variable parameters analysis about the column base connection were carried out.The variable parameters were axial compression,the distance between bolts center and the edge of column web,the distance between bolts center and the edge of the baseplate,the distance between bolts center and the edge of column flange(equal to the distance between 2 bolts on the same side),thickness of the baseplate,and diameter of the bolts.Increasing the axial compression at the top of the column can significantly improve the flexural capacity and initial rotation stiffness of the exposed column base connections subjected to minor-axial bending moments.Increasing the distance between the bolt and the edge of the column web has little effect on the flexural capacity of the joints.But it can improve the initial rotation stiffness of the joints.Changing the distance between the bolt and the bottom edge has little effect on the flexural capacity and initial rotation stiffness of the joints.Increasing the distance between the bolt and the edge of the flange(reducing the distance between bolts on the same side)has little effect on the bearing capacity of joints,but it will reduce the initial rotational stiffness of joints.Increasing the thickness of floor can greatly improve the flexural bearing capacity and initial rotational stiffness of joints.Increasing the diameter of bolts can greatly increase the flexural bearing capacity of joints,but it will reduce the ductility of joints and make the initial rotational stiffness slightly reduced.P-△ effect and thickness of the grout were also considered.P-△ effect can be eliminated by considering additional bending moment when processing data.The mechanical properties of joints are almost unaffected when the thickness of mortar layer is thinner,but the flexural capacity,initial rotational stiffness and ductility of joints are reduced when the thickness is larger.Based on the method of probabilistic design,formulas for calculating the flexural capacity and initial rotation stiffness of the column base connections were proposed,and based on the formulas,moment-rotation relations were also proposed.Based on the cyclic simulation analysis of MATLAB and ANSYS,the semi-analytical solution of the initial rotation stiffness and flexural capacity formula of column base connections is obtained by combining the response surface method,and the moment-rotation model is obtained by combining the three-parameter model of the joints.