Nonlinear Finite Element Analysis Of Beam-column Joints For The Steel Frame In Fire Condition

Beam-column joints is an important part of the steel frame. Fire resistance of the joints has a close relationship with the fire resistance of the entire steel frame. The destruction of the beam-column joints under fire loads can easily lead to the collapse of the entire structure. Currently, fire resistance research on steel structure focuses on the fire resistance of single component. The fire resistance of joints has been sparsely studied. Judging from the related specification, fire resistant design of steel structure is very rough. The research on fire resistance of steel beam-column joints has important practical and theoretical significance. In order to study the problem, the following research work was done:(1)Based on validation of finite element model at room temperature, extended end plate connection flush end plate connection and the flexible end plate connection joints are studied by non-linear finite element method under static load conditions at room temperature. The moment-rotation curves and failure modes of the end plate connections are obtained. The value of ultimate bearing capacity and initial rotational stiffness of the joints can be obtained from the curves. The results show that the value of the ultimate bearing capacity and initial rotational stiffness of the extended end plate connection is maximum in the three kinds of joints, and the value of the ultimate bearing capacity and initial rotational stiffness of the flexible end plate connection is minimum.(2) Based on validation of finite element model at elevated temperature, three beam-column end plate connections are studied by nonlinear finite element analysis under elevated temperature. the temperature-rotation curves of three end-plate connections are obtained. Taking into account the rotation particularity of the flexible end plate connection, temperature-rotation curve of flexible end plate connection is specially discussed. In addition, large number of parameters are discussed. The discussion show that load ratio and stiffeners have a greater impact on the critical temperature of the joints; the thickness of end plate、types of connection the friction coefficient and the thickness of the column flange have no influence on the critical temperature of joints.(3) Furthermore, the actual steel frame beam-column joints are spatial. Based on finite element model validation, the fire resistance of four welded joints for spatial steel frames are simulated and the temperature-rotation curves are obtained. The temperature-rotation curves show that corner joints and exterior joints which are in the direction of the column flange connected to the beam have lower critical temperature; inner joints and exterior joints which are in the direction of the column web connected to the beam have higher critical temperature; the lateral loads acting on the end of beam When the lateral load acting on the end of beam which is in the direction of the column web connected to the beam is greater than that in the direction of the column flange connected to the beam, the critical temperature of corner joints is significantly reduced. Consequently,in fire resistant design, fire protection on corner joints and exterior joints which are in the direction of the column flange connected to the beam should be strengthened.