Study On Flexural Capacity And Fire Resistance Of High Strength Q690 Steel Flush End-plate Beam-to-column Connections

High strength steel has been gradually applied in building structures.The damage and collapse of high strength steel structures caused by fire will not only cause casualties,but also cause huge economic losses.Beam-to-column connection is a key part of the structure,and its reliable connection under fire is an important prerequisite to ensure the fire safety of the structure.At present,there is little research on the fire resistance of high strength steel beam-to-column connections.In this paper,the high strength Q690 steel flush end-plate connections were taken as the research object,and the experimental study and finite element analysis of the flexural and fire resistance of the connections were carried out,and the rotation-temperature curve model suitable for predicting the fire response of high strength Q690 steel flush end-plate connections was established.The main research contents and conclusions are as follows:(1)Experimental study on flexural behavior of high strength Q690 steel flush endplate connections: two flexural tests were carried out to investigate the influence of endplate thickness on flexural behavior of flush end-plate connections.It is found that increasing the thickness of end-plate can effectively improve the flexural capacity and initial rotational stiffness of connections,but the rotational capacity is significantly reduced,and the failure components turn from end-plate to bolts.(2)Experimental study on fire resistance of high strength Q690 steel flush end-plate connections: three constant load heating tests were completed,and the effects of end-plate thickness and axial compression ratio of column on fire resistance of flush end-plate connections were investigated.It is found that increasing the thickness of end-plate has no obvious effect on the improvement of fire endurance and critical temperature,but obvious effect on the reduction of rotational capacity.Increasing the axial compression ratio of column will greatly reduce the fire endurance and rotational capacity.(3)Finite element analysis of flexural performance of high strength Q690 steel flush end-plate connections: the finite element analysis model of flexural performance of connections was established,and the reliability of the analysis results was proved by the test results.The effects of axial compression ratio of column,thickness of end-plate and diameter of bolts on flexural behavior of flush end-plate connections were investigated by parametric analysis.It is found that increasing the axial compression ratio will make the failure mode of the connection transform from end-plate buckling to bolt failure.Increasing the thickness of end-plate can significantly improve the initial rotational stiffness and flexural capacity,but the rotational capacity will decrease.Increasing bolt diameter properly can significantly improve bending capacity and rotational capacity.(4)Finite element analysis of fire resistance performance of high strength Q690 steel flush end-plate connections: the finite element thermo-mechanical coupling analysis model of fire resistance performance of connections considering the thermal creep effect of steel was established,and the accuracy and reliability of finite element analysis were verified by comparing the test results.The influences of axial compression ratio of column,bending moment ratio of beam,thickness of end-plate and diameter of bolts on fire resistance of flush end-plate connections were investigated by parameter analysis.It is found that increasing the axial compression ratio will lead to the change of the failure components and reduce the rotational capacity.Increasing bending moment ratio has obvious adverse effects on fire endurance,critical temperature and rotational capacity.Increasing the thickness of the end-plate and the diameter of bolts can greatly improve the fire resistance and the rotational capacity,but the excessive thickness of the end-plate or the diameter of bolts will obviously reduce the rotational capacity.(5)Derivation of angle-temperature relationship of high strength Q690 steel flush end-plate connections: based on the moment-rotation curves of high strength Q690 steel flush end-plate connections obtained by finite element parametric analysis,the shape coefficient formula of the moment-rotation relationship of high strength flush end-plate connections was obtained by fitting the three-parameter power function model,and the expression of the rotation-temperature relationship of high strength flush end-plate connections at elevated temperatures was derived.