Research On Performance Of Flange-Welded/Web-Bolted Connections Of Steel Frames In Fire

In recent years,with the rapid development of China’s economy,the construction industry has shown a vigorous development trend.Steel structure is increasingly favored by owners and engineers because of its advantages of light weight,high strength,environmental protection,energy saving and fast construction speed.As an important part of steel frame structure,flangewelded/web-bolted connections of steel frames are mainly used to transmit the internal force under loads such as gravity,wind and earthquake.However,in the ultimate condition,the connections not only bear the above loads,but also have a vital impact on the overall performance of the structure and the prevention of continuous collapse.Among all kinds of disasters in the world today,building fire is still one of the disasters with the highest frequency and the widest influence range,which has caused huge losses to the safety of society and people’s property.Therefore,it is extremely urgent to study the fire resistance performance of engineering structures.However,at present,there are few researches on the fire resistance of steel frame flange-welded/web-bolted connections at home and abroad,especially the experimental research on the fire resistance of steel frame flange-welded/web-bolted composite connections.Based on the above background,this paper carries out experimental research and numerical simulation on the fire resistance of steel frame flange-welded/web-bolted connections,and reveals their behavior response and failure mode under fire.On the basis of parametric analysis and the principle of component method,a practical calculation method for flexural capacity and initial rotational stiffness of flange-welded/web-bolted connections at high temperature was established.Fire tests were carried out on three flange-welded/web-bolted connections of steel frames.The effects of load ratio and slab thickness on the fire resistance of steel frame flangewelded/web-bolted connections were investigated,and the connection performance and failure mechanism of steel frame flange-welded/web-bolted connections under fire were revealed.Through the fire resistance test,the furnace temperature-time curve,specimen temperature-time curve,specimen displacement-time curve,joint rotation-time curve,joint rotation-temperature curve,fire resistance time and critical temperature of each specimen of the steel frame flangewelded/web-bolted connections were obtained.Based on the finite element software ABAQUS,the accurate finite element analysis models of three steel frame flange-welded/web-bolted connections were established,and the mechanical properties and failure modes of each specimen under fire were numerically simulated.By comparing the finite element results with the test results,the accuracy of the finite element analysis model was verified.Using the verified finite element analysis model,a large number of analyses were carried out on the fire resistance of flange-welded/web-bolted connections of steel frames.The analysis results show that the load ratio,beam flange thickness,beam web thickness,concrete plate thickness and steel bar diameter are the key parameters affecting the fire resistance of steel frame flange-welded/web-bolted connections.Column axial force,column flange thickness,column web thickness,connecting plate thickness,transverse stiffener thickness and bolt diameter have little influence on its fire resistance.The initial rotational stiffness of the joint decreases rapidly with the increase of temperature.When the joint temperature reaches 200 ℃,the initial rotational stiffness of the joint decreases by about 1/3.When the joint temperature reaches 300 ℃,the initial rotational stiffness of the joint is about 1/2 of the initial rotational stiffness of the joint at normal temperature.When the joint temperature reaches 600 ℃,the joint basically loses the rotational stiffness.Based on the principle of component method,the flange-welded/web-bolted connections of steel frame is decomposed into several components which contribute greatly to the bearing capacity and stiffness of the joint.Then,the temperature calculation method of steel beam upper and lower flanges,steel beam web and steel bars in concrete floor in the flange-welded/webbolted connections of steel frame under fire was established,and the bearing capacity and stiffness of each component at high temperature were analyzed.Finally,a practical calculation method for flexural capacity and initial rotational stiffness of flange-welded/web-bolted connections of steel frames at high temperature is established through integrated analysis of various components.Comparing the calculation results of the practical method with the finite element results,the accuracy of the practical method in calculating the flexural capacity and initial rotational stiffness of steel frame flange-welded/web-bolted connections under fire was verified,which provides an important reference for the follow-up research of fire resistance of flange-welded/web-bolted connections of steel frames.