Study On Fire Behaviour Of Steel-concrete Composite Bending Members

Steel-concrete composite structures are widely used in actual civil engineering.The mechanical properties of structural materials change rapidly in fire,which adversely affect the load-bearing capacity and cause safety risks.Therefore,the study of the fire resistance of the combined structure under fire has essential significance.At present,domestic and foreign scholars have carried out a series of research on fire performance of steel-concrete composite structures.However,most researches focus on the single building members,and there are few researches on the components or the whole structure with boundary constraints.Moreover,the restrained forces by the surrounding boundary conditions was not tested in the previous experiments,which has an important influences on the mechanism of composite bending members.The fire resistance of a single building member is significantly different from that of the same member in the overall structure.Therefore,the fire resistant performance of bending members(composite beams and composite floors)were studied by using the experimental and theoretical method in this thesis.Combined The main research work is conducted as follows:(1)The fire resistant performance tests of four full-scale steel-concrete composite beams with end restraints were carried out.The connection mode of composite beam was stud connection,and the one of concrete floor was embedded connection.Tension-compression type sensors were set at the end of composite beams to measure the restrained internal force.During the experiments,the temperature field,displacement and end restraint internal force of steel-concrete composite beams under fire were measured.The end restraint internal force,midspan axial force and failure mode of end restrained composite beam under fire were studied.The influence of parameters like load ratio and connection mode on the fire performance of end restrained composite beams was analyzed.(2)Considering secondary beams in different span directions in two common conditions respectively,the fire performance tests of two steel-concrete composite floors with integral space constraint were carried out.During the experiments,The temperature field distribution,midspan deformation,horizontal displacement,floor edge corner and floor corner constraint internal force of the steel-concrete composite floor under fire were measured.The temperature distribution of the composite floors under two common conditions,the failure mode of the composite floors under two common conditions and the influence of the internal force of the floor corner constraint on the fire performance of the composite floors were studied.(3)The acoustic emission characteristics of composite floor under fire were analyzed,and the basic acoustic emission parameters,RA-AF correlation analysis and value-energy change of composite floor under fire were studied.The relationship between the acoustic emission characteristics and the failure mode and the internal damage mechanism of the composite floor under fire was established,respectively.(4)According to the basic assumption of classical yield-line theory,the calculation principle of segment equilibrium method and virtual work method,the calculation model of ultimate bearing capacity,based on segment equilibrium method and virtual work method considering membrane effects was put forward.The ultimate bearing capacity of composite bending members under fire was calculated by the parameters including restrained internal force,reinforcement temperature and vertical deflection.Compared with the calculation results of classical yield-line theory,the improved ultimate bearing capacity calculation model is more reasonable.