Calculation Method Of Middle Span Deflection Of Castellated Composite Beams Under High Temperature

Castellated composite beams are widely used in engineering,and the research on the fire resistance of castellated composite beams is also increasingly abundant at home and abroad,but the research on the mid-span deflection change process and ultimate deformation of castellated composite beams under high temperature is less.The failure process of castellated composite beams under high temperature is as follows: the strength and rigidity of the members are reduced by high temperature,the web yields between holes,the lower flange moves sideways,the section rigidity is redistributed,the web buckles,the lower flange twists,and the mid-span deflection of the members suddenly increases,reaching the deformation that is not suitable for continuous bearing.It can be seen that the failure of castellated composite beams at high temperature is mainly determined by their deformation.It is necessary to study the development process of mid-span deflection and ultimate deformation of castellated composite beams.Based on the fire resistance experiment of the castellated composite beam,the model of the castellated composite beam is established by using finite element software,and the thermal-mechanical coupling simulation is carried out on the model.The deformation and failure of the components in the test and simulation are compared and analyzed to verify the reliability of the simulation.The finite element model is analyzed by parameterization,The dimension parameters of steel beams(pass,hole height ratio,distance to height ratio,web height to thickness ratio,flange width to thickness ratio,beam length)are studied.The influence of concrete slab size parameters(concrete slab thickness)and load parameters(load ratio)on the development process of mid-span deflection and ultimate deformation of castellated composite beams is studied.It is concluded that load ratio and beam length are the main parameters that affect the development of mid-span deflection and ultimate deformation of castellated composite beams under high temperature.The larger the load ratio and the longer the beam length,the faster the mid-span deflection of the castellated composite beam develops and the greater the ultimate deformation.Other steel beam size parameters and concrete floor size parameters are secondary parameters that affect the mid-span deflection development and ultimate deformation of castellated composite beams under high temperature.Extracting the time-displacement curve obtained by thermal-mechanical coupling analysis of the model,it is found that the deformation rate of castellated composite beams starts to accelerate when the mid-span deflection reaches about1/20,and the deflection changes abruptly when the mid-span deflection reaches about 1/10,resulting in instability and failure of members.However,the common solid web composite beam has reached the fire resistance limit when the mid-span deflection reaches 1/30.Therefore,it is proposed that the mid-span deflection reaches 1/20-1/10 of the beam length as the criterion for judging the failure of castellated composite beams at high temperature.The equivalent bending stiffness calculation formula of castellated composite beams is established by using reduced stiffness method and fee truss method.Models of different web height-to-thickness ratio,flange width-to-thickness ratio and concrete floor width-to-thickness ratio are established to verify the formula.The results show that the formula has high accuracy.The effects of temperature,web height-thickness ratio,flange width-thickness ratio and concrete floor width-thickness ratio on the equivalent bending stiffness reduction coefficient of castellated composite beams at high temperature are compared and studied,of which temperature and concrete floor width-thickness ratio have greater effects.The fitting formula of stiffness reduction factor is established,and the calculation method of mid-span deflection of castellated composite beams within a certain application range is proposed.According to the actual application size of castellated composite beams in engineering,a model is established to verify it.The results show that the formula has high accuracy and can provide reference for fire resistance design of castellated composite beams.