Experimental Research On Fire Resistance Of Reinforced Concrete Beam-supporting Column Transfer Structure

The importance of reinforced concrete beam-supporting column transfer structure is now amplified because it has advantages such as direct force transfer and good mechanical performance.The research on fire resistance of this kind of structures is of great significance for fire resistance design,fire safety evaluation and post-fire evaluation of existing structures.This paper uses the method of combining experimental research and finite element analysis to conduct a systematic and in-depth study on the fire resistance performance of this kind of structure,including the following aspects:(1)According to the characteristics of a reinforced concrete beam-supporting column transfer structure,two types of beam-supporting column joint unit specimens are designed,and fire resistance tests are conducted on the specimens.Adopting the ISO 834 international standard heating curve for temperature control,the temperatures at the measuring points on the concrete and rebar inside each joint unit specimen and the vertical deformation at the feature position of each joint unit specimen are recorded.Also,the corresponding curves are drawn and the fire resistance rating is obtained.The experimental results show the following contributions: after the reinforced concrete beam-supporting column transfer structure joint unit specimen reaches the fire resistance rating,the bending deformation of the transfer girder becomes more concentrated;the damage modes of the beam sections are failed to be bended.;the cracks are largely located approximately 400-500 mm from the maximum bending moment section;and the longitudinal rebar in the crack section of the transfer girder can be broken under high temperatures.(2)The post-fire residual mechanical behavior of two types of full-size beam supporting column transfer structure joints is tested.We observe the deformation and crack propagation of joint specimens under load,construct the load-deflection curve,record the post-fire residual strength and analyze the failure mode.The results show that longer exposure to high temperature is correlated with a smaller residual capacity of the joint units.When the specimens reach the ultimate bearing capacity,cracks form in the A-type joint unit specimens are aligned with the central axis of the supported column,and several vertical cracks appear in the core area of the joint.The cracks of the B-type joint unit are primarily on the compressionside of the supported column,and many prominent inclined cracks are in the core area of the joint that extend from the bottom to the compression zone of the transfer beam.(3)Based on the test results,using ABAQUS-a finite element analysis software,considering the influence of raising temperature curve,loading ratio,protection thickness,additional bars,surface number in fire of transfer girder,the fire resistance of beam supporting column transfer structure joints is analyzed and the influence rule to beam supporting column transfer structure joints of fire resistance is summarized.Analysis shows that,raising temperature curve and maximum temperature have a great influence to fire resistance;Moreover,fire resistance tend to decrease with the increase of loading ratio and the reduction of cover thickness.Hanging steel bars in the transfer girder can improve the fire resistance and avoid sudden failure.The more surfaces of the transfer girder in fire,the shorter the fire resistance.In the same loading ratio,two kinds of beam supporting column transfer structure joints have different damage characteristics and fire resistance.(4)Based on the test results,by using finite element analysis software,considering the influence of the increasing,decreasing temperature curve,heating time,and number of transfer girder surfaces exposed to fire,the residual mechanical behaviours of the beam-supporting column transfer structure joint units after fire are analysed and the influence rules of parameter variation summarized.According to the increasing temperature trends in ISO834,the analysis shows that longer heating times,higher temperatures and having more surfaces exposed to fire decrease the bearing capacity of the beam-supporting column joint unit after fire and increase the deflection rate under load.With the same maximum temperature in the increasing and decreasing temperature curve and different variation trends,the residual load-bearing capacity of the beam-supporting column joint unit is hardly changed,but the deflection increase rate is significantly changed.The shorter the duration of the high temperature is,the slower the deflection increase rate.Besides,the lower the cooling rate is,the faster the deflection increase rate.(5)ABAQUS software is used to analyze the fire resistance of the beam supporting column transfer structure.The characteristic point displacement and the whole deformation mode of reinforced concrete beam supporting column transfer structure under different fire conditions are studied.The results show that with the increase of heating time,the deflection of the transfer girder and beams of upstairs increases gradually.The bending deformation ofthe frame column also increases gradually,pointing outside of the structure,and the maximum deformation occurs near the fire floor.Under asymmetric fire condition,the whole structure tends to incline to the side of the fire room.Due to the interaction among the members in the transfer structure,no matter how the fire condition is,the whole deformation of the transfer structure will occur,just the deformation mode is different.(6)On the basis of studying the high temperature deformation of beam supporting column transfer structure,the internal force redistribution law and plastic hinge development of the whole structure are further analyzed.The research result shows that the internal force redistribution of the transfer structure at high temperature starts from the thermal expansion of the members directly exposed to fire,and transfers from the high temperature area to the normal temperature area under the interaction between the members.The changes of internal forces of structural members are mainly manifested as follows: The beam section produces obvious pressure or tension;The bending moment of beam and column members in high temperature zone changes obviously and the direction may change;The internal forces of beam and column members that not directly affected by the high temperature will change to varying degrees under the interaction between the members.Plastic hinge will be produced when the internal force of member section changes to a certain extent.In order to ensure that the transfer structure can continue to bear the load after the local section plastic hinge is produced,measures should be taken to improve the rotation capacity of beam,column ends and increase ductility,as well as to improve the deformation and internal force redistribution capacity of the whole transfer structure under high temperature.(7)Finally,through the test and theoretical analysis of the reinforced concrete beam-supporting column transfer structure,the fire-resistant design proposal of this kind of structure is put forward.