Similarity Of Thermal-Structural Response Of Reinforced Concrete Columns With Different Scales And Influence Of Concrete Spalling Under Fire

At present,most of the experimental research on fire resistance of concrete structures focus on single full-scale components.Although fire tests on reinforced concrete(RC)full-scale components could reflect the response of independent components precisely,it costs a lot and requires large test equipment,which limits the scale of specimens.Meanwhile,it is difficult to conduct repetitive tests and hard to promote the fire research of RC specimens from independent component level to structure level.Reduced scale fire tests make repetitive tests feasible due to its economically.The key point of conducting reduced scale fire tests and predicting the response of prototypes from models is establishing the similarity between prototypes and models.The fire resistance of RC columns as the main load-bearing member of structure had significant influence on the safety of structure.Experimental and finite element analysis(FEA)study on RC columns with different scales under fire were conducted to establish similarity in thermal-structural response between prototype and model columns,and the influence of concrete spalling was also studied.The main research contents and conclusions are as follows:1.Similarity conditions in temperature field of RC columns with different scales under fire and the corresponding method of model furnace curve were introduced.After verifying the ABAQUS model,a preliminary numerical simulation of the thermal-structural responses of three-scaled RC columns was conducted to compare the similarity in thermal-structural responses of three-scaled columns under three different model furnace curve calculation methods.The results show that:(1)Three calculation methods of model furnace curve require higher heating rate of the model than that of prototype,especially at the early stage.If the prototype furnace curve was determined by standard fire curve,then the equipment cannot achieve the heating rate of the model column at the early stage of fire test.(2)Compared with the method proposed by Ng and the convective model proposed by O'Connor,radiative model is more appropriate in determining the furnace curve of model since better temperature field similarity between model and prototype was exhibited when using radiative model.2.Fire tests on six RC columns with three scales were carried out to investigate the similarity in thermal-structural response of three-scaled RC columns under fire condition,meanwhile,the influence of load ratio on the similarity was examined.The results show that:(1)The temperature similarity of concrete of different scaled columns was favorable.However,the temperature similarity of longitudinal reinforcement was unfavorable due to the deviation of thermocouple position and concrete spalling;(2)Concrete spalling of large-scaled columns was more serious than that of small-scaled columns;(3)The axial deformation of three-scaled RC columns under same load ratio were similar at the early thermal expansion stage,however,with the rise of temperature at the later stage,the structural response exhibited nonlinear behavior,the axial deformation and fire resistance of specimens with different scales were not similar due to the concrete spalling and size effect of mechanical properties at high temperature.3.Finite element analysis(FEA) with concrete spalling considered were conducted on RC columns with different scales in fire based on “birth-death element” method of ABAQUS.The results show that:(1)Without considering concrete spalling,the calculated temperature field and fire resistance of the small-scaled specimens not experienced serious spalling were in good agreement with that of test.For large-scaled specimens with severe spalling,the calculated temperature of the concrete was in good agreement with test,while the calculated temperature of reinforcement deviated greatly from that of test,which means that concrete spalling had a great influence on the temperature of reinforcement,axial deformation and fire resistance,while only the local part of concrete temperature was affected.(2)With considering concrete spalling,calculated temperature of large-scaled specimens experienced severe concrete spalling was in good agreement with test,and the calculated fire resistance was closer to the test.(3)The simplified method of concrete spalling size and the modeling method considering spalling proposed in this paper can simulate the influence of concrete spalling more precisely.