Similarity In Thermal-structural Response Of Reinforced Concrete Beams With Different Scales In Fire

At present,most of the experimental researches 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 fire behavior of independent components precisely,it requires large-scale site and test equipment and costs a large amount of money.Also,the size of the test specimens is limited,and it is difficult to carry out repetitive tests or controlled trials,so the test data obtained are limited.Fire tests on RC scaled models could help establish the similarity relationship in thermal-structural response between models and prototypes,so that the fire response of prototypes could be predicted from test results of models.Besides,reduced-scale fire tests could save costs and resources,making repetitive tests easy to implement,and promote the research on fire behavior of RC specimens from component level to structure level.Based on existing similarity theory,experimental study and finite element analysis(FEA)on RC beams with different scales under fire conditions are conducted to establish similarity relationship in thermal-structural response between prototype beams and model beams.The main research contents and conclusions of this paper are as follows:1.This paper introduces the conditions for similarity in thermal and structural response of RC beams with different scales and the calculation methods for model fire curve,then FEA software is used to simulate and analyze the fire behavior of three-scaled RC beams at two load ratios,0.4 and 0.6.Two calculation methods for model fire curve,radiative model and convective model,are used to determine the similarity in temperature distribution,mid-span deflection and fire resistance among three-scaled beams,verifying the similarity theory of RC beams under fire conditions.The results of FEA show that:(1)Whether convective model or radiative model is used to determine the fire curve for model beam,the heating rate of the model beam is much higher than that of the prototype beam,especially at the early stage of the test.If standard fire curve is used as the fire curve for prototype beam,then the required furnace temperature for model beam is too high to be achieved at the early stage of the test.(2)In comparison with convective model,radiative model is better in determining the fire curve for model beams,since FEA results show that temperature distribution and mechanical behavior of the specimens exhibit better similarity when using radiative model.So radiative model is adopted as the calculation method of fire curve in the subsequent fire tests.2.Fire tests and numerical simulation on six RC beams with three scales without pre-damage are carried out to investigate the similarity in temperature distribution and mechanical behavior of three-scaled RC beams under fire conditions,meanwhile,the influence of load ratio on the similarity of specimens is examined.Results show that:(1)The similarity in temperature distribution of different scaled beams is good.However,the similarity in mechanical behavior and fire resistance under fire conditions is influenced by size effect;(2)Load ratio has no significant influence on the similarity in temperature distribution inside the beam,but it has an effect on the similarity in mechanical behavior and fire resistance of the beams.The larger the load ratio,the worse the similarity in fire resistance;(3)There is some difference between FEA results and test results since FEA doesn't consider the size effect in mechanical property of the beams at high temperatures.3.Static load tests and fire tests on nine RC beams with three scales are carried out successively to investigate the effects of different degrees of damage(crack and residual deformation)on fire behavior of RC beams,also the similarity in temperature distribution and mechanical behavior of pre-damaged RC beams under fire conditions is explored.Test results show that:(1)The mechanical response of RC beams with different scales under static load at room temperature is influenced by size effect.(2)The cracks formed in static load tests have no significant effect on the overall temperature distribution in the specimens,only the temperature of local concrete could be affected.(3)The similarity in temperature distribution of pre-damaged RC beams in fire is good,while the similarity in mechanical behavior and fire resistance is affected by the degree of damage and size effect at high temperatures.(4)When adjusting the proportion index of fire exposure time from 2 to 1.7-1.8,the fire resistance of different scaled beams could achieve good similarity.