| Cold-formed steel(CFS)has the advantages of easy production and processing,convenient installation and high ratio of strength to weight.It is widely used in construction products such as light-gauge steel skeleton,profiled steel sheet and shelf in the field of civil engineering.High-temperature material constitutive model is an important input parameter for the study of the fire resistance of steel structures.In fact,the structural failure of steel structures might occur during full-range compartment fires.However,most previous experimental studies on the high-temperature material properties of CFS focus on the heating stage of steel,corresponding to the growth and fully developed phases of compartment fires,and few studies focus on the cooling stage of steel corresponding to the decay phase of fire.In this paper,the high-temperature steady-state and transient-state test methods are used to study the material properties degradation law of Q345 CFS during heating and cooling stages,and the effects of test procedure parameters on the high-temperature material properties of CFS commonly used in China are quantitatively investigated.On the base of test results,data-driven constitutive models of G550 and Q345 CFS during full-range compartment fires are constructed by using the BP neutral network technology.The main contents and conclusions are as follows :(1)Forty-four typical test conditions are selected to carry out the high-temperature steady state tests of Q345 CFS during heating and cooling stages.The results show that: 1)The material property variations with changing tensile temperature for Q345 CFS during cooling stage are similar to those during heating stage,but the values are significantly different.Under the same tensile temperatures,the differences in the elastic modulus reduction factors of Q345 CFS between the heating and cooling stages are within-0.16 to 0.057;meanwhile,the yield strength reduction factors of Q345 CFS during cooling stage show overall downward trends with increasing peak temperature,and the differences in the yield strength reduction factors between the heating and cooling stages are within-0.009 to 0.142.From the perspective of fire resistance design,if the material properties during heating stage are used to replace those during the corresponding cooling stage,the calculation results of the fire-induced deformation for Q345 CFS members may be conservative,but the calculation results of high-temperature bearing capacity will be over-predicted.2)The differences in the material properties of different grades of CFS during cooling stage are also extremely obvious,especially when the peak temperature is higher than600 ℃,the effects of peak temperature on the reduction factors of yield strength and ultimate strength of Q345 CFS are significantly lower than those of G550 CFS under the same conditions.3)A peak temperature influence coefficient is introduced and unified expressions are proposed,which can be used to predict the material properties and stress-strain curves of Q345 CFS under full-range compartment fires.(2)Fifty-two typical test conditions are selected to quantitatively investigate the effects of four kinds of test procedure parameters including peak temperature holding time,cooling rate,temperature history,and steady-state and transient test methods on the high-temperature mechanical properties of G550 and Q345 CFS during full-range compartment fires.The results show that : 1)For the high-temperature steady-state tests,the effects of the peak temperature holding time and cooling rate on high-temperature mechanical properties of CFS are insignificant,and most of the relative percentage deviations are within-10% to 10%;Meanwhile,under the same tensile temperature and highest temperature of the peak temperatures of each heating and cooling stages in the temperature history,the high-temperature mechanical properties of CFS under multiple heating and cooling stages are close to those of the cooling stage section of one heating and cooling stages.2)For the high-temperature transient tests,the effects of the temperature history on the high-temperature strain of G550 and Q345 CFS are significant;in these experiments,under the same tensile temperature,the relative percentage deviation between the strain of the heating and cooling stages of G550 CFS can reach to 99642%,and the relative percentage deviation between the strain of the heating and cooling stages of Q345 CFS can reach to 12904%;3)The steady-state and transient test methods also have remarkable affects on the high-temperature strain of G550 and Q345 CFS;under the same tensile temperature,the relative percentage deviation between the strain of the steady-state and transient test methods of G550 CFS can reach to 105183%,and the relative percentage deviation between the strain of the steady-state and transient test methods of Q345 CFS can reach to 14851%.Therefore,the high-temperature constitutive model of CFS under full-range compartment fires should consider the effects of the highest temperature of the peak temperatures of each heating and cooling stages in the temperature history and the tensile temperature.In addition,the high-temperature constitutive models constructed by the steady-state and transient tests are not equivalent.(3)Combined with the tests in this paper and the previous relevant experimental research of our research group,653 groups of high-temperature material properties data of CFS are obtained,and the sample data are preprocessed by correlation analysis and elimination of abnormal data.Based on the BP neutral network technology,data-driven constitutive models of G550 and Q345 CFS during full-range compartment fires are constructed.Four kinds of test procedure parameters: peak temperature,tensile temperature,steel position and steel thickness are taken as input parameters of the models,and the typical stress-strain curve fitting formula coefficients are used as output parameters of the models.The trainlm function is selected as the training function,and the numbers of hidden layer neural node in the models are determined not to exceed 13.These data-driven models can give reasonable prediction to the high-temperature mechanical properties and stress-strain curves of G550 and Q345 CFS during full-range compartment fires.The innovation points of this paper are as follows:(1)The high-temperature steady-state tests of Q345 CFS during full-range compartment fires are carried out,which quantitatively reveal the material properties degradation law of Q345 CFS in the whole fire process.(2)It is confirmed that the peak temperature and tensile temperature could represent the influence of temperature history in the high temperature material properties of CFS,and the constitutive data-driven models of CFS during full-range compartment fires are constructed. |
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