Study On The Load Capacity And Fire Resistance Of Cold-formed Steel Built-up Box Columns

Cold-formed steel quadruple-limb built-up box section column（CFS-QBC）is a built-up column mainly used to bear the concentrated load.It is composed of two channel sections and two lipped-channel sections connected by self-tapping screws.The CFS-QBC has excellent mechanical and convenient assembly performance,so it is popular in CFS multi-story and high-rise buildings system.However,the existing research on the mechanical performance and fire resistance of CFS-QBC is insufficient,mainly in the test and numerical simulation stage,and there is a lack of corresponding design methods.This study takes CFS-QBC as the research object,systematically studies its mechanical performance at room temperature and fire resistance,and proposes the design method for engineering application.The main research contents are as follows:（1）Mechanical properties of CFS at elevated temperatures.The accurate material properties are essential for the later numerical simulation and theoretical analysis.Therefore,the high-temperature（including room temperature）steady-state tensile tests（22 in total）of CFS steel are carried out in this study.The stress-strain relationship curve of CFS at high temperature is obtained,and the elastic modulus,yield strength,and ultimate strength at high temperature are extracted.High-temperature creep tests（21 in total）were carried out,and the creep versus time curves of CFS under different temperature and stress levels were obtained.Based on the power function creep model,a three-stage creep model of CFS is proposed.By compiling the creep subroutine,the creep model is introduced into ABAQUS for numerical analysis,where the influence of creep can be effectively considered in the analysis of fire resistance.（2）Load bearing capacity test and numerical simulation of CFS-QBC at room temperatureLoad-bearing capacity tests for the CFS-QBC flexural buckling specimens were taken at ambient temperature（a total of 12）.The tests considered different screw arrangements and slenderness ratios.The CFS-QBC numerical model is established,and a"flexible boundary condition"setting method suitable for CFS built-up columns is proposed.It is verified that this boundary condition is more accurate and more practical than the traditional"rigid boundary condition"setting method.This paper also carries out the finite element modeling for the tests of local buckling and local-global interaction buckling CFS-QBC in other research,which verifies the accuracy of the model.（3）Design method of CFS-QBC bearing capacity at room temperatureBased on the finite strip method and parameter analysis,the exact solution and approximate solution of CFS-QBC local buckling elastic critical load(P_crl)are derived,respectively.Based on the energy method,the elastically flexural buckling load(P_crg)of the CFS-QBC,considering the screw spacing and screw shear stiffness is derived.The approximate solution of P_crg is also given based on parameter analysis.Then P_crl and P_crg are brought into the direct strength method（DSM）,and the coefficients of the DSM are calibrated and corrected based on a large number of parameter analyses.The accuracy of the modified calculation method is verified by comparing it with the numerical simulation results and test results.（4）Tests and numerical simulation on fire resistance of CFS-QBCThe transient fire-resistance test is adopted in the experimental research,and the factors such as initial load ratio,heating rate,longitudinal non-uniform heating,axial restraint,buckling mode,screw arrangement and so on are considered in the test（a total of 38 tests）.Based on the test results,the effects of these factors on the fire resistance of the specimen are preliminarily explored.Then,the finite element model of CFS-QBC fire resistance performance analysis is established in this study.Through the comparative analysis with the test results,the accuracy of the numerical model is verified.（5）Design method of the load-bearing capacity of CFS-QBC at elevated temperaturesThe calculation method of CFS-QBC high temperature load-bearing capacity with uniform longitudinal temperature is developed based on DSM.The calculation formulas at elevated temperatures from 100℃ to 800℃ are calibrated and modified,respectively.Based on the equilibrium differential equation and the energy method,the elastically flexural buckling load of the column with longitudinally non-uniform temperature are derived,and then introduced into the DSM.The coefficients in the formula are calibrated based on a large number of parameter analyses,and the accuracy of the calculation method is verified.By comparing with the test results,it is verified again that the calculation method of load-bearing capacity under uniform and non-uniform elevated temperatures has high accuracy.（6）Design method of fire resistance for CFS-QBCConcerning the interaction of the non-uniform temperature distribution and axial restraint,based on the equilibrium equation,the internal force temperature relationship curve of the specimen during temperature rise is established.Combined with the calculation method of load-bearing capacity at elevated temperatures of the specimen,the critical temperature T_cr-nc is calculated（regardless of high-temperature creep）.The corresponding critical temperature correction coefficient is proposed based on numerical fitting to correct T_cr-nc,and finally T_cr-cr（considering high-temperature creep）is obtained.The design method is compared with the test results to verify the accuracy of the design method.