Research On Mechanical Properties Of Cold-formed Steel And Local Buckling Of Cold-formed Thin-walled Channel Steel Columns At Elevated Temperatures

Cold-formed steel has unique advantages in the steel structure system: good processing performance;efficient cross-sectional form;low material consumption;green and recyclable;ultra-thin and high strength.As our country vigorously promotes the application of steel structure systems to residential and commercial buildings,the cold-formed steel light structure system with unique advantages has become more and more widely used in civil buildings.Although the domestic demand for cold-formed thin-walled steel has continued to increase in recent years,the research on cold-formed steel has started much later than abroad.Especially in recent years,frequent fire events at home and abroad have caused people to doubt whether steel structures can have sufficient safety under fire.The fire damage to buildings can be reduced by spraying fire protection coatings on steel components and using automatic spray systems.Unlike ordinary hot-rolled steel,although cold-formed steel can obtain higher strength and thinner cross-section through cold working,its ductility is limited accordingly.In particular,the rapid deterioration of its mechanical properties at high temperatures has caused the components to fail rapidly under fire,which has become a pain point for the promotion of light steel structural systems.Cold-formed thin-walled steel columns made of extremely thin cold-formed steel plates are the main members in light steel structural systems,and their bearing capacity is more easily affected by various buckling modes.Among them,short columns are easily affected by local buckling.However,the existing design standards for cold-formed thin-walled steel members do not give the design guidline for the local buckling bearing capacity of cold-formed steel axial stub columns at elevated temperatures.Therefore,it is very urgent and necessary to study the bearing capacity of cold-formed thin-walled steel stub columns subjected to local buckling under fire.In order to enrich the related research content,the following researches are conducted in this paper:(1)In this paper,a steady-state tensile test at elevated temperatures was performed on G550 cold-formed steel with thicknesses of 0.42 mm,0.55 mm,and 0.95 mm.According to the stress-strain curve obtained from the test,the main mechanical property parameters of the steel are analyzed,including the elastic modulus,yield strength,ultimate strain,ultimate strength and fracture strain.The obtained mechanical property parameters were compared with the experimental results obtained by other researchers through experiments and the recommended values of design standards,and it was found that the predicted values given by the standards were not safe.The design standards can’t accurately predict the reduction of mechanical properties parameters of cold formed steel at elevated temperatures.The results of other researchers are different from the experimental results of this paper.Finally,based on the experimental data in this paper,reduction factor prediction models for elastic modulus,yield strength and ultimate strength is proposed.(2)Based on the mechanical parameters obtained through experiments,the Ramberg-Osgood model is used as the theoretical basis,and a high-temperature constitutive model suitable for high-strength cold-formed steel is established by using fitting software.By comparison,the established constitutive model curve agrees well with the stress-strain curve obtained from the test.(3)Based on the established constitutive model of high-strength cold-formed steel and the local buckling test of cold-formed thin-wall channel steel axial compression columns at elevated temperatures,ABAQUS finite element models were established for finite element analysis at elevated temperatures.After verification,the established finite element model at elevated temperatures has accuracy and applicability.(4)Based on the validated and effective finite element model and the established material constitutive model using experimental results at elevated temperatures,a parameterized analysis of the local buckling of cold-formed thin-walled channel steel columns at elevated temperatures was conducted to study the effect of web width,flange width,and lip width on ultimate bearing capacity of columns affected by local buckling.Comparing the finite element analysis value with the standards prediction values,it is found that the prediction values given by the standards are conservative.Finally,based on the direct strength method,the design euqations of local buckling bearing capacity at anbient temperature was revised to make it applicable to high temperature.