The Post-fire Mechanical Properties Of Cold-rolled Steel With A Numerical Study On Distortional Buckling Of Cold-formed Steel Members

As China enters the "14th Five-Year Plan" economic development period,the“Green and Economic”has become the new direction of the development of the construction industry.Cold-formed steel is gradually being promoted and applied with a series of advantages such as various cross-sectional forms,light weight,good seismic performance,mass production,short construction cycle,and easy recycling.In addition to meeting the requirements of vertical load-bearing and lateral resistance,the cold-formed steel structure system also puts forward higher requirements for fire-resistant design.By studying the changes of member performance under and after high temperature,it can provide theoretical support for structural fire resistance design.Therefore,in this paper,the experimental study of post-high temperature mechanical properties of two cold formed steels commonly used at home and abroad is carried out and the post-high temperature stress-strain curve model of cold formed steel is established,followed by the numerical simulation study of axial compression deformation buckling properties of short columns of cold formed steel after high temperature,the main contents of which are as follows.(1)A total of 300 test specimens of two strengths of cold formed steel(G450 and G550)were tested for post-high temperature mechanical properties,and various variables were set in the tests,such as different specimen thicknesses,different specimen sampling directions,and different thermal immersion holding times.The key mechanical properties(modulus of elasticity,yield strength,ultimate strength,ultimate strain)were extracted from the stress-strain curves obtained from the tests,and the effects of the test variables on the mechanical properties were analyzed and compared with other experimental studies on cold-formed steel and high-strength steel.The ultimate strains of the specimens with different sampling directions are different after high temperature,but the rest of the important mechanical properties are basically the same.In addition,it is proved that the thermal immersion time has less effect on the mechanical properties of cold formed steel after high temperature.In comparison with other scholars,it is found that the modulus of elasticity of cold formed steel has a decreasing trend after high temperature but the decreasing interval is within 20%,for yield strength and ultimate strength there is a sharp decreasing trend in the overheating temperature 500℃-600℃ interval,and the strength at 700℃-900℃ is only 40%-50%at room temperature,and the above rule can be found from the cold formed steel made by other scholars.(2)Based on the Mahendra model,a stress-strain curve model applicable to cold formed steel after high temperature is proposed,and a prediction equation for the discount factor of key mechanical property parameters of cold formed steel after high temperature is proposed through the test data.Through this model,the predicted values of relevant mechanical property parameters and stress-strain prediction curves after a certain high temperature can be obtained by simply knowing the modulus of elasticity E,yield strength fy and ultimate strength fu of cold formed steel at room temperature.By comparing and analyzing with the test data,the results show that the prediction curves match well,thus verifying the applicability and reliability of the stress-strain curve model.(3)Based on the cold-formed steel post-high temperature stress-strain curve model,the ultimate load carrying capacity of cold-formed steel short columns after distortional buckling at high temperature in uniform temperature field is further investigated.ABAQUS numerical software and CUFSM finite bar software were used for simulation analysis to verify the applicability of the existing code normal temperature distortional buckling calculation formula after high temperature,which includes the domestic Technical Specification for Low-rise Cold-formed Thin-walled Steel House Construction(JGJ 227-2011),Australian/New Zealand code(AS/NZS 4600),American code(AISI-NAS 2007).Research shows that: the existing code can be applied to the calculation of distortional buckling load capacity after high temperature,but there is a conservative code after 600℃ high temperature,and the error is about 20%,in addition,the direct strength method is more accurate to predict the distortional buckling load capacity after high temperature for cold formed steel short columns.Therefore,the prediction equation of post-high temperature deformation buckling load capacity is proposed according to the code design value,finite element value and test value to provide some reference for the design of axial compression members against fire.