Research On The Mechanical Properties Of Cold-formed Thin-walled Steel After High Temperature And Its Axial Compression Short Column Stressing Properties

Fire is one of the basic problems in structural design,but in the current research,the performance of cold-formed thin-walled steel structures after high temperature and different cooling methods is rarely involved.Based on this,this paper carried out experimental research on the mechanical properties of S280 cold-formed thin-walled steel under different high temperature temperatures and cooling methods(natural cooling and water immersion cooling),and established the constitutive model of S280 cold-formed thin-walled steel and the different cooling methods after high temperature.The mechanical performance index prediction formula,on this basis,the numerical simulation of the axial compression performance of the C-shaped steel column after high temperature and different cooling methods,based on the direct strength method theory,is suitable for the cold-formed thin-walled steel C-section column Calculation formula for local buckling capacity of different cooling methods after high temperature.The main research contents and conclusions are as follows:(1)180 test pieces were designed,with three thicknesses of 1.0mm,1.5mm and 2.0mm;ten temperatures ranging from 20°C to 800°C,and two cooling methods,natural cooling and water immersion cooling,were considered.Uniaxial tensile tests were carried out on the specimens.Experimental research shows that the influence of fire temperature and cooling method on the elastic modulus of S280 cold-formed thin-walled steel is within 4%,which can be ignored.When the fire temperature is lower than 600°C,the yield strength and ultimate tensile strength of S280 cold-formed thin-walled steel under the two cooling methods fluctuate within 6%,and there is almost no weakening;It shows a trend of first increasing and then decreasing,and the maximum value appears between 500°C and 600°C;when the fire temperature is higher than 600°C,the yield strength,ultimate tensile strength,and elongation decrease under natural cooling,and the corresponding Compared with normal temperature,the maximum drop rate is 58%,41% and 39%,respectively,and most of the specimen fractures show obvious necking phenomenon.Under water immersion cooling,the yield strength and ultimate tensile strength increased by 18% to 26% and 17% to 23% respectively compared with normal temperature,and the elongation dropped sharply,with a maximum drop of 53%,and the surface color of the test piece was obviously darker for the natural cooling specimens,the fracture of the specimens is mostly brittle fracture failure,indicating that cooling by immersion in water will reduce the plastic deformation capacity of the steel.(2)The reduction coefficients of various mechanical performance indicators,such as yield strength reduction coefficients,are given under different cooling methods of S280 cold-formed thin-walled steel after high temperature,and the reduction coefficients are fitted.At the same time,the uniaxial tension constitutive model of S280 cold-formed thin-walled steel is established.This constitutive model is suitable for S280 cold-formed thin-walled steel under natural cooling or water immersion cooling from room temperature to 800 °C.Compared with the stress-strain curve obtained from the test,the constitutive model established in this paper is in good agreement with the test results,indicating that the established constitutive model has high accuracy.(3)The finite element model of cold-formed thin-walled steel C-shaped axial compression column was established.After verifying the correctness of the model,variable parameter analysis was carried out on the local buckling capacity under different cooling methods after high temperature.Finally,based on the theory of the direct strength method,the calculation formula of the local buckling capacity in the original direct strength method was revised,and a temperature And the direct strength method formula of cooling mode parameters.The modified direct strength method formula is suitable for different cooling methods after high temperature,and can more accurately calculate the local buckling capacity of cold-formed thin-walled steel axially compressed short columns under different cooling methods after high temperature.