Study On Local Buckling Of Cold-formed Steel Columns Under Axial Compression After High Temperature

Cold-formed steel structures have the characteristics of recyclability,energy saving,material saving,environmental protection,fast construction speed and easy industrialization.The vigorous development and promotion of cold-formed steel structural systems has become an vital direction for China to promote the industrialization of residential structures.Extending the low-rise cold-formed steel building structural system to a multi-layer structure,in addition to complying with the requirements of the structural vertical load bearing and lateral resistance,it also needs to meet stricter structural fire resistance requirements and post-fire mechanical performance evaluation.In this paper,the experimental research of post-fire cold-formed steel and numerical simulation of post-fire cold-formed steel short columns under axial compression are carried out respectively,and the corresponding prediction formulas are proposed for the evaluation of structural mechanical performance after fire,the main contents are as follows:(1)The post-fire mechanical properties of cold-formed steel(CFS)structure are vital parameters for evaluating its performance after fire,besides,special significance reflected in the two critical heat treatment temperatures at which the post-fire yield strength began to decrease and it was lower than the nominal value of the steel provided by the manufacturer.In this paper,230 tests were performed cumulatively for the post-fire mechanical properties of commonly used Q345 and G550cold-formed steels with the thicknesses ranging from 1.0mm to 1.9mm,0.75 mm to1.0mm,respectively.The electric furnace was used to heat up the test specimens in batches.And the protective isolation was set up in the electric furnace to avoid the non-uniform temperature distribution in the specimens,which might be caused by the direct thermal radiation of the heating elements.The single specimen was also heated in the MTS 653 high-temperature furnace to accurately obtain the critical heat treatment temperatures and analyze their variation characteristics.Furthermore,the effect of thickness and steels' types taken into consideration,the post-fire appearance features of steels were analyzed.The results showed that: the effect of thickness on the residual factors of post-fire mechanical properties(elastic modulus,yield strength,ultimate strength)was generally within 10%;Two critical heat treatment temperatures of Q345 cold-formed steels were significantly higher than those of G550 cold-formed steels;The predictive equations for post-fire mechanical properties were proposed due to the differences in the steels with similar thickness and yield strength.(2)Based on the material parameters provided by the above-mentioned post-fire mechanical properties research of cold-formed steel,the cold-formed steel short columns with non-uniform temperature distribution and uniform temperature distribution were further studied for the ultimate bearing capacities of local buckling after high temperature.The ABAQUS finite element method and CUFSM finite strip method were used for numerical simulation to verify the applicability of the current formula for calculating ultimate bearing capacity of local buckling at ambient temperature,including the direct strength method and the effective width method at the AISI standard,and the effective width method at the Chinese standard GB50018-2002.Based above research,put forward corresponding affirmative equation.Numerical simulation analysis shows that: the existing direct strength method predicts cold-formed steel short columns with uniform temperature distribution and non-uniform temperature distribution after high temperature,the ultimate bearing capacity of local buckling has a large dispersion,and corresponding adjustment coefficients are proposed.The accuracy of the adjusted direct strength method is significantly improved.Compared with the direct strength method and effective width method of the AISI standard and the Chinese standard GB50018-2002,the effective width method at the Chinese standard is most conservative and has the largest dispersion.The innovation of this paper are as follows:(1)The critical temperatures for material degradation of Q345 and G550cold-formed steel after high temperature are obtained through the detailed test,that is,the critical heat treatment temperature at which the yield strength of steel begins to decline after high temperature and the critical heat treatment temperature at which the yield strength of steel after high temperature is lower than the nominal value,excluding the possibility of error about linear interpolation based on the results of other temperature conditions in previous studies.(2)The prediction method of local buckling for the cold-formed steel columns with non-uniform temperature distribution after high temperature is proposed.