Local Stability Study Of Columns Under Axial Compression At Elevated Temperatures

High strength is an important characteristic for construction steel structure, in orderto take advantage of it,the slenderness and the width to thickness ratio is comparisonlylarge. The bearing capacity is always reached due to its overall or local buckling. Thecode CECS200have issued recommendations for overall stability of steel column atelevated temperature, but for local buckling, there is no related proposal. Local bucklingof steel members at elevated temperature have been observed in fire and fire test inresent years. Therefore, it is of great significance in both academic and practical forstudying the local buckling of steel members at elevated temperature. On the other hand,the difference in manufacturing standards, smelting process and alloy composistionslead to the difference performance of structure members at both room and elevatedtemperature, the local stability of Q235steel stubs and Q460steel stubs at elevatedtemperature should be studied separately.Twelve stubs has been tested at room temperature,450℃and650℃. CalibratedFEM has been used to calculate the local buckling capacity of different sections atvarious temperature. The simplified calculation method has been proposed based on theFEM results. The main work in this paper includes the following parts:(1) Material properties: tensile test has been conducted on the Q235and Q460specimen to measure the mechanical properties at room temperature, research progresson Q235and Q460steel properties at elevated temperature have been summarized.(2) Theoretical analysis:the achievement in calculating the buckling load of platesat room temperature was introduced and extended to calculating the buckling load atelevated temperature.(3) Steady experiment of Q235and Q460stubs at elevated temperature:fourgroups of steady test was conducted to study the performance of flange and webbuckling in Q235H-section and Q460H-section at room temperature，450℃，650℃respectively. The load capacity, the axial load-displacement curve andload-defection curve were obtained.(4) Finite element analysis based on abaqus6.10: the finite element model wasestablished and then validated by the experiment data. Using the calibrated FEM, theload capacity due to local stability of different H-section stubs was calculated at varioustemperatures. Parametric analysis was conducted to study effect of the web-flange interaction and the initial imperfection on the load capacity.(5) Simplified calculation method:simplified equations was proposed based on thefinite analysis result. For Q235H-section stubs,simplified calculation method isconservative,but for Q460H-section stubs,simplified calculation method may lead tounsafe result when temperature is or higher than650℃.(6) Based on the equivalence of overall stability stress and local buckling stress,web and flange slenderness limits of Q235H-section and Q460H-section columnswere derived. Compare to that at room temperature, the web and flange slenderness ofQ235H-section at elevated are relatively low, and the web and flange slenderness arerelatively low when member slenderness is small while web and flange slenderness arerelatively large when member slenderness is small big.The result of study shows that:ideal elastic-plastic stress-strain model can not beused when calculating the local buckling load capacity of steel columns at elevatedtemperature. The restrained effect of flange on web was more obvious, the restrainedeffect must be considered quantitatively. for some compact and non-compact section,the load capacity may be govered by strength or overall stability at room temperature,local buckling can happen in the members mentioned above.