Research On Mechanical Properties Of Stiffened Rectangular Short CFST Columns

The constraint role of rectangular CFST column mainly focuses in the corners, and italmost does not exist in the four central sides. So the overall confinement effect on the coreconcrete is much lower than circular CFST column. Compared with that of square CFSTcolumn, rectangular steel tubular section presents the long and short sides, so the bindingeffect of the long side on the core concrete is weaker than the short side. The long side is moresusceptible to local buckling than the short side. However, due to the presence of bindingeffect, if the setting is proper, its bearing capacity is higher than the simple uniaxialsuperposition of steel strength and concrete strength.In order to improve mechanical properties of rectangular CFST column components,binding bars (reinforcements) are set at cross section of the columns at regular intervals. Onthe one hand it can limit the lateral deformation of core concrete; on the other hand it canprovide lateral supporting effect for steel pipe, so the local buckling strength is improved. Theset of binding bars can greatly improve mechanical properties of rectangular CFST column,but area between binding bars still appears elastic-plastic local buckling phenomenon, so theconfinement role to internal filled concrete is weakened.Stiffened rectangular CFST column forms harmonious collocation by adding stiffeners tobinding bars, with minimal increase of steel consumption without increasing the difficulty ofconstruction and affecting the construction schedule. It can maximize to slow elastic-plasticbuckling between the binding bars, improve the lateral constraint ability and the mechanicalproperties of rectangular CFST column.Learning from the existing research results, the stiffening design of rectangular CFSTcolumn in the paper can achieve the higher strength, stiffness and ductility to a large extentwith thinner steel tubular wall in the premise of satisfying the structural safety of high-risebuilding. Based on the above analyses, the author has a series of researches on the mechanicalproperties of stiffened rectangular CFST column under axial load, eccentric compression, andlow cycling quasi-static loading.Specific content includes the following main aspects:(1) The tests of10square CFST columns under axial load were conducted by varying the parameters such as horizontal spacing, vertical spacing of the binding bars, the section ofstiffeners and its setting style. The paper analyzes the influence of various parameters on themechanical properties such as failure mode, stress, strain, bearing capacity and ductility,providing the basic experimental data for the following research.(2)Taking the steel tubular thickness, the diameter and strength of binding bars, the crosssection and setting style of stiffeners as parameters, low cycling quasi-static tests were appliedfor16square CFST columns specimens. The paper analyzes the hysteresis curve, skeletoncurve of each member under different parameter to calculate the bearing capacity and itsdegradation, rigidity degradation, ductility factor of displacement, energy dissipation capacity.Skeleton curve parameters of stiffened square CFST columns are determined with three linearregression analysis from the experimental results. It will provide references for the design andanalysis of the new component in the ultra-high-level engineering design and analysis.(3)The constraint mechanisms of stiffened rectangular CFST columns are analyzed. Theconstraints to concrete from the long and short sides are different. The number of binding barsand stiffeners along the long and short side is also different. Based on three axial compressioncharacteristics of core concrete, the equivalent uniaxial constitutive relationship of coreconcrete is proposed. The whole process load-deformation curves of rectangular CFSTcolumns under axial compression are calculated by the equivalent uniaxial constitutiverelationship of the core concrete. It verifies the agreement of the calculated and experimentalcurves.(4) For verifying the consistent of experiment and finite element analysis, according tothe proposed constitutive relationship of stiffened rectangular steel tubular concrete, threedimensional nonlinear finite-element analyses are conducted on all specimens. For revealingthe effects of the different setting style of stiffeners and binding bars on stress distribution ofcore concrete and steel tube, all specimens are compared. The finite-element analysis and thetest results are basically consistent. Lateral binding capacities are analyzed under the differentstiffening parameters.(5) The axial bearing capacity of stiffened structure in each style is very different, and itcannot be expressed with a uniform formula. The axial bearing capacity formula in each wayis deduced by adoption of the proposed rectangular steel tubular concrete constitutive relationship. The bearing capacities of stiffened rectangular CFST columns under axialcompression are calculated by the formula in the paper. It shows the calculations are in goodagreement with the test results and FEM analysis. Based on existing research results, themethod can reasonably evaluate axial bearing capacity of stiffened rectangular CFST column.(6) Based on the constitutive relationship of stiffened rectangular CFST concrete, meshelements’ method is presented for numerical analysis of stiffened rectangular CFST columnsunder eccentric compression, and verified with finite element method. The result shows betterconsistent. It can provide a reasonable assessment for rectangular CFST column undereccentric compression.