コンクリートの圧縮破壊を考慮した角形コンクリート充填鋼管柱の繰り返し曲げせん断挙動の有限要素解析

抄録

 In this paper, we conduct a finite element analysis of a square concrete-filled-tube (CFT) column to replicate cyclic shear-bending behavior under constant axial load. CFTs are used as columns of architectural and civil engineering structures. CFTs usually have large deformation capacity due to lateral constraints from steel tubes providing ductility. There are several existing studies on numerical analysis of CFTs. To the best of authors' knowledge, however, no numerical analysis method for simulating cyclic bending behavior of CFTs, in which interaction between steel tube and filled concrete, softening of concrete and local buckling of steel tube are simultaneously considered, has been proposed.
 The authors have been developing a detailed finite element analysis system called E-Simulator. In this research project, we simulate structural behavior only by combining detailed solid finite element model and material constitutive laws, which can represent damages and fractures of materials. By using E-Simulator, we can consider interaction between filled concrete and steel tube without any additional constraint effects.
 The concrete constitutive model, which has been proposed by the authors, is updated and applied to the analysis. The concrete model is a combination of extended Drucker-Prager model and a simple damage criterion. Here, we improve the concrete constitutive model so that damage progresses only in compressive stress state.
 A cyclic shear-bending loading experiment of a square CFT column is replicated. The height and width of the column are 1440 mm and 240 mm, respectively. A pseudo-static cyclic lateral displacement and a constant vertical load are given at the top of the column. The ratio of lateral displacement amplitude to the column height is increased from 1/400 to 1/33, gradually. The CFT consists of high strength concrete and steel, whose nominal strengths are 150 N/mm² and 780 N/mm², respectively. The material parameters are identified from material tests. The load-deformation relation obtained from the numerical result is compared to the experimental one. The interaction between the filled concrete and the steel tube is also discussed.
 The results of the analysis are summarized as follows:

  1. The load-displacement relationships of the analysis and experiment have been almost linear and elastic until the distortion angle has reached 1/100. The stiffness of the analysis has agreed well to that of the experiment. The load measured in the experiment is not proportional to the displacement. In positive loading direction, as a result, the load of the analysis is larger than the experimental one. The ratio of load from the analysis to the experimental one is 0.979 – 1.142 in positive loading direction, 0.715 - 1.004 in negative loading direction.
 2. When the distortion angle has exceeded 1/100 first, strength deterioration due to local buckling of the tube and damage of the concrete has been seen in the numerical result. In the experiment, local buckling of the steel tube has been observed in the same loading cycle. Although the strength deterioration in the analysis has occurred later than that in the experiment, the load-deformation relation of the experiment after plasticizing has been replicated, qualitatively. The ratio of load from the analysis to the experimental one is 1.076 – 1.154.
 3. The contact between the filled concrete and the steel tube due to the local buckling of steel tube has been first observed. After that, the large bending deformation has caused another contact. The contact pressure gave the concrete confinement, which made the equivalent stress higher than the uniaxial compressive strength of the damaged concrete. From this result, it can be concluded that interaction between filled concrete and steel tube can be represented by using detailed solid FE mesh model and E-Simulator.