T形钢管混凝土截面在双向弯矩和轴力联合作用下的相互作用曲线

Interaction Curves for Concrete-Filled T-Shaped Multi-Celled Steel Tube Sections Under Combined Biaxial Bending and Axial Force

T形钢管混凝土柱由翼缘宽矩形钢管和腹板矩形钢管内填混凝土构成,其中翼缘宽钢管又分成腹板左侧、与腹板相对和腹板肢右侧等三个腔,各腔壁板的宽厚比满足矩形钢管混凝土柱中对宽厚比的限值,因而钢板不发生局部屈曲。按照弹性材料假定确定组合截面的弹性形心主轴;采用截面变形符合平截面假定,钢材为理想弹塑性,并假设混凝土在钢管的约束下混凝土在到达强度标准值后不退化,略去混凝土拉应力,采用计算程序确定了T形钢管混凝土柱(T-CFT)在双向弯矩和轴力作用下形成塑性铰时的极限曲面。计算截面在塑性中和轴平行于翼缘时的轴力-弯矩相关曲线,论证了轴力-弯矩相关曲线的旋转对称性;分析了曲线的特点,根据弹性形心轴和塑性中性轴的相对位置,识别出4个关键点,分别是全截面受压、全截面受拉、塑性中性轴与弹性形心轴重合和塑性中性轴位于翼缘钢管的下表面,计算这4个关键点的轴力和弯矩;计算并且理论分析也表明,当塑性中性轴与弹性形心轴重合时,抗弯承载力达到最大值。根据不同截面宽高比、不同钢材与混凝土强度等级的组合下相关曲线的不同特点,将相关曲线分为两类,分别拟合了轴力和弯矩的相关关系。对于塑性中性轴平行于腹板的情况,存在绕两个形心轴的弯矩,分别画出了轴力与绕两个坐标轴的弯矩的两组相关曲线,这些相关曲线上同样可以识别出5个关键点,分别对应于全截面受拉、全截面受压、塑性中性轴在腹板肢左侧和腹板肢右侧、塑性中性轴与弹性形心轴重合,利用这5个关键点的轴力和弯矩值,拟合了轴力与弯矩相关曲线的两组近似计算公式。对最大抗弯承载力进行了参数分析,结果表明,影响最大抗弯承载力的最大因素是混凝土的分担率,为此拟合了最大抗弯承载力与混凝土分担率的关系曲线。最后,分析了T形钢管混凝土截面在双向弯矩和轴力作用下的相关曲线,考察了给定轴力下绕两个坐标轴的弯矩之间的相关关系曲线,发现相关曲线的左、右、上、下4个极值点的弯矩值对应的塑性中性轴分别平行于坐标轴,基于此特性,利用已经提出的塑性轴平行于坐标轴时的轴力-弯矩相关关系的计算式,可以计算出这4点的弯矩值;根据不同轴力下双向弯矩相关曲线,拟合了双向弯矩相关关系式。通过与大量算例结果的对比,发现该公式精度良好且偏于安全。

Concrete filled T-shaped columns are composed of a wide-flange rectangular box and a rectangular web box filled with concrete,the flange box is divided into 3 cells:each on both sides of the web box and the middle cell right above the web box.The width-tothickness ratio of the plates of each cell meets the requirement in concrete-filled square tube columns to avoid local buckling.The wellknown assumption in the theory of beams that plane section remains plane after deformation is adopted,the steel is assumed to be elastic and ideally plastic,it is postulated that the concrete strength does not degenerate after it reaches the prismatic strength due to the confining effect,tensile strength of concrete is neglected.The ultimate strength surfaces of T-shaped concrete filled T-shaped steel tube columns under biaxial bending moments and axial force are computed in this paper.First the axial force-bending moment relation is studied when the plastic neutral axis is parallel to the flange box,the rotational symmetry of the axial force-moment curves is theoretically proved.The characteristics of the interactive curves are analyzed,and it is found that,based on the positions of the plastic neutral axis,there are 4 points on the interactive curve,representing uniform compression,pure tension of the full cross-section,coincidence of the plastic neutral axis with the elastic centroid axis and the plastic neutral axis on the bottom face of the flange box.The axial forces and bending moments are computed at these 4 points.It is found that the maximum bending capacity occurs when the plastic neutral axis is coincident with the elastic centroid axis.Varying the sectional dimensions and the strengths of steel and concrete,the axial force-bending moment interactive curves are computed and are divided into two groups,simplified approximate equations for the two groups of the curves are put forward.The axial force-bending moment relation when the plastic neutral axis is parallel to the web box is then studied,In this case,moments exist on both centroid axes,so interactive curves of the axial force-both bending moments are depicted respectively.Again on these interactive curves,5 key points are identified:they are:uniform compression,pure tension of the full cross-section,coincidence of the plastic neutral axis with the elastic centroid axis and the plastic neutral axis on the both faces of the web box.The axial forces and bending moments are computed at these 5 points,simplified approximate equations for the interactive curves are put forward.Again the maximum bending moment capacity occurs when the plastic neutral axis is coincident with the elastic centroid axis,approximate equations for this maximum moments are proposed.Finally,the interaction curves of T-CFT between both bending moments under a series of axial forces are analyzed,four maximum points on each of these interactive curves are found to be corresponding to the plastic neutral axis parallel to the centroid axis respectively,thus these 4 maximum points can be computed using the already-established approximate equations for the cases of plastic neutral axis parallel to the centroid axis.Approximate formulas are then proposed for each of the 4 segments divided by the four maximum points.Comparison of a large number of examples shows that the proposed formulas have good accuracy and are on the safe side.