轴力和双向受弯作用下短钢管混凝土梁柱的非线性分析

提出一种非线性纤维元分析法用于确定在轴力和双向受弯作用下短钢管混凝土梁柱的轴力-弯曲强度的交互图。针对被约束混凝土和结构钢的非线性本构模型对组合截面中性轴深度和方向采用有效正切算法进行迭代,以满足平衡条件。通过纤维单元分析结果与试验数据和现有解决方案的对比,验证了纤维元分析程序的有效性。采用该程序研究含钢量、混凝土抗压强度和钢屈服强度对于轴力-弯曲交互作用和钢管混凝土梁柱的影响。     

High strength circular concrete-filled steel tubular slender beam-columns, Part II: Fundamental behavior

There is relatively little experimental and numerical research on the fundamental behavior of high strength circular concrete-filled steel tubular (CFST) slender beam-columns. In a companion paper, a new numerical model for predicting the nonlinear inelastic behavior of high strength circular CFST slender beam-columns under axial load and bending was presented. The numerical model developed accounts for confinement effects on the strength and ductility of the concrete core and on circular steel tubes as well as initial geometric imperfections of beam-columns. This paper presents the verification of the numerical model and extensive parametric studies on the fundamental behavior of high strength circular CFST slender beam-columns. The ultimate strengths and axial load-deflection responses of circular CFST slender beam-columns under eccentric loading predicted by the numerical model are verified by corresponding experimental results. The computer program implementing the numerical model is used to investigate the fundamental behavior of high strength circular CFST slender beam-columns in terms of load-deflection responses, ultimate strengths, axial load-moment interaction diagrams, and strength increase due to concrete confinement. Parameters examined include column slenderness ratio, eccentricity ratio, concrete compressive strengths, steel yield strengths, steel ratio and concrete confinement. It is demonstrated that the numerical model developed is an efficient computer simulation and design tool for high strength circular CFST slender beam-columns. Benchmark numerical results presented in this paper are valuable in the development of composite design codes for high strength circular CFST slender beam-columns.     

Performance-based analysis of concrete-filled steel tubular beam-columns, Part I: Theory and algorithms

This paper presents a performance-based analysis (PBA) technique based on fiber element formulations for the nonlinear analysis and performance-based design of thin-walled concrete-filled steel tubular (CFST) beam-columns with local buckling effects. Geometric imperfections, residual stresses and strain hardening of steel tubes and confined concrete models are considered in the PBA technique. Initial local buckling and effective strength/width formulas are incorporated in the PBA program to account for local buckling effects. The progressive local buckling of a thin-walled steel tube filled with concrete is simulated by gradually redistributing normal stresses within the steel tube walls. Performance indices are proposed to quantify the section, axial ductility and curvature ductility performance of thin-walled CFST beam-columns under axial load and biaxial bending. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a thin-walled CFST beam-column section to satisfy equilibrium conditions. The analysis algorithms for thin-walled CFST beam-columns under axial load and uni- and biaxial bending are presented. The PBA program can efficiently generate axial load-strain curves, moment-curvature curves and axial load-moment strength interaction diagrams for thin-walled CFST beam-columns under biaxial loads. The proposed PBA technique allows the designer to analyze and design thin-walled CFST beam-columns made of compact or non-compact steel tubes with any strength grades and normal and high-strength concrete. The verification and applications of the PBA program are given in a companion paper.     

High strength circular concrete-filled steel tubular slender beam-columns, Part I: Numerical analysis

High strength circular concrete-filled steel tubular (CFST) slender beam-columns are frequently used in high-rise composite buildings because they possess higher strength and stiffness than normal strength ones. Most nonlinear inelastic methods of analysis for circular CFST slender beam-columns have not considered the effects of high strength materials and concrete confinement that significantly increases the strength and ductility of the concrete core. As a result, these methods produce computational solutions that deviate considerably from experimental results. This paper presents a new numerical model for predicting the nonlinear inelastic behavior of high strength circular CFST slender beam-columns under axial load and bending. The numerical model developed not only accounts for confinement effects on the concrete core and circular steel tubes but also incorporates initial geometric imperfections of beam-columns. Axial load-moment-curvature relationships obtained from the fiber element analysis of column cross-sections are utilized to determine the equilibrium states in the inelastic stability analysis of slender beam-columns. Computational algorithms are developed for determining the axial load-deflection and axial load-moment interaction curves for slender beam-columns. The numerical model is implemented in a computer program, which is shown to be an efficient and accurate simulation tool that can be used to investigate the fundamental behavior of high strength circular CFST slender beam-columns. The verification and applications of the numerical model are given in a companion paper.     

高强圆钢管混凝土压弯构件(1):数值分析

由于具有比普通构件强度高、刚度大等特点,高强圆钢管混凝土压弯构件被广泛用于高层建筑中。然而,针对此类构件的大多数非线性分析方法都没有考虑高强材料属性和混凝土约束的影响,这很大程度上高估了核心混凝土的强度和韧性。因此,这些方法的求解结果与试验结果相差很大。针对高强圆钢管混凝土压弯构件的非线性性能,提出新的数值模型。该模型不仅考虑了混凝土约束对核心混凝土和钢管的影响,还考虑了压弯构件的初始几何缺陷。根据通过有限元分析求得的轴力-弯矩曲线,确定压弯构件非线性稳定分析中的平衡状态。为确定轴力-变形及轴力-弯矩曲线,提出了计算准则。在计算机程序中应用该数值模型,可研究高强圆钢管混凝土压弯构件的基本性能。在后续文章中,将验证该模型的正确性,并应用此模型。     

High strength thin-walled rectangular concrete-filled steel tubular slender beam-columns,Part I: Modeling

High strength thin-walled rectangular concrete-filled steel tubular (CFST) slender beam-columns under eccentric loading may undergo local and overall buckling. The modeling of the interaction between local and overall buckling is highly complicated. There is relatively little numerical study on the interaction buckling of high strength thin-walled rectangular CFST slender beam-columns. This paper presents a new numerical model for simulating the nonlinear inelastic behavior of uniaxially loaded high strength thin-walled rectangular CFST slender beam-columns with local buckling effects. The cross-section strengths of CFST beam-columns are modeled using the fiber element method. The progressive local and post-local buckling of thin steel tube walls under stress gradients is simulated by gradually redistributing normal stresses within the steel tube walls. New efficient Müller's method algorithms are developed to iterate the neutral axis depth in the cross-sectional analysis and to adjust the curvature at the columns ends in the axial load–moment interaction strength analysis of a slender beam-column to satisfy equilibrium conditions. Analysis procedures for determining the load–deflection and axial load–moment interaction curves for high strength thin-walled rectangular CFST slender beam-columns incorporating progressive local bucking and initial geometric imperfections are presented. The new numerical model developed is shown to be efficient for predicting axial load–deflection and axial load–moment interaction curves for high strength thin-walled rectangular CFST slender beam-columns. The verification of the numerical model and parametric studies is given in a companion paper.     

Nonlinear analysis of circular concrete-filled steel tubular short columns under eccentric loading

This paper presents an effective theoretical model for the nonlinear inelastic analysis of circular concrete-filled steel tubular (CFST) short columns under eccentric loading. Accurate material constitutive relationships for normal and high strength concrete confined by either normal or high strength circular steel tubes are incorporated in the theoretical model to account for confinement effects that increase both the strength and ductility of concrete. The predicted ultimate bending strengths and complete moment-curvature responses of circular CFST columns under eccentric loading are compared with existing experimental results to examine the accuracy of the theoretical model developed. The fundamental behavior of circular CFST beam-columns with various diameter-to-thickness ratios, concrete compressive strengths, steel yield strengths, axial load levels and sectional shapes is studied using the verified theoretical model. Based on extensive numerical studies, a new design model for determining the ultimate pure bending strengths of circular CFST beam-columns is proposed. The theoretical model and formulas developed are shown to be effective simulation and design tools for the nonlinear inelastic behavior of circular CFST beam-columns under eccentric loading.     

Strength and ductility of high strength concrete-filled steel tubular beam-columns

The ultimate strength and ductility of high strength thin-walled concrete-filled steel tubular (CFST) beam-columns with local buckling effects, are investigated in this paper, using a performance-based analysis (PBA) technique. The PBA technique accounts for the effects of geometric imperfections, residual stresses, strain hardening, local buckling and concrete confinement on the behavior of high strength thin-walled CFST beam-columns. The accuracy of the PBA technique is further examined by comparisons with experimental results. The PBA program is employed to study the effects of depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility of high strength thin-walled CFST beam-columns under combined axial load and biaxial bending. The results obtained indicate that increasing the depth-to-thickness ratio and axial load levels significantly reduces the stiffness, strength and ductility of CFST beam-columns. Increasing concrete compressive strengths increases the stiffness and strength, but reduces the axial ductility and section performance of CFST beam-columns. Moreover, the steel yield strength has a significant effect on the section and strength performance of CFST beam-columns but does not have a significant effect on their axial and curvature ductility.     

Nonlinear analysis of circular concrete-filled steel tubular short columns under axial loading

The confinement effect provided by the steel tube in a circular concrete-filled steel tubular (CFST) short column remarkably increases the strength and ductility of the concrete core. The reliable prediction using nonlinear analysis methods for circular CFST columns relies on the use of accurate models for confined concrete. In this paper, accurate constitutive models for normal and high strength concrete confined by either normal or high strength circular steel tubes are proposed. A generic fiber element model that incorporates the proposed constitutive models of confined concrete is created for simulating the nonlinear inelastic behavior of circular CFST short columns under axial loading. The generic fiber element model developed is verified by comparisons of computational results with existing experimental data. Extensive parametric studies are conducted to examine the accuracy of various confining pressure models and the effects of the tube diameter-to-thickness ratio, concrete compressive strengths and steel yield strengths on the fundamental behavior of circular CFST columns. A new design formula accounting for concrete confinement effects is also proposed for circular CFST columns. It is demonstrated that the generic fiber element model and design formula adequately predict the ultimate strength and behavior of axially loaded circular CFST columns and can be used in the design of normal and high strength circular CFST columns.     

Behavior of biaxially-loaded rectangular concrete-filled steel tubular slender beam-columns with preload effects

In composite construction, rectangular hollow steel tubular slender beam-columns are subjected to preloads arising from construction loads and permanent loads of the upper floors before infilling of the wet concrete. The behavior of biaxially loaded thin-walled rectangular concrete-filled steel tubular (CFST) slender beam-columns with preloads on the steel tubes has not been studied experimentally and numerically. In this paper, a fiber element model developed for CFST slender beam-columns with preload effects is briefly described and verified by existing experimental results of uniaxially loaded CFST columns with preload effects. The fiber element model is used to investigate the behavior of biaxially loaded rectangular CFST slender beam-columns accounting for the effects of preloads and local buckling. Parameters examined include local buckling, preload ratio, loading angle, depth-to-thickness ratio, column slenderness, loading eccentricity and steel yield strength. The results obtained indicate that the preloads on the steel tubes significantly reduce the stiffness and strength of CFST slender beam-columns with a maximum strength reduction of more than 15.8%. Based on the parametric studies, a design model is proposed for axially loaded rectangular CFST columns with preload effects. The fiber element and design models proposed allow for the structural designer to efficiently analyze and design CFST slender beam-columns subjected to preloads from the upper floors of a high-rise composite building during construction.     

High strength thin-walled rectangular concrete-filled steel tubular slender beam-columns,Part II: Behavior

Experimental and numerical research on full-scale high strength thin-walled rectangular steel slender tubes filled with high strength concrete has not been reported in the literature. In a companion paper, a new numerical model was presented that simulates the nonlinear inelastic behavior of uniaxially loaded high strength thin-walled rectangular concrete-filled steel tubular (CFST) slender beam-columns with local buckling effects. The progressive local and post-local buckling of thin steel tube walls under stress gradients was incorporated in the numerical model. This paper presents the verification of the numerical model developed and its applications to the investigation into the fundamental behavior of high strength thin-walled CFST slender beam-columns. Experimental ultimate strengths and load-deflection responses of CFST slender beam-columns tested by independent researchers are used to verify the accuracy of the numerical model. The verified numerical model is then utilized to investigate the effects of local buckling, column slenderness ratio, depth-to-thickness ratio, loading eccentricity ratio, concrete compressive strengths and steel yield strengths on the behavior of high strength thin-walled CFST slender beam-columns. It is demonstrated that the numerical model is accurate and efficient for determining the behavior of high strength thin-walled CFST slender beam-columns with local buckling effects. Numerical results presented in this study are useful for the development of composite design codes for high strength thin-walled rectangular CFST slender beam-columns.     

Biaxially loaded high-strength concrete-filled steel tubular slender beam-columns,Part I: Multiscale simulation

The steel tube walls of a biaxially loaded thin-walled rectangular concrete-filled steel tubular (CFST) slender beam-column may be subjected to compressive stress gradients. Local buckling of the steel tube walls under stress gradients, which significantly reduces the stiffness and strength of a CFST beam-column, needs to be considered in the inelastic analysis of the slender beam-column. Existing numerical models that do not consider local buckling effects may overestimate the ultimate strengths of thin-walled CFST slender beam-columns under biaxial loads. This paper presents a new multiscale numerical model for simulating the structural performance of biaxially loaded high-strength rectangular CFST slender beam-columns accounting for progressive local buckling, initial geometric imperfections, high strength materials and second order effects. The inelastic behavior of column cross-sections is modeled at the mesoscale level using the accurate fiber element method. Macroscale models are developed to simulate the load-deflection responses and strength envelopes of thin-walled CFST slender beam-columns. New computational algorithms based on the Müller's method are developed to iteratively adjust the depth and orientation of the neutral axis and the curvature at the column's ends to obtain nonlinear solutions. Steel and concrete contribution ratios and strength reduction factor are proposed for evaluating the performance of CFST slender beam-columns. Computational algorithms developed are shown to be an accurate and efficient computer simulation and design tool for biaxially loaded high-strength thin-walled CFST slender beam-columns. The verification of the multiscale numerical model and parametric study are presented in a companion paper.     

Nonlinear analysis of concrete-filled thin-walled steel box columns with local buckling effects

Local buckling of steel plates reduces the ultimate loads of concrete-filled thin-walled steel box columns under axial compression. The effects of local buckling have not been considered in advanced analysis methods that lead to the overestimates of the ultimate loads of composite columns and frames. This paper presents a nonlinear fiber element analysis method for predicting the ultimate strengths and behavior of short concrete-filled thin-walled steel box columns with local buckling effects. The fiber element method considers nonlinear constitutive models for confined concrete and structural steel. Effective width formulas for steel plates with geometric imperfections and residual stresses are incorporated in the fiber element analysis program to account for local buckling effects. The progressive local and post-local buckling is simulated by gradually redistributing the normal stresses within the steel plates. Two performance indices are proposed for evaluating the section and ductility performance of concrete-filled steel box columns. The computational technique developed is used to investigate the effects of the width-to-thickness ratios and concrete compressive strengths on the ultimate strength and ductility of concrete-filled steel box columns. It is demonstrated that the nonlinear fiber element method developed predicts well the ultimate loads and behavior of concrete-filled thin-walled steel box columns and can be implemented in advanced analysis programs for the nonlinear analysis of composite frames.     

Performance-based analysis of concrete-filled steel tubular beam–columns,Part II: Verification and applications

The theory and algorithms of a performance-based analysis (PBA) technique for the nonlinear analysis and performance-based design of thin-walled concrete-filled steel tubular (CFST) beam–columns with local buckling effects were presented in a companion paper. Initial local buckling and effective strength/width formulas for steel plates are incorporated in the PBA program to account for local buckling effects. Performance indices are used in the PBA program to quantify the section, axial ductility and curvature ductility performance of thin-walled CFST beam–columns. This paper presents the verification and applications of the PBA program developed. The axial load–strain curves, ultimate axial loads and moment–curvature curves for thin-walled CFST columns predicted by the PBA program are verified by experimental data. The PBA program is then utilized to investigate the effects of local buckling, depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility performance of thin-walled CFST beam–columns under axial load and biaxial bending. The PBA technique developed is shown to be efficient and accurate and can be used directly in the performance-based design of thin-walled CFST beam–columns and implemented in advanced analysis programs for composite columns and frames.     

Local buckling of steel plates in concrete-filled thin-walled steel tubular beam-columns

The availability of high strength steels and concrete leads to the use of thin steel plates in concrete-filled steel tubular beam-columns. However, the use of thin steel plates in composite beam-columns gives a rise to local buckling that would appreciably reduce the strength and ductility performance of the members. This paper studies the critical local and post-local buckling behavior of steel plates in concrete-filled thin-walled steel tubular beam-columns by using the finite element analysis method. Geometric and material nonlinear analyses are performed to investigate the critical local and post-local buckling strengths of steel plates under compression and in-plane bending. Initial geometric imperfections and residual stresses presented in steel plates, material yielding and strain hardening are taken into account in the nonlinear analysis. Based on the results obtained from the nonlinear finite element analyses, a set of design formulas are proposed for determining the critical local buckling and ultimate strengths of steel plates in concrete-filled steel tubular beam-columns. In addition, effective width formulas are developed for the ultimate strength design of clamped steel plates under non-uniform compression. The accuracy of the proposed design formulas is established by comparisons with available solutions. The proposed design formulas can be used directly in the design of composite beam-columns and adopted in the advanced analysis of concrete-filled thin-walled steel tubular beam-columns to account for local buckling effects.     

高强圆钢管混凝土压弯构件(2):基本性能

目前,有关高强圆钢管混凝土压弯构件基本性能的试验及数值研究相对较少。在之前文章中,已经提出评估此类压弯构件非线性性能的数值模型,考虑约束和初始几何缺陷对核心混凝土强度和韧性及钢管的影响。验证该模型的正确性,并进行压弯构件基本性能的参数化研究。通过相应的试验结果,验证根据此模型求得的偏心荷载作用下极限承载力和轴力-变形性能的正确性。在计算机程序中应用该模型,研究高强圆钢管混凝土压弯构件的基本性能,如:荷载-位移曲线,极限承载力,轴力-弯矩曲线及由混凝土约束引起的强度增量。参数包括:构件长细比,偏心率,混凝土抗压强度,钢材屈服强度,钢材百分比,混凝土约束条件等。结果表明:提出的数值模型能有效模拟和设计高强圆钢管混凝土压弯构件。本基准数值结果对完善组合结构设计规范中有关高强混凝土压弯构件的规定很有意义。     

A force-based model for composite steel-concrete beams with partial interaction

This paper presents a new force-based beam-column element for the nonlinear analysis of composite steel-concrete beams with partial interaction. The element is made up of three components: (a) a fiber beam-column element that models the behavior of the steel girder, (b) a fiber beam-column element that models the behavior of the concrete deck, and (c) a bond element that models the transfer of forces between the steel and concrete elements through shear connectors. The model neglects uplift and frictional effects. The fiber beam-columns are force-based elements that depend on force interpolation functions. A linear bending moment and a constant axial force serve as the interpolation functions. An important factor that favors the use of force-based elements in modeling composite structures is their ability to treat any type of distributed element loads. Distributed element loads are applied internally in a continuous manner by force superposition at the control sections. The state determination of these elements is based on an iterative solution that determines the element resisting forces and stiffness matrix. The bond element is a spring-type element that assumes a linear bond stress variation along the length. The nonlinear behavior of the composite element derives entirely from the constitutive laws of the steel, the concrete and the shear connectors. The paper concludes with a correlation study to investigate the validity of the model. Good agreement between analysis and experimental results was observed.     

钢管混凝土叠合柱轴压性能研究

为研究钢管混凝土叠合柱轴压性能,基于合理的钢材和混凝土本构关系模型,采用纤维模型法和有限元法分析方法计算叠合柱轴压荷载-变形关系曲线。将理论计算结果与试验结果进行对比,验证了理论分析模型的正确性。在此基础上,对叠合柱的破坏模态、轴向荷载分配以及组成钢管混凝土叠合柱的外围钢筋混凝土、钢管和钢管内部混凝土之间相互作用等进行分析,提出了叠合柱的轴压承载力简化计算式,简化计算结果与试验结果吻合较好。为保证外围钢筋混凝土和内部钢管混凝土较好地协同工作,建议外围钢筋混凝土中箍筋的约束指标与内部钢管混凝土的约束效应系数比值不应小于0.188。     

圆钢管混凝土结构非线性有限元分析

基于合理的钢管混凝土拉、压材料数值本构模型,采用U.L.列式单元增量平衡方程,引入分层梁单元材料非线性分析理论,通过调整截面形心应变和曲率,使梁端内外力平衡,完善了分层单元法,编制了相应的非线性有限元程序,并对已有钢管混凝土结构面内受力,如钢管混凝土偏压柱、不等端弯矩钢管混凝土偏压柱、钢管混凝土压弯构件和钢管混凝土模型拱肋等试验资料进行双重非线性有限元分析。结果表明:几组钢管混凝土模型拱和钢管混凝土压弯构件的荷载-变形曲线和极限承载力与试验结果最接近,验证了本文方法与程序的可靠性,通过与其他学者计算结果相比,表明本文方法具有较高的计算精度。     

钢管混凝土X形交叉柱轴压性能研究

为研究圆形和长圆形截面钢管混凝土柱交叉相贯形成的变截面不规则钢管混凝土组合柱,即钢管混凝土X形柱的轴压性能,采用有限元软件ABAQUS建立上述两种截面钢管混凝土X形柱的轴压模型,并通过与试验结果对比验证有限元模型的合理性,在此基础上,对圆形和长圆形截面钢管混凝土X形柱的受力全过程进行分析,并对钢管混凝土X形柱进行参数分析。研究表明,钢管混凝土X形柱在节点交叉区域钢管对混凝土产生不均匀的约束作用,混凝土中心区域受到的约束作用最大,其次是左右区域,钢管相交部位的混凝土受到约束最小,部分截面甚至在混凝土与钢管之间发生脱离。随着混凝土强度提高、钢材强度提高、钢管壁厚增加以及交叉角度加大,钢管混凝土X形柱的轴压承载力相应提高。