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 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.     

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.     

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.     

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.     

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.     

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。     

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.     

偏心荷载下钢管混凝土圆形短柱的非线性分析

提出了一个有效的理论模型,用于钢管混凝土圆形短柱在偏心荷载作用下的非线性弹塑性分析。理论模型中考虑了普通或高强钢管中填充的普通和高强混凝土的精确材料构成关系,用于计算混凝土强度和延性升高时对约束效应的影响。将偏心荷载下对钢管混凝土圆柱预测的极限抗弯强度和完整的弯矩-曲率曲线,与现有的试验数据进行对比以验证理论模型的准确性。利用经过验证的理论模型对圆形钢管混凝土梁柱的基本性能包括径厚比、混凝土抗压强度、钢筋屈服强度、轴向荷载水平和截面形状进行了研究。基于大量的数值研究,针对圆形钢管混凝土梁柱提出了一个用于确定极限纯弯强度的设计模型。理论模型和公式可以对偏心荷载下圆形钢管混凝土梁柱的非线性弹塑性性能进行有效的模拟和设计。     

火灾作用后圆钢管混凝土柱荷载-位移滞回性能研究

火灾作用后,钢管混凝土柱的强度和刚度均会遭受损失,需进一步考察其抗震能力。本文介绍了7个火灾后圆钢管混凝土试件滞回性能试验研究,采用数值方法对荷载位移滞回关系曲线进行全过程分析,计算分析了轴压比、长细比、含钢率、钢材屈服极限、混凝土抗压强度、受火时间等参数对荷载位移关系骨架线的影响。研究结果表明,火灾作用后圆钢管混凝土柱滞回关系曲线形状饱满,无明显的捏缩现象,但试件的极限承载力和弹性刚度会有所降低。本文的研究成果可为火灾后该类结构的抗震修复加固提供参考。     

Experimental research on concrete-filled square steel tubular slender columns with inner I-shaped CFRP under axial compression

对7个内置工字形CFRP型材的方钢管混凝土长柱进行了轴压性能试验研究,得到了荷载-跨中挠度曲线和荷载-应变曲线,并分析了其受力过程及破坏特征。试验结果表明：内置工字形CFRP型材的方钢管混凝土轴压长柱的破坏模式为弯曲失稳破坏。随着长细比的增大,试件的承载力降低。试件的荷载-挠度曲线可分为弹性段、弹塑性段以及下降段。采用ABAQUS有限元分析软件对内置工字形CFRP型材的方钢管混凝土轴压长柱受力性能进行有限元模拟,模拟结果与试验结果吻合良好。在此基础上,分析了长细比、混凝土强度、钢材强度、含钢率和有无CFRP对组合柱受力性能的影响。分析结果表明,长细比对柱的受力性能影响较大,随着长细比的增加柱的承载力明显下降;提高混凝土强度可以提高柱的承载力,但延性有所降低;提高钢材强度和增大含钢率均可以提高柱的承载力;设置工字形CFRP型材可以提高柱的承载力和变形能力。     

Nonlinear analysis of short concrete-filled steel tubular beam-columns under axial load and biaxial bending

This paper presents a nonlinear fiber element analysis method for determining the axial load-moment strength interaction diagrams for short concrete-filled steel tubular (CFST) beam-columns under axial load and biaxial bending. Nonlinear constitutive models for confined concrete and structural steel are considered in the fiber element analysis. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a composite section to satisfy equilibrium conditions. The accuracy of the fiber element analysis program is verified by comparisons of fiber analysis results with experimental data and existing solutions. The fiber element analysis program developed is employed to study the effects of steel ratios, concrete compressive strengths and steel yield strengths on axial load-moment interaction diagrams and the C-ratio of CFST beam-columns. The proposed fiber element analysis technique is shown to be efficient and accurate and can be used directly in the design of CFST beam-columns and implemented in advanced analysis programs for the nonlinear analysis of composite columns and frames.     

冻融循环-酸雨锈蚀交替作用后圆钢管混凝土轴压短柱力学性能分析

基于材料强度折减及钢管壁厚折减的方法,对冻融循环-酸雨锈蚀交替作用后圆钢管混凝土轴压力学性能采用有限元法进行了研究。基于合理的有限元分析模型,对冻融循环-酸雨锈蚀交替作用后圆钢管混凝土柱的破坏模态、轴向荷载-位移关系、钢管与混凝土相互作用进行了分析,研究了含钢率、截面尺寸、钢管屈服强度、混凝土轴心抗压强度以及冻融循环-酸雨交替次数对试件轴压极限承载力的影响。结果表明:有限元模拟结果与试验结果吻合良好,验证了模型的有效性; 冻融循环-酸雨锈蚀交替作用后轴压圆钢管混凝土短柱的破坏模态与普通试件相似,轴向荷载-位移曲线变化趋势一致,试件均为塑性破坏; 圆钢管混凝土轴压短柱随冻融循环-酸雨锈蚀交替次数的增加,材料性能劣化严重,外钢管对核心混凝土约束作用减弱,试件极限承载力明显下降。     

钢管约束的钢管混凝土短柱轴压性能试验研究

为增强核心混凝土约束作用并改善其力学性能,在钢管混凝土外部增设圆钢管形成钢管约束的钢管混凝土组合柱,以构件类型、内层和外层钢管含钢率及核心混凝土强度等级为参数,设计并完成了14个钢管约束的钢管混凝土短柱和14个钢管混凝土短柱的轴压试验;观察试验现象和不同试件的破坏模式,研究各关键参数对试件轴压力学性能的影响,分析内层和外层钢管应力和应变发展规律,对比分析钢管混凝土和钢管约束的钢管混凝土的承载力和变形性能。研究结果表明:当钢管约束的钢管混凝土套箍系数不小于0.87,可使钢管混凝土柱的剪切脆性破坏转为截面压溃破坏,外观表现为腰鼓形破坏;达到承载力时,外层钢管横向应力可以达到钢材屈服强度,对核心混凝土的约束作用较强;钢管约束的钢管混凝土短柱轴压承载力较含钢率相近的常规钢管混凝土承载力可以提高20%左右,较内部钢管混凝土承载力可提高约70%。基于叠加法和GB 50936—2014《钢管混凝土结构技术规范》,提出两种钢管约束的钢管混凝土轴压承载力计算方法,预测结果与试验结果吻合良好。     

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.     

长周期地震动下高层钢管混凝土框架侧移性倒塌性能

开发了新的高精度钢与钢管混凝土(concrete-filled steel tube,CFST)的损伤与性能退化模型,研究了钢管混凝土柱在超高轴压下的抗震性能及其退化规律,基于该退化本构模型开发了纤维有限元仿真程序。基于长周期地震波峰值速度,分析了高层钢管混凝土框架在长周期地震作用下的倒塌行为。通过建立了高层CFST框架倒塌模型,考虑长周期地震动特性、退化效应和P-Δ效应对侧移性倒塌的影响,发现了残余变形集中的倾覆性倒塌机理。     

Simulation and design recommendations of eccentrically loaded slender concrete-filled tubular columns

This paper proposes an efficient numerical model for the simulation of the behavior of slender circular concrete-filled tubular columns subjected to eccentric axial load with single curvature, for the cases of both normal and high strength concrete. The paper focuses on the study of the influence that the variables affecting beam-column behavior (length and relative slenderness) and the variables affecting section behavior (diameter/thickness ratio, mechanical capacity of steel) have on the overall buckling of this type of column. An extended parametric study is carried out to propose design recommendations, primarily to establish the importance of the use of high strength concrete compared with that of normal strength concrete. The results show that for slender elements the optimum design is reached when the mechanical capacity of the steel is slightly lower than that of the concrete contribution.     

钢管高性能混凝土恒高温后力学性能研究

在确定高温后钢材和混凝土应力-应变关系的基础上,建立了恒高温后钢管高性能混凝土轴压构件的理论分析模型,对恒高温后该类构件的荷载-变形关系由线进行了计算,并与以往试验结果进行了比较,二者符合较好。分析结果表明,恒高温后钢管高性能混凝土轴压构件的荷载-变形关系弹性段变短,弹塑性段变长,极限承载力降低,极限承载力对应的应变增大,曲线下降段趋于不明显,这一特性与钢管普通混凝土构件高温后性能较为相似。