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 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.     

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。     

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.     

Nonlinear analysis of concrete-filled square stainless steel stub columns under axial compression

Concrete-filled stainless steel tubes (CFSST) can be considered as a new and innovative kind of composite construction technique, and have the potential to be used extensively in civil engineering. This paper employs a nonlinear analysis of square CFSST stub columns under axial compression. A three-dimensional nonlinear finite element (FE) model is developed using ABAQUS, where nonlinear material behaviour, enhanced strength corner properties of steel, and initial geometric imperfections are included. Close agreement is achieved between the test and FE results in terms of load-deformation response and ultimate strength. In light of the numerical results, the behaviour of stainless steel composite columns is compared with that of carbon steel composite columns. A simple model is proposed to calculate the ultimate strength of square CFSST stub columns.     

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.     

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.     

Experimental study of high strength concrete-filled circular tubular columns under eccentric loading

The paper describes 37 tests conducted on slender circular tubular columns filled with normal and high strength concrete subjected to eccentric axial load. The test parameters were the nominal strength of concrete (30, 70 and 90 MPa), the diameter to thickness ratio D/t, the eccentricity ratio e/D and the column slenderness (L/D). The experimental ultimate load of each test was compared with the design loads from Eurocode 4, which limits the strength of concrete up to 50 MPa. The aim of the paper is to establish the advisability of the use of high strength concretes as opposed to that of normal strength concretes by comparing three performance indices: concrete contribution ratio, strength index and ductility index. The results show for the limited cases analyzed that the use of high strength concrete for slender composite columns is interesting since this achieves ductile behavior despite the increase in load-carrying capacity is not greatly enhanced.     

Behaviour of short and slender concrete-filled stainless steel tubular columns

In this paper, a series of tests were carried out on short and slender concrete-filled stainless steel tubular columns to explore their performance under axial compression or combined actions of axial force and bending moment. Empty short steel hollow sections were also tested for comparison. The test results showed that the performance of the composite columns was quite good and have the potential to be used extensively as structural members. Comparisons of the test results were also made with several existing design methods for conventional concrete-filled carbon steel tubular columns as presented in Australian standard AS 5100 (2004), American code AISC (2005), Chinese code DBJ/T 13-51-2010 (2010), and Eurocode 4 (2004), which indicates that all the codes are somewhat conservative in predicting the load-carrying capacities of both short and slender 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.     

A novel steel section for concrete-filled tubular columns

In modern structural constructions, concrete-filled steel tubular (CFT) columns have gradually become a central element in structural systems like tall buildings, bridges and so forth. The effective parameters on load carrying capacity of CFT columns are the bond between the steel and internal concrete, local buckling strength of steel tube, creep of concrete and loading conditions of column at connections. Considering these effective parameters, a novel section is suggested which can be used for columns of tall buildings and bridges with large spans. The main characteristic of the suggested steel section is internal longitudinal symmetric stiffeners. In the present study, a comparative investigation into the behavior of this novel section (with circular and octagonal shapes) and the most common used sections of CFT columns has been carried out under axial and cyclic loading. Having verified the finite element modeling, several different analyses have been undertaken. The results of the analyses clearly exhibit the increase in strength and ductility of the suggested novel section under axial and cyclic loading and therefore, its application is recommended in construction practice.     

Seismic performance improvement of circular steel columns using precompressed concrete-filled steel tube

This paper presents a new method aimed at improving seismic resisting characteristics of circular-shaped steel columns representing highway bridge piers and an experimental investigation carried out to validate it. In this method, a special compression member is placed in the middle of the pier in order to take the axial load from the superstructure. As a result, the influence of axial load on the inelastic buckling deformation of plates can be greatly controlled. The special compression member consists of a precompressed concrete-filled steel tube (PC-CFT). Six specimens were tested to check the seismic performances of the proposed column system. It was clear from the test results that the specimens with PC-CFT could deform even up to ten times of their yield displacements without significant load deterioration. They also showed improved ultimate strength, ductility and energy absorption capacities than the corresponding benchmark specimens.     

Analysis of circular concrete-filled double skin tubular slender columns with external stainless steel tubes

This paper investigates the strength and behaviour of concrete-filled double skin steel tubular (CFDST) slender columns under axial compression. The lean duplex stainless steel material (EN 1.4162) which has recently gained significant attention is considered herein as the external jacket of such columns. Finite element (FE) analyses of several CFDST columns are conducted. Careful consideration is taken in the modelling for the concrete behaviour, for which both of the compressive and the tensile behaviours and the non-linear behaviour due to cracking are fully considered. The accuracy of the current FE models is ensured through the comparison with the existing columns in literature. A parametric study is then conducted to investigate the behaviour of such columns under different affecting factors; the slenderness ratio, the concrete confinement effect, the hollow ratio, the concrete compressive strength and the thickness ratio. The behavioural differences between intermediate length and very long CFDST columns are carefully addressed. Analytically obtained ultimate strengths are compared with design strengths calculated by European and American specifications. European design strength is found to give better predictions compared to the American specifications. However, it is shown that both strengths cannot be used in design because they overestimate the ultimate strengths and thereby do not satisfy the safety requirements. Therefore, a modification is suggested to the European design model which is shown to be able to estimate the compressive resistance of the CFDST columns more accurately than other methods.     

Post-fire bond between the steel tube and concrete in concrete-filled steel tubular columns

Push-out tests were carried out on 64 concrete-filled steel tubular columns, which had been exposed to ISO 834 standard fire for 90 min or 180 min, respectively. At the same time, 12 unheated specimens were also prepared and tested for comparison. The variables investigated in the bond tests were selected as (a) fire exposure time; (b) cross-section type; (c) cross-sectional dimension; (d) interface length to diameter (or width) ratio; (e) concrete type; (f) fly ash type; and (g) concrete curing condition. The effects of the above different parameters on the bond behaviour are discussed. The test results indicate that fire exposure had a significant effect on the bond between a steel tube and its concrete core. A decrease in bond strength was generally observed for specimens after a fire exposure of 90 min; however, bond strength recovery was found when the fire exposure time was extended to 180 min. Other factors also had influence to some extent.     

Cyclic performance of concrete-filled steel CHS columns under flexural loading

Concrete-filled steel CHS (circular hollow section) columns are currently being increasingly used in the construction of buildings. Limited information is available on the models for the moment (M) versus curvature (?) response, and the lateral load (P) versus lateral displacement (Δ) relationship of these columns subjected to axial load and cyclically increasing flexural loading.Eight concrete-filled steel CHS specimens were tested under constant axial load and cyclically increasing flexural loading. The parameters in the study included the concrete strength (f_cu) and the axial load level (n). A mechanics model is developed in this paper for concrete-filled steel CHS columns subjected to constant axial load and cyclically increasing flexural loading. The predicted cyclic responses for the composite columns are in good agreement with test results.Based on the theoretical model, parametric analysis was performed on the behaviours of the moment (M) versus curvature (?) response, and the lateral load (P) versus lateral displacement (Δ) relationship, as well as the ductility coefficient (μ) for the composite columns. Finally, simplified models for the moment (M) versus curvature (?) response, and the lateral load (P) versus lateral displacement (Δ) relationship are suggested. A formula for the calculation of the ductility coefficient (μ) of the composite columns under constant axial load and cyclically increasing flexural loading is developed.     

Experimental behaviour of concrete-filled stiffened thin-walled steel tubular columns

An experimental study on the structural behaviour of concrete-filled stiffened thin-walled steel tubular columns is presented in this paper. The stiffening was achieved by welding longitudinal stiffeners on the inner surfaces of the steel tubes. Companion tests were also undertaken on 12 unstiffened concrete-filled steel tubular (CFST) columns, with or without steel fibres in the infill concrete. The test results showed that the local buckling of the tubes was effectively delayed by the stiffeners. The plate buckling initially occurred when the maximum load had almost reached for stiffened specimens, thus they had higher serviceability benefits compared to those of unstiffened ones. Some of the existing design codes were used to predict the load-carrying capacities of the tested composite columns.     

Behaviors of recycled aggregate concrete-filled steel tubular columns under eccentric loadings

The paper investigates the behaviors of recycled aggregate concrete-filled steel tubular (RACFST) columns under eccentric loadings with the incorporation of expansive agents. A total of 16 RACFST columns were tested in this study. The main parameters varied in this study are recycled coarse aggregate replacement percentages (0%, 30%, 50%, 70%, and 100%), expansive agent dosages (0%, 8%, and 15%) and an eccentric distance of compressive load from the center of the column (0 and 40 mm). Experimental results showed that the ultimate stresses of RACFST columns decreased with increasing recycled coarse aggregate replacement percentages but appropriate expansive agent dosages can reduce the decrement; the incorporation of expansive agent decreased the ultimate stresses of RACFST columns but an appropriate dosage can increase the deformation ability. The recycled coarse aggregate replacement percentages have limited influence on the ultimate stresses of the RACFST columns and has more effect than that of the normal aggregate concrete-filled steel tubular columns.     

In-plane strength of concrete-filled steel tubular circular arches

More than 400 concrete-filled steel tubular (CFST) arch bridges have been constructed worldwide so far. However, design codes or guidance for the in-plane strength design of CFST arches are yet to be developed. In current design practice, the philosophy for the in-plane strength design of reinforced and prestressed concrete arches is widely adopted for CFST arches. For this, the CFST arches are considered under central or eccentric axial compression and are treated similarly to CFST columns, and the classical buckling load of CFST columns is used as the reference elastic buckling load of CFST arches. However, under transverse loading, the in-plane elastic buckling behaviour of CFST arches, particularly shallow CFST arches, is very different from that of CFST columns under axial compression. In addition, different from CFST columns under central or eccentric axial compression, CFST arches are subjected to significant nonlinear bending actions and transverse deformations prior to buckling and these will influence the strength of CFST arches greatly. Therefore, it is doubtful if the current method for in-plane strength design of CFST arches can provide correct strength predictions. In this paper, a method for the in-plane strength design of CFST circular arches, which is consistent with the current major design codes for steel structures, is developed by considering both geometric and material nonlinearities. A design equation for the in-plane strength capacity of CFST arches under uniform compression, and a lower-bound design equation for the in-plane strength check of CFST arches under combined actions of bending and compression are proposed.     

Nonlinear static and cyclic analysis of concrete-filled steel columns

This paper presents the formulation of a one-dimensional (1D) composite frame element for the nonlinear static and cyclic analysis of concrete-filled steel (CFS) beam-columns. A two-node frame element is formulated using the force interpolation concept, and the material nonlinearity at section level is taken into account by employing a total secant stiffness and modified fibre element approach. The size effect and steel tube confinement on the concrete strength and ductility are taken into account. Further, the effect of steel tube local buckling on the member strength is addressed. Concerning geometrical nonlinearities at the element level, the equilibrium equations are satisfied for the deformed element to take account of P-Δ effects. The formulation accuracy and efficiency of the model are verified by some numerical examples of the static and cyclic analysis of CFS members.