Nonlinear analysis of biaxially loaded rectangular concrete-filled stainless steel tubular slender beam-columns

Rectangular concrete-filled stainless steel tubular (CFSST) beam-columns utilized as supporting members for building frames may experience axial compression and biaxial moments. A numerical simulation considering the local buckling effects for thin-walled rectangular CFSST slender beam-columns has not been performed. This paper reports a stability modeling on the structural characteristics of rectangular CFSST slender beam-columns accounting for different strain-hardening of stainless steel under tension and compression. The influences of local buckling are considered in the simulation utilizing the existing effective width formulations. The developed numerical model simulates the strength interaction and load-deflection behavior of CFSST slender beam-columns. Comparisons of computed results with test data provided by experimental investigations are performed to validate the proposed fiber model. The influences of different geometric and material property on ultimate strengths, ultimate pure moments, concrete contribution ratio, strength interaction and load-deflection responses of CFSST slender beam-columns are examined by utilizing fiber model. A design formula considering strain hardening of stainless steel is derived for calculating the ultimate pure moment of square CFSST beam-columns.

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title: Developments and advanced applications of concrete-filled steel tubular (CFST) structures: Members
abstract: Concrete-filled steel tubular (CFST) structure offers numerous structural benefits, and has been widely used in civil engineering structures. This paper reviews the development of the family of concrete-filled steel tubular structures to date and draws a research framework on CFST members. The research development on CFST structural members in most recent years, particularly in China, is summarized and discussed. The current design approaches from various countries are examined briefly. Some projects in China utilizing CFST members are also introduced. Finally, some concluding remarks are made for CFST members.

title: Stainless steel in construction: A review of research, applications, challenges and opportunities
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title: AESTHETICS, ECONOMICS AND DESIGN OF STAINLESS STEEL STRUCTURES
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title: Local buckling of steel plates in concrete-filled thin-walled steel tubular beam–columns
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title: Performance-based analysis of concrete-filled steel tubular beam–columns, Part I: Theory and algorithms
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title: Performance-based analysis of concrete-filled steel tubular beam–columns, Part II: Verification and applications
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title: Behaviour of short and slender concrete-filled stainless steel tubular columns
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title: Axially Loaded Concrete-Filled Steel Tubes
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title: Seismic behavior and modeling of high-strength composite concrete-filled steel tube (CFT) beam–columns
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title: Behaviour of eccentrically loaded high-strength rectangular concrete-filled steel tubular columns
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title: Experimental study of high strength concrete-filled circular tubular columns under eccentric loading
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title: Column buckling tests of hot-rolled concrete filled square hollow sections of mild to high strength steel
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title: Experimental investigation of concrete-filled cold-formed high strength stainless steel tube columns
abstract: This paper presents an experimental investigation of concrete-filled cold-formed high strength stainless steel tube columns. The high strength stainless steel tubes had a yield stress and tensile strength up to 536 and 961 MPa, respectively. The behaviour of the columns was investigated using different concrete cylinder strengths varied from 40 to 80 MPa. A series of tests was performed to investigate the effects of the shape of the stainless steel tube, plate thickness and concrete strength on the behaviour and strength of concrete-filled high strength stainless steel tube columns. The high strength stainless steel tubes were cold-rolled into square and rectangular hollow sections. The depth-to-plate thickness ratio of the tube sections varied from 25.7 for compact sections to 55.8 for relatively slender sections. The columns had different lengths so the length-to-depth ratio generally remained at a constant value of 3. The concrete-filled high strength stainless steel tube specimens were subjected to uniform axial compression. The column strengths, load–axial strain relationships and failure modes of the columns were presented. The test strengths were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards that consider the effect of local buckling using an effective width concept in the calculation of the stainless steel tube column strengths. Based on the test results, design recommendations were proposed for concrete-filled high strength stainless steel tube columns.

title: Structural design of stainless steel concrete filled columns
abstract: a b s t r a c t This paper presents the behaviour and design of axially loaded concrete filled stainless steel circular and square hollow sections. The experimental investigation was conducted using different concrete cube strengths varied from 30 to 100 MPa. The column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with existing design methods for composite carbon steel sections — Eurocode 4 and ACI. It was found that existing design guidance for carbon steel may generally be safely applied to concrete filled stainless steel tubes, though it tends to be over-conservative. A continuous strength method is proposed and it is found to provide the most accurate and consistent prediction of the axial capacity of the composite concrete filled stainless steel hollow sections due largely to the more precise assessment of the contribution of the stainless steel tube to the composite resistance.

title: Experimental investigation of eccentrically loaded fibre reinforced concrete-filled stainless steel tubular columns
abstract: This paper presents an experimental investigation of axially and eccentrically loaded plain and fibre reinforced (FR) concrete-filled stainless steel circular tubular columns. The composite columns were pin-ended subjected to axial and eccentric loads. The stainless steel tubes were relatively slender having a diameter-to-plate thickness ratio of 50. The composite columns had different lengths varied from 3 D to 12 D . The column ultimate loads, load–axial shortening relationships, load–strain relationships, load–mid-height lateral deflection relationships and failure modes of the concrete-filled stainless steel circular tubular columns were measured from the tests. The study has shown that FR concrete-filled stainless steel tubular columns offer a considerable increase in column ductility compared with plain concrete-filled tubular columns. The test ultimate loads were compared with the design ultimate loads calculated using the Eurocode 4 for composite columns. Generally, it has been shown that the EC4 accurately predicted the ultimate loads of axially loaded concrete-filled stainless steel circular tubular columns, but were quite conservative for predicting the ultimate loads of the eccentrically loaded columns. It has also been shown that the conservatism of the EC4 predictions is increased as the eccentricity is increased. The test results provide useful information regarding the behaviour of FR concrete-filled stainless steel columns.

title: Tests on plain and steel fiber concrete-filled stainless steel tubular columns
abstract: This paper reports the experimental behavior of plain and steel fiber concrete-filled stainless steel tubular columns under biaxial bending and axial compression. In the experimental study, the length-to-diameter ratios of the stainless steel tubes were 12, 15, and 20. The effects of the main test parameters of concrete compressive strength, cross-section, load eccentricity, steel fiber material and slenderness on the structural behavior of plain and steel fiber concrete-filled stainless steel tubular columns were examined. The ultimate strength capacities, load–deflection relations, and load–axial strain behavior were observed in the tests. The plain and steel fiber concrete-filled tube columns have been analyzed with a theoretical method based on the nonlinear behavior of the materials. The experimental and theoretical biaxial load–deflection diagrams have been obtained and compared in the study. The experimental results have exposed significant knowledge to describe the behavior of plain and steel fiber concrete-filled stainless steel tubular columns.

title: A distributed plasticity model for concrete-filled steel tube beam-columns with interlayer slip
abstract: A fiber-based distributed plasticity finite element formulation is presented to perform three-dimensional monotonic analysis of square or rectangular concrete-filled steel tube (CFT) beam-columns. This stiffness-based beam-column element fomulation accounts comprehensively for all significant geometric nonlinearity exhibited by CFT beam-columns as part of composite frame structures, and the steel and concrete constitutive models account for the significant inelastic phenomena which are seen in CFT experiments. In addition, the finite element formulation accounts for slip between the steel and concrete components of the CFT by incorporation of a nonlinear slip interface. This formulation is able to capture behavior ranging from perfect bond to immediate slip. The calibration and verification of the slip formulation are presented, and the finite element model is verified against experiments of CFT beam-columns subjected to monotonic loading. Results from a preliminary study are presented on the effect of slip on CFT beam-column and composite frame behavior. A related paper extends this formulation to cyclic analysis of composite CFT frames, and provides details of the steel and concrete constitutive models.

title: Uniaxial stress–strain relationship of concrete confined by various shaped steel tubes
abstract: A method is presented to predict the complete stress–strain curve of concrete subjected to triaxial compressive stresses caused by axial load plus lateral pressure due to the confinement action in circular, box and octagonal shaped concrete-filled steel tubes. Available empirical formulas are adopted to determine the lateral pressure exerted on concrete in circular concrete-filled steel columns. To evaluate the lateral pressure exerted on the concrete in box and octagonal shaped columns, FEM analysis is adopted with the help of a concrete–steel interaction model. Subsequently, an extensive parametric study is conducted to propose an empirical equation for the maximum average lateral pressure, which depends on the material and geometric properties of the columns. Lateral pressure so calculated is correlated to confined concrete strength through a well known empirical formula. For determination of the post-peak stress–strain relation, available experimental results are used. Based on the test results, approximated expressions to predict the slope of the descending branch and the strain at sustained concrete strength are derived for the confined concrete in columns having each type of sectional shapes. The predicted concrete strength and post-peak behavior are found to exhibit good agreement with the test results within the accepted limits. The proposed model is intended to be used in fiber analysis involving beam–column elements in order to establish an ultimate state prediction criterion for concrete-filled steel columns designed as earthquake resisting structures.

title: An analytical model for thin-walled steel box columns with concrete in-fill
abstract: Concrete-filled steel box columns are widely used as a major structural element in tall buildings as they provide efficient structural performance in resisting axial compression. Local buckling of steel plates is a common failure criteria when plate slenderness ratio is large in box sections. A method using an effective width principle is proposed in this paper to predict the behaviour and load carrying capacity of thin-walled steel tubes with concrete in-fill. The present investigation addresses columns pinned at their ends and subjected to biaxial loading. Columns tested by other researchers have been analysed using the proposed method and the predicted results are compared with the corresponding test results. The proposed model is also verified against Eurocode 4. The comparison shows that the method could predict the ultimate load with sufficient accuracy.

title: Simulation and design recommendations of eccentrically loaded slender concrete-filled tubular columns
abstract: 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.

title: High strength thin-walled rectangular concrete-filled steel tubular slender beam-columns, Part I: Modeling
abstract: 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 Muller'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.

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

title: Biaxially loaded high-strength concrete-filled steel tubular slender beam-columns, Part I: Multiscale simulation
abstract: 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 Muller'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.

title: Nonlinear behaviour of eccentrically loaded FR concrete-filled stainless steel tubular columns
abstract: This paper investigates the nonlinear behaviour of eccentrically loaded fibre reinforced (FR) concrete-filled stainless steel tubular composite columns. A nonlinear 3-D finite element model for the axially loaded composite columns, recently reported by the author, was extended to study the structural performance of the eccentrically loaded composite columns. The columns were pin-ended subjected to an eccentric load acting along one axis. The model accounted for the inelastic behaviour of the composite column components, effect of FR concrete confinement and interface between the stainless steel section and concrete. The measured initial local and overall geometric imperfections were carefully incorporated in the model. The finite element model has been validated against tests previously reported by the author. Furthermore, the variables that influence the eccentrically loaded composite column behaviour and strength comprising different eccentricities, different column slenderness and different concrete strengths were investigated in an extensive parametric study comprising 72 columns. The composite column strengths and moment resistances predicted from the finite element analysis were compared with the design composite column strengths and moment resistances calculated using the Eurocode 4. The study has shown that finite element modelling could effectively assess the accuracy of the design rules in current codes of practice.

title: New insights into the mechanism of load introduction into concrete-filled steel tubular column through shear connection
abstract: This paper presents the results of an extensive numerical and analytical study to re-investigate the mechanism of load introduction into concrete-filled tubular column through shear connection. The new mechanism of load introduction revealed from this research indicates that the shear load from the connection is introduced through the column length above and within the connection. This is different from the currently assumed mechanism of load introduction from underneath the connection. In fact, a simple free-body diagram analysis would demonstrate that the currently assumed load introduction mechanism is not possible. The results of the numerical simulations also indicate that the load introduction length above the connection is unlimited and can be as long as the entire column height. Based on the new load introduction mechanism and the numerical simulation results, the maximum amount of load introduced into the concrete core can be calculated as the bond strength multiplying the entire steel tube/concrete core contact surface area for the entire column length above and including the connection. Under the new load introduction mechanism and using representative values of column dimensions and concrete cylinder strength, it has been demonstrated that complete load introduction can be achieved in almost all practical arrangements of concrete filled tubular construction.

title: Biaxially loaded high-strength concrete-filled steel tubular slender beam-columns, part II: Parametric study
abstract: Biaxially loaded high strength rectangular concrete-filled steel tubular (CFST) slender beam-columns with large depth-to-thickness ratios, which may undergo local and global interaction buckling, have received very little attention. This paper presents the verification of a multiscale numerical model described in a companion paper and an extensive parametric study on the performance of high strength thin-walled rectangular CFST slender beam-columns under biaxial loads. Comparisons of computer solutions with existing experimental results are made to examine the accuracy of the multiscale numerical model developed. The effects of the concrete compressive strength, loading eccentricity, depth-to-thickness ratio and columns slenderness on the ultimate axial strength, steel contribution ratio, concrete contribution ratio and strength reduction factor of CFST slender beam-columns under biaxial bending are investigated by using the numerical model. Comparative results demonstrate that the multiscale numerical model is capable of accurately predicting the ultimate strength and deflection behavior of CFST slender beam-columns under biaxial loads. Benchmark numerical results presented in this paper provide a better understanding of the local and global interaction buckling behavior of high strength thin-walled CFST slender beam-columns and are useful for the development of composite design codes.

title: Effective stress-strain relationships for analysis of noncompact and slender filled composite (CFT) members
abstract: Fiber-based section analysis methods are widely used to model and predict the fundamental axial force-bending moment-curvature ( P - M - ϕ ) behavior and the strength interaction ( P - M ) of concrete-filled steel tube (CFT) members. The accuracy of these predictions is governed by the uniaxial effective stress-strain relationships assumed for the steel tube and concrete infill of the CFT section. Prior research has developed these effective stress-strain relationships for compact CFT members. This paper presents the development and verification of effective stress-strain relationships for the steel tube and concrete infill of noncompact and slender CFT members. These relationships are developed using results from comprehensive 3D finite element analyses of CFT members with a wide range of geometric and material parameters. The 3D finite element models, which were developed and benchmarked previously by the authors, accounted explicitly for the effects of steel yielding and tube local buckling, concrete cracking and compression inelasticity, and the transverse interaction leading to steel hoop stresses and concrete confinement. As a result, the developed effective stress-strain relationships also accounted (implicitly) for these complexities (yielding, local buckling, confinement, etc.) of behavior. These effective stress-strain relationships are implemented in a nonlinear fiber analysis (NFA) macro model, and used to predict the behavior of noncompact and slender CFT members in the experimental database. The conservatism of the predictions using the effective stress-strain relationships is evaluated.

title: Design and behaviour of concrete-filled cold-formed stainless steel tube columns
abstract: This paper presents a nonlinear finite element model to investigate the behaviour and design of axially loaded concrete-filled cold-formed high strength stainless steel tube columns. The study was conducted over a wide range of concrete cylinder strengths from 20 to 100 MPa. The depth of flat portion-to-plate thickness (d/t) ratio of the square and rectangular hollow sections ranged from 16 to 96, covering compact and slender sections. The columns had different lengths so that the length-to-depth ratio remained a constant of 3. Nonlinear material models for confined concrete and stainless steel tubes were used in the finite element model. The results obtained from the finite element analysis were verified against experimental results. An extensive parametric study was conducted to investigate the effects of cross-section geometry and concrete strength on the behaviour and strength of the columns. The column strengths obtained from the finite element analysis were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards. A design equation was proposed for concrete-filled stainless steel tube columns. It is shown that the design strengths obtained using the proposed modified equation are more accurate compared to the American and Australian/New Zealand predictions.

title: Nonlinear analysis of concrete-filled square stainless steel stub columns under axial compression
abstract: 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.

title: Behaviour of circular concrete-filled lean duplex stainless steel tubular short columns
abstract: Lean duplex stainless steel material (EN 1.4162) has recently gained significant attention for its higher structural performance and corrosion resistance compared to the austenitic type. Circular lean duplex stainless steel tubes filled with concrete are innovative composite columns which have not been studied experimentally or numerically. This paper presents the fundamental behaviour of circular concrete-filled lean duplex stainless steel tubular (CFSST) short columns under axial compression. Three dimensional finite element (FE) models for CFSST columns subjected to axial compression are developed using the FE package ABAQUS. The lean duplex stainless steel material is modelled using the two-stage constitutive laws while the concrete is simulated using accurate concrete confinement models. The FE models are verified by comparisons with existing experimental results on hollow stainless steel columns, concrete-filled steel tubular columns and CFSST columns. Parametric studies are undertaken to investigate the effects of concrete compressive strength and diameter-to-thickness (D/t) ratio on the behaviour of CFSST columns. The results show that the ultimate axial strength of circular CFSST columns increases with increasing the concrete compressive strength but decreases with an increase in the D/t ratio. Circular CFSST columns with different D / t ratios exhibit the same initial stiffness. The lean duplex stainless steel tubes cannot provide good confinement on the concrete when D / t ratio is large. The ultimate axial strengths of CFSST columns predicted by the FE models are also compared with those calculated by the Eurocode 4, ACI code, the continuous strength method (CSM) by Lam and Gardner and Liang and Fragomeni's design formulas. The comparative study shows that Eurocode 4 and the CSM give good estimates of the ultimate axial strengths of CFSST columns with D / t 40 but overestimates the strengths of columns with D / t ≥ 40 . The ACI code gives too conservative estimates of the ultimate loads of CFSST columns as it does not consider the concrete confinement effects. Finally, it was found that the modified Liang and Fragomeni's design formulas yield the best predictions of the ultimate axial strengths of CFSST columns over the entire range of D / t ratios.

title: Behaviour of circular concrete-filled lean duplex stainless steel–carbon steel tubular short columns
abstract: Recently, a concrete-filled austenitic stainless steel–carbon steel tubular column has been introduced as a new form of composite members. This column is composed of an external stainless steel tube and an internal concentrically placed carbon steel tube with concrete filling in the whole tubular section. However, the excellent performance of such composite columns can additionally be achieved by utilising the low-cost lean duplex stainless steel material. In this paper, the behaviour of circular concrete-filled lean duplex stainless steel–carbon steel tubular (CFSCT) short columns under axial loading is investigated by using the general purpose finite element analysis program ABAQUS. A new constitutive model for the concrete core confined by both the external stainless steel tube and the internal carbon steel tube is proposed and incorporated in the finite element model developed. The accurate two-stage stress–strain relationships are employed to model cold-formed lean duplex stainless steels. The finite element results obtained by the proposed concrete model are shown to be in good agreement with test results. Parametric studies are then conducted to investigate the effects of concrete compressive strength, carbon steel yield stress and cross-section geometry on the behaviour of CFSCT columns. The ACI code is found to significantly underestimate the ultimate axial strengths of CFSST columns. Therefore, a new design model is proposed for CFSCT short columns. It is shown that the proposed design model provides excellent predictions of the ultimate axial strengths of circular CFSCT columns over a wide range of diameter-to-thickness ratios of the stainless steel tubes.

title: Inelastic analysis for the post-collapse behavior of concrete encased steel composite columns under axial compression
abstract: This paper proposes a simple inelastic analysis approach to efficiently map out the complete nonlinear post-collapse (strain-softening) response and the maximum load capacity of axially loaded concrete encased steel composite columns (stub and slender). The scheme simultaneously incorporates the influences of difficult instabilizing phenomena such as concrete confinement, initial geometric imperfection, geometric nonlinearity, buckling of reinforcement bars and local buckling of structural steel, on the overall behavior of the composite columns. The proposed numerical method adopts fiber element discretization and an iterative Muller's algorithm with an additional adaptive technique that robustly yields solution convergence. The accuracy of the proposed analysis scheme is validated through comparisons with various available experimental benchmarks. Finally, a parametric study of various key parameters on the overall behaviors of the composite columns is conducted.

title: Nonlinear analysis of axially loaded circular concrete-filled stainless steel tubular short columns
abstract: Experiments show that the ultimate compressive strength of stainless steel is much higher than its tensile strength. The full-range two-stage constitutive model for stainless steels assumes that stainless steels follow the same stress–strain behavior in compression and tension, which may underestimate the compressive strength of stainless steel tubes. This paper presents a fiber element model incorporating the recently developed full-range three-stage stress–strain relationships based on experimentally observed behavior for stainless steels for the nonlinear analysis of circular concrete-filled stainless steel tubular (CFSST) short columns under axial compression. The fiber element model accounts for the concrete confinement effects provided by the stainless steel tube. Comparisons of computer solutions with experimental results published in the literature are made to examine the accuracy of the fiber element model and material constitutive models for stainless steels. Parametric studies are conducted to study the effects of various parameters on the behavior of circular CFSST short columns. A design model based on Liang and Fragomeni's design formula is proposed for circular CFSST short columns and validated against results obtained by experiments, fiber element analyses, ACI-318 codes and Eurocode 4. The fiber element model incorporating the three-stage stress–strain relationships for stainless steels is shown to simulate well the axial load–strain behavior of circular CFSST short columns. The proposed design model gives good predictions of the experimental and numerical ultimate axial loads of CFSST columns. It appears that ACI-318 codes and Eurocode 4 significantly underestimate the ultimate axial strengths of CFSST short columns.

title: Nonlinear analysis of circular high strength concrete-filled stainless steel tubular slender beam-columns
abstract: Concrete-filled stainless steel tubular (CFSST) slender columns are increasingly used in composite structures owing to their distinguished features, such as aesthetic appearance, high corrosion resistance, high durability and ease of maintenance. Currently, however, there is a lack of an accurate and efficient numerical model that can be utilized to determine the performance of circular CFSST slender columns. This paper describes a nonlinear fiber-based model proposed for computing the deflection and axial load-moment strength interaction responses of eccentrically loaded circular high-strength CFSST slender columns. The fiber-based model incorporates the accurate three-stage stress-strain relations of stainless steels, accounting for different strain hardening characteristics in tension and compression. The material and geometric nonlinearities as well as concrete confinement are included in the computational procedures. Existing experimental results on axially loaded CFSST slender columns are utilized to verify the proposed fiber-based model. A parametric study is conducted to examine the performance of high-strength slender CFSST beam-columns with various geometric and material parameters. It is shown that the fiber-based analysis technique developed can accurately capture the experimentally observed performance of circular high-strength CFSST slender columns. The results obtained indicate that increasing the eccentricity ratio, column slenderness ratio and diameter-to-thickness ratio remarkably decreases the initial flexural stiffness and ultimate axial strength of CFSST columns, but considerably increases their displacement ductility. Moreover, an increase in concrete compressive strength increases the flexural stiffness and ultimate axial strength of CFSST columns; however, it decreases their ductility. Furthermore, the ultimate axial strength of CFST slender columns is found to increase by using stainless steel tubes with higher proof stresses.

title: Theoretical stress strain model for confined concrete
abstract: A stress‐strain model is developed for concrete subjected to uniaxial compressive loading and confined by transverse reinforcement. The concrete section may contain any general type of confining steel: either spiral or circular hoops; or rectangular hoops with or without supplementary cross ties. These cross ties can have either equal or unequal confining stresses along each of the transverse axes. A single equation is used for the stress‐strain equation. The model allows for cyclic loading and includes the effect of strain rate. The influence of various types of confinement is taken into account by defining an effective lateral confining stress, which is dependent on the configuration of the transverse and longitudinal reinforcement. An energy balance approach is used to predict the longitudinal compressive strain in the concrete corresponding to first fracture of the transverse reinforcement by equating the strain energy capacity of the transverse reinforcement to the strain energy stored in the concret...

title: Full-range stress–strain curves for stainless steelalloys
abstract: The paper develops an expression for the stress–strain curves for stainless steel alloys which is valid over the full strain range. The expression is useful for the design and numerical modelling of stainless steel members and elements which reach stresses beyond the 0.2% proof stress in their ultimate limit state. In this stress range, current stress–strain curves based on the Ramberg–Osgood expression become seriously inaccurate principally because they are extrapolations of curve fits to stresses lower than the 0.2% proof stress. The extrapolation becomes particularly inaccurate for alloys with pronounced strain hardening. The paper also develops expressions for determining the ultimate tensile strength ( σ u ) and strain ( ϵ u ) for given values of the Ramberg–Osgood parameters ( E 0 , σ 0.2 , n ). The expressions are compared with a wide range of experimental data and shown to be reasonably accurate for all structural classes of stainless steel alloys. Based on the expressions for σ u and ϵ u , it is possible to construct the entire stress–strain curve from the Ramberg–Osgood parameters ( E 0 , σ 0.2 , n ).

title: Three-Stage Full-Range Stress-Strain Model for Stainless Steels
abstract: Advanced numerical modeling of cold-formed stainless steel members, from manufacturing to full-range response under applied loading, requires knowledge of the stress-strain relationship of the material over a wide range of tensile and compressive strains. Although a number of stress-strain models have been developed for stainless steels, they are only capable of accurate predictions either over a limited strain range or for the tensile stress-strain behavior only. This paper presents a three-stage stress-strain model for stainless steels, which is capable of accurate predictions over the full ranges of both tensile and compressive strains. The new stress-strain model is defined using the three basic Ramberg-Osgood parameters and is based on a careful interpretation of existing experimental data. The accuracy of the proposed model is demonstrated by comparing its predictions with experimental stress-strain curves. These comparisons also clearly demonstrate the advantage of the proposed model over the only existing full-range stress-strain model.

title: Inversion of three-stage stress–strain relation for stainless steel in tension and compression
abstract: Presented in this paper is a new stress–strain relation for stainless steel alloys that provides the stress as an explicit function of the strain. The relation is an approximate inversion of a recently proposed three-stage stress–strain relation based on a modified Ramberg–Osgood equation. The three-stage relation is a much more accurate formulation than the previous two-stage formulations and is applicable to both tensile as well as compressive stresses. The new relation is derived by making a rational function assumption on the fractional deviation of the actual stress–strain curve from an idealized linear elastic behaviour. The new expression is valid over the full range of the stress well beyond the elastic region. The validity of the inverted expression is tested over a wide range of material parameters. These tests demonstrate that, the new expression results in stress–strain curves which are both qualitatively and quantitatively in excellent agreement with the fully iterated numerical solution of the full-range stress–strain relation with a maximum error below 4%.