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• A new computational model is developed for the simulation of RDCFST short columns. • The model incorporates the local and post-local buckling of outer and inner tubes. • The behavior of RDCFST columns is investigated by using the computational model. • An expression is proposed for the strength design of RDCFST short columns. • The proposed modeling method predicts well the performance of RDCFST columns. Both the external and internal thin-walled steel sections of a rectangular double-skin concrete-filled steel tubular (RDCFST) short column loaded axially may be susceptible to progressive local buckling, which is rarely included in mathematical modeling programs employing the fiber discretization scheme for such composite members. This paper provides a description of a new computational simulation technology, which is developed for the nonlinear fiber analysis of short RDCFST columns axially loaded to failure. The progressive localized-buckling failure of thin-walled steel sections are included in the formulation of the computational simulation method. Experimental measurements and finite element analysis results obtained by ABAQUS software by other researchers are employed to evaluate the accuracy of the computer algorithms developed. Numerical studies are undertaken to ascertain the responses of RDCFST columns to the change of design parameters. Proposed is a mathematical expression for the determination of the axial resistances of RDCFST columns considering the post-localized buckling strength of thin-walled steel sections. It is demonstrated that the proposed computational modeling and design technologies yield good predictions of the responses of RDCFST columns and can be used to undertake the nonlinear simulation of RDCFST columns with any class of steel sections. [ABSTRACT FROM AUTHOR]

This study investigated the behaviour of short square concrete filled fibre reinforced polymer tube specimens under concentric, uniaxial eccentric, biaxial eccentric and four-point bending loads. Seven concrete filled glass fibre reinforced polymer (GFRP) tube specimens (GFRP-CFFT) and seven traditional steel bar reinforced concrete (steel-RC) specimens of 200 mm x 200 mm cross-section were tested. The GFRP-CFFT specimens obtained a larger P-M interaction surface (higher maximum axial loads and higher bending moments) than the steel-RC specimens. In addition, the GFRP-CFFT specimens exhibited a lower average reduction in ductility under eccentric axial loads than the steel-RC specimens. • Behaviour of GFRP-CFFT columns under biaxial eccentric load has been investigated. • Load–displacement responses of biaxially loaded GFRP-CFFT specimens are reported. • CFFT specimens achieved larger P-M interaction surfaces than steel-RC specimens. • FRP tubes could be used instead of steel reinforcement in biaxially loaded columns. [ABSTRACT FROM AUTHOR]

• A new computer model for predicting behavior of RDCFST slender columns is proposed. • The local and global interaction buckling is considered in computational formulations. • The effects of design parameters on responses of RDCFST columns are investigated. • The computer simulation technique captures well the behavior of RDCFST slender columns. Rectangular double-skin concrete-filled steel tubular (RDCFST) slender columns made of non-compact or slender steel sections under axial load and bending may undergo local–global interaction buckling, which is rarely considered in existing fiber-based nonlinear modeling procedures for RDCFST columns. In this paper, a new computational model is presented, which can simulate the load–deflection responses of uniaxially loaded thin-walled RDCFST slender columns that are discretized into fiber elements. The important features associated with thin-walled RDCFST slender columns are explicitly accounted for in the computational model, including the interaction between the local buckling of outer and inner steel tubes and the column global buckling, initial geometric imperfections, material nonlinearities and second order effects. An incremental-iterative computational algorithm is developed to capture the nonlinear buckling displacements of RDCFST columns. The nonlinear equilibrium equation generated at each iteration is solved by means of implementing Müller's numerical scheme. The computational model, which is verified by experimental results, is utilized to investigate the interaction buckling responses of RDCFST columns loaded eccentrically, considering a wide range of design parameters. It is demonstrated that the developed computational modeling and simulation technique can detect the initial local buckling of double-skins and capture the influences of the interaction of localized and global instability on the performance of nonlinear RDCFST slender columns. The benchmark numerical results obtained from the parametric study provide a new insight into the interaction buckling behavior of RDCFST columns. [ABSTRACT FROM AUTHOR]

• A new strain model for discretely FRP confined concrete was proposed. • Energy balance principle was used to develop the strain model. • A strain reduction factor of discretely FRP confined concrete was proposed. • An energy absorption factor for discretely FRP confined concrete was developed. • Ultimate axial strain of discretely FRP confined concrete was predicted accurately. This study proposes a new strain model for concrete with discrete fiber-reinforced polymer (FRP) confinement. An energy absorption factor for discretely FRP confined concrete and a strain reduction factor of FRP were proposed. The energy absorption factor and strain reduction factor in conjunction with an available strength model of concrete with FRP confinement were then used to develop a new strain model for concrete with discrete FRP confinement based on the strain energy balance principle. The new strain model was compared with available strain models suggested for concrete with FRP confinement. It was found that the strain model proposed in this study was more accurate than the available strain models in predicting the ultimate axial strain of concrete with discrete FRP confinement. [ABSTRACT FROM AUTHOR]

• Analytical failure envelopes of discretely CFRP confined columns were developed. • Parameters that influence the failure envelopes of columns were evaluated. • Clear spacing and CFRP band width had pronounced effect on the failure envelope. The failure envelopes of square and circularized reinforced concrete (RC) columns discretely confined with CFRP have been analytically investigated in this paper. The analytical failure envelopes of the columns were developed using a strip-by-strip integration approach incorporating stress-strain models of constituent materials. The experimental results published in the literature were utilized to examine the accuracy of the analytical model. It was found that the analytical failure envelopes matched well with the experimental results. A parametric investigation was undertaken to evaluate the influence of different parameters on the failure envelopes of square and circularized RC columns discretely confined with CFRP. It was found that the clear spacing of CFRP band, CFRP band width and CFRP confinement ratio had pronounced influences on the failure envelopes of square and circularized RC columns with discrete FRP wrapping. For the same ratio of the clear spacing and the band width, the change in the CFRP band width had a negligible influence on the failure envelope of the discretely CFRP confined square RC column but had a slight influence on the failure envelope of the discretely CFRP confined circularized RC column. In addition, for the same amount of CFRP, discretely FRP confined concrete columns with lower clear spacing of CFRP band obtained higher failure envelope. [ABSTRACT FROM AUTHOR]

Concrete-filled aluminum tubular (CFAT) columns have the advantages of a lightweight, high resistance to corrosion, good appearance and ease of maintenance over concrete-filled steel tubular (CFST) columns. However, experimental and computational studies on the performance of CFAT columns have rarely been reported. This paper concerns with the fiber-based numerical analysis, structural behavior, and design of circular high strength CFAT stub columns loaded concentrically. A new concrete confinement model for computing the lateral pressures on the concrete core in CFAT circular columns is proposed and implemented in the computational model. The numerical analysis procedure is given for the determination of the nonlinear load-strain behavior of CFAT stub columns. The available experimental results of circular CFAT columns are used to assess the accuracy of the developed model. The computer model is employed to ascertain the significance of various geometric parameters in addition to material strengths on the responses of CFAT stub circular columns. The design equation based on Liang-Fragomeni's expression is proposed for quantifying the ultimate axial strengths of axially compressed CFAT short columns. The verification reveals that the computer modeling approach accurately quantifies the ultimate loads and nonlinear load-strain performance of CFAT columns with circular sections. The proposed design equation yields good calculations of the ultimate axial strengths of CFAT columns. • A numerical model is developed for predicting the responses of CFAT short columns loaded concentrically. • A new confinement model is proposed for concrete confined by the circular aluminum tube. • The numerical investigations into the structural behavior and load distributions in circular CFAT columns are undertaken. • A design equation is proposed for computing the ultimate axial loads of CFAT columns. • The numerical and design models predicts well the behavior and strength of circular CFAT columns. [ABSTRACT FROM AUTHOR]

The repair of fire-damaged thin-walled rectangular concrete-filled steel tubular (CFST) columns in engineering structures after fire exposure requires the assessment of their residual strength and stiffness. Existing numerical models have not accounted for the effects of local buckling on the post-fire behavior of CFST columns with rectangular thin-walled sections. This paper describes a nonlinear post-fire simulation technique underlying the theory of fiber analysis for determining the residual strengths and post-fire responses of concentrically loaded short thin-walled rectangular CFST columns accounting for progressive local buckling. The post-fire stress-strain laws for concrete in rectangular CFST columns are proposed based on available test data and implemented in the theoretical model. An innovative numerical scheme for modeling the progressive local and post-local buckling of CFST thin-walled columns is discussed. The nonlinear post-fire simulation model is verified by experimental data and then used to investigate the significance of local buckling, material strengths and width-to-thickness ratio on the post-fire responses of CFST stub columns. The proposed post-fire computer model is shown to be capable of predicting well the residual stiffness and strength of concentrically loaded thin-walled CFST columns after fire exposure. A design formula is proposed that estimates well the post-fire residual strengths of CFST columns. Computational results presented provide a better understanding of the post-fire behavior of CFST columns fabricated by thin-walled sections incorporating local and post-local buckling. • Nonlinear post-fire simulation model is presented for rectangular CFST stub columns. • A new post-fire stress-strain model for concrete in CFST columns is proposed. • A design formula is proposed for computing the post-fire strengths of CFST columns. • The post-fire behavior of CFST columns is investigated using the verified model. • The computer and design models predict well the residual strengths of CFST columns. [ABSTRACT FROM AUTHOR]

Slender rectangular thin-walled concrete-filled steel tubular (CFST) columns in composite building structures exposed to fire may experience the interaction of local and global buckling. Numerical investigations on the interaction buckling responses of such columns under fire exposure have been rarely reported. This paper describes a fiber-based computational model for the prediction of the fire-resistance and interaction responses of local and global buckling of concentrically-loaded slender CFST columns made of rectangular sections exposed to fire. The thermal analysis is undertaken to calculate the distribution of temperatures in the column cross-section considering the effects of the air gap between concrete and steel, exposure surface emissivity as well as moisture content in concrete. The local and post-local buckling models proposed previously for steel tube walls at elevated temperatures are incorporated in the inelastic analysis of cross-sections to model the progressive post-local buckling. The global buckling analysis of slender CFST columns exposed to fire accounts for the effects of material and geometric nonlinearities as well as local buckling. Efficient computational procedure and solution algorithms are developed to solve the nonlinear equilibrium dynamic functions of loaded slender CFST columns exposed to fire. Independent experimental and numerical results on slender CFST columns are utilized to validate the computational model. The interaction behavior of local and global buckling and fire-resistance of slender rectangular CFST columns are investigated. It is shown that the developed computational model provides a reasonably accurate and efficient method for the prediction of the interaction buckling responses as well as the fire-resistance of slender CFST columns subjected to axial loading and fire. • A numerical model is presented for rectangular CFST slender columns exposed to fire. • Local buckling, air gap, moisture content and surface emissivity are included. • Computation algorithms for predicting strength and fire-resistance are described. • The interaction buckling behavior and fire-resistance of columns are investigated. • The computational model predicts well the fire-resistance of CFST slender columns. [ABSTRACT FROM AUTHOR]

• A new numerical simulation model for circular DCFAT stub columns is described. • Double-skin confinement on sandwiched concrete in DCFAT columns is considered. • The verified numerical model is used to conduct parametric study on DCFAT columns. • The applicability of current design codes and model to circular DCFAT columns is evaluated. • The simulation technique developed and design model yield accurate predictions. The sandwiched concrete in a circular double-skin concrete-filled aluminum tubular (DCFAT) column is subjected to the lateral confinement from inner and outer aluminum tubes. The effects of double-skin confinement have not been considered in the existing numerical models for the analysis of DCFAT stub columns. This paper describes a numerical model for the simulation of concentrically compressed circular DCFAT short columns. The numerical model is developed using the fiber element methodology. A new expression for determining the lateral confining pressures on the sandwiched concrete in circular DCFAT stub columns is proposed based on experimental results and incorporated in the computational technique. The stress-strain relations for determining the material performance of aluminum and confined sandwiched concrete are described. The numerical model is validated through comparisons with the experimental results of circular DCFAT stub columns. The numerical predictions correlate well with the tested column results, especially the aluminum stress-strain responses, load-strain responses, and ultimate axial load. A parametric study is performed to ascertain the influences of geometric and material variables on the behavior of DCFAT stub columns. The numerical results reveal that the use of aluminum instead of steel in a composite column could reduce the column weight by about 22.5%. The comparison of experimental results with the ultimate loads obtained by the design approaches specified in AISC 360-16, Eurocode 4, and Liang's design model indicates that the codified methods generally either underestimate or overestimate the strengths of DCFAT columns, and Liang's design model gives accurate predictions. [ABSTRACT FROM AUTHOR]

Abstract Circular high-strength concrete-filled double steel tubular (CFDST) columns are high performance members where the internal and external steel tubes offer significant confinement to the concrete infill. The confinement remarkably improves the concrete compressive strength and ductility. However, no fiber element models have been formulated for computing the responses of CFDST columns with circular steel tubes filled with high-strength concrete incorporating accurate confinement to the core and sandwiched concrete. In this paper, a new fiber-based numerical model is developed that computes the axial load-strain responses of circular high-strength CFDST short columns under axial loading. Based on existing experimental results, a new confining pressure model is developed for the determination of the confining pressures on the core-concrete in CFDST columns with circular sections. A new strength degradation parameter is also proposed that allows the concrete post-peak characteristics to be quantified. The fiber-based numerical model validated by experimental data is used to assess the responses of high-strength CFDST columns considering important parameters, which include the inner steel tube, external tube diameter-to-thickness ratio and concrete and steel strengths. A simple expression is derived for the estimation of the axial load-carrying capacities of circular short CFDST columns and comparisons with several design codes are made. The proposed fiber-based analysis technique and design equation can accurately determine the responses of short circular high-strength CFDST columns. Highlights • A new mathematical model for simulating circular CFDST short columns is presented. • A new confining pressure model for core concrete in CFDST columns is proposed. • The strength degradation factor is derived for quantifying concrete post-peak behavior. • The verified model is used to study the behavior of high-strength CFDST columns. • The numerical and design models predict well the performance of CFDST columns. [ABSTRACT FROM AUTHOR]

Highlights • Load deformation behaviour of partially FRP confined concrete with different gaps. • Significant strain localization was observed within the non-wrapped region. • A new stress-strain model was developed for partially FRP confined concrete. Abstract This study provides new insight on the compressive behaviour of partially fibre reinforced polymer (FRP) confined concrete with either strain-hardening or strain-softening responses. Fully FRP confined concrete, partially FRP confined concrete with different strip gaps, and unconfined concrete were tested under axial compression. Four types of axial load-axial deformation behaviours were observed for specimens with different strip gaps. Even though a high volumetric ratio of FRP was applied, the confinement effectiveness was negligible when the strip gap exceeded the diameter of the specimens. Moreover, the axial stress-axial strain behaviours of wrapped and non-wrapped concrete were observed to be different, and significant strain localization was observed within the non-wrapped region. Based on the experimental observations and an extensive literature review, a confinement effectiveness coefficient was proposed for partially FRP confined concrete. A stress-strain model was then developed by considering the proposed confinement effectiveness coefficient. The developed stress-strain model provided better predictions than other existing stress-strain models. [ABSTRACT FROM AUTHOR]

Highlights • A fiber-based model is developed for simulating rectangular CFDST short columns. • The progressive local and post-local buckling of the outer steel tube is considered. • The model includes concrete confinement offered by the inner circular steel tube. • The performance of rectangular CFDST columns with various parameters is studied. • The fiber element and design models predict well the behavior of DCFST columns. Abstract Rectangular concrete-filled double steel tubular (CFDST) columns with inner circular steel tube possess higher structural performance than conventional concrete-filled steel tubular (CFST) columns. However, the local buckling of the outer steel tube of thin-walled rectangular CFDST columns has not been accounted for in the existing fiber element models and design codes that may overestimate the column ultimate axial strengths. This paper describes a computationally efficient fiber-based modeling technique developed for determining the behavior of concentrically-loaded rectangular CFDST short columns including the local buckling effects of the external steel tube and the confinement offered by the internal circular steel tube. The effective width concept is used to simulate the post-local buckling of the outer steel tube. Comparative studies are undertaken to verify the fiber-based model with the relevant test results. The computational model is then employed to investigate the axial load–strain responses of rectangular CFDST short columns with various key design variables. A design equation is developed for computing the ultimate axial loads of short rectangular CFDST columns and compared with design methods given in several international design codes. It is shown that the fiber-based modeling technique and the proposed design model predict well the structural performance of short CFDST columns. [ABSTRACT FROM AUTHOR]

Steel-Equal Angle (SEA) has recently been utilized as longitudinal reinforcement in high-strength concrete columns to improve their strength and ductility. However, investigations have been very limited on the responses of eccentrically loaded SEA Reinforced Concrete (SEARC) columns. This paper presents the development of a computer simulation model based on fiber element discretization for determining the structural behavior of eccentrically loaded SEARC short columns. The inelastic buckling of SEA sections and the concrete confinement induced by SEA sections are explicitly taken into account. The modeling procedures are given for axial load moment-interaction curves and moment-curvature curves of SEARC columns. The accuracy of the developed fiber model is verified against the available test data. The performance of eccentrically loaded SEARC columns incorporating various design parameters is extensively investigated. It is shown that the developed computer simulation model captures well the experimentally measured responses of short SEARC columns and can be utilized in the analysis and design of such columns in practice. It has been found that the behavior of SERAC columns under eccentric loads is significantly influenced by the longitudinal steel ratio, the compressive strength of concrete, and the axial load level. The width of SEA sections and tie spacing considerably influence the ductility of the columns. • A fiber model is developed for the analysis of eccentrically-loaded SEARC columns. • Validation of the developed fiber model of SEARC columns is presented. • Effects of various column parameters on load-moment interaction curves are studied. • Effects of various column parameters on moment-curvature responses are studied. [ABSTRACT FROM AUTHOR]

• The characteristic of AAM raw material is extremely varied. • Relative standards for AAM raw material selection and quality control are limited. • A universal testing method is crucial to the wide application of AAM. • Alkali leaching test is a suitable method to quantify the reactive content of APs. • Further development is needed to optimize the parameters of leaching tests. The utilization of industrial waste as an aluminosilicate precursor (AP) for alkali-activated material (AAM) production can provide an outlet for the growing waste stream and relief to landfill. However, to achieve the transformation of extremely varied waste into normalized APs with stable performance on an industrial scale, a universal testing method and criteria for quantifying the geopolymerization reactivity (GR) of APs are crucial. To facilitate the establishment of a more consistent method for determining the GR of APs, this paper reviewed and compared four mainstream methods for GR quantification: quantitative X-ray diffraction (QXRD)–based methods, energy-dispersive X-ray spectroscopy (EDS)–based methods, selective dissolution, and leaching tests. QXRD-based and EDS-based methods, as conventional and robust methods for material characterization, have gained excellent agreement with each other on the determination of inherent amorphous phases of APs, which have been proven indicative of GR to some extent. However, the inability of QXRD-based and EDS-based methods to evaluate GR under real reaction environments should also be highlighted, as not all the amorphous phases are chemically involved in the reaction. Selective dissolution and leaching tests have also been developed to address this issue, and these have proved more indicative. However, the parameters of these two methods—such as reagent type, contact time, and temperature—should be optimized further and unified to improve their accuracy and viability. [ABSTRACT FROM AUTHOR]

• Strain localization occurs for partially FRP confined concrete with both strain hardening and strain softening responses. • An ultimate strain model that considers the strain localization phenomenon is developed. • The proposed strain model can provide more accurate predictions and outperform other existing strain models. This study experimentally and analytically investigated the compressive behaviour of partially FRP confined concrete, with emphasis on its strain localization phenomenon. An experimental programme was conducted first, in which both fully and partially FRP confined concrete specimens were tested under axial compression. The global deformation as well as the local deformation of the nonwrapped region were used to quantitively investigate the differences in the axial strains between the wrapped and nonwrapped regions. It was observed that significant strain localization occurred in the partially confined concrete specimens with either strain-hardening or strain-softening responses, and the arching action assumption could reasonably reflect the reduced confinement effectiveness due to the partial wrapping. Afterwards, an ultimate strain model that considers the strain localization behaviour was proposed. The proposed strain model was compared with other existing strain models based on a collected test database. It was found that the proposed strain model outperforms other strain models, and it is the only strain model that captures the strain localization behaviour. [ABSTRACT FROM AUTHOR]

• A numerical model for circular CFDST slender columns with preloads is developed. • The model incorporates concrete confinement and deformations caused by preloads. • The preload effects on the behavior of circular CFDST slender columns is studied. • The load distributions in circular CFDST columns with preload effects are examined. • The model predicts well the behavior of preloaded circular CFDST slender columns. The hollow circular steel tubes support the permanent and construction loads of several upper floors during the construction of a high-rise building structure before the concrete is filled into the steel tubes to form circular concrete-filled double steel tubular (CFDST) columns. The preloads acting on the steel tubes may cause a marked reduction in the performance of slender CFDST columns. This paper presents a numerical modeling technique underlying the method of fiber elements for quantifying the behavior of circular slender CFDST beam-columns loaded eccentrically with preloads acting on the hollow steel tubes. The numerical model considers the concrete confinement, second-order effects, initial geometric imperfection, deformations induced by preloads as well as geometric and material nonlinearities. The dynamic nonlinear equilibrium functions are solved using efficient computational solution algorithms developed. The validation of the numerical modeling technique is made by comparing computations with existing results obtained by experiments and finite element analyses. The influences of preloads on the structural behavior of slender CFDST beam-columns with various important parameters are investigated by means of utilizing the developed computer model. It is shown that the numerical modeling method is an efficient computational simulation and design tool for slender CFDST columns including preloads. [ABSTRACT FROM AUTHOR]

• Computational model for simulation of circular CFDST slender columns is presented. • The model considers confinement, geometric imperfection and second-order effects. • Parametric studies on the behavior of CFDST slender columns are undertaken. • Design models are proposed for the design of CFDST slender columns. • The computational and design models predict well the strengths of CFDST columns. The concrete confinement provided by the circular steel tubes improves the performance of circular concrete-filled double steel tubular (CFDST) columns, which have increasingly been employed as high-performance structural members in tall buildings. This paper presents computational and design models for the simulation and design of high strength CFDST slender columns composed of circular thin-walled sections that are eccentrically loaded. The formulation of the computational model is described, which takes into account the influences of concrete confinement, geometric imperfection, second-order, gradual plasticity of steel, and geometric and material nonlinearities. The inverse quadratic method is implemented in computational algorithms which solve the highly nonlinear equilibrium function of slender CFDST circular columns. The computations are compared with experimentally measured results to validate the mathematical modeling procedure developed. The behavior of slender circular CFDST columns made of high-strength concrete is investigated by utilizing the developed computational model taking into consideration the important material and geometric parameters. Proposed are design models that determine the ultimate loads of CFDST slender columns loaded concentrically and the axial load-moment strength interaction curves of slender thin-walled CFDST columns under eccentric loads. It is shown that the computational simulation method proposed can efficiently and accurately capture the experimentally measured behavior of CFDST slender columns. The design models give good strength predictions of CFDST columns and can be employed in designing CFDST composite columns. [ABSTRACT FROM AUTHOR]

Rectangular concrete-filled double steel tubular (CFDST) columns have increasingly been utilized in high-rise buildings to support heavy loads. However, the responses of eccentrically loaded rectangular CFDST short columns have rarely been studied. This paper reports experimental and numerical investigations into the behavior of rectangular thin-walled short CFDST columns composed of a rectangular inner steel tube loaded eccentrically. Tests on rectangular and square CFDST short columns under eccentric loading and axial loading were carried out to examine their responses to various design parameters, including the cross-sectional dimensions, loading eccentricity, and width-to-thickness ratios of the external and internal tubes. The experimental program and results are described. A mathematical model underlying the theory of fiber elements is developed for simulating the moment-curvature responses as well as axial load-moment interaction envelopes of short CFDST columns. The computational model explicitly incorporates the experimentally observed failure mode of the progressive local buckling of the outer thin-walled rectangular steel tube. A computer simulation procedure is proposed for capturing the nonlinear behavior of short CFDST columns loaded eccentrically together with efficient solution algorithms that implement the inverse quadratic method for solving nonlinear equations. The experimental verification of the computer modeling technique is undertaken. The verified modeling technique is utilized to ascertain the significance of various design parameters on the structural behavior of CFDST columns made of rectangular thin-walled sections. It is confirmed that the computer model developed is capable of predicting well the experimentally observed responses of CFDST short columns. • Test results on eccentrically loaded rectangular CFDST short columns are reported. • Fiber model is developed for simulating moment-curvature and strength envelopes. • The model incorporates the progressive local buckling of the outer steel tube. • The behavior of CFDST columns with local buckling effects are investigated. • The fiber model is capable of predicting well the responses of CFDST columns. [ABSTRACT FROM AUTHOR]

• Computational model for square CFDST slender columns with preloads is presented. • The model incorporates preloads, local-global interaction buckling and confinement. • Behavior of CFDST slender columns with preloads and local-buckling is examined. • Numerical and design models simulate well the behavior of square CFDST columns. Thin-walled square and circular hollow steel tubes are designed to support the permanent and construction loads of several upper composite floors before filling the concrete into the tubes to form concrete-filled double steel tubular (CFDST) columns. The influences of preloads acting on the steel tubes on the structural responses of slender square CFDST columns have not been investigated either experimentally or numerically. This paper presents a fiber-based computational model for the determination of the interaction behavior of local and global buckling in axially and eccentrically loaded CFDST thin-walled square slender columns including preload effects. The computational modeling method accounts for the influences of the deformations induced by preloads, local-global interaction buckling, geometric imperfections, second-order, and geometric and material nonlinearities. The accuracy of the computational algorithms developed is validated by comparing computations with test data on concrete-filled steel tubular (CFST) columns and finite element results of double-skin CFST (DCFST) columns with preload effects. The computer algorithms are employed to quantify the influences of preloads on the local-global interaction buckling responses of CFDST columns with various important parameters. Proposed is a design method for calculating the ultimate loads of concentrically loaded slender square CFDST columns considering preloads. The computational and design models developed are shown to be efficient modeling and design tools for square CFDST slender columns taking into account the construction method of high-rise composite buildings. [ABSTRACT FROM AUTHOR]

• Experiments on circular CFDST short columns loaded eccentrically are presented. • Mathematical model is described that predicts the behavior of CFDST short columns. • The model incorporates the concrete confinement provided by inner and outer tubes. • The behavior and moment distributions in circular CFDST columns are investigated. • The model predicts well the behavior of eccentrically loaded CFDST short columns. Circular concrete-filled double steel tubular (CFDST) columns in high-rise building structures possess high ductility and strength performance owing to the concrete confinement exerted by the external and internal circular steel tubes. However, the behavior of circular CFDST short columns that are loaded eccentrically has not been investigated either experimentally or numerically. Particularly, numerical studies on the moment-curvature responses, strength envelopes, confinement effects and moment distributions in circular CFDST beam-columns have not been reported. In this paper, experimental and computational investigations into the structural responses of circular CFDST short columns loaded eccentrically are presented. Nineteen short circular CFDST columns with various parameters under axial and eccentric loads were tested to failure to measure their structural responses. Test results are presented and discussed. A mathematical simulation model underlying the method of fiber analysis is proposed that simulates the axial load-moment-curvature relationships as well as the strength interaction curves of CFDST beam-columns composed of circular sections. The mathematical modeling technique explicitly takes into account the confinement of concrete on the responses of CFDST columns. The computational procedure and solution method are given. The accuracy of the computer simulation model is evaluated by comparing computations against experimental data. The significance of material and geometric properties, concrete confinement and axial load ratio on the responses of moment-curvature and strength envelopes of CFDST columns and the moment distributions in concrete and steel components are investigated. The mathematical model proposed not only simulates well the experimentally observed responses of CFDST columns but also can monitor the moment distributions in the steel and concrete components of such composite columns. [ABSTRACT FROM AUTHOR]

• Test results on square CFDST short columns under eccentric loads are presented. • A numerical model is developed for the prediction of the column strength envelops. • The model accounts for the progressive local buckling of the outer steel tube. • The confinement on the core concrete is considered in the material constitutive laws. • The behavior of square CFDST columns with various parameters is investigated. • The numerical model is shown to simulate well the behavior of DCFST columns. Square concrete-filled double steel tubular (CFDST) beam-columns consisting of an internal circular steel tube have increasingly been utilized in composite building structures because of their high structural performance. This paper describes experimental and numerical studies on the structural responses of square thin-walled CFDST columns loaded eccentrically. Tests on twenty short square CFDST columns were undertaken that included sixteen columns under eccentric loading and four columns under concentric loading. The parameters examined in the experiments included the cross-sectional dimensions, the width-to-thickness ratios of outer and internal tubes and loading eccentricity. The measured ultimate strengths, load-shortening responses, load-lateral displacement curves, stress-strain curves and observed failure modes are presented. A numerical model incorporating the fiber analysis is developed that predicts the moment-curvature responses and axial load-moment strength envelops of CFDST columns. The model explicitly accounts for the influences of the confinement exerted by the internal circular steel tube on the core concrete and the progressive post-local buckling of the external steel tube. Efficient computer algorithms implementing the inverse quadratic method is developed to produce converged solutions to the nonlinear dynamic equilibrium equations generated in the analysis. Measurements from the tests are employed to validate the proposed numerical model. It is shown that there is a good agreement between theory and experiment. The computer model is utilized to demonstrate the significance of various parameters on the behavior of thin-walled short CFDST beam-columns. [ABSTRACT FROM AUTHOR]

High-strength square concrete-filled double steel tubular (CFDST) slender beam-columns with a circular internal steel tube subjected to eccentric loads may undergo interaction local-global buckling. No computational studies on the interaction local-global buckling of slender square CFDST beam-columns have been reported and their behavior has not been fully understood. This paper describes a mathematical model for the simulation of the interaction local-global buckling behavior of square high-strength CFDST slender beam-columns under axial compression in combination with uniaxial bending. The mathematical model is formulated by the fiber approach, accounting for confinement provided by the internal circular steel tube, and geometric and material nonlinearities. An incremental-iterative numerical procedure is designed to quantify the local-global interaction buckling responses of slender CFDST columns. Efficient numerical solution algorithms implementing the inverse quadratic method are developed for solving the nonlinear equilibrium dynamic functions of CFDST columns. The formulation proposed is verified by existing experimental data on CFDST columns as well as double-skin concrete-filled steel tubular (DCFST) slender columns. The developed computational model is employed to study the local-global interaction buckling behavior of CFDST columns made of high-strength materials with various important parameters. Simplified design models are proposed for determining the ultimate axial strengths of slender square CFDST columns under axial compression and the interaction curves of CFDST slender beam-columns loaded eccentrically. It is demonstrated that the computational and design models predict well the interaction local-global buckling behavior and strength of slender square CFDST beam-columns. • A new mathematical model for simulating square CFDST slender columns is described. • The model formulation considers local-global interaction buckling and confinement. • New solution algorithms are developed for solving nonlinear equilibrium equations. • The local-global interaction buckling behavior of CFDST slender columns is studied. • The numerical and design models predict well the strengths of CFDST slender columns. [ABSTRACT FROM AUTHOR]