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Highlights • A new form composite piles is proposed. • The new pile is made of pervious geopolymer concrete, geogrid, PVC and FRP. • Axial compression experiments of this new form piles were conducted. • The mechanical behaviour of this new form piles is significantly improved. Abstract The use of fibre reinforced polymer (FRP) and polyvinyl chloride (PVC) as strengthening materials for piles was found to be a promising scheme, due to their high strength-to-weight ratio, high durability and high anti-corrosion ability. This study presents an experimental investigation of a new form of composite piles: geogrid-confined pervious geopolymer concrete piles (GPGCPs) with fibre reinforced polymer (FRP)-polyvinyl chloride (PVC)-confined concrete core (FPCC). The GPGCP with FPCC consists of a circular geogrid outer tube, a FRP-PVC-confined normal geopolymer concrete core, and pervious geopolymer concrete (PGC) filled in between. The reason for applying PGC into piles is to increase the rate of consolidation. The aim of using FPCC is to improve the compressive strength and ductility of the concrete. In this study, two groups of GPGCPs (without and with FPCC) were prepared and tested under axial compression. In each group, one layer, two layers, and three layers of geogrid were used to investigate the influence of the outer tube. The test results show that the FPCC can significantly improve the mechanical behaviour of the GPGCPs. In comparison with GPGCPs without FPCC, the maximum axial loads of GPGCPs with FPCC were higher, and the ductility was improved significantly. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the influence of steel fibres inclusion on the behaviour of Reactive Powder Concrete (RPC) columns. Micro steel fibre (MF) and deformed steel fibres (DF) were used. Steel fibres were hybridized to produce hybrid steel fibre (HF). Sixteen RPC specimens were cast and tested under axial loading, eccentric loading (25 mm and 50 mm) and four-point bending. Results of testing demonstrated that RPC specimens that included MF exhibited 8–58% higher load carrying capacity compared to the reference NF specimens. Moreover, RPC specimens that included HF showed 29–408% higher ductility under different loading conditions compared to the reference specimens (NF). Also, the RPC specimens containing steel fibres exhibited 2–32% higher axial deformation under different loading conditions compared to NF specimens. Finally, it was observed that the RPC specimens reinforced with HF showed delayed spalling of concrete cover more than the RPC specimens that included MF and DF. [ABSTRACT FROM AUTHOR]

Fiber reinforced polymer (FRP) pultruded materials are available in a wide variety of shapes, including bars, I-sections, C-sections and other structural sections. Due to their high durability, low self-weight and reduced maintenance costs, these FRP materials are becoming a competitive option for replacing steel as structural materials especially in corrosive environments. This paper summarizes an experimental program on the axial and flexural behaviour of square concrete members reinforced with glass fiber reinforced polymer (GFRP) bars and embedded with pultruded GFRP structural sections under different loading conditions. Furthermore, an analytical model is presented to predict the axial load-bending moment interaction diagrams of the experimentally tested specimens. It can be concluded from this study that the analytical models provide reliable estimates of the maximum load and bending moment capacities of GFRP reinforced and GFRP encased concrete columns. In addition, a parametric study was conducted to study the effects of concrete compressive strength and longitudinal GFRP reinforcement ratio on the structural performance of GFRP reinforced square concrete columns. [ABSTRACT FROM AUTHOR]

Fiber Reinforced Polymer (FRP) composites have emerged as a viable alternative of steel reinforcement due to higher ultimate tensile strength to weight ratio and corrosion resistance of FRP composites. Concrete Filled Fiber Reinforced Polymer Tube (CFFT) technique for new column construction has attracted significant research attention. This paper investigated the axial and flexural behavior of Concrete Filled Carbon FRP Tube (CFRP CFFT) columns with and without CFRP bar and concrete filled Glass FRP (GFRP) tube columns with and without GFRP bar. The test results of 16 circular CFFT and four steel Reinforced Concrete (RC) specimens of 203–205 mm diameter and 800–812 mm height have been reported in this paper. The test results showed a larger reduction in the confinement effectiveness of FRP tube than steel helices under increasing applied load eccentricity. FRP bar reinforced CFFT specimens exhibited higher axial loads, flexural loads, and deformations at peak loads than unreinforced CFFT and steel RC specimens. Experimental axial load-bending moment interaction curves of tested specimens also showed the improved performance of FRP bar reinforced CFFT specimens. [ABSTRACT FROM AUTHOR]

Highlights • A fiber model is presented for simulating rectangular CFST short columns in fire. • The sequential coupled nonlinear thermal and stress analysis procedure is given. • Local buckling, air gap, moisture content and surface emissivity are considered. • The fire-resistance and behavior of loaded CFST short columns in fire is studied. • The numerical model developed predicts well the fire performance of CFST columns. Abstract Elevated temperatures significantly reduce the local buckling strengths of steel tubes and the ultimate strengths of rectangular concrete-filled steel tubular (CFST) columns exposed to fire. No fiber-based models have been developed that include local buckling effects on the fire-resistance of rectangular CFST columns. This paper presents a new fiber element model for the fire-resistance predictions of axially loaded rectangular CFST short columns at elevated temperatures considering local buckling. The thermal analysis problem of a CFST column is solved by the finite difference method to determine the temperature distribution within its cross-section including an air gap, concrete moisture content and the emissivity of exposure surfaces. The nonlinear stress analysis of axially loaded short CFST columns under fire recognizes the stress-strain behavior of concrete and steel at elevated temperatures. The expressions for initial local buckling and effective widths of steel plates are incorporated in the computational model to include the effects of local and post-local buckling on the fire responses of CFST columns. The existing experimental and numerical results are utilized to examine the accuracy of the fiber-based model. The fiber model developed is used to undertake parametric studies on the effects of local buckling, geometric and material properties and loading ratio on the thermal and structural responses of CFST short columns and the load distribution in steel tube and concrete. The numerical model proposed is demonstrated to simulate well the fire and structural performance of axially loaded CFST short columns under fire. Moreover, computational solutions presented provide a better understanding of the thermal and structural responses of CFST columns in fire. [ABSTRACT FROM AUTHOR]

In this study, the efficiency of Glass Fibre Reinforced-Polymer (GFRP) bar reinforced normal strength concrete (NSC) and high strength concrete (HSC) columns in sustaining axial and flexural loads was analytically investigated. Experimental data from available literature were used as benchmarks for the analytical investigations conducted in this study. In addition, a comprehensive parametric study was carried out to investigate the effect of different parameters (i.e., compressive strength of concrete, mechanical properties and reinforcement ratio of GFRP bars, and slenderness ratio of the columns) on the performance of concrete columns reinforced with GFRP bars. It was observed that under concentric axial load, the improvements in the axial load carrying capacity due to increasing GFRP longitudinal reinforcement ratio were more pronounced in GFRP bar reinforced NSC columns than in GFRP bar reinforced HSC columns. It was also observed that HSC columns reinforced longitudinally with GFRP bars with small longitudinal reinforcement ratio or low tensile modulus of elasticity might experience a tensile failure of the GFRP bars located on the tension side of the column cross-sections, especially if the columns are subjected to a high level of axial load eccentricity. [ABSTRACT FROM AUTHOR]

This study investigates the behaviour of square reinforced concrete (RC) columns partially and fully wrapped with CFRP under different loading conditions. The experimental results of twelve specimens with 150 mm × 150 mm cross-section and 800 mm height tested under concentric axial load, eccentric axial loads and four-point bending are presented in this study. The experimental results showed that partial and full wrapping increased the strength and ductility of square RC column specimens. The increase in the strength and ductility of fully wrapped square RC column specimens was higher than the increase in the strength and ductility of partially wrapped square RC column specimens under all loading conditions. However, the increase in the axial load eccentricity (concentric, 25 mm eccentric and 50 mm eccentric axial loads) resulted in a significant decrease in the maximum axial load with the largest reduction observed for fully wrapped specimens compared to partially wrapped specimens. The experimental axial load-bending moment interaction diagrams showed the better performance of partially and fully CFRP wrapped square RC specimens compared to non-wrapped square RC specimens. [ABSTRACT FROM AUTHOR]

The axial and flexural behaviors of Concrete Filled Carbon Fiber Reinforced Polymer Tube (CFRP-CFFT) columns have received significant research attention in the last two decades. One of the most attractive advantages of Carbon FRP (CFRP) tube is the high confinement which results in substantial increase in peak axial and flexural loads and deformations. Despite large research efforts, the behavior of CFRP-CFFT with and without CFRP reinforcing bars under different applied axial load eccentricity has not yet been adequately investigated. This study investigates the experimental and analytical axial-flexural ( P − M ) interactions of CFRP-CFFT columns with and without CFRP reinforcing bars. A total of 12 specimens of 204–205 mm outer diameter and 800–812 mm height were tested under concentric axial load, 25 mm and 50 mm eccentric axial loads and four-point load. The effectiveness of CFRP reinforcement (tube and bar) was observed to be reduced with the increase in the applied axial load eccentricity. Analytical P − M interactions were constructed using available FRP confined concrete design codes which matched well with the experimental P − M interactions. The parametric study showed that the actual confinement ratio, orientation of fibers and CFRP bar reinforcement ratio have significant influences on P − M interactions of CFRP-CFFT specimens. [ABSTRACT FROM AUTHOR]

Circularizing a square column by bonding concrete segments onto the sides of the column and then wrapping with fibre reinforced polymer (FRP) is considered an effective technique in strengthening square solid columns. This paper investigates the suitability of the circularization technique for strengthening square hollow concrete specimens. Eight specimens in two groups (four solid specimens and four hollow specimens) were cast and tested under axial compression. The first specimen from each group was the reference specimen. The corners of the second specimen were rounded to 20 mm radius and wrapped with two layers of carbon fibre reinforced polymer (CFRP). The third specimen was circularized with full length concrete segments and wrapped with two layers of CFRP. The fourth specimen was circularized with concrete segments shorter than the length of the specimen and wrapped with two layers of CFRP. The test results demonstrate that circularization of hollow specimens similar to the circularization of solid specimens reduces the stress concentration at the corners and enhances the ultimate load carrying capacity and ductility. The circularization with short concrete segments is more effective for hollow specimens than the circularization with full length concrete segments when the ductility is of main concern. [ABSTRACT FROM AUTHOR]

This study presents the results of an experimental investigation on high strength concrete (HSC) and steel fibre high strength concrete (SFHSC) circular column specimens reinforced longitudinally and transversely with Glass Fibre-Reinforced Polymer (GFRP) bars and helices, respectively. The Influence of the type of the reinforcement (steel and GFRP), the pitch of the transverse reinforcement, the addition of the steel fibres and the loading condition (concentric, eccentric and four-point loading) on the performance of the specimens was investigated. The study showed that the GFRP bar reinforced HSC (GFRP-HSC) specimen is as efficient as the steel bar reinforced HSC (steel-HSC) specimen in sustaining concentric axial load. However, the maximum load sustained by the GFRP-HSC specimens under eccentric axial load was 10–12% lower than the maximum load sustained by the steel-HSC specimens. GFRP bar reinforced SFHSC (GFRP-SFHSC) specimens sustained 3–13% higher axial load and 14–27% greater ductility than GFRP-HSC specimens under different loading conditions. Furthermore, reducing the pitch of the GFRP helices in GFRP-SFHSC specimens resulted in a significant improvement in the ductility and the post-peak axial load-axial deformation behaviour of the specimens. [ABSTRACT FROM AUTHOR]

• New concrete confinement model for concrete in SCRCFST columns is proposed. • An efficient fiber model of SCRCFST short columns with local buckling is developed. • Accuracy of the existing lateral pressure models for confined concrete is evaluated. • A design formula for predicting the strength of SCRCFST short columns is proposed. Square spiral-confined reinforced concrete-filled steel tubular (SCRCFST) columns have recently been proposed as a new form of composite columns. The additional confinement exerted by the spiral reinforcements improves the strength and ductility of a square SCRCFST column compared to a square concrete-filled steel tubular (CFST) column. However, there is a lack of a computational model of SCRCFST columns considering the confinement provided by spirals and the localized interaction buckling of the outer steel tube. Moreover, the application of existing material laws for concrete or the design models originally proposed for square CFST columns may underestimate the performance of SCRCFST columns. This paper investigates the behavior of axially loaded square SCRCFST short columns by utilizing a computationally efficient fiber model developed in this study. Accurate material laws of concrete are developed that incorporate the proposed formulae for the compressive strength of confined concrete and a degradation factor for estimating the residual strength of concrete confined by spirals. The gradual interaction local buckling of the outer steel tube is taken into account in the nonlinear simulation of SCRCFST columns. The computational fiber model is validated against a large test database. The accuracy of various models that determines the lateral pressure on concrete confined by the spiral is evaluated. A parametric study is carried out to investigate the influences of important design parameters on the responses of SCRCFST columns. Moreover, a comparison of the ultimate axial loads of SCRCFST columns calculated using design standards with test results is made. Furthermore, a simple design model is proposed for calculating the axial strengths of short SCRCFST columns. It is demonstrated that the developed fiber modeling technique incorporating the new confinement model simulates well the structural behavior of SCRCFST columns; in addition, the proposed design model can be used to design such innovative composite columns in practice. The parametric study shows that the performance of SCRCFST columns is significantly influenced by the width-to-thickness ratio and yield stress of the steel tube as well as the concrete compressive strength. The ductility of the columns can be improved by using a smaller spacing of spiral reinforcements. [ABSTRACT FROM AUTHOR]

• A fiber model of self-compacting concrete columns with steel tubes is presented. • The concrete confinement provided by the steel tubes and spirals is considered. • The behavior of steel-tube reinforced concrete short columns is investigated. • The developed numerical model predicts well the structural behavior of the columns. This paper presents an investigation on the efficiency of steel tubes in reinforcing self-compacting concrete (SCC) columns. A theoretical model is developed to study the axial behavior of SCC columns reinforced with steel tubes, considering the effects of confinement on the filled concrete by steel tubes and concrete core confined by spirals. The computational results are validated against the existing test results of SCC columns. The effects of various column parameters on the performance of SCC short columns under axial concentric loading are investigated. It is shown that the axial performance of SCC columns is influenced by the dimension and number of steel tubes, pitch of spirals, compressive strength of concrete, and yield stress of the steel tubes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Müller''s method are developed to iteratively adjust the depth and orientation of the neutral axis and the curvature at the column''s ends to obtain nonlinear solutions. Steel and concrete contribution ratios and strength reduction factor are proposed for evaluating the performance of CFST slender beam-columns. Computational algorithms developed are shown to be an accurate and efficient computer simulation and design tool for biaxially loaded high-strength thin-walled CFST slender beam-columns. The verification of the multiscale numerical model and parametric study are presented in a companion paper. [Copyright &y& Elsevier]

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 Müller''s method algorithms are developed to iterate the neutral axis depth in the cross-sectional analysis and to adjust the curvature at the columns ends in the axial load–moment interaction strength analysis of a slender beam-column to satisfy equilibrium conditions. Analysis procedures for determining the load–deflection and axial load–moment interaction curves for high strength thin-walled rectangular CFST slender beam-columns incorporating progressive local bucking and initial geometric imperfections are presented. The new numerical model developed is shown to be efficient for predicting axial load–deflection and axial load–moment interaction curves for high strength thin-walled rectangular CFST slender beam-columns. The verification of the numerical model and parametric studies is given in a companion paper. [Copyright &y& Elsevier]

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]

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]

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]

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]

• 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]

• 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]