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An FRP (fiber-reinforced polymer) -confined concrete filled steel tube is formed by the combination of FRP material and a concrete filled steel tube, and it combines the advantages of FRP-confined concrete and a concrete filled steel tube (CFT); however, there is still no effective model to generally describe the complete stress-strain curve of a concrete filled FRP-confined steel tube. The stress-strain response of a concrete filled FRP-confined steel tube features the dual characteristics of both FRP confined concrete and steel confined concrete. In this paper, ninety-six test results of FRP confined circular CFT columns under axial compression were collected from different researchers. The steel tube confinement index is taken as an additional key influencing parameter on the strength and deformation, and calculation methods for the ultimate strain, ultimate stress, peak strain and peak stress of a concrete filled FRP-confined steel tube are established by taking into account the steel tube confinement index. Moreover, the stress-strain model was developed to describe the complete stress-strain curve consisting of the four stages of a concrete filled FRP-confined steel tube. The proposed model reflects the characteristics of the stress-strain behavior of a concrete filled FRP-confined steel tube with good accuracy and simplicity.

The behaviour of square steel tube confined reinforced concrete columns after fire exposure was studied experimentally and numerically in this paper. Eighteen stub columns were first heated following the ISO 834 standard fire including both heating and cooling phases, and were subsequently loaded to failure after cooling to ambient temperature. Failure modes, temperatures in specimens, axial load versus deformation curves and strains in steel tube were monitored and discussed. A finite element model was developed using the sequentially coupled thermal-stress analysis method and was validated against tests found in literatures and this study. Parametric study was performed to identify influences of key parameters, where are heating time, cross-sectional dimension, strengths of materials, steel tube to concrete area ratio and reinforcement ratio, on residual capacity and compressive stiffness. Finally, a simplified method is proposed for predicting residual cross-sectional capacity and compressive stiffness of square steel tube confined reinforced concrete columns after fire exposure.

This paper presents an experimental investigation of the mechanical behaviour and working mechanism of the concrete-filled double-skin steel tube (CFDST) with stiffeners. A total of 40 CFDST columns were prepared and subjected to axial and eccentric loading. The failure modes and ultimate load-bearing capacities, together with the deformation and strain responses of the specimens, were analysed. The effects of the stiffener type, slenderness ratio, and eccentricity ratio were researched in this study. The test results indicated that the ductility of the CFDST specimens was improved greatly by the presence of stiffeners, which gave a lower load-bearing capacity owing to the reduction in the effective area of the inner concrete. Moreover, an analytical-numerical model was developed to analyse the mechanical behaviour of the CFDST specimen, and an improving mechanism was considered. In addition, theoretical estimations to predict the ultimate load-bearing capacities of CFDST specimens subjected to axial and eccentric loading were established. These estimations showed good agreement with the theoretical results and the test results.

In this study, an innovative exposed concrete filled steel tube (CFST) column base with embedded reinforcing bars is proposed. Based on the conventional exposed CFST column base comprised of a base plate and anchor rods, additional reinforcing bars are embedded in the proposed column base. Eight column bases are fabricated and tested under constant axial loading and horizontal cyclic loading. The test parameters include the width-to-thickness (B/t) ratio of the steel tube, axial force ratio, with or without embedded reinforcing bars, and two different types of the base plates. Failure modes, hysteretic curves, strength and stiffness, cyclic behavior and strain characteristics are discussed and analyzed. The results show that the strength, stiffness and energy dissipation of the new column bases with embedded reinforcing bars are considerably greater than those of the conventional exposed CFST column bases. The difference in base plate types leads to a slight difference in seismic behavior of the column bases; however, both types of specimens have stable and sufficient seismic behavior, and can be used in design practice as needed. Finally, a simplified approach for predicting the bending moment versus axial force (M-N) interaction curve at the ultimate state is proposed.

Concrete-filled steel tubular (CFST) structures have been widely applied in modern construction industry owing to the composite action between the concrete and the steel tube. The benefits of CFST structures can be further achieved if high-strength materials are used. However, when high-strength steel is applied to CFST structures, the thin-walled steel section may not develop its yield strength owing to the early crushing of the cover concrete. Therefore, this paper presents an experimental study on concrete-filled circular high-strength steel stub tube (CFHST) focusing on the effect of the diameter-to-thickness (D/t) ratio. Fifteen specimens were tested under axial compression, with D/t ratios ranging from 50 to 130. The effects of the D/t ratio on the failure mode, load-versus-deformation relationship, and axial compression strength were investigated. Test results showed that the CFHST with a large D/t ratio can still reach the plastic section design. The fibre element method proposed by Liang et al. was revised to predict the load-axial deformation curves and then validated by using the ultimate bearing strength of CFST columns from 215 collected data. Finally, the bearing capacity of CFHST columns was compared with the present design codes, namely, Eurocode 4, American Institute of Steel Construction standard (AISC 360-10), Architectural Institute of Japan (AIJ) standard, and the proposed model, to evaluate the feasibility of the current codes for predicting the axial compressive strength of CFHST under axial compression.

Prestressed tube stayed columns with one and two crossarms are investigated. The analyses are performed using SCIA Engineer software and ANSYS package and validated by tests of four stayed columns. First, the investigations concern the ideal (perfect) columns and cover linear buckling analyses (LBA) and geometrically nonlinear analyses (GNIA) to obtain critical loads and buckling modes under various prestressing levels. Second, the collapse loadings of the imperfect columns with initial deflections required by Eurocode EN 1993-1-1 are analysed with respect to relevant initial deflection modes and material behaviour (elastic for mild steel, nonlinear for stainless steel). All the analyses are performed for geometrical characteristics of the tested columns and the results concerning both the column with one or two crossarms are set against each other to assess the respective effectivity. In addition, the geometrical analysis of the prestressed ideal column with two crossarms and respective formulas concerning optimal prestressing and maximal critical loading are presented. Finally some recommendations for practical use are suggested.

In the present study, the crushing mechanism of two types of thin walled structure, holed tube and grooved tube, were analyzed by analytical, numerical and experiments. Plastic deformation was occurring on thin walled tubes by introducing grooves and holes predetermined intervals along the tube. For this purpose, crushing performance of grooved model and holed model were analyzed through finite element simulation and then verified with experiment. Firstly, the comparison of crushing performance, load-displacement curve, energy absorption, mean crushing load and Mass specific energy (SEA) was performed for grooved and holed thin walled tube of mild steel analytically, numerically and experimentally. Secondly, crushing mechanics of holed tube with grooved tube were investigated. At last, the load-displacement curve of holed steel tube and grooved steel were investigated and then the load displacement curve of holed thin walled tube which were made by mild steel and aluminum were investigated. The results indicate that the holed thin walled tube which was made from aluminum has the maximum crushing performance, SEE and energy absorption.

Steel tube filled with ultra-high performance concrete (UHPCFST) is an innovative and efficient structural form. To promote its application, a comprehensive experimental program was conducted to investigate the structural behavior of UHPCFST columns subjected to axial compression. The key issue is to clarify the differences in mechanical behavior between UHPCFSTs and CFSTs, and to evaluate whether the current design guidelines related to CFSTs are applicable to UHPCFSTs. To address this, the compression characteristics of UHPCFSTs were analyzed, including failure mode, load versus deformation relationship, axial compressive strength and strain development. The test results showed that the steel tube and UHPC worked well together, but the enhancement effect of the steel tube on the core UHPC strength was not as significant as that of ordinary concrete. Moreover, an experimental database of UHPCFSTs including this study was established, and the experimental results were compared with the predictions by various design codes. Based on the regression analysis of the database results, a simplified model for predicting the ultimate strength of UHPCFSTs was developed. It was concluded that the proposed model could accurately predict the axial compressive strength of UHPCFSTs with circular and square cross-sections, and was also applicable to UHPCFSTs with high-strength steel (HSS).

This paper investigates the axial compressive behaviour of T-shaped concrete-filled steel tube (CFST) stub columns with blinding bars. In this column system, binding bars are applied cross-through the section and produce confinement effects. Eleven specimens with binding bars and five ones without binding bars were tested under axial compressive loading. The experimental results demonstrate that, by setting binding bars, the local buckling failure modes are changed and the occurrence of local buckling is delayed, and the global outward bulge of the steel tube at the concave corners can be effectively restrained. The ultimate strength and ductility of the columns with binding bars can achieve up to 1.53 and 7.5 times higher than those of the columns without binding bars, respectively. Taking into consideration the contribution of binding bars, the confinement effects of steel tube and the other sectional characteristics, a method for predicting the ultimate axial compressive strength of the columns with or without blinding bars is established. The accuracy of the method is validated through comparisons of the experimental results reported in this paper and in other available open literatures.

Concrete-filled double skin steel tubular (CFDST) stub columns with external stainless steel and internal carbon steel tubes can be considered as new types of composite members and expected to combine the advantages of all three kinds of materials. This paper presents non-linear finite element (FE) analysis and design of circular and square CFDST stub columns with external stainless steel under axial compression. FE models are developed, where non-linear material property of stainless steel is considered, and verified through comparisons with experiments in terms of failure modes, load-deformation histories and ultimate strength. Behaviors of stainless steel composite columns are compared with that of columns with both carbon steel tubes. Parametric studies are conducted to investigate the influence of the outer stainless steel tube strength, concrete strength, inner carbon steel tube strength and hollow ratio on structural behavior of axially loaded columns in terms of loading and interaction performance.

The axial compressive behavior of carbon fiber reinforced polymer (CFRP)-confined post heated square concrete-filled steel tube (CFST) stub columns is experimentally investigated in this paper. Twenty CFRP-confined post heated square CFST stub columns and one CFST stub column left untreated at ambient temperature were axially tested. Subsequently, the failure mode, ultimate strength, load versus displacement curves, load versus strain distribution curves, initial stiffness, and ductility of the specimens were obtained and analyzed. The maximum temperature (600 ℃，800 ℃,1000 ℃, 1100 ℃) CFST stub columns exposed to and the number of the layer of CFRP sheets (zero, one, two, three and four) were considered as main parameters. The test results showed that, in general, the CFST stub columns wrapped with CFRP sheets shown a better mechanical behavior than those without CFRP sheets wrapping. Furthermore, the more the layers of CFRP sheets are, the higher the ultimate strength and initial stiffness of the CFST stub columns are. Based on the regression of the test data, the simplified formulae for ultimate strength of CFRP-confined post heated square CFST stub columns was proposed. Accuracy of the formulae was evaluated by comparison between the calculated and experimental results.

This paper presents an experimental and theoretical study of CFRP-confined concrete-filled circular steel tube (CFT) stub columns, which aims at investigating the effects of different numbers of CFRP layers and concrete strengths on the mechanical performance of CFRP-confined CFT stub columns. Based on continuum mechanics, a mechanical model of concentric cylinder consisting of CFRP, circular steel tube and concrete core under concentric loading was established and the corresponding elasto-plastic methods were obtained through a FORTRAN program. The influence of the number of CFRP layers on the ultimatecapacity, ductility and confinement effect of the steel tube on the core concrete wasdiscussed and identified. Finally,based on the limit equilibrium and elasto-plastic methods, a simplified formula for the ultimatecapacity of CFRP-confined CFT stub columns was proposed. Good agreement of the ultimate capacity was found between the elasto-plastic methods and the proposed formula, with the maximum discrepancy less than 12%.

Under coupled bending-shear-torsion (B-S-T) loading combinations, the ultimate bearing capacity of concrete filled steel tube (CFST) short columns has not been studied. In this paper, the test on eight CFST short columns was carried out. The load-deformation relations and failure modes of CFST short columns were obtained and analyzed. Based on experimental results, the failure mechanism of CFST short columns were obviously affected by the ratio of bending moment to torque, and CFST short columns under coupled B-S-T loading combinations had good ductility. The ultimate bending-shear bearing capacity of CFST short columns was significantly decreased by increasing torsion moment. Moreover, nonlinear finite element analysis was performed, which generated an accurate prediction of the ultimate capacity of concrete filled steel tube short columns under coupled B-S-T. As a conclusion, a simplified design equation for calculating the coupled B-S-T bearing capacity of CFST short columns was proposed according the regression results of the finite element parametric analysis.

This paper presents a comparative study of circular concrete-filled steel tubular stub columns with three different stirrup confinement types: bidirectional stirrups, loop stirrups and orthogonal stirrups. Axial compression tests have been carried out aiming at investigating the effects of the stirrup form and volume-stirrup ratio on the mechanical behavior of the stirrup-confined circular CFT stub columns, and ABAQUS was used to carry out the 3D numerical modelling. Radial stress of the core concrete and the composite action among the steel tube, stirrups and the core concrete have been investigated. It is found that the confinement provided by stirrups on core concrete strongly outperforms that provided by steel tube, steel sections or steel reinforcement. Furthermore, a simplified approach was developed to predict the ultimate bearing capacity of stirrup-confined circular CFT stub columns, which agreed well with the experimental and numerical results.

To research the web crippling behavior of welded galvanized steel tubes with square and rectangular hollow sections under concentrated load, SHS and RHS carbon welded galvanized steel tubes under four different loading conditions were tested. This paper also examined an approach to enhance web crippling strength of the tubes, by infilling the mortar as composite section. Although very relevant, the understanding about the web crippling behavior in carbon welded galvanized composite section is still very limited, as attested by the lack of information available in design codes and guidelines. End-two-flange (ETF) and interior-two-flange (ITF) loading conditions were adopted, with specimens seated on a bearing plate. Specimens were also placed on the ground with end (EG) or interior (IG) bearing load to simulate the loading conditions of floor joist members. The effects of the loading positions (end loading or interior loading) as well as the supporting conditions (on a bearing plate or on the ground) on the web crippling behavior are discussed and the enhancements of infilling mortar are evaluated. It was found that infilling the mortar in galvanized rectangular steel tube is a very effective approach to enhance the web crippling strength of the tubes, especially for specimens with end bearing with solid ground. Based on the results of the parametric study, a number of design formulas proposed in this paper can be successfully employed as a design rule for predicting web crippling ultimate capacity of grouted galvanized steel tubes under four loading and boundary conditions.

An experimental study into the axial compressive behaviour of concrete-filled circular hollow section (CHS) steel columns with internal fibre reinforced polymer (FRP) tubes is presented in this paper. A total of 17 concrete-filled steel tubular (CFST) columns were tested, 15 with an inner FRP tube and 2 with no inner tube. Complementary material tests and tests on 15 FRP-confined concrete (FCC) columns were also carried out. The varied test parameters included the concrete strength, the ratio of the diameter of the steel tube to that of the FRP tube, the diameter to wall thickness ratio of the inner FRP tube and the type (influencing principally the rupture strain) of the FRP. It was found that the presence of the inner FRP tube led to considerably improved axial compressive behaviour due to the greater levels of confinement afforded to the ‘doubly-confined’ inner concrete core; the load-bearing capacity was increased by between about 10% and 50% and the ductility was also enhanced. Greater benefits arose with (1) increasing diameter of the inner FRP tube due to the increased portion of the cross-section that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. The load-deflection responses of all tested specimens were reported, revealing that failure was generally gradual with no sharp loss in load-bearing capacity, implying that the embedment of the inner FRP tube within the concrete enables it to continue to provide a reasonable degree of confinement even after the initiation of fibre rupture; this is different to the sudden loss of confinement typically observed in FRP externally jacketed concrete columns.

The paper reports an experimental investigation on the bond stress–slip relationship in concrete-filled stainless steel tube (CFSST) structures. Push-out tests were conducted on twelve specimens covering two stainless steel grades, two concrete grades, two cross-sectional dimensions and two internal ring conditions (with and without the internal ring). The effects of these factors on the bond stress–slip relationship in CFSST members were analysed and discussed. A theoretical model, based on the assumption that no restraining force from the steel tube was applied to the concrete core, was developed to predict the bond stress–slip response of CFSSTs and was evaluated against the test results. An empirical model was also proposed based on regression analysis to replicate the experimental bond stress–slip relationships, which can be used as input data for analytical and numerical simulations in future studies of CFSST structures.

This paper investigates the compressive behaviour of prefabricated concrete-filled steel tube (CFST) columns with prefabricated column–column (PCC) connections. The innovative PCC connections are designed using conventional structural bolts instead of expensive blind bolts to connect two prefabricated CFST columns, simplifying the construction process and rendering the construction cost-effective. The compressive behaviour of prefabricated CFST columns with or without PCC connections has been investigated numerically. Parametric studies have been conducted to investigate the effect of different parameters of PCC connections, including connection configurations, horizontal and vertical stiffeners of connections, diameter of the coupling bolt, steel tube thickness, yield strength of steel tube, and compression strength of concrete, on the compressive behaviour of CFST columns with PCC connections. The prefabricated CSFT columns with the developed PCC connections are able to achieve similar compressive strengths that are achieved by the CFST columns without connections. Therefore, the developed PCC connections can be used to construct prefabricated CFST columns for the prefabricated steel–concrete composite structures.

This study experimentally explored the torsional behaviour of 33 specimens, which comprises of 18 concrete filled steel tube (CFST), 9 steel tube (ST) and 6 concrete (C) specimens. The length to diameter (L/D) ratio was 7.14 while the ratios of diameter to thickness (D/t) were 60.9, 46.7 and 40.0. The compressive strength of the infill concrete was 24.2 MPa (normal) and 33.3 MPa (relatively high). All specimens were tested to failure and the obtained data was analysed. The results indicated that buckling failure of CFST specimens was prevented due to the presence of concrete infill. Diagonal cracks of concrete infill observed on CFST specimens after cutting steel tubes are the evidence for the collaboration between steel tube and concrete. This evidence also reveals that the concrete infill contributes to increase the torsional moment capacity of CFST and helps steel tubes to work effectively. The superposition method fails to apply for CFST members under torsion because it does not take into account the mutually beneficial collaboration and interaction between steel tubes and concrete infill which resulted in the improvement on the failure mode, mechanical properties and behaviour of CFST specimens under torsion. Details of the test results and above-mentioned findings are reported in this paper.

Concrete-filled double-tube (CFDT) columns, which are a new type of composite columns, have high fire resistance and the potential to be widely used in high-rise buildings. It is expected that the amount of steel used in a regular CFDT column will be relatively high due to the use of double tubes. To reduce the steel consumption, a thin-walled steel tube with longitudinal stiffeners may be adopted for the outer tube in a CFDT column. This paper studies the behaviour of concrete-filled stiffened double-tube (CFSDT) stub columns under axial compression. Tests on 12 CFSDT stub columns and two reference columns were carried out accordingly, and the test results confirm that the stiffened columns have high strength and good deformation capacity. A finite element model was developed to analyse the interaction between the steel tubes and concrete. Based on further parametric studies, a superposition model was proposed to predict the axial compressive strength of CFSDT stub columns.

In this paper, thirty-six short columns with different diameters (150 mm ≤ d ≤ 460 mm) and steel ratios (4.0% ≤ α ≤ 10.0%) were tested to failure to investigate the size effect of circular concrete-filled steel tubular short columns subjected to axial compression. Size effects on the peak axial stress, peak axial strain, composite elastic modulus, and ductility coefficient were studied. The experimental results showed that the peak axial stress, peak axial strain and ductility coefficient of the specimens tended to decrease with the increase in the column diameter. The values of the composite elastic modulus remained almost constant when the diameter of the specimens increased, indicating that size effect on the composite elastic modulus was not obvious. Meanwhile, size effect on the peak axial stress was influenced by the steel ratio in the range of 4–10%. Furthermore, the size effect tended to decrease as the steel ratio increased. By comparing with the current codes, a reduction coefficient was introduced to consider the size effect of concrete core. Based on the reduction coefficient, the size effect of the concrete core inside the steel tube is found to be weaker compared with that of the unconfined concrete columns because of the confinement effect.

Compressive tests of the CFST stub columns with artificial notches in the steel tubes were conducted in this paper to investigate the effects of the material imperfections of steel tubes on column performances. The load-strain responses, column strength and confining effects were discussed in detail. A parametric study, including the notch length, notch orientation, notch location and ductility, was also conducted. The results indicate that the notched CFST specimens have quite different failure modes from the intact CFST specimens. A notched CFST specimen may fail by bulging or closing of the notch, which was determined by the notch orientation. The results also show that the notched CFST specimens had lower mechanical performances than the intact CFST specimens because the notch steel tube could not offer sufficient confining effect on the concrete core inside it. Based on the experimental results, a practical calculation formula for the bearing capacity of notched CFST columns was proposed, which gave well agreement with the experimental results.

Because of its rational mechanical performances, concrete-filled steel tubes (CFT) have been widely used in construction. To improve the reliability of CFT structures, steel tubes are filled with fly ash concrete (FAC) to increase the workability and compactness of the concrete core. Because the hydration process of concrete changes with the addition of fly ash, creep in fly ash concrete shows obvious differences compared with common concrete. As an important long-term behavior of CFT structures, creep has significant effects such as extra deflection, or stress redistribution between steel tube and concrete core. These effects become more complicated after using FAC, and studies of the effects are limited. In this study, the compressive levels of creep in plain FAC and fly ash concrete-filled steel tubes (FACFTs) were measured. The effects of the two fly ash-replacement ratios, 20% and 40%, on creep were investigated. On the basis of an inverse analysis method, a creep model of plain FAC was regressed according to the test data. After a linear stress–strain relationship of compressive FACFTs was established, the creep model was introduced to predict the creep of FACFTs. By comparing the predicted results and by testing the creep of FACFTs, this approach for predicting the creep of compressive FACFTs has shown desirable accuracy. The findings illustrate that fly ash has un-ignorable effects on the creep of FACFTs and that using a linear model to predict the creep of CFTs under low stress levels is feasible.

This paper presents the first study of its kind on the torsional moment behavior of a new dumbbell-shape concrete-filled steel tube (DCFST) system. Experiments were conducted to investigate the effect of the dimensions of the web connecting the two circular CFSTs, in terms of its width-to-diameter (b/D) and its minimum depth-to-diameter (h/D) ratios, on torsional strength (Tu). Torsion tests were also conducted on circular CFSTs (CCFSTs), on hollow tubes and on plain concrete, as control specimens. It was shown that Tu of DCFSTs ranged from 1.3 to 2.2 that of the CCFST, depending on (b/D) and (h/D) ratios. Increasing (b/D) from 0.31 to 0.54 resulted in only 7% increase in Tu. However, a further increase from 0.54 to 1.0 resulted in an additional 31% increase in Tu. The effect of (h/D) on Tu was more pronounced at higher (b/D) ratios. In general, increasing the web width appears to be more effective than web height. A rigorous nonlinear finite element model was developed and a simple mechanics-based design equation have been proposed to predict Tu of CCFSTs and DCFSTs and resulted in an average predicted-to-experimental Tu of 0.98.

This paper presents an experimental study into the axial compressive behaviour of self-compacting concrete filled elliptical steel tube columns. In total, ten specimens, including two empty columns, with various lengths, section sizes and concrete strengths were tested to failure. The experimental results indicated that the failure modes of the self-compacting concrete filled elliptical steel tube columns with large slenderness ratio were dominated by global buckling. Furthermore, the composite columns possessed higher critical axial compressive capacities compared with their hollow section companions due to the composite interaction. However, due to the large slenderness ratio of the test specimens, the change of compressive strength of concrete core did not show significant effect on the critical axial compressive capacity of concrete filled columns although the axial compressive capacity increased with the concrete grade increase. The comparison between the axial compressive load capacities obtained from experimental study and prediction using simple methods provided in Eurocode 4 for concrete-filled steel circular tube columns showed a reasonable agreement. The experimental results, analysis and comparison presented in this paper clearly support the application of self-compacting concrete filled elliptical steel tube columns in construction engineering practice.

The fiber reinforced polymer (FRP) has attracted increasing attention as a strengthening method for concrete-filled steel tube (CFT) columns because of its lightweight and high strength. Nevertheless, related investigations on seismic performance of FRP confined high-strength rectangular CFT columns are rarely reported. Herein, a series of experimental and numerical analysis on the seismic performance of FRP confined rectangular CFT columns using high-strength thin-walled steel tubes and high-strength concrete are presented considering the following parameters: aspect ratio, axial compression ratio, number of FRP layers and fiber direction. The seismic performance of specimens under reversed cyclic lateral loading was evaluated by discussing the failure modes, hysteretic behaviors, capacity degradation, stiffness degradation, ductility, and strains. The numerical models were developed and verified by the test results, which could accurately simulate the mechanical behaviors and the failure of specimens. The observations drawn from this research might provide a good basis for the application of high-strength material and FRP-strengthened CFT structures.

Twenty four specimens were designed to investigate the axial compressive stiffness of the recycled aggregate concrete(RAC)-encased recycled aggregate concrete-filled steel tube (RACFST) composite columns. The working mechanism of the RAC-encased RACFST composite column was investigated and the full range load–displacement curves of the composite columns were obtained. The interaction of each component of the composite column, i.e. the effect of the outer RAC from the stirrup and the steel tube, and the effect of the core RAC from the stirrup and the steel tube, were analyzed. The constitutive models of the outer unconfined RAC, the outer confined RAC, the core RAC were discussed. Thereupon, simple formulas were proposed to calculate the axial compressive stiffness of composite columns. These formulas can assist engineers to predict the axial deformation of structure in high-rise buildings more accurately.

Applications of ultra-high performance fiber-reinforced concrete (UHPFRC) in composite columns for high-rise buildings or bridges keep increasing to achieve improved structural performances. A testing program with 16 specimens was carried out to study the behaviour of UHPFRC-filled square steel tubular stub columns (UFST-SCs) under eccentric compression. Besides, influences of eccentric ratio (e/r) and thickness of steel tube (ts) on the behaviours of UFST-SCs were studied. The UFST-SCs under eccentric compression exhibited three working stages categorized as linear, nonlinear, and recession. Compression-flange yielding of steel tube and crushing of UHPFRC occurred at the elastic limit and peak load point of load-displacement curves, respectively. Despite the increasing e/r ratio decreased the stiffness and ultimate resistance of the UFST-SCs, its ductility was improved. Moreover, increasing thickness of steel tube enhanced the structural resistance of UFST-SCs under eccentric compression. The validations showed that EC4, AISC and GB50936 were conservative on estimating the Nu-Mu interaction relationships of UFST-SCs under eccentric compression. Considering the accuracy and reliability, EC4 was recommended to predict the Nu-Mu interaction curves of UFST-SCs.

In this work, experimental and numerical investigations were performed on the behaviours of circular concrete filled double steel tubular (CFDST) slender columns and beams, in which the external tube employed stainless steel tube. Eighteen specimens, 12 slender columns and 6 beams, were tested to obtain the failure patterns, load versus deflection relationships and strain developments of stainless steel tube. A finite element (FE) model was developed and verified by experimental results. The validated FE model was then employed to investigate the effects of key parameters, including hollow ratio, eccentric ratio and material strength, on the load-carrying capacity. The load distribution among the components and contact stress between steel tube and sandwiched concrete were also analyzed. Finally, the design methods for CFDST, hollow CFST and solid CFST members with carbon steel external tube respectively suggested by Han et al. (2018), Chinese GB 50936-2014 (2014) and AISC 360-16 (2016) were employed to evaluate their applicability for the circular CFDST slender columns and beams with stainless steel outer tube.

Concrete-filled double skin steel tubular (CFDST) columns with galvanized corrugated steel tubes are a novel type of composite members. Such column consists of a layer of concrete sandwiched between an outer corrugated steel tube and an inner flat or a corrugated steel tube. It is expected to combine the advantages of each component, and have additional advantages in corrosion resistance, local stability, and ductility. In this study, a total of 21 large-scale stub specimens were fabricated to investigate the axial behaviour of the proposed columns. The test results indicated that such columns are feasible and competitive in local stability and ductility. Generally, a larger axial bearing capacity can be obtained by retaining the inner flat steel tube in the proposed CFDST members while better ductility is gained if corrugated steel tubes are used as both inner and outer skins. Strain/stress analysis of the column surfaces indicated that the confinement provided by the thin-walled corrugated steel tubes on the sandwiched concrete was sufficient. The axial bearing capacity resisted by the corrugated steel tubes was negligible due to its unique configuration together with the accompanied local bending effect. Based on the test results, four available design models for predicting the axial bearing capacities of traditional CFDSTs were adopted to assess their applicability for the newly proposed columns with galvanized corrugated steel tubes, with specific design suggestions proposed eventually.

Concrete-filled double-skin steel tubes (CFDSTs) find increasing use as bridge piers and building columns. This paper treats the impact response of such columns using numerical simulations. The modelling techniques are validated using experimental results to establish the capability of the numerical model to capture the impact response and failure modes of the column. The validated numerical model was used to carry out a parametric study on the effects of several structure-related parameters on the impact response of the CFDST columns. The findings of this paper can be used to develop appropriate design information for CFDST columns under lateral impact loading.

Rectangular concrete-filled steel tube (CFST) may be subjected to local bearing forces transmitted from brace members while being used as a chord of a truss, and thus development of finite element analysis (FEA) and simple design calculation method for rectangular CFSTs under local bearing forces are very important to ensure the safety and reliable design of such a truss with rectangular CFST chords in engineering practices. A three-dimensional FEA model was developed using ABAQUS software package to predict the performance of thin-walled rectangular CFST under local bearing forces. The preciseness of the predicted results was evaluated by comparison with experimental results reported in the available literature. The comparison and analysis show that the predicted failure pattern, load versus deformation curves and bearing capacity of rectangular CFST under local bearing forces obtained from FEA modelling were generally in good agreement with the experimental observations. After the validation, the FEA model was adopted for the mechanism analysis of typical rectangular CFSTs under local bearing forces. Finally, based on the parametric analysis, simple design equations were proposed to be used to calculate the bearing capacity of rectangular CFST under local bearing forces.

This paper presents an investigation of the behavior of CFDST members under low velocity lateral impact using ABAQUS, where nonlinear material behavior and strain rates of steel and concrete are included. The finite element analysis (FEA) models of CFDST members are described and verified using existing experimental results. Strain and stress states of concrete and steel tube in CFDST members are analyzed over the full loading range, followed by a parametric study investigating the influence of impact height, hollow ratio, nominal steel ratio, diameter-to-thickness ratio of inner steel tube and strength of materials on the impact force and global residual lateral deformation. Furthermore, the dynamic increase factor (DIF) of CFDST members under lateral impact is proposed, and the influence of the confinement factor on the dynamic increase factor (DIF) is analyzed. The results show that when the confinement factor is larger than 1.03, the dynamic increase factor needs to be considered to accurately estimate the dynamic resistance capacity of CFDST members subjected to lateral impact.

Concrete-filled steel tubes (CFTs) exhibit superior performance under static and dynamic loads, due to composite action. Factors, such as the thickness of the steel tube and the concrete core condition, have a significant effect on the structural behaviour of CFT columns, particularly in the post-yield behaviour. Studies show that if the concrete core is reinforced with steel bars, the new composite member has better characteristics compared to CFT columns. In this study, the axial compressive behaviour of reinforced concrete-filled steel tube (RCFT) columns using spirally reinforced concrete (SRCFT) was investigated and compared with CFTs. The main variation was the pitch spacing of the spiral shear reinforcement. Fifteen specimens, including three CFTs and twelve SRCFTs, were tested in five groups. The test results indicated that a SRCFT column has much better post-yield behaviour than a CFT column. A reduction in the pitch spacing rate further improves the post-yield behaviour of the SRCFTs. A comparison of the measured strength of the specimens with corresponding values predicted by two international codes (ACI 318-11 and EC4-1994) shows a good prediction of EC4 and a conservative estimation of ACI.

This paper presents a comparative study and a 3D finite element (FE) model of square stirrup-confined concrete-filled steel tubular (stirrup-confined CFT) stub columns. The FE results agreed well with the experimental results. The results indicated that: (1) The ultimate load-bearing capacity of CFT stub columns with internal studs was no improvement; (2) All three forms of stirrup confinement effectively reduced local bucking of the steel tube, and the cross ties was optimal, followed by spiral stirrups; (3) With the same overall steel ratio, the behavior of the square stirrup-confined CFT stub columns was considerably higher than in the square CFT stub columns.

The behavior of thin-walled dodecagonal section double skin concrete-filled steel tubes under bending was studied in this paper. Both experimental investigation and finite element analysis (FEA) were carried out. A total of 7 specimens were tested with the beam length of 2000 mm. The width to thickness ratio of the outer steel tube ranged from 75 to 133. The load–displacement curves, failure mode and ultimate capacity of test specimens were obtained. A finite element model was developed for the thin-walled dodecagonal section double skin concrete-filled steel tube subjected to bending. The FEA results were verified against the test results and parametric study was conducted using the verified model. In addition, the suitability of Han's method, Uenaka's method and proposed method for thin-walled circular section double skin concrete-filled steel tubes subjected to bending was also evaluated.

External confinement in the form of steel rings, tie bars, spirals and FRP wraps has been widely adopted for strengthening concrete-filled-steel-tube (CFST) columns. Previous experimental and theoretical studies have proved that it can improve the strength, elastic stiffness, ductility and interface bonding of CFST columns. However, in real engineering practise, CFST columns need to be strengthened are usually under pre-compressed axial load. The stress-lagging effect between the CFST columns and external confinement due to pre-loading has not yet been justified. In this paper, external confinement in the form of circular steel jacket is proposed to improve the uni-axial behaviour of CFST columns with and without pre-compressed axial load. An experimental study, consisting of 5 hollow-steel-tubes and 10 thin-walled CFST columns was conducted to examine the effectiveness of the proposed strengthening scheme. The main parameters were the concrete cylinder strength, jacket spacing and pre-compressed axial load level. Test results revealed that the steel jacket could improve the uni-axial behaviour of CFST columns and the stress-lagging could degrade this beneficial effect. In addition, a theoretical model developed by the authors previously was adopted to predict the uni-axial behaviour of the strengthening columns. Very good agreement has been obtained between the theoretical and experimental results.

In this paper, the mechanical behaviors and failure mechanism of steel-reinforced concrete-filled steel tubular (SRCFST) columns were numerically investigated with the software of ABAQUS/standard solver. Existing experimental results were employed to verify the validity of the finite element models of SRCFST columns. A total of 22 specimens were numerically studied to examine the effect of steel tube ratio ( α t ), section steel ratio ( α s ), concrete strength ( f c ), yield strength of steel tube ( f y t ) and yield strength of section steel ( f y s ) on the mechanical behaviors and ultimate resistance of the SRCFST columns under axial compression. The model in Eurocode 4 significantly underestimated the load bearing capacity of SRCFST columns, and a new model was then proposed to predict the strength of the SRCFST columns. Based on the comparison between the calculation results from the proposed model and experimental results, the proposed model can provide reliable prediction of the ultimate load carrying capacity for the SRCFST columns.

An experimental study of 22 concrete-filled round-ended steel tubular (CFRT) stub columns under axial compression is conducted compared with 4 circular concrete-filled steel tubular (CFT) stub columns. The influences of width–thickness ratio, concrete strength, steel yield strength and wall-thickness of steel tube on the ultimate bearing capacity of the CFRT columns are discussed. The 3D finite element (FE) model is also developed to analyze the behavior of the CFRT columns under axial compression. From the results, local buckling of the round-ended steel tube associated with shear failure of in-filled concrete could be observed. With the increasing width–thickness ratio, the corresponding load–strain curves have a shorter elastic–plastic stage. The parametric studies indicate that the concrete strength, tube thickness and width–thickness ratio of the steel tube also have a great effect on the ultimate bearing capacity. The numerical results also show that the confinement effect of the stub columns decreases with the increasing width–thickness ratio. A practical calculation formula for the bearing capacity of the CFRT stub columns is proposed, which is well in agreement with the experimental results.

This study addresses some key concerns on using steel tube and sandwiched concrete jackets to strengthen intact concrete-filled steel tubular (CFST) columns. The research background lies in engineering cases, in which the functional purpose changes when the structure remains intact. Within the parameters investigated, the load-bearing capacity of columns was enhanced by 114%–199% after being jacketed with steel tube and sandwiched concrete. The observable enhancement depended on section enlargement and the confinement effects of the outer steel tube, the latter of which could account for as large as 19% of the nominal load-bearing capacity. A finite element (FE) model was developed and verified, and the load distribution on components of the strengthened columns was presented. The radial stress between the inner concrete and inner steel tube was validated to increase substantially after the CFST column is jacketed with steel tube and sandwiched concrete. The decrease in the diameter-to-thickness ratio of the outer steel tube contributes to the synchronisation of the inner and sandwiched concrete in reaching peak load and benefits the load increase. A parametric study based on the FE model was conducted, and the effects of estimation error with respect to the properties of existing CFST columns were evaluated. A reasonable match between the outer steel tube grade and sandwiched concrete class was advised.