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A comprehensive experimental and numerical study of concrete-filled double skin tubular (CFDST) stub columns is presented in this paper. A total of 23 tests was carried out on CFDST specimens with austenitic stainless steel circular hollow section (CHS) outer tubes, high strength steel CHS inner tubes, and three different grades of concrete infill (C40, C80 and C120). The ultimate load, load-deflection histories and failure modes of the stub columns are reported. The test results were employed in a parallel numerical simulation programme for the validation of the finite element (FE) model, by means of which an extensive parametric study was undertaken to extend the available results over a wide range of cross-section slendernesses, inner tube strengths and concrete grades. The experimentally and numerically derived data were then employed to assess the applicability of the existing European, Australian and North American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections. Overall, the existing design rules are shown to provide generally safe-sided (less so for the higher concrete grades) but rather scattered capacity predictions. Use of an effective concrete strength is recommended for the higher concrete grades and shown to improve the consistency of the design capacity predictions.

The steel-concrete composite structure using the high-strength steel (HSS) is featured with high strength, lightweight and economic benefits. In this study, a finite element model was established to simulate the compressive behavior of concrete-filled double-skin steel tubular (CFDST) stub column using high-strength steel. The material nonlinearity, the confinement effect and the contact behavior were considered. A total of four large-scale CFDST stub columns using HSS in the outer tube were tested under axial compression. The model was validated using both current and previous test results. The verified numerical model was then used to analyze the influences of various parameters, such as the yield stress of steel, the hollow ratio and the confinement factor. Furthermore, the feasibility of current design equations for CFDST columns using HSS was assessed. It was found that the HSS tube worked well with the sandwiched concrete, and the current methods could provide reasonable yet conservative predictions on the compressive strength of CFDST stub columns using HSS.

This paper presented axial compression tests on the behavior of the circular concrete filled steel tube stub columns with double inner square steel tubes (DIS-CFST). A total of 27 DIS-CFST stub columns with different eccentricity ratio (e), hollow area ratio (ψ) and concrete compressive strength (C) were tested. The study mainly investigated the failure modes, ultimate bearing capacity, load-strain curves, load-displacement curves, initial stiffness at elastic stage (ki) and ductility of the DIS-CFST stub column. The failure mode of DIS-CFST stub columns observed from the tests was local buckling and no crack was observed on the surface of the specimens. It was found that concrete compressive strength had remarkable influence on ductility and ultimate bearing capacity of DIS-CFST stub columns. However, eccentricity ratio had little influence on the behavior of DIS-CFST stub columns. Experimental results showed that when the hollow ratio was small, it had little effect on the ultimate load-carrying capacity of DIS-CFST stub columns. However, the ultimate bearing capacity of specimens significantly decreased when hollow area ratio was large. The eccentricity ratio, hollow area ratio and the concrete compressive strength had slight influence on initial stiffness at elastic stage. Furthermore, the ductility of DIS-CFST stub columns increased with the decrease of hollow area ratio. A simplified formula was proposed to calculate the ultimate bearing capacity of DIS-CFST stub columns. The predicted ultimate bearing capacities were in good agreement with DIS-CFST stub column test results. By comparing with the existing strength calculation formula, it can be concluded that the ACI code, EC4 and Han's method were more satisfied with the requirements of the design.

Both concrete-filled single-skin and double-skin steel tubular stub columns were investigated experimentally in this study. The skin of the composite columns was constructed using steel oval hollow section (OHS) tubes. A series of tests was performed to investigate the effects of the geometric dimension of steel OHS tube and concrete strength on the behaviour and strength of composite columns. The overall width-to-thickness ratio of the OHS tube sections ranged from 15.8 to 62.6. The structural performance of the composite columns was investigated using different nominal concrete cylinder strengths of 40, 70 and 100 MPa. The composite column lengths were chosen so that the length-to-width ratio generally remained at a constant value of 3 to prevent overall column buckling. The concrete-filled OHS tube specimens were subjected to uniform axial compression. The test strengths, load-axial shortening relationship and failure modes of the composite columns were presented in this paper. The test strengths were compared with the design strengths calculated using the American Specification and European Code for composite steel and concrete structures. It was shown that the design strengths do not agree well with the experimental results.

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.

Cold-drawing is one of the typical cold-forming techniques in producing stainless steel circular hollow sections (CHS). Compared with cold-rolling, cold-drawing can produce large scale stainless steel tubes which have wide applications in highly corrosive environment. Nevertheless, few researches have been carried out on the duplex stainless steel CHS columns. In this paper, cold-drawn duplex S22053 stainless steel CHS of three types of cross-sections were tested, including 8 material tensile coupon tests, 6 axially compressed stub column tests and 7 axially compressed long column tests. The material stress-strain curves of the cold-drawn CHS tubes, ultimate loads and failure modes of the stub and long columns were obtained. It has been revealed that material properties of cold-drawn stainless steel CHS tubes are close to those of the virgin plate in the annealed state. Moreover, all the stub columns show local buckling in “elephant foot” mode, while all the long columns undergo overall flexural buckling, among which a long column specimen with the nominal length of 700 mm（CHS-700） shows local buckling after the peak load due to the interaction of axial compression and second-order bending moment. Furthermore, comparing the test values with the predicted values based on the European, American, Australian/New Zealand and Chinese design codes, and the calculation methods available in literatures, it is concluded that the predicted values of the stub columns and the long columns with relatively large member slenderness are all lower than the test values, while the predicted values using the design methods except that in the Chinese design code are higher than the test values for the long columns with relatively small member slenderness. Chinese design code could provide safety predictions for cold-drawn duplex stainless steel CHS columns.

To reveal more information and understand the difference in failure mechanisms and mechanical behaviors of welded T-stub joints with different steel grades under bolt preloading cases, a systematic experimental study was conducted in this paper. The tensile performance of the welded T-stub joint with Q690 high strength steel (HSS) and Q345 normal steel (NS) was compared and evaluated. Especially, the properties of stiffness, resistance and ductility varying with the bolt diameter, strength grade and the ratio of bolt edge distance e to bolt distance d from the web. Moreover, the Faella stiffness formulations and the provisions of Eurocode 3 Part 1–8 were validated against the test results of HSS welded T-stub joints. It was found that, the failure of only the flange occurred near the weld toe can ensure a good ductility for the welded T-stub joints with NS, but for HSS welded T-stub joints there is still a brittle rupture mode due to little necking developed. The plastic resistance of welded T-stub joints with HSS is higher than that of those joints using NS by about 24–66%, while its ultimate bearing capacity is slightly more than that of those joints with NS by around 1–23%. The empirical stiffness coefficient ψ in Faella stiffness formulation significantly overestimates the bolt restraining action of the HSS welded T-stub joints. In addition, EC3 resistance equations may be not safe to predict the actual resistance of the HSS welded T-stub joints when the flange fracture near the weld toe governs the collapse, due to the influence of welding process.

The round-ended concrete-filled steel tube (RCFST) column behaves favorable prospects in engineering application due to its advantages of aesthetical properties and structural efficiency. Nevertheless, relevant studies were commonly focused on the mechanical performance of RCFST stub column. This paper developed a numerical analysis method for predicting the structural behaviour and failure mechanism of thin-walled RCFST slender column under axial compression. The nonlinear finite element (FE) modelling was established on the basis of the equivalent constitutive model and was verified well by the existing experimental results. Following this, several crucial geometric and material parameters were designed to explore the influences on the strength, stiffness and ductility of axially loaded thin-walled RCFST slender column. The impacts of various parameters on second order effect and buckling reduction factor of RCFST slender column were also identified. Furthermore, the mechanical performance of thin-walled RCFST slender column was evaluated by means of the axial force versus mid-height deflection characteristic and the contact stress. The numerical analysis results indicated that the axial strength of thin-walled RCFST slender column was obviously heightened by increasing the strength of concrete and steel in addition to the cross-section area, while contrary results were discovered with the increase of the slenderness ratio and the diameter-to-thickness ratio. Lastly, design recommendations for the axially-loaded thin-walled RCFST slender column were presented by comparing the numerical and previous experimental data with design provisions in specifications EC4, ANSI/AISI 360-16 and GB 50936.

Material properties, residual stress distributions and cross-sectional behavior of cold-formed steel elliptical hollow sections are investigated in this study. Four cross-section series with the nominal section aspect ratio ranging from 1.65 to 3 were included in the experimental investigation. The material properties for each cross-section series and material properties distribution on half of the cross-section profile of a representative section were measured through tensile coupon tests. The distributions of bending and membrane residual stresses in both longitudinal and transverse directions were measured on the half-section profile of the same representative section. Initial local geometric imperfections were measured on five stub column specimens. Besides, stub column tests were conducted between fixed ends to ascertain the material properties of the complete cross-section in the cold-worked state as well as to study the structural behavior of cold-formed steel elliptical hollow section stub columns. In addition to experimental investigation, a finite element model was developed and verified against the test results, with which an extensive parametric study covering a broad range of cross-section geometries was carried out. Currently, there is no codified design rule for elliptical hollow section compression members. The stub column strengths obtained from experimental program and numerical analysis were only compared with the predicted strengths by the equivalent diameter method and equivalent rectangular hollow section approach proposed by previous researchers for design of hot-finished steel elliptical hollow sections, the existing traditional design rules originally developed for circular hollow section with equivalent diameter incorporated as well as the Direct Strength Method and the Continuous Strength Method that the equations were not calibrated for cold-formed steel elliptical hollow sections. The comparisons show that the Direct Strength Method offers the most accurate and reliable design strength predictions among the existing design methods, but further improvement remains possible. In this study, modifications on the Direct Strength Method and the Continuous Strength Method are proposed, which are shown to improve the accuracy of the design strength predictions.

Concrete-filled double-tube (CFDT) columns have the potential to be widely used in high-rise buildings due to their very high load-carrying capacity, excellent ductility and good fire resistance. This new type of composite construction has attracted increasing research attention in the last decade, but there is a need to develop practical design recommendations for this construction. This paper aims to study the ultimate strength, the ultimate strain corresponding to the ultimate strength (ductility), and the stiffness of CFDT stub columns under axial compression. An existing finite element model is directly used to study square CFDT stub columns and this model is further modified to simulate circular CFDT stub columns under axial compression. Numerical data are generated to cover a wide range of parameters for CFDT stub columns. Through regression analysis, simple equations are proposed to calculate the ultimate strength, the compressive stiffness and the ultimate strain of CFDT stub columns. These equations can assist engineers to conduct structural analysis and design of CFDT columns.

Face-to-face built-up cold-formed steel channel sections are becoming increasingly popular for column members in cold-formed steel structures; its applications include cold-formed steel trusses, space frames and portal frames. In such an arrangement, the independent buckling of the members is resisted by intermediate fasteners. In the literature, no research is available for such face-to-face built-up cold-formed steel columns. The issue is addressed herein. This paper presents the results of 36 experimental tests, conducted on face-to-face built-up cold-formed steel channel-sections covering a wide range of slenderness from stub to slender columns. A nonlinear finite element model is then described that shows good agreement with the experimental results. The finite element model includes material non-linearity, initial imperfections and modelling of intermediate fasteners. Both finite element and experimental results are compared against the design strengths calculated in accordance with the American Iron and Steel Institute (AISI), Australian and New Zealand Standards (AS/NZS) and Eurocode (EN 1993-1-3). The verified finite element model is used for the purposes of a parametric study comprising 90 models. The effect of fastener spacing on the axial strength was investigated. From the results of experiments and finite element investigations, it is shown that the design in accordance with the AISI & AS/NZS and Eurocode (EN 1993-1-3) is generally conservative by around 15%, however, AISI & AS/NZS and Eurocode (EN 1993-1-3) can be un-conservative by 8% on average for face-to-face built-up columns failed through local buckling.

This paper reports the numerical axial behaviour of cold-formed ferritic stainless steel (CFFSS) stub columns with two circular perforations on opposite faces located centrally along the width and height of the column specimens. Numerical models are established using the finite element (FE) package ABAQUS and validated by using the test results of cold-formed carbon steel stub columns with perforations and CFFSS shallow stub columns reported in the literature. The modelling of the material properties and geometric imperfections has carefully been considered in the developed FE models and validated rigorously against the available test results. The validated FE models are adopted to perform a comprehensive parametric analysis by varying the cross-sectional geometries and the diameter-to-width ratio of the circular perforation. The parametric results are used to examine the accuracy of the current direct strength method (DSM) and the DSM proposed by other researchers in the literature on CFFSS tubular stub columns. The comparison of the strengths indicates the need for proposing specified design rules for the compression capacity of the perforated CFFSS stub columns. Accordingly, new design recommendations are proposed for DSM for accurate compression strength predictions for CFFSS tubular stub columns which are verified through reliability analysis.

A series of low-temperature tests on axial compression behaviours of thirty stub columns were presented in this paper, which included eight hollow GFRP tubes (HFTs), six unconfined plain seawater and sea sand concrete columns (UPCCs), and sixteen seawater and sea sand concrete-filled GFRP tubes (CFFTs). Regarding the compression tests on CFFTs, axial compression load was applied to inside confined plain seawater and sea sand concrete column (CPCC) and GFRP tube concurrently. The studied parameters of this testing programme included loading schemes of monotonically or cyclically axial compression, low temperatures of 20, − 30, − 60, and − 80 °C, and diameter-to-thickness ratios of 20, 21, 30, 40, and 50. The failure modes, axially compressive load–strain curves, and compressive stress–strain curves of all columns at low temperatures have been revealed. Meanwhile, the influences of different parameters on ultimate compression strength and ultimate axial strain of CFFTs at low temperatures were detailly discussed. These results indicated that axial compression behaviours of CFFT stub columns have been improved at low temperatures. According to the test results, prediction formulae have been proposed to determine the ultimate compression resistance and ultimate axial strain of CFFTs reasonably but conservatively at low temperatures.

This paper presents experimental and numerical investigations and design of concrete-filled double-skin aluminum stub columns with circular hollow sections (CHS) as both outer and inner skins. A series of tests was carried out to investigate the effects of the geometric dimension of the aluminum CHS and concrete strength on the behaviour and strength of the composite columns. The CHS tubes were fabricated by extrusion using 6061-T6 heat-treated aluminum alloy having nominal 0.2% proof stress of 240 MPa. The structural performance of the composite columns was investigated using different nominal concrete cylinder strengths of 40, 70 and 100 MPa. A non-linear finite element model is developed and verified against the experimental results. The test results and the composite column strengths predicted from the finite element analysis (FEA) were compared with the design strengths to evaluate the reliability of the design rules in the current American specifications for aluminum and concrete structures. Furthermore, design equations were proposed to consider the benefits of the composite columns due to the composite action between the aluminum tubes and concrete. The proposed design equations accurately predicted the ultimate strengths of the concrete-filled double-skin aluminum CHS stub columns.

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.

This study aims to investigate the effect of manufacturing process on the post-fire mechanical response of Grade 1200 ultra-high strength steel (UHSS) tubes. To this end, the post-fire mechanical properties of “direct-quenched” UHSS (UHSS-DQ) standard tensile coupons are compared to those made of “quenched and tempered” UHSS material (UHSS-QT) with similar original room temperature stress-strain responses. Thus, to compare the post-fire compression behaviour of UHSS-DQ tubular stub columns with those made of UHSS-QT material, a finite element (FE) model is developed in ABAQUS FE software with precise material properties extracted from the results of the post-fire tensile coupon tests. Quasi-static compression tests are then conducted on UHSS-QT tubular stub columns cooled from different fire temperatures to room temperature to validate the FE analysis. Using the results of the tensile coupon tests and the FE analysis on UHSS stub columns, it is shown that the manufacturing process substantially affects the mechanical properties of UHSS stub columns under cooling phase of a fire.

This paper describes an experimental study to estimate the stub columns capacity of cold–formed square (SHS) and rectangular hollow sections (RHS) containing two opposite central circular perforations at column mid–height. The stub columns were extracted from commercially available Tata Structura–YSt 310 tubular sections, with minimum yield strength of 310 MPa, conforming to Indian Standard 4923. The influence of two opposite central circular perforations on the structural performance of tubular steel stub columns under concentric loading was investigated for perforation size to flat width ratios up to 0.9 The local geometric imperfections, load–end shortening curves, strain distributions at mid height of column and typical failure modes observed from the present test programme are documented in this paper. Further, the ultimate column capacities recorded from the test programme are compared with codified design predictions as well as design equations proposed by various researchers. Based on the comparison, it is observed that the predictions made by most of the currently available design equations are conservative and reliable but generally scattered for design of cold–formed tubular structural steel stub columns having central circular perforations.

The behaviour of square concrete filled stainless steel tubular (CFSST) stub columns after exposure to elevated temperatures under axial compression is presented in this paper. A total of sixty-nine specimens were tested, including forty-eight square CFSST stub columns at elevated temperatures cooled in air, nine square CFSST stub columns at elevated temperatures cooled in water and twelve square CFSST stub columns left untreated at ambient temperature. A total of twelve square CFSST stub columns were carried out load-strain tests. The main parameters explored in the test include thickness (1.0 mm, 1.2 mm, 1.5 mm, 2.0 mm), concrete strength (C20, C30, C40), temperatures ranging from 400 °C to 1000 °C and cooling methods. This paper presents the failure modes, ultimate load-bearing capacity, load-strain curves, load versus displacement curves, initial compressive stiffness at the elastic stage and ductility of the specimens. It was found that high temperature had the greatest influence on the ultimate load-bearing capacity of the columns, and the cooling methods also had some effect on it. As the elevated temperature to which the specimens were subjected increased, the percentage of the ultimate load-bearing capacity decrease had an obvious increase and the ductility of the specimens was enhanced. The initial compressive stiffness decreased evidently as the temperature increased to 800 °C. A new failure mode was identified without visible corner crack in the specimens after exposure to elevated temperatures compared with the specimens at ambient temperature. The ultimate load-bearing capacity increased and the axial deformation versus load curves had a longer elastic stage as the concrete strength increased.

This paper presents an experimental investigation on a wide range of aluminium alloy stub, intermediate and slender columns containing multiple circular openings. Three series of compression tests including six different cross-section dimensions of columns with different lengths were conducted by applying uniform axial load to the pin-ended specimens, which were fabricated by extrusion of square and rectangular hollow sections (SHS and RHS) using 6061-T6 and 6063-T5 heat-treated aluminium alloys. In test series I, the specimens were fabricated by welding aluminium alloy plates at both ends of the columns for pin-ended compression tests. In test series II, the specimens were further reinforced by carbon fiber-reinforced polymer (CFRP) at the heat-affected zone resulted from the welding. In test series III, the special aluminium alloy sleeves were designed instead of welding at both ends of the columns to preclude the form of the heat-affected zone. The column strengths, failure modes, load versus axial shortening curves, and strain distributions along the circular openings of test specimens were all obtained from the experimental investigation. In addition, the test strengths of aluminium alloy SHS and RHS columns with or without openings were compared with the design strengths calculated using the design rules given in the current design guidelines. It is shown from the comparison that the first design method proposed by Zhu and Young for aluminium alloy SHS and RHS welded columns is comparatively appropriate for the design of aluminium alloy SHSs and RHSs under axial compression. Whereas, the current design rules for cold-formed steel structural members with openings may be inapplicable to the design of perforated aluminium alloy SHSs and RHSs under axial compression.

Special-shaped CFST columns are becoming increasingly attractive as alternative solutions to engineering design. Three-dimensional FE models are developed and verified against experimental results in terms of failure modes, load-deformation curves and ultimate loads, where circular, triangular, Fan-shaped, D-shaped, 1/4 circular and semi-circular sections are considered. In light of the FE simulations, the composite actions between the special-shaped steel tubes and concrete cores have been investigated through load-deformation and interaction stress-deformation histories. Possible parameters affecting specimens loading behaviors have been studied. The studies generally show that the failure modes, composite behaviors and load-deformation histories of the axially loaded special-shaped CFST stub columns are similar to those of SHS/RHS specimens.

Round-ended elliptical concrete-filled steel tube (RECFST) column is a novel type of composite member which is gradually used in engineering practices (i.e. piers and arches) because of its low flow resistance coefficient and reasonable distribution of major-minor axis. However, little attentions to the structural behavior of RECFST columns have been paid. This paper makes an attempt to conduct a numerical analysis on the axial compressive performance of RECFST stub columns. Firstly, an equivalent constitutive model for the confined round-ended concrete of the RECFST column was proposed. Following this, a nonlinear finite element (FE) modeling considering contact interactions of the type of thin-walled CFST column was developed and verified by the test results in terms of axial load (N) - longitudinal shortening (δ) curves and failure modes. The influence of extensive parameters, including diameter-to-thickness ratio, aspect ratio and cross-section area etc., on the performance of thin-walled RECFST stub columns under axial compression was estimated as well. The analytical results demonstrated that the axial compressive strength of thin-walled RECFST stub columns was substantially improved with the increase of the steel strength, the concrete strength and the cross-section area, and the trend of the N-δ curves was obviously affected by the confinement factor. Finally, two simplified design methods to predict the axial compressive strength of thin-walled RECFST stub columns were established on the basis of the simple superposition approach and the unified theory. The studies may provide a considerable reference for designing this type of structures in engineering practice.

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.

This paper presents an investigation on the structural performance of welded S460 steel columns under axial compression at elevated temperatures using finite element analysis. Stub and long columns with box and H- sections were considered. A new stress-strain curve model for S460 steel at elevated temperatures was proposed and adopted to obtain the stress-strain curves for the finite element analysis. A finite element model was developed and verified against the available test data for welded S460 steel columns subject to axial compression at room and elevated temperatures. A parametric study was carried out to generate additional data of the axial compressive strength of box and H-section columns with various cross-section slenderness and column slenderness ratios at elevated temperatures. The obtained numerical results of the column strength were compared with the design predictions using the European and American specifications and the direct strength method by substituting the material properties at elevated temperatures. It was found that the provisions in European and American standards for elevated temperature conditions provide relatively conservative predictions. The direct strength method overestimates the strength of stub columns while accurately predicts the strength of long columns at elevated temperatures. Modifications are proposed for the European and American standards and direct strength method and these modified design rules are recommended to be used to more accurately estimate the design strength for welded S460 steel columns at elevated temperatures.

This paper presents a series of twelve buckling tests on rectangular and circular hollow section beam-column of nominal steel grade S355. The columns were fabricated by either the hot-rolling or the cold-forming process, and were subjected to different load cases through the application of eccentric compression: mono-axial bending (My) or bi-axial bending (My + Mz) combined with axial compression (N). Preliminary measurements of cross-section geometry, material properties, geometrical imperfections, residual stresses as well as stub column tests are also reported in this paper. The (imperfect) initial geometry was measured along the whole column by means of two different procedures; the first method relied on the use of a set of equally spaced Linear Variable Displacement Transducers (LVDTs) displaced on each specimen's plates. The second method consisted in scanning the specimen's plates by means of a laser Tracker AT401. Residual stresses were also determined experimentally, and the sectioning technique was classically used to record the deformations of the released material; these results have been compared to measurements taken with electrical strain gauges. Finally, beam-column buckling strengths were plotted in an O.I.C.-type format, and complemented with an extensive experimental data collected from the literature that comprised various load cases, fabrication processes, yield strengths, cross-sections shapes, and elements’ lengths. The O.I.C. approach was shown to adequately capture the behavior of hollow section beam-columns in a straightforward and simple manner, and its potential for efficiently and rationally predicting carrying capacities was evidenced.

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.

Carbon fiber reinforced polymer (CFRP) strengthened concrete-filled thin-walled steel tubular (CFST) column can improve its load carrying capacity and resolve local bugles or corrosion of thin-walled steel tube. However, little attention to CFRP strengthened CFST columns has been paid. This paper reported an experimental and numerical analysis on eccentric compressive behavior of circular CFST stub columns partially-wrapped by CFRP strips. A series of circular composite columns, including nine CFRP strengthened CFST stub columns and one bare CFST stub column, were tested subjected to eccentric compression. Moreover, a nonlinear finite element (FE) modeling in considering contact interactions of the composite columns was developed and verified by the test results in terms of eccentric load (N) - longitudinal shortening (δ) curves and failure patterns. Then, the influence of extensive parameters on the eccentric compressive behavior of CFRP strengthened circular CFST columns was also evaluated. Meanwhile, a simplified empirical method on the eccentrically-loaded stub composite columns was proposed on the basis of unified theory. The experimental and analytical data indicated that the eccentric compressive strength of CFRP strengthened circular CFST stub column was obviously influenced by load eccentricity, CFRP confinement factor, steel strength, core concrete strength and CFRP strength. The proposed simplified empirical formulas may provide a considerable approach for designing this type of composite structures in engineering practice.

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.

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%.

This paper mainly presents a combined experimental, numerical and theoretical study on the mechanical behaviors of both conventional concrete-filled rectangular steel tubular stub columns (RST) and stirrup-confined concrete-filled rectangular steel tubular stub columns (SST) under compressive load. 16 RST stub columns and 16 SST stub columns were tested, with consideration of parameters including internal stirrup, concrete strength, and cross-sectional aspect ratio B/D. The failure patterns, bearing capacity, stiffness and ductility of specimens were analyzed based on the experimental results. A 3D finite element (FE) model was established for numerical simulation and parametric study to investigate the composite action among the steel tube, stirrups and the core concrete. The improvement of energy dissipation capability due to the stirrups confinement on the core concrete is also discussed. In addition, a unified theoretical formula to predicted ultimate bearing capacity for SST stub columns and RST stub columns subjected to compressive load was developed according to the superposition principle with rational simplification. It is shown that the proposed formula for these columns has a higher accuracy compared with the existing formulas in current literatures and codes.

The behaviour of dune sand concrete-filled steel tubular (CFST) stub columns under axial compression was experimentally investigated in this paper, where the nominal cube strength of concrete was 60 MPa. A total of 24 specimens including 20 dune sand CFST stub columns and 4 reference hollow steel tubular stub columns were tested. The effects of tube thickness, cross-sectional dimension and concrete infill on the behaviour of the composite stub columns were investigated. The results showed that, similar to conventional CFST stub columns, the tested dune sand CFST stub columns behaved in a ductile manner and displayed an outward buckling failure model with high compressive strength. It indicates that it is feasible to use dune sand to replace normal sand to fabricate CFST columns in regions close to deserts. Comparisons were made between the test results and the predicted ultimate strength using the existing codes AIJ (2008), AISC360-10 (2010), BS5400 (2005), DBJ/T13-51-2010 (2010) and EC4 (2004). Generally, these codes are conservative but acceptable for predicting the ultimate strength of dune sand CFST stub columns.

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.

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 objective of the present study is to study the failure and size effect of squared concrete-filled steel tubular (CFST) columns subjected to eccentric loading. To model the failure of squared CFST stub columns, a three-dimensional meso-scale simulation approach taking into consideration of both concrete heterogeneity and the concrete–steel tube interaction was established. The meso-scale method was first verified by comparing the simulation results with the available test ones. The verified numerical model was then employed to perform the investigations on the influence of load eccentricity ratio ranging from 0.1 to 0.9 and confinement effect on the failure mode and nominal strength of squared CFST stub columns having the constant slender ratio of 3.0 and having different structural sizes (i.e. 200 mm, 400 mm, and 800 mm). The simulation results indicate that, as the magnitude of the eccentricity increases, the failure mode of squared CFST stub columns changes from brittle compression-shear failure to ductile bending failure, making the size effect on nominal compressive strength be weakened. Moreover, as the confinement effect described by confinement coefficient arises, the nominal strength as well as the ductility of CFST columns improve and the corresponding size effect is also weakened. Finally, a simplified model of axial force–bending moment (N-M) interaction curve for predicting the bearing capacity of squared CFST stub columns was proposed considering the influence of structural size, which can provide conservative predictions when compared to the simulated results.

This paper presents a numerical investigation on the performance and design of cold-formed steel square (SHSs) and rectangular hollow sections (RHSs) stub columns having two opposite central circular perforations located at column mid-height. Finite element (FE) models are initially developed and validated against the experimental results on perforated YSt-310 cold-formed steel stub columns reported by the authors. A total of 264 experimental and numerical column capacities, collected from previous experimental investigations and current parametric study, has been utilized to compare with the design strengths predicted by available codified and proposed design equations. Based on the analysis, most of the presently available design equations for perforated stub columns provide overly-conservative and highly scattered design predictions. In particular, the Direct Strength Method in American Standard provides the maximum overly conservative prediction of about 60% with high scatterings. Hence, a modified DSM design equation has been proposed for various perforation size ratios (i.e. diameter of perforation to flat element width) considering the previous test results and current FE column capacities through least square regression analysis. The newly proposed design equation is shown to offer improved mean (conservative) and reliable predictions, and therefore recommended for inclusion in future revisions of design codes for perforated cold-formed steel tubular stub columns.

To investigate the local buckling behavior and obtain critical buckling stress of flange and web in Q690 steel stub columns at elevated temperatures, a total of 8 stub columns made of H-shape welded sections were tested under simultaneous axial compression and high temperatures. 2 stub columns were tested at ambient temperature and critical stress of flange and web were determined. 4 stub columns were tested in steady-state at 450°C and 600°C, respectively. 2 stub columns were tested in the transient state based on the critical stress of flange and web at 600°C. During the tests, temperature evolution, applied load, axial displacement, lateral deflection, failure modes of each specimen were recorded. The results showed that the critical buckling stress decreases significantly with the increase of temperature. At the same load level, the steady-state test’s critical temperature was lower than that obtained in the transient state test. In parallel with the tests, a finite element model was established using ABAQUS software under high temperatures to evaluate the effects of the temperature on local buckling stress, and the model was validated by the test results. A parametric study was carried out to investigate the effect of numerous factors, including width-to-thickness ratio, residual stresses, initial imperfection, and temperature. The simulation and test results were compared with the current design provisions in Eurocode 3, and the comparison showed that the Eurocode 3 design method could not be applied to the Design of the local buckling bearing capacity and ultimate stress, and it might not be precise enough at elevated temperatures.

This paper presents an experimental study on blind bolted T-stub to square hollow section (SHS) connections made by stainless steel. Experiments were conducted to investigate the structural behaviour (e.g., failure mode, load–displacement curve and load-carrying capacity) of T-to-SHS connections and their basic components, including bolts, T-stubs and SHSs. The effects of key parameters, such as bolt types, bolt layouts, tube thicknesses and specimen configurations, on the performance of T-to-SHS connections and the components were identified. Design formulas were proposed to estimate the load-carrying capacities of T-stubs and SHSs, which agree well with the experimental results. The experimental results on connections between T-stub and steel hollow sections will provide a basis for verifying numerical simulations and theoretical analysis to derive design equations for such stainless-steel connections.

This paper presents an experimental investigation into the compressive behaviour of compact to slender octagonal concrete-filled steel tubular (OctCFST) stub columns. In this study, a total of twenty OctCFST stub column specimens were tested. Five octagonal hollow sections with various cross-sectional slenderness were fabricated from 3 mm and 6 mm 460 steel plates. The measured yield strengths of the two batches of steel plates are 546.5 and 580.7 MPa, respectively. Three grades of concrete with measured cylinder strengths varying from 27.9 to 92.1 MPa were used to infill the hollow tubes. The test results, in terms of failure modes, axial load versus axial strain responses and cross-sectional capacities of the stub column tests are fully presented and discussed. The experimental results were then used to evaluate the applicability of current design methods in Eurocode EN 1994-1-1, American standard ANSI/AISC 360-16, Chinese code GB 50936-2014 and equivalent circle methods. It is shown that the circumcircle method and GB 50936-2014 approach produce excellent cross-sectional strength predictions for the OctCFSTs and the current cross-section slenderness limit of 135 × 235/f y for circular concrete-filled steel tubes specified in GB 50936-2014 can be safely extended to the design of OctCFSTs when the equivalent circumcircle method is used.

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 paper describes experimental and numerical investigations conducted to characterize the basic material properties and design of YSt–310 cold–formed structural steel sections at Indian Institute of Technology, Guwahati. Square (SHS) and rectangular (RHS) hollow sections with minimum yield strength of 310 MPa manufactured by Tata Steel India, were considered in the study. Initially, results from elemental analysis via optical emission spectrometer (OES) investigation and metallographic examination using optical microscope are presented. Key stress–strain parameters viz., Young's modulus, proof stress, ultimate strength, percentage elongation, strain hardening exponent etc. were generated based on flat, corner and weld coupon tests data. Extent of corner strength enhancement due to cold – forming determined using Vickers's microhardness test are reported. Additionally, cross – section capacity of the stub columns were also investigated experimentally and numerically. The column capacities generated from test and finite element study are compared with the existing design code – EN 1993–1–1 (EN 1993-1-1, 2005) [1] and design rules – continuous strength method (CSM) (Zhao et al., 2017) [2] and direct strength method (DSM) (North American Specifification for the Design of Cold-Formed Steel Structural Members, 2016; Arrayago et al., 2017) [3,4]. Based on the comparison, modifications on the existing design code and rules are suggested to provide a more accurate and reliable compressive design prediction.

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.

A complete stress-strain model for cold-formed circular hollow section (CHS) stub columns under compression is proposed with emphasis placed on the modeling of post-local-buckling behavior. The proposed model comprises two mathematical expressions. One of the two equations is to define the ascending portion of the proposed stress-strain curve and depict the nonlinearity caused by residual stress induced during manufacturing process of cold-formed CHS, while the other is to define the descending portion and trace the post-local-buckling behavior. The proposed stress-strain model is a single parameter model. Only if the generalized outer diameter-to-thickness ratio is given, the compressive stress-strain relationship of a CHS can be completely determined. To calibrate the proposed model and verify its accuracy, test results of forty-eight CHS stub columns under compression are collected. The previous tests cover a wide range of structural factors such as outer diameter-to-thickness ratio and yield strength, particularly including CHS stub columns with yield strength of up to 1180 MPa. Comparisons between the measured results and the calculated ones indicate that the proposed model can not only predict the local buckling strength as well as corresponding strain, but also trace the post-local-buckling behavior up to large strain with high accuracy.

The objectives of this paper is to investigate the mechanical behavior of steel reinforced concrete-filled square steel tubular (SRCFT) stub columns under axial loading through combined experimental and numerical studies. In total of six specimens are tested to investigate the effect of the concrete strength and steel ratio on the mechanical behavior of SRCFT stub columns. The ultimate bearing capacity, ductility, and confinement effects are discussed and clarified based on the experimental results. The inserted steel section can effectively prevent shear cracks in the core concrete from propagating quickly. The strength-weight-ratio of SRCFT stub column is larger than SCFT stub column. In addition, ABAQUS is used to establish the 3D finite element (FE) model and analyze the composite action of SRCFT stub columns under axial loading. Based on the experimental and numerical results, a simplified formula is proposed to estimate the ultimate bearing capacity of SRCFT stub columns using superposition method. The predicted results show satisfactory agreement with both experimental and FE results.

Concrete-filled bimetallic tubes (CFBT) could be recognized as a type of steel-concrete composite members, and have the potential to be applied in engineering structures. A three-dimensional finite element analysis (FEA) model is established to further study the axial compressive behaviour of circular CFBT stub columns in this paper. The preciseness of the FEA predictions is evaluated by comparison with experimental results collected from ten CFBT column specimens tested in a companion paper (Ye et al., 2016) [15]. Good agreement is obtained between the FEA predictions and the test results in terms of failure pattern, load versus deformation response, ultimate strength, and strain development of the two steel tube layers. In light of the FEA results, the stresses of concrete and steel, as well as the interactions and load distribution between different components of the composite section are investigated. Finally,parametric study is performed and a simplified model is proposed to be used to calculate the ultimate strength of circular CFBT stub columns.

This paper deals with developing advanced, yet design-oriented expressions for ultimate strength of the newly developed hollow corrugated stub columns with or without ultra-high strength corner tubes under uniaxial compressive loading. This is achieved by individually analysing comprising self-strengthened corrugated plates using the effective width concept whilst the effect of corner enhancement is taken into calculation. For stiffened panels such as corrugated plates, the ultimate limit state design is equated to the lowest value among all collapse provisions. However, the finite element analyses show that Euler buckling state may govern the ultimate load capacity of corrugated plates used in the proposed columns. This is verified by comparing the results obtained from the proposed analytical formulations with those demonstrated by the finite element modelling and experiments. The results are also compared with the guidelines for design of corrugated panels available in the literature. Finally, several cases with different corrugation geometrical parameters are defined and the capacity of corresponding columns is evaluated and compared to each other.

Cold-formed steel hollow flange sections are structurally efficient compared to the conventional open channel sections. They can be of different forms including hollow flange I- and channel sections, which are generally fabricated by first cold-forming a single sheet to form the desired shape and then connecting their flange and web elements by continuous welding. Instead of welding, intermittently screw/rivet fastened or spot welded connection can also be used. A detailed investigation is currently under way on the structural performance characteristics of such intermittently screw/rivet fastened steel hollow flange sections. This paper presents the details of an experimental study performed to investigate the behaviour of built-up cold-formed steel hollow flange sections in compression, where more than 45 stub columns were tested. Both hollow flange I- and channel sections built-up using either single steel sheet or three steel elements were tested. Tests also included built-up hollow flange channel sections with stiffened web elements. Test results including the failure loads, load versus axial deformation plots and failure characteristics are presented in this paper. Test failure loads are compared with the predictions based on both effective width and direct strength methods to verify their adequacy in predicting the section compression capacity of built-up hollow flange sections. This paper has shown that although hollow flange I-sections made of three elements with a 100 mm fastener spacing can perform similar to continuously welded sections, hollow flange channel sections undergo premature failures even with a 50 mm fastener spacing, and the reasons are discussed.

It is well accepted that, for a long column, the effect of end conditions on the local buckling load can be ignored due to its less impact. However, such an assumption may be inappropriate for a stub column. According to the available literature, the research dealt with such a problem was found to be quite rare. In this scenario, this paper aims to derive novel local buckling formulae for cold-formed C-section column, which takes both the effects of cross-section plate element interactions and end conditions into account. Initially, an equivalent simplified web local buckling model was proposed, in which the cross-section plate element interaction of the flanges and the lips on the web were modeled by rotational springs and the loaded edges were assumed to be uniformly compressed and to be simply supported or fixed. Subsequently, based on the energy method and the theory of Bleich, explicit expressions for the local buckling load of the column were derived. Further, simplifications to the rigorous formulae were made to allow them to be easily used by the engineers. Finally, in order to verify the accuracy of the derived formulae, the results obtained from the derived formulae were compared with the numerical results obtained from the computer software CUFSM. The comparison results show that the formulae proposed in this paper can be used successfully in estimating the local buckling loads for pin-ended or fix-ended C-section columns with a practical length.

The grout is often used to fill the gap between two tubes as binding material in some specific structures, which may actually form a grout-filled double skin steel tube. However, little literature has been found regarding the behaviour of the grout-filled double skin steel tubular (GFDST) member, while the influence of the in-filled grout could be underestimated in the steel-grout composite structure. This paper thus presents an experimental investigation of the compressive and flexural behaviour of GFDST members. A total of 14 specimens are tested, including 8 stub columns, 4 beam-columns and 2 beams. For the axially compressed stub columns, the main test parameter is the hollow ratio of the cross section, and 2 corresponding hollow steel counterparts are designed to study the effect of the grout filling. For the beam-columns, the influences of load eccentricity are analyzed through test results. The failure modes, the strain distribution and the ductility of the GFDST members are studied in order to sufficiently evaluate the performance of the steel-grout composite member. The ultimate strengths of the GFDST members are estimated by the equations of concrete-filled double skin steel tubular members tentatively. It is found that all tested specimens failed in a ductile way, and the stiffness, the ultimate strength and the ductility of GFDST member is improved attributing to the in-filled grout. In addition, the predicted stiffness and strength results generally match well with the measured ones.

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.

This paper presents the experimental and numerical studies of concrete-filled cold-formed elliptical hollow sections with varying aspect ratios subjected to axial compression load. Twenty one stub columns were tested to investigate the fundamental behaviours of these elliptical concrete-filled steel tubular (CFST) columns. The axial load versus displacement curves, longitudinal and transverse strains in steel tube and failure modes were obtained and discussed. A constitutive model for concrete in the elliptical CFST columns was proposed. Finite element (FE) models were developed and validated against the test results. Parametric studies were carried out to identify the influence of key parameters on the load-bearing capacity. Key parameters included aspect ratio, steel tube to concrete area ratio, yield strength of steel and compressive strength of concrete. Finally, the applicability of relevant design methods to elliptical CFST columns was assessed on the basis of the test and the FE results of this research and other related studies. Results indicated that the design method recommended in the Chinese Standard (GB50936-2014), and the design method for circular CFST columns in EC4 were found to generate accurate predictions.

An experimental investigation was conducted to evaluate the axial compressive behaviour of circular hollow section (CHS) stub columns with initial crack defects. A total of 96 specimens – 12 stub columns with one crack, 48 stub columns with two cracks, and 36 stub columns with three cracks – were tested under axial compression loading. The failure modes, load-carrying capacities, overall deflections, and strain intensities of all the specimens were examined. The design parameters were the tube thickness, long crack length, short crack length, crack spacing, and crack number. Additionally, a finite-element analysis was performed using the verified models in combination with the experimental data to evaluate the effects of the main influential factors. The initial cracks formed in the circular thin-walled stub columns accelerated the local buckling and changed both the bearing capacity and deformation capacity. The ultimate loads of the stub columns increased with the tube thickness, decreased slightly with an increase in the long crack length, and decreased with an increase in the crack spacing. The effects of the short crack length and crack number on the bearing capacity were complex and were coupled with the tube thickness. An adjustment coefficient was introduced into the predictive formula to estimate the ultimate bearing capacity. Through this, the ultimate bearing capacity can be accurately predicted.