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Cross-section behaviour for cold-formed high strength steel (HSS) irregular hexagonal hollow section (IHexHS) stub columns under combined compression and bending is studied and presented in this paper. Finite element models were developed and validated using the existing experimental data collated from the previous research. Upon the validated finite element models, extensive parametric studies were subsequently carried out to generate more numerical data covering a wider range of cross-section dimensions, steel grades and load combinations from pure compression to pure bending. The obtained numerical results were utilised to assess the accuracy and the applicability of the current design codes, such as Eurocode of EN 1993-1-12 (EC3) and the North American code of ANSI/AISC 360-16 (AISC) for cold-formed HSS IHexHS stub columns under combined loading. It was demonstrated that the existing design codes can be safely applied and can be extended for cold-formed HSS IHexHS stub columns design under combined loading. In cross-sectional resistance predictions, conservative results were provided from the existing design codes. The over-predictions were primarily due to the neglect of the strain hardening and plate element interaction. The end points used in the interaction curves of EC3 and AISC adopt an idealised elastic-plastic material model to derive the corresponding resistance in cross-sectional level. The employment of Continuous Strength Method (CSM) leads to improved accuracy in cross-sectional resistance prediction with updated end points in the interaction curve. More consistent and reliable predictions were revealed by carrying out reliability analysis in accordance with EN 1990. [ABSTRACT FROM AUTHOR]

An experimental and numerical study on the octagonal high-strength steel tubular stub columns under combined compression and bending is presented in this paper. Octagonal high-strength steel tubular stub column specimens with different sizes were prepared. Experiments were performed on these specimens under concentric compression or eccentric compression, which induced various combinations of compression and bending on the structures. In addition, a numerical study through finite-element modeling was also conducted. A finite-element model was developed and validated to be capable of accurately replicating the experimental results. Subsequent parametric studies using the validated model were performed on the structures with a wide range of dimensions and under various combinations of compression and bending. Based on both experimental and parametric studies results, the applicability of existing design approaches in European and American Standards to the structures subject to combined compression and bending was evaluated. The evaluation results show that the design approaches from the standards, especially the compression and bending interaction relationship, provide conservative strength predictions and can be safely applied to the structures. [ABSTRACT FROM AUTHOR]

Highlights • Octagonal, circular and square tubular steel stub columns were tested under compression. • Finite element models were developed and verified by the test results. • Cross-section classification in design codes were assessed by the tests and FE results. • A new cross-section slenderness limit and an alternative design approach were proposed. Abstract This paper presents an experimental investigation on the compression behaviour of cold-formed octagonal, circular and square tubular steel stub columns. Three octagonal hollow steel tubes were examined under axial compression while three circular steel tubes and two square steel tubes were tested for comparison. Nine existing test data on octagonal hollow steel tubes were also collated for comparison. The hollow tubes were fabricated by welding two cold-formed half-section steel plates. To examine the material characteristics, flat, curved and corner coupons were extracted and tested. Residual stress distribution in longitudinal direction was also measured by sectioning method. Fifty longitudinal strips with a nominal width of 10 mm were machined by a wire cutting machine. The test results obtained from the tensile coupon tests and residual stress measurements were examined to evaluate the effect of the fabrication process on different cross-sectional shapes. The geometric imperfections of each specimen were carefully measured. Cross-sectional capacity, load-deformation relationships and failure modes of the stub columns were presented by the stub column tests. Finite element model was developed and validated against the experimental results. The experimental cross-sectional capacities were accurately predicted by the finite element models. The test data accompanied with the FE results were assessed with reference to current design provisions on cross-section classification and a suggested cross-section slenderness limit and an alternative design approach for octagonal cross-sections were proposed. [ABSTRACT FROM AUTHOR]

Concrete filled steel tubes (CFST) have been widely used in modern constructions. The cross-section shapes of circular and rectangular are mostly used. This paper presents an investigation on CFST with a new cross-section shape - octagonal shape which combines both advantages from circular and square cross-sections. In this study, 21 CFST stub columns were tested accompanying with 9 plain concrete columns under uniaxial compression. Three cross-section shapes, octagonal, circular and square sections were considered. In parallel, 10 associated CFST stub columns with octagonal cross-sections from literature were also complied. The measured steel yield strength was between 383 MPa to 485 MPa. Both normal and high strength concrete were used with measured cylinder compressive strengths ranging from 38 MPa to 112 MPa. The key investigation focuses on the relationship between the cross-section shapes and confinement effectiveness which can provide an understanding on the difference in load bearing capacity of CFST with those three section shapes. Design formulae for the cross-section capacity in the current code of practice were assessed by the experimental results and modification was proposed to the existing formula for circular cross-section which could be adopted for the CFST with octagonal cross-section. [ABSTRACT FROM AUTHOR]

A novel fiber-reinforced polymer (FRP)–concrete double tube composite column, consisting of an outer filament winding FRP tube, an inner pultruded FRP tube and infilled core concrete and ring concrete, has been proposed and experimentally investigated under monotonic compression by the authors. It exhibited obviously improved deformability than the traditional FRP-confined concrete columns. In this study, cyclic compression tests were conducted on the composite column to examine its structural behavior under cyclic loadings. Effects of different outer and inner FRP tube thicknesses and ring concrete types were investigated. Failure modes, cyclic load–strain responses and hoop strain behavior were presented and analyzed. Cyclic load–strain model, including the envelope model, unloading and reloading models, plastic strain equation and stress deterioration equation, was proposed to predict the cyclic compressive behavior of the FRP–concrete double tube composite columns. The proposed model was verified against the test results and exhibited good performance. • FRP–concrete double tube composite columns were tested under cyclic compression. • Ultimate axial strain of the composite column is obviously improved. • Axial load prediction equation was proposed for the composite column. • Cyclic stress–strain models were proposed for FRP-confined HSC, NC and ECC. • Cyclic load–strain model was proposed for the composite column and exhibited good performance. [ABSTRACT FROM AUTHOR]

• Database of concrete-filled steel tubes under eccentric loading was created. • Machine learning models were used to predict the ultimate strength of CFSTs. • Support Vector Machine, Random Forest, Neural Network were adopted. • Machine learning models are more accurate than methods in design codes. • Hyperparameters were optimized through random and grid search using cross-validation. Concrete-filled steel tubes (CFSTs) are popularly used in structural applications. The accurate prediction of their ultimate strength is a key for the safety of the structure. Extensive studies have been conducted on the strength prediction of CFSTs under concentric loading. However, in real situation CFSTs are usually subjected to eccentric loading. The combined compression and bending will result in more complex failure mechanisms at the ultimate strength. The accuracy of methods in design codes is usually limited due to their simplicity. In this study, three machine learning (ML) methods, namely, Support Vector Regression (SVR), Random Forest Regression (RFR), and Neural Networks (NN), are adopted to develop models to predict the ultimate strength of CFSTs under eccentric loading. A database consisting of information of 403 experimental tests from literature is created and statistically analyzed. The database was then split to a training set which was used to optimize and train the ML models, and a test set which was used to evaluate performance of trained ML models. Compared with the methods in two typical design codes, the ML models achieved notable improvement in prediction accuracy. The parametric study revealed that the trained ML models could generally capture the effect of each primary input feature, which was verified by the relevant experimental test results. [ABSTRACT FROM AUTHOR]

• Eighteen FRP-confined ECC columns were tested under monotonic and cyclic loadings. • Compressive behavior of FRP-confined ECC columns was presented and analyzed. • Existing models for FRP-confined normal concrete were evaluated for FRP-confined ECC. • Design equations on ultimate conditions were developed for FRP-confined ECC columns. • Analysis-oriented model was proposed for FRP-confined ECC and exhibited good performance. A total of 18 specimens of fiber-reinforced polymer (FRP) confined engineered cementitious composite (ECC) were tested under axial compression in this study. Both monotonic loading and cyclic loading were considered. Effects of different FRP materials, glass FRP (GFRP) tube and carbon FRP (CFRP) jacket, as well as different FRP thicknesses were investigated. All the specimens exhibited the typical strain hardening behavior, indicating the effective enhancement of compressive strength and strain of ECC under FRP confinement. Failure modes, stress–strain behavior and dilation behavior were presented and analyzed. The FRP-confined ECC showed large ultimate axial strains, demonstrating the superior deformability and ductility performance. Design equations were developed to predict the ultimate compressive strength and strain. Based on the test results obtained from this study and other test results collected in the literature, an analysis-oriented model was proposed to predict the overall compressive stress–strain behavior of FRP-confined ECC columns. [ABSTRACT FROM AUTHOR]

This paper presents experimental findings of ten full-scale blind-bolted concrete-filled steel tubular (CFST) connection tests under predominant shear loading. Previous works show that, the performance of blind-bolts with extended shank and headed nut performed significantly well under tensile loads, but research on such connections under shear loading is at scarce. In this testing programme, the performance of individual blind-bolted CFST column connection is assessed where various parameters including presence of infill concrete, bolt embedment length, tube wall thickness and concrete strength are considered. Material tests for all the elements in the connection assembly have also been conducted. The study is accompanied by assessment of international codes. It is observed that, with higher bolt embedment length, the concrete contribution can be enhanced up to 58% of the total load applied, where the load is transmitted to the concrete core by bearing, and the tube wall yielding could also be delayed. Test results also shows that, ultimate capacity of the bolts can be achieved with CFST connections under shear load. For all the connections, the failure mode was consistent to shear fracture of bolt. At the end, a modified prediction equation for blind-bolt shear resistance is proposed. The test results presented in the investigation provides scientific data for further numerical investigations and can also provide insights towards load-introduction mechanism in bolted CFST column connections. • Conducted ten full-scale experimental investigation of blind-bolted CFST connections. • Tests were carried out under predominant shear loading on single bolted connections. • Influence of extended hollo-bolt in shear load transfer to CFST columns studied. • Higher bolt embedment length led to enhanced concrete contribution under shear loads. • Full ultimate strength of single hollo-bolt can be achieved with CFST connections. [ABSTRACT FROM AUTHOR]

This paper presents an experimental programme of single anchored blind-bolted concrete filled square steel tube connections under tension to understand the joint behaviour, with emphasis on combined failure mode. A total of eight full-scale specimens were fabricated and tested. Parameters including infill concrete, length and diameter of anchored blind-bolt, tube wall thickness and concrete strength were considered. The experimental programme is accompanied by bolt preload tests and a range of material tests for all the elements in the connection. Primarily, three failure modes of the connection were identified, viz., tube wall deformation, bolt fracture and combined failure. It was observed that, apart from the beneficial effect of bolt anchorage length in enhancing the connection performance, the combined failure mode may be preferred over other failure modes for higher ductility and collapse prevention. Concrete strength is identified as the primary influential factor determining the failure modes. A component model based on spring theory is developed for prediction of global force-deformation behaviour of the bolted connection. Based on this analytical model, the strength and stiffness of such a complex connection can be predicted with good accuracy. • Experimental investigation of anchored blind-bolted CFST connections under tension • Influence of different parameters are studied and reported observed failure modes. • Concrete strength is the primary influential factor determining the failure modes. • High ductility and progressive failure of components are observed in combined mode. • Component model is presented for prediction of bolted-connection global behaviour. [ABSTRACT FROM AUTHOR]

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. • Twenty OctCFST stub column tests with compact to slender sections were carried out. • Results indicate that the strengths of both steel and concrete are fully utilised, even in slender sections. • Cross-sectional slenderness limits and capacities of OctCFSTs were discussed. • Experimental results were compared with predictions from current design codes. • Equivalent circle methods were discussed for the design of OctCFSTs. [ABSTRACT FROM AUTHOR]

• Behaviour of concrete-filled CFHSS octagonal tube stub columns was studied. • Stub column tests and numerical modelling were performed for the study. • The applicability of existing design approaches was assessed and discussed. • A design approach was proposed to accurately estimate the structural strengths. This paper presents a comprehensive experimental and numerical study on the structural performance of concrete-filled cold-formed high-strength steel octagonal tubular stub columns. Stub column specimens formed using high-strength steel and infilled concrete with grades C50 and C90, and with three different plate width-to-thickness ratios were tested. The ultimate loads, load-displacement responses and failure modes of the structures were observed and discussed. In addition to the experimental investigations, a finite element model validated using the stub column test results was developed. A series of parametric studies were subsequently conducted to obtain supplementary data for concrete-filled cold-formed high-strength steel octagonal tubular stub columns with a wide range of plate width-to-thickness ratios and different concrete compressive strengths. The applicability of existing design approaches in European and American standards and in literature provided for concrete-filled steel tubular structures with rectangular, circular or octagonal cross-sections were evaluated using the results obtained from the experimental and numerical study. The accuracy of design predictions for concrete-filled cold-formed high-strength steel octagonal tubular stub columns using different design approaches was discussed. A design approach that more accurately incorporates the strength contributions from steel tubes and concrete infill was also proposed for structural design. [ABSTRACT FROM AUTHOR]

• Investigation on tensile behaviour of hollo-bolted connections with CFST columns. • FE models have been generated for validation and conducting parametric studies. • Holistic behavior of EHB-CFST connection with varied combination is studied. • FE models can predict accurately behavior of single bolted CFST connections. • New observations that counter and complement previous findings. Achieving a strong and stiff bolted connection with concrete filled steel tube (CFST) has been a challenge to structural engineers, and therefore to enhance the connection performance, blind-bolts that are extended to anchor in the concrete core have been recently developed. Though some experimental tests to investigate the performance of extended blind-bolts were conducted, a holistic understanding of extended hollo-bolts remains to be at scarce because of certain limitations in the experimental program. In this work, the tensile pull-out behaviour of extended hollo-bolt, has been extensively investigated for its performance with CFST column connections. The study is conducted initially by validating numerical models with existing experimental works, and later by conducting extensive finite element parametric studies to predict and understand the influence of various connection components. It is observed that, not only the presence of concrete in the hollow steel tube has led to reduced deformation of the connection, but also the bolt embedment length into the concrete core has significantly improved the strength and stiffness. The study observes significant change in connection behaviour due to influence of change of parameter profiles. In this study, the various failure modes that can be altered as per combinations of the connection component strength are elaborately discussed. [ABSTRACT FROM AUTHOR]

• A study on cold-formed high strength steel tubular beam-columns was conducted. • Second order effects were considered by taking into account of the mid-height deflections. • The compression and bending capacities were assessed against predictions from the American, Australian and European standards. • Finite element modelling methodology on cold-formed high strength steel tubular beam-columns was presented. High strength steel is used more often in a variety of civil engineering applications due to its high strength-to-weight ratio and cost effectiveness. This paper presents the experimental investigation on cold-formed high strength carbon steel tubular members subjected to combined compression and bending. The nominal 0.2% proof stresses of the test specimens were 700 and 900 MPa. The test specimens consisted of square hollow sections (SHS), rectangular hollow sections (RHS) and circular hollow sections (CHS). The material properties, global geometric imperfections of the specimens were measured. The behaviour of the beam-column members was investigated through testing 32 specimens which had a nominal member length of 1480 mm. The second order effects were also considered by measuring the mid-height deflections for all specimens. The compression and bending capacities, load-deformation histories and failure modes of the test specimens were also reported. The test results were compared with the values predicted from the American, Australian and European standards. Improved design recommendation is provided for cold-formed high strength steel tubular beam-columns. Based on the experimental results, finite element modelling methodology is also proposed. [ABSTRACT FROM AUTHOR]

• Eight CFST beams with octagonal cross-section were tested. • Three concrete grades were considered in the test programme. • The test results revealed ductile behaviour of the specimens. • Test results were compared with different design provisions. • Strength and stiffness deteriorations of the specimens were discussed. This paper presents an experimental investigation into concrete-filled steel tubular beams with octagonal cross-sections (OCFST) under monotonic or cyclic flexural loading. A total of eight tests, including four cyclic specimens and four monotonic counterparts, were conducted. Three concrete grades with measured compressive cylinder strength (f c ′) varying from 54.5 MPa to 105.6 MPa were used to infill the OCFST specimens. The failure modes, ultimate bending moments, effective flexural stiffness, cumulative dissipated energy and deteriorations of test specimens were discussed. Test results indicate that OCFST beams exhibit a ductile plastic mode and excellent energy dissipation. Concrete grades seem to have limited influence on the ultimate strength and energy dissipation capacity. The comparison results of the ultimate bending moments and effective flexural stiffness between predictions using EN 1994-1-1 and AISC 360-16 and test results reveal their applicability to the design of OCFST beams. An energy-based hysteretic rule was adopted to assess the strength and stiffness deteriorations of the OCFST beams, and the results indicate that the predictions match well with the test observations. [ABSTRACT FROM AUTHOR]

The global buckling resistance of welded high strength steel box section members subject to combined compression and bending was investigated through a numerical modelling programme. Finite element models were developed with the capability to accurately replicate the experimental results of the box section members under combined compression and bending. Extensive parametric studies were carried out to examine the global buckling behaviour of welded high strength steel box section members with a wide range of dimensions and member slenderness and steel grades of S460, S690 and S960 and subject to varying combinations of compression and bending. The effects of residual stresses and ultimate tensile strength-to-yield strength ratio on the global buckling behaviour of those members were investigated. The applicability of existing design rules to welded high strength steel box section members subject to combined compression and bending was evaluated using the results from parametric studies and the available experimental results in literature. The European, American and Australian standards provide conservative strength predictions for the structures. More accurate and safe strength predictions can be obtained based on European standard using the suggested column curves for the members while the design methods in Australian and American standards are safely applicable to the members. • Welded high strength steel box tubular members under combined loading were studied. • Validated FE models were applied in parametric studies for the investigation. • Effects of residual stresses and yield-to-tensile strength ratio were discussed. • Existing design methods in standards provide conservative strength predictions. • Reliability analysis was conducted and design recommendations were proposed. [ABSTRACT FROM AUTHOR]

An experimental investigation on steel tube confined concrete (STCC) stub columns with circular, square and octagonal cross-section shapes under monotonic axial compression was conducted. Different concrete grades were examined with measured compressive cylinder strengths ranging from 38 MPa to 112 MPa. The nominal yield strength of the steel tube was 355 MPa. The load capacity, load-axial shortening curves and axial-hoop strain behaviour of the STCC stub columns are compared to that of the counterparts, concrete filled steel tubes (CFST), with different cross-section shapes based on the same material and geometric properties. Comparisons indicated that the load capacity of STCC stub columns are very close to that in the CFST with circular and octagonal cross-sections, while the performance of square STCC is not as good as the CFST counterparts due to low effectiveness of confinement. According to the experimental results of lateral behaviours of STCC, CFST and plain concrete columns, it was concluded that with very high strength concrete (cylinder strength over 100 MPa), the axial performance of STCC stub columns is better than that in CFST. A design assessment was also conducted on STCC stub columns and shows that the current design formulae for load capacity of CFST are also applicable to that of STCC stub columns. • Octagonal, circular and square STCC stub columns were tested under compression. • Effect of confinement in STCC with different cross-section shapes was discussed. • Behaviours of STCC and CFST with different cross-section shapes were compared. • Current design methods were assessed with the experimental data of STCC stub columns. [ABSTRACT FROM AUTHOR]

The continuous strength method (CSM) adopts base curves which relate the deformation capacity to its overall cross-section slenderness to take into account the element interaction and employs elastic, linear hardening material models to exploit the strength enhancement from strain hardening. This paper extends the CSM for structural design of hot-finished and cold-formed high strength steel tubular sections under bending. Results of 146 tests in the literature were compiled and a parametric study on 660 high strength steel tubular beams was conducted using validated finite element models. Base curves and resistance functions were proposed for non-slender and slender high strength steel circular hollow sections (CHS), elliptical hollow sections (EHS), square hollow sections (SHS) and rectangular hollow sections (RHS). Bi-linear and tri-linear material models were adopted for cold-formed and hot-finished steel tubular sections, respectively. Experimental and numerical results of 806 tubular beams were used to assess the proposed CSM, the direct strength method (DSM) and codified design methods. The resistance prediction of the proposed CSM is more accurate and less scattered. • Test results of 146 existing tests on CHS, EHS, SHS and RHS tubular beams were compiled. • Finite element analysis on 660 high strength steel (HSS) tubular beams was carried out. • Continuous strength method (CSM) was extended for the design of HSS tubular sections in bending. • The resistance predictions of the proposed CSM are accurate and consistent. [ABSTRACT FROM AUTHOR]

This paper proposes two types of semi-precast reinforced-concrete (RC) beams with steel connections embedded at each end. The proposed beams are designed to address the problems of complex detailing and construction difficulties between concrete-filled steel tubular columns and conventional RC beams. The proposed beams are composed of two steel I-beams partially embedded into the end sections of the RC beams. Full-scale experiments were carried out on the two proposed beams and a reference conventional RC beam to study the failure mode, crack propagation and flexural and shear strengths. Results indicated that the flexural strength of the proposed beams was well predicted by the current design codes on concrete structures including GB 50010, ACI 318-11 and BS EN 1992-1-1 (Eurocode 2). To further examine the shear behaviour of proposed beams, six experiments with two different shear span-to-depth ratios were conducted. The influence of the embedded steel connections on the shear behaviour was investigated and a correction coefficient for the shear span-to-depth ratio was proposed. Based on the current design codes on concrete structures, modified equations for calculating the shear strength were proposed. Finite-element methodology was also adopted to simulate the structural behaviour of the embedded steel connections under combined bending and shear force. Based on numerical findings, design methods for the embedded steel connections were recommended. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies were conducted to investigate the behaviours of the concrete-filled cold-formed elliptical hollow section beam-columns. A total of 11 specimens were tested to evaluate the failure modes, load-deformation histories and strains development in the steel tube. Complementary finite element (FE) models were developed and validated against experimental results. Validated FE methodology was then used to study the influence of key parameters, including aspect ratio, slenderness ratio, load eccentricity ratio, yield strength of steel, compressive strength of concrete and steel tube to concrete area ratio, on the load carrying capacity. As a result, the design method for elliptical concrete-filled steel tubular (CFST) columns in Chinese code - GB50936-2014 and the design method for circular CFST columns in EC4 were assessed to confirm their applicability for cold-formed elliptical CFST columns with aspect ratio ranging from 1.0 to 2.5. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation on the compressive behaviour of thin-walled concrete-filled steel tube (CFST) columns with encased built-up latticed angles. The additional confinement provided by encased angles was considered in the finite element (FE) model, in addition to the contact between tube wall and inner concrete and the effect of tube-wall confinement on concrete model. The established FE model is verified against the existing test results that it can well predict the behaviour of reinforced composite columns under axial compression. The composite action between encased angles and concrete and the efficiency of built-up angle configurations on performance improvement are extensively analyzed through the validated FE models. The influence of three parameters, namely confinement factor (ξ), reinforcement ratio ( ρ sr ) and slenderness ratio (λ), to the angle-contributions on lateral confining pressure and the ultimate strength are evaluated and quantified. It is found the relationship between the strengths of CFST columns and built-up member can be defined as an exponential equation. Finally the design equation based on AISC 360-10 design provisions with consideration of angle-contributions is proposed, and the comparison results show it has a desirable accuracy and satisfactory correlation with both experimental and numerical results. [ABSTRACT FROM AUTHOR]

The local buckling behaviour of cold-formed high strength steel (HSS) irregular hexagonal hollow section (IHexHS) stub columns under concentric compression is studied numerically in this paper. Finite-element (FE) models were developed and validated against the experimental data. The validated FE models were employed to conduct extensive parametric studies. The obtained numerical results together with the experimental data were used to evaluate the applicability of the existing design codes and design approaches to the cold-formed HSS IHexHS under concentric compression. The evaluation shows that the current slenderness yield limit values codified in design codes cannot be applied to cold-formed HSS IHexHS stub columns. The Eurocode 3 (EC3) and the North American code AISC 360-16 provide slightly over-estimated predictions for slender sections whereas the Australian code AS 4100 yields conservative predictions. For stocky sections, all the design codes show high degree of conservatism due to the lack of consideration for strain hardening. Furthermore, design approaches of Continuous Strength Method (CSM) and Generalised Slenderness Resistance Method (GSRM) have been found to provide more accurate predictions than the Direct Strength Method (DSM). Modified design approaches are also proposed to improve the accuracy of the strength predictions. • Numerical models for HSS cold-formed IHexHS stub columns were developed. • Extensive parametric studies were carried out. • The current design codes were assessed. • The existing design approaches of DSM, CSM, and GSRM were assessed. • Modified design codes and design approaches are proposed. [ABSTRACT FROM AUTHOR]

A systematic experimental and numerical investigation on the behaviour of octagonal concrete-filled steel tube short columns fabricated with S690 high-strength steel and C90 high-strength concrete, and subjected to combined compression and bending is reported in this paper. Experiments were firstly performed on twelve short column specimens with different plate width-to-thickness ratios. The experimental results and observations including the ultimate loads, load versus displacement responses and failure modes of the specimens were discussed. In supplementary to the experimental investigation, numerical investigation using a validated finite element model was performed to examine the behaviour of the structures with a wide range of plate width-to-thickness ratios, yield strength of high-strength steel and compressive strength of high-strength concrete that subjected to various combinations of compression and bending. Based on the experimental and numerical results, the applicability of existing design approaches in standards was also evaluated. The assessment shows that the design approaches provide conservative strength predictions for the high-strength octagonal concrete-filled steel tube short columns under combined compression and bending and can be applied to provide safe designs for the structures. • High-strength octagonal CFST short columns under combined loading were investigated. • Both experiments and numerical modelling were performed for the investigation. • The variation of compressive resistance with increasing moment was observed. • The suitability of existing design methods was assessed using the research results. • The design methods in standards can be safely applied for the structures. [ABSTRACT FROM AUTHOR]

• Three different fabrication routes were examined. • 18 HSS HexHS stub columns were tested under axial compression. • Material properties within the cross sections were measured. • The slenderness yield limits in design codes were assessed. • Design approaches of DSM and CSM were evaluated and compared. This paper presents a comprehensive experimental investigation into the structural behaviour of the high strength steel (HSS) hexagonal hollow sections (HexHS) stub columns under concentric compression. A total of 18 HSS HexHS stub columns encompassing three fabrication routes were performed. The HSS plates used to fabricate the specimen were delivered in Quenched and Tempered (QT) condition. The material properties within the cross sections of the HSS HexHS were measured. Initial local geometric imperfections for all the examined stub columns were measured. The cross-section slenderness yield limits in the existing design codes of the EN 1993-1-12, ANSI/AISC 360-16, AS 4100 and ASCE/SEI 48-11 were compared and assessed against the experimental results. It was found that the current limits for internal compression plates in HSS rectangular and square sections cannot be extended to cover the cross-section classification for HSS HexHS. In terms of the cross-sectional resistance predictions, all the design codes tend to under-estimate the resistance of HSS HexHS under axial compression. For sections failed by local buckling prior to the attainment of yield load, the AS 4100 yields more accurate predictions compared with those predictions based on the other design codes. Compared with the design approach of Direct Strength Method (DSM), Continuous Strength Method (CSM) provides more consistent and satisfactorily predictions, particularly for stocky sections due to the consideration of strain hardening. [ABSTRACT FROM AUTHOR]

• 16 Experimental tests have been performed. • Numerical models for HSS welded T-section have been developed. • Extensive parametric studies have been carried out. • Design methods from design codes, and from DSM and CSM have been assessed. This paper presents a comprehensive experimental and numerical investigation into the material properties and stub column behaviour of Q690 high strength steel (HSS) welded T-sections. A total of 16 concentrically loaded stub column tests were performed. The Q690 HSS welded T-sections were fabricated from 6 mm and 10 mm thick plates, delivered in quenched and tempered (QT) conditions by means of gas metal arc welding (GMAW). Material properties measurements were carried out on the coupon specimens taken from the parent plates. Initial local geometric imperfection measurements were conducted. In conjunction with experimental tests, finite element (FE) models were developed to replicate the test results and failure modes. Afterwards, the validated FE models were adopted to conduct parametric studies to supplement the experimental data by generating further structural performance data covering a broader range of cross-section slenderness. Cross-section slenderness limits set out in design codes such as EN 1993-1-12, ANSI/AISC 360-16, AS 4100 and design methods of DSM as well as CSM were evaluated against the experimental and numerical data. It was found that the current limits are generally accurate and safe in three design codes and the cross-section slenderness of DSM is accurate and comparably conservative. Cross-section capacities predictions obtained from EN 1993-1-12, ANSI/AISC 360-16, AS 4100, DSM and CSM were also compared with the tests and numerical results. It is shown that the established local buckling design provisions in EN 1993-1-12 and ANSI/AISC 360-16 result in more precise and consistent predictions compared with AS 4100, DSM and CSM. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical investigation on the tensile behaviour of concrete-filled double-skin steel tubular (CFDST) member. A comprehensive finite element (FE) model is established and calibrated against the test results for the CFDST members under concentric and eccentric tension. The comparison shows that the proposed FE model is capable to capture the structural behaviour of the tensile specimens. Parametric studies are then conducted to investigate the influence of some key parameters, such as the material strength, the nominal steel ratio and the hollow ratio on the tensile behaviour of composite members. The steel–concrete composite action, the effect of loading paths and the tension–bending interaction diaphragm are also discussed. It is found that the tensile strength of the member depends on the composite action between steel tubes and sandwich concrete, and the tension–bending interaction diaphragm can be represented by a simple linear relationship. Finally, the design equation to predict the tensile strength of CFDST member is proposed, and the comparison results show that the proposed equation has a good accuracy. [Copyright &y& Elsevier]

• The initiation of confinement in CFST is captured and modelled. • A confinement-related analysis model of CFST is proposed. • Reduction of confining pressure in CFST with high strength concrete is proposed. • The proposed analysis model well predicts the axial and lateral stress–strain relationships of CFST. It is well known that the effect of confinement in concrete filled steel tube (CFST) can enhance the axial compressive strength of the concrete core. The confining pressure is a key issue to determine the compressive strength of concrete while the development of confining pressure is related to the lateral responses of steel tube and concrete core. This paper firstly presents a review on the lateral behaviour of circular CFST and existing lateral-axial strain models for concrete. These models are assessed by the authors' previous experimental observations. Assessment results reveal deficiencies of current lateral-axial strain models, especially on the CFST with high strength concrete. A modified lateral-axial strain model for both normal and high strength concrete core in CFST was then proposed to accurately capture the interaction behaviour of the steel tube and the concrete. An analytical model of CFST with uniformly confined concrete core (i.e. with circular section) was then proposed to capture the development of confining pressure in CFST. A reduction factor was considered when the high strength concrete was used. The proposed analytical model can well predict the load bearing capacity, load-shortening curve, axial strain-lateral strain relationship of CFST with normal and high strength concrete. [ABSTRACT FROM AUTHOR]

With the growing applications of high strength steel in modern constructions, thin-walled steel tubes attract many attentions. Local buckling failure is the most typical failure mode in a thin-walled steel tube, and concrete filled steel tubes (CFSTs) are excellent composite members in which the occurrence of local buckling is mitigated by the infilled concrete. This paper aims to investigate the compressive strength of concrete filled thin-walled octagonal steel tubes, which have the advantages of confinement efficiency over rectangular CFSTs, and constructability over circular CFSTs. Finite element models are established and validated by the experimental results from existing literatures. A parametric study is subsequently conducted and the result shows that the buckling strength of octagonal tube is significantly improved by the infilled concrete and the enhancement percentage increases with the increase of section slenderness coefficient, and the local buckling behaviour has a minor influence to the effect of confinement. In addition, a database consisting of 37 experimental specimens and 37 FE models are developed and used to assess the design methods in current codes of practice. It is found that the design methods from ANSI/AISC 360-16 and GB 50936-2014 underestimate the load capacity of octagonal CFSTs while the design formula from EN 1994-1-1 which considers the effect of confinement overestimates the capacity. Finally, a design model considering the buckling strength of octagonal steel tube, the influence of infilled concrete core and the effect of confinement is proposed and this model can well capture the compressive strength of octagonal CFSTs. • Finite element models of thin-walled octagonal CFSTs are developed and validated. • Influence of infilled concrete to the buckling strength of octagonal steel tube is investigated. • A design model considers effect of local buckling is proposed for octagonal CFSTs. • The proposed formula can well predict the load bearing capacities of octagonal CFSTs with slender and compact sections. [ABSTRACT FROM AUTHOR]

The local buckling behaviour of high strength steel (HSS) hexagonal hollow section (HexHS) stub columns encompassing three different fabrication routes under axial compression is numerically investigated in this study. Finite-element (FE) models were developed and validated against the experimental results. The validated FE models were subsequently employed to carry out parametric studies to expand structural performance data covering a wider range of geometries and cross-section slenderness as well as two different steel grades. The obtained results from FE models in line with the experimental data were used to evaluate the applicability of the existing design codes and the design approaches in literature to HSS HexHS under pure compression. The comparison indicates that the current slenderness yield limiting values in design codes cannot be simply extended to HSS HexHS stub columns. All the design codes exhibit a high degree of conservatism due to the lack of consideration for strain hardening whereas the Continuous Strength Method (CSM) was found to provide more accurate predictions than the Direct Strength Method (DSM). Modified design approaches are proposed to improve the accuracy of the strength predictions for HSS HexHS stub columns. • Numerical models were developed and validated. • Extensive parametric studies were carried out. • The current design codes were evaluated. • The design approaches of DSM and CSM were assessed. • Modifications to design approaches are proposed. [ABSTRACT FROM AUTHOR]

This paper presents an extensive experimental investigation of the circular recycled aggregate concrete filled steel tubes (RACFST) stub columns under to axial compression to assess their cross-section behaviour. Recycled aggregate concrete is a sustainable concrete produced with recycled coarse aggregate (RAC) from demolition wastes generally used to fabricate non-structural elements. When compared with natural coarse aggregate (NCA), the recycled coarse aggregate presents lower mechanical properties due to physical and chemical changes that occurred during the recycling process, and thus the resulting RAC possess lower strength than the conventional natural aggregate concrete. However, the confinement provided by the circular steel tube can increase the strength and ductility of RAC in recycled aggregate concrete filled steel tubes (RACFST) columns. The test programme presented in this paper comprised twenty-three composite columns with replacement ratios (the mass percentages of the NCA replaced by RCA in concrete) corresponding to 0% (nature coarse aggregate), 30 and 50% (recycled aggregate concrete), plus four steel columns used for comparison. The experiments have shown that, as anticipated, the concrete core enhances the composite section load-carrying capacity. However, the control over the contents of this concrete core influences the composite section's ductility. If the fill material presents a brittle behaviour, stress redistributions occur in both steel and concrete to compensate for this effect. Since the behaviour of natural aggregate concrete filled steel tubes (NACFST) and RACFST columns are similar, the present paper starts from a study based on a comparative assessment of the recommendations presented in ABNT NBR 8800, ABNT NBR 16239, AISC 360–16, Eurocode 4 and Australian/New Zealand AS/NZS 2327 design standards. These design recommendations proved to be consistent and in line with the performed experiments. Additionally, the results indicated that the cross-section slenderness ratio (D/t) and the confinement ratio directly influence the composite section response. [Display omitted] • An experimental programme was carried out comprising recycled aggregate concrete-filled steel tubular (RACFST) columns. • Different replacement ratios of the nature coarse aggregate replaced by recycled aggregate concrete were assessed. • The steel tubes were produced using a high-frequency induction welding process minimizing the residual stress effects. • The RACFST stub columns presented similar resistances with NACFST stub columns due to confinement contribution. • EC4 and AS/NZS 2327 predictions provide a good approach to be used to RACFST stub columns' design. • NBR 8800 and AISC 360-16 presented very conservative predictions for the columns' strength. [ABSTRACT FROM AUTHOR]