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This paper presents a reusable steel column base connection in moment resisting frame (MRF), which involves simple yet effective pinned steel plates to dissipate seismic energy and makes full use of the self‐weight of the column to provide re‐centring force. Permanent deformations of dissipative plates may induce non‐negligible lateral drifts of the column. By virtue of the pinned design, additional rotational degrees of freedom are added to mitigate the resistance to the restoring behaviour. Based on the proposed connection, the column can rock back to its original position after earthquake‐generated deformations while remaining within the elastic range. Moreover, the proposed connection can be conveniently installed and replaced to facilitate adaptability and reusability. The mechanical performances of the proposed column base connection under monotonic and cyclic loadings are described by simplified analytical models. A refined three‐dimensional finite element (FE) model is developed. The results of FE modelling help verify the feasibility of the simplified analytical models. Besides, the self‐centring capacity, energy dissipation capacity and low damage behaviour of the proposed connection are then evaluated by the FE models. [ABSTRACT FROM AUTHOR]

An innovative composite column, which consists of a high strength concrete (HSC) core, engineered cementitious composites (ECC) ring, and fiber-reinforced polymer (FRP) tube, has recently been developed and subjected to monotonic axial compression testing by the authors. In this study, the cyclic compressive behavior of this proposed FRP-ECC-HSC composite column was examined. Test parameters, including HSC core strength, FRP tube thickness, and ECC ring thickness, were investigated. Typical failure modes, dilation behavior, and axial load versus axial strain behavior were discussed and analyzed. It was found that the FRP-ECC-HSC composite columns exhibited improved deformability, compared with the counterpart traditional FRP-confined HSC columns, with the ultimate axial compressive strain increased by 0.7%–69.1% for the tested specimens. Meanwhile, the ultimate axial strain for cyclically loaded specimens is larger than that for monotonically loaded specimens in general, indicating a delayed column failure. Cyclic axial load-axial strain models, including the envelope model, unloading and reloading models, plastic strain equation, and stress deterioration equation, were proposed to predict the cyclic compressive behavior of the tested specimens. The proposed model was verified with the test results and exhibited good performance. [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]

A novel composite column, consisting of an outer fiber reinforced polymer (FRP) tube, an engineered cementitious composite (ECC) ring and an inner high strength concrete (HSC) core, has been proposed and experimentally investigated in this study. Due to the high brittleness of HSC, localized cracks may occur and lead to premature failure for conventional FRP-confined HSC columns. With the excellent tensile and cracking behavior, ECC ring is used to redistribute the hoop stress and strain from HSC core to FRP tube in the proposed novel FRP-ECC-HSC composite column. A total of 12 stub columns with different HSC core strengths and ECC ring thicknesses were tested under axial compression. It is found that FRP-ECC-HSC composite columns can develop larger FRP confining efficiency with more uniform hoop strain distribution in comparison to the corresponding normal FRP-confined HSC columns. The ultimate axial strain is obviously enhanced as well for this composite column, leading to an improved ductile compressive behavior. Based on the test results obtained from this study, design equations are proposed to predict the ultimate loading capacity and ultimate axial strain for the FRP-ECC-HSC composite column. [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]

A novel glass fiber-reinforced polymer (GFRP) — concrete double tube composite column, which consists of an outer filament winding GFRP tube, an inner pultruded GFRP tube and infilled core concrete and ring concrete, is proposed in this study. A total of 20 specimens were tested to investigate the structural behavior of the composite column. High strength concrete (HSC) was used as the core concrete filled in the inner pultruded GFRP tube, while engineered cementitious composite (ECC) or normal concrete (NC) with medium compressive strength was used as the ring concrete. Different outer and inner GFRP tube thicknesses were considered. Test results reveal that overall performance of the GFRP-concrete double tube composite columns, especially the deformability, is effectively enhanced in comparison to the corresponding normal GFRP-confined HSC columns. Axial load–strain responses and dilation behavior of the composite column were carefully analyzed. Based on the test results, equations are developed to predict the ultimate load carrying capacity and ultimate axial strain for the proposed GFRP-concrete double tube composite column. • A novel GFRP-concrete double tube composite column was proposed and investigated. • Columns with thicker inner pultruded GFRP tube can develop larger load capacity. • Inner pultruded GFRP tube and ECC ring lead to more uniform hoop strain distribution. • Deformability of the proposed composite column is obviously enhanced. • Equations are proposed to predict the ultimate load carrying capacity and axial strain. [ABSTRACT FROM AUTHOR]

This article provides an in-depth review on the understanding of load transfer mechanism in concrete-filled steel tubular (CFST) columns, with emphasis on tube-concrete interface behaviour, secondly, load transfer to encased concrete by bond strength, and lastly, load transfer by blind-bolt bearing. This article summarizes the diverse observations on obtaining a reliable bond strength between infill concrete and steel tube of CFSTs, as this is the key component that ensures the load transfer from the beam to the column. After assessing the bond strength, the identification of the zone for load introduction from fin-plate connection to CFST column is pertinent, as it is assumed to occur through bond strength via an introduction length, and studies on this aspect have been reviewed in detail. The review further extends to understanding of load transfer through bearing mechanism of blind-bolts, and provides an overview of the current developments in blind-bolted connections, which might overcome some of the challenges related to uncertainties in load transfer through bond strength. The article aims to highlight the authors' reflections on exploring and connecting the above-stated three sub-topics related to load transfer in CFST columns, discussing the challenges, and concludes by presenting the avenues for futue research. • Comprehensive review on load transfer mechanisms in CFST column is conducted. • Analysis of CFST bond strength from wide range of observations and data is made. • Current understanding of load introduction mechanism in CFST is presented. • Load transfer by bolt bearing mechanism and its systematic development is reviewed. • Future research avenues for enhanced composite action in CFST column is presented. [ABSTRACT FROM AUTHOR]

It has been known that thin plate can carry loads considerably more than the buckling loads. This phenomenon is defined as the post-buckling behaviour, which is affected by material properties, geometric dimensions and boundary conditions of the plate. Hybrid I-section is a design concept where flanges are made of high strength steel and web is made of lower steel grade. In order to investigate the effect of slender web on the local buckling and post buckling of hybrid I-section under pure compression, experimental studies of twelve fixed-ended I-section stub columns with slender web were conducted. Tensile coupon tests and initial local geometric imperfection measurement were carried out before stub column tests. Experimental and theoretical methods for determining the critical local buckling load of plate elements were discussed, followed by a post-test finite element analysis to investigate the effects of stub column length and sectional steel combination on the buckling behaviour. Based on the experimental and numerical results, it was observed that for specimens which web plate failed by elastic local buckling, the ultimate test loads are affected by the web strength grade, indicating that the unbuckled portion of slender web plate still provides the post-buckling strength for I-section stub columns even after yielding. By comparing the existing design methods with the numerical ultimate loads, it was found that the Eurocode 3 gives the less scattered predictions than the North American code, Australian code and Chinese code, as well as the existing direct strength method and continuous strength method. • 12 I-section stub column tests with slender web are conducted. • Three steel grades are considered for the web plate of I-section. • Critical local buckling load of plate elements is evaluated. • The current codified design methods are assessed. • The existing direct strength method continuous strength method are assessed. [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]

• 15 hybrid welded T-section stub columns were tested under axial compression. • Material properties and residual stresses were measured. • Numerical models were developed. • Extensive parametric studies were carried out. • Design codes and design approaches of DSM and CSM were evaluated and compared. Comprehensive experimental and numerical investigations on hybrid T-section stub columns under compressive load were performed and are presented in this paper. The hybrid welded T-section stub columns comprise high strength steel Q690 flange and normal strength steel Q355 web. Material property studies were implemented through tensile coupon tests. Residual stress in membrane type was measured and recorded and its impact on structural performance was also assessed. Initial local geometric imperfection measurements were conducted for all the specimens. In conjunction with experimental studies, numerical investigation was performed to generate more test specimens extending the range of cross-section dimensions by which test data pool can be supplemented. The Eurocode of EN 1993-1-1, EN 1993-1-5 and EN 1993-1-12, the North American code of ANSI/AISC 360-16 and the Australian code of AS 4100 together with design approaches of Direct Strength Method and Continuous Strength Method were examined for its applicability for cross-section classification and compression resistance predictions of hybrid T-sections. Overall, the current specified limits were applicable for hybrid welded T-section and strength predictions from design codes were comparatively consistent and reliable. Strength predictions from Direct Strength Method and Continuous Strength Method were relatively appropriate though overly conservative predictions were provided for cross-section with large slenderness. In this study, modifications on the Direct Strength Method and Continuous Strength Method are proposed, which is shown to provide accurate predictions at cross-sectional level in a reliable manner. [ABSTRACT FROM AUTHOR]

Comprehensive experimental and numerical investigations on mechanical properties of Q690 and Q460 high strength steel (HSS) sections and stub column behaviour of the 14 hybrid HSS welded T-section stub column specimens are presented in this paper. A total of six coupon specimens were longitudinally and transversely machined from HSS Q690 and Q460 steel parent plates respectively. Furthermore, initial local geometric imperfections measurements were also carried out. In parallel with the experimental investigation on stub column behaviour, extensive numerical studies were carried out by developing finite element (FE) models to replicate the experimental tests results. The validation of the FE models was confirmed against the obtained experimental data. To supplement the experimental data, validated FE models were used in the parametric studies, by which numerical specimens with a larger range of cross-section dimensions can be generated. Subsequently, cross-section classification slenderness limits for outstand plate elements under compressive load set out in structural steel design codes of EN 1993-1-12, ANSI/AISC 360-16, AS 4100 were evaluated against the experimental and numerical data. In addition to the design codes, design methods such as direct strength method (DSM) and continuous strength method (CSM) were also assessed to evaluate the applicability and suitability for hybrid HSS welded T-section stub columns. It was found that the current codified slenderness limits in EN 1993-1-12, ANSI/AISC 360-16 as well as AS 4100 for slender/non-slender outstand flat elements can generally be adopted for hybrid HSS welded T-section under compression. CSM provide under-estimated compressive resistance in comparison to the design codes and DSM for slender sections. • 14 experimental tests of hybrid HSS welded T-section stub columns have been performed. • Numerical models for hybrid HSS welded T-section stub columns have been developed. • Extensive parametric studies have been carried out. • Design codes as well as design approaches of DSM and CSM have been assessed. [ABSTRACT FROM AUTHOR]

• 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]

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]

As inspired by the concept of modular integrated construction, component-assembly-structure, it leads to a great opportunity to reassess the basic structural elements made by an uncommon manufacturing process. Rectangular hollow sections (RHS) can be fabricated by tip-to-tip welding two press-braked channels together to form an integrated cross-section. This unique technique gains more flexibilities and economic efficiencies regarding the setup of production equipment as compared to the mainstream manufacturing process. In this paper, a comprehensive investigation into press-braked RHSs comprising material tests, residual stress measurements, stub column tests, and numerical studies is presented. A total of 40 tensile coupons, 108 sectioned residual stress strips, 10 stub columns, and 290 numerical models were combined to investigate various characteristics of press-braked RHSs. Of the results observed, the strength enhancement of corner materials due to the cold-working can be conservatively captured by the predictive model from AISI S100-16. A simplified predictive residual stress pattern is proposed to represent residual stress distributions of press-braked RHSs. The applicability of Eurocode 3, ANSI/AISC 360-16, and the direct strength method (DSM) for the design of press-braked RHS stub columns under compression was evaluated based on the experimental and numerical results. And modified design approaches were also proposed and shown to have a more accurate, consistent, and safe prediction of compressive resistance for press-braked RHSs. [Display omitted] • Cold-formed rectangular hollow sections of Q355 and Q460 steels were fabricated through press-braking. • Material properties and residual stresses of press-braked RHS were measured. • 10 press-braked RHS stub columns were tested under axial compression. • Finite element analyses on press-braked RHS stub columns were carried out. • Design methods in EN 1993-1-1 and direct strength method (DSM) were evaluated. [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]