Tubular Columns Research Articles

Multilinear regression model for predicting the ultimate load of slender circular CFDST columns subjected to concentric and eccentric loading

AbstractThis paper presents a novel approach to predict the ultimate load of slender circular concrete‐filled double skin tubular (CFDST) columns subjected to concentric and eccentric loading. The proposed approach was developed using a multilinear regression model through analyzing a database containing 165 CFDST column configurations. The data set was obtained by varying the magnitude of the parameters to determine its effect on the ultimate load using FE analysis. The variables considered in the model formulation are concrete strength, steel tube strengths, tube diameters, tube thicknesses, load eccentricity, column curvature, and column length. The accuracy and efficiency of the proposed models were validated with previous experimental investigations. The results demonstrate that the proposed models provide a practical, simple, and efficient approach to predict the ultimate load of slender circular CFDST columns subjected to concentric and eccentric loading. When compared to existing experimental work on slender circular CFDST columns subjected to concentric and eccentric loading, the proposed model over predicts the ultimate load by a maximum of 6%.

Finite Element Modelling of Circular Concrete-Filled Steel Tubular Columns Under Quasi-Static Axial Compression Loading

This paper presents a modified finite element analysis (FEA) model for predicting the axial compression strength of large-diameter concrete-filled steel tubular (CFST) stub columns, addressing the gap in research that has often focused on smaller diameters. The size effect, which significantly impacts the structural performance of large-diameter CFST columns, is a key focus of this study. The goal is to validate the accuracy and reliability of the modified FEA model by comparing its predictions with experimental data from the literature, specifically examining ultimate axial load capacity, failure modes, and deformed shapes. In addition to validating the model, this study includes a comprehensive parametric analysis that explores how critical geometric parameters such as the diameter-to-thickness (D/t) ratio and length-to-diameter (L/D) ratio affect the axial compressive behavior of CFST stub columns. By systematically varying these parameters, the research provides valuable insights into the load-bearing capacity, deformation characteristics, and failure mechanisms of CFST columns. Furthermore, the material properties of the steel tube—particularly its yield strength—and the compressive strength of the concrete core are investigated to optimize the design and safety performance of these columns. The results indicate that the FEA model shows excellent agreement with experimental results, accurately predicting the axial load-strain response. It was observed that as the diameter of the steel tube increases, the peak stress, peak strain, strength index, and ductility index tend to decrease, underscoring the size effect. Conversely, an increase in the yield strength and thickness of the steel tube enhances the ultimate strength of the CFST columns. These findings demonstrate the reliability of the modified FEA model in predicting the behavior of large-diameter CFST columns, offering a useful tool for optimizing designs and improving safety margins in structural applications.

Numerical analysis of axially loaded concrete-filled steel tubular columns strengthened with CFRP grid-reinforced ECC

Carbon fiber-reinforced polymer (CFRP) and engineered cementitious composites (ECC) are commonly used to reinforce structural elements, with proven effectiveness in enhancing the performance of concrete-filled steel tube (CFST) columns as shown in prior experimental studies. However, the complex interaction among the components of the reinforced columns, including CFRP, ECC, the steel tube, and the concrete core, as well as the underlying mechanical mechanisms, remain insufficiently understood. This research aims to fill these gaps by developing a 3D finite element (FE) model of circular CFST short columns reinforced with CFRP grid-reinforced ECC for comprehensive numerical analysis. Sensitivity analyses are conducted to determine the governing parameters in the numerical simulation, including mesh size and critical plasticity parameters for the concrete damage model, such as the ratio of the second stress invariant on the tensile meridian to that on the compressive meridian (Kc), and the dilation angle (ψ) for both the concrete core and ECC. The model also accounts for the effects of concrete confinement provided by the CFRP grid-reinforced ECC layer and the circular steel tube. The accuracy of the FE model is validated against existing experimental data, and the model is subsequently used to investigate the behavior of reinforced CFST columns under varying geometric and material properties. The study examines failure modes, axial load-displacement responses, stress distributions, and the contribution of each material component to the load-bearing capacity of the strengthened columns. The results indicate an increase in ultimate axial strength ranging from 13% to 20% for the reinforced CFST columns. While the compressive strength of the ECC has a moderate effect on axial resistance, increasing ECC thickness, concrete strength, and steel tube yield strength significantly enhances the columns’ load-bearing capacity. Finally, a novel design model is proposed and validated, which accounts for the confinement effects provided by the CFRP grid and ECC on the concrete core.

Compressive behavior of elliptical concrete-filled steel tubular short columns using numerical investigation and machine learning techniques

This paper presents a non-linear finite element model (FEM) to predict the load-carrying capacity of three different configurations of elliptical concrete-filled steel tubular (CFST) short columns: double steel tubes with sandwich concrete (CFDST), double steel tubes with sandwich concrete and concrete inside the inner steel tube, and a single outer steel tube with sandwich concrete. Then, a parametric and analytical study was performed to explore the influence of geometric and material parameters on the load-carrying capacity of elliptical CFST short columns. Furthermore, the current study investigates the effectiveness of machine learning (ML) techniques in predicting the load-carrying capacity of elliptical CFST short columns. These techniques include Support Vector Regressor (SVR), Random Forest Regressor (RFR), Gradient Boosting Regressor (GBR), XGBoost Regressor (XGBR), MLP Regressor (MLPR), K-nearest Neighbours Regressor (KNNR), and Naive Bayes Regressor (NBR). ML models accuracy is assessed by comparing their predictions with FE results. Among the models, GBR and XGBR exhibited outstanding results with high test R2 scores of 0.9888 and 0.9885, respectively. The study provided insights into the contributions of individual features to predictions using the SHapley Additive exPlanations (SHAP) approach. The results from SHAP indicate that the eccentric loading ratio (e/2a) has the most significant effect on the load-carrying capacity of elliptical CFST short columns, followed by the yield strength of the outer steel tube (\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:{f}_{yo}$$\\end{document}) and the inner width of the inner steel tube (\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:2{a}_{ii}$$\\end{document}). Additionally, a user interface platform has been developed to streamline the practical application of the proposed ML.

Novel Graphene‐Epoxy Composite with Aligned Architecture and Ultrahigh Thermal Conductivity

AbstractBecause of the surge in the power density of electronic devices, there is an urgent need for improvements in the thermal conductivity of packaging materials. Nowadays, epoxy composites supplemented with thermally conductive fillers are widely used for this purpose, but unfortunately, none of them can satisfactorily meet industrial requirements. Herein, this article reports on a novel method to prepare well‐shaped and highly ordered graphene‐epoxy composite architecture; that is, the epoxy infiltrates into tubular graphene columns that have been restored to the high thermally conductive structure of graphene. As a result, the newly developed graphene‐epoxy composite exhibits record‐high thermal conductivity of 69.74 W m−1 K−1 with filler content of only 11.22 wt.%. The unprecedentedly high thermal conductivity stems from the construction of thermally conductive channels and restoration of the inherent π–π conjugate structure of graphene. These findings not only offer a breakthrough in packaging materials with significantly enhanced thermal conductivity but also provide a promising avenue for the development of other advanced graphene‐added composites.

A Restoring Force Correction Model for Enveloped Steel Jacket-Confined Seismic-Damaged Rectangular Recycled Aggregate Concrete-Filled Steel Tubular Columns

In order to establish a suitable restoring force correction model for enveloped steel jacket (ESJ)-confined seismic-damaged rectangular recycled aggregate concrete-filled steel tubular (RRACFST) columns, based on experimental research, a study of the seismic performance and parameters of ESJ-confined seismic-damaged RRACFST columns was carried out. The restoring force theory, model test, and OpenSees simulation of ESJ-confined seismic-damaged RRACFST columns were conducted. Firstly, a trilinear model of the skeleton curve and a suitable restoring force model for ESJ-confined seismic-damaged RRACFST columns were established. The results were compared with the model test results, and it was found that the two results had good consistency. Secondly, the initial damage of the RRACFST column was simulated by the reducing material properties method, and a correct numerical model for ESJ-confined seismic-damaged RRACFST columns was proposed. The influence mechanism of seismic parameters of the RRACFST column was clarified. Finally, the seismic parameter combination with the best seismic performance for ESJ-confined seismic-damaged RRACFST columns was established; namely, the replacement rate of recycled coarse aggregate is 50%, the concrete strength is C40, the axial compression ratio is 0.3, the strength of the rectangular steel tube is Q345, the wall thickness of the steel tube is 4 mm, and the slenderness ratio is 7.5.

A multiaxial thermo-plasticity model for concrete in a transient fire condition

Lateral confinement and high temperature affect the load-carrying capacity of a concrete member significantly. In a fire condition, a concrete member exhibits not only mechanical deformation but also thermal deformation. Therefore, accurately modeling the fire behavior of confined structural concrete is challenging. In this paper, a new three-dimensional thermo-plasticity model for concrete is developed, taking both high temperature and confinement into account. A temperature-dependent yield surface and a non-associate flow rule are formulated. The hardening rules for concrete are proposed as a function of confinement and temperature. The load-induced strain of concrete at elevated temperatures is calculated by using a stress-triaxiality factor to consider the confinement effect. The proposed concrete model is implemented in the user-subroutine (UMAT) of Abaqus, in which Runge-Kutta-England scheme with sub-stepping is used to update stress increment. The accuracy of the proposed model is validated by experiments on concrete specimens, circular concrete-filled steel tubular (CFST) columns, double-skin CFST (DCFST) columns, and concrete-filled double steel tubular (CFDST) columns. It is shown that the developed thermo-plasticity model of concrete accurately captures the behavior of confined concrete at ambient and elevated temperatures and can be incorporated in nonlinear procedures for simulating the fire resistance of composite columns exposed to fire.

Compressive behavior of manufactured sand recycled coarse aggregate concrete-filled steel tubular short columns

To study the mechanical properties of manufactured sand recycled aggregate concrete-filled steel tubular (RACFST) columns, a total of 81 square short RACFST column specimens with different types of manufactured sand and natural sand were created and subjected to axial compression tests. The variables included sand type, core concrete strength, slenderness ratio, and steel tube thickness. The failure mode, axial load-displacement curve, axial compressive capacity, and deformation capacity of the specimens were analyzed in detail. The results indicate that the failure mode of the specimen is generally waist drum-type failure and shear-type failure, and both of them are local buckling. The load-displacement curve of the manufactured sand specimen is similar to that of the natural sand specimen, but the steel tube of the manufactured sand specimen yields later than that of the natural sand specimen. Generally, the limestone manufactured sand specimen has the highest axial compressive capacity and the best ductility ratio, while the pebble manufactured sand specimen has an axial compressive capacity similar to the natural sand specimen but a lower ductility ratio. The effects of concrete compressive strength, slenderness ratio, and steel tube thickness on specimens with manufactured sand are similar to those with natural sand. Based on the results of this experimental study, a formula for calculating the axial compressive capacity of square RACFST columns is provided.

Seismic performance on PSPC beam - concrete encased CFST column frame with a built-in reduced beam section

The concrete-filled steel tubular (CFST) column is widely used to connect with a steel beam owing to the quick dry connection and acceptable seismic performance. Therefore, a partially steel-reinforced precast concrete (PSPC) beam with an embedded H-beam was developed to combine the merits of both the prefabrication in the precast concrete beam and the quick assembly in the steel beam. In this study, the developed beam was proposed to connect the concrete encased CFST column with a reduced beam section. A half-scaled frame substructure specimen with two floors and two bays was designed and tested under the cyclic loading to investigate the seismic performance. A series of seismic properties were analyzed, including the failure phenomena, hysteretic performance, stiffness degradation, and energy dissipation capacity. The dog-bone configuration was found to benefit the beam hinge failure mechanism and an acceptable deformation capacity (the ductility coefficient up to 2.77). The function of the H-beam contributed to stable energy absorption (the equivalent damping coefficient from 0.33 to 0.37). Finally, the proposed frame was compared with a reinforced concrete frame and a steel-reinforced concrete frame through numerical analysis. The partial steel-reinforced configuration had a positive influence on the bearing capacity, hysteretic performance, and safety storage. In addition, it could improve the cost-effectiveness with little decrease in the bearing capacity.

Dynamic behavior of concrete filled steel tubular columns internally strengthened with I-shaped CFRP against vehicular bombs

Square concrete filled steel tubular columns internally strengthened with I-shaped carbon fiber reinforced polymer (SCFST-CFRP) demonstrate promising potential for application in building structures, particularly in high-risk buildings and critical structural elements, due to their exceptional static and impact resistance. High-risk buildings may be vulnerable to vehicular bomb attacks over their lifespan, resulting in blast loading. In this study, the blast resistance of SCFST-CFRP columns is investigated through field blast tests and numerical simulation. The plastic deformation of SCFST-CFRP members is low, and they exhibit promising blast resistance in explosion tests with a scaled distance of 0.07m/kg1/3. Subsequently, SCFST-CFRP columns with varying steel ratios were designed according to specifications to investigate their deformation and damage under typical vehicular bomb scenarios. It was found that the blast resistance of SCFST-CFRP columns is superior to that of ordinary SCFST columns, particularly when the steel ratio is smaller. Additionally, a protection distance of 3.95m (1.01H) is sufficient to ensure that the composite columns do not fail under these scenarios, highlighting favorable blast resistance for SCFST-CFRP columns. Based on the parametric analysis of 115 sets of data, a prediction formula for the critical failure distance of SCFST-CFRP columns under typical vehicular bomb scenarios is proposed. This study serves as a valuable reference for designing SCFST-CFRP columns with blast resistance.

Development of Circular Multicell Double-Skin Tubular Columns: Testing and Improved Axial Performance

Development of Circular Multicell Double-Skin Tubular Columns: Testing and Improved Axial Performance

Axial compressive behaviour and design of concrete-filled wire arc additively manufactured steel tubes

The axial compressive behaviour of concrete-filled wire arc additively manufactured (WAAM) steel tubular columns is investigated experimentally in this paper. Firstly, the manufacture of a series of WAAM steel plates and tubes is described. The results of tensile testing performed on coupons cut from the WAAM plates, to obtain the mechanical properties of the printed material, are summarised. 3D laser scanning was employed to generate digital models and to capture the geometric features of the WAAM steel test specimens. Concrete was then cast into the WAAM steel tubes, creating a total of nine concrete-filled steel tubular (CFST) specimens of different diameters, thicknesses and lengths that were subjected to compressive loading. The axial compressive load-deformation responses and ultimate loads of the specimens were obtained and the influence of the as-built surface undulations of the WAAM sections was assessed. Comparisons of the test results against existing structural design provisions highlight the need to consider the influence of the weakening effect of the geometric undulations that are inherent to the WAAM process on the structural response of CFST sections, in order to achieve safe-sided strength predictions.

Comparative Study of Life-Cycle Environmental and Cost Performance of Aluminium Alloy–Concrete Composite Columns

As is widely known, the construction industry is one of the sectors with a large contribution to global carbon emissions. Despite numerous efforts in the construction industry to develop low-carbon materials, there is a limited number of studies quantifying and presenting the overall environmental impact when these materials are applied in a construction project as structural members. To address this gap, this study focuses on assessing the life-cycle performance of novel structural aluminium alloy–concrete composite columns. In this paper, the environmental impacts and economic aspects of a concrete-filled aluminium alloy tubular (CFAT) column and a concrete-filled double-skin aluminium alloy tubular (CFDSAT) column were assessed using life-cycle assessment (LCA) and life-cycle cost analysis (LCCA) approaches, respectively. The cradle-to-grave system boundary is considered for these analyses to cover the entire life-cycle. A concrete-filled steel tubular (CFST) column is also assessed for reference. All columns are designed to have the same load-carrying capacity and, thus, are compared on a level-playing basis. A comparison is also made of the self-weight of these columns. In particular, the self-weight of the CFST column is reduced by around 17% when the steel tube is replaced by an aluminium alloy tube, and decreased by 47% when the double-skin technique is adopted in CFDSAT columns. The LCA results indicate that the CO2 emission of CFST and CFAT is almost the same, which is 21% less than the CFDSAT columns due to the use of high aluminium in the latter. The LCCA results show that the total life-cycle cost of CFAT and CFDSAT columns is around 29% and 14% lower, respectively, than that of the CFST column. Finally, a sensitivity analysis was carried out to evaluate the effects of data and assumptions on the life-cycle performance of the examined columns.

Performance of self‐compacting alkali‐activated slag concrete‐filled cold‐formed steel tubular stub columns

AbstractThis study comprehensively investigates the self‐compacting alkali‐activated slag concrete‐filled cold‐formed steel tubes (SACFST) stub columns under axial compression. The innovative combination of alkali‐activated slag concrete (AASC) with concrete‐filled steel tubes (CFST) enhances both sustainability and structural performance. AASC, known for its eco‐friendly benefits compared to conventional concrete, was developed using industrial wastes, contributing to green construction practices. Based on Taguchi's L‐9 orthogonal array, nine mixes of self‐compacting alkali‐activated slag concrete (SASC) were developed, achieving impressive flowability exceeding 700 mm and compressive strengths ranging from 48 to 69 MPa. Additionally, split‐tensile strengths between 3.8 and 5.7 MPa, flexural strengths from 6.3 to 8.2 MPa, and a modulus of elasticity between 27.6 and 32.9 GPa were observed. Nine circular CFSTs containing SASC mixes were tested under axial compression, and a finite element model was developed to simulate their results. The experimental results were compared with standards from ACI 318, AIJ‐97, AISC‐360, AS 5100, and EC‐4, to evaluate the accuracy of axial capacity predictions. Among these standards, EC‐4 and AS 5100 provided the closest estimates, with mean PTest/PEC4 and PTest/P5100 ratios of 1.03 and 1.04, respectively, indicating a high level of accuracy. These findings highlight the potential of SACFST in advancing sustainable and resilient construction practices, laying the groundwork for future applications in structural engineering.

Residual behaviour and damage assessment of UHPC-filled double-skin steel tubular columns after lateral impact

Ultra-high performance concrete (UHPC)-filled double-skin steel tubular (DST) column has great potential to be used in the protective structures. Although its lateral impact behaviour has been well understood, the residual behaviour after lateral impact remains unexplored. As a result, this study extensively investigated the residual behaviour and damage assessment of UHPC-filled DST columns after lateral impact. Firstly, a set of six DST columns were designed and tested under lateral impact, followed by static axial compression. In addition, two intact columns were subjected to static axial compression for comparative analysis. Secondly, the refined finite element models were developed and validated using the current test data, and the impact resistant mechanism of UHPC-filled DST columns with different impact locations was analysed. Thirdly, the suitability of different damage indexes for the damage assessment of impacted UHPC-filled DST columns was evaluated. Two damage indexes, the ratio of mid-height deflection to column height (R1), and the ratio of local deflection to the column diameter (R2), were proposed for the DST columns. Finally, two types of machine learning-based models were developed to predict the impact damage of UHPC-filled DST columns. The prediction models were interpreted locally and globally using the additive feature attribution method Shapley Additive Explanation (SHAP). The machine learning-based prediction models can rapidly evaluate the damage extent of impacted UHPC-filled DST column, which hold great significance for the selection of strengthening and retrofitting schemes.

Axial hysterestic behavior of prestressed CFDST columns for lattice-type wind turbine towers

The escalating power outputs of wind turbines necessitate enhanced load-bearing capabilities in their support structures. A new type of prestressed concrete-filled double skin steel tubular (CFDST) lattice-type wind turbine tower has been proposed to replace the original steel-concrete hybrid tower. The corner columns of the tower are made of prestressed CFDST columns, with the prestressing steel strands situated within the hollow area. While numerous studies have researched the axial characteristics of concrete-filled steel tubular (CFST) columns, investigations into prestressed CFDST columns subjected to axial cyclic loading remain sparse. To address this research gap, this study carried out the experimental and finite element studies of eight prestressed CFDST columns under axial tensile, axial compressive, and tensile-compressive cyclic loads. Detailed analyses of failure modes, hysteresis curves, stiffness degradation, skeleton curves and ductility were conducted. The test results indicate that the prestressing enables the concrete to establish good contact with the steel tubes, thereby preventing the premature cracking. At a cost of approximately 6.8 % reduction in axial compressive load, the axial tensile load of the structure is enhanced by about 47.2 %. Furthermore, an advanced finite element (FE) model, refined based on the test, closely matched the experimental data, thereby validating its accuracy for subsequent mechanism and parameter investigation.

Influence of in-plane transverse compression on the behaviour of the face plate component in weak axis and tubular steel joints

The behaviour of the face-plate component under out-of-plane loading has been previously studied, and analytical formulations characterizing the initial stiffness have been proposed. However, research investigating the response of this component under simultaneous transverse loading is limited. This study examines the influence of in-plane transverse compression on the out-of-plane behaviour of the face-plate component in weak axis and tubular beam-to-column steel joints. An experimental test campaign was conducted, covering five representative joint configurations for connections to both I-section and tubular columns. Finite element models of the tested specimen were generated and validated based on the test results. Subsequently, these results were supplemented by two sets of numerical parametric studies, encompassing a total of 329 connections under different levels of transverse loads. The results indicate the detrimental effect of transverse compression on the stiffness and resistance of the face-plate component. Analytical expressions for the reduction of initial stiffness under compressive in-plane transverse loads were proposed and validated, facilitating easy incorporation into design standards.

Prediction of the Strength of the Concrete-Filled Tubular Steel Columns Using the Artificial Intelligence

Introduction. The machine learning algorithms are highly promising for predicting the load-bearing capacity of the building structures. The paper aims at building the predictive models for calculating the strength of the concrete-filled steel tubular (CFST) columns to enable a highly accurate prediction of the ultimate loads for the entire possible range of parameters affecting the load-bearing capacity of the eccentrically compressed columns.Materials and Methods. The article studies the eccentrically compressed short concrete-filled steel tubular (CFST) columns of circular cross-section. Model input parameters: column outer diameter, pipe wall thickness, yield strength of steel, compressive strength of concrete, relative eccentricity. Output parameters: the ultimate loads without taking into account and taking into account the random eccentricities. The models were trained on synthetic data generated based on the theoretical principles of the limit equilibrium method. Two machine learning models were built. When training the first model, the ultimate loads were determined at a given eccentricity of the longitudinal force without taking into account the additional random eccentricity. When training the second model, the additional random eccentricity was taken into account. The effect of the features on the model predictions was assessed using the Feature Importance function. The Optuna method was used to select the hyperparameters. The machine learning models were implemented in the Jupiter Notebook environment using the Gradient Boosting learning method. The total volume of the training sample was 179 025 samples.Results. The importance of the features most affecting the predictive values of the model have been determined. For both models, the outer diameter of the column and the relative eccentricity have proved to be the most important features, which is consistent with the existing experience of designing and calculating such structures. Optimisation of the hyperparameters using the Grid Search method enabled getting the improved results. The high accuracy of prediction has been ascertained by the low values of the regression metrics: MSE = 9.024; MAE = 9.250; MAPE = 0.004 — for the model built without taking into account the additional random eccentricity; MSE = 8.673; MAE = 8.673; MAPE = 0.004 — for the model built taking into account the additional random eccentricity.Discussion and Conclusion. The developed Gradient Boosting models for predicting the ultimate loads of the eccentrically compressed short concrete-filled steel tubular (CFST) columns of circular cross-section, both without taking into account and taking into account the additional random eccentricities, have demonstrated high accuracy and stability of prediction, they can be applied for assessing the strength of the columns during design and construction, which will reduce the time and resources involved in physical testing. In the future, it is planned to expand the data range by including other materials, different cross-section geometries of the columns and a slenderness parameter, which may improve the generalization ability of the model.

A unified confinement-based direct design approach for novel double-skin composite columns

Concrete-filled stiffened steel tubular short columns, with cold-formed stiffened steel tubes for the outer section, have been widely used in Asia in large commercial facilities and warehouses. This paper focuses on the response of concrete-filled double-skin stiffened steel tubular (CFDSST) short columns and concrete-filled dual stiffened steel tubular (CFDST) short columns, with either square or circular inner tubes. A detailed database of existing information is assembled and presented, as well as a thorough analysis of existing design expressions. Additional examination of the concrete confinement has been undertaken which highlight the differences in behaviour between CFDSST and CFDST cross-sections. The major aim in this work is to provide a unified design approach, which accounts for concrete confinement and accounts for the material and geometric properties of the section, and offers improved accuracy over the existing approaches given in the European, American, British and Chinese design codes. The newly proposed unified confinement-based direct design approach for axially-loaded CFDSST and CFDST short columns is discussed, including the consideration given to the lateral confining pressure provided by the steel tubes. This method is shown to provide more accurate depictions of the load resistances of CFDSST and CFDST short columns relative to the international specifications, with lower scatter and greater range of applicability. It has also been shown that the CFDSST sections with a stiffened outer steel tube provides greater confinement and additional strength compared with equivalent unstiffened circular tubes of high relative tube slenderness. Given the easier fabrication and installation of beam-to-square column joints compared with circular columns, this is likely to extend their range of application in the future.

Group resistances of tensile-loaded anchored-bolted connections with CFST columns: Tests, simulations, and theoretical models

The performance of the connections in concrete-filled steel tubular (CFST) columns fabricated with a group of two anchored blind-bolts under tensile forces is evaluated. Based on the bolt group arrangement, two sets of experiments, comprising eight full-scale specimens, were tested. Test parameters include the presence of concrete, bolt anchorage length, pitch distance, and gauge distance. Further, numerical models were developed, which, after validation, were used for parametric studies. Key findings indicate that bolt gauge distance, anchorage length, concrete grade, and column cross-section slenderness significantly influence the connection behaviour, but pitch distance has the least influence. Tube and concrete failure modes, as well as critical gauge and pitch distances, are identified, which govern tube deformation and concrete crushing zones. Finally, a bi-linear theoretical model is developed that can provide a fair prediction of the connection strength and stiffness.