Compound concrete is produced by mixing fresh concrete with demolished concrete lumps. This study focuses on compound concrete-filled stainless steel tubular (CCFSST) sections, for which experimental data are scant. A comprehensive experimental investigation looking into the axial compressive behaviour of square CCFSST sections is presented in this paper. A total of 17 specimens was tested under uniform axial compression, and detailed observations from the tests are reported. The experimental failure loads were used to assess the applicability of existing design provisions for composite members specified in Eurocode 4 (EC4), the American Specification (AISC 360-22), and Han's method to the studied CCFSST cross-sections. The results indicate that the existing design provisions produce conservative and highly scattered capacity predictions, with EC4 and AISC 360-22 exhibiting particularly pronounced conservatism. Modifications to EC4 have been proposed by incorporating the Continuous Strength Method (CSM) to account for the strain hardening behaviour of the stainless steel tubes. Overall, the modified design provisions demonstrated enhanced accuracy and reduced conservatism for the design of studied CCFSST cross-sections.

Structural performance of cold-formed high-strength steel (CFHSS) square and rectangular hollow section (SHS and RHS) columns at elevated temperatures up to 1000 degrees C was investigated in this study. A finite element model was developed and verified against prior test results of cold-formed steel SHS and RHS columns at ambient and elevated temperatures. Upon validation, a parametric study was conducted using constitutive models developed for CFHSS tubular members of S700 and S900 steel grades at various temperatures. A total of 700 CFHSS tubular columns of S700 and S900 steel grades were studied, covering various cross-sectional classes and member slenderness ratios. Applicability of the existing design rules given in Eurocode 3, AISC 360, and AISI S100 specifications was evaluated for the investigated columns at elevated temperatures. Overall, it has been inferred that the Eurocode 3 and AISC 360 design provisions can be used to predict the ultimate strengths of the investigated CFHSS tubular columns at elevated temperatures, whereas the DSM provision given in AISI S100 yielded unconservative predictions. Therefore, a modified DSM design rule has been put forward, and reliability assessments were conducted to evaluate the safety levels of the current and proposed design provisions.

The structural performance of concrete-filled double skin tubular (CFDST) beams is examined in this study. Stainless steel rectangular hollow sections (RHS) were used as the outer tubes, while steel circular, square and rectangular hollow sections (CHS, SHS and RHS) were used as the inner tubes. The inner tubes were eccentrically positioned in the tensile zone of the cross-sections to enhance the bending moment capacity. A total of 30 CFDST specimens was tested under three-point bending. Three levels of inner tube eccentricity were investigated. The nominal compressive strength of the infilled-concrete ranged from 40 to 120 MPa. The full moment-deflection responses, bending moment capacities and failure modes of the investigated CFDST beams are reported and discussed herein. It was observed that increasing the inner tube eccentricity enhanced the bending moment capacity by up to 18% and increasing the concrete compressive strength from C40 to C120 led to a maximum improvement of 50% in bending moment capacity. Additionally, the experimental bending moment capacities were compared against predictions determined from current European, American and Chinese standards for composite carbon steel members. It was shown that the existing design provisions yielded conservative and scattered predictions for the investigated CFDST beams. Developments required in the current design provisions in relation to their application to the studied cross-sections include improved allowance for the strain hardening of stainless steel, concrete confinement and shear-bending interaction. These factors are to be addressed in future research.

The durability of carbon fibre-reinforced polymer (CFRP) strengthened steel structures is a critical issue affecting their long-term reliability in practical engineering applications. However, the degradation behavior and durability of CFRP-strengthened circular hollow section (CHS) joints have not been quantified. This study proposed a numerical analysis method for assessing the durability of CFRP strengthened CHS joints based on experiments. First, durability tests in a neutral salt spray environment and tensile failure tests were conducted on the adhesive and CFRP sheets to obtain moisture diffusion parameters and establish mechanical performance formulae. Static tests were conducted on unaged and aged CFRP-reinforced CHS T-joints. Then, FE models of bare and CFRP-reinforced CHS joints were developed. In the models of CFRP-reinforced joints, mechanical property degradation of the CFRP composite layers and the CFRP-to-steel bonding interfaces were considered. FE models of CFRP composite layers and the bonding interface were developed for mass diffusion analysis. A coupled diffusion-mechanical analysis program was proposed to predict the joint’s durability year, which was verified by experimental results. Finally, the effects of interface moisture diffusion coefficient, CFRP length, and number of CFRP layers on the durability year were analyzed. Suggestions were proposed to improve the durability performance of CFRP-reinforced CHS joints. The proposed method provides an effective tool for predicting the long-term performance of CFRP-reinforced CHS joints and supports durability design.

Compound concrete (CC) can be cast by mixing fresh concrete with coarsely crushed demolished concrete lumps that have been obtained from a demolished structure. A significant challenge for the broader application of CC is the lower compressive strength and ductility stemming from the weak interface between the demolished concrete lumps and the fresh concrete. However, the use of CC in Concrete-Filled Steel Tubes (CFSTs) has the potential to mitigate this issue due to the lateral confinement provided by the steel tube. Therefore, an innovative composite cross-section, i.e., compound concrete-filled steel tubular section, is proposed and investigated in this study. The research project involved the compression testing of 22 circular CFST stub columns, employing two cross-section dimensions (244.5 x 6.3 mm and 244.5 x 5.0 mm) in combination with a range of CC mixes. The CFST experimental failure loads were shown to be strongly correlated with the values of the CC compressive strengths. Furthermore, the applicability of the design rules for conventional CFST members in the European Code (EC4), the American Specification (AISC360) and Han's method are evaluated by comparing the experimental failure loads to the axial compressive capacity predictions.

Corrosion of reinforced concrete (RC) columns, particularly in marine environments, threatens structural integrity by reducing load-bearing capacity and heightening the risk of sudden failure. Chloride ions and carbonation expedite degradation, leading to cracking, spalling, and fragile rust layers that compromise both the concrete cover and the steel reinforcement. Traditional retrofitting methods often prove inadequate, underscoring the need for innovative solutions. This study evaluates the effectiveness of externally bonded reinforcement (EBR) systems, particularly Fiber-Reinforced Polymer (FRP) and Fabric-Reinforced Cementitious Matrix (FRCM) jacketing systems (including Engineered Cementitious Composites (ECCs) and Ultra-high-performance concrete (UHPC)) in strengthening corroded RC columns. A comprehensive experimental database was compiled from eighteen peer-reviewed studies, encompassing approximately 300 specimens subjected to up to 35

The behavior of cold-formed stainless steel (CFSS) square and rectangular hollow section (SHS and RHS) stub columns at elevated temperatures is investigated. A total of 19 specimens, cold-rolled from austenitic and lean duplex stainless steel (ASS and LDSS) strips, were tested at various temperatures up to 800°C. A finite element (FE) model was developed and validated against the obtained test results. Upon verification, a parametric study covering 288 FE analyses was conducted. The obtained test and FE results were used to examine the suitability of current yield slenderness limits for the CFSS SHS and RHS at elevated temperatures. The applicability of existing design provisions in ASCE/SEI 8, AISC 370 and EN 1993-1-2 for strength predictions of the CFSS tubular stub columns at elevated temperatures was also evaluated. Reliability analysis was performed to assess the reliability levels of the evaluated design rules. Overall, it is shown that ASCE/SEI 8 provisions cannot be applied for the ASS and LDSS stub columns tested at elevated temperature conditions and it also overestimated the resistance of the ASS specimens at ambient temperature. A modified approach was therefore proposed and shown to provide conservative and reliable strength predictions for the studied stub columns. AISC 370 and EN 1993-1-2 provisions provided conservative and reliable strength predictions for the stub columns at elevated temperatures.

Concrete-filled stainless-steel tubes (CFSSTs) exhibit good resistance to compressive loading and possess a high level of corrosion resistance. CFSSTs can be cast using compound concrete (CC) by incorporating coarsely crushed demolished concrete lumps (DCL). The inclusion of DCLs in CFSST members reduces the demand for Portland cement and natural aggregates, which provides clear environmental benefits. CFSST members incorporating DCLs, for which there are currently little experimental data, are the focus of the present study. A comprehensive experimental investigation into the axial compressive behaviour of rectangular CFSST sections is presented in this paper. A total of 17 rectangular CFSST stub columns were tested under axial compression. The influence of the key parameters, including the strengths of DCLs and fresh concrete, as well as the replacement ratio, on the axial compressive behaviour of CFSST sections were investigated. The axial capacities of the investigated CFSST members are predicted using the European Code (EC4), American Specifications (AISC360-22) and Han's method. The applicability of the considered design codes has been assessed by comparing the predicted axial capacities to the experimental failure loads of the CFSST members.

A comprehensive numerical investigation looking into the static structural behaviour and design of cold-formed steel elliptical hollow section X-joints (CFS-EHS-XJs) is reported. Detailed finite element (FE) models were developed for three distinct brace-chord configurations, incorporating cold-forming effects and heat-affected zones. The numerical models were validated against experimental results of 30 specimens through comparisons of load-deformation responses, joint resistance and failure mode. Parametric analyses encompassing 199 CFS-EHS-XJs were performed to examine the influence of key geometric parameters on joint static behaviour. The combined experimental and numerical dataset of 229 joints was used to evaluate the existing design provisions, including CIDECT and Eurocode 3 methods with equivalent circular and rectangular section approaches, and Wardenier's EHS joint design method. These comparisons revealed significant scatter and inaccuracy in joint resistance predictions using the current design methods. A new design methodology based on the ring model theory has been proposed and calibrated against the developed dataset. The proposed approach demonstrates superior accuracy and consistency in predicting CFS-EHS-XJ resistances compared to existing design methods.

Elliptical hollow sections (EHS) have recently gained significant interest due to their aesthetic appeal and promising structural performance. Despite their growing popularity, there remains a lack of dedicated design provisions and limited research on their structural behaviour, especially concerning tubular joints. This paper presents a numerical investigation into the static structural behaviour and design of cold-formed steel elliptical hollow section T-joints (CFS-EHS-TJs) under brace axial compression. Complementing previous experimental investigation on CFS-EHS-TJs, finite element models were meticulously developed for three T-joint configurations, considering cold-forming effects and heat-affected zones. These models were validated with existing test data of 31 T-joint specimens, by duly focusing on load-deformation responses, failure modes and joint strengths. A parametric study, involving 200 CFS-EHS-TJs, was conducted to assess the impact of critical geometric parameters on joint performance. The combined experimental and numerical dataset, encompassing 231 T-joints, was used to evaluate current design rules, indicating significant discrepancies in strength predictions. Reliability analysis was also conducted. A new design methodology was proposed, offering more consistent and reliable strength predictions for the investigated CFS-EHS-TJs.

An innovative demountable tapered iron bolt shear connector (TIBC) for steel-concrete composite beams has been proposed and tested by push-out test in previous research. In this study, finite element (FE) model of push-out test specimens using TIBC has been developed to further investigate the load transfer mechanism. The accuracy of the proposed FE model is validated by comparing FEA results with test results. Parametric study has been performed using the verified FE model. Effect of parameters including diameter and length of bolt, material strength of bolt and tapered iron plug (TIP), size of TIP as well as compressive strength of concrete on the shear capacity and load-slip relationship of push-out specimens are investigated. It was found that the size of the tapered iron plug must be proportionate to the bolt diameter to avoid the concrete crush failure. It is also shown that the aspect ratio of bolt and the yield strength of the TIP have minor impact on the load-slip relationships of TIBC and the concrete damage. Critical size of the TIP has been proposed for two different failure models, namely concrete crushing failure and bolt shear failure. Practical design recommendations in algebraic form have been proposed based on the FEA results. Additionally, a severe damage criterion for concrete in steel-concrete beam has been proposed.

This study investigated the durability of CFRP-to-steel bonded joints. First, durability tests were conducted on adhesive, CFRP sheets, and CFRP-to-steel double-lap joints in a neutral salt spray environment. Three types of influencing factors were considered in the joints, including steel surface treatment methods, number of CFRP layers, and types of CFRP sheets. The salt spray exposure durations were set at 500 h, 1000 h, 1500 h, and 2000 h. Subsequently, tensile tests were conducted on adhesive, CFRP sheets, and CFRP-to-steel joints. The effects of neutral salt spray on appearance, failure modes, and mechanical properties of adhesive and CFRP sheets were discussed. The influence of the steel plate surface treatment methods, the number of CFRP layers, and the types of CFRP sheets on shear strength, stiffness, and fracture energy of CFRP-to-steel joints, as well as their degradation mechanisms, were analyzed. Finally, aging duration-dependent equations were proposed by regression analysis to calculate the normalized mechanical performance of the adhesive, CFRP sheets, and CFRP-to-steel joints. The results indicate that the degradation of the adhesive's mechanical properties was much greater than that of the CFRP sheets over the same aging duration. The deterioration in the bonding performance of CFRP-to-steel joints was related to the degradation in the mechanical properties of adhesive-steel interface and adhesive. The surface treatment of the steel plate was crucial to the mechanical performance and durability of the joints. A rough steel plate surface, dense CFRP sheets, and multiple layers of CFRP could enhance the durability of CFRP-to-steel joints.

This study numerically analyses the load-bearing capacity of Carbon Fibre Reinforced Polymer (CFRP) strengthened circular hollow section (CHS) multiplanar KT-joints based on experiments. Compared to unreinforced joints, the load-bearing capacity improvement coefficient k was proposed and investigated. First, the experiments on bare and CFRP-reinforced CHS KT-joints were briefly described. Three-dimensional numerical models were then established for bare and CFRP-strengthened CHS KT-joints. Refined and simplified CFRP models were established. The effectiveness of the simplified CFRP model was validated by comparing it with experimental results. To improve computational efficiency, the contact between CFRP and steel tubes was optimised by the combination of surface-based cohesive behaviour and tie. A parametric study was carried out to analyse the effect of 20 independent parameters on k. We found that k is primarily influenced by nondimensional geometric parameters, load of brace-T, number of CFRP layers, and CFRP properties. Parametric formulae for k were established through nonlinear regression analysis. The proposed parametric formulae agree well with numerical analysis and experimental results.

Extensive numerical studies and analysis are presented to gain insights into the mechanism of load transfer in T- and K-type concrete-filled steel tube (CFST) connections. The new mechanism of load transfer investigated in this paper manifests that the shear load is transferred non-uniformly in the CFST column. Using validated finite element (FE) models, contact pressure stresses of the concrete-tube and angle-concrete interfaces are investigated to examine the interfacial interaction between each component. It indicates that the friction resistances between three components occur almost throughout the loading process, which is non-uniformly distributed along the column length. In addition, stress in each component is evaluated to determine the region of non-uniform load transfer in the column. For T-type connection, non-uniform load transfer shall only be considered for concrete-tube interface. For K-type connection, non-uniform load transfer should be considered for both concrete-tube and angle-concrete interfaces. Non-uniform load transfer mainly occurs in the connecting region and one time diameter of column above the connecting region for theses connections. Finally, the uneven transferred load of each component for the connections is analysed, whilst different force allocation schemes are compared. Based on the shear stresses at the tube-concrete and concrete-angle interfaces, non-uniform load transfer factor of each interface is calculated to quantitatively evaluate the non-uniformity of load transfer for the T- and K-type CFST connections.

This paper summarises the investigations carried out by the authors on traditional and member-rotated T- and Xjoints made of cold-formed S900 and S960 steel grades tubular members with nominal 0.2% proof stresses (equivalent to yield strengths) of 900 MPa and 960 MPa, respectively. Static structural performance of two traditional and three member-rotated configurations of tubular T- and X-joints undergoing axial compression load through brace members was investigated. The experimental investigations included 165 tests and targeted different failure modes in the chord members of the investigated joints. In addition, 32 tensile and compression coupon tests were also carried out by the authors to determine the tensile mechanical properties of these tubular members. Finite element (FE) models were developed and verified against the test results by duly showing the capability of reciprocating experimental joint strengths, failure modes, and load-deformation curves. Upon validations of the developed FE models, comprehensive parametric studies comprising 2254 finite element models were performed. In order to examine the applicability of existing design rules for the investigated high strength steel tubular joints, static strengths obtained from tests and numerical studies were compared with nominal strengths calculated from design equations given in Eurocode 3, CIDECT, and literature. Generally, it has been demonstrated that the existing design rules cannot provide accurate strengths for the investigated joints. Therefore, the authors have proposed user-friendly, accurate, and reliable design rules to predict the static strengths of cold-formed S900 and S960 steel grades traditional and member-rotated tubular T- and X-joints.

In this paper, the design calculations of axially loaded CFST slender columns with latticed annular steel parts (SRCFST) are proposed based on the separated model. Finite Element (FE) analysis is conducted to predict the axial compressive performance of circular CFST slender columns with latticed annular steel parts. The test data are used to validate the FE model and to assess the suitability of design guidelines given in AISC 360-10, Eurocode 4, and DL/T 5629-2021. The validated FE model extensively analyzes the composite interaction between the concrete and the latticed annular steel parts as well as investigates the effect of different configurations of the latticed annular steel on compression performance. The influence of various parameters, including the constraint coefficient (xi), steel reinforcement ratio (rho), and slenderness ratio (lambda), on the lateral confinement stress and ultimate bearing capacity of the latticed annular steel parts were assessed and quantified. Additionally, using the results of numerical simulations performed in this study, design procedures were proposed by modifying the separation model. The predictions from proposed design rules have demonstrated good agreement with the strengths of test and numerical specimens.

The detailed numerical investigation and design of cold-formed S960 steel grade square bird-beak (SBB) T- and X-joints have been presented in this paper. The SBB joint is one of the novel bird-beak tubular joint configurations and obtained by rotating the chord member of a conventional square hollow section (SHS) joint along its centroidal axis by 45°. In this investigation, accurate finite element (FE) models were developed for SBB T- and X-joints using the tests carried out by the authors. The developed FE models successfully replicated the static strengths, load vs deformation curves and failure modes of test specimens. In order to gain an in-depth understanding on the static behaviour of SBB joints, a comprehensive FE parametric study was performed using the verified FE models. The joint failure strengths and joint ultimate capacities of a total of 220 SBB T- and X-joints specimens, including 200 FE specimens investigated in this study, were evaluated against the nominal strengths predicted from the literature and European code. All SBB T- and X-joints test and FE specimens were failed by the chord crown failure (C) mode. It has been shown that the design provisions given in the literature and European code are unsuitable and uneconomical for the design of cold-formed S960 steel grade SBB T- and X-joints investigated in this study. Therefore, accurate and reliable design equations are proposed in this study for predicting the static strengths of the investigated SBB T- and X-joints.