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This paper presents the experimental results from nine full-scale concrete-filled double-skin tubular (CFDST) beam-columns. The test specimens exploited two fabrication strategies, featuring either hollow steel inner skins with corrugated geometry or ultrahigh-strength steel corner tubes to enhance the seismic performance of noncompact CFDST beam-columns for potential use in low-to-moderate seismicity regions. The effects of loading sequence, axial load ratio, and cross-sectional geometry were investigated. The experimental results suggested that the current AISC specification may be used to predict the axial strength of composite members with a relatively good accuracy. In the postbuckling range, conventional CFDST beam-columns and those with corrugated inner skins are prone to fracture at the corner welds of the built-up cross section. However, the latter exhibited up to two times larger drift capacities than conventional CFDST counterparts prior to losing axial load carrying capacity. Noncompact beam-columns retrofitted with ultrahigh-strength steel corner tubes exhibited a 4% lateral drift demand without experiencing more than 25% flexural strength loss. The presence of ultrahigh-strength steel increases the plastic hinge length of CFDST beam-columns by up to four times relative to CFDST beam-columns with corrugated inner skin, regardless of the employed loading history. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical study on the performance of concrete-encased concrete-filled steel tube (CFST) box columns subjected to eccentric axial loading. Finite-element analysis (FEA) was used to analyse the composite columns and experimental data reported recently by the authors was used to verify the FEA modelling. Analysis of the behaviour of the composite, including the full-range load versus lateral mid-height deflection, stress distributions, column failure modes and the interactions between steel and concrete, was carried out. The behaviour of the concrete-encased CFST box columns was compared with that of corresponding reinforced concrete columns and CFST built-up columns. Finally, a parametric study was carried out and the moment magnification approach adopted to propose design formulae for eccentrically loaded concrete-encased CFST box columns. [ABSTRACT FROM AUTHOR]

Abstract: Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism. [Copyright &y& Elsevier]

The welded truss composed of circular hollow section (CHS) braces and concrete-filled circular hollow-section (CFCHS) chords is a new kind of structural system that has been increasingly applied in large span arch bridges in China. It is necessary to have a good knowledge of fatigue strength of the welded CHS-to-CFCHS joints for the design of this kind of composite bridge. This paper reports on a series of tests on welded CHS-to-CFCHS T-joints subjected to axial cyclic fatigue loading in the brace. Eleven joints were designed to investigate various influence factors such as different nondimensional geometric parameters of circular hollow sections and different concrete strength grades. The quality of welds connecting brace and chord members were examined using the magnetic particle and radiographic inspection methods. The conditions of hot spot stress at both the crown and saddle positions in brace and chord members were determined by means of linear and nonlinear extrapolation methods. During the fatigue testing process, the number of cycles relating to several stages of failure, the crack initiation positions, crack propagation patterns, and the final failure modes were recorded. Fatigue strength of the CHS-to-CFCHS T-joints was compared with that of CHS-to-CHS T-joints. It is concluded that the CHS-to-CFCHS T-joints have a much lower stress concentration factor and consequently have better fatigue strength than the CHS-to-CHS T-joints, when both kinds of joints have the same nondimensional geometrical parameters and same nominal stresses on the brace. The curves in the Comité International pour le Développement et l'Étude de la Construction Tubulaire guidelines used for CHS-to-CHS joints are not appropriate for the reliable fatigue assessment of CHS-to-CFCHS T-joints based on the current test data. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports a series of tests on CFDST (concrete-filled double skin tube) stub columns subjected to large deformation axial loading. Both the outer and inner tubes are circular hollow sections (CHS) with a diameter-to-thickness ratio ranging from 19 to 96. It was found that CFDST significantly improved the behaviour of CHS in terms of failure modes, load carrying capacity and energy absorption. Multiple folds were formed in CFDST under large deformation axial loading. The load drop during cyclic loading was found to be insignificant (about 20 to 30%). The behaviour after the cyclic loading was influenced by the stage of the cyclic loading and the ratio. [Copyright &y& Elsevier]

This paper describes a series of tests to failure of fix-ended tubular braces subjected to cyclic concentric axial loading. The braces were made from cold-formed steel grade C350L0 (350 MPa nominal yield stress) circular hollow sections (CHS). Nine different diameter-to-thickness ratios in the range of 19

This paper describes a series of tests on empty and void-filled rectangular hollow section (RHS) braces subjected to static and cyclic axial loading. The braces were made from cold-formed C450 (450 MPa nominal yield stress) RHS with two different plate slenderness ratios. The filler material used was normal concrete and light-weight concrete. Two loading protocols were applied, namely Cyclic-Direct (full axial displacement applied and load oscillated) and Cyclic-Incremental (load oscillated at several accumulating axial displacement increments). First-cycle buckling loads were compared with design loads predicted using various national standards. The effects of filler material and section slenderness on the load-deflection response, first cycle peak load, postpeak residual strength, ductility, and energy absorption of void-filled RHS braces are studied in the paper. The effect of the loading scheme is also discussed. [ABSTRACT FROM AUTHOR]

Abstract The concrete-encased concrete filled steel tubular (CFST) column to RC beam joint is composed of the concrete-encased concrete filled steel tubular (CFST) column and the reinforced concrete (RC) beam. Concrete-encased CFST column consists of a CFST component and an outer reinforced concrete (RC) component. This paper presents an experimental investigation of the seismic performance of this kind of composite joints. Thirteen joints were tested under a constant axial load on the column and a cyclic vertical load on the beam. The test parameters include the joint types, the connection types, the axial load level, the joint core volumetric stirrup ratio, the beam longitudinal rebar ratio and the sectional dimension of the beam. Four types of typical failure modes are found, and the corresponding hysteretic relationships are analyzed based on the test results. The seismic performance of the composite joint is also evaluated by several indexes, such as rigidity degradation, strength degradation, ductility, and energy dissipation. Finally, a comparison with other typical joints is conducted to exhibit the seismic performance of the concrete-encased CFST column. Highlights • Thirteen concrete-encased CFST column to RC beam joints were tested under constant axial load and cyclic vertical load. • The failure modes and hysteretic relations of concrete-encased CFST column to RC beam joints were investigated. • Several aspects reflecting the seismic performance of this composite joints were analyzed. • The seismic performance of this composite joint was compared with other conventional joints. [ABSTRACT FROM AUTHOR]

Highlights • Bernstain-Bézier functions are used to develop a material model for UHSS cooled from fire. • The model considers the effect of fire temperature and sustained axial load ratio. • The model is calibrated and validated against experimental tests results. • The model is capable of extrapolating the results out of the range of tests data. • The creep strain of UHSS under a transient fire and a constant stress is derived. Abstract Ultra-high strength steels (UHSS) have significant potential applications in the engineering fields due to their unique specifications. In recent years, it has been experimentally shown that the post-fire stress-strain response of this material is highly dependent on the maximum steel temperature and the sustained load applied to it during fire. This paper employs the Bernstain-Bézier functions to present the relationship between the stress, strain, the maximum fire temperature and the sustained axial load ratio (β) for Grade 1200 UHSS cooled for fire temperatures to room temperature. The experimental results are used to verify and validate the proposed model throughout the paper. The model showed to be capable of not only interpolating the stress-strain curves, but also extrapolating them out of the range of the available experimental tests data. Also, taking advantage of the stress-strain-temperature response of the UHSS tested at elevated temperatures, the instantaneous stress-induced strain and consequently the creep strain of UHSS subjected to different constant sustained load values during transient fire are obtained. [ABSTRACT FROM AUTHOR]

Bird-beak joints are a type of welded square hollow section (SHS) joint, the static strength of which is higher than that of conventional joints. Experimental analyses were conducted on the stress concentration factors (SCFs) of square bird-beak joints to obtain more information regarding their fatigue behavior. Twelve specimens, i.e., three conventional joints and nine square bird-beak joints, are separately tested by considering chord and brace axial force load cases. Both conventional and square bird-beak joints tested have a T-shape and are simply supported at their chord ends. Elastic strain distributions near the crown and saddle areas are measured, and strain concentration factors (SNCFs) at potential hot spots are calculated using an extrapolation approach. Quadratic extrapolation is suggested for the square bird-beak joints because linear extrapolation may underestimate SNCFs by up to 35%. Furthermore, critical locations are identified for square bird-beak joints subjected to chord and brace axial forces. A comparison indicates that the square bird-beak joints can produce lower stress concentration factors than the conventional or the diamond bird-beak joints with identical nondimensional parameters, especially when brace-to-chord width ratios are relatively small. [ABSTRACT FROM AUTHOR]

In recent years, the risk of damage or failure of axial load bearing structural members has increased rapidly due to increase of accidental vehicle/ship collision events. Therefore, suitable strengthening technique needs to be developed to minimise the casualty and economic loss caused by vehicular collisions with structural columns. In this study, numerical simulations are carried out to evaluate the effect of carbon fibre reinforced polymer (CFRP) strengthening of full scale concrete-filled steel tubular (CFST) columns under vehicular impact. Numerical models of bare and CFRP strengthened CFST columns were first developed and validated in a recent study of the authors. The validated finite element (FE) models are extended to full scale columns. Realistic vehicle behaviour is simulated with simplified mass-spring vehicle model. The outer diameter of steel section is kept same and the wall thicknesses are changed to account the slenderness effects of hollow steel sections. Both vehicle and column deformations are considered during the impact simulation as observed in practical situation. The dynamic impact analysis results show that adhesively bonded CFRP sheets provide enhanced impact resistance capacity of strengthened columns by reducing lateral displacement about 40% compared to ordinary CFST columns. A comprehensive parametric study is conducted by varying the vehicle velocity, vehicle mass, axial static loading, vehicle stiffness and CFRP bond length to observe the effects of these parameters on the structural responses of bare and wrapped columns. CFRP wrapping is found to be a promising strengthening technique to control global failure of full scale CFST columns subjected to vehicular impact. [ABSTRACT FROM AUTHOR]

The compressive behaviour of Concrete-Filled Double-Skin tubular (CFDST) sections consisting of corrugated plates is investigated numerically in this study. Finite element simulations are employed to decipher more details on the compressive behaviour of numerous archetypes with different geometries and material properties. The results of these simulations reveal how the ultimate static compressive capacity of the intended sections may be affected by changing the geometric and material properties. Different cross-section shapes, dimensions and heights within the practical range are considered for the proposed archetypes. In addition, a wide range of concrete compressive strength values are assigned to the concrete infill of the archetypes. The ultimate axial capacity of almost all the intended CFDST archetypes is mainly dominated by concrete compressive strength. In conjunction with numerical simulations, different design formulations are also employed to predict the ultimate axial load-bearing capacity of CFDST sections. [ABSTRACT FROM AUTHOR]

Concrete-filled double-skin tubes (CFDST) have been increasingly popular in the field of engineering in recent years. A lot of research has been carried out to investigate the behaviour of CFDST members under a variety of loading conditions. This paper presents an experimental investigation on ultra-high performance concrete infilled double-skin tube columns subjected to close-in blast loading. Two types of CFDST columns were investigated in the experiments – one with both inner and outer tubes circular and the other one with both square. The main test parameters included the explosive charge weight and the magnitude of axial load. After the blast tests, there was no visible buckling nor ruptures found on the steel tubes and only minor cracks, of no more than 1mm width, were observed in the core concrete when the outer steel tube was removed. Based on the findings of the experiments, it is evident that CFDST column has excellent blast resistance. This feature has the potential to be used in high-value structures which may be the targets of terrorist attacks, such as embassies, government buildings and critical infrastructures. [ABSTRACT FROM AUTHOR]

This paper deals with the static compression behaviour of concrete-filled double-skin tubular (CFDST) columns with corrugated plates. These sections take advantage of the high strength of their infill concrete as well as the ductility of their steel skins. Static compression tests were performed on CFDST column specimens with inner or outer skins built up from corrugated plates. The steel hollow sections fabricated from corrugated plates exhibited a more stable response under axial compression loading. Hence, implementing them in CFDST columns is expected to enhance the overall behaviour of these columns. These tests reveal how the overall behaviour of CFDST columns is affected by replacing their inner/outer skin with one fabricated from corrugated plates. The corrugated plates have an insignificant effect on the strength of column specimens. However, the ductility and energy absorption capacity of CFDST columns are improved dramatically. A finite element modelling framework was also developed to simulate the response of CFDST columns with corrugated plates and validated against the results of the experiments. [ABSTRACT FROM AUTHOR]

Geopolymeric recycled concrete (GRC) is a new construction material which takes environmental sustainability into account. In this paper, an experimental study was carried on 12 concrete filled steel tubular columns under axial loading, in order to fill a knowledge gap on the engineering and structural properties of GRC filled steel tube (GRCFST). Two section sizes of square hollow sections filled with GRC and recycled aggregate concrete (RAC) respectively, with different recycled aggregate (RA) replacement ratios of 0%, 50% and 100%, were used in the experiments. The test results indicated that the ultimate strength was reduced when adding more RAs in the columns, while the peak strain increased. The ductility of the columns was improved by increasing the RA replacement ratio. Overall, the influence of RA on the strength and ductility of GRCFST columns is greater than that of RAC filled steel tubular (RACFST) columns. The assumed theoretical model for predicting load versus deformation relation of GRCFST columns under axial loading was examined, and a revised theoretical model proposed. The results of the new model show good correlation with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the effects of curing conditions on the performance of steel/CFRP double strap joints subjected to combined loading, cyclic temperature and humidity. A series of joints, cured respectively at an elevated temperature (120°C) and room temperature (about 23°C) were exposed to different combined environmental and loading conditions. It was found that curing at an elevated temperature had no effects on the joint ultimate strength tested at room temperature. However, at 50°C, the joints cured at 120°C showed almost no strength reduction, in comparison to those cured at room temperature with a 50% strength reduction. Curing at elevated temperature also helped to improve the time-to-failure of joints: joints cured at room temperature failed within less than 2h when exposed to the environmental conditions, while joints cured at 120°C could survive up to 270h, dependent on the applied load levels. Partial safety factors proposed in design guidelines for FRP composites under environmental effects were used to evaluate the experimental results obtained, suggesting a very critical degradation of joint mechanical properties due to the environmental effects investigated in this study. [Copyright &y& Elsevier]

Abstract: Concrete filled double skin steel tubular (CFDST) columns have some advantages when compared to conventional concrete filled steel tubular (CFST) columns, such as lighter self-weight and better cyclic performance. CFDST columns have the potential to be used in structures such as bridges, high-rise buildings, viaducts and electricity transmission towers. The inner and outer steel tubes will be subjected to preloads from upper structures and wet filled concrete. The initial stresses and extra deformations on steel tubes caused by these preloads would affect the capacity of the CFDST members. This paper thus discusses the behaviour of axially loaded CFDST columns with preload either on the outer tube alone or on both tubes. A general purpose finite element analysis (FEA) model is developed to predict the behaviour of CFDST columns with the steel tubes subjected to preload. The FEA model is verified using test results of CFDST columns without preload and those of CFST columns with preload. The influences of preload ratio, slenderness ratio, hollow ratio and material strength on axial strength are discussed. Finally, formulas are proposed for calculating the ultimate strength of CFDST columns with preload on steel tubes. [Copyright &y& Elsevier]

Abstract: This paper studies the behavior of very slender thin-walled concrete filled steel tubular (CFST) columns under axial compression. A finite element analysis (FEA) was developed to carry out the behavior of compressive columns. Initial out-of-straightness imperfection was discussed. Generally good agreement was obtained between the predicted and measured results. The FEA model was then used to perform mechanism analysis on very slender circular CFST columns. Parametric studies were carried out and the ultimate strengths from tested results and design codes were compared and discussed. The reliability analysis method was used to calibrate the existing design formulas given in DBJ/T13-51-2010, ANSI/AISC 360-05 and Eurocode 4. [Copyright &y& Elsevier]

This paper presents an experimental study on the fatigue behaviour of circular hollow section (CHS) gap K-joints strengthened by carbon fibre-reinforced polymer (CFRP) sheets (CFRP-CHS K-joint). Eight CFRP-CHS K-joints and three reference un-strengthened CHS gap K-joints were tested under cyclic axial loading. Fatigue failure modes of the CFRP-strengthened and un-strengthened joints were determined and compared. Fatigue lives in three fatigue failure criteria were recorded: crack-initiation fatigue N 2 , through-thickness fatigue N 3 , and end-of-test fatigue N 4. The comparisons indicate that CFRP-CHS K-joints have a longer fatigue life than un-strengthened joints in the same nominal stress range, regardless of fatigue failure criteria (N 2 , N 3 , or N 4). The increase in N 3 was approximately 138%, on average. No significant difference in fatigue data in terms of hot spot stress range was found between the CFRP-strengthened and un-strengthened joints. Based on the experimental fatigue data, a modified S r , h s - N curve based on the CIDECT recommended curve was proposed for CFRP-CHS K-joints. • Fatigue tests were conducted on CFRP-strengthened and un-strengthened CHS gap K-joints. • Failure modes and fatigue life of CFRP-CHS K-joints are reported and discussed. • CFRP strengthening significantly increases the fatigue life of CHS gap K-joints. • A modified S r , h s - N curve is proposed for CFRP-CHS K-joints. [ABSTRACT FROM AUTHOR]

• FE models for CFRP-CHS K-joints are developed and validated by experimental data. • FE models are analysed to extract SCFs and SCF reduction coefficients of CFRP-CHS K-joints. • The effects of various parameters on SCF reduction coefficients are investigated. • Parametric formulae are proposed for the SCF reduction coefficients of CFRP-CHS K-joints. The focus of this paper is on estimating stress concentration factor (SCF) reduction coefficients (ψ) of circular hollow section (CHS) gap K-joints strengthened with carbon fibre reinforced polymer (CFRP) sheets (CFRP-CHS K-joints), so that the SCFs can be calculated on the basis of the existing formulae for unstrengthened joints. Three-dimensional finite element models are developed for both CFRP-strengthened and unstrengthened CHS gap K-joints to calculate the SCFs, which are verified using previous test data. A parametric study is then carried out to explore the effects of various parameters (11 independent CFRP strengthening parameters and 5 basic geometric parameters describing the steel K-joints) on the efficiency of CFRP strengthening in terms of SCF reduction coefficient ψ. It is found that ψ is primarily affected by the CFRP relative reinforcement rates, the CFRP-to-steel tensile modulus ratios, the adhesive modulus, and non-dimensional geometric parameters. Through regression analyses, parametric formulae are finally developed for estimating ψ at six critical hot spots in CFRP-CHS K-joints under balanced axial loading. The proposed parametric formulae agree well with the experimental data. [ABSTRACT FROM AUTHOR]

This research investigated the stress concentration factor (SCF) superposition of bird-beak square hollow section (SHS) X-joints under asymmetrical axial brace forces. Three typical non-dimensional parameters (i.e., β = 0.6, 2 γ = 20, τ = 0.5) were employed for tubular joints. Finite element models of conventional, square bird-beak, and diamond bird-beak SHS X-joints were constructed by accurately considering the geometries of welds, and the structural hot spot stresses at weld toes were obtained using the extrapolation method. Numerical results show that, in case of single-side brace axial force, the stresses on the unloaded brace side of bird-beak X-joints are obviously higher the values at the corresponding locations of conventional X-joints, and as a result the SCF errors induced by the existing superposition formula of conventional joints are significant for bird-beak joints. The mechanism was revealed to be the interactions between adjacent walls of the chord by further investigating the compatible deformations of these joints. A modified superposition formula, in which a new load case considering both single-side brace axial force and reversed chord in-plane bending has been employed, was introduced as the solution. The SCF variations with non-dimensional parameters under the new load case were also identified by conducting parametric analysis. The proposed superposition approach gives reasonable predictions of SCF of bird-beak SHS X-joints under asymmetrical brace axial forces. [ABSTRACT FROM AUTHOR]

A diamond bird-beak square hollow section (SHS) T-joint is different from a conventional SHS T-joint as the brace and chord members both rotate 45°. This paper deals with fatigue behavior of such joints through fatigue tests and proposed fatigue design curves ( S r,hs – N curves). Diamond bird-beak SHS T-joints (DBB T-joints) with a unique set of geometric non-dimensional key parameters ( β , 2 γ and τ ) were used for fatigue tests. Cracking patterns were observed during the fatigue tests. Based on the fatigue data and stress concentration factors (SCFs) derived by the authors of this paper, new fatigue design curves ( S r,hs – N curves) using hot spot stress concept were discussed by means of deterministic method and least-square method, respectively. Finally, fatigue strength was compared between DBB T-joints and conventional SHS T-joints. The results indicated that DBB T-joints have better fatigue behavior when β is not greater than 0.7. [ABSTRACT FROM AUTHOR]

Abstract: T-joints made of hollow structural sections (e.g. circular hollow section (CHS) or square hollow section (SHS)) are often found in welded trusses that may be subjected to fatigue loading. The brace member of conventional T-joints (e.g. CHS-to-CHS, SHS-to-SHS and CHS-to-SHS T-joints) is perpendicular to the chord member. When the brace member and the chord member both rotates 45° a conventional SHS-to-SHS T-joint becomes a so-called “diamond bird-beak” SHS T-joint. Most of the research work on bird-beak joints has been focused on the static strength. Very limited information is available with regard to the fatigue strength of diamond bird beak joints. This paper will fill the knowledge gap in understanding the stress concentration factor (SCF) of this type of T-joints under brace loading (i.e. axial load in the brace and in-plane bending in the brace). A series of tests were conducted to measure the SCF of diamond bird-beak T-joints with a reasonable range of three key parameters, i.e. β (ratio of brace diameter to chord diameter), 2γ (ratio of chord diameter to chord thickness) and τ (ratio of brace thickness to chord thickness). It was found that the SCFs for the diamond bird beak T-joints are much lower than those for SHS-to-SHS and CHS-to-SHS T-joints, but just slightly smaller than those for CHS-to-CHS T-joints. [Copyright &y& Elsevier]

The grouted T-joint in offshore structure is usually subjected to low-velocity impact caused by ship collision. The behaviour of these joints under lateral impact loading is investigated. Tests on 12 specimens including eight grouted T-joints are conducted. The composite T-joint consists of grout-filled double-skin steel tubular chord and hollow steel section brace. The constant axially compressive load is applied along the axis of the composite chord and the impact load is applied along the axis of the hollow steel tubular brace. A finite element model is established and verified. The influences of various parameters on the T-joint behaviour are discussed. • We provide new test data of the impact behaviour of the T-joint consisted of GFDST chord and hollow steel brace. • We establish the FE model to simulate the impact performance of GFDST T-joint. • We discuss effects of grout and other key parameters on the impact performance of GFDST T-joint. [ABSTRACT FROM AUTHOR]