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High-strength steel (HSS) has been increasingly applied in structural engineering, while the work on residual stress of HSS sections is still insufficient. Limited research is available for welded H-sections above 690 MPa while with a plate thickness of more than 10 mm. This paper deals with the residual stress distributions of welded H-sections with flame-cut edges, which are fabricated from HSS plates with yield strength of 690 MPa and 960 MPa and thickness of 10 mm–20 mm, using both experimental and numerical method. The experiments showed that the welded tensile residual stresses in the weld region are not significantly affected by sectional dimensions and steel grade. Verified by comparing with the experimental results, three-dimensional (3D) finite element (FE) models using sequentially coupled thermal-stress analysis was adopted for parametric study of compressive residual stress of welded H-sections made of 690 MPa HSS. The results showed that the compressive tensile stress is negatively correlated with plate thickness as well as width-thickness ratio. Through regression analysis, formulae were proposed for estimation of compressive residual stress in the flange and web, respectively. Finally, residual stress distribution model was developed, which was proved effective by comparing with the experimental and FE results. [ABSTRACT FROM AUTHOR]

Axial load-bearing structural members often experience significant damage or failure when subjected to moving-vehicle or vessel collisions. Hollow steel tubular columns are highly vulnerable under transverse impact loading. Thus, strengthening/retrofitting of existing steel tubular columns may be required if these members are not designed to withstand expected transverse impact from transport accidents. This paper investigates the performance of full-scale circular-hollow-section (CHS) tubular columns strengthened with fiber-reinforced polymer (FRP) and subjected to vehicular impact. Initially, finite-element (FE) models of bare and FRP-strengthened CHS medium-scale specimens were developed to conduct transverse impact analysis for the model validation purpose. The impact simulation results were compared with the drop-mass impact test results and good agreements were found between the FE and experimental tests. The validated FE models were extended to full-scale bare and FRP-wrapped CHS columns. The full-column vehicle collisions were simulated using a realistic vehicle model by considering varying axial static forces and vehicle impact velocities. The results showed that strengthening with carbon-fiber-reinforced polymer (CFRP) improved the impact resistance capacity of a bare CHS column by preventing plastic hinge formation due to excessive local buckling when subjected to accidental vehicular impact. Three-layer CFRP strengthening proved to be an effective strengthening system compared with two-layer CFRP strengthening system. The effect of load eccentricity was assessed further, and it was found that CFRP strengthening contributed significantly to preventing the failure of CHS columns with varying eccentricities when subjected to credible vehicular impact events. [ABSTRACT FROM AUTHOR]

Research into carbon fibre reinforced polymers (CFRP)'s application to steel as a retrofitting technique has shown very promising results for improving mechanical properties. However, the limitation of these materials in a corrosion inducing environment has generally been overlooked. In this study, the interaction of CFRP and steel in extreme weather conditions and the material characteristics in causing localised corrosion (e.g. pitting) were investigated, with the primary aim of quantifying pit depth. Pits are known to initiate fatigue cracks in steels. Steel tiles placed in direct contact with different varieties of CFRP were exposed to 5% NaCl solutions at two temperatures for different durations. The specimens were microscopically examined to find the existence of pits, their depth and density. Steel with prior grinding suffered negligible pitting, whereas the sand-blasting created distinct isolated regions where pitting was more commonly witnessed. However, the most relevant observation was that even after 6 months of exposure to the aggressive NaCl environment, whatever pitting occurred, it was found not to be overly critical in reducing the mechanical strength of the CFRP/steel bonds. [ABSTRACT FROM AUTHOR]

Abstract: The efficiency of the application of adhesively-bonded CFRP materials to rehabilitate and/or strengthen steel structures is completely dependent on the bond between the CFRP composites and the steel. As the repaired and/or upgraded structural elements are likely to be exposed to impact tensile loads during service, this necessitates the importance of understanding the bond strength between CFRP materials and steel under dynamic loadings. This paper presents the experimental procedure, the equipment and the test results for double strap joints at four speeds of loading to highlight the effect of loading rate on the bond strength. Two different CFRP layouts with various bond lengths were examined. It was found that the loading rate has significant influence on bond strength and failure modes, although it has little influence on the effective bond length. [Copyright &y& Elsevier]

Carbon fibre reinforced polymer (CFRP) has shown promise for improving the fatigue performance of steel structures. Previous studies have indicated that increasing the Young's modulus of CFRP can be beneficial for decreasing the stress intensity factor at the fatigue crack tip. In this project, ultra high modulus (UHM) CFRP plates with Young's modulus of 460 GPa were adopted to study their fatigue repair effectiveness. A series of fatigue tension tests was carried out on steel plates with an initial crack in the centre. Five strengthening configurations were used and a constant amplitude fatigue loading was applied to all the specimens. The beach marking technique was utilized to record the fatigue crack propagation. The effects of CFRP bond length, bond width and bond locations on the fatigue performance of cracked steel plates were also studied. The experimental results show that UHM CFRP plates can greatly increase the fatigue life of cracked steel plates by a factor ranging from 3.26 to 7.47. When CFRP plates cover the whole crack surface, the fatigue crack of the steel plate is arrested. The strengthening effectiveness of UHM CFRP plates is also compared with those using high Young's modulus CFRP sheeting and normal Young's modulus CFRP plates with or without prestressing. [ABSTRACT FROM AUTHOR]

Abstract: External bonding of fibre reinforced polymer (FRP) composites has emerged as a popular technique for strengthening steel structures in recent years. In this study, the non-linear behaviour of circular hollow steel beams bonded with thin carbon FRP sheets was investigated theoretically by including the effect of the amount of CFRP reinforcement, fibre configuration, fibre and adhesive volume fractions and material non-linearity. This paper presents a cross-sectional analysis for the moment–curvature response of CFRP-reinforced steel tubular beams. Numerical results are presented to illustrate the strengthening effect of fibre composites, and are shown to be in reasonable agreement with previous experimental data. [Copyright &y& Elsevier]

Abstract: This paper presents the results of a series of tension tests on CFRP bonded steel plate double strap joints. The main aim of this research is to provide detailed understanding of bond characteristics using experimental and numerical analysis of strengthened double strap joints under tension. A parametric study has been performed by numerical modelling with the variables of CFRP bond lengths, adhesive maximum strain and adhesive layer thicknesses. Finally, bond–slip models are proposed for three different types of adhesives within the range of the parametric study. [Copyright &y& Elsevier]

Abstract: This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of concrete-filled tubes (CFT). Controlled-rotation, symmetrical cyclic bending tests were performed using a variable amplitude loading history on different sizes of CFT with different section slenderness. The CFT beams exhibited stable hysteresis behaviour up to local buckling and then showed considerable degradation in strength and ductility depending on the ratio. Seismic capacity parameters are presented including strength, hysteresis curves and modes of failure for the specimens. Peak moments obtained in the cyclic tests were compared with design moments predicted using a number of steel and concrete specifications. New section slenderness limits suitable for design and construction of seismic resisting structural systems were determined. A comparison is made between these seismic slenderness limits and the limits available in the design codes. [Copyright &y& Elsevier]

Abstract: This paper presents a plastic mechanism analysis to predict the complete load-shortening behaviour of fabricated square and triangular section (FSTS) stub columns. Very high strength (VHS) circular steel tubes with a nominal yield stress of 1350MPa are placed at each corner of the section and welded to Grade 350 steel plates to form a FSTS. Three stages are considered in the analysis: (1) both the VHS tube and steel plate are elastic, (2) plastic mechanism starts to form in steel plates while the VHS tube remains elastic (3) plastic mechanism occurs in both steel plates and the VHS tubes. Discussions are made on the factors affecting the types of mechanism (“flip-disc” versus “roof-type”) in steel plates and the effect of initial geometrical imperfection of the plates on the analysis. Simplified formulae were given for an easy application of the theory developed. The predicted load-axial shortening curves are in good agreement with those obtained by experiments. The predicted ultimate load carrying capacity is about 6% to 8% lower on average than the experimental capacity. [Copyright &y& Elsevier]

This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of concrete-filled tubular (CFT) beams made of cold-formed circular hollow sections and filled with normal concrete. Cyclic bending tests were performed using a constant amplitude loading history on different CFT specimens with diameter-to-thickness ratios (D/t) ranging from 20 to 162. The CFT beams exhibited stable hysteresis behaviour up to the formation of plastic ripples and then showed considerable degradation in stiffness, strength, and ductility depending on the D/t ratio. Seismic capacity parameters are presented including strength, stiffness, rotation capacity, hysteresis loops and modes of failure of the specimens. Peak moments obtained in the cyclic tests are compared with those obtained previously in monotonic tests and also with design moments predicted using the available design rules for composite beams. The deformation ductility demand was determined and used to derive new fully ductile section slenderness limits suitable for seismic design. [Copyright &y& Elsevier]

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]

This paper presents stub columns tests on concrete-filled double skin sandwich tubes (CFDT) constructed using cold-formed steel tubes. The annulus is filled with micro high-strength concrete having compressive cylinder strength of 64 MPa. The outer skin is made of circular hollow sections (CHS), while the inner skin is made of square hollow sections (SHS). Eight different section sizes were used for the outer skin with diameter-to-thickness ratio ranging from 19 to 55. Three section sizes were chosen for the inner skin with width-to-thickness ratio in the range of 20 to 26. The CFDT construction was found to have significant increase in strength, ductility and energy absorption over the outer jacket. A simplified formula is derived to determine the compressive capacity of CFDT and compared against the current design rules. The proposed formula was found in good agreement with experimental results. This paper also verifies the yield slenderness limit (λey) of 82 specified in AS 4100 for cold-formed CHS stub columns. [Copyright &y& Elsevier]

This literature review has examined the use of FRP composite materials as a potential retrofitting technique for civil structures. Importantly, the various material properties, bond mechanisms, durability issues and fatigue resistance have been discussed. Studies exploring the performance of CFRP repaired steel have strongly indicated its potential as a rehabilitation material. These systems offer many improvements over the current bulky and less chemically resistant methods of bolting or welding steel plate patches. This review has established and highlighted the factors that affect CFRP/steel bond durability, namely surface preparation, curing, corrosion, fatigue loading, temperature and moisture ingress through studies that focus on their effect. These studies, however, often focus on a single influencing factor or design criteria. Only limited studies have investigated multiple parameters applied simultaneously, even though they commonly occur together in industrial practice. This review aimed to summarise the numerous influencing parameters to give a clearer understanding of the relevance of CFRP repaired steel structures. [ABSTRACT FROM AUTHOR]

• Bond tests were conducted between high/ultra-high strength steel and ultra-high modulus CFRP laminates. • Both single-sided and double-sided schemes were adopted. • Numerical simulation was performed on bond behaviour. • A theoretical bond model was developed to estimate the bond strength. With the increasing applications of high and ultra-high strength steel (HSS/UHSS) in engineering structures, there is a need to address rehabilitation and strengthening of such steel grades using carbon fibre reinforced polymer (CFRP). The bond between HSS/UHSS and CFRP is vital to ensure the efficiency of the strengthening. The existing bond study was focused on mild steel as substrate, with very limited work on HSS up to the grade of S690. Since HSS/UHSSs are designed to undergo much higher loading in service, much higher shear stress is expected in the adhesive layer, leading to a higher chance of premature debonding. In this paper, the bond between HSS/UHSS plates and ultra-high modulus (UHM) CFRP laminates under static tensile loading is studied experimentally, numerically and theoretically. Both single-sided and double-sided schemes were adopted. The numerical simulation using LS-DYNA software package was implemented and a reasonable agreement with the experimental results is found. A theoretical bond model was developed to relate the bond strength to the imposed strain in the steel member outside the bonded region. [ABSTRACT FROM AUTHOR]

Abstract This study aims to investigate the effect of manufacturing process on the post-fire mechanical response of Grade 1200 ultra-high strength steel (UHSS) tubes. To this end, the post-fire mechanical properties of "direct-quenched" UHSS (UHSS-DQ) standard tensile coupons are compared to those made of "quenched and tempered" UHSS material (UHSS-QT) with similar original room temperature stress-strain responses. Thus, to compare the post-fire compression behaviour of UHSS-DQ tubular stub columns with those made of UHSS-QT material, a finite element (FE) model is developed in ABAQUS FE software with precise material properties extracted from the results of the post-fire tensile coupon tests. Quasi-static compression tests are then conducted on UHSS-QT tubular stub columns cooled from different fire temperatures to room temperature to validate the FE analysis. Using the results of the tensile coupon tests and the FE analysis on UHSS stub columns, it is shown that the manufacturing process substantially affects the mechanical properties of UHSS stub columns under cooling phase of a fire. Highlights • The post-fire behaviour of Ultra-High Strength Steel (UHSS) tubes is investigated. • Effect of manufacturing process on post-fire behaviour of UHSS tubes is studied. • Direct Quenched (DQ) and Quenched and Tempered (QT) UHSS tubes are studied. • Post-fire tensile/compressive behaviour of UHSS-DQ and UHSS-QT tubes are compared. • Manufacturing process has a major effect on post-fire behaviour of UHSS tubes. [ABSTRACT FROM AUTHOR]

Ductile fracture of steel components cannot be avoided particularly due to large inelastic strain in extreme loading scenarios. Without considering damage model, conventional elasto-plastic constitutive material model cannot lead to accurate finite element (FE) modelling results when damage due to fracture exists. Metal fracture is a complex damage process in which damage onset, material deterioration and final fracture are related to development of material plasticity. Uniaxial tensile fracture and shear fracture are two typical failure modes of structural members. In this study, experimental investigations of monotonic tensile fracture and in-plane shear fracture under different strain rates in quasi-static loading range are presented for low-alloy structural carbon steel. The characteristic steel material properties are obtained insensitive to the chosen strain rate. A semi-empirical process is used to calibrate the damage properties (damage onset and softening criteria) through FE modelling of the coupon samples based on a continuum damage mechanics (CDM) model. Deterioration of the carbon steel properties was found related to the evolution of equivalent plastic strain, and particularly there is a linear relationship for the in-plane shear scenario. The verified damage onset and shear softening are then used for modelling of a welded steel tubular connection experiencing punching shear fracture phenomenon. It is shown that modelling result of the steel connection is significantly improved when the proposed fracture modelling is adopted. [ABSTRACT FROM AUTHOR]

Fatigue failure is brittle and sudden and is one of the main problems with steel members and connections. Carbon fiber reinforced polymer (CFRP) sheets and laminates have been shown to be effective and practical for strengthening steel under fatigue loading regardless of the existence of initial cracks. Many studies have examined the fatigue behaviors of CFRP strengthened steel, but fatigue design guides or available programs for designers and engineers are limited. Thus, based on existing design codes and guidance for pure steel under fatigue loading (e.g., Design Guide for Circular and Rectangular Hollow Section Welded Joints under Fatigue Loading and Recommendations for Fatigue Design of Welded Joints and Components), this paper proposes fatigue design guides and programs for CFRP strengthened steel structures. First, for steel without initial fatigue cracks, Classification method is adopted along with a related calculation method for obtaining the reduced stress range of steel after strengthening. Then, a classification table for hybrid CFRP-steel members is given to illustrate where to glue CFRP sheets or laminates and the correct fiber orientation. Second, for steel with initial fatigue cracks, fracture mechanics are adopted to obtain the reduced stress range. This paper considers debonding at the crack tip using the finite element method (FEM) and introduces a coefficient d to enlarge the range of the stress intensity factor (SIF). Then, a program called “EasyFatigueforFSS” (Easy Fatigue design for FRP Strengthened Steel) is developed to calculate the available life or allowable stress. Finally, typical design examples are given for reference. [ABSTRACT FROM AUTHOR]

Natural extreme events such as earthquake and fire are destructive for building structures. Earthquake is not only a destructive extreme event but also can trigger other extreme events such as fire. The possible post-earthquake fire (PEF) can be a more destructive scenario for building structures. For safety reasons, it is necessary to capture the behaviour of structural components under PEF scenario. This study aims to investigate the behaviour of flange-welded/web-bolted steel I-beam to box column connections subjected to PEF scenario. Experimental tests were carried out on two groups of steel connections under ISO fire. In each group, four connections were fabricated with the same specifications in which one connection was tested under fire only while the other three connections were tested under cyclic loading and subsequent fire. During the fire stage, the connections were subjected to constant static load equivalent to 30% of their ultimate monotonic loading strength. The furnace temperature, temperature distribution of connection and the beam deflection of connection were measured during the fire tests. It was found that the load-carrying capacity of the flange-welded/web-bolted connections decreases significantly with the increase of pre-damage level induced by cyclic loading. Additionally, in order to study the temperature distribution of the connections under fire, preliminary finite element modelling was carried out for a heat transfer analysis on the basis of uniform fire exposure. [ABSTRACT FROM AUTHOR]

Carbon fibre reinforced polymers are widely used to strengthen steel structural elements. These structural elements are normally subjected to static, dynamic and fatigue loadings during their life-time. A number of studies have focused on the characteristics of CFRP sheets bonded to steel members under static, dynamic and fatigue loadings. However, there is a gap in understanding the bonding behaviour between CFRP laminates and steel members under impact loading. This paper shows the effect of different load rates from quasi-static to 300 × 10 3 mm/min on this bond. Two types of CFRP laminate, CFK 150/2000 and CFK 200/2000, were used to strengthen steel joints using Araldite 420 epoxy. The results show a significant bond strength enhancement under impact loading, whereas the high loading rates have little influence on the effective bond length. [ABSTRACT FROM AUTHOR]

Carbon fibre reinforced polymers (CFRPs) have been widely used in the last few decades. The bond between steel members and CFRP is the main issue in rehabilitation. Rehabilitation is considered when structures have cracks or damage. The CFRP section that needs to be used in rehabilitation depends on the damage amount or crack thickness. This paper reports the experimental and numerical effects of CFRP properties and sections on the bond between CFRP laminate and steel members under quasi-static loading. Three CFRP moduli (low CFRP modulus, normal CFRP modulus and ultra-high CFRP modulus) and two CFRP sections (20 × 1.4 mm and 10 × 1.4 mm) were used in this research. The results show that small CFRP sections are very sensitive to evaluate the bond properties between CFRP and steel, and the ultra-high CFRP modulus with low tensile strength has significant effects on the bond between CFRP and steel. [ABSTRACT FROM AUTHOR]

• Fatigue crack growth rates in base metal and heat-affected zone of butt-welded high-strength steels Q960, Q890, and HG785 were obtained. • Fatigue behavior of butt-welded high-strength steel connections with semielliptical surface cracks was studied experimentally. • An accurate theoretical method is proposed to predict the fatigue life and crack propagation trajectories of surface crack specimens. This paper presents a study on the fatigue behavior of butt-welded high-strength steels (HSSs) Q960, Q890, and HG785. Fatigue crack growth rate (FCGR) experiments in the base metal (BM) and heat-affected zone (HAZ) are conducted, and the fatigue parameters are fitted using the Paris Law. Scanning electron microscopy (SEM) is also performed. The results reveal that the FCGR in the HAZ is slower than that in the BM for HSS; Q960 exhibits the highest fatigue crack growth (FCG) resistance, followed by Q890 and HG785. Surface crack propagation experiments are conducted at a maximum stress of half yield strength and 200 MPa. At half yield strength, the fatigue life decreases as the stress range increases, while at 200 MPa, the fatigue life decreases as the steel yield strength decreases. Furthermore, it is found that the surface cracks initially grow faster in the depth direction and then shift to the through thickness crack along the width direction. Finally, a theoretical method for predicting the fatigue life and crack propagation trajectories of surface crack specimens is proposed based on the Paris Law to consider multidimensional crack propagation behavior. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the numerical simulation of both CFRP/steel double strap joints with 1 and 3 CFRP layers per side at quasi-static and three dynamic tensile loading speeds of 3.35, 4.43 and 5m/s. Simulations are implemented using both the implicit and explicit codes respectively using non-linear finite element (FE) package ABAQUS. In these analyses, failures of both CFRP sheet and adhesive are considered and a cohesive element is utilised to model the interface. The developed FE models for both types of joints were validated by comparing their quasi-static and dynamic findings with those obtained from previous experimental program. This comparison includes four different variables such as the ultimate joint strength, effective bond length, failure pattern and strain distribution along the bond length. It was found that FE models proved to be able to predict all these parameters for both quasi-static and dynamic analyses and their prediction matched well with test results. [Copyright &y& Elsevier]

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: Although extensive studies of the bond between steel and CFRP under various environmental exposures have been carried out, the effects of UV have not been examined to date. This knowledge gap is addressed in this paper. Specimens (epoxy adhesive, CFRP laminates and steel/CFRP adhesively-bonded joints) were exposed to UV for various time periods and identical reference specimens were exposed to only thermal environments without UV. It was found that UV exposure does not influence the tensile strength of CFRP composites. The tensile strength of the adhesive reduced by 13.9% while modulus showed a significant increase by 105% after 744h of exposure. The tensile modulus of adhesive exposed to only thermal environment also increased by 38%, considerably less than that induced by UV exposure. The UV exposure also led to a decrease in joint strength but an increase in stiffness, caused by the temperature effect rather than the UV rays. [Copyright &y& Elsevier]

Abstract: This paper discusses the mechanical performance of steel/CFRP double strap joints subjected to harsh environments. Seventy-five joint specimens were exposed to various conditions, including (1) simulated sea-water at 20°C and 50°C up to 1year in a temperature-controlled sea-water tank, (2) constant and (3) cyclic temperatures with a high level of relative humidity (RH) up to 1000h in an environmental chamber. Tensile tests were undertaken to determine the mechanical properties of the unexposed and exposed specimens. Significant reductions in joint strength and stiffness were found for the specimens exposed to simulated sea-water after 1year of exposure, while the specimens exposed to combinations of temperature and humidity in the environmental chamber showed little decrease in joint strength and stiffness after 1000h of exposure. A model was developed to predict the strength and stiffness degradation of the joints in sea-water and the results compared well with the experimental results. [Copyright &y& Elsevier]

Abstract: This paper examines the mechanical performance of steel/CFRP adhesively-bonded double strap joints at elevated temperatures around the glass transition temperature (Tg , 42°C) of the adhesive. A series of joints with different bond lengths were tested to failure at temperatures between 20°C and 60°C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature approached Tg . In addition, the ultimate load and joint stiffness decreased significantly at temperatures near to and greater than Tg , while the effective bond length increased with temperature. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modelling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. The modelling results were validated by the corresponding experimental measurements. [Copyright &y& Elsevier]

Abstract: Concrete-filled double skin steel tubes (CFDST) have a great potential to be used in the construction of bridges and buildings. Limited information is available on the models for the moment (M) versus curvature (φ) response, and the lateral load (P) versus lateral displacement (Δ) relationship of these columns subjected to axial load and cyclically increasing flexural loading. A mechanics model is developed in this paper for CFDST beam-columns subjected to constant axial load and cyclically increasing flexural loading. The predicted cyclic responses for the composite columns are in good agreement with test results. Based on the theoretical model, parametric analysis was performed on the behaviours of moment (M) versus curvature (φ) response and lateral load (P) versus lateral displacement (Δ) relationship for the composite columns. Finally, simplified models for the moment (M) versus curvature (φ) response, and the lateral load (P) versus lateral displacement (Δ) relationship were suggested. [Copyright &y& Elsevier]

Abstract: This paper presents a finite element analysis (FEA) modeling to study the flexural performance of circular concrete-filled thin-walled steel tubular (CFST) beam. A set of test data was used to verify the FEA modeling; generally, good agreement was achieved. The FEA modeling was then used to investigate the stress and strain distributions across the composite section in the whole loading procedure. The composite action between the steel tube and its concrete core was analyzed. A strut–tie model was proposed for the load transfer mechanism of the circular composite member subjected to pure bending. [Copyright &y& Elsevier]

Abstract: This paper presents the behaviour of the composite frame with concrete-filled square hollow section (SHS) columns to steel beam. Finite element modeling (FEM) was developed to carry out the behaviour of composite frames under a constant axial load on columns and a lateral cyclic load on the frame. Accurate material and geometrical nonlinear for confined concrete and steel were considered in the analysis. The finite element program ABAQUS was adopted. A damage plastic model for concrete and elasto-plastic model for steel were used respectively. Six composite frame tests were carried out to verify the FE model. Each test frame specimen consisted of two concrete-filled SHS columns and a steel beam to represent an interior frame in a building. The results obtained from the finite element model were verified against those obtained from the experimental results. [Copyright &y& Elsevier]

Abstract: The behaviour of self-consolidating concrete (SCC) filled hollow structural steel (HSS) stub columns subjected to an axial load was investigated experimentally. A total of 50 specimens were tested. The main parameters varied in the tests are: (1) sectional types: circular and square; (2) steel yielding strength: from 282 to 404 MPa; and (3) tube diameter or width to wall thickness ratio ( or ): from 30 to 134. A mechanics model is developed in this paper for concrete-filled HSS stub columns. A unified theory is described whereby a confinement factor is introduced to describe the composite action of the steel tube and the filled concrete. The predicted load versus deformation relationship was in good agreement with test results. The theoretical model was used to investigate the influence of important parameters that determine the ultimate strength of the composite columns. The parametric and experimental studies provide information for the development of formulae for the calculation of the ultimate strength and the axial load versus axial strain curves of the composite columns. Comparisons are made with predicted stub column strengths using the existing codes, such as ACI-1999, AISC-LRFD-1999, AIJ-1997, BS5400-1979 and EC4-1994. [Copyright &y& Elsevier]

• Transient-state tests are performed to study the thermal creep of Grade 1200 UHSS. • A robust creep model is developed, exclusively for transient-state fire conditions. • The yield strength of the UHSS in fire is significantly overestimated by the available design standards. • For the safe fire design of UHSS components, thermal creep needs to be explicitly considered in calculations. The literature on the thermal creep of steels with nominal yield strengths greater than 700MPa is scarce. Conventionally, mathematical models simulating the thermal creep of steel are developed based on steady-state tests. Transient-state testing, however, replicates structural fire conditions more realistically, and consequently, yields more reliable results for the fire design of structures. Hence, in response to the existing knowledge gap, the thermal creep of Grade 1200 ultra-high strength steel (UHSS) is examined and modeled in this study using the transient-state testing approach. First, by performing transient-state tests at a 10°C/min heating rate, the stress-strain diagrams of the UHSS at elevated temperatures are obtained, in which thermal creep has been implicitly included. Thermal expansion, elastic modulus, and yield strength of the material are derived at temperatures up to 750°C and compared with the predictions given by three world-leading design codes of practice. Due to the distinct microstructure and chemical composition of the UHSS, a significant disparity is observed between the results obtained for the UHSS and specifications provided by the design codes for mild steel. Then, based on transient-state tests at the heating rates of 5 and 20°C/min, an analytical thermal creep model is developed for the UHSS. Exclusively tailored to the transient-state conditions such as real fires, the proposed creep model is capable of simulating tertiary creep, and hence, ideal for structural fire design applications. The model is validated against the experimental data obtained for the heating rate of 10°C/min, where the model predictions are found to be in very good agreement with experimental results. By the explicit inclusion of thermal creep using the creep model, it is then shown that under 5 and 20°C/min heating rates, the yield strength of the UHSS can experience a difference as great as 30% at fire temperatures; hence, the necessity of the explicit modeling of thermal creep for the safe and accurate fire design of UHSS components is concluded. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights • A novel friction-based mechanical clamping system was developed to anchor CFRP plates to steel. • No reduction in the ultimate capacity of the system was observed after 10 million fatigue cycles. • Performance of the proposed PUR system was experimentally compared to that of the PBR solution. • A very good correlation was found between the experimental results and the proposed analytical models. • Prestressed NM CFRP reinforcements are much more effective than nonprestressed UHM CFRPs. Abstract In this study, a novel unbonded mechanical clamping system was developed for the strengthening of tensile metallic members using prestressed carbon fiber reinforced polymer (CFRP) plates. The system clamps a pair of prestressed CFRP reinforcement to a metallic substrate and provides an almost uniform contact pressure over the CFRP plate along the anchorage length. A finite element simulation was used to optimize the design of the mechanical components of the system. Subsequently, a set of static and fatigue tests was performed to evaluate the performance of the optimized design. Experimental results revealed that the proposed mechanical clamping system is capable of transferring the entire tensile capacity of the CFRP plates to the steel substrate, even after experiencing 10 million fatigue cycles. The comparative performance of the developed clamps was further investigated by a set of static tests on steel plate specimens strengthened with the prestressed bonded reinforcement (PBR) and the newly developed prestressed unbonded reinforcement (PUR) systems. Furthermore, simple analytical models are proposed to formulate the stress state in prestressed unbonded and bonded CFRP-strengthened tensile metallic members. The accuracy of the proposed analytical formulations was verified by the experimental results obtained during the current study. Experimental results revealed that the efficacy of having relatively high prestressing forces in the normal modulus (NM) CFRP reinforcements is much higher than the stiffness improvement obtained by using ultra-high modulus (UHM) CFRPs. However, the available capacity of the PBR system before debonding failure is far lower than that of the developed PUR solution. [ABSTRACT FROM AUTHOR]

Highlights • CFRP-steel double-lap joints subjected to environment and fatigue attack was studied. • Effects of salt fog and high RH at the curing stage of adhesive were focused. • The bond strength declined from 1% to 11% after harsh environment and fatigue cycles. • Silane treatment was promising to maintain the bond strength and stiffness. • Harsh environment at curing was detrimental to the properties of the adhesive. Abstract External bonding with carbon fiber-reinforced polymer (CFRP) has been considered great potential in fatigue strengthening of defected steel components. Generally, it takes one to two weeks for a structural adhesive to achieve full strength in such a repair scheme. The potential risk of an environmental attack at the curing stage of adhesive has not been well understood. This paper presents an experimental study on the interfacial behavior of CFRP-steel double-lap joints subjected to salt fog spray or high relative humidity (RH) at the curing stage. Afterwards, the specimens were statically loaded to failure or applied by a pre-set number of fatigue cycles followed by a static test. CFRP laminate-patched specimens had the ability to resist six million high-stress range cycles while CFRP sheet-patched specimens only survived two million low-stress range cycles. After environmental exposure and fatigue loading, the bond strength loss ranged from 1% to 11%. Proper silane treatment was promising to maintain both the strength and stiffness of the bonded joints. Exposure to the harsh environment at the curing stage was detrimental to the mechanical properties of the structural adhesive Araldite 420. The unfavorable effects on the elastic modulus and ultimate strain were comparable to that of long-term exposure for well cured structural adhesive. [ABSTRACT FROM AUTHOR]

Abstract Impact toughness properties of four kinds of high strength steel (HSS) were investigated with the nominal yield strength 460, 690, 800 and 960 MPa, respectively. The specialty of impact toughness among base metal, heat affected zone (HAZ) and weld metal were also compared and discussed. The correlations between the impact toughness of HSS and their nominal yield strength, plate thicknesses, testing temperature, welding methods were further studied. Combined with the scanning electron microscope (SEM) fracture surface observation, the microstructural features and corresponding fracture mechanisms have been analyzed. The results show that the impact toughness for HSS base metal deteriorated with the increasing nominal yield strength. The impact toughness of weld metal for HSS is much lower than that of the corresponding base metal. The impact performance of HAZ is discretized due to the welding thermal cycling on the HAZ where significant changes in grain structure and properties occur. The microcosmic fracture surface of HSS is examined to explain the reduction in impact performance under low temperature. Highlights • Four types of high strength steel (460, 690, 800 and 960 MPa) were investigated. • Impact toughness among base metal, HAZ and weld metal were measured and compared. • The fracture mechanism was examined using SEM microstructure analysis. [ABSTRACT FROM AUTHOR]

The use of carbon fibre reinforced polymer (CFRP) for strengthening of steel members has attracted increasing attentions in the structural engineering community. However, there is still a lack of understanding on the mechanisms of CFRP strengthening of steel tubular sections which are subjected to torsion. This paper presents a comprehensive experimental study on the effectiveness of the CFRP material for strengthening of steel circular hollow section (CHS) under pure torsion. Five different types of CHSs with various diameters and wall thicknesses were selected. Furthermore, five different types of CFRP strengthening schemes were used with the fibers aligned in different angles respect to the axial direction of the CHSs. In order to investigate the wrapping sequence, spiral (S) and reverse spiral (R), different mixed S/R schemes, such as SRRR vs. RRRS and etc., were applied to the CHSs. The abbreviations of the wrapping scheme, such as SRRR, refer to four strengthening layers with the spiral (S) and reverse spiral (R) wrapping directions. Various failure modes of the CFRP-strengthened specimens were observed including CFRP rupture, CFRP crushing and adhesive cracking without fiber failure. It was found that the CHS with larger diameter and thickness benefit the most from the CFRP strengthening scheme. In addition, S wrapping was found to be the most effective strengthening scheme. Finally, in order to achieve the best strengthening performance of the system for a mixed S/R CFRP strengthening scheme, it was recommended to apply the S wrapping inside. A theoretical model was proposed for the prediction of the torsional capacities of the CHS specimens. Reasonable agreement was achieved between the predictions and the experimental results. [ABSTRACT FROM AUTHOR]

There is an increase in the use of steel reinforced concrete (SRC) beams in high-speed railway platforms and bridges that are subjected to fatigue loading. It was found by the authors in their previous research on SRC beams under fatigue loading that flexural fatigue failure was the dominating failure mode when a large shear span-to-beam depth ratio ( λ ) of 2.63 was used. Research on reinforced concrete (RC) beams under fatigue loading and research on SRC beams under static loading revealed that shear failure may dominate when the shear span-to-beam depth ratio gets smaller. It is unknown if shear fatigue failure will happen in SRC beams with smaller shear span-to-beam depth ratio. This paper presents an experimental investigation of fatigue behavior of SRC beams with shear span-to-beam depth ratio of 1.8, 1.5, 1.1 and 1.0. It was found that shear fatigue failure did not occur in all twenty-seven SRC beams tested even for λ ratio as low as 1.0. Because the failure mode is found to be the same (i.e. dominated by flexural fatigue failure) for all the SRC beams with λ ranging from 1.0 to 2.63, the test results of all 39 SRC beams under fatigue loading were grouped together to derive fatigue strength curves for SRC beam design. [ABSTRACT FROM AUTHOR]

Numerous studies in the literature have shown that the strengthening of steel members using carbon fiber reinforced polymer (CFRP) composites can significantly extend the fatigue life of these structures. However, not enough attention has been focused on the potential of prestressed CFRP reinforcements for fatigue crack arrest in such members. In the current study, a simple analytical model is proposed to calculate the required prestressing level in the CFRP reinforcements in order to arrest the propagation of an existing fatigue crack in tensile steel members. Furthermore, a novel mechanical unbonded system is developed to anchor the high prestressing forces in CFRP reinforcements to the steel substrate using friction. A set of fatigue tests are performed on unstrengthened and strengthened precracked steel plates to verify the proposed model. The experimental results of the current study showed that the application of nonprestressed ultra-high modulus CFRP plates as externally bonded reinforcements can increase the fatigue life of precracked steel plates by a factor of 4.3. However, fatigue crack arrest is only possible when prestressed CFRPs of a certain prestressing level are used. Based on the analytical, numerical, and experimental results of the current study, it can be concluded that existing fatigue cracks in tensile steel members can be arrested using the proposed prestressed unbonded reinforcement system with the initial prestressing level calculated using the proposed model. In addition, some design recommendations are provided for fatigue crack arrest in practical cases. [ABSTRACT FROM AUTHOR]

The behaviour of Carbon Fibre Reinforced Polymers (CFRP) reinforced mild steel isolated plate (strut) under uniform axial compression is not well understood because there is a lack of experimental research. This paper presents experimental results for a series of struts reinforced with CFRP sheets subjected to uniform axial compression. The main parameters examined were the thickness of the steel plates, and the location and the number of the CFRP sheets. Three different thicknesses of steel plates were examined, namely, 2, 3 and 4 mm. The testing program included 21 struts with three different width-to-thickness ratios of 50, 66.7, and 100. The CFRP sheets were adhesively bonded where the fibres were aligned in the longitudinal direction parallel to the direction of the applied force. The results showed that the debonding failure strain in the compression face occurred in the range of 0.3–0.6% for all the composite specimens. It was found that the amount of permanent damage in the composite strut identified by the residual displacement is significantly less than that of the bare strut. Experimental results showed a global increase in strength, energy absorption and ductility for all test specimens where the greatest percentage increases were 452%, 259% and 107%, respectively. The 2 and 3 mm composite specimens showed greater percentage increases than the 4 mm composite specimens due to the relative thickness of the carbon fibre sheets verses steel. All experimental specimens failed by overall buckling, achieving maximum capacity at the end of their elastic range showing that the specimens were successfully bonded and acting as composites within such range. An existing rigid plastic theory considers initial imperfections (defects) was modified and used to predict the response of the struts. Experimental and theoretical curves showed consistency where initial defects controlled the elastic-inelastic behaviour and the load bearing capacity of the strut. In general, specimens with initial defects produced a smooth elastic-plastic curve, while specimens with little to no initial defects showed a sharp peak at the buckling load. The rigid plastic theory well predicted the response of the bare specimens whereas it reasonably predicted the plastic collapse curve of the composite struts. [ABSTRACT FROM AUTHOR]

This paper presents a detailed experimental study on the mechanical properties of very high strength (VHS) steel at elevated temperatures. Applications of VHS steel as a structural material are currently limited due to a lack of research supporting its use and relevant design guidelines, such as loading under fire. Samples of VHS steel underwent testing to failure at temperatures ranging from room temperature up to 600°C. The yield strength, ultimate strength and strength relating to 0.5, 1.5 and 2.0% strain were compared with corresponding values for mild and high strength steels available in the literature, and also with empirical equations available in Australian and American standards. Comparisons were made based on the suitability of the standards for application of plastic analysis of the section made of VHS steel at both ambient and elevated temperatures. It is shown that the empirical relations recommended by the standards cannot be used to predict the mechanical properties of VHS steel. The deterioration in VHS steel strength at elevated temperatures is not similar to mild steel or high strength steel. A greater reduction in strength at elevated temperatures is observed and this is because the ‘quench and temper’ manufacturing process used to create the VHS generates a fundamentally different microstructure, with a different thermal stability, compared to mild and high strength steels. Predictive equations of the reduction factors were proposed for yield strength and ultimate strength for temperatures up to 600°C. The results presented in this paper have the potential for inclusion in the prescriptive code rules for rational fire engineering based designs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thermomechanical simulation using Gleeble proved to be a reliable approach to replicate and reconstruct HAZ subzones. • Ferrite and granular bainite as the primary cause behind the HAZ softening. • Acceptable reliability of the hardness models based on microstructures to predict soft zones. • Controversial simultaneous decrease in hardness and toughness was associated with carbide precipitation in tempered phases. Ultra-high strength steels (UHSS) have a determining role in construction and industry. Furthermore, welding as the primary joining process for steel has a similar role in promoting its applications. Therefore, welded UHSS have a vital role in related applications. However, due to their complex microstructures, these steels are more prone to harmful effects of welding heat input on the mechanical properties compared to mild steels. Thus, identifying the correlations between the microstructural transformations triggered by the heat input and the mechanical properties can lead to new insights and hindering the drawbacks. This study investigates the microstructures and mechanical properties of S960 (with a severe softening after welding) and S1100 (with a negligible decrease of the mechanical properties after welding) to understand the mechanisms behind the softening of welded UHSS. Microstructural analysis showed the formation of soft phases, e.g., ferrite and granular bainite, as the primary reason for the softening. Furthermore, tempered forms of martensite and bainite resulted in the simultaneous decrease of hardness and notch toughness. Finally, the applicabilities of two experimental approaches to predict hardness from microstructural constituents were evaluated for welded S960 and S1100 and proved to have relatively good reliability to detect their HAZ softened spots. [ABSTRACT FROM AUTHOR]

• Conducted tensile tests on CFRP-strengthened butt-welded ultra-high strength steel (UHSS) • Performed FE analysis of CFRP-strengthened butt-welded UHSS under tension. • Compared with results of CFRP-strengthened mild steel under tension. • Compared with results with high frequency mechanical impact treatment and TIG-dressing. Due to the softening at the heat-affected zone (HAZ) in ultra-high strength steels (UHSS), the mechanical properties can deteriorate depending on the steels' grade, welding heat input, and its associated cooling rate. Hence, a design based on the properties of the base material (BM) may not be appropriate, and there is a need to retain the lost properties of the weldment. This study presents an effort to strengthen butt-welded UHSSs using ultra-high modulus (UHM) carbon fiber reinforced polymer (CFRP) plates. Although the rehabilitation and strengthening of steel structures using CFRPs have been available during the last two decades, they have not been used to strengthen higher grades of steel such as UHSSs. For this aim, a series of tensile tests were carried out on butt-welded UHSSs strengthened with adhesively bonded UHM CFRP plates on both sides of the weldment. Two welding processes, i.e. the gas metal arc welding (GMAW) and laser welding, were implemented separately to join the plates. Microhardness measurements and tensile tests were carried out for the experimental part, as well as finite element (FE) analysis of the joints. The results were also compared with other available methods, i.e. high-frequency mechanical impact (HFMI) treatment and TIG-dressing (tungsten inert gas) of the weld toe that are commonly used to increase the fatigue life of UHSS joints. It was found that CFRP strengthening can not only retain the lost properties of the weldment but can also increase the joint strength up to 32% compared to the BM, and a considerable 50% higher stiffness. In contrast, the other methods (HFMI and TIG-dressing) had no impact on the tensile characteristics of the weldments. [ABSTRACT FROM AUTHOR]

Abstract: Degradation of structural adhesives at elevated temperatures makes the time-dependent behaviour of adhesively-bonded steel/CFRP joints a critical issue for safety considerations of CFRP strengthened steel structures. This paper reports the examination of specimens at different load levels (i.e. 80%, 50%, and 20% of their ultimate load measured at room temperature) and constant temperatures from 35°C to 50°C (i.e. temperatures below and above the glass transition temperature Tg , 42°C of the adhesive). Furthermore, a scenario of cyclic thermal loading between 20°C and 50°C was included to represent more realistic exposure. Joint time-dependent behaviour was demonstrated by the stiffness and strength degradation as a function of not only temperature but also time. At the same temperature level close to or above Tg , a higher load level corresponded to a shorter time-to-failure. In addition, up to 47% of strength recovery was found for the specimens subjected to cyclic temperatures compared with those under constant 50°C which failed at the same load level. Based on the proposed temperature and time-dependent material property models, the time-dependent failure time of steel/CFRP double strap joints was well described and validated by the experimental results. [Copyright &y& Elsevier]

The mechanical properties and fatigue performance of four typical high-strength structural steels (HSSs) with grades of 460, 550, 690, and 960 MPa were experimentally investigated. The influence of the HSS plate thickness t (e.g., 10 mm and 20 mm) was also considered. The results show that as the HSS steel grade increases, the scale of the yield plateau in the tensile test gradually decreases and becomes insignificant. The developments of the strain and stress are also limited, leading to a significant increase in yield ratio, i.e., from 0.68 in Q460 to 0.95 in Q960. In a high-cycle fatigue test, the fatigue strength of the HSS increases significantly along with an increase in steel grade and is remarkably higher than that recommended by current standards. In the natural S − N fitting results, the fatigue strength of the HSS increases in the steel grade order of Q460, Q550, Q690, and Q960, with the values of fatigue strength being 177 < 227 < 233 < 251 MPa, respectively. The ratio of the fatigue strength to yield strength gradually decreases from 0.43 in Q460–10 to 0.26 in Q960–10 as the steel grade increases, signifying the lag effect of the growth in fatigue strength compared with the static strength. The slope m of the HSS S − N curves in the natural fitting is closer to 3.0 when the steel grade is relatively lower (Q550 MPa and below) and is closer to 4.0 or even 5.0 at higher steel grades (690 MPa and above). Based on the experimental analysis, S−N design curves are recommended for the each HSS material analyzed in the study, to reflect their actual fatigue performance. [Display omitted] • Fatigue tests of high-strength structural steel series were performed with steel grades of Q460, Q550, Q690 and Q960. • Fatigue failure modes of these HSSs including crack initiation and propagation were discussed. • Different approaches to S-N curve fitting of these HSSs were studied. • The S-N curves suitable for fatigue design of these HSSs were recommended. • Fatigue strength of HSSs is significantly higher than those for normal-strength structural steel in current design codes. [ABSTRACT FROM AUTHOR]

Abstract: The transverse distribution of wheel loads in orthotropic decks generates significant out-of-plane bending moments in the deck plate and rib wall at the rib-to-deck joint. Due to the relatively small thickness of both the deck plate and rib wall, the out-of-plane bending moments result in high local flexural stresses causing fatigue cracks to develop at the joint. In this study, the transverse stresses within the joint region that arise under the action of wheel loads are investigated with finite element analyses. Based on the stress results and the basic theories of linear elastic fracture mechanics, the design fatigue strength is determined for the joint. Factors affecting the stress range are also studied. The analyses show that the surface stresses in the deck plate are much larger than those in the rib wall in the case of 75% weld penetration into the rib wall, indicating that the fatigue strength of the joint is governed by the fatigue cracks propagating into the thickness of deck plate. It is also shown with finite element analyses that increasing the distribution area of wheel load or the thickness of deck plate can reduce the stress range of the deck plate and increase significantly the fatigue life of the joint. [Copyright &y& Elsevier]

Column-to-column bolted end-plate connections with stiffeners welded on the end plates and column side faces for steel modular buildings were investigated, to understand the effect of stiffener configurations on tensile capacity and stiffness. The connection performance was experimentally examined in tension, focusing on the flexure behaviour responses of the end plates, especially the yield lines on the end plates. Detailed finite element analyses (FEA) were also conducted to model the connection behaviours, considering bolt pretensions, contact behaviours and detailed bolt geometry, showing good comparison with experiments. The experimental and FE studies indicated that stiffeners at locations such as the corners of the steel tube could improve the tensile capacity by about 60%. On the basis of the FE modelling, a comparative study was subsequently performed to clarify the effects of stiffener configurations, end-plate thickness, and number of bolts on the tensile capacities and stiffness of such bolted end-plate connections. The effects of stiffener configuration on tensile capacity were also described in detail, based on yield-line theory. • Bolted end-plate connections with various stiffener configurations for tubular members. • Welded stiffeners for improving connection tensile performance. • Tensile failure of the connections governed by flexural yielding of the end-plates. • Detailed FE analysis parametric study for understanding of connection behaviour. • Yield line pattern on the end-plate analysed to clarify the effect of stiffeners. [ABSTRACT FROM AUTHOR]

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]