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Concrete-filled double-skin FRP tubes (CFDST) are increasingly attracting researchers’ interests due to the advantages of their reduced self-weight and higher bending stiffness than fully filled tubes. However, the structural behaviour of CFDST, especially the non-uniform confinement in annular concrete, has not ever been well addressed. This paper presents an analytic study on axial compressed circular stub CFDST with FRP wrap/tube as outer tube and steel/FRP as inner tube. Based on existing studies on actively confined concrete, a constitutive model for non-uniformly FRP-confined concrete is developed in this paper. The dilation model for concrete fully filled FRP tubes is modified to account for the effects of void ratio so that the hoop-axial strain curve of CFDST could be reasonably predicted. Behaviours of steel and FRP tubes in CFDST are investigated and proper stress-strain models are proposed to estimate the loads shared by tubes. The stress state in annular concrete is theoretically studied by dividing the cross-section into multiple circular layers. Finally, an analysis-oriented load-strain model, which accounts for the non-uniform confinement, effects of void ratio, buckling of FRP tube, and strain hardening of stainless steel, is proposed for CFDST. As validated by the experimental data from a wide range of literature, the proposed model is reasonable and of high accuracy.

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.

Seawater and sea sand concrete (SWSSC) filled double-skin stainless steel (SS) tubes consist of two concentric circular SS tubes, sandwiching SWSSC between them. This paper presents an experimental study on SWSSC-filled double-skin SS tubes (CFDSTs) under axial compression. Unfilled circular hollow sectional (CHS) specimens, fully-filled tubes (CFSTs) and double-skin tubes without inner tube (CFHT) were also tested for comparison purpose. Load-axial strain curves, stress in concrete, post-peak behaviour and energy absorption are investigated in this study. The effects of some key parameters, including confinement factor, void ratio and tube slenderness ratio, on the structural behaviour of SWSSC-filled SS tubes are discussed. It is found that the ultimate stress in concrete is mainly affected by confinement factor and the post-peak behaviour depends on both confinement factor and inner tube slenderness. Formulas are proposed to estimate the load carrying capacity of SWSSC-filled SS circular tubes in compression.

This paper presents an application of prestressed carbon fiber-reinforced polymer (CFRP) plates for the strengthening of metallic girders of a roadway bridge in Melbourne, Australia. The study also describes the application of a wireless sensor network (WSN) system for long-term structural monitoring of the retrofitted bridge girders. A flat prestressed unbonded retrofit (FPUR) system was developed to apply prestressed CFRP plates to the steel girders of the Diamond Creek Bridge (122 years old), which is subjected to daily passenger and heavy vehicles. The first section explains the results of sets of static and fatigue tests performed in the laboratory to examine the efficiency of the proposed FPUR system prior to its installation on the bridge. The second section presents details of different aspects of the CFRP strengthening of the bridge girders, fatigue design criteria, and layouts for short- and long-term monitoring. For the short-term measurements, the bridge was loaded with a 42.5-ton semi-trailer before and after strengthening. The CFRP plates were prestressed up to approximately 980 MPa (≈38% of the CFRP ultimate tensile strength), which resulted in about 50% reduction in the maximum tensile stress in the bridge girders. The third section discusses the development, installation, and preliminary results of the WSN system used to monitor the pre-stress level in the CFRP plates. The results of the short- and long-term measurements in this study show that the FPUR system is very effective for flexural and fatigue strengthening of bridge girders. Finally, a set of recommendations for long-term structural monitoring is provided.

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-Bezier 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.

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.

Employing advanced material such as carbon‐fibre‐reinforced‐polymer (CFRP) in tension fatigue strengthening of aged roads and railway bridges have shown a great capability of arresting or delaying crack initiation and/or propagation in steel structures. However, it is not clear whether the fatigue behaviour and the CFRP strengthening efficiency is the same when the cracked-steel elements exhibit a state of complex loading. The aim of this paper is to investigate the fatigue behaviour of the CFRP strengthening of steel plates with central initial inclined cracks with a focus on the effect of the CFRP properties. The initial slit-like cracks were oriented to introduce a state of combined action of tension (mode-I) and shear (mode II) stresses at the crack tips. The key parameters in this study are the mixed‐mode (shear to tension stresses) ratio, the crack-starter length ratio (initial crack length to the plate width), patching configurations, and mechanical properties of the composite material. All the test specimens were artificially notched with central cracks of different damage levels. This study covered the fatigue performance of steel plates strengthened with two configurations of composite materials of different tensile stiffness (high modulus CFRP sheets, and normal modulus CFRP plates). Furthermore, the efficiency of strengthening systems of different fibre orientation relative to the initial crack angle was investigated. The outcomes of this study are extending the current knowledge of the CFRP strengthening to its applications on metal plates contain defects subjected to mixed-mode fatigue loading.

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.

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.

The use of FRP with seawater and sea sand concrete (SWSSC) holds great potential for marine and coastal infrastructure, and concrete-filled FRP tubular columns are among the attractive forms of structural members for such applications. This paper presents a theoretical model for the compressive behaviour of seawater and sea sand concrete-filled circular FRP tubular stub columns. FRP tubes can be manufactured to possess considerable strength and stiffness in the longitudinal direction, so the behaviour of concrete-filled FRP tubes differed substantially from that of concrete columns with an FRP wrap (also referred to as “concrete-filled FRP wraps”) which commonly contains fibres only in the hoop direction. Many theoretical models have been proposed for concrete-filled FRP wraps, but very limited work has been conducted on the theoretical modelling of concrete-filled FRP tubes. In the present study, an existing dilation model for concrete-filled FRP wraps is combined with a biaxial stress analysis of the FRP tube so that the effect of the Poisson’s ratio of the FRP tube is properly accounted for. In order to predict the buckling of the FRP tube, a maximum strain buckling failure criterion is proposed and is shown to be in reasonable agreement with the experimental results. Moreover, the load carried by the FRP tube is studied, and a simplified model is proposed to determine the load shared by the FRP tube during the entire loading process. Finally, a theoretical model for SWSSC-filled FRP tubular columns is proposed, in which the behaviour of both the concrete and the FRP tube as well as their interactions are explicitly modelled (i.e., an analysis-oriented model). The proposed model gives reasonably close predictions of the existing experimental data.

A CHS-CFSHS T-joint is fabricated by welding a circular hollow section (CHS) brace to a concrete-filled square hollow section (CFSHS) chord. This paper presents an investigation into stress concentration factors (SCFs) of CHS-CFSHS T-joints through experimental testing as well as finite element (FE) analysis. A series of well-designed CHS-CFSHS T-joints are first tested under axial loading (tension and compression respectively) and in-plane bending in the brace so that the strain concentration factors (SNCFs) of the joints are determined. By comparing the experimental results with those extracted from previous research on empty CHS-SHS T-joints, it is found that the in-filled concrete can effectively reduce the maximum SNCF and therefore SCF. Three-dimensional FE models are subsequently developed and validated. Then an extensive parametric study is conducted to evaluate the influences of three key non-dimensional geometric parameters (i.e. β, 2γ and τ) using the validated FE modeling strategy. Finally, SCF formulae corresponding to typical load conditions are established for fatigue design of CHS-CFSHS T-joints.

Hybrid fabricated columns (HFCs) have been investigated by researchers during the past two decades. Many researchers have reported higher load-bearing capacity, post-buckling strength and better energy absorption as the main advantages of this type of column compared to other structural sections. This provides an opportunity for the construction industry and designers to utilise these superior properties in building more reliable structures. However, no matter how capable and reliable a column is, it still needs a robust connection to make the use of this column in construction viable. Since the topic of connections between these columns and I-beams has been untouched, the authors decided to study this knowledge gap. In this research, a new generation of modular connections is proposed by the authors, referred to hereafter as M-HFC. The performance of this connection has been tested experimentally. In addition, a three-dimensional finite element model has been developed to simulate the behaviour of the connection. The results show that this connection is a fully resistant and semi-rigid connection according to the standard classifications, which makes it a good choice for use in moment frames.

Due to the increasing popularity of concrete-filled steel tubular (CFST) members, there will be more chances of vehicles/vessels or terrorist attacks on these structures in near future. Fibre-reinforced polymer (FRP) strengthening can be an effective option to reduce impact damage or failure of CFST members. However, existing knowledge is very limited in understanding the behaviour of FRP strengthened CFST structures under lateral impact loading. This paper outlines drop hammer impact test results of a series of experimental programs of bare and FRP strengthened CFST specimens. A total of sixteen CFST specimens were prepared and tested under lateral impact at their mid-span. The results indicate that permanent lateral displacement of CFST members can be reduced up to 18.2% by externally bonded FRP sheets. The effects of FRP type, FRP wrapping direction, carbon fibre-reinforced polymer (CFRP) wrapping layers, wrapping length, and impact velocity were investigated to understand the influences of these parameters on the behaviour of strengthened CFST specimens. CFRP laminates were found to be weak under impact loading when wrapped in only longitudinal direction. However, a combination of longitudinal and hoop layers of CFRP laminates, or only GFRP wrapping, can remarkably minimise the severity of damage and failure of FRP in CFST specimens under lateral impact. A comparison of current test results with recent works has been presented to understand the effect of impact energy on the lateral displacement control ability of FRP strengthened CFST members.

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.