Strengthened Steel Members Research Articles

Effectiveness of Fan Anchors in Preventing Debonding in FRP-Strengthened Steel Members

Fibre Reinforced Polymers (FRPs) are extensively employed to strengthen existing structures because of their several advantages over other strengthening techniques. On the other hand, the premature debonding of FRP reduces its effectiveness in strengthening steel structures. Anchoring FRP composites is an effective solution to delay or even prevent their debonding. Very limited anchorage methods, however, have been introduced for FRP-strengthened steel structures and the need for an effective anchorage system remains. Fan anchor has been validated as one of the remedies against debonding failure in FRP-strengthened concrete structures. Considering the advantages that fan anchors offer, the use of fan anchors for FRP-strengthened steel structures is proposed and evaluated in this paper. Since FRP-steel joints have a different bond-slip law than FRP-concrete joints and the strengthened steel members are prone to buckling-debonding interactions, this study focuses on the efficiency of fan anchors in delaying FRP debonding by assuming that an adequate mechanical connection between the dowel and the steel substrate is provided. Three experimental studies involving shear, flexural and buckling strengthening of steel components were simulated through finite element modelling, and fan anchors were added to the models after validation. The effect of fan anchors on strength, failure mode and FRP's strain distribution of the models was examined. The study showed that the fan anchor was successfully able to delay debonding mode, which increased the strength and ductility and exploited a higher strain capacity of FRP plates.

Experimental Investigation on the Buckling Capacity of Angle Steel Strengthened at Both Legs Using VaRTM-Processed Unbonded CFRP Laminates

Strengthening steel structures by using carbon fiber reinforced polymer (CFRP) laminates showed a growth trend in the last several years. A similar strengthening technique, known as adhesive bonding, has also been adopted. This paper presented a promising alternative for strengthening steel members against buckling by using vacuum-assisted resin transfer molding (VaRTM)-processed unbonded CFRP laminates. A total of thirteen slender angle steel members (L65x6), including two control specimens, were prepared and experimentally tested. The specimens were strengthened only at both legs and were allowed to buckle on their weak axes. The test showed that the unbonded CFRP strengthening successfully increased the buckling capacity of the angle steel. The strengthening effect ranged from 7.12% to 69.13%, depending on various parameters (i.e., number of CFRP layers, CFRP length, and angle steel’s slenderness ratio). Flexural stiffness of the CFRP governed the failure modes in terms of location of plastic hinge and direction of buckling curvature.

Development of nail-anchor strengthening system with iron-based shape memory alloy (Fe-SMA) strips

This work discusses the development process of a direct fastening technique for strengthening steel members using iron-based shape memory alloy (Fe-SMA) strips. First, several lap-shear tests were performed to examine the efficiency of five nail-anchor systems in terms of load-carrying capacity, slippage, and convenience of installation. To this end, 3D optical digital image correlation (DIC) measurements (using an ARAMIS system) and numerical simulation were conducted. Different failure modes including tensile rupture of the strips, shear rupture of the nails, and/or sliding of the strips were observed and discussed. The optimum nail-anchor system was then determined. This system has a very short anchorage length of approximately 50 mm and only 12 nails (i.e., 3 × 4). Second, to demonstrate the efficiency of the proposed system, a steel I-beam was strengthened with the selected nail-anchor system and activated using an infrared heating (IRH) technique. The SMA-strengthened beam was subjected to static and then high-cycle fatigue loadings with a loading ratio of R=0.2 for 2.5 million cycles without any failure or damage, indicating reliable performance of the system. Owing to the larger coefficient of thermal expansion of the Fe-SMA material compared to that of steel, it was observed that a decrease and increase in the environmental temperature resulted in an increase and decrease of the pre-stress level in the Fe-SMA strip, respectively (i.e., an inverse correlation).

Behavior analysis of FRP tube/filling strengthened steel members under axial compression

Abstract Influence of the ultimate strength of stee This study presents a new fiber reinforced polymer tube/filling strengthening method (i.e., FRP tube/filling strengthening) that creates a new composite compression member with a novel mechanical behavior, “bilinear behavior.” Previous experimental and theoretical studies have been conducted based on the ultimate state of the section. To thoroughly study the stiffness and entire loading process of this novel member, finite element analysis (FEA) models are built in this paper. First, the model development process considers the initial defections and interface properties, and these models are verified by the experimental results. Two models of typical load-axial displacement curves describe the experimental results: a Bilinear model and a Trilinear model. Second, finite element modeling (FEM) is used to conduct a parametrical analysis that considers 7 parameters, and the results identify the mechanism of the strengthened member. Specifically, the Bilinear model corresponds to an unreinforced or relatively weakly reinforced specimen, and the Trilinear model corresponds to a relatively strongly reinforced specimen. If the exterior restraint system is weak, then the core steel buckles and subsequently yields; this behavior is described by the Bilinear model. If the exterior restraint system is strong (e.g., the length or the bending stiffness of the exterior restraint system is great), then the specimen can continue to bear a higher force after steel yielding until elastic-plastic buckling occurs; this behavior is described by the Trilinear model. Third, the FEA results and previous experimental and theoretical studies are used to obtain the stiffness and load-axial displacement curves, which are helpful for the design process. In general, relatively accurate FEA models are built in this paper that are capable of describing the mechanical behaviors of the novel member.

Short-term bond behavior and debonding capacity of prestressed CFRP composites to steel substrate

In this study, the short-term bonding behaviors of prestressed CFRP plates to steel substrates and their debonding capacities have been investigated. For this purpose, single lap-shear and prestress release tests were performed on adhesively bonded CFRP-to-steel joints. The feasibility of accelerated curing (AC) of the adhesive by heating was also investigated based on the conducted tests. Moreover, bond tests with partial prestress release and subsequent lap-shear loading were conducted to examine the feasibility of prestressed strengthening of steel members using AC. A three-dimensional (3D) digital image correlation (DIC) technique was utilized to monitor the bond behavior of CFRP-to-steel joints. Experimental results demonstrated that a mixed-mode I/II (tensile/shear) fracture governs the debonding failure of CFRP-to-steel joints during the prestress release. However, given that the steel substrate cannot undergo tensile failure, a relatively high prestressing force can be transferred to the steel substrate prior to debonding. The experimental results also revealed that the AC of the epoxy adhesive can be an advantageous alternative to the conventional room temperature curing (RTC) for strengthening steel members with prestressed bonded CFRP plates.

Strengthening Steel Members with Holes Under Tension Using Unidirectional GFRP Sheets

Steel structure sections are often connected using bolts and rivets, which necessitates the need for holes. These holes provide a zone of failure during loading. In the present work an attempt is made to strengthen the steel members under tensile loading with hole provision for bolts, using Glass Fiber Reinforced Polymer (GFRP) sheets. The performance of FRP wrapping is assessed by changing the angle of orientation of fiber. The experimental investigation is conducted for one hole, two hole and staggered hole arrangement and the results are compared with finite element analysis. The difference between experiment and finite element modeling are within allowable limits. From the study it is found that GFRP wrapping with 0° orientation of fiber is the most suitable in terms of ultimate strength for all the three hole configurations.

Fatigue design of CFRP strengthened steel members

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.

Effect of Thermal Distress on Residual Behavior of CFRP-Strengthened Steel Beams Including Periodic Unbonded Zones

This paper presents the residual behavior of wide-flange steel beams strengthened with high-modulus carbon fiber-reinforced polymer (CFRP) laminates subjected to thermal loading. Because the coefficients of thermal expansion of the steel and the CFRP are different, temperature-induced distress may take place along their interface. Periodic unbonded zones are considered to represent local interfacial damage. Five test categories are designed depending on the size of the unbonded zones from 10 to 50 mm, and corresponding beams are loaded until failure occurs after exposing to a cyclic temperature range of ΔT = 25 °C (−10 to 15 °C) up to 84 days. The composite action between the CFRP and the steel substrate is preserved until yielding of the beams happens, regardless of the thermal cycling and periodic unbonded zones. The initiation and progression of CFRP debonding become apparent as the beams are further loaded, particularly at geometric discontinuities in the vicinity of the unbonded zones along the interface. A simple analytical model is employed to predict the interfacial stress of the strengthened beams. A threshold temperature difference of ΔT = 30 °C is estimated for the initiation and progression of CFRP debonding. Multiple debonding-progression stages in conjunction with the extent of thermal distress appear to exist. It is recommended that high-modulus CFRP be restrictively used for strengthening steel members potentially exposed to a wide temperature variation range.

Comparative durability study of CFRP strengthened tubular steel members under cold weather

In strengthening systems, the carbon fibre reinforced polymer (CFRP) materials typically have excellent resistance against environmental conditions; however, the performance of adhesives between CFRP and steel is generally affected by various environmental conditions such as marine environment, cold and hot weather. This paper presents the comparative durability study of CFRP strengthened tubular steel structures by using two different adhesives such as MBrace saturant and Araldite K630 under four-point bending. The program consisted of testing twelve CFRP strengthened specimens having treated with epoxy based adhesion promoter, untreated surface and one unstrengthened specimen and conditioned under cold weather for 3 and 6 months to determine the environmental durability. The beams were then loaded to failure in quasi-static manner under four-point bending. The structural responses of CFRP strengthened tubular steel beams were compared in terms of failure load, stiffness and modes of failure. The research findings show that the cold weather immersion had adversely affected the durability of CFRP strengthened steel members. Design factor is also proposed to address the short-terms durability performance under cold weather.

Numerical studies on CFRP strengthened steel columns under transverse impact

Strengthening of metallic structures using carbon fibre reinforced polymer (CFRP) has become a smart strengthening option over the conventional strengthening method. Transverse impact loading due to accidental vehicular collision can lead to the failure of existing steel hollow tubular columns. However, knowledge is very limited on the behaviour of CFRP strengthened steel members under dynamic impact loading condition. This paper deals with the numerical simulation of CFRP strengthened square hollow section (SHS) steel columns under transverse impact loading to predict the behaviour and failure modes. The transverse impact loading is simulated using finite element (FE) analysis based on numerical approach. The accuracy of the FE modelling is ensured by comparing the predicted results with available experimental tests. The effects of impact velocity, impact mass, support condition, axial loading and CFRP thickness are examined through detail parametric study. The impact simulation results indicate that the strengthening technique shows an improved impact resistance capacity by reducing lateral displacement of the strengthened column about 58% compared to the bare steel column. Axial loading plays an important role on the failure behaviour of tubular column.

Crack-dependent response of steel elements strengthened with CFRP sheets

This paper presents the crack-dependent response of steel members strengthened with carbon fiber reinforced polymer (CFRP) composite sheets subjected to axial tension. A modeling approach is proposed to represent debonding of the CFRP and crack propagation across the steel section. Such an approach is incorporated to a three-dimensional finite element analysis (FEA) model and validated with experimental data. A total of nine specimens are constructed to study the effect of CFRP-strengthening on the behavior of strengthened steel members having various crack properties. Investigation parameters include the level of damage in the members represented by a0/h ratios (a0=notch size and h=specimen height) and CFRP-reinforcement ratios (ρCFRP). The strengthening effect is noticeable when the a0/h ratio increases. The crack propagation rate of the strengthened members is dependent upon the debonding characteristics of the CFRP and the size of the a0/h ratio. CFRP-strengthening results in significant improvement in energy release rate, including moderate increases in J integral. Overall, the interaction between the a0/h ratio and ρCFRP is substantial for the strengthened members.

Convergent Stresses at the Adhesive Ends of a Steel Member Strengthened with Carbon Fiber Reinforced Polymer Plates

Recently, repairing or strengthening steel members with externally bonded Carbon Fiber Reinforced Polymer (CFRP) plates has been widely reported. In these reports, some criteria for preventing the debonding of the CFRP plates were proposed. In Japan, although CFRP plates have been employed to enhance steel members that are deteriorating from corrosion attack or fatigue cracks or have been applied to upgrade the load-carrying capacity of steel members, there is no clear standard for preventing the debonding of CFRP plates from steel members. In this study, to assist in determining the design criterion for preventing debonding, we derived and solved dierential equations that calculate the stresses in steel members with CFRP plates under concentrated and uniformly-distributed loads. Further, this paper presents the convergent stresses in the adhesive at the ends of the CFRP plates that were strengthening steel members. The convergent stresses at the adhesive ends are sucient to allow CFRP plates to be used under any load conditions.

Bond characteristics between ultra high modulus CFRP laminates and steel

The use of high modulus CFRP laminates in strengthening steel members has the advantage of increasing the strength and stiffness of such members. In this paper, the bond characteristics between ultra high modulus (UHM) CFRP laminates with a modulus of 460 GPa and steel were studied. A series of experiments with double strap steel joints bonded with UHM CFRP laminates were conducted. Experimental results presented in this paper include failure modes, bond strength, effective bond length, CFRP strain distribution, adhesive layer shear stress distribution and bond slip relationship. Comparison was made with previous research on CFRP sheet–steel and normal modulus CFRP laminate–steel systems and different aspects of bond characteristics were discussed. Theoretical models were employed for the prediction of the specimen bond strength and effective bond length, and their applicability for UHM CFRP–steel joints was verified by comparisons with experimental results. Nonlinear finite element analysis was carried out to simulate the experimental specimens. The FEA results agreed well with those from experiments.

Analysis of adhesive joints used to bond FRP laminates to steel members – A numerical and experimental study

The strengthening of existing structures using bonded fiber reinforced polymer laminates has attracted a great deal of attention in recent years. However, when compared with concrete structures, the application of this method for strengthening structural steel members is somewhat limited. This is mainly due to the lack of an established design code for treating FRP-strengthened members. Issues such as the complexity of failure modes and the lack of knowledge of the force-transfer mechanism are obstacles that contribute to the difficulty associated with developing accurate design models. A few studies have underlined the need for new design approaches to strengthening steel members, since the existing design methods are not accurate enough. The first step towards developing new design models is to study these adhesive joints. This paper deals with analyses of adhesive joints used to bond CFRP laminates to steel substrates using a numerical and experimental approach. A numerical study of joints has been made utilizing the FE method, while, in the experimental part of the study, an optic measurement technique has been used. Different aspects of joint behavior, such as strain distributions along the bond line and through the thickness of the adhesive layer and failure mechanisms are discussed.

State-of-the-art review on FRP strengthened steel structures

The use of FRP (Fibre Reinforced Polymer) to strengthen steel structures has become an attractive option which may produce confident retrofitting of existing structures. This paper reviews the following areas that have received only small coverage in previous review articles, but have developed rapidly: the bond between steel and FRP, the strengthening of steel hollow section members, and fatigue crack propagation in the FRP–Steel system. Future research topics have also been identified, such as the as bond–slip relationship, the stability of CFRP strengthened steel members, and fatigue crack propagation modeling.