International Journal of Structural Stability and Dynamics

This paper presents the flexural behavior of notched steel beams repaired with carbon fiber-reinforced polymer (CFRP) strips. A combined experimental and computational approaches are used to examine local plasticity near the damage and the effects of CFRP-repair. A modeling approach is proposed to take into account the bond-slip behavior of CFRP-steel interface. The experimentally validated models are further used to conduct a parametric study addressing various engineering properties of CFRP composites and adhesives. The CFRP-repair is shown to restore the strength of the damaged beam. The CFRP strip relieves the stress concentration resulting from the presence of the notch, reducing the high local plasticity. The parametric study confirms the improved effectiveness of high modulus CFRP (i.e., exceeding 150 GPa) in affecting repairs of steel members. Under static loading conditions, the stiffness of the adhesive bond line influences the local behavior of the CFRP-steel interface but has little effect on the overall member behavior.

Results from finite element modeling (FEM) of large-scale steel-concrete composite beams strengthened in flexure with prestressed carbon fiber-reinforced polymer (CFRP) plate were validated with experimental results and presented in this paper. The effect of varying the level of prestressing as percentage of the ultimate tensile strength of the CFRP plate was investigated. Comparison was carried out in terms of overall load-deflection behavior, strain profile along the length of the CFRP plate, and strain distribution across the depth of the beam at mid-span section. Very good agreement was observed between the finite element (FE) and the experimental results. The validated FE models were used to perform a comprehensive parametric study to investigate the changes in the behavior through wider range of prestressing levels and then, determine the optimum prestressing level that maintain the unstrengthened beams' original ductility (or energy absorption). An iterative analytical model was also developed, validated with both the FE model and the experimental results, and showed good agreement. A parametric study was carried out to investigate the effect of changing the yield strength of the steel and the concrete compressive strength on the moment of resistance of the section and the strain in the CFRP plate at ultimate.

The use of bonded carbon fiber-reinforced polymer (CFRP) laminates to strengthen and upgrade existing structures has attracted a great deal of attention during the past two decades. Fiber-reinforced polymer (FRP) bonding has been widely researched and practiced in the strengthening of concrete members. However, when it comes to steel structures, it is somewhat limited in terms of field applications. One of the most important obstacles to the widespread use of FRP bonding in steel structures is the lack of design codes. This is mainly due to the lack of suitable design models for adhesive joints used to bond FRP laminates to steel substrates. Issues such as the lack of knowledge about the behavior of adhesive joints, the lack of suitable material models for structural adhesives, and analyzing adhesive joints are contributing to the difficulty associated with establishing design models.

This paper is mainly concerned with a proposal and verification of a new design model for adhesive joints used to bond FRP laminates to steel beams. The paper, first, shortly reviews the most commonly used failure criteria and presents the background to the newly proposed model. Quasi-static tests were then performed on steel plate and full-scale beam specimens bonded with CFRP laminates to evaluate the new design model. The new design model presented in this paper was found to be accurate in terms of predicting the ultimate load and failure mode of the joints. To illustrate the application of the new design model, an example is appended to this paper.

The confinement of concrete columns using fiber-reinforced polymer (FRP) jackets or wraps is a popular structural retrofitting technique. More recently, the benefits of FRP confinement of concrete-filled steel tubes have also been explored by researchers. Failure of such FRP-confined concrete-filled steel tubes is usually governed by the rupture of the FRP jacket in the hoop direction. However, the observed FRP hoop strain at failure (i.e. the hoop rupture strain) is typically lower than the ultimate tensile strain from a flat coupon test. Many factors may contribute to this phenomenon, one of which is the geometrical discontinuities at both the starting and finishing ends of the wrapping process commonly used to form an FRP jacket. This paper examines the effect of these geometrical discontinuities on the hoop rupture strain of FRP jackets in FRP-confined concrete-filled circular steel tubes. Detailed finite element (FE) analyses conducted using both linear elastic and elastic-perfectly plastic adhesive constitutive models are presented. Comparison between the FE predictions and available test results shows that the hoop rupture strains of FRP jackets predicted by FE analysis using an elastic-perfectly plastic adhesive model are in reasonable agreement with the test results. The influence of parameters such as the FRP thickness, FRP orthotropy, FRP elastic modulus, adhesive yield strength, adhesive thickness, and column size are examined.

This paper presents an experimental investigation of the performances of concrete-filled glass fiber-reinforced polymer (GFRP) and GFRP externally wound steel circular tubes, subjected to freeze-thaw cycles ranging from -18°C to 18°C. The variation in hoop strains of the tubes during the freeze-thaw cycles was monitored by embedded fiber Bragg Grating (FBG) strain sensors in GFRP layers or between GFRP and steel tube. The residual hoop strain after each freeze-thaw cycle indicates the possible degradation of GFRP materials, such as cracks, debonding of GFRP-concrete or GFRP-steel due to mismatch of the coefficient of thermal expansion, as well as water immersion. A synergistic effect of FRP and steel tubes on the confinement of inside concrete was revealed, resulting in well-improved ductility. After 56 freeze-thaw cycles, remarkable degradation were found in the axial strength, modulus, and strain for concrete-filled GFRP tubes. However, the GFRP-steel tube system showed a negligible reduction in the ultimate axial strain by the freeze-thaw cycles with less degradation in the axial strength and modulus.

Web crippling is the major failure mode of thin-walled members when they are subjected to concentrated loading. Carbon fiber-reinforced polymer (CFRP) is found to be promising for strengthening metallic structural members. This paper reports improved web-crippling capacity of sharp-corner aluminum tubular sections: rectangular hollow section (RHS) and square hollow section (SHS), by attaching CFRP to their webs. Twenty four specimens were tested with four CFRP strengthening configurations applied on each of six different aluminum RHS and SHS sections. Significant increase in load-carrying capacity was obtained. Further comparison is made between CFRP strengthened aluminum tubular sections and cold-formed steel counterparts in respect of strengthening efficiency. Underlying mechanism of different failure modes and strengthening efficiencies of various strengthening configurations are discussed with the assistance of FEM simulation.

This paper presents a novel way of strengthening thin-walled steel cylindrical shells against buckling during axial compression in which a small amount of fiber-reinforced polymer (FRP) composite, coated from both sides can increase the buckling strength effectively. The effects of the reinforcement and the angle of fiber orientation as well as initial geometric imperfections on the buckling load-carrying capacity have been made clear through the three kinds of analytical procedures; the conventional linear eigen value buckling analysis, the reduced stiffness (RS) buckling analysis and the fully nonlinear numerical experiments. These multiple treatments suggest obtaining valuable information for the design of FRP-based hybrid structural elements and discusses influence of FRP to increase the load-carrying capacity of the thin-walled metallic structures having complex buckling collapse behavior. This paper also discusses how the angle of fiber orientation affects on the buckling strength and the associated buckling modes of the thin-walled shells.

Fiber-reinforced polymer (FRP) strengthening technique to improve buckling resistance of steel members is presented in concept and experimental demonstration. The conceptual design of this method is introduced through the preliminary experiments on three specimens. Then, another 14 specimens are tested under axially compressive loading, by which the compressive behavior and the strengthening effects are investigated considering different design parameters and configuration, including the slenderness ratio, the confinement detail, the filled materials and the end connection. The strengthening effects are analyzed by the comparison of both theoretical and test results, which show that the overall buckling failure of steel members can be prevented by FRP strengthening and the ultimate loading capacity and deformation capacity of steel members are enhanced considerably. The maximum load-bearing capacity of strengthened members is 2.86 times of the nonstrengthened ones, and the failure maintains a ductile behavior. In addition, the load-bearing capacity of the members strengthened in this way is compared with the Euler loads of the original steel member and the composite member.

This paper reports an experimental study on the use of carbon fiber-reinforced polymer (CFRP) sheets to strengthen non-load-carrying cruciform welded joints subjected to fatigue loading. Failure modes and corresponding fatigue lives were recorded during tests. Scatter of test results was observed. Thereafter, a series of numerical analyses were performed to study the effects of weld toe radius, the number of CFRP layers and Young's modulus of reinforced materials on local stress concentration at a weld toe. It was found that fatigue life of such welded connections can be enhanced because of the reduction of stress concentration caused by CFRP strengthening. Parametric study indicates that the weld toe radius and the amount of CFRP are the key parameters influencing the stress concentration factors and stress ranges of the joint. Enhancement of modulus for adhesive and CFRP sheets can also be beneficial to the fatigue performance to some extent.

Cracked cross-beam connections made of thin-walled rectangular hollow sections (RHS) are repaired with carbon fiber-reinforced polymer (CFRP) sheets. Constant amplitude fatigue experiments are conducted on the repaired specimens. A pilot test is conducted on a T-connection of square hollow sections (SHS) to explore the effective method of applying CFRP. Due to the peeling effect at the corner region of the SHS-to-SHS connection, early debonding happens in the pilot test that only results in a slight extension in fatigue life. In the following experiments of repairing RHS-to-RHS cross-beam connections, circumferential or transverse restraining CFRP patches are applied in the corner region that prevents early debonding successfully and leads to significant increase in fatigue life. Finally, an improvement is made with the addition of steel strengthening plates that increase both the flexural stiffness and fatigue life of the cracked specimens significantly. The proposed retrofitting method may be useful for repairing other similar fatigue-cracked welded tubular connections.