Search

One of the common deficiencies of pre-70s reinforced concrete (RC) moment frames is the shear failure of beam–column (BC) joints. In addition to that, joints with high tension demand have limited shear capacity to resist seismic actions, according to the New Zealand (NZ) seismic assessment guidelines. Fiber-reinforced polymers (FRP) have been widely used in research and practice for seismic retrofitting of BC joints prone to shear failure. However, in many cases, the tests were conducted on two-dimensional BC joints, which does not necessarily represent the limitations structural engineers face in practice, such as the presence of the diaphragm slab, transverse beams, and access limitations to the joint panel. The American Concrete Institute guidelines for FRP strengthening only mention that strengthening joints is possible but do not provide design guidance. The European fib Bulletin 90 provides design guidance but assumes that the joint panel is always available to install FRP, which is rarely the case in real structures. We have proposed a design method for shear strengthening of beam–column joints using FRP sheets and FRP anchors based on the adaptation of aforementioned design documents, published research, and first principles of engineering. The intention is for this case study to inspire other engineers who face the same issue and to encourage further research in this area. [ABSTRACT FROM AUTHOR]

Short columns with deficiencies of low-strength concrete and large spacing of transverse reinforcement are quite common in construction practice in many developing countries in seismically active regions. Therefore, it is important to retrofit this type of column to avoid negative consequences in the case of severe damage. External fiber–reinforced polymer (FRP) jacketing is one of the most practical methods to retrofit reinforced concrete (RC) frame members against shear actions. However, knowledge of the seismic behavior of FRP-retrofitted RC short columns is still limited. Thus, the aim of this study was to investigate the seismic response of FRP-retrofitted low-strength RC short columns with and without predamage through the testing of 11 short column specimens representing old and deficient RC buildings. Accordingly, the results of a research program on the seismic behavior of such columns and on their repair and FRP retrofitting, covering experimental and analytical phases, are presented. In the experimental phase, test results obtained for short columns, FRP-retrofitted short columns, and repaired and retrofitted short columns are documented. Test results provide important knowledge about the effects of extremely low-strength concrete, and of different levels of predamage on the behavior of FRP-retrofitted substandard short columns. In the analytical phase, the performance of relevant design documents in predicting the shear strength of such columns retrofitted with FRP jacketing was tested through the comparison of predicted and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents investigations on the tensile behavior of several textile-reinforced concrete (TRC) material compositions after fire exposure. First, experimental investigations are presented, aiming to provide insights into the thermomechanical performance of fire-exposed TRC, and data that can be used for the development and implementation of analytical or numerical models to design fire-exposed TRC elements. The applicability of the Aveston–Cooper–Kelly (ACK) theory is assessed for ambient and increased temperatures. TRC specimens with various cases of reinforcement (carbon or glass fibers, uncoated or coated textiles, low or high fiber volume fraction) were manufactured and subjected to temperatures reaching 700°C. The residual tensile capacity of the exposed specimens is discussed, focusing on the effect of the varying reinforcement. It is verified that the use of uncoated carbon fibers is the most promising solution for maintaining the residual capacity after fire exposure. Finally, it is concluded that the ACK predictions are reliable for TRC specimens with good bond conditions and when adopting a representative thermal degradation law for the reinforcement. [ABSTRACT FROM AUTHOR]

The study presents the mechanics associated with the short-term behavior of glass-fiber reinforced-plastic (GFRP) box beams that include a layer of concrete and a carbon-fiber-reinforced-plastic laminate (CFRP) in the compression and the tension zone, respectively. This innovative concept results in cost-effective composite members with pseudoductile characteristics and high stiffness and strength properties. It can be thought of as a better way of producing composite structural members for simply supported spans, through an automated manufacturing technique such as pultrusion. The analytical results are verified by a series of bending tests on large-scale specimens and by the finite-element technique. The agreement between experiments and analysis was found quite satisfactory. A preliminary design procedure for the hybrid members is also presented, based on a complete set of stiffness, strength (flexural strength, web shear failure by either crushing or buckling, lateral instability), and ductility design requirements.

The time-dependent behavior of a new class of hybrid elements consisting of glass-fiber-reinforced-plastic (GFRP) box beams combined with a layer of concrete in the compression flange and a carbon-fiber-reinforced-plastic laminate (CFRP) as additional tension reinforcement is presented in this study. Shrinkage, creep, and fatigue models for concrete are combined with creep and fatigue models for GFRP (based on both laminate theory and semiempirical approaches) to yield a general analysis procedure for predicting the deformation and residual strength response of hybrid GFRP/CFRP/concrete beams subjected to sustained and/or alternating loading. The analytical model predictions are compared with experimental results obtained from creep and fatigue testing of large-scale members, giving very good agreement. The present study shows that the new hybrid beams possess very good time-dependent response characteristics, and the models described here, along with those given in the companion paper for short-term behavior, can be used as analytical tools for the optimum design of such members under both short-term and long-term loading actions.

Strengthening of concrete structures in flexure with externally epoxy-bonded carbon-fiber-reinforced plastic (CFRP) laminates is becoming an increasingly popular retrofit technique among researchers and engineers worldwide. A reliability study of such structures is the topic of this paper. First, considering the statistical character of the design variables (geometry, material properties), the effect of changes in these variables on the flexural capacity of CFRP-strengthened members is established. Variabilities in concrete strength and CFRP failure strain and area fraction are most influencial on flexural strength. Next, strength reduction factors are derived, with the aim of achieving a uniform reliability index of about three over a broad spectrum of design conditions. A general strength reduction factor [Phi] = 0.80 is proposed. Finally, the effect of each design variable on the reliability of the system is examined and quantified in terms of the reliability index.

In this study, the writers aim to provide a basic understanding of the creep behavior of wood members reinforced with fiber-reinforced plastic (FRP) materials epoxy-bonded to the tension faces. An analytical method is presented first for the deformation of cross sections based on a Burger model combined with a mechanosorptive element for wood, Findley's model for composite materials, and a stress-relaxation procedure to yield the cross-section stresses and strains as a function of time, for both constant and variable temperature and humidity conditions. Parametric studies assessing the effect of the type and area fraction of composite material on the creep response of FRP-reinforced wood members are also presented, for both constant and variable environment. Finally, the analytical model is employed to predict the time-dependent deflections of wood beams reinforced with carbon fiber-reinforced plastic (CFRP) laminates of different thicknesses, and the results of a short experimental program involving FRP-reinforced wood beams under constant environment conditions are also described. The (limited) results are in good agreement with the analytical model.

Strengthening of concrete structures with fiber-reinforced plastic (FRP) composites epoxy-bonded to their tension faces appears to be a method with a high potential for increasing the strength and stiffness of existing members. The writers aim at developing a fundamental understanding of the time-dependent (creep and shrinkage) behavior of reinforced-concrete beams strengthened with FRP laminates. An analytical procedure is given first for the deformation of cross sections based on the age-adjusted effective-modulus method for concrete, Findley's model for composite materials, and a stress-relaxation procedure to yield the cross-sectional stresses and strains as a function of time. Parametric studies assessing the effect of the type and the area fraction of composite material and of the area fraction of steel reinforcement on the long-term response of strengthened elements are also presented. Finally, the analytical model is used to predict the deflections of concrete beams strengthened with FRP laminates of different thicknesses, and an experimental program is described confirming the analysis.

The objective of this paper is to verify a mechanical model for the failure of plain or fiber-reinforced concrete shear key joints through the comparison of the model's prediction with both experimental measurements and finite element method (FEM) analysis results. In the comparison with experimental results, both entire load-displacement relations and the maximum shear stress or the shear strength of shear keys are considered. The comparison with nonlinear FEM analysis results is also carried out in terms of the load-displacement relations to supplement the assessment of the mechanical model's validity. The model is found in good agreement with both experiments and FEM analysis, indicating that it can be used as a reliable tool for predicting the response of shear key joints.

The objective of this paper is to develop a simple mechanical model for the analysis and design of plain or fiber-reinforced concrete shear key joints. The method makes use of well-known results of fracture mechanics and truss model theory, combined in a simple model. The analysis employs a single discrete crack model under wedging force and a smeared crack model under remote shear force. The proposed formulation identifies two main fracture mechanisms for shear-off failure of key joints: single curvilinear cracking and development of multiple diagonal cracks. Furthermore, as a first step in developing design aids for the shear strength of shear keys, a simple design formula is obtained. The procedures developed in this study would be useful in designing shear key joints according to the precast concrete segmental method of construction, which is becoming increasingly popular.

Wood has been used as a construction material historically but has a lasting reputation for impermanence and its limited uses. Improvements in design and treatment have evolved through the years, the most recent being FRP or fiber-reinforced plastics composite, that when mated to wood produces a new type of hybrid construction that is lightweight yet low-cost, increasing strength, stiffness and ductility. New studies show that even in very small areas the reinforced wood performs significantly better.

Several questions must be answered regarding Nikolaos Plevris et al's research on the creep characteristics of FRP-reinforced wood members. One is how the wood is generally assumed to have a modular ratio of about 1.05 when the modulus of elasticity is for bending. Also, shrinkage in the transverse direction will decrease the shear capacity of the reinforcement wood interface. Another is the need to consider that contraction results into strain-induced stresses.

The present study investigates experimentally the behavior of concrete confined with fiber reinforced polymers (FRP) in the form of jackets which are applied according to a number of nonconventional techniques. First, the effectiveness of various jacketing configurations combined with anchors as a measure of increasing the strength and deformability of L-shaped columns is investigated. It is concluded that easy to install and low-cost anchors made of resin impregnated fibers properly placed at the reentrant corner of L-shaped columns enable excellent mobilization of confining stresses supplied by the FRP jackets. Next, a number of alternative confinement methods are investigated on concrete cylinders, aimed at quantifying the effectiveness of (1) unbonded jacketing, (2) spirally applied strips attached only at their ends, and (3) jacketing directly on concrete with mortar plastering. Although the study may be regarded as preliminary, it provides useful experimental support to a number of techniques which have the potential to open new horizons in the field of externally applied FRP for enhancing concrete confinement. [ABSTRACT FROM AUTHOR]

The application of fiber-reinforced polymer (FRP) as a means of increasing the axial capacity of masonry through confinement, a subject not addressed before, is investigated in this study. Four series of uniaxial compression tests, with a total of 42 specimens, were conducted on model masonry columns with these variables: number of layers, radius at the corners, cross-section aspect ratio, and type of fibers. It is concluded that, in general, FRP-confined masonry behaves very much like FRP-confined concrete. Confinement increases both the load-carrying capacity and the deformability of masonry almost linearly with the average confining stress. The uniaxial compression test results enabled the development of a simple confinement model for strength and ultimate strain of FRP-confined masonry. This model is consistent with the test results obtained here but should attract further experimental verification in the future to account for types of masonry materials other than those used in this study. [ABSTRACT FROM AUTHOR]

The results of a comprehensive experimental program, aimed at providing a fundamental understanding of the behavior of shear-critical exterior reinforced concrete (RC) joints strengthened with fiber reinforced polymers (FRP) under simulated seismic load, are presented in this study. The role of various parameters on the effectiveness of FRP is examined through 2/3-scale testing of 18 exterior RC joints. Conclusions are drawn on the basis of certain load versus imposed displacement response characteristics, comprising the strength (maximum lateral load), the stiffness, and the cumulative energy dissipation capacity. The results demonstrate the important role of mechanical anchorages in limiting premature debonding, and they provide important information on the role of various parameters, including: area fraction of FRP; distribution of FRP between the beam and the column; column axial load; internal joint (steel) reinforcement; initial damage; carbon versus glass fibers; sheets versus strips; and effect of transverse beams. [ABSTRACT FROM AUTHOR]

Analytical models are presented in this study for the analysis of reinforced concrete joints strengthened with composite materials in the form of externally bonded reinforcement comprising unidirectional strips or flexible fabrics. The models provide equations for stresses and strains at various stages of the response (before or after yielding of the beam or column reinforcement) until the ultimate capacity is reached, defined by concrete crushing or fiber-reinforced polymer (FRP) failure due to fracture or debonding. Solutions to these equations are obtained numerically. The models provide useful information on the shear capacity of FRP-strengthened joints in terms of the quantity and configuration of the externally bonded reinforcement and may be used to design FRP patching for inadequately detailed beam-column joints. A number of case studies are examined in this article, indicating that even low quantities of FRP materials may provide significant enhancement of the shear capacity. The effectiveness of external reinforcement increases considerably if debonding is suppressed and depends heavily on the distribution of layers in the beam and column. The latter depends on the relative quantities of steel reinforcement crossing the joint panel and the level of axial load in the column. Analytical shear strength predictions were in good agreement with test results found in the literature, thus adding confidence to the validity of the proposed models. [ABSTRACT FROM AUTHOR]

The present study describes a simple design model for the calculation of the fiber-reinforced polymer (FRP) contribution to the shear capacity of strengthened RC elements according to the design formats of the Eurocode, American Concrete Institute, and Japan Concrete Institute. The key element in the model is the calculation of an effective FRP strain, which is calculated when the element reaches its shear capacity due to concrete diagonal tension. Diagonal tension failure may be combined with FRP debonding or tensile fracture, and the latter also may occur at a stage beyond the ultimate shear capacity. An upper limit (maximum) to the FRP effective strain also is defined and aimed at controlling crack opening. The effective strain, obtained through calibration with >75 experimental data, is shown to decrease with the FRP axial rigidity divided by the concrete shear strength. It also is demonstrated that the contribution of FRP to shear capacity is typically controlled by either the maximum effective strain or by debonding and, for a given concrete strength, it increases linearly with the FRP axial rigidity until the latter reaches a limiting value beyond which debonding controls and the gain in shear capacity is relatively small. However, proper anchoring (e.g., full wrapping) suppresses the debonding mechanism and results in considerable increases in shear capacity with the FRP axial rigidity. Finally it is demonstrated that, when compared with others, the proposed model gives better agreement with most of the test results available. [ABSTRACT FROM AUTHOR]

Presents study results of the mechanical behavior of wood members either reinforced or strengthened with fiber-reinforced plastic (FRP) materials in the form of sheets externally bonded to the shear-critical zones. Demonstration of the effectiveness of FRP reinforcement; Reinforcement of 21 wood beams designed to fail in shear with carbon-FRP fabrics.

The present study presents a systematic analysis for the short-term strength of masonry walls, strengthened with externally bonded fiber reinforced polymer (FRP) laminates under monotonic out-of-plane bending, in-plane bending, and in-plane shear, all combined with axial load, within the framework of modern design codes (e.g., Eurocode 6). The results, which are presented in the form of both design equations and normalized interaction diagrams, show that: (1) when out-of-plane bending response dominates, as in the case of upper story levels, where axial loads are low, the increase in bending capacity is quite high; (2) in the (rather rare) case of in-plane bending, high area fractions of reinforcement placed near the highly stressed zones give considerable strength increases (regardless of the axial load), provided that premature peeling-off of the laminates is prevented; and (3) the in-plane shear capacity of FRP-strengthened walls can be quite high, too, especially in the case of low axial loads. The results obtained from monotonic four-point bending tests on wallettes confirm the analysis of both out-of plane and in-plane response and emphasize the important role of failure through FRP shearing beneath the bond. [ABSTRACT FROM AUTHOR]