Jin Guang Teng

DESIGN-ORIENTED STRESS-STRAIN MODEL FOR FRP-CONFINED CONCRETE

Abstract External confinement by the wrapping of FRP sheets (or FRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. For the reliable and cost-effective design of FRP jackets, an accurate stress–strain model is required for FRP-confined concrete. In this paper, a new design-oriented stress–strain model is proposed for concrete confined by FRP wraps with fibres only or predominantly in the hoop direction based on a careful interpretation of existing test data and observations. This model is simple, so it is suitable for direct use in design, but in the meantime, it captures all the main characteristics of the stress–strain behavior of concrete confined by different types of FRP. In addition, for unconfined concrete, this model reduces directly to idealized stress–strain curves in existing design codes. In the development of this model, a number of important issues including the actual hoop strains in FRP jackets at rupture, the sufficiency of FRP confinement for a significant strength enhancement, and the effect of jacket stiffness on the ultimate axial strain, were all carefully examined and appropriately resolved. The predictions of the model are shown to agree well with test data.

Interfacial stresses in plated beams

Fibre-reinforced plastic (FRP) or steel plates can be bonded to the soffit of a beam as a means of retrofitting the beam. In such plated beams, tensile forces develop in the bonded plate and these have to be transferred to the original beam via interfacial shear and normal stresses. Consequently, debonding failure may occur at the plate ends due to a combination of high shear and normal interfacial stresses. This paper starts with a review of approximate closed-form solutions for interfacial stresses, identifying their assumptions and limitations, thereby clarifying the differences between these solutions. This review also establishes the need for a similar but more accurate solution, and such a solution is presented next in the paper. This new solution is intended for application to beams made of all kinds of materials bonded with a thin plate, while all existing solutions have been developed focusing on the strengthening of reinforced concrete (RC) beams, which allowed the omission of certain terms. Finally, numerical comparisons between the existing solutions and the present new solution enable a clear appreciation of the effects of various parameters.

FRP-strengthened RC beams. I: review of debonding strength models

Bonding of a fibre-reinforced polymer (FRP) plate to the tension face of a beam has become a popular flexural strengthening method in recent years. As a result, a large number of studies have been carried out in the last decade on the behaviour of these FRP-strengthened beams. Many of these studies reported premature failures by debonding of the FRP plate with or without the concrete cover attached. The most commonly reported debonding failure occurs at or near the plate end, by either separation of the concrete cover or interfacial debonding of the FRP plate from the RC beam. This and the companion paper are concerned with strength models for such plate end debonding failures. In this paper, a comprehensive review of existing plate debonding strength models is presented. Each model is summarised and classified into one of the three categories based on the approach taken, and its theoretical basis clarified. The review not only brings together for the first time all existing plate end debonding strength models into a unified framework for future reference, but also provides the necessary background information for them to be assessed in the companion paper using a large test database assembled by the authors from the published literature.

Shear capacity of FRP-strengthened RC beams: FRP debonding

Abstract Many studies have been undertaken on shear strengthening of reinforced concrete (RC) beams by externally bonding fibre-reinforced polymer (FRP) composites. These studies have established clearly that such strengthened beams fail in shear mainly in one of two modes: FRP rupture; and FRP debonding, and have led to preliminary design proposals. This paper is concerned with the development of a simple, accurate and rational design proposal for the shear capacity of FRP-strengthened beams which fail by FRP debonding. Existing strength proposals are reviewed and their deficiencies highlighted. A new strength model is then developed. The model is validated against experimental data collected from the existing literature. Finally, a new design proposal is presented.

FRP-strengthened RC beams. II: assessment of debonding strength models

Abstract Reinforced concrete (RC) beams strengthened in flexure by the bonding of a fiber reinforced polymer (FRP) plate to the tension face are susceptible to brittle debonding failures. Such failures commonly initiate at or near one of the plate ends at a load below that to achieve flexural failure of the plated section. For a successful design of flexural strengthening using FRP composites, it is important to be able to predict such plate end debonding failures. In the first of these two companion papers, 12 plate end debonding strength models have been reviewed and summarized (Engng Struct 24(4) (2002) 385–395). The aim of the present paper is to provide a comprehensive assessment of the strengths and weaknesses of all the 12 models. To this end, a large test database containing the test results of 59 beams reported to have failed by plate end debonding is first presented. This database was carefully constructed from an extensive survey of the published literature. Both statistical and graphical comparisons between test results and the predictions of the debonding strength models are next presented. A new simple debonding strength model which is superior to existing models is also proposed.

Buckling of Thin Shells: Recent Advances and Trends

This paper provides a review of recent research advances and trends in the area of thin shell buckling. Only the more important and interesting aspects of recent research, judged from a personal view point, are discussed. In particular, the following topics are given emphasis: (a) imperfections in real structures and their influence; (b) buckling of shells under local/non-uniform loads and localized compressive stresses; and (c) the use of computer buckling analysis in the stability design of complex thin shell structures. I INTRODUCTION Thin-shell structures find wide applications in many branches of engineering. Examples include aircraft, spacecraft, cooling towers, nuclear reactors, steel silos and tanks for bulk solid and liquid storage, pressure vessels, pipelines and offshore platforms. Because of the thinness of these structures, buckling is often the controlling failure mode. It is therefore essential that their buckling behavior be properly understood so that suitable design methods can be established. This paper provides a review of recent research advances and trends in the area of thin shell buckling. The paper is not intended to be an exhaustive review of the field, nor is it possible to do so in a single paper of limited length. Instead, only the more important and interesting aspects of recent research, judged from a personal viewpoint, will be discussed. In particular, the following topics are given emphasis: (a) imperfections in real structures and their influence; (b) buckling of shells under local/non-uniform loads and localized compressive stresses; and (c) the use of computer buckling analysis in the stability design of complex thin shell structures. The author wishes to apologize in advance for any inadvertent omission of relevant publications.

Interfacial stresses in reinforced concrete beams bonded with a soffit plate: a finite element study

Abstract This paper presents a careful finite element investigation into interfacial stresses in reinforced concrete (RC) beams strengthened with a bonded soffit plate, with the aims of assessing the accuracy of existing approximate closed-form analytical solutions based on simplifying assumptions and highlighting aspects which are omitted by them. Finite element modelling issues are first discussed, with particular attention on stress singularities in such beams and appropriate finite element meshes for the accurate determination of interfacial stresses. Finite element stresses are then carefully examined and compared with the predictions from a closed-form solution recently derived by the authors. The finite element results show that stresses vary strongly across the adhesive layer, with the stresses along the adhesive-to-concrete (AC) interface being very different from those along the plate-to-adhesive (PA) interface. In particular, near the end of the plate, the interfacial normal stress is tensile along the AC interface but compressive along the PA interface, offering a plausible explanation for the fact that PA interfacial failure in tests has rarely been if at all reported. The closed-form solution, being based on the assumption of uniform stresses in the thickness direction in the adhesive layer, is incapable of predicting such complex stresses, but nevertheless provides a reasonably close prediction of stresses along the middle-thickness section of the adhesive layer, particularly when the adhesive layer is not very thin. This and other similar closed-form solutions are therefore believed to still form a useful basis on which design rules against debonding failures can be built, with appropriate tuning using experimental data. Finally, the effect of spew fillets on interfacial stresses is examined.

Shear-Bending Interaction in Debonding Failures of FRP-Plated RC Beams

Fibre reinforced polymer (FRP) plates can be bonded to the tension face of a reinforced concrete (RC) beam to increase its flexural capacity. Many studies have reported premature failure by debonding of the FRP plate before the ultimate flexural capacity of the plated section is reached, and the most commonly reported debonding failure mode occurs at or near the plate end. This paper presents the results of an experimental investigation into the interaction between shear force and bending moment at the plate end at debonding for FRP plated beams by varying the length of plate. The experimental results are compared with a plate end debonding strength model recently proposed by the authors in which this interaction is ignored. A bi-linear approximation is then proposed to describe the interaction between the shear force and the bending moment at the plate end at debonding based on the present results. The effect of U strip end anchorage, used to delay or prevent plate end debonding, is also assessed in this study but it is shown that U strip end anchorage provides only limited strength enhancement.

Buckling of thin metal shells

List of contributors. Preface 1. Buckling of thin shells: an overview 2. Cylindrical shells under axial compression 3. Cylindrical shells above local supports 4. Settlement beneath cylindrical shells 5. Cylindrical shells under uniform external pressure 6. Cylindrical shells under non-uniform external pressure 7. Cantilever cylindrical shells under wind loading 8. Cylindrical shells under torsion and transverse shear 9. Cylindrical shells under static and cyclic shear 10. Cylindrical shells under combined loading 11. Stiffened cylindrical shells 12. Large diameter fabricated steel tubes 13. Shell junctions 14. Rings at shell junctions 15. A probabilistic design approach for shell structures

Numerical models for nonlinear analysis of elastic shells with eigenmode-affine imperfections

Nonlinear finite-element analysis provides a powerful tool for assessing the buckling strength of shells. Since shells are generally sensitive to initial geometric imperfections, a reliable prediction of their buckling strength is possible only if the effect of geometric imperfections is accurately accounted for. A commonly adopted approach is to assume that the imperfection is in the form of the bifurcation buckling mode (eigenmode-affine imperfection) of a suitable magnitude. For shells of revolution under axisymmetric loads, this approach leads to the analysis of a shell with periodically symmetric imperfections. Consequently, sector models spanning over one or half the circumferential wave of the imperfection may be considered adequate. This paper presents a study which shows that a simple nonlinear analysis of the imperfect shell may not deliver the correct buckling load, due to the tendency of the shell to develop mode changes in the deformation process before reaching the limit point. This inadequacy exists not only with short sector models (half-wave or whole-wave models) but also with more complete models (half-structure or whole-structure models) for different reasons. The paper concludes with recommendations on the proper use of the four different kinds of models mentioned above in determining shell buckling strengths.