Jesús M. Bairán

Shear-flexural strength mechanical model for the design and assessment of reinforced concrete beams

A conceptual model for the prediction of the shear-flexural strength of slender reinforced concrete beams with and without transverse reinforcement is presented. The model incorporates the shear transferred by the un-cracked concrete chord, along the cracks length, by the stirrups, if they are, and, in that case by the longitudinal reinforcement. After the development of the first branch of the critical shear crack, failure is considered to occur when the stresses at any point of the concrete compression chord reach the assumed biaxial stress failure envelope. A physical explanation is provided for the evolution of the shear transfer mechanisms, and the contribution of each one at ultimate limit state is formulated accordingly. Simple equations are derived for shear strength verification and for designing transverse reinforcement. The method is validated by comparing its predictions with the results of 1131 shear tests, obtaining very good results in terms of mean value and coefficient of variation. Because of its accuracy, simplicity and theoretical consistency, the proposed method is considered to be very useful for the practical design and assessment of concrete structures subjected to combined shear and bending.

Efficient 1D model for blind assessment of existing bridges: simulation of a full-scale loading test and comparison with higher order continuum models

Assessing the structural performance of existing concrete bridges is nowadays a major task. Nonlinear finite element (FE) analysis can quantify their capacity, evaluate strengthening interventions and prevent premature dismantle. However, this technique, mainly performed with 2D/3D FE, is seldom used at true scale due to the great complexity and computational costs involved. In this paper, the loading test of a strengthened concrete bridge in Sweden (Örnsköldsvik) is simulated using a 1D model. The bridge failed in combination of shear–bending–torsion triggered by fibre-reinforced polymer bond failure. Consecutive levels of refinement of the 1D model are presented and available results from higher order models are compared. The study of the structural response involved comparing displacements, strains, cracking patterns and failure mechanisms. The demonstrated robustness and efficiency of the proposed model makes it adequate for blind assessments of existing bridges.

Nonlinear analysis of RC beams using a hybrid shear-flexural fibre beam model

Purpose – A nonlinear finite element (FE) beam-column model for the analysis of reinforced concrete (RC) frames with due account of shear is presented in this paper. The model is an expansion of the traditional flexural fibre beam formulations to cases where multiaxial behaviour exists, being an alternative to plane and solid FE models for the nonlinear analysis of entire frame structures. The paper aims to discuss these issues. Design/methodology/approach – Shear is taken into account at different levels of the numerical model: at the material level RC is simulated through a smeared cracked approach with rotating cracks; at the fibre level, an iterative procedure guarantees equilibrium between concrete and transversal reinforcement, allowing to compute the biaxial stress-strain state of each fibre; at the section level, a uniform shear stress pattern is assumed in order to estimate the internal shear stress-strain distribution; and at the element level, the Timoshenko beam theory takes into account an av...

Shear Design and Assessment of Reinforced and Prestressed Concrete Beams Based on a Mechanical Model

Safe and economical design and assessment of reinforced (RC) and prestressed concrete (PC) beams requires the availability of accurate but simple formulations which adequately capture the structural response. In this paper, a mechanical model for the prediction of the shear-flexural strength of PC and RC members with rectangular, I, or T sections, with and without shear reinforcement, is presented. The model is based on the principles of concrete mechanics and on assumptions supported by the observed experimental behavior and by the results of refined numerical models. Compact, simple, and accurate expressions are derived for design and verification of the shear strength, which incorporate the most relevant shear transfer actions. Excellent agreement between the predictions of the model and the results of the recently published ACI-DAfStb databases, including more than 1,287 tests on RC and PC beams with and without stirrups, has been observed. The theory behind the model provides consistent explanations for many aspects related to the shear response that are not clearly explained by current code formulations, making it a very helpful tool for daily engineering practice.

Numerical Analysis of Shear Critical RC Beams Strengthened in Shear with FRP Sheets

AbstractThe objective of this paper is to contribute to the understanding of the shear resisting mechanisms in RC beams shear-strengthened by externally bonded fiber-reinforced polymer (FRP) sheets. For this purpose, a fiber beam model of RC frames subjected to combined normal and shear forces, previously developed by the authors, has been extended to include the response of externally bonded FRP shear reinforcement in a wrapped configuration. No FRP delamination phenomena or tensile strength reductions in the corner zones are taken into account in the model. The numerical results have been compared with eight existing experimental results and the influence of the FRP sheets on the shear strength of the beam has been studied. The effects of the contribution of FRP ratio on the concrete, on the transversal steel strains and stresses, on the longitudinal tensile steel stresses, and on the diagonal compression struts have been analyzed. It is concluded that the presence of FRP reinforcement modifies the incl...

Predicting the Response of FRP-Strengthened Reinforced-Concrete Flexural Members with Nonlinear Evolutive Analysis Models

To design efficient and economical strengthening solutions, the structural performance before and after the intervention must be accurately evaluated. In the case of statically indeterminate structures or when the structure has suffered damage or deterioration, linear elastic analysis methods are not adequate to obtain the residual capacity and the structural effects of the intervention because of the nonlinear response of the structure. In such cases, refined analytical models able to capture the structural nonlinear behavior, the effects of previous damage, and those produced by any intervention are required to design safe and economical strengthening solutions. In this paper, a nonlinear and time-dependent evolutive analysis model, previously developed by the authors, is applied to the prediction of the response of fiber-reinforced polymer (FRP)-strengthened concrete structures in flexure. The model can take into account the structural effects of changes in geometry, structural scheme, material properties, and applied loads that may occur along the structure service life, including those attributable to strengthening. A criterion to predict peeling failure in FRP-strengthened beams on the basis of nonlinear fracture mechanics consisting in evaluating the maximum shear force that can be transmitted to the concrete by the FRP laminate between cracks or at the laminate end is incorporated in the model. Two previous experimental programs have been used to validate the model. First, four RC continuous beams, three of them strengthened with FRP laminates and tested to study the influence of the FRP arrangement, are analyzed. Second, two beams previously precracked owing to service loads and strengthened with FRP are analyzed under increasing load up to failure. In all cases, very good agreement between the theoretical and the experimental results is obtained in terms of deflections, strains, reactions, internal forces, and failure mode, showing the capabilities of the model to evaluate the efficiency of proposed strengthening solutions.

Shear strengthening of reinforced concrete beams by means of vertical prestressed reinforcement

Strengthening reinforced concrete (RC) elements critical to shear with prestressed transversal reinforcement can be an efficient method to increase the shear resistance of structures, allowing the development of the full flexural capacity. However, research on the performance of this technique is very limited, and methods for designing the optimum amount of prestressed transversal reinforcement and assessing the retrofitted structure have not been produced yet. Nonlinear finite element models are an important tool regarding predicting the efficiency of these interventions. In this paper, a shear-sensitive fibre beam formulation is extended in order to account for the effects of unbonded vertical external prestressed reinforcement in the structural response of RC beams. The model is validated with experimental tests available in literature, succeeding in capturing the gain of shear strength brought by different strengthening solutions. A parametric study is performed to find the optimal quantity of transversal reinforcement that ensures flexural failure mechanism in a beam with insufficient internal shear reinforcement. The relative simplicity of the numerical model makes it suitable for engineering practice.

Assessment of prestressed concrete bridge girders with low shear reinforcement by means of a non-linear filament frame model

The safety of existing bridges and the efficiency of strengthening measures can be accurately studied through non-linear numerical models, assisting decisions of dismantle, repair or change of use and avoiding unnecessary or inappropriate interventions. In this ambit, filament beam models due to their inherent simplicity and low computational demand are adequate for the engineering practice. Accordingly, in this paper, the structural assessment of a prestressed concrete bridge presenting low shear reinforcement, the Wassnerwald Viaduct in Switzerland, is presented. The bridge was dismantled due to, among other reasons, not complying with the safety standards related to shear. The girders of the bridge, which were submitted to full-scale in situ load tests, were numerically simulated by means of a non-linear filament beam model considering axial force (N)–shear (V)–bending (M) interaction. Hypothetical strengthening solutions for this bridge were also numerically studied: a shear strengthening through vertical prestressing and a bending strengthening through external longitudinal prestressing.

Analysis of FRP shear strengthening solutions for reinforced concrete beams considering debonding failure

AbstractIn this paper, a fiber beam model previously developed by the authors for the nonlinear analysis of strengthened elements, including the effects of shear, is used to predict the response of reinforced concrete (RC) beams strengthened in shear with fiber reinforced polymers (FRP) sheets. In the previous version of the model, debonding failure of FRP was not included; hence, its application was limited to the simulation of wrapped configurations. The model is now extended to account for debonding failure in order to allow for its application to beams strengthened with U-shaped and side-bonded configurations. Existing experimental tests on RC beams strengthened in shear by FRP sheets in both wrapped and U-shaped configurations were numerically simulated. The model reproduces, with reasonable accuracy, the experimental failure loads, the load-deflection behavior, and the strains in FRP and stirrups with increasing load. The advantages of this proposal are related with the simplicity and straightforwar...

Partial Safety Factor for the Residual Flexural Strength of FRC Precast Concrete Segments

The use of fibre reinforced concrete (FRC) to produce segmental linings of TBM-constructed tunnels is an increasing tendency. So far, more than 50 tunnels have been constructed with this structural material, in some of these even using solely fibres as reinforcement. Moreover, several design guidelines (e.g., fib Model Code 2010) already include the FRC as structural material. There also exist specific guidelines for the design of FRC precast segment linings (e.g., ITAtech Report/7-15 and ACI 544.7R-16). These guidelines deal with the design of FRC considering the traditional limit state safety format. Therefore, partial safety factors for both the loads (γ L ) and material strengths (γ M ) must be considered. In particular, the magnitude of γ M considered for compressive and tensile FRC strengths are assumed to be the same. Nonetheless, this assumption can be unrealistic, particularly in terms of flexural residual strength (f R ) since this property present higher scatter than the compressive strength (f c ). This is particularly true for elements with a reduced cracking surface (e.g., beams) due to the higher impact that uncertainties like fibre orientation and distribution have on the variability of f R . Therefore, this assumption can lead to lower reliability indexes (β) than those established for traditional reinforced concrete structures. However, this variability tends to decrease with the increase of the width of the cracked sections (e.g., slabs). The results of a structural reliability analysis carried out to calibrate partial safety factors for f R is presented. Full-scale bending tests on precast segments with different dimensions, amounts and type of fibers were considered. This partial safety factors could be used in the design of future precast FRC tunnel linings.