Miguel Fernández Ruiz

Shear strength of members without transverse reinforcement as function of critical shear crack width

This paper investigates the shear strength of beams and one-way slabs without stirrups based on the opening of a critical shear crack. The shear-carrying mechanisms after the development of this crack are investigated. On this basis, a rational model is developed to estimate the shear strength of members without shear reinforcement. The proposed model is based on an estimate of the crack width in the critical shear region, taking also into account the roughness of the crack and the compressive strength of concrete. The proposed model is shown to properly describe a large set of available test data. A simplified method adopted by the Swiss code for structural concrete (SIA 262) is also introduced. Comparisons with other codes of practice are finally presented, with a highlight on the main differences between them.

Applications of Critical Shear Crack Theory to Punching of Reinforced Concrete Slabs with Transverse Reinforcement

The traditional approach of codes of practice for estimating the punching strength of shear-reinforced flat slabs is based on the assumption that concrete carries a fraction of the applied load at ultimate while the rest of the load is carried by the shear reinforcement. Concrete contribution is usually estimated as a fraction of the punching strength of members without shear reinforcement. The ratio between the concrete contribution for members with and without shear reinforcement is usually assumed constant, independent of the amount of shear reinforcement, flexural reinforcement ratio, and bond conditions of the shear reinforcement. The limitations of such an approach are discussed in this paper and a new theoretical model, based on the critical shear crack theory, is presented to investigate the strength and ductility of shear-reinforced slabs. The proposed approach is based on a physical model and overcomes most limitations of current codes of practice. Its application to various punching shear reinforcement systems is also detailed in the paper and its results are compared to available test data.

Experimental Investigation on Punching Strength and Deformation Capacity of Shear-Reinforced Slabs

This paper presents the results of an extensive experimental campaign on 16 flat-slab specimens with and without punching shear reinforcement. The tests aimed to investigate the influence of a set of mechanical and geometrical parameters on the punching shear strength and deformation capacity of flat slabs supported by interior columns. All specimens had the same plan dimensions of 3.0 x 3.0 m (9.84 x 9.84 ft). The investigated parameters were the column size (ranging between 130 and 520 mm [approximately 5 and 20 in.]), the slab thickness (ranging between 250 and 400 mm [approximately 10 and 16 in.]), the shear reinforcement system (studs and stirrups), and the amount of punching shear reinforcement. Systematic measurements (such as the load, the rotations of the slab, the vertical displacements, the change in slab thickness, concrete strains, and strains in the shear reinforcement) allow for an understanding of the behavior of the slab specimens, the activation of the shear reinforcement, and the strains developed in the shear-critical region at failure. Finally, the test results were investigated and compared with reference to design codes (ACI 318-08 and EC2) and the mechanical model of the critical shear crack theory (CSCT), obtaining a number of conclusions on their suitability.

Strengthening of flat slabs against punching shear using post-installed shear reinforcement

Keywords: armature de poinconnement ; dalle ; poinconnement ; theorie de la fissure critique ; punching shear reinforcement ; slab ; punching ; critical crack theory Reference EPFL-ARTICLE-174480 URL: http://ibeton.epfl.ch/util/script/sendArticle.asp?R=Fernandez11 Record created on 2012-01-26, modified on 2017-11-16

On development of suitable stress fields for structural concrete

Strut and tie models and stress fields are methods that can be used for the dimensioning and detailing of reinforced and prestressed concrete structures as well as for the check of existing ones. This paper presents an innovative approach towards the automatic development of stress fields based on a nonlinear finite element analysis. Strut and tie models can also be easily developed from the resulting stress fields. Most of the difficulties of the existing methods for developing stress fields and strut and tie models based on elastic uncracked analyses are overcome. Its application to the dimensioning of structural members in practical cases is detailed and several comparisons with experimental results are discussed.

Post-Punching Behavior of Flat Slabs

Reinforced concrete flat slabs are a common structural system for cast-in-place concrete slabs. Failures in punching shear near the column regions are typically governing at ultimate. In case no punching shear or integrity reinforcement is placed, failures in punching develop normally in a brittle manner with almost no warning signs. Furthermore, the residual strength after punching is, in general, significantly lower than the punching load. Thus, punching of a single column of a flat slab overloads adjacent columns and can potentially lead to their failure on punching, thus triggering the progressive collapse of the structure. Over the past decades, several collapses have been reported due to punching shear failures, resulting in human casualties and extensive damage. Other than placing conventional punching shear reinforcement, the deformation capacity and residual strength after punching can also be enhanced by placing integrity reinforcement to avoid progressive collapses of flat slabs. This paper presents the main results of an extensive experimental campaign performed at the Ecole Polytechnique Federale de Lausanne (EPFL) on the role of integrity reinforcement by means of 20 slabs with dimensions of 1500 x 1500 x 125 mm (≈5 ft x 5 ft x 5 in.) and various integrity reinforcement layouts. The performance and robustness of the various solutions is investigated to obtain physical explanations and a consistent design model for the load-carrying mechanisms and strength after punching failures.

Effect of load distribution and variable depth on shear resistance of slender beams without stirrups

The shear resistance of elements without stirrups has mainly been investigated by test setups involving simply supported beams of constant thickness subjected to one- or two-point loading, and most of the formulas included in codes have been adjusted using this experimental background. It is a fact, however, that most design situations involve constant or triangular distributed loading (such as retaining walls or footings) on tapered members. Furthermore, there seems to be few shear tests involving cantilever structures subjected to distributed loading. These structures, which are common in everyday practice, fail in shear near the clamped end, where the shear forces and bending moments are maximum (contrary to simply supported beams of tests, where shear failures under distributed loading develop near the support region for large shear forces but limited bending moments). In this paper, a specific testing program undertaken at the Poly- technic University of Madrid (UPM), Madrid, Spain, in close collab- oration with Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland, is presented. It was aimed at investigating the influence of load distribution and tapered beam geometrics on the shear strength. The experimental program consists of eight slender beams without stirrups. Four specimens had a constant depth, whereas the others had variable depths (maximum depth of 600 mm [23.6 in.]). Each specimen was tested twice: one side was tested first under point loading, and then (after repairing) the other side was tested under either uniform loading or triangular loading. The setup allowed direct comparisons between point and distributed loading. The experimental results showed a significant influence of the type of loading and of tapered geometries on the shear strength. On the basis of these results, and using the funda- mentals of the critical shear crack theory, a consistent physical explanation of the observed failure modes and differences in shear strength is provided. Also, comparisons to current design provisions (ACI 318-08 and EC2) are discussed.

Levels-of-Approximation Approach in Codes of Practice

Abstract Civil engineering projects typically involve a number of design phases, such as preliminary design, tender design and executive design. The required degree of accuracy for the analysis of the behaviour and strength of the structural members increases as a project evolves. Codes of practice, however, seldom reflect this situation and propose a single design expression to be used at all phases of a project. This is not convenient in a number of situations, leading to lengthy analyses for initial design phases and sometimes not allowing to consider all potential load-carrying mechanisms or strength reserves for advanced analyses (design of complex structures and assessment of critical existing structures). In opposition to this approach, codes of practice can privilege a design strategy named the levels-of-approximation (LoA) approach. It is based on the use of physically sound theories where the accuracy of the mechanical parameters used in the design expressions can be refined, if needed, in successive LoA. In the initial design phases, simple and safe hypotheses allow performing preliminary design tasks within limited time, checking principal dimensions and identifying critical regions and failure modes. Refinements on the values of the mechanical parameters in successive LoA thereafter allow to increase the accuracy of the estimate of the strength and behaviour. In this paper, the main ideas and advantages of the LoA approach are introduced and explained. An example of how this approach can be used with reference to the shear design of bridge deck slabs is also presented and discussed.

Shear Strength of Thin-Webbed Post-Tensioned Beams

Keywords: beton arme ; effort tranchant ; fissure ; Modele de treillis / champs de contraintes ; precontrainte ; securite structurale ; reinforced concrete ; shear force ; crack ; strut-and-tie model / stress fields ; prestressing ; structural safety Reference EPFL-ARTICLE-143413 Record created on 2010-01-15, modified on 2017-08-01

Interaction between Bond and Deviation Forces in Spalling Failures of Arch-Shaped Members without Transverse Reinforcement

This paper investigates the structural behavior of reinforced concrete (RC) arch-shaped members without transverse reinforcement subjected to bending. Such members have typical applications in tunnels, cut-and-cover structures, shells, vaults, ducts, silos, tanks, and off-shore structures. Although such members are mostly subjected to axial forces, bending moments may also develop when the shape of the structure does not perfectly match the ideal funicular shape. In this case, when the intrados reinforcement is in tension, deviation forces developed by the reinforcement increase the splitting stresses originated by bond and can lead to spalling of the reinforcement cover. Such a failure mode is particularly brittle and dangerous, leading to a sudden loss of load-carrying capacity of the structure. In this paper, a series of six tests on 400 mm (15.7 in.) thick arch-shaped beams are presented. They are aimed at investigating spalling failures before and after yielding of the tensile reinforcement. These results, as well as others taken from the literature, were compared to an analytical model accounting for the interaction between bond and deviation forces, showing a good agreement and explaining the various failure modes observed. On that basis, a practical formula for the design of such members is proposed.