Dependency of punching shear resistance and membrane action on boundary conditions of reinforced concrete continuous slabs

The design of reinforced concrete flat slabs can be governed at failure by punching shear close to concentrated loads or columns. Geometrical features, reinforcement layouts and in-plane forces provided by external vertical elements, such as shear walls, can affect the membrane action and, consequently, the punching shear resistance. This paper presents a study on reinforced concrete continuous flat slab whose lateral expansion is restrained by the presence of vertical elements considering also shrinkage effects.

Static and Fatigue Shear Strength of Reinforced Concrete Slabs Under Concentrated Loads Near Linear Supports

Francisco Manuel Maciel Natário

Reinforced concrete slabs without shear reinforcement under concentrated loads near linear supports are typical cases of bridge deck slabs, transfer slabs or pile caps. Such members are often designed or assessed in shear/punching shear with code provisions calibrated on the basis of tests on beams or one-way slabs loaded over the full width, as well as tests on isolated flat slab elements supported on columns in axis-symmetric conditions. However, these tests are not representative of the actual behavior of slabs under concentrated loads near linear supports. Moreover, concentrated loads of heavy vehicles have a repetitive nature, causing loss of stiffness and potential strength reductions due to fatigue phenomena. In this thesis, two experimental campaigns are presented. The first one consists of twelve static tests on six full-scale cantilever slabs subjected to a concentrated load with a central line support, that allows tracing the linear reaction evolution. Parameters such as the location of the concentrated load, and the presence, material and injection of ducts were varied. All slabs failed in shear and significant redistributions of the linear reactions were observed prior to failure. The second campaign has a similar test setup and consists of four static tests on two full-scale cantilever slabs (reference tests) and other eleven fatigue tests on eight identical slabs. The results show that cantilever slabs are significantly less sensitive to shear-fatigue failures than beams without shear reinforcement. The static reference tests presented shear failures. Some of the fatigue tested slabs failed due to rebar fractures. They presented significant remaining life after the first rebar failure occurred and eventually failed due to shear. The shear failures exhibited by the static tests on cantilever slabs from this thesis and others from the literature can be reasonably predicted with the Critical Shear Crack Theory (CSCT), provided that the influence of direct load strutting and redistribution of internal forces is accounted for, and that no contribution due to the inclination of tapered members to shear transferring is considered. Simply supported slabs under concentrated loads near linear supports may exhibit shear or punching shear failures. Factors like the ratio between the dimension of the load parallel to the support and the slab width, or the type of loading (monolithic or not) seem to be crucial to determine the failure mode. In this thesis it is proposed to use the CSCT for both shear and non-axis-symmetric punching shear combined with certain proposals on how to determine the internal forces and punching perimeter lengths to assess tests from the literature. The proposed approaches are not fully capable of predicting the correct failure mode, but allow a safe design. A reasonable accuracy is obtained, either knowing or not a priori the correct failure mode. A consistent design approach for shear-fatigue failures of reinforced concrete members without shear reinforcement is also presented, based on FM applied to quasi-brittle materials in combination with the CSCT. This leads to simple, yet sound and rational design equation incorporating the different influences of fatigue actions (minimum and maximum load levels) and shear strength (size and strain effects, material and geometrical properties). The accuracy of the design expression is checked against available test data in terms of Wöhler (S-N) and Goodman diagram

EPFL2015

The mechanics of shear failures in RC slabs based on refined measurements

Raffaele Cantone

Reinforced concrete planar members, as slabs and shells, are structural elements commonly used in the construction technique, which are typically designed without the arrangement of shear reinforcement. Despite the fact that this solution allows for fast and economic construction, the absence of shear reinforcement can give rise to the potential localization of strains within a critical shear crack and eventually to the shear failure of the member. In the case of redundant systems, most research on the mechanics of shear failures has been devoted to the strength of the member, neglecting in many cases the development of shear deformations due to inclined cracking as well as the redistributions of internal forces, which are instrumental for the analysis of the response of these members. The contribution to the state-of-the-art includes a series of theoretical works explaining the observed responses for a series of experimental programmes. These experimental campaigns comprise tests in tension, shear tests in one- and two-way slabs as well as punching tests. For their instrumentation, in addition to classical measurement devices, Fibre-Optic Measurements and Digital Image Correlation were intensively used.This thesis starts by revisiting the basis of the interaction between reinforcement and concrete. A series of bond tests show the stress concentrations occurring near the ribs and its complex transfer of forces with the surrounding concrete. In addition, tests on beams failing in shear show a complex interaction between bond stresses and kinking on the reinforcement due to the development of dowel action. These phenomena are normally neglected for concrete design due to the ductile nature of reinforcement, but may be relevant for fatigue and negative tension-stiffening effects.An important step in the knowledge is performed on the understanding of the shear response with respect to the characterization of the deformations in concrete members. Based on a series of test results, a complete description of the deformation field (including shear strains) is presented. On that basis, a rational model is proposed, consistent with the mechanical model of the Critical Shear Crack Theory. This model allows for a precise description of the response and also to describe the through-thickness distribution of the shear deformation.A general frame for modelling of reinforced concrete slabs is thus presented accounting for the redistribution of internal forces during propagation of the shear crack. This approach is used to investigate a testing programme performed on three wide slabs, analysing in a scientific manner the influence of the width of the member on the shear resistance. The detailed experimental data allow to capture the crack propagation and internal forces redistributions. Clear conclusions and answers are obtained, showing the influence of the shape of the failure surface and of its propagation on the load-carrying capacity.The research ends with a final investigation on the dowelling action of compression reinforcement, with an application to slabs failing in punching. Based on a large testing programme including eleven axisymmetric punching tests, an analytical approach is developed to estimate the contribution of the dowel action on the load-carrying capacity. This approach is formulated within the frame of the Critical Shear Crack Theory, and is incorporated in a consistent and efficient manner for design purposes.

EPFL2021

The mechanics of shear failures in RC slabs based on refined measurements

Raffaele Cantone

EPFL2021

Static and Fatigue Shear Strength of Reinforced Concrete Slabs Under Concentrated Loads Near Linear Supports

Francisco Manuel Maciel Natário

EPFL2015