No abstract

Redistribution of shear forces in reinforced concrete members without shear reinforcement is a key aspect for the assessment of the shear capacity of wide beams and slabs. Such redistributions are due to the nonlinear response of reinforced concrete (both in bending and shear) and have the potential to significantly modify the internal forces during loading. This phenomenon allows in many cases, as for slabs linearly supported and subjected to concentrated forces, to increase the level of load even when some regions have already attained their local shear resistance. This work introduces the results of an experimental programme performed on three cantilever slabs subjected either to strip loads or to concentrated loads. Shear redistributions close to failure are investigated on the basis of refined measurements performed on the concrete surface and on the reinforcement bars. The results show the significance of several mechanical parameters, as well as how shear redistributions occur when some regions are in softening (post-peak behavior) while others have still not attained their local shear resistance. On this basis, a comprehensive approach is presented for determining the redistributions of internal forces and to predict the shear capacity of reinforced concrete slabs subjected to concentrated loads near linear supports. The performance of such approach is eventually validated against test data and practical recommendations are proposed for design and assessment.

Type of publication:Peer reviewed conference paper AbstractThe shear resistance of RC slabs without shear reinforcement subjected to concentrated loads near linear support is usually calibrated on the base of tests on one -way slabs with rectangular cross section. However, the actual behavior of slabs subjected to concentrated loads is described properly by a two-way slab response. The aim of this paper consists in the evaluation of the shear resistance of bridge deck slabs using analytical formulations and Nonlinear Finite Element Analyses (NLFEA). The obtained numerical results are consequently compared with experimental observations from two test campaigns. The case studies were analysed by NLFE analyses carried out using the constitutive Crack Model PARC_CL (Physical Approach for Reinforced Concrete under Cycling Loading) implemented in the user subroutine UMAT.for in Abaqus Code. In order to predict properly global and local failure modes through a NLFE model, a multi -layered shell modelling has been used. As shell element modelling is not able to detect out -of -plane shear failures, the ultimate shear resistance of these slabs is evaluated by means of a post -processing method according to the Critical Shear Crack Theory (CSCT).

Punching reinforcement systems have significantly developed in recent years as they allow enhancing the punching resistance of slab-column connections as well as their deformation capacity. These systems, with varying geometry and layout, normally consist of vertical or inclined shear reinforcement with both ends anchored on the compression and tension side of the slab. For very high levels of load, when even common punching reinforcement systems cannot safely ensure the transfer of loads, steel shear heads are usually embedded in the slab to enhance the resistance of the connection. Yet, shear heads might be expensive and difficult to place in construction sites. Following the principle of the dowel action of the compression reinforcement, this paper introduces a novel system to efficiently reinforce slabs against punching shear by using large-diameter double-headed studs acting as shear dowels. This system enhances the performance of shear-reinforced slabs with respect to conventional solutions and might be an efficient alternative to shear heads for a large number of practical situations. The system is validated by means of a specific experimental program including 11 axisymmetric punching tests on interior slab-column connections. The results demonstrate not only the increase of the punching strength but also the deformation capacity of the connection. It is also shown that the system can be consistently designed accounting for the doweling forces by making use of the theoretical frame of the Critical Shear Crack Theory (CSCT), allowing to understand the activation of the shear dowels on the basis of the deformation of the member.

The design of reinforced concrete flat slabs in practice can be governed at failure by punching shear close to concentrated loads or columns. Punching shear resistance formulations provided by codes are calibrated on the basis of experimental tests on isolated slabs supported on columns in axisymmetric conditions. Nevertheless, the behavior of flat slabs can be different than isolated specimens due to the potentially beneficial contributions of moment redistributions and compressive membrane actions. Accounting for the significance of these effects, nonlinear finite element analyses are performed with the crack model PARC_CL implemented in Abaqus. This paper aims to investigate a series of punching shear tests on slabs with and without shear reinforcement, different reinforcement ratios and loading conditions accounting for the potential contribution to the enhancement of the punching strength due to compressive membrane action (CMA). The numerical results with a multi – layered shell modeling are then post – processed adopting the failure criterion of the Critical Shear Crack Theory (CSCT). The results pointed out the significant outcomes and differences between standard specimens and actual members showing how the current codes of practice may underestimate the punching capacity.