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Publication# Applications of bent-up bars as shear and integrity reinforcement in R/C slabs

Miguel Fernández Ruiz, Stefan Lips, Aurelio Muttoni, Luca Tassinari

*fib Symposium Prague 2011, *2011

Article de conférence

Article de conférence

Résumé

Bent-up bars were extensively used as shear reinforcement in beams and slabs from the first R/C developments up to the 1970’s. Their use was justified by the fact that they allowed enhanced development of the tensile reinforcement at the same time they acted as shear reinforcement. New reinforcing bars (with reduced development lengths) in combination with shear reinforcing systems led however to the abandon on their use. Recently, interest has again grown on the use of bent-up bars in flat slabs. This is explained by their efficiency both as shear reinforcement (alone or in combination with stirrups) and as integrity reinforcement. In this paper, the results of 7 full-scale specimens (3.0×3.0×0.25 m) reproducing the support region of an actual flat slab with different layout of bent-up bars are presented. The tests show different potential failure modes and allow understanding the contribution of this reinforcement to the shear-carrying capacity of the specimens. Additionally, the performance of bent-up bars as integrity reinforcement is discussed with reference to 9 half-scale specimens (1.5×1.5×0.125 m) where the influence of the development conditions of the bent-up bars is clearly assessed.

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Reinforced concrete bridge deck slabs without shear reinforcement can be subjected to concentrated or distributed loads of important magnitude. Under these loads their structural response is not always ductile. In particular under concentrated loads their deformation capacity can be limited by shear or punching shear failures, which prevent them from reaching the ultimate load predicted by pure flexural analysis. This problem has been studied in this research by means of an important experimental program and theoretical modeling. The limited ductility of bridge decks was investigated by means of full scale tests on bridge deck cantilevers under groups of concentrated loads. Six large scale laboratory tests were performed on two bridge deck cantilevers with a span of 2.8 m and a length of 10.0 m. All slabs failed in a brittle manner, in shear or punching shear. The theoretical flexural failure load estimated using the yield-line method was never attained. Despite the brittle failures, the results of tests on cantilevers have shown that some amount of yielding can occur before the shear failure and therefore reduce the shear strength. This effect was quantified on eleven full scale tests on slab strips without shear reinforcement with a length of 8.4 m. The results clearly show that the increase of plastic strains in the flexural reinforcement leads to a reduction of the shear strength. The measured rotation capacity of the plastic hinge was thus limited by a shear failure. A particular problem of bridge deck slabs is the introduction of concentrated loads applied by wheels with pneumatic pressure. Punching shear with these loads is usually treated in a manner similar to punching by a column. A punching shear test was performed with a concentrated load simulating a vehicle wheel with pneumatic pressure to investigate the differences. It appears that punching shear with a wheel with pneumatic pressure is less critical because curvatures tend to be distributed over the surface of the applied load rather than concentrated near the edges of the column. In order to investigate the experimental results on slab strips without shear reinforcement, a mechanical model is proposed to predict the shear strength and rotation capacity of plastic hinges. The shear strength is formulated as a function of the opening of the shear crack and of the strength of the concrete compression zone. The results of the mechanical model are in good agreement with the measured values, both for the shear strength and for the shear carried across the shear crack. Based on the mechanical model, a simplified equation is proposed. The model can also be used to predict the shear capacity of yield-lines. A non linear finite element model was implemented during this work and used to correctly predict the measured rotations and load-displacements curves of the tested cantilevers and other full scale tests performed by other researchers. The measured failure loads are accurately estimated by using the results of the non-linear model and the one-way shear and punching shear criteria proposed by Prof. A. Muttoni (Muttoni 2003).

Miguel Fernández Ruiz, Francisco Manuel Maciel Natário, Aurelio Muttoni

Reinforced concrete slabs without shear reinforcement subjected to concentrated loads near linear sup- ports are usually designed or assessed in shear using design provisions that have been calibrated on the basis of tests on one-way slabs or beams with rectangular cross-section (slabs loaded over their full width). This approach may however be inconsistent, as the actual behavior of slabs under concentrated loads reflects a two-way slab response, with the shear forces not developing in a parallel manner in the failure region and with potential redistribution of the internal forces for increasing levels of load. In this research, a series of 12 tests on 6 full scale slabs (3.00 m 3.00 m 0.18 m) is presented. The slabs were centrally supported on an aluminum profile and were subjected to two concentrated symmet- rical loads. The support profile was equipped with vertical strain gauges, allowing to trace the distribu- tion of reactions at the supported line and its redistributions as the value of the concentrated loads increased. Significant redistributions of the reactions were actually measured prior to failure, which are presented and discussed within the paper. The aim of these measurements was to improve the under- standing of the mechanical behavior of such members and the role of shear cracking on the distribution of internal forces. A number of parameters was investigated, such as the location of the concentrated loads and the presence of ducts in the slab. This latter case (typical of prestressed concrete balanced cantilever bridges and residential buildings) has not been investigated in the past, despite its potential influence on the shear strength. The performance of these specimens is finally investigated and compared to the prediction of the fib - Model Code 2010 and the Critical Shear Crack Theory. This analysis shows that accurate predictions of the strength can be obtained if the influence of direct load strutting (for loads acting near supports) and redistribution of internal forces is accounted.

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