Infoscience

Thesis

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

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

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