Estudo numérico-experimental da influência dos mecanismos na resistência à força cortante de lajes alveolares protendidas

Abstract

The prestressed hollow core slabs present uncertainties regarding the evaluation of the resistance to the resistant shear force. This is because several parameters influence the shear-resistant mechanism of prestressed hollow core slabs, such as the loss of prestressing, the length of transfer of the prestressing force, the position of the critical breaking section, the height of the hollow core slab and the core shape. This work seeks to evaluate the influence of these variables through tests and computational modelling. The tests were performed on a slab with an non-circular cores and height of 200 mm, from segments cut longitudinally from the slab in order to isolate the rib, and thus avoid the torsion effect during the test. The concrete was characterized in terms of compressive strength, elasticity modulus, fracturing energy, and tensile strength. The latter was compared with the tensile strength obtained from bending tests on hollow core slab segments without prestressing. A series of prestressed segments were brought to ruin by flexion in order to determine the actual loss of prestressing. In addition, slab segments, with and without balance at its end, were brought to ruin by shear force in order to identify the influence of the prestressing force transfer length. A group with non-prestressed segments, only 1.0 meter long, were tested with a notch at a certain height. The purpose of this notch was to force a shear crack. A computational model based on the finite element method was developed, which was calibrated by testing the segments and the 200 mm high hollow slab, as well as a 160 mm high hollow slab tested by another author. From these computational models it was possible to determine the transfer length, the position of the critical section and the portion of the prestressing force in the shear capacity of these slabs. Subsequently, these analyses were also performed on a 320 mm and 400 mm slab. Also, in the numerical models, the prestressing intensity installed was varied to analyse its influence. Thus, it was possible to analyse the concrete and prestressing plots in hollow core slabs with different heights. The results of the experiments showed that it is possible to determine the shear force of the model with and without prestressing for shear tests in the segment. The presence of prestressing modifies the cracking panorama of the segments. Segment testing reduced the effects of torsion. From the flexion tests of the segment with and without prestressing, it was possible to determine the tensile strength in flexing the segments of the slab itself, in addition to the loss of prestressing. The computational models showed the behaviour of concrete and strands deformations, making it possible to identify the transfer length and anchoring length. The prestressing portion, in the resistant shear force, varies from slab to slab, and depends on the properties of the cross section and the eccentricity of the strand.