1995

Pajari, Matti

Six full-scale loading tests for slim floors comprising prestressed hollow core slabs and spine beams have been carried out. In these tests, the observed shear resistance of the slabs was only 40 to 70% of the shear resistance observed in the reference tests in which slab units were supported on rigid bearings. In one floor test, yielding of the beam preceded the shear failure of the slabs. In five tests, the slab units underwent a shear failure before the yielding of the beam. According to the conventional calculation models, the beams should have failed first in all tests. At failure, the deflection of the supporting beams was relatively small, typically 1/400 to 1/250 of the span of the beam. However, a three dimensional finite element simulation and the observed failure mode, web shear failure, confirm that the reduction in shear resistance is due to transverse deformation in the hollow core units caused by the deflection of the beams. A calculation model for the analysis of the web shear failure of the slabs is proposed. In the model, the maximum principal stress in the web is calculated from three non zero stress components and compared with the tensile strength of the concrete. Two of the stress components, namely the axial stress and vertical shear stress, are calculated as in the conventional model. The third one, the transverse horizontal shear stress, is calculated using a linear composite beam model. The load-bearing capacity of floors can be enhanced by reducing the deflection of the beams, for example, by making the beams stiffer or continuous. Another approach is to reduce the transverse shear deformation of the slab ends, for example, by filling the voids with concrete or by providing the floor with a reinforced concrete topping.