Shear capacity of hollow core slabs on flexible supports

Six full-scale loading tests for slim floors comprising prestressed hollow core slabs and spine beams were carried out. In one test, yielding of beam preceeded the shear failure of the slabs. In five tests the slab units failed in shear before yielding of the beam. The observed shear capacity of the slabs was 40 - 70% of the shear capacity observed in the reference tests with non-flexible supports. The failure modes and finite element simulations suggested that the reduction in shear capacity was due to transverse deformation in the hollow core units. A design method against web shear failure of the slabs was proposed. In this method, principal stress in the web is calculated from three stress components and compared with the tensile strength of the concrete. Two of the stress components, namely axial stress and vertical shear stress, are calculated as in the conventional design method. The third one, transverse shear stress, is calculated using a linear composite beam model. The tests and theoretical considerations show that deflection of the beams leads to strong reduction in the shear capacity of the hollow core slabs. However, thanks to their small structural depth, the deflection or bearing capacity of the spine beams, rather than the shear capacity of the slabs, has often been the critical design criteria. Therefore, the risk against failure in slim floors designed according to the conventional method is considerably lower than what might be expected based on the reduction of shear capacity of the slabs. According to present knowledge, total safety factors below 1.0 do not occur in conventionally designed slim floors. No information is available, either, about poor structural behaviour of hollow core slabs in slim floors on site.