Durchstanzen von Flachdecken bei hohen Stützenlasten

Interaction between column loading and punching shear strength in flat slabs without transverse reinforcement

Flat slabs supported by columns are cost efficient solutions for high-rise buildings. To reduce the cross section of the columns, the columns are typically precast with high strength concrete and include large amounts of longitudinal reinforcement. On the contrary, the slab between columns is typically cast-in-place with normal strength concrete. Thus, the slab is subjected in the support region of the columns to large transverse compressive stresses, which may exceed its uniaxial compressive strength. In addition, the flat slab around the column region has to carry large shear stresses, which may potentially lead to punching shear failures. In this paper, the interaction between column loads and shear forces in the support region of a flat slab is investigated by means of an experimental programme consisting of 6 slabs without transverse reinforcement (3.0 x 3.0 x 0.25 m). The flexural reinforcement of the slab was varied as well as the ratio between the column loads and the applied shear forces. On the basis of these results, a theoretical model based on the punching shear approach of DIN 1045-1 is presented. This model accounts for the compressive stresses in the support region due to column loads and for the potential increase of punching shear strength due to it. A very good agreement between the test results and the theoretical model is obtained both in terms of strength and observed failure modes.