Punching shear behaviour of structural slabs on top of ribbed foundation piles

Abstract

The excavated building pit is a frequently used building technique during the construction of structures below ground level. A temporary structure of sheet piles and underwater concrete is designed to retain soil and water. Ribbed tension piles are driven to compensate the hydrostatic pressure after de-watering of the construction pit. Generally, the top of the foundation piles are crushed before concreting the structural. The uncovered pile reinforcement is combined with the slab reinforcement which results in a monolithic floor with an optimal connection between slab and pile. During some construction projects it is desirable to avoid pile head crushing for economic reasons or to prevent the stray current phenomena. The question that arises is to what extend does the partly penetrated ribbed foundation pile influence the punching shear behaviour of the structural slab when the pile is loaded in compression during the exploitation phase. At the heart of this debate lies the problem of the effectiveness of the present ribs and the associated punching cone perimeter. In order to investigate the punching shear behaviour, simulations by nonlinear finite element analyses have been performed using the software ATENA developed by Cervenka Consulting. Initially, conducted experiments were simulated in order to validate the modelling technique. The contact area is modelled with interface material, based on the Mohr-Coulomb failure criterion for shear planes. The FEM analyses showed good agreement with the conducted experiments regarding to crack propagation which induces punching shear. The case study is made up of a number of subsequent modelling steps to investigate each influence separately. The case study starts with a parameter study to investigate the influence of the shear transfer capacity from slab to pile on the punching behaviour. The maximum allowable shear stress depends of a cohesion parameter and a coefficient of friction parameter. Various combinations of both parameters are investigated with respect to the punching shear behaviour. A clear distinction in failure behaviour is observed between a friction coefficient of 1.0 and 1.4. Up to a friction coefficient of 1.0, the critical shear crack propagates from pile head towards the slab surface which indicates that only the top part of the slab is responsible for punching shear resistance. From a friction coefficient of 1.4 the critical shear crack propagates from slab bottom towards slab surface, which indicates that the full slab height is active in punching resistance. This change in punching cone size results in a failure load which is more than twice as high. No significant increase of failure load is observed during analyses with different cohesion values. The second step consists of adding reinforcement to the initial model. The influence of bending reinforcement was neglectable due to the limited slab deflections. However, the influence of shear reinforcement was significant. For friction coefficients from 0.0 to 1.0, the failure load increased with a magnitude of roughly four compared to the failure load without shear reinforcement. An increase of roughly two is observed for friction coefficients of 1.4 or greater. Also a change in failure behaviour is observed. For friction coefficients of 1.0 or greater, the brittle punching behaviour changes into ductile flexural behaviour. This is highly desirable because large structural deflections insinuates an upcoming failure. Although full scale laboratory tests are needed to verify the numerical results, the recommendation for practical use is that for structural slabs provided with shear reinforcement and a foundation pile with at least two ribbed sides, a fully punching cone develops.