Multiaxial compressive stress states within a concrete onshore wind turbine foundation

Abstract

With the rising demand for renewable energy sources the amount of wind turbines which are built on land are growing rapidly. Because of the repetitive nature of the foundation small design improvements can add up to large savings in material usage and building cost. The most critical part of the foundation is the connection between the metal mast and the concrete foundation. This connection is made by implementing an anchor cage. The application of an anchor cage results in complex multiaxial stresses in the surrounding area. Confinement plays a large role in the compressive strength of the concrete subjected to the partially loaded area created by the anchor flange. The main goal of this thesis is investigating the best way of modelling the concrete surrounding the anchor cage at confinement levels being present in the wind turbine foundation. This is done by investigating the theory regarding confinement and the effects of it. But also the way this is captured by analytical models. To make a comparison between DIANA and the analytical models the present stress situation in the wind turbine foundation is analysed. Looking at the stress distribution and the confinement levels. Next a comparison of the ideal case of confinement is made. This is later expanded to a larger scale model, to compare the effects of confinement. The confinement present will enhance the concrete strength and strain properties. The concrete specimen can resist larger loads and reacts more ductile. These properties are captured in analytical confinement models, these are compared to the way DIANA approaches confinement. The anchor rods between the anchor flanges are prestressed, this prestressing results in a permanent stress situation. Within this stress situation there is a small part beneath the anchor flange subjected to confinement. With increasing pressure, originating from the moment load which results from the wind on the tower and it’s blades, the confined area increases to the top half of the foundation. With significant confinement in the pedestal. Comparing the DIANA compressive behaviour models with the analytical confinement models and experimental data within a one element model. It is seen that compressive behaviour models do react on the confinement. Although they show a lower peak strength than the analytical models. A larger difference is noted in the underestimation of the peak strain. This difference in outcomes is the result of the different approaches between DIANA and the analytical models. As well as some assumptions that DIANA makes concerning the strength and strain increase factor. The Parabolic compressive curve is most suitable for modelling the confinement in this ideal case. A case study is conducted to further investigate how the confinement is represented in a total non-linear model. Modelling the top part of the foundation and checking the effect that confinement has on this model. For this model the prestressing of the anchor rods is increased until failure. Without confinement the concrete elements directly beneath the anchor flange fail in compression. Running the model with confinement it is noticed that the underlying weaker concrete is failing in compression first. This is the result of the confinement being present surrounding the anchor flange. The model with confinement is also able to resist a 30% larger load than the model without confinement. The parabolic compression curve is the most suitable curve currently available inside the strain based crack model for modelling the effects of confinement. Overall DIANA is able to capture the effects of confinement well enough to see the positive effects in the case study. Due to limitations in the analyses performed and in their interpretations, it is not possible to give an unequivocal answer on the effect of confinement within the foundation of the wind turbine.