Limits of Current Design Approaches in the Analysis of Wind Turbine Foundations

The design of massive structures such as the shallow foundations of wind turbines is not a trivial task also in light of the fact that this structural typology is not specifically covered in technical Standards. The structural modelling of onshore wind turbine shallow foundations deals with combined complex phenomena, such as material nonlinearities, soil-structure interaction and local stress concentration in both reinforced concrete foundation and soil. The design practice basically adopts two methods: (i) an elastic approach where wind tower foundations are usually designed by modelling the foundation as a slab with variable thickness using shell finite elements and by introducing very simplified assumptions for the soil behaviour (i.e. Winkler model), and (ii) an approach where wind tower foundations are designed by using limit analysis. The paper aims to highlight the limits and the safety margins of the current design practice by comparing the aforementioned methods (i) and (ii) with nonlinear numerical simulations fully accounting for soil-structure interaction based on three-dimensional elements for both concrete foundation and soil, contact surfaces for their interface, and beam elements for reinforcing bars.