Design of wind turbine foundation slabs

Abstract: In this study the structural behaviour of wind turbine foundation slabs is
analysed with various numerical and analytical models. The studied methods
include models suitable for hand-calculations, finite element models with
plate elements resting on springs as well as three dimensional models of
both the foundation slab and the soil. Linear elastic as well as nonlinear
behaviour including cracking of concrete and the complex load transfer from
the tower into the foundation through a steel ring is considered in the
study.

The elastic analyses show, for example, that whereas in a concentrically
loaded foundation slab a significant part of the load is carried through
diagonal compression struts thus resulting in less flexure than what was
found with the FE-models, the largest section forces and moments in a slab
subjected to large overturning moment are obtained with a three-dimensional
FE-model of both the slab and the underlying soil: i.e. the section forces
increase together with the accuracy of the model.

An important issue when designing members according to nonlinear analyses
is to consider proper choice of material parameters. The results of a
nonlinear plate element analysis verify the assumption that considerable
redistribution of the section forces takes place due to flexural cracking
of concrete. However, because of the large amount of simplifications of a
simple plate element model no major conclusions of the structural behaviour
should be made.

A three-dimensional elastic analysis of a typical wind turbine foundation
slab considering the complex load transfer through a steel ring reveals
that the global flexural behaviour of the structure can be modelled
sufficiently well by simpler models. This model, however, yields the
largest section forces and moments: this has to be considered when
simplifications are made. Additionally, the high local stress
concentrations and the relative movement of the steel ring anchorage have
to be taken into consideration when designing the reinforcement. A
complete, three-dimensional nonlinear analysis of the foundation slab shows
that the steel ring anchorage in the slab is the most critical part of the
structure.