ABSTRACT

With an increasing demand for renewable energy sources worldwide, a promising alternative is wind power. During the last decades the number of wind power plants and their size have increased. Wind power plant foundations are subjected to a centric load, resulting in a 3D stress distribution. Even though this is known, the common design practice today is to design the foundation on the basis of classical beam-theory. There is also an uncertainty of how to treat the fatigue loading in design. Since a wind power plant is highly subjected to large variety of load amplitudes the fatigue verification must be performed.
The main objective with this master thesis project was to study the possibility and suitability of designing wind power plant foundations with 3D strut-and-tie modelling. The purpose was also to investigate the appropriateness of designing wind power plant foundations with beam-theory. In addition possible differences in reinforcement arrangement was studied.
A reference case with fixed loads and geometry was designed according to Eurocode with the two different methods, i.e. beam-theory and strut-and-tie modelling. Fatigue assessment were performed Palmgren-Miners law of damage summation and the use of an equivalent load. The shape of the foundation and reinforcement layout was investigated to find appropriate recommendations.
The centric loaded foundation results in D-regions and 3D stress flow which make the use of a strut-and-tie model an appropriate design method. The 3D strut-and-tie method properly simulates the 3D stress flow and is appropriate for design of D-regions. Regarding the common design practice there is a need for a local analysis in the interface between the foundation and tower. We found that the transition between the local and global analysis is neglected in the common design practice. Which makes the analysis incomplete. Therefore there is a need to determine the linear-elastic stress distribution.
Further, clarifications of fatigue assessment regarding the use of an equivalent load for reinforced concrete need to be established. The method of using an equivalent load in fatigue calculations would considerably simplify the calculations for both reinforcement and concrete.
We found the use of 3D strut-and-tie method appropriate for designing wind power plant foundations. It shows advantages in the fatigue design but the need for computational aid or an equivalent load becomes necessary.

Opponent: Niklas Larsson and Dan Nilsson
Examiner: Professor Björn Engström, Chalmers
Supervisor: Anders Bohlin, Norconsult AB and Rasmus Rempling, Chalmers