Windmill foundations

Recently, I designed concrete foundations for several windmills in the NW of Spain using nonlinear finite element analysis.

First of all, I did a simple linear static analysis with nonlinear friction to estimate the amount of steel and the stability. Then, I confirmed the reinforcement using a 3D model with nonlinear materials for concrete and steel. The main problem was the deadline; the job must have been done in three weeks and running a nonlinear simulation took a while.

Square reinforced foundation are inefficient because the wind can blow in all directions and the diagonal load causes high moments. Circular reinforced rings are much more cost effective for wind turbine foundations. The reason why square windmill foundations are still used is because to dig a square hole is simple. An octogonal foundation was my proposal: it is nearly as efficient as a circular one and it is also easy to build. They would probably dig a square hole and backfill the corners. After long and hard talking it was impossible to change the mind of my client and the choice was a square foundation.
Sketch of the two load cases considered. The dimensions of the box are 12.5 x 12.5 x 1.6 m
Nonlinear model for the diagonal load case. (Only a half was modelled)

Nonlinear model for the parallel load case. (Only a half was modelled)
Drawing of the resultant reinforcement