Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 109

Fig. 4.2 Reaction in the subsoil to the tower loads acting on an annular foundation

where
M found fixed-end moment at soil/structure interface

c s foundation modulus

I found second moment of area for area of foundation

Only the rotation of the foundation dependent on a short-term increase in the fixed-end
moment at the soil/structure interface is significant for the vibration calculation (see
Section 4.3) and the calculation of deformations at the serviceability limit state.
According to [53], the foundation modulus for determining the subsoil deformations
due to overturning amounts to

E s ; dyn E s ; dyn

t found

p

A found

c s ; dyn ¼ ¼

f 0

where
E s,dyn dynamic modulus of compressibility

A found area of foundation

f 0 ¼ 0.25

shape factor for overturning

p

A found t found ¼ 0 : 25 effective depth for antisymmetric action effect

Non-elastic rotations of the foundation must be considered as well when calculating
the deformations according to second-order theory at the ultimate limit state (see
Section 4.7.1). In the author's experience, these non-linear effects can be taken into
account approximately by assuming the static modulus of compressibility E s,stat instead
of the dynamic one.
We get the following ratios depending on the type of soil [53]:
a) Non-cohesive soils: 2 < E s,stat /E s,dyn < 4
b) Cohesive soils: 6 < E s,stat /E s,dyn < 20

Accordingly, the static foundation modulus is as follows:

E s ; stat E s ; stat

t found

c s ; stat ¼ p

A found

¼

f 0