Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 117

Fig. 4.7 Campbell diagram for a 5 MW turbine

A turbine structure design where the first natural frequency of the total tower 4) lies
below the blade passing frequency (3P) and above the rotor frequency (1P) is designated
“soft-stiff”.
A turbine structure design where the first natural frequency of the total tower 4)

lies

above the blade passing frequency (3P) is designated “stiff-stiff”.
Such stiff designs are uneconomic and should be avoided.
Note 1:
The natural frequency in the example taken from Section 5.2 is f ¼ 0.289Hz and
therefore within the resonance range of the 3P excitation. If the rotational speed range
of the wind turbine is not to be restricted, then the stiffness of the structure must be
reduced or the arrangement of the mass modified.
Note 2:
The natural frequency should be calculated using the secant modulus of the concrete
E cm according to DIN 1045-1 [33] 9.1.5 and Table 9 5) because the excitation lies on the
level of the frequent actions ( s c 0.4 f c ), see Figure 4.8.

4) Taking into account a 10% safety margin.
5) The higher E cm values according to [36,55] result in a theoretically stiffer structure!