Environmental Engineering Reference
In-Depth Information
• Deutsches Institut für Bautechnik-DIBt, Richtlinie für Windenergieanlagen
Einwirkungen und Standsicherheitsnachweise für Turm und Gründung, 2012 [ 13 ].
The majority of the standard codes suggests an idealization of wind turbine as a
vertical beam with a concentrated mass at the top. The latter includes the weight of
rotor, nacelle, gear box, and a part of the tower.
Any analysis procedure is permitted for the evaluation of horizontal seismic
forces: Equivalent lateral force, modal response spectrum, and response history
analysis. The seismic forces are then combined with the operational loading equal
to the greatest between:
1. Design load during normal power production at the rated wind speed and
2. Design load calculated for an emergency stop at rated wind speed.
The partial safety factors for all load components are set to 1.0.
A very conservative method is presented in Annex C of the IEC [ 19 ], where the
natural frequencies higher than the first are neglected and the wind turbine is
idealized as an SDOF system. Once the natural period of the SDOF system is
determined, a correspondent spectral ordinate is selected from the standard design
response spectrum. Multiplying the spectral acceleration by the mass of the SDOF,
an equivalent static seismic load is obtained. In this case, a standard design
response spectrum with a damping coefficient of 1 % is applied.
As described above, standard codes tend to suggest simplified techniques, which
are suitable for normal civil buildings. However, a more detailed modeling of the
seismic response of wind turbines can be achieved with the aid of computer-aided
aerodynamic tools. Nowadays, aerodynamic computational tools are widely used
and have become an indispensable support for design considerations, especially
when dealing with nondeterministic phenomena, such as earthquake and wind.
12.2.3 Soil-Structure Interaction
The first step in any dynamic analysis is the determination of the structural
dynamic properties. These depend on the performance of all components, from the
tip of the rotor blades to the underlying ground. During this step, the interaction
between the soil and the structure so-called soil-structure interaction (SSI) must be
taken into account because of several interrelated effects:
• First of all the dynamic natural properties of the structure are modified by the
presence of a compliant soil.
• Consequently, the minimum frequency separation between the natural frequency
of the structure and the rotor operational frequency (1P) as well as the blade
passing frequency (2P or 3P) may be violated and resonance effects may raise.
• On the one hand, the frequency content of the dynamic load may lead to
vibration amplification or attenuation phenomena, with possible high shear
force and overturning moment at the tower base.
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