This paper aims to report a simple and fast procedure to determine translational, rotational and torsional stiffness of wind tower foundations. The manufacturers of these structures establishing stiffness as a very important parameter for the foundations design. Usually the stiffness is estimated in the design stage considering empirical formulations, but several cases are reported pointing out that empirical procedures have conduced to significant differences from real values. At this paper the real stiffness of two wind tower foundations, one operating and another stopped, were obtained by constructing numerical models and fitting its modal parameters until reach real ones. Real modal parameters were obtained by conducting operational modal analysis over those structures. Results indicates that vibrations due tower under operation did not affect extraction of experimental values and also it was showed that modal analysis represents the structural behaviour of wind towers with good reliability, becoming an efficient and simple procedure to determine real stiffness of foundations.
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