Comparison of two different OMA techniques applied to vibration data measured on a Stadium grandstand

During the last years the increasing reliability of OMA techniques has made ambient vibration modal testing and analysis a good tool to monitor the structural health conditions and to improve maintenance strategies. A large number of robust output-only modal identification techniques are available in the literature; among these methods, the Stochastic Subspace Identification (SSI) and the Poly-Reference Least Square Frequency Domain (PolyMAX) techniques are widely used, as they are fast and reliable also at the commercial stage. In this paper, both SSI and PolyMAX identification techniques are used on a real test case to extract the modal parameters of a stadium grandstand. The stadium is the San Siro Meazza, in Milan, which, since some years, is the object of an extensive continuous monitoring activity. Lots of ambient vibrations data are available as the stand is monitored on a 24h-7 days a week basis. A compete comparison of the two methods is presented pointing at reliability and repeatability of identification results and uncertainty issues in the modal parameter identification. The proposed benchmark will focus attention on the identification of those modes with close frequencies and high damping levels.