Dynamic testing of a helicopter landing pad: comparison between operational and experimental approach

The ‘Altra Sede’ Tower is one of the most recent buildings in Milan, but it has already become one of the strongest features of the city skyline. On 22 January 2010, an opening ceremony dedicated the skyscraper to the citizenship as the base of the ‘Regione Lombardia’ government. A helicopter landing surface was built for institutional use beside the main tower and the adjacent buildings. This paper deals with the dynamic testing of this surface, describing all the steps followed to identify the main vibration modes in the vertical and horizontal directions. Two different forcing systems are employed and the results from the forced response, experimental modal analysis are compared to the ones obtained via operational modal analysis, leaving the same sensors in place for a few days and continuously measuring the ambient vibrations. The obtained results describe the surface dynamic behavior in the range 0–10 Hz. All the resonant frequencies and the associated mode shapes are correctly identified. This modal parameter set is the reference to which future measurements should be compared, to perform a structural health-monitoring campaign on the surface.