Dynamic behavior of ‘Palazzo Lombardia’ helisurface, comparison between Operational and Experimental Modal Analysis

The ‘Altra Sede’ Tower, located in the heart of the city of Milan, has recently been completed. On January 22, 2010 an official ceremony marked the end of the construction phase and unveiled the new building to the citizens of Milano, a building which is currently the tallest in Italy, and one of the strongest features of the skyline of the city for years to come. Besides the main tower and the adjacent buildings an helicopter landing surface has been built. This paper deals with the dynamic testing of the surface, describing all the steps taken to identify the main vibration modes in the vertical and horizontal planes. Two different forcing systems have been employed and then the results coming from the forced response, experimental modal analysis results, have been 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 associated mode shapes have been correctly identified. This modal parameter set is the reference to which future measurements should be compared in order to perform a structural health monitoring campaign on the surface