The two new high rise buildings for the European Court of Justice in Luxembourg have been tested by harmonic shakers and by Operational Modal Analysis. The background for the tests is to estimate the influence on the damping of one of the towers from an array of Tuned Liquid Dampers (TLDs) placed on top of the building. The TLDs have been designed to minimise the response of the buildings to wind loading, resulting in an increase in occupancy comfort. The harmonic excitation is performed to estimate the damping at a response level corresponding to moderate wind loading whereas the OMA has been performed under minimum wind loading. The paper presents the testing programme and the main results of which one is a clear non-linear behaviour of the introduced viscous damping system. To the authors’ knowledge, no other structural excitation of this scale has been undertaken, before and after the application of tuned damping systems, in which the damping system is completely independent of the excitation system.