Elastic period of vibration calculated experimentally in buildings hosting permanent GPS stations

During the earthquake in Emilia (Italy) of 2012, ca. 30 permanent GPS stations were in operation within a radius of about 100 km from the epicentre, each equipped with an antenna rigidly fixed to the host building and sampling the GPS signal at a high rate (> 1 Hz). From the recording of the GPS measurements we calculated, in kinematic mode, the instantaneous displacements s(t) in the North-South and East-West directions of the phase centres of the single GPS antennas at each permanent station during the most important seismic sequences. Subsequently, for each of the two displacements considered as two distinct external forces, we determined the elastic response spectra of the building and from them we extracted the two periods of vibration T along two orthogonal directions coinciding, respectively, with the walls of the building. The experimentally obtained periods of vibration were compared with those inferable from the technical literature. In this way, we obtained a sufficiently large sample per building type, geometry (square, rectangular, regular or irregular planimetry), height (from a minimum of 4 to a maximum of 20 metres), materials (masonry, reinforced concrete, etc.). From the computational point of view, the study confirmed that GPS is an emerging tool for monitoring dynamic displacements and the experimentally estimated value of T is always lower than the one estimated with the formulae reported in the literature. The limitations of the study lie in the impossibility to choose a priori the geometry and/or structural type of the building hosting the GPS station.