Framed Structures: Detection of Building Torsional Modes Using a Simplified Experimental Approach

Heavy structural damage on buildings subjected to seismic motion is frequently due to torsional effects. These effects have been extensively studied in the last years and incorporated in seismic codes. The standard approach for the experimental evaluation of such effects involves the installation of a multi-channel accelerometric system on buildings, either for continuous monitoring of earthquakes or to record forced vibrations. This time- and resource-consuming approach is usually applied to single buildings, but cannot be used in large scale studies in order to validate, as example, building code provisions or for a quick estimations of building dynamic properties during large scale vulnerability evaluations. In this paper a simplified experimental approach based on ambient vibration recordings, which is widely used for the dynamic characterization of structures, has been proposed. In order to test the proposed approach experimental tests have been performed. Several sensor configurations have been tested. Using just two three-directional seismic stations installed at the top floor of a building it is possible to identify the most important building eigenfrequencies related to torsional modes. On the contrary, installing at least one station for each floor it is possible to identify also the mode shapes. In order to validate the proposed technique a comparison between the results provided from the proposed technique (using velocimeters), and those obtained from another full experimental campaign (using accelerometers) performed on a two-story steel frame at the University of Basilicata, have been compared.