GNGTS 2017 - 36° Convegno Nazionale

372 GNGTS 2017 S essione 2.2 Damping ratioswere computed using theRandomDecrement technique toolbox implemented in Geopsy (Fig. 3). This technique is based on the assumption that at each time step, the recorded signal is the sum of a random signal and the impulse response function of the study- system. Stacking many time-windows with the same initial condition implies the enhancing of the impulse response function component with respect to the zero-mean random part. The algorithm selects all the windows of the given length starting with a 0 amplitude and a positive derivative and averages them. Then, the impulse response function is fitted by an exponentially decreasing sine function (starting at 0) depending on an amplitude α, the resonance frequency f=ω/2π and the damping ratio ξ (see for details: http://www.geopsy.org/ ). Fig. 3 - Examples of damping graphs obtained from measurements performed, for the NS and EW components, at the recording sites located in the “Palazzo CLMA-COF” (upper panels) and “Palazzo Ingrassia” (lower panels). Results and concluding remarks. In the studied buildings, ambient noise was recorded at each floor locating the sensors both at their center of mass and their edges. This settlement allowed us to point out the potential existence of complex dynamic behaviour of the investigated structures. Going into more details, such complex behavior is often observed in the buildings erected in downtown area where MA edifices, being adjacent, interact each other therefore showing the presence of several SSNR peaks (e.g. Palazzo CLMA-COF in Fig. 2). This complex behavior can also be ascribed to the influence of new elements, such as floor tiling, floating floors, piping, out-of-plane movements of detached walls and stairwells detached from the frame of the building, added across different periods in time. Conversely, isolated buildings having a regular plant (e.g. Palazzo Ingrassia in Fig. 2) are characterized by the presence of a dominant single SSNR peak. As concerns the damping values, although they are obtained through a weak motion input (ambient noise), they can achieve significant information. Results set indeed into evidence a link with the soil foundation and the building geometry. In our study, it appears in particular evident a variability linked to the different rigidity of the investigated structures, as a function of the longitudinal or transversal orientation of the structural elements. Moreover, the comparison between the buildings and the site frequencies, achieved through HVSR, allowed us to put into evidence the potential existence of site-to-structure resonance effects.