GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.2 371 the damping ratio (ζ). The latter parameter represents the energy loss of an oscillating system that can be either internal (material damping) or due to another system (radiated damping). The damping ratio is important in seismic design since it allows to evaluate the ability of a structure to dissipate the vibration energy during an earthquake. Such energy causes a structure to have the highest amplitude of response at its fundamental frequency, which depends on the structure’s mass and stiffness. Therefore, the damping level, as well as the knowledge of the fundamental period (T) of the building are particularly important for estimating the seismic base shear force F in designing earthquake resistant structures. The seismicperformanceof abuildingobviouslydepends on theprogressionof the frequencies along the input time-history, nevertheless the knowledge of its fundamental frequency at low amplitude values and the associated damping are of primary importance to characterize the initial seismic behavior of a structure. These parameters can usually be obtained either through numerical modelling or experimental monitoring of the building using different input motions. In the present study, the standard noise spectral ratio (SSNR) and horizontal to vertical spectral ratio (HVSR) techniques were used to identify the building and site fundamental frequencies. Measurements are going to be performed in 70 buildings distinguished according to their construction typology into masonry buildings (MA) and reinforced concrete (RC) buildings. Ambient noise was recorded using a three-component velocimeter, sampling the signal at a frequency of 128 Hz. In each building, 20 minutes length ambient noise samples were recorded both at the top and at the ground floor. According to the guidelines suggested by the European project Site Effects assessment using AMbient Excitations (SESAME, 2004), time windows of 20 s were considered, selecting the most stationary parts and not including transients associated to very close sources. Fourier spectra were calculated in the frequency band 0.5-20 Hz and smoothed using a triangular average on frequency intervals of ± 10% of the central frequency. In Fig. 2 example of results, obtained for measurements performed in the area of “Palazzo CLMA-COF” (Uffici in Fig. 1) and ”Palazzo Ingrassia” (comparto Benedettini in Fig. 1) are shown. As concern the measurement performed inside the buildings to observe the influence of the geometry, the two main axes of sensors are oriented as coincident with the main directions of the building (NS ≡ transverse; EW ≡ longitudinal) in order to better highlight their respective contribution. The spectral ratio peak having the highest amplitude in the SSNR was considered as representing the building fundamental period (Fig. 2). Fig. 2 - Example of SSNR graphs concerning the ambient noise measurements performed at the different floors of “Palazzo CLMA-COF” (upper panels) and “Palazzo Ingrassia” (lower panels).