Issue 46

S. Ivorra et alii, Frattura ed Integrità Strutturale, 46 (2018) 203-215; DOI: 10.3221/IGF-ESIS.46.19 209 When all the bells are swinging in N-S direction, a high level of acceleration has been detected, not only in N-S direction, but also in E-W direction. Fig. 7 shows that the increment of acceleration in N-S direction is immediately observed after the bells start to swing, while a similar level is achieved in the E-W direction after 35 s from the starting of high levels of accelerations in N-S direction. This response is generated by the torsional effects produced by the non-symmetrically excitation by the swing of bells with different weights and different swing velocities. Tab. 2 shows that the amplification generated by the swing of all the bells is higher in N-S direction than in E-W direction. All bells swinging vibration Ambient vibration Amplification 1 (N-S) 2 (E-W) 1 (N-S) 2 (E-W) 1 (N-S) 2 (E-W) Max (g) 0.0983 0.1251 0.0025 0.0061 38.66 20.45 Min (g) -0.0977 -0.1126 -0.0034 -0.0062 28.35 18.24 Table 2 : Amplification generated by bells in N-S and E-W directions. Figure 7. Acceleration registered at the top of the tower with 1024 Hz sampling frequency and under the swinging of all the bells at the same time. Up: E-W direction. Bottom: N-S direction In order to delete the high frequency accelerations, which may be due to the acoustical excitations and not related to the bells swinging, a digital filter has been applied that neglets the frequencies higher than 100 Hz. Fig. 8 represents this applied filter in which it is guaranteed that the frequencies of the structure and their interaction with the mechanical frequencies of the bells themselves are far from the filtering and therefore remain in the analyzed signals. In order to have a considerable reduction of frequencies higher than 100 Hz an attenuation of 80 dB has been applied (Fig. 8, up). In the same way, in order to achieve an adequate transition in the proximities of 100 Hz, after testing with different configurations, a Log 200 has been assumed to guarantee that the filtered accelerograms maintain the maximum values of the original signals in the range of 0-100 Hz. Figure 8. Digital filter applied. Up: Real filter applied. Bottom: Impulse applied. Time [s] 420s 400s 380s 360s 340s 320s 300s 280s 260s 240s 220s 200s 180s 160s 140s 120s 100s 80s 60s 40s 20s 0s Accelertaion (g) 0,1 0,05 0 -0,05 -0,1 Time [s] 420s 400s 380s 360s 340s 320s 300s 280s 260s 240s 220s 200s 180s 160s 140s 120s 100s 80s 60s 40s 20s 0s Accelertaion (g) 0,1 0,05 0 -0,05 -0,1 Frequency [Hz] 500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 Filter (dB) 0 -50 -100 -150 -200 Time [s] 200s 190s 180s 170s 160s 150s 140s 130s 120s 110s 100s 90s 80s 70s 60s 50s 40s 30s 20s 10s 0s Acceleration (g) 0,2 0,15 0,1 0,05 0 -0,05