## 5 Experimental Results
### Dynamic Tests
It is possible to observe that the first mode has the highest amplification, the following being less amplified, especially for the displacement response. By comparing the PFRFs of the non-segmented structure with the experimental PFRFs of the absolute acceleration and relative displacement at the fourth floor (TMD level) of the FTMD configuration, as a general result, it can be stated that the dynamics of the two systems greatly differ. For the FTMD configuration both acceleration and displacement signals are strongly reduced with respect to the uncontrolled case: the first mode splits into two smaller  
Figure 8: WN. Frequency response functions: (**a**) absolute acceleration at support plane, (**b**) relative displacement at support plane, (**c**) 4 th floor absolute acceleration, (**d**) 4 th floor relative displacement. Legend: F3S, F4, FTMD configurations at RMS amplitudes WN01 = 0.002 g, WN05 = 0.008 g, WN12 = 0.025 g. Circles in the figure indicate additional modes due to asymmetric base boundary conditions. amplitude modes, with frequencies lower and higher than the first uncontrolled one, respectively, and amplifications dramatically reduced. The first peak generally is always greater than the second one, and this is more evident for the displacement response. A third modest amplification is evident only in the acceleration response (Figure 8c). All peaks subsequent to the first appear dramatically reduced with the insertion of the non-conventional TMD. By increasing the input intensity, the curves move to the left, evidencing the softening behavior induced by the HDRB isolators, already noticed in the responses at the support level; moreover, the first amplification attenuates, while the second one modestly increases. This phenomenon is due to the variations in the first two modal damping factors with the intensity, which produce an increase in damping in the first mode and a decrease in the second one. The control appears effective in all the range of frequencies of the uncontrolled non-segmented structure. Figure 9 reports the force displacement cycles of the isolators in the FTMD configuration at the three input intensities investigated. It can be noticed that at low intensity the isolators behave as rigid plastic elements, the friction effects being predominant due to the interaction with the support plane. Instead, at higher intensities the cycles become elliptic, denoting that most of the damping is due to viscous effects. Moreover, it is possible to observe that the effective stiffness of the isolators decreases by increasing the input intensity. From the force displacement cycles, the secant stiffness of the isolators in the range of displacements considered ranges from 75 kN/m (Figure 9a) to 25 kN/m (Figure 9c). Making use of the experimental FFRFs shown in Figure 8, a procedure for the structural identification based on the ERA/OKID algorithm [24; 25] was applied, providing an identified first-order representation of each configuration. In the case of the FSS configuration, the identified relevant frequencies obtained are five, at about 3.98, 11.85, 16.71, 17.49, and 20.65 Hz, and the identified damping factors are around 0.48, 0.46, 0.40, 0.45, and 0.51 (%). While repeating tests at increasing intensities for this configuration, it was observed that frequencies modestly decreased, whereas damping factors had a modest increase; the results are not reported for sake of brevity. It can be concluded that the three-story frame structure with support was within the elastic range and behaved linearly. In the case of the F4 configuration, the identified relevant frequencies are six, at about 3.25, 9.70, 12.95, 14.90, 16.30, and 18.25 Hz, and the identified damping factors are 0.92, 0.50, 0.40, 0.42, 0.44, and 0.34 (%).