## 4 Numerical Results
For this purpose, the maximum shear strain of the elastomer due to seismic displacements (\(\gamma_{s,max}\)), the maximum total shear strain (\(\gamma_{tot,max}\)), the ratio between the maximum axial load and the critical axial load \((N/N_{\sigma})\), and the ratio \((N/F_{cat})\) between the maximum axial load and the cavitation load provided by the CM2 CR model are compared with those provided by the other HDRB models. As with beams and columns, the above quantities are plotted as a function of the submultiple \(\alpha\) of the peak ground acceleration of the selected  
Figure 9: Response parameters of the base-isolation system for the B3 (**a**,**d**,**g**,**l**), B7 (**b**,**e**,**h**,**m**) and B10 (**c**,**f**,**i**,**n**) test structures subjected to the Chi-Chi EQ. EQs, for each test structure, nominal stiffness ratio, isolator, and HDRB model. For the sake of brevity, only the results for the most stressed isolators are presented below, denoting each device by the reference number in Figure 2a. In particular, the plots of Figure 9 show the shear strain \(\gamma_{s,max}\) of the corner isolator 1 when the IDA of the test structures B3, B7, and B10 are carried out considering the nominal stiffness \(\alpha_{K0}=1200\) and the Chi-Chi EQ. These results prove that as the value of the submultiple \(\alpha\) of the peak ground acceleration increases, the shear strains \(\gamma_{s,max}\) obtained by the CM2 CR model and other models diverge progressively although their starting values, corresponding to \(\alpha=0.1\), are close. Specifically, percentage variations of \(\gamma_{s,max}\) range from +29.1% for the B3 (Figure 9a) to +33% for the B10 (Figure 9c). The same remark can be made for the Imperial Valley EQ while for the Northridge EQ the different HDRB models lead to similar evaluations of the shear strains affecting the elastomeric bearings. A similar situation can be observed when comparing the total shear strains \(\gamma_{tot,max}\), as shown by Figure 9 where the plots corresponding to the perimetral isolator 7 and to the Chi-Chi EQ are presented. Specifically, percentage variations of \(\gamma_{tot,max}\), range from +40.7% for the B3 (Figure 9d) to +21.3% for the B10 (Figure 9f). Certain differences can be noted in the plots of Figure 9 where the ratio \((N/N_{cr})\), provided by the IDAs for the Imperial Valley EQ, is presented for isolators 8 and 9. They highlight that the CM2 CR and VEL models provide the highest and lowest values of the ratio \((N/N_{cr})\) for all the test structures, with a percentage variation of about +38.1% and +19.2% for the B3 and B10 test structures respectively, while the CM1 and CM1C models lead to intermediate results. These differences are less evident for the Chi-Chi and Northridge EQs. Finally, the plots where the ratios \((N/N_{cav})\) provided by the different HDRB models are presented for the Imperial Valley and Northridge EQs. They show that substantial differences are obtained when using different nonlinear models for HDRBs, also depending on the nominal stiffness ratio of the isolation system. Further results omitted for the sake of brevity highlight similar behavior for the Chi-Chi and Northridge EQs.