Comparison for EQSB isolators

6.2.1 Available data and selection of representative results

As presented in Table 17 (Section 4.3.1), hybrid tests were performed for the benchmark cases 1 and 3 only, corresponding to RG 1.60 spectra excitation at DBE level (0.5 g peak ground acceleration) with, respectively, one and five macro-isolators. For each of these cases, two subcases are defined corresponding to:

 Excitation in real time but in the two horizontal directions only.

 Excitation in slowed time in the three directions simultaneously.

As for the LRB system in the previous section, and for the same reasons, the comparison will focus on the two tests representative of the Case 1 and noted Test 1_1 (two directions real time) and Test 1_2 (three directions slowed time).

In the vertical direction, and due to practical testing reasons, the excitation applied during the hybrid tests on the EQSB isolator was lower than the one specified for the numerical benchmark (see Section 5). This change can potentially affect both the vertical and the horizontal behaviour, because of the dependency of friction forces on the vertical loads. This fact is acknowledged when comparing simulation to tests results.

6.2.2 Comparison of displacement time histories

Comparisons of the experimental and numerical displacement time histories at the central node of the upper basemat are given in Figure 177 for the three directions X, Y and Z and for Case 1.

In both horizontal directions, and as it was observed for the LRB system, the main sliding phases occur at the same time in the simulation and in the experiments. On the other hand, and unlike what was

185 observed with LRB, some small sliding phases occurring at the beginning and at the end on the experiment seem not to be reproduced by any of the participants simulation. This phenomenon is

FIG. 177. EQSB system – Comparison of hybrid tests and benchmark results for Case 1 –Relative displacement at centre of upper basemat.

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associated to the experimental process itself, as explained in Section 5 and it will not be further considered here.

The maximum amplitude of the sliding phase is correctly predicted by both participant P5 and P7. Unlike for the LRB system, no apparent conservatism is observed for this group of participants with the EQSB system. Participant P3 results are 2 to 3 times lower than the experimentally observed ones.

In the vertical direction, and even though the test excitation applied was lower than the one applied to the numerical models, all participants results are significantly lower than the experimental ones. For participants P3 and P5, this is clearly due to their rigid representation of the EQSB isolator in the vertical direction. For participant P7, who updated his vertical stiffness based on the isolator characterization tests, and whose results still seem to be one order of magnitude below the experimental one, it seems to be that the global vertical mode of the superstructure is affected by the same overdamping of all structural modes that was identified in all of these participants results.

The conclusions for Case 3 are strictly the same as those stated above for Case 1.

6.2.3 Comparison of floor response spectra at centre of upper basemat

Participants floor response spectra computed at the upper basemat central node are compared to the test ones for directions X, Y and Z in Figure 178.

In the horizontal directions, and up to 2 Hz, participants P5 and P7 (first group of participants) response spectra do fit the test response spectra rather well, highlighting a same globally well represented effect of the isolation system. For reasons that have already been explained in Section 4, results of participant P3 (second group of participants) are significantly different in this frequency range.

Around 3 Hz, a peak appears on the test response spectra, which exact frequency seems to be dependent on the loading rate. This peak occurs at different frequencies for Test 1_1 and Test 1_2 and it may partially be a side effect of the testing process. This peak is not observed on the participants response spectra.

For higher frequencies, where the influence of the internal structures and auxiliary building structural modes is felt at the basemat level, the peak appearing around 8 Hz on the test results is 2 to 3 times higher than the one predicted by participants P3 and P5. Participant P7 spectra, as in other cases, show no peak at all at the structural frequencies.

In the vertical direction, a clear peak appears in the test response spectra between 7 and 8 Hz, which is non-existent on the participant predicted response spectra. This shows the tremendous importance of not neglecting the isolators vertical stiffness calibration when defining a numerical model.

The comparison of floor response spectra at the upper basemat level for Case 3 leads to slightly different conclusions from the ones presented above for Case 1. For this reason, results for Case 3 are presented in Figure 179. The peak observed in Case 1 around 3 Hz and reaching 1 g is now smaller and its frequency does not vary with the test loading speed anymore. The peak at 8 Hz, which was largely above the participants predicted peak is now reduced to a comparable order of magnitude.

In the vertical direction the conclusions for Case 3 are identical to those for Case 1.

As a global conclusion, the test procedure itself has a significant effect on the upper basemat floor response spectra in the frequency range of the structural modes. Significant differences are observed between tests in real time and slowed time around 3 Hz and a 50% variability are observed on peaks resulting from the structural modes around 8 Hz.

187 FIG. 178. EQSB system – Comparison of hybrid tests and benchmark results for Case 1 –Floor response spectra at upper basemat level.

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FIG. 179. EQSB system – Comparison of hybrid tests and benchmark results for Case 3 – Floor response spectra at upper basemat level.