Comparison of results for all bearing types

Lastly, hybrid simulation results from the three isolator types (LRB, EQSB, and TPFB) are compared in order to investigate differences in the isolator behaviours and their effect on the in-structure response of the seismically isolated nuclear power plant superstructures.

5.7.4.1 2D tests: LRB (Run 56), EQSB (Run 104), TPFB (Run 217)

Firstly, results from hybrid simulations with 2-component ground motion input are compared for the three different isolator types. The three hybrid simulations were executed at a test rate 2x-slower than real time using the 1-bearing equivalent model excited by the same design basis earthquake (DBE) motions. The isolator responses are compared in Figure 166. Significant differences in the horizontal isolator displacement demands are observed for the three isolation systems. The peak displacement amplitude for the LRB is 254 mm, while for the EQSB, the maximum isolator displacement is 205 mm, which is somewhat smaller than the LRB displacement demand. However, because the design displacement of the EQSB is only 152 mm, nonlinear hardening behaviour of the MER springs in the EQSB is observed. This significantly increased the horizontal shear forces in the bearing. For the TPFB, the peak isolator displacement amplitude is 456 mm, almost twice the displacement demand obtained for the other two bearings. However, as stated before, the design displacement of the TPFB is 584 mm, therefore the displacement demand is only 78% of the displacement capacity and no stiffening behaviour is observed for the TPFB. Nevertheless, it is important to remember that the large displacement demand of the TPFB requires that the umbilical systems, which cross the seismic isolation gap, are designed based on this large possible deformation.

169 Due to the greatly different displacement amplitudes and horizontal force-deformation behaviours of these three bearings, the horizontal shear force amplitudes are also very different. For the LRB and EQSB, the characteristic strengths are similar (1010 kN vs. 1090 kN). However, as discussed earlier, nonlinear hardening is triggered in the EQSB due to its smaller design displacement, leading to a peak

FIG. 165. Comparison of floor response spectra for 2D vs. 3D ground motion inputs. (Both hybrid simulations were executed 10x-slower than real time using the TPFB 1-bearing equivalent model under OBE seismic excitation levels).

horizontal resisting force in the EQSB which is almost twice that of the LRB (4227 kN vs. 2235 kN).

For the TPFB, due to its low horizontal sliding stiffness of the main sliding stage, which is significantly smaller than the post-yield stiffness of the other two bearings, the maximum shear force is only 1561 kN.

This is the case despite the much greater displacement demand of the TPFB in

relation to the other two cases. The significantly smaller shear force amplitude of the TPFB is beneficial for reducing the in-structure response of the upper plant because a smaller force demand is being

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transmitted into the superstructure. However, as a trade-off, the large displacement demand needs to be considered in the design of umbilical and other systems that cross the seismic isolation gap.

FIG. 166. Comparison of isolator response for three different isolator types. (All hybrid simulations were executed 2x-slower than real time with 2D bidirectional ground motion input using the 1-bearing equivalent model).

Floor spectra are compared in Figure 167. As can be observed, the in-structure responses are also significantly different for the three isolation systems. To provide a reference for the comparison and to better asses the benefits of the isolation systems, the original ground motion spectra and the fixed base spectra without isolators are also plotted. For the RCB, INS, and ACB buildings, above frequencies of 0.6 Hz spectral accelerations from the EQSB are up to two times larger than those from the LRB.

Spectral accelerations from the TPFB are even lower than the ones from the LRB for frequencies above 0.6 Hz. The only frequency range where the response spectra of the TPFB are somewhat larger than the ones from the LRB and EQSB is between 0.3 Hz and 0.6 Hz, which corresponds to the post-sliding frequency range of the TPFB. Because most of the equipment in the power plant is sensitive to accelerations with frequencies above 1 Hz (except for possible fluid sloshing effects), the TPFB is the most effective isolation system among the three systems considered to reduce the in-structure response in the power plant superstructures. This in-structure response, assessed in terms of floor response spectra, is consistent with the conclusions drawn based on the isolator responses where it was observed that the TPFB transmits the lowest shear force demands into the superstructure, followed by the LRB and then the EQSB transmitting the largest shear force demands. Finally, it is important to notice that all three bearing types tremendously reduce in-structure responses above frequencies of 0.7 Hz compared to the fixed-base power plant design. Peak floor accelerations are reduced by up to a factor of 3 for the EQSB isolation design and by up to a factor of 6 for the LRB and TPFB designs. Only at very low frequencies, which correspond to the effective periods of the isolators, the spectral acceleration amplitudes of the seismically isolated plants are slightly larger than the ones of the fixed base plant.

5.7.4.2 3D tests: LRB (Run 57), EQSB (Run 121), TPFB (Run 198)

To further assess the effectiveness of the different isolation systems, an additional comparison between the LRB, EQSB, and TPFB systems is performed. However, for this comparison the hybrid models were subjected to 3-component ground motion input at the DBE level. The three hybrid simulations were executed at a test rate 10x-slower than real time using the 1-bearing equivalent model. Firstly, the isolator responses are compared in Figure 168. Similar to the 2-component ground motion input case discussed above, the horizontal behaviour of the isolation systems is significantly different for the three bearing types. The TPFB generated a displacement demand (405 mm) that is almost twice as large as

171 the LRB demand (254 mm) and the EQSB demand (211 mm). In comparison to the previously discussed 2D input case, much larger shear force fluctuations can be observed, especially for the EQSB and the TPFB. For the LRB the shear force fluctuations are somewhat less pronounced.

FIG. 167. Comparison of floor response spectra for three different isolator types. (All hybrid simulations were executed 2x-slower than real time with 2D bidirectional ground motion input using the 1-bearing equivalent model).

These shear force oscillations are mainly due to the vertical-horizontal force coupling of the TPFB and EQSB where the sliding friction force directly depends on the instantaneous axial force in the bearing.

For the TPFB there is additional vertical-horizontal kinematic coupling due to the curved sliding surface.

Hence, the frequency of the oscillations in the shear forces is related to the frequency content of the axial force fluctuations. Part of this depends on the frequency content of the horizontal and vertical ground motion components, but part of it also depends on the vertical frequency of the seismically isolated plant and the vertical tuning parameters of the SRMD control system. For the LRB, shear force fluctuations are also observed, but they are much less pronounced in comparison to the other two

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isolation systems because the LRB only exhibits vertical-horizontal kinematic coupling effects. In addition, due to the poor vertical force tracking performance of the SRMD, the EQSB and TPFB behaviour is also greatly affected because of the pronounced vertical-horizontal coupling effects.

FIG. 168. Comparison of isolator response for three different isolator types. (All hybrid simulations were executed 10x-slower than real time with 3D ground motion input using the 1-bearing equivalent model).

The maximum horizontal resisting force also varies a lot for the three isolator types. For the LRB, the maximum shear force is only 2058 kN, while for the EQSB it is 5065 kN. The TPFB has a peak shear force amplitude of 2267 kN. Because the displacement demand of the EQSB was well beyond the design displacement of the bearing, nonlinear hardening behaviour of the MER springs was observed. This is the main cause for the very larger shear force demands produced by the EQSB. Because of the shear force fluctuation effects, the largest isolator force is greatly affected by the oscillation amplitudes.

Unlike the 2-component ground motion input case discussed above, the TPFB isolator shear force amplitude is similar to the LRB in the 3-component input case. The main reason is that the significant shear force fluctuations due to vertical ground motion input result in spikes, which in turn increase the maximum value of shear. Therefore, the effectiveness of the TPFB system to reduce the in-structure response in the power plant superstructure is substantially affected when 3D excitations are considered.

Superstructure floor response spectra are compared in Figure 169. Similar to the results obtained for the 2-component ground motion input, for most of the frequency range above 0.6 Hz, the TPFB provides the greatest reduction of spectral acceleration amplitudes. The EQSB provides the smallest reduction in relation to the fixed base power plant. However, all three isolation systems deliver significant benefits in comparison to the fixed base case. Because of the strong vertical-horizontal coupling behaviour of the sliding bearings, when vertical ground motion input is included, the shear force oscillations in the EQSB and TPFB result in a significant increase of the spectral accelerations around 9-10 Hz. From the results shown in Figure 169, all three cases have a larger spectral response in the 9-10 Hz frequency range when vertical ground motion input is considered in comparison to the 2D input case discussed earlier. The amplification effect for the EQSB and TPFB systems is more pronounced than for the LRB system. As discussed, the vertical-horizontal coupling for these two sliding bearings is mainly a force coupling, while the vertical-horizontal coupling effect of the LRB is mainly a kinematic or second order effect, which is less pronounced than the friction effect in sliding bearings.

173 FIG. 169. Comparison of floor response spectra for three different isolator types. (All hybrid simulations were executed 10x-slower than real time with 3D ground motion input using the 1-bearing equivalent model).

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