Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC

code stress by using a damping ratio of 4% rather than 5% decreases from 8% to 2%. However, the case
for the US2-1 shows just the opposite.
To evaluate the impact of the damping ratio in a slightly different way, the reactions to the piping systems
are utilized in this appendix for comparison. For the DM model, there are 17 reaction forces and 9
reaction moments. For the US2-1 model, there are 16 reaction forces and 9 reaction moments. The
comparison was made by either considering all reactions or considering only the significant ones, which
are defined as the top half reactions when ranked by magnitude.
Figure A-5 and Figure A-6 show histograms for the ratio of the reaction forces and the ratio of the
reaction moments using all data. The mean and the standard deviation of the percentage increase in these
reactions (PIR) using 4% damping versus 5% damping are about 11~12% and 4~5%, respectively. If the
top half significant reactions (ranked by magnitude) are used in the comparison to merit more weight on
the large reactions, as shown in Figure A-7 and Figure A-8, the mean of the same percentage increase
decreases slightly to 10~11% while the standard deviation remains about the same. The four comparisons
shown in these figures also show that the maximum percentage increase is about 20%. The PIR appear to
be somewhat larger than those for the maximum code stress, partly because the internal pressure and
gravity load used in the maximum code stress calculation do not change with damping. It should be noted
that the internal pressure, gravity load, and seismic load in other directions are not considered in the
analyses to obtain the reactions.
The effect of spectrum broadening on the PIR is demonstrated through Figure A-9 through Figure A-16.
By using broadened input spectra instead of the unbroadened input spectra, the mean of the PIR
uniformly increases by 3% for all cases, the standard deviation of the PIR slightly decreases, and the
maximum of the PIR increase in various degrees but remains close to 20%. Therefore, using the
broadened spectra as in the current design practice is unambiguously conservative in assessing the
difference in dynamic responses due to 4% damping versus 5% damping.
In summary, compared to 5% damping, the reactions of piping systems subjected to just seismic
excitations can be 10~16% [mean ~ mean + standard deviation] higher using 4% damping, while the
maximum increase in reactions can be 20%. These observations should be applicable to the internal
forces/moments in the piping systems as the linear analyses implies. Comparing cases using 4% damping
versus 5% damping, the slightly lower increase in the maximum code stress (9%) is believed to be caused
by the loads other than the subject seismic load. Broadening of the input spectra can add an additional
3% percentage increase in reactions, compared to using unbroadened input spectra.
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