Current US requirements are based on a response spectrum with 5% damping. If the site specific regulations require a different damping value, and a site-specific spectrum is not required, the following factors may be applied to the impulsive spectral component to adjust the 5% damped values to other values of damping. The convective multiplier, K, from API 650 Appendix E is unchanged and equal to 1.5. Alternative adjustment factors to damping ratios are permitted providing they are based on local geotechnical data and rational analysis.
Table L-4 Impulsive Damping Ratio Adjustment
Damping ratio | Adjustment factor, Ki |
20% | 0.45 |
10% | 0.6 |
5% | 1.0 |
2% | 1.65 |
1% | 2.0 |
0.5% | 2.2 |