1. Site Specific Response Analyses and Design Spectra for Soft Soil Sites Steven F. Bartlett, Ph.D., P.E. I-15 NATIONAL TEST BED TECHNOLOGY TRANSFER SYMPOSIUM Highway Bridge and Geotechnical Innovations Salt Lake City, Utah Sept 17-18, 2003

2. Amplification of pga on Soft Soil Sites

4. Approach 1.Development of Subsurface Profile and Dynamic Soil Properites 2.Development of a Target Rock Spectrum 3.Selection of Time Histories 4.Scaling or Spectral Matching of Time Histories to Target Spectrum 5.1-D Equivalent Linear Convolution Analysis 6.Development of Final Enveloping Design Spectra

5. Step 1 - Example I-80 and 600 S. Soil Profile

6. Deep Profile Depth To site class C (approx. 70 m) The Vs model should be developed to a depth corresponding to site class C soils (1200 ft/s or 365 m/s)

7. Step 1 Selection of Modulus and Damping Curves Vucetic and Dobry (1991) for shallow clayey and silty soils (requires estimate of PI) Electric Power Research Institute (1993) sands for deeper sediments (or other curves that account for effective stress dependency)

8. Step 2 Development of Target Rock Spectrum 2% Exceedance Probability in 50 yrs – Site Class B

9. Step 3 Selection of Time Histories Considerations a.fault mechanism and tectonic regime b.earthquake magnitude (characteristic event) c.fault distance d.duration of strong ground motion e.site conditions (select rock records) f.fault directivity pulse (sites < 10 km) g.number of independent time histories required (5 to 7)

10. Step 3 Selection of Time Histories

11. Step 4 Spectral Matching Spectral matching insures that the design time history has the proper amplitude and frequency content of the target spectrum. RSPMATCH (Abrahamson, 1992) RSPMATCH adjusts the initial accelerogram iteratively in the time domain to achieve compatibility with a specified target acceleration response spectrum. RSPMATCH adds wave packets having specified period ranges and limited durations to the input time history.

12. Step 4 Spectral Matching Comparison of Matched Spectrum to Target Spectrum

13. Step 5 – 1 D Convolution Analysis Surface Soil Response Bedrock Site-Specific Profile Spectrally matched time history assigned to bedrock layer as an outcropping rock motion. Assign 1 to 2 percent damping in the elastic bedrock layer.

14. Step 5 Results of 200 m deep convolution Analysis

15. Step 6 – Development of Final Design Spectrum