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Local Site Effects

Seismic Site Response Analysis. Local Site Effects. CEE 531/ESS 465. Local Site Effects. Ground surface motions are affected by local site conditions Site effects can influence: Amplitude - may amplify or de-amplify motion Frequency content - may shift to higher or lower w

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Local Site Effects

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  1. Seismic Site Response Analysis Local Site Effects CEE 531/ESS 465

  2. Local Site Effects • Ground surface motions are affected by local site conditions • Site effects can influence: • Amplitude - may amplify or de-amplify motion • Frequency content - may shift to higher or lower w • Duration - may extend duration of strong shaking

  3. Important Aspects of Earthquake Motion • Earthquake damage is influenced by ground motion: • Amplitude • Frequency content • Duration Low amplitude High amplitude

  4. Important Aspects of Earthquake Motion • Earthquake damage is influenced by ground motion: • Amplitude • Frequency content • Duration FAS Low frequency motion w FAS High frequency motion w

  5. Important Aspects of Earthquake Motion • Earthquake damage is influenced by ground motion: • Amplitude • Frequency content • Duration Short duration Long duration

  6. Path Source Site Response Problem: Predict the response of a soil deposit due to earthquake excitation Site

  7. Site Response • Soil response depends on: • Type of soil • Thickness of soil • Stiffness of soil Bedrock • Results: • Some soil deposits amplify bedrock motion • Some soil deposits de-amplify bedrock motion • Some soils do both

  8. Communications Building 30 m soft clay University Rock Site Response • 1985 Mexico City Earthquake • M = 8.1 • Over 200 miles away Young lake deposits

  9. Site Response Soft clay amplified motions at periods of ~2 sec. even more • 1985 Mexico City Earthquake • M = 8.1 • Over 200 miles away Rock – 0.03g Soft clay – 0.15g Soft clay amplified bedrock motions by factor of 5

  10. Yerba Buena Island Treasure Island San Francisco Yerba Buena Island Treasure Island Rock Soft soil Site Response • 1989 Loma Prieta Earthquake • M = 7.1 • Over 60 miles away Oakland

  11. Soft soil Rock Site Response Amplification is even larger in response spectrum at some periods • 1989 Loma Prieta Earthquake • M = 7.1 • Over 60 miles away Rock – 0.06g Soft soil – 0.15g Soft soil amplified bedrock motions by factor of 2-3

  12. Rock Dense sand Rock Rock Evidence 1957 San Francisco Earthquake (M = 5.3) Soft clay

  13. Calculations Loma Prieta Mexico City When? Why? Evidence Compilation of data from multiple events Rock Stiff soils Peak acceleration (g) Peak acceleration on rock (g) • Peak acceleration • For low loading levels, strong amplification can occur • For high loading levels, deamplification can occur

  14. Short-period Long-period Evidence Compilation of data from multiple events Soft to medium clay and sand Deep cohesionless soils Sa/amax Stiff soils Rock

  15. Summary • Local site conditions can strongly influence: • Amplitude • Frequency content • Duration • Shown by: • Measured surface motions • Compilations of data on local site effects • Measured amplification functions • Theoretical analyses

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