local site effects seismic site response analysis cee 531/ess 465
TRANSCRIPT
Local Site EffectsLocal Site Effects
Seismic Site Response Analysis
CEE 531/ESS 465
Local Site EffectsLocal 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 • Duration - may extend duration of strong shaking
Important Aspects of Earthquake MotionImportant Aspects of Earthquake Motion
Earthquake damage is influenced by ground motion:
• Amplitude
• Frequency content
• Duration
Low amplitude
High amplitude
Important Aspects of Earthquake MotionImportant Aspects of Earthquake Motion
Earthquake damage is influenced by ground motion:
• Amplitude
• Frequency content
• Duration
Low frequency motion
High frequency motion
FAS
FAS
Important Aspects of Earthquake MotionImportant Aspects of Earthquake Motion
Earthquake damage is influenced by ground motion:
• Amplitude
• Frequency content
• Duration
Short duration
Long duration
Site Response
Problem:
Predict the response of a soil deposit due to earthquake excitation
SourceSource
PathPath
SiteSite
Site Response
Soil response depends on:• Type of soil• Thickness of soil • Stiffness of soil
Results:• Some soil deposits amplify bedrock motion• Some soil deposits de-amplify bedrock motion • Some soils do both
Bedrock
Site Response1985 Mexico City Earthquake
M = 8.1Over 200 miles away
Younglake
deposits
University
CommunicationsBuilding
30 m soft clay
Rock
Site Response1985 Mexico City Earthquake
M = 8.1Over 200 miles away
Rock – 0.03g
Soft clay – 0.15g
Soft clay amplified bedrock motions by
factor of 5
Soft clay amplified bedrock motions by
factor of 5
Soft clay amplified motions at periods of ~2 sec. even more
Site Response1989 Loma Prieta Earthquake
M = 7.1Over 60 miles away
Oakland
Yerba Buena IslandTreasure Island
San Francisco
Yerba Buena Island
Treasure Island
Rock Soft soil
Site Response1989 Loma Prieta Earthquake
M = 7.1Over 60 miles away
Rock – 0.06g
Soft soil – 0.15g Rock
Soft soil
Soft soil amplified bedrock motions by
factor of 2-3
Amplification is even larger in response spectrum at some
periods
EvidenceEvidence
1957 San Francisco Earthquake (M = 5.3)
Rock RockRockDense sand Soft clay
EvidenceEvidence
Compilation of data from multiple events
RockStiff soils Loma
Prieta
MexicoCity
Calculations
Peak acceleration on rock (g)
Pea
k ac
cele
ratio
n (g
)
Peak acceleration
For low loading levels, strong amplification can occur
For high loading levels, deamplification can occur
When?
Why?
EvidenceEvidence
Compilation of data from multiple events
Soft to medium clay and sand
Deep cohesionlesssoils
Stiff soils
Rock
Sa/a
ma
xShort-period
Long-period
SummarySummary
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