Seismic Design & Retrofit of Bridges- Geotechnical Considerations MULTIDISCIPLINARY CENTER FOR EARTHQUAKE ENGINEERING RESEARCH Seismic Design & Retrofit of Bridges Seismic Design & Retrofit of Bridges Part 4: Geotechnical Considerations Part 4: Geotechnical Considerations Presented by Dr. Ken Fishman,P.E. McMahon & Mann Consulting Engineers, P.C. 2 MCEER Seismic Design and Retrofit of Bridges GEOTECHNICAL CONSIDERATIONS Pittsburgh International Bridge Conference June 2006 McMahon & Mann Consulting Engineers, P.C.
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Seismic Design & Retrofit of Bridges- Geotechnical Considerations
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1MULTIDISCIPLINARY CENTER FOR EARTHQUAKE ENGINEERING RESEARCH
Seismic Design & Retrofit of BridgesSeismic Design & Retrofit of BridgesPart 4: Geotechnical ConsiderationsPart 4: Geotechnical Considerations
Presented by Dr. Ken Fishman,P.E.McMahon & Mann Consulting Engineers,
P.C.
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MCEERSeismic Design and Retrofit of
Bridges
GEOTECHNICAL CONSIDERATIONS
Pittsburgh International Bridge ConferenceJune 2006
McMahon & MannConsulting Engineers, P.C.
Seismic Design & Retrofit of Bridges- Geotechnical Considerations
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INTRODUCTION
• NCHRP 12-49
• FHWA Retrofit Guidelines
• New seismic requirements
• More input from geotechnical engineer
• Detailed geotechnical studies can save $
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Performance Based Design
Design Earthquake
Lower Level Upper Level
Performance Objective
OperationalLife safety
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Minimal to None
MinimalDamage50% PE in 75 yrs
ImmediateImmediateServiceExpectedEarthquake
MinimalSignificantDamageMCE3% PE in 75 yrs
ImmediateSignificantDisruption
ServiceRare Earthquake
OperationalLife SafetyDesign Earthquake
LevelPerformance
Performance Based Design
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Geotechnical Seismic Issues
• Determining Site Class– Site-specific Seismic Analyses
• Liquefaction Susceptibility• Ground Improvement to Improve Site Class• Foundation Elements• Abutments, Retaining Walls• Approaches
Seismic Design & Retrofit of Bridges- Geotechnical Considerations
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Nomenclature
• Spectral Ordinates – What ?????
• Site Class (A to F)
• Seismic Hazard Level (I to IV)
• SDAP (A to E)
• SDR (1-6)
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SPECTRAL ORDINATES
• SS – short period spectral response acceleration
• S1 – long period spectral response acceleration at a period of 1 s
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SINGLE-DEGREE-OF-FREEDOM OSCILLATOR
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Seismic Design & Retrofit of Bridges- Geotechnical Considerations
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Site Class
Different subsurface profiles attenuate or increase earthquake motions differently
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Hsoil
rock
SoilColumn
Base Motion
Surface Motion
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Site Class
• Based on the subsurface profile for the top 100 feet
• Six Site Classes (A to F)
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Site Class is determined based on measurements of:
• Standard Penetration Test (N-values)• Shear Wave Velocity• Undrained Shear Strength
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Measurement of Standard Penetration Test
Courtesy of GZA
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Standard Penetration Test (SPT)
• Consists of advancing a split spoon soil sampler with a 140lb. hammer falling freely 30 inches.
• Values reported on the boring logs are the blows required to advance successive 6-inch increments.
• The first increment is a seating operation and is not considered in the engineering evaluation of the soils.
• The sum of the number of blows for the second and third increments is the "N" value that is an indication of soil relative density.
Courtesy of GZA
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Cross-Hole Up-Hole Down-Hole
Energy Source Geophone
Recorder
GeneratedWave
Measurement of Shear Wave Velocity
Courtesy of GZA
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Seismic Cone Penetration Tests (SCPT)
Courtesy of GZA
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Note & Limits on Values
• N-values uncorrected as measured in field• N-values cannot exceed 100 bpf• Su determined by U or UU triaxial tests• Su cannot exceed 5,000 psf• Weighted average N for soil with PI< 20• Weighted average Su for soil with PI > 20• Use Site Class of softer soil
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Vs METHOD
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N METHOD
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3.3 Nch cohesionless soil layers (PI < 20) in the top 100 feet (30 480 mm) and average, su for cohesive soil layers (PI > 20) in the top 100 feet (30 480 mm) (su method).
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<1,000<15<600Soft soilE
1,000-2,00015-50600-1,200Stiff soilD
>2,000>501,200-2,500Very dense or soft rock
C
Not applicableNot applicable2,500-5,000RockB
Not applicableNot applicable>5,000Hard rockA
Undrained Shear Strength
(psf)
Standard Penetration Resistance
Shear Wave Velocity (fps)Profile
NameSite Class
Site class based on properties of top 100 feet of soil/rock
Site Classes A to E
Seismic Design & Retrofit of Bridges- Geotechnical Considerations