seismic design & retrofit of bridges part 4: geotechn part...
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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.
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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
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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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DESIGN EARTHQUAKES
Expected Earthquake
50% PE in 75 years
MCE
3% PE in 75 years
•Mapped Spectral ordinates•http://eqhazmaps.usgs.gov
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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
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Site Class E – Soft Clay(all of following)
• PI>20• Moisture content > 40%• Undrained Shear strength
< 500 psf
Courtesy of GZA
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Site Class F –Any one of the following
• Liquefiable, quick clay or collapsible soil• >10 feet peat or highly organic clay• >25 feet clay with PI>75• >120 feet soft to medium clay
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Seismic Hazard Level (SHL)
• Used to assess SDAP and SDR • Need:
– Site Class (from Geotechnical )– Response Spectra Accelerations (from code maps or
site-specific analysis)
• Site-Specific Analysis required for Site Class F & E in high seismic areas (>.75g)
• Engineer may use Site-Specific Analysis for other Classes
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RESPONSE SPECTRA RATIOS
SDS = Fa x Ss
SD1 = Fv x S1where,
SDS and SD1 are the short and long period spectral response adjusted for site class;Fa and Fv are site coefficients
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Fa AS A FUNCTION OF SITE CLASS AND SS
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Fv AS A FUNCTION OF SITE CLASS AND S1
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0.60 < SDS0.40 < SD1IV
0.15 < SDS ≤0.350.25< SD1 ≤0.40III
0.15 < SDS ≤0.350.15< SD1 ≤0.25II
SDS ≤0.15SD1 ≤0.15I
Value of SDS
Value of SD1
SeismicHazardLevel
SEISMIC HAZARD LEVELS
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6C/D/E4C/D/EIV
5C/D/E3B/C/D/E
III
3C/D/E2A2II
2A21A1I
SDRSDAPSDRSDAPLevel
OperationalSafetyLifeSeismicHazard
SDAP and SDR REQUIREMENTS
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GENERAL DESIGN SPECTRUM
T0 = 0.2 X SD1/SDS
TS = SD1/SDS
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One Dimensional Site-Specific Response Analysis
ELASTIC FREE FIELD RESPONSE SPECTRA AT TOP OF SOIL PROFILES
0.0
0.1
0.2
0.3
0.4
0.5
0.0 0.5 1.0 1.5 2.0 2.5Period (sec)
Free
Fie
ld A
bsol
ute
Spec
tral
Acc
eler
atio
n, g
Based on Measured Shear Wave Velocities
Courtesy of GZA
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Site Response Analysis
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.5 1 1.5 2 2.5Period (sec)
Free
Fie
ld A
bsol
ute
Spec
tral
Acc
eler
atio
n, g
Code
Design Response Spectrum
Based on Measured Shear Wave Velocities
Courtesy of GZA
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Advantages of Site-Specific Seismic Analysis
• More accurate approach for spectral accelerations• Amplification analysis shows where greatest
amplification occurs• Can treat poor zones to improve Site Class• Cost of Improvement << cost of more stringent
seismic requirements
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Liquefaction Damage—Niigata, Japan 1964
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Liquefaction Assessment
• Evaluation required for SDR 3,4,5 & 6• More detailed assessment required for SDR 4, 5
and 6• Assessment based on peak ground acceleration• Site-specific analysis for amplification effects
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Liquefaction Potential
• Assessment by geotechnical engineer• Water table• N-values corrected for energy transmission &
overburden• Silt content• Magnitude of maximum design earthquake
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Liquefaction Assessment
• Seed-Idriss Simplified Liquefaction Evaluation Procedure
– CSR = tav/s’vo = 0.65(amax/g)(σvo/σ’vo)rd
Site Specific
– CSR – Obtained Shear Stress (t) from One-Dimensional, Level Ground Site-Specific Dynamic Soil Response Analyses considering actual soil conditions
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Liquefaction Assessment
0 0.5 1 1.5 2 2.5Factor of Safety
70
80
90
100
Elev
atio
n (ft
)
BORINGB-1B-2B-3B-4B-5B-6B-7B-8
FACTOR OF SAFETY AGAINST LIQUEFACTIONFOR 2,500-YEAR EARTHQUAKE
LIQUEFIABLEImpacts:
• Settlement of footings
• Loss of support to piles
• Increased pressure on basement walls
Courtesy of GZA
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Ground Improvement
• Reduce liquefaction potential
• Improve site classification
• Typical methods:
– Grouting
– Deep Densification
– Rammed Aggregate Piers
– Soil Mixing
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FOUNDATION ELEMENTS
• Spring Constants for Spread Footings and Deep Foundations
• Capacity When Exposed to Overturning Moments
• Contribution of Pile Cap in Lateral Capacity and Displacement Evaluation
• Implications of Soil Liquefaction
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ABUTMENT DESIGN
EarthquakeResisting
System
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SCREENING, SEISMIC EVALUATION AND RETROFIT OF EXISTING REINFORCED
CONCRETE, INVERTED T-TYPE RETAINING WALLS
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RETROFIT STRATEGY
• Preliminary Screening• Detailed Evaluation• Consider Alternatives• Evaluate RetrofitMeasures
• Selection of Retrofit andDetailed Design
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SCREENING
• Importance Classification• Seismic Hazard - MCE
– Ag effective peak ground acceleration, PGA
– kh effective peak ground acceleration at ground surface; includes site effects
• Existing Condition• Wall Geometry, height (H), foundation
width (B/H)
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EVALUATION
• Ground Motions• Hazards
– Liquefaction• Collapse Mechanism
– External Stability- tilting, global failure– Structural Failure of Reinforced Concrete
• Permanent Deformations
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Collapse #1- Excessive Tilt
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Collapse #2 - Structural Failure
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SOURCES OF PERMANENT DEFORMATION
• Grain-slip Induced Settlement (densification)
• Deep Seated Global Mechanism (slope movement)
• Movement of Retaining Wall – sliding– tilting
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Movement of Retaining Wall-Serviceability
• Seismic Resistance– yield acceleration, threshold or cutoff
acceleration• Allowable displacement
– settlement– translation– tilt
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Conclusions
• Thorough subsurface assessment can save construction $
• Need proper selection of seismic design parameters
• Need good communication between GE and SE
• Site-specific analyses may save $, especially on soft soil sites
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QuestionsOr
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