seismic design & retrofit of bridges part 4: geotechn part...

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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• 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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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

Comments

seismic design & retrofit of bridges part 4: geotechn part...

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