Seismic Design & Retrofit of Bridges- Geotechnical Considerations

Transcription

1 Seismic Design & Retrofit of Bridges Part 4: Geotechnical Presented by Dr. Ken Fishman,P.E. McMahon & Mann Consulting Engineers, P.C. 1 MULTIDISCIPLINARY CENTER FOR EARTHQUAKE ENGINEERING RESEARCH MCEER Seismic Design and Retrofit of Bridges GEOTECHNICAL CONSIDERATIONS Pittsburgh International Bridge Conference June McMahon & Mann Consulting Engineers, P.C.

2 INTRODUCTION NCHRP FHWA Retrofit Guidelines New seismic requirements More input from geotechnical engineer Detailed geotechnical studies can save $ 3 Performance Based Design Design Earthquake Performance Objective Lower Level Upper Level Operational Life safety 4

3 Performance Based Design Performance Level Design Earthquake Life Safety Operational Rare Earthquake Service Significant Disruption Immediate MCE 3% PE in 75 yrs Damage Significant Minimal Expected Earthquake Service Immediate Immediate 5 50% PE in 75 yrs Damage Minimal Minimal to None Geotechnical Seismic Issues Determining Site Class Site-specific Seismic Analyses Liquefaction Susceptibility Ground Improvement to Improve Site Class Foundation Elements Abutments, Retaining Walls Approaches 6

4 Nomenclature Spectral Ordinates What????? Site Class (A to F) Seismic Hazard Level (I to IV) SDAP (A to E) SDR (1-6) 7 SPECTRAL ORDINATES S S short period spectral response acceleration S 1 long period spectral response acceleration at a period of 1 s 8

5 SINGLE-DEGREE-OF-FREEDOM OSCILLATOR 9 10

6 DESIGN EARTHQUAKES Expected Earthquake 50% PE in 75 years MCE 3% PE in 75 years Mapped Spectral ordinates

7 13 Site Class Different subsurface profiles attenuate or increase earthquake motions differently 14

8 Surface Motion Soil Column H soil rock 15 Base Motion Site Class Based on the subsurface profile for the top 100 feet Six Site Classes (A to F) 16

9 Site Class is determined based on measurements of: Standard Penetration Test (N-values) Shear Wave Velocity Undrained Shear Strength 17 Measurement of Standard Penetration Test 18 Courtesy of GZA

10 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. 19 Courtesy of GZA Measurement of Shear Wave Velocity Recorder Generated Wave Energy Source Geophone Cross-Hole Up-Hole Down-Hole 20 Courtesy of GZA

11 Seismic Cone Penetration Tests (SCPT) 21 Courtesy of GZA 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 22

12 V s METHOD 23 N METHOD 24

13 3.3 N ch cohesionless soil layers (PI < 20) in the top 100 feet ( mm) and average, s u for cohesive soil layers (PI > 20) in the top 100 feet ( mm) ( s u method). 25 Site Classes A to E Site Class Profile Name Shear Wave Velocity (fps) Standard Penetration Resistance Undrained Shear Strength (psf) A Hard rock >5,000 Not applicable Not applicable B Rock 2,500-5,000 Not applicable Not applicable C Very dense or soft rock 1,200-2,500 >50 >2,000 D Stiff soil 600-1, ,000-2,000 E Soft soil <600 <15 <1,000 Site class based on properties of top 100 feet of soil/rock 26

14 Site Class E Soft Clay (all of following) PI>20 Moisture content > 40% Undrained Shear strength < 500 psf 27 Courtesy of GZA 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 28

15 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 29 RESPONSE SPECTRA RATIOS S DS = F a x S s S D1 = F v x S 1 where, S DS and S D1 are the short and long period spectral response adjusted for site class; F a and F v are site coefficients 30

16 F a AS A FUNCTION OF SITE CLASS AND S S 31 F v AS A FUNCTION OF SITE CLASS AND S 1 32

17 SEISMIC HAZARD LEVELS Seismic Hazard Level I Value of S D1 S D Value of S DS S DS 0.15 II 0.15< S D < S DS 0.35 III 0.25< S D < S DS 0.35 IV 0.40 < S D < S DS 33 SDAP and SDR REQUIREMENTS Seismic Hazard Life Safety Operational Level SDAP SDR SDAP SDR I A1 1 A2 2 II A2 2 C/D/E 3 III IV B/C/D/ E C/D/E 3 4 C/D/E C/D/E

18 35 GENERAL DESIGN SPECTRUM T 0 = 0.2 X S D1 /S DS T S = S D1 /S DS 36

19 One Dimensional Site-Specific Response Analysis ELASTIC FREE FIELD RESPONSE SPECTRA AT TOP OF SOIL PROFILES Free Field Absolute Spectral Acceleration, g Based on Measured Shear Wave Velocities 37 Courtesy of GZA Period (sec) 0.7 Site Response Analysis Free Field Absolute Spectral Acceleration, g Design Response Spectrum Based on Measured Shear Wave Velocities Code 38 Courtesy of GZA Period (sec)

20 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 39 Liquefaction Damage Niigata, Japan

21 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 41 Liquefaction Potential Assessment by geotechnical engineer Water table N-values corrected for energy transmission & overburden Silt content Magnitude of maximum design earthquake 42

22 Liquefaction Assessment Seed-Idriss Simplified Liquefaction Evaluation Procedure CSR = t av /s vo = 0.65(a max /g)(σ vo /σ vo )r d Site Specific 43 CSR Obtained Shear Stress (t) from One- Dimensional, Level Ground Site-Specific Dynamic Soil Response Analyses considering actual soil conditions Liquefaction Assessment 100 LIQUEFIABLE FACTOR OF SAFETY AGAINST LIQUEFACTION FOR 2,500-YEAR EARTHQUAKE Impacts: 90 Settlement of footings Elevation (ft) 80 BORING B-1 B-2 B-3 B-4 B-5 B-6 B-7 B Factor of Safety Loss of support to piles Increased pressure on basement walls 44 Courtesy of GZA

23 Ground Improvement Reduce liquefaction potential Improve site classification Typical methods: Grouting Deep Densification Rammed Aggregate Piers Soil Mixing 45 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 46

24 ABUTMENT DESIGN Earthquake Resisting System 47 SCREENING, SEISMIC EVALUATION AND RETROFIT OF EXISTING REINFORCED CONCRETE, INVERTED T-TYPE RETAINING WALLS 48

25 RETROFIT STRATEGY 49 Preliminary Screening Detailed Evaluation Consider Alternatives Evaluate Retrofit Measures Selection of Retrofit and Detailed Design SCREENING Importance Classification Seismic Hazard - MCE A g effective peak ground acceleration, PGA k h effective peak ground acceleration at ground surface; includes site effects Existing Condition Wall Geometry, height (H), foundation width (B/H) 50

26 EVALUATION Ground Motions Hazards Liquefaction Collapse Mechanism External Stability- tilting, global failure Structural Failure of Reinforced Concrete Permanent Deformations 51 Collapse #1- Excessive Tilt 52

27 Collapse #2 - Structural Failure 53 SOURCES OF PERMANENT DEFORMATION Grain-slip Induced Settlement (densification) Deep Seated Global Mechanism (slope movement) Movement of Retaining Wall sliding tilting 54

28 Movement of Retaining Wall- Serviceability 55 Seismic Resistance yield acceleration, threshold or cutoff acceleration Allowable displacement settlement translation tilt 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 56

29 Questions Or Comments 57