Geotechnical Aspects of ODOT Seismic Bridge Design

# Geotechnical Aspects of ODOT Seismic Bridge Design

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## Geotechnical Aspects of ODOT Seismic Bridge Design

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1. Geotechnical Aspects of ODOT Seismic Bridge Design Jan Six P.E. ODOT Bridge Section

2. Seismic Design Standards ODOT Geotechnical Manual AASHTO Guide Specifications for LRFD Seismic Bridge Design ODOT Bridge Design & Drafting Manual

3. Topics When is a Site Specific Response Analysis Needed? What does a Ground Response Analysis consist of? How is liquefaction and lateral spread quantified? How are these results used in design? When is liquefaction mitigation needed?

4. When is a Site Specific Response Analysis Needed? Site Specific Analysis ????? Hazard Analysis vs. Ground Response Analysis

5. When is a Site Specific Response Analysis Needed? Seismic Hazard Analysis • Probabilistic seismic hazard analysis (PSHA) or • Deterministic seismic hazard analysis • A deterministic hazard analysis (DSHA) involves evaluating the seismic hazard at a site for an earthquake of a specific magnitude occurring at a specific location, considering the attenuation of the ground motions with distance. The DSHA is usually conducted without regard for the likelihood of occurrence.

6. When is a Site Specific Response Analysis Needed? Probabilistic Seismic Hazard Analysis (PSHA) • Focuses on the spatial and temporal occurrence of earthquakes, and evaluates all of the possible earthquake sources contributing to the seismic hazard at a site with the purpose of developing ground motion data consistent with a specified uniform hazard level. • Quantifies the uncertainties associated with the seismic hazard, including the location of the source, extent and geometry, maximum earthquake magnitudes, rate of seismicity, and estimated ground-motion parameters. • Produces a uniform hazard acceleration response spectrum based on a specified uniform hazard level or probability of exceedance within a specified time period (i.e., 7% probability of exceedance in 75 years).

7. When is a Site Specific Response Analysis Needed? Seismic Hazard Analysis • Site specific hazard analysis are typically not performed on routine ODOT projects. Only if new information on new or existing sources was uncovered and documented. • The 2002 USGS Probabilistic Seismic Hazard Maps are typically used.

8. When is a Site Specific Response Analysis Needed? Ground Response Analysis • Usually done to either: • Develop acceleration response spectra (ARS) or • For liquefaction analysis

9. When is a Site Specific Response Analysis Needed? • AASHTO “General Procedure” usually adequate • Use 2002 USGS Seismic Hazard Maps to obtain bedrock PGA, S0.2 and S1 for 500 and 1000 yr return periods • Determine soil site class designation (A – F) • Develop Response Spectra

10. When is a Site Specific Response Analysis Needed? General Procedure for determining Response Spectrum Use the program: SeismicDesignUtility_2002.mde

11. When is a Site Specific Response Analysis Needed? • A site-specific ground motion response analyses should be performed if any of the following apply (AASHTO): • The site consists of Site Class F soils, as defined in Article 3.4.2.1. • The bridge is considered critical or essential according to Article 4.2.2, for which a higher degree of confidence of meeting the seismic performance objectives of Article 3.2 is desired.

12. When is a Site Specific Response Analysis Needed? Near–Fault Effects AASHTO 3.4: If the site is located within 6 mi of a known active fault capable of producing a magnitude 5 earthquake and near fault effects are not modeled in the development of national ground motion maps, directivity and directionality effects should be considered as described in Article 3.4.3.1 and its commentary. AASHTO 3.4.3.1 For sites located within 6 mi of an active surface or shallow fault, as depicted in the USGS Active Fault Map, near-fault effects on ground motions should be considered to determine if these could significantly influence the bridge response.

13. When is a Site Specific Response Analysis Needed? Near–Fault Effects • AASHTO 3.4 definition: • An active fault is defined as a near surface or shallow fault whose location is known or can reasonably be inferred and which has exhibited evidence of displacement in Holocene (or recent) time (in the past 11,000 yr, approximately). • Use USGS Quaternary Fault database to determine if fault is considered “active” (<15ka) and for description of fault characteristics.

14. When is a Site Specific Response Analysis Needed? Near–Fault Effects • Directivity effects that increase ground motions for periods greater than 0.5 sec if the fault rupture propagates toward the site, and • Directionality effects that increase ground motions for periods greater than 0.5 sec in the direction normal (perpendicular) to the strike of the fault. AASHTO 3.4.3.1: These effects are significant only for periods longer than 0.5 sec and normally would be evaluated only for essential or critical bridges having natural periods of vibration longer than 0.5 sec.

15. When is a Site Specific Response Analysis Needed? Near–Fault Effects • • Currently no ODOT classification of “essential” or “critical” bridges. • All bridges considered subject to near fault effects if criteria is met. • Ground Response Analysis typically not required. Currently researching procedures to use for modifying general response spectrum.

16. When is a Site Specific Response Analysis Needed? • Site Class F soils, as defined in Article 3.4.2.1: • Peat or highly organic clays, greater than 10 ft in thickness, • Very high plasticity clays (H > 25 ft with PI > 75) • Very thick soft/medium stiff clays (H >120 ft),

17. When is a Site Specific Response Analysis Needed? A site-specific ground motion response analyses should be considered if any of the following apply: • Evaluation of Liquefiable Soil Conditions • (vs. Simplified Methods, when FOSliq ≈ 1.0) • Very deep soil deposits or thin (<40 – 50 feet) soil layers over bedrock. • Obtain better information for evaluating lateral deformations, near surface soil shear strain levels or deep foundation performance. • Obtain ground surface PGA values for abutment wall or other design.

18. When is a Site Specific Response Analysis Needed? • Ground Response Analysis • Primary uses: • Developing Site Specific Design Acceleration Response Spectra (ARS) • Developing ground motion data for use in liquefaction evaluation

19. What does a Ground Response study consist of? Evaluates the response of a layered soil deposit subjected to earthquake motions. One-dimensional, equivalent-linear models are commonly utilized in practice.

20. What does a Ground Response study consist of? This model uses an iterative total stress approach to estimate the nonlinear elastic behavior of soils. Modified versions of the numerical model SHAKE (e.g., SHAKE2000, ProSHAKE, SHAKE91) are routinely used to simulate the propagation of seismic waves through the soil column

21. What does a Ground Response study consist of? • Output consists of: • acceleration response spectra at ground surface or at depths of interest, • time histories at selected depths in the soil profile, • plots of ground motion parameters with depth (e.g., PGA, maximum shear stress and shear strain), • induced cyclic shear stresses in individual soil layers, which may be used in liquefaction analysis.

22. What does a Ground Response study consist of? Acceleration Response Spectra Development Steps • Earthquake Source Characterization (deaggregation of uniform seismic hazard) • Develop input ground motions (time-histories) • Develop soil profile and dynamic properties for soil model • Run program and develop response spectrum from output

23. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization • Develop Uniform Hazard Spectrum from 2002 USGS Seismic Hazard maps (“target bedrock spectrum”) • Use the deaggregation information from the 2002 USGS Seismic Hazard database to obtain information on the primary sources that affect the site. • Review USGS deaggregation data to: • Determine and characterize primary seismic sources • Determine magnitude (M) and distance (R) of each source

24. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization • All seismic sources (M-R pairs) that contribute more than about 5% to the hazard in the period range of interest should be considered. • Scale (or spectrally match) earthquake time histories to the “target” spectrum

25. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization 2002 USGS PSHA maps

26. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization USGS Web Site: http://earthquake.usgs.gov/research/hazmaps/ Custom Mapping Analysis Tools

27. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization (Deaggregation)

28. Design Response Spectra from Ground Response Analysis

29. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization

30. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization

31. Design Response Spectra from Ground Response Analysis Earthquake Source Characterization Most Significant Contributors to Seismic Ground Motion Hazard • 0 – 0.5s period: Shallow Crustal • 0.5 – 2s period: Subduction Zone Mega-Thrust In areas where the hazard has a significant contribution from both the Cascadia Subduction Zone (CSZ) and from crustal sources, both earthquake sources need to be included in the analysis and development of a site specific response spectra.

32. Design Response Spectra from Ground Response Analysis Selection of Time Histories • considering tectonic environment and style of faulting (subduction zone, Benioff zone, or shallow crustal faults), • seismic source-to-site-distance, • earthquake magnitude, • duration of strong shaking, • peak acceleration, • site subsurface characteristics, • predominant period, • spectral shape

33. Design Response Spectra from Ground Response Analysis Selection and Scaling of Time Histories • AASHTO (2009) allows two options for the selection of time histories to use in ground response analysis. The two options are: • a) Use a suite of 3 response-spectrum-compatible time histories with the design response spectrum developed enveloping the maximum response, or • b) Use of at least 7 time histories and develop the design spectrum as the mean of the computed response spectra.

34. Design Response Spectra from Ground Response Analysis Selection and Scaling of Time Histories • Use at least three (3) spectrum-compatible time histories, representing the seismic source characteristics. • Used for single primary source sites • Match the selected time-histories to the “target” spectrum using response spectrum matching techniques. • Develop the design response spectrum by enveloping the caps of the resulting response spectra.

35. Design Response Spectra from Ground Response Analysis Selection and Scaling of Time Histories: • Sites with multiple primary sources • Difficult to match time histories from every source to the entire target spectrum (gives unrealistic results) • Use a collection of time histories that include at least three (3) ground motion records representative each primary source (typically subduction zone events and shallow crustal earthquakes) • Scale the records associated with each primary source so that the average of the records closely matches the target spectrum in the period range of significance. • Develop the mean spectrum for each primary source • Design response spectrum is developed as an envelope with minor reductions in the spectral peaks (mean + one standard deviation).

36. Design Response Spectra from Ground Response Analysis Scaling of Time Histories Four earthquake records based on PSHA deaggregation, deterministic specta • Two Shallow Crustal (SC-1, SC-2) • Two Subduction Zone (CSZ-1, CSZ-2)

37. Design Response Spectra from Ground Response Analysis Scaling of Time Histories Scaling to get the geometric mean matched to period range of predominate hazard contribution

38. Design Response Spectra from Ground Response Analysis Scaling of Time Histories Once the time histories have been scaled or spectrally matched, they can be used directly as input into the ground response analysis programs to develop response spectra and other seismic design parameters. Five percent (5%) damping is typically used in all site response analysis.

39. Design Response Spectra from Ground Response Analysis Site Characterization • Select bent location • Develop input parameters • dependent on type of analysis, total or effective stress (nonlinear) • Shear wave velocity profile • static and dynamic soil properties

40. Design Response Spectra from Ground Response Analysis • Total Stress Analysis • SHAKE91 Computer Program (Shake2000, Proshake) • One Dimensional Wave Propagation Theory • Vertical Propagation of Shear Waves • Equivalent Linear Analysis • Effective Stress, Nonlinear Analysis • D-MOD, DESRA Computer Program • One Dimensional Wave Propagation Theory • Vertical Propagation of Shear Waves • Models pore water pressure generation • Models nonlinear soil degradation

41. Design Response Spectra from Ground Response Analysis

42. Design Response Spectra from Ground Response Analysis

43. Design Response Spectra from Ground Response Analysis

44. Design Response Spectra from Ground Response Analysis

45. Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures (AASHTO 6.8 and GDM Section 6.5.2.2) • Preliminary Screening • Liquefaction Assessment not required if: • The bedrock PGA (or Acceleration Coefficient, As) is less than 0.10g, • The ground water table is more than 75 feet below the ground surface, • The soils in the upper 75 feet of the profile have a minimum SPT resistance, corrected for overburden depth and hammer energy (N’60), of 25 blows/ft, or a cone tip resistance qc of 150 tsf.

46. Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures (AASHTO 6.8 and GDM Section 6.5.2.2) • Preliminary Screening (cont.) • Liquefaction Assessment not required if: • All soils in the upper 75 feet are classified as “cohesive”, and • Have a PI ≥ 18. • Note that cohesive soils with PI ≥ 18 may still be very soft or exhibit sensitive behavior and could therefore undergo significant strength loss under earthquake shaking. This criterion should be used with care and good engineering judgment.

47. Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures (AASHTO 6.8) • Simplified (empirical-based) Procedures (Seed & Idriss and others) • Limited to depths of about 50 feet • Total stress ground response analysis methods, used to obtain parameters for use in simplified procedures • Limited to low to moderate cyclic strain and moderate peak accelerations • Effective stress, nonlinear ground response analysis methods are used to obtain pore pressure ratio to assess liquefaction potential • More sophisticated analysis, requires peer review

48. Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures • Simplified Procedures (Seed & Idriss and others) • Limited to depths of about 50 feet • Stress reduction factor (rd), becomes • highly variable and uncertain with depth

49. Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures • Simplified Procedures • (Seed & Idriss and others) Cyclic Resistance Ratio (CRR)

50. Liquefaction Assessment from Ground Response Analysis Ground Response Analysis for Liquefaction Assessment • Earthquake Source Characterization • Identify primary sources contributing to the hazard • Attenuate PGA from primary source(s) to site (given M-R pairs) • Develop soil profile and dynamic properties for soil model • Apply soil amplification factors to obtain surface PGA for use with simplified procedures • OR • Perform ground response analysis • total stress or • effective stress, nonlinear analysis