3Topics 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?
4Hazard Analysis vs. Ground Response Analysis When is a Site Specific Response Analysis Needed?Site Specific Analysis ?????Hazard Analysisvs.Ground ResponseAnalysis
5When is a Site Specific Response Analysis Needed? Seismic Hazard AnalysisProbabilistic seismic hazard analysis (PSHA) orDeterministic seismic hazard analysisA 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.
6When 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).
7When is a Site Specific Response Analysis Needed? Seismic Hazard AnalysisSite 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.
8When is a Site Specific Response Analysis Needed? Ground Response AnalysisUsually done to either:Develop acceleration response spectra (ARS) orFor liquefaction analysis
9When is a Site Specific Response Analysis Needed? AASHTO “General Procedure” usually adequateUse 2002 USGS Seismic Hazard Maps to obtain bedrock PGA, S0.2 and S1 for 500 and 1000 yr return periodsDetermine soil site class designation (A – F)Develop Response Spectra
10When is a Site Specific Response Analysis Needed? General Procedure for determining Response SpectrumUse the program:SeismicDesignUtility_2002.mde
11When 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 ArticleThe 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.
12When is a Site Specific Response Analysis Needed? Near–Fault EffectsAASHTO 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 and its commentary.AASHTOFor 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.
13When is a Site Specific Response Analysis Needed? Near–Fault EffectsAASHTO 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.
14When 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 :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.
15When 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.
16When is a Site Specific Response Analysis Needed? Site Class F soils, as defined in Article :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),
17When 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.
18When is a Site Specific Response Analysis Needed? Ground Response AnalysisPrimary uses:Developing Site Specific Design Acceleration Response Spectra (ARS)Developing ground motion data for use in liquefaction evaluation
19What 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.
20What 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
21What 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.
22What does a Ground Response study consist of? Acceleration Response SpectraDevelopment StepsEarthquake Source Characterization (deaggregation of uniform seismic hazard)Develop input ground motions (time-histories)Develop soil profile and dynamic properties for soil modelRun program and develop response spectrum from output
23Design Response Spectra from Ground Response Analysis Earthquake Source CharacterizationDevelop 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 sourcesDetermine magnitude (M) and distance (R) of each source
24Design Response Spectra from Ground Response Analysis Earthquake Source CharacterizationAll 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
31Design Response Spectra from Ground Response Analysis Earthquake Source CharacterizationMost Significant Contributors to SeismicGround Motion Hazard0 – 0.5s period: Shallow Crustal0.5 – 2s period: Subduction Zone Mega-ThrustIn 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.
32Design Response Spectra from Ground Response Analysis Selection of Time Historiesconsidering 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
33Design Response Spectra from Ground Response Analysis Selection and Scaling of Time HistoriesAASHTO (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, orb) Use of at least 7 time histories and develop the design spectrum as the mean of the computed response spectra.
34Design Response Spectra from Ground Response Analysis Selection and Scaling of Time HistoriesUse at least three (3) spectrum-compatible time histories, representing the seismic source characteristics.Used for single primary source sitesMatch 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.
35Design Response Spectra from Ground Response Analysis Selection and Scaling of Time Histories:Sites with multiple primary sourcesDifficult 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 sourceDesign response spectrum is developed as an envelope with minor reductions in the spectral peaks (mean + one standard deviation).
36Design Response Spectra from Ground Response Analysis Scaling of Time HistoriesFour earthquake records based on PSHA deaggregation, deterministic spectaTwo Shallow Crustal (SC-1, SC-2)Two Subduction Zone (CSZ-1, CSZ-2)EarthquakeStationDirectionMagnitudeDistanceSC-1Northridge, CASanta Monica City Hall360 deg.6.718 kmSC-290 deg.CSZ-1Michoacán, MEXLa Union8.183.9 kmCSZ-2Zihuatenejo132.6 km
37Design Response Spectra from Ground Response Analysis Scaling of Time HistoriesScaling to get the geometric mean matched to period range of predominate hazard contribution
38Design Response Spectra from Ground Response Analysis Scaling of Time HistoriesOnce 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.
39Design Response Spectra from Ground Response Analysis Site CharacterizationSelect bent locationDevelop input parametersdependent on type of analysis, total or effective stress (nonlinear)Shear wave velocity profilestatic and dynamic soil properties
40Design Response Spectra from Ground Response Analysis Total Stress AnalysisSHAKE91 Computer Program (Shake2000, Proshake)One Dimensional Wave Propagation TheoryVertical Propagation of Shear WavesEquivalent Linear AnalysisEffective Stress, Nonlinear AnalysisD-MOD, DESRA Computer ProgramOne Dimensional Wave Propagation TheoryVertical Propagation of Shear WavesModels pore water pressure generationModels nonlinear soil degradation
41Design Response Spectra from Ground Response Analysis
42Design Response Spectra from Ground Response Analysis
43Design Response Spectra from Ground Response Analysis
44Design Response Spectra from Ground Response Analysis
45Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures(AASHTO 6.8 and GDM Section )Preliminary ScreeningLiquefaction 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.
46Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment Procedures(AASHTO 6.8 and GDM Section )Preliminary Screening (cont.)Liquefaction Assessment not required if:All soils in the upper 75 feet are classified as “cohesive”, andHave 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.
47Liquefaction 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 feetTotal stress ground response analysis methods, used to obtain parameters for use in simplified proceduresLimited to low to moderate cyclic strain and moderate peak accelerationsEffective stress, nonlinear ground response analysis methods are used to obtain pore pressure ratio to assess liquefaction potentialMore sophisticated analysis, requires peer review
48Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment ProceduresSimplified Procedures (Seed & Idriss and others)Limited to depths of about 50 feetStress reduction factor (rd), becomeshighly variable and uncertain with depth
49Liquefaction Assessment from Ground Response Analysis Liquefaction Assessment ProceduresSimplified Procedures(Seed & Idriss and others)Cyclic Resistance Ratio (CRR)
50Liquefaction Assessment from Ground Response Analysis Ground Response Analysis for Liquefaction AssessmentEarthquake Source CharacterizationIdentify primary sources contributing to the hazardAttenuate PGA from primary source(s) to site (given M-R pairs)Develop soil profile and dynamic properties for soil modelApply soil amplification factors to obtain surface PGA for use with simplified proceduresORPerform ground response analysistotal stress oreffective stress, nonlinear analysis
52Liquefaction Assessment from Ground Response Analysis Earthquake Source CharacterizationThree Primary Sources for considerationShallow CrustalGridded (random)Subduction ZoneFor the crustal and “gridded” sources, review the individual fault details to select fault characteristics (M, R, fault mechanism, etc.) most relevant to the hazard.
53Liquefaction Assessment from Ground Response Analysis Attenuate PGA from Source to SiteSummary of Magnitude, Distance and PGA (1000-yr return period)SourceMagnitudeDistance, (km)Depth, (km)Crustal6.728.410.5Subduction9.09820Magnitude and Distance pairs represent weighted averages ofthe individual sources
54Liquefaction Assessment from Ground Response Analysis Ground motion attenuation relationships used in 2002 USGS PHSAShallow Crustal:Boore et al. (1997)Abrahamson and Silva (1997)Sadigh et al. (1997)Spudich et al., 1999,Campbell and Bozorgnia (2003).Cascadia Subduction Zone:Youngs et. al. (1997)
55Liquefaction Assessment from Ground Response Analysis Attenuate PGA from Source to Site
56Liquefaction Assessment from Ground Response Analysis Attenuate PGA from Source to Site
57Liquefaction Assessment from Ground Response Analysis Attenuate PGA from Source to SiteSummary of Magnitude, Distance and PGA (1000-yr return period)SourceMagnitudeDistance, (km)Depth, (km)PGA rockCrustal6.728.410.50.38Subduction9.098200.09Magnitude and Distance pairs represent weighted averages ofthe individual sources
58Liquefaction Assessment from Ground Response Analysis Site CharacterizationSelect bent locationDevelop input parametersdependent on type of analysis, total or effective stress (nonlinear)shear wave velocity profilestatic and dynamic soil properties
59Liquefaction Assessment from Ground Response Analysis Total Stress AnalysisSHAKE91 Computer Program (Shake2000, Proshake)Calculate cyclic shear stress ratio (CSR) with depthCalculate cyclic resistance ratio (CRR) with depthFOS against liquefaction equals (CRR/CSR)Effective Stress, Nonlinear AnalysisUsed in areas of high accelerations and high cyclic shear strainsD-MOD, DESRA or other computer ProgramCalculates pore pressure ratio, Ru, with depth in soil profileDetermine where Ru ≥ 0.80 – 0.90 for liquefaction
60Liquefaction Assessment from Ground Response Analysis Selection of Time Histories use at least:3 motions representative of subduction zone events and3 motions appropriate for shallow crustal earthquakesScaled to the bedrock PGA determined from attenuation relationships
61Liquefaction Assessment from Ground Response Analysis Shake Analysis; Peak Acceleration
62Liquefaction Assessment from Ground Response Analysis CSR(Shake) vs. CSR Simplified Procedure
63Liquefaction Assessment from Ground Response Analysis Shake Analysis; FOS Against LiquefactionSubduction Zone
64Liquefaction Assessment from Ground Response Analysis Shake Analysis; FOS Against LiquefactionSubduction Zone
65Liquefaction Assessment from Ground Response Analysis Shake Analysis; FOS Against LiquefactionSubduction Zone
66Liquefaction Assessment from Ground Response Analysis Shake Analysis; FOS Against LiquefactionCrustal EQs
67Liquefaction Assessment from Ground Response Analysis Shake Analysis; FOS Against LiquefactionCrustal EQs
68Liquefaction Assessment from Ground Response Analysis Shake Analysis; FOS Against LiquefactionCrustal EQs
69Liquefaction Assessment from Ground Response Analysis FOS Against LiquefactionFOS < 1.1 » Liquefaction (also indicates the potential for liquefaction-induced ground movement (lateral spread and settlement).FOS between 1.1 and 1.4 » reduced soil shear strengths due to excess pore pressure generation.FOS > 1.4 » excess pore pressure generation is considered negligible and the soil does not experience appreciable reduction in shear strength.
70Liquefaction Assessment from Ground Response Analysis DMOD Analysis; Liquefaction AssessmentCrustal andSubduction Zone EQs
71RecommendationsUse deepest liquefaction depth with UHS Design Response Spectra (from either AASHTO General Procedure or Ground Response Analysis)Design Response Spectrum cannot be lower than 2/3rd of spectrum from the AASHTO General Procedure
72Lateral Spread Assessment Use conventional limit equilibrium analysis to assess slope failure potential• Residual shear strengths are used in liquefiable layers,• Typically don’t use Kh or Kv (de-coupled analysis),• If FOS < 1.0; Flow failureIf FOS ≥ 1.0; Deformation Analysis
73Lateral Spread Assessment Methods to estimate the magnitude of seismically induced lateral slope deformation include:• Empirically-based displacement estimates for lateral spreading(Youd et al. (2002),• Newmark-type analyses using acceleration time histories generated from site-specific soil response modeling.• Simplified charts based on Newmark-type analyses (Makdisi and Seed, 1978)
74Lateral Spread Assessment Methods to estimate the magnitude of seismically induced lateral slope deformation include:• Simplified procedures based on refined Newmark-type analyses (Bray and Travasarou 2007, Saygili and Rathje 2008)• Simplified charts based on nonlinear, effective stress modeling (Dickenson et al, 2002)• Two-dimensional numerical modeling of dynamic slope deformation.
75Lateral Spread Assessment Several of these methods should be used as appropriate, and engineering judgment applied to the results, to determine the most reasonable range of predicted displacements
76How are these results used in design? Liquefaction effects include:reduced axial and lateral capacities and stiffness in deep foundations,ground settlement and possible downdrag effectslateral spread, global instabilities and displacements of slopes and embankments,loads transferred to foundation piles and shafts from lateral displacements
77How are these results used in design? Bridge Approach Fills:Assess performance requirements (no-collapse & serviceability)Global stabilitySettlementAllowabledeformation andfoundation damage
78Abutment Resistance for Seismic Loads Designer has two options:Use passive resistanceDon’t use passive resistanceIf using presumptive values in AASHTO the longitudinal passive soil pressure shall be less than 0.70 of the value obtained using the procedure given in Article 5.2.3
79Abutment Resistance for Seismic Loads Presumptive Pp (AASHTO 5.2.3)
80Abutment Resistance for Seismic Loads Presumptive Pp
81Abutment Resistance for Seismic Loads Passive soil pressure less than 0.70 of the value obtained using presumptive method = no Agency Approval RequiredPassive soil pressure greater than 0.70 of the value obtained using presumptive method = Agency Approval Required
82How are these results used in design? Bridge Foundations(Extreme Limit State I):Loss of strength due to liquefaction generally assumed to be concurrent with the peak loads in the structureUnless nonlinear effective stress analysis is performed.
83How are these results used in design? Bridge Foundations(Extreme Limit State I):For bridge sites where liquefaction occurs bridges should be analyzed and designed in two configurations as follows:• Nonliquefied Configuration: no liquefaction occurs, using the ground response spectrum appropriate for the site soil conditions in a nonliquefied state.• Liquefied Configuration: The structure as designed in the nonliquefied configuration should be reanalyzed assuming that the layer has liquefied and the liquefied soil provides the appropriate residual resistance for lateral and axial deep foundation response analyses consistent with liquefied soil conditions The design spectrum should be the same as that used in a nonliquefied configuration.
84How are these results used in design? Bridge FoundationsSpread Footings:Not recommended over liquefiable soils unless ground improvement providedPiles & Drilled Shafts:Tips located below deepest liquefiable layerFriction resistance in liquefiable layer not included in Extreme Event I state loading caseProvide modified soil parameters for modeling p-y curves in liquefied soil layers (don’t use built-in DFSAP program option for estimating liquefied lateral stiffness parameters)Provide estimates of downdrag loads due to liquefaction settlement
85How are these results used in design? Bridge FoundationsPiles & Drilled Shafts (cont.):Assess effects of lateral spread deformations on deep foundations and the ability of the pile/shaft foundation to resist these loadsATC/MCEER reports: Recommended LRFD Guidelines for the seismic design of bridges (Design Examples & Liquefaction Study Report); MCEER/ATC 49-1/49-2.Determine if mitigation is necessary
86Earthquake Resisting Elements not permitted Full plastic hinging of pile foundations under seismic loads is not permitted (BDDM)
87When is liquefaction mitigation needed? Mitigation is required when the bridge performance requirements cannot otherwise be met.Design deviations can be considered by the Bridge SectionAll mitigation designs are to be reviewed by the Bridge Section
88When is liquefaction mitigation needed? Performance Requirements (New Bridges)1000-year “No-Collapse” CriteriaUnder this level of shaking, the bridge and approach structures, bridge foundation and approach fills must be able to withstand the forces and displacements without collapse of any portion of the structure.If large embankment displacements (lateral spread) or overall slope failure of the end fills are predicted, the impacts on the bridge end bent, abutment walls and interior piers should be evaluated to see if the impacts could potentially result in collapse of any part of the structure.Slopes adjacent to a bridge or tunnel should be evaluated if their failure could result in collapse of a portion or all of the structure.
89When is liquefaction mitigation needed? Performance Requirements (New Bridges)500-year “Serviceability” CriteriaUnder this level of shaking, the bridge and approach fills, are designed to remain in service shortly after the event (after the bridge has been properly inspected) to provide access for emergency vehicles.In order to do so, the bridge is designed to respond semi-elastically under seismic loads with minimal damage. Some structural damage is anticipated but the damage should be repairable and the bridge should be able to carry emergency vehicles immediately following the earthquake. This holds true for the approach fills leading up to the bridge.
90When is liquefaction mitigation needed? Performance Requirements (New Bridges)500-year “Serviceability” Criteria (cont.)Approach fill settlement and lateral displacements should be minimal to provide for immediate emergency vehicle access for at least one travel lane.For mitigation purposes approach fills are defined as shown on Figure 6-12.As a general rule of thumb, an estimated lateral embankment displacement of up to 1 foot is considered acceptable in many cases as long as the “serviceable” performance criteria described above can be met. Vertical settlements on the order of 6” to 12” may be acceptable depending on the roadway geometry and anticipated performance of the bridge end panels.
91When is liquefaction mitigation needed? Performance Requirements (New Bridges)500-year “Serviceability” Criteria (cont.)These displacement criteria are to serve as general guidelines only and engineering judgment is required to determine the final amounts of acceptable displacement that will meet the desired criteria. It should be noted that these estimated displacements are not at all precise values and may easily vary by factors of 2 to 3 depending on the analysis method(s) used. The amounts of allowable vertical and horizontal displacements should be decided on a case-by-case basis, based on discussions and consensus between the bridge designer and the geotechnical designer and perhaps other project personnel.