FHWA Seismic Retrofitting Seminar - Indiana

FHWA Seismic RetrofittinggSeminarIndianapolis,p , INOctober 1919-20,20 2010

AgendaSeminarSiOOverviewiLesson 1 - Introduction to Seismic Retrofitting Manual– Philosophy– Methods for Screening– Evaluation MethodsLesson 2 - Seismic Ground Motion Hazards and Geotechnical Hazards– Develop Response Spectrum– DiscussDiGGeotechnicalt h i lHHazardsd iincludingl di LiLiquefactionf tiLesson 3 - Retrofitting Methods for SuperstructuresLesson 4 - Retrofitting Methods for SubstructuresLesson 5 - Retrofitting Methods for Abutments & FootingsQuestions and Answers Session and Final Exam

Instructors Tom Saad, PE, Structural Engineer,FHWA Resource Center JJusticeti Maswoswe,MPE GeotechnicalPE,G t h i lEngineer, FHWA Resource Center DDerrellll MManceaux, PEPE, StructuralSt t lEngineer, FHWA Resource Center

Seismic retrofittingg manuals forhighway bridges1983: Seismic Retrofitting Guidelines for Highwayg (FHWA(Reportp83/007)/)Bridges1995: Seismic Retrofittingg Manual for HighwaygyBridges (FHWA Report 94-052)2006: Seismic Retrofitting Manual for HighwayStructures (FHWA Report ) Part 1: BridgesPart 2: Tunnels, walls, slopes, culverts.

FHWA Seismic Retrofittingg ManualPublication No. FHWA-HRT-06-032 (January 2006)

FHWA Seismic Retrofittingg ManualPublication No. FHWA-HRT-05-067 (August 2004)

FHWA Manual and AASHTO SpecificationsTerminology and Philosophy FHWA Seismic Retrofit Manual– Dual level ground motions (100 and 1000 yr. event)– Seismic Retrofit Category A to D (SHL and SRC) AASHTO LRFD Seismic Design Provision(2008)– 1000 yr.yr design event– Seismic Zones 1-4 AASHTOS O SeismicSe s c Designes g GuideGu de SpecificationSpec cat o– 1000 yr. design event– Seismic Design Category A to D Standard Specifications– 500 yr. event– SPC A-D

State Earthquake Activity Ranking

CCommonEQ FailureF ilMechanismsM h iUnseating (most common) Column Shear Column Confinement Reinforcing Embedment and Laps Inadequate Foundation Capacity

UnseatingLarge displacements encountered during EQ can lead topunseating.gsuperstructure

Unseatingg

Unseatingg

Unseatingg

Column ShearLarge shear forces encountered duringEQQ can lead to column shear failure.

Column Shear

Loss of ConfinementLarge compressive stresses encounteredduring EQ can lead to concrete crushingeventual loss of confinement.confinement

Loss of ConfinementIs this a failure?

Inadequate Reinforcing Embedment& LapsLarge forces encountered during EQ canlead to ppull out of reinforcing.g

I dInadequatet ffoundationd ti capacityit

I dInadequatet ffoundationd ti capacityitCollapse due to liquefaction

Learning Outcomes EExplainl i theh philosophyhilh ffor seismici i retrofittingfi i structures iinaccordance with the FHWA manual Developp a designg responsepspectrumpto determine the demand onthe structure Understand when liquefaction may be a consideration and discussmitigation measures Explain strategies for increasing capacity of existing structurespstrategiesgfor decreasingg demand on existingg structures Explain Establish State-wide policy and procedure for retrofitting structures

FHWA/NHI Bridge Design andAnalysis Courses (www.nhi.fhwa.dot.gov)NHI Course 130081: LRFD for Bridge SuperstructuresNHI CCourse 130082130082: LRFD ffor BBridgeid SubstructuresS b t tandd ERSNHI Course 130092: LRFR for Highway BridgesNHI Course 130093: LRFD Seismic Analysis and Design of BridgesNHI Course 130094: LRFD Seismic Analysis and Design of Tunnels,Walls and other Geotechnical FeaturesNHI Course 130095: LRFD: Design and Analysis of Skewed andHorizontally Curved Steel Bridges

Audience Expectations

Lesson 1 –Introduction to FHWA SeismicRetrofitting Manual

YesIsBridgeExempt?NNoPassScreen / prioritizeFailPassEvaluateF ilFailNext bridgeRetrofitReview

IsBridgeExempt?Exempt bridges include those that are: Near end of service life ( 15 years remainingservice life)Temporary (less than a 15-year life)Closed, but not crossing active roads, rail-lines, orwaterwaystIn the lowest seismic zone

Performance-based retrofitExplicit attempt to satisfy public expectations ofbridge performance for earthquakes ranging fromsmall to large large for example:PerformanceNo interruptionLimited accessClosed forrepairsEarthquakeSmall IntermediateLarge

Seismic Retrofit PhilosophySmall to Moderate Earthquakes:- resisted in the elastic range- no significant structural damageLarge Earthquakes:- avoid collapse- damage rapidly detected & accessiblefor inspection and repair

Upper and lower level earthquakesLower Level earthquake (LL):100-year100year return period(50% probability of exceedance in 75years)Upper Level earthquake (UL):1000-year return period(7% probability of exceedance in 75years)

Performance-based retrofitApplication of performance-baseddesign to bridge retrofitting two earthquake levels (Lower Level, UpperLevel)two bridge types (standard, essential)2 3)three service life categories (ASL 11,-2,-3)two performance levels (life safety,operational)

Relative Effort 109876543210PL2 PerformancePL1LevelHL1HL2PL0HL3HL4Hazard LevelPL0PL1PL2

Seismic retrofit categoriesSeismic Retrofit Categories, SRC, are used torecommend minimum levels of: screeningevaluationretrofittinggIf these minima are satisfied, therequired performance levels will besatisfied.SRCs are similar to Seismic Design Categories((SDC)) used in new designg

BridgeImportanceAnticipatedService Life, ASLSpectralAccelerations,Ss and S1PERFORMANCELEVEL, PLSoil Factors,Fa and FvSEISMIC HAZARDLEVEL, SHLSEISMIC RETROFITCATEGORY, SRC

Bridge importanceA bridge is essential if it satisfies oneor more of the following: Provides access for emergency vehiclesand is required for secondary life safetyWould result in major social and / oreconomic loss if collapsed or was closedRequired for security / defenseCrosses an essential routeAll other bridges are standard

Service life categories (ASL)AgeServiceSeice LifeCategoryAnticipatedAti i t dService Life(if notrehabilitated))ASL 10 – 15 yrs60 - 75 yrsASL 215 – 50 yrs25 - 60 yrsASL 3 50 years 25 yrs

BridgeImportanceAnticipatedService Life, ASLSpectralAccelerations,Ss and S1PERFORMANCELEVEL, PLSoil Factors,Fa and FvSEISMIC HAZARDLEVEL, SHLSEISMIC RETROFITCATEGORY, SRC

Performance levels: PL0 and PL3PL0: No minimum performancespecified.specifiedPL3: Fully Operational: No collapse,damage no interruption to trafficno damage,flow. No repair required.

Performancelevels for bridge retrofittingBRIDGE IMPORTANCEand SERVICE LIFEEARTHQUAKEStandardEssentialASL1 ASL2 ASL3 ASL1 ASL2 ASL3Lower LevelPL0PL3 PL3PL0PL3 PL3

Performance levels: PL1 and PL2PL1: Life-safety: No collapse and life-safetypreserved but damage will be severeparticularly after UL event. Service issignificantly disrupted. Bridge may needreplacementlt afterft UL event.tPL2: Operational: No collapse, life-safetypreserved, damage is minor, almostiimmediatedi t access forf emergency vehicles,hi lrepairs feasible but with restrictions on trafficflow.flow

Performancelevels for bridge retrofittingBRIDGE IMPORTANCEand SERVICE LIFEEARTHQUAKEStandardEssentialASL1 ASL2 ASL3 ASL1 ASL2 ASL3Lower LevelUpper LevelPL0PL3 PL3PL0PL3 PL3PL0PL1PL0PL1PL1PL2

BridgeImportanceAnticipatedService Life, ASLSpectralAccelerations,Ss and S1PERFORMANCELEVEL, PLSoil Factors,Fa and FvSEISMIC HAZARDLEVEL, SHLSEISMIC RETROFITCATEGORY, SRC

USGS hazard maps

S i i hazardSeismichd levels:ll I - IVS ilSoilAcc

BridgeImportanceAnticipatedService Life, ASLSpectralAccelerations,Ss and S1PERFORMANCELEVEL, PLSoil Factors,Fa and FvSEISMIC HAZARDLEVEL, SHLSEISMIC RETROFITCATEGORY, SRC

Seismic retrofit category (SRC)PERFORMANCE LEVELHAZARD LEVELUpper Level EQLower Level EQPL0:No AABCIIABBCIIIABCCIVACDD

Minimum requirementsqSEISMIC RETROFIT nnections,liquefactionB columns,wallswalls,footingsC E

Example:Data:Essential bridge30-year service life remainingBridge CityDense soils (vs 1000 ft/sec)Find:Seismic Retrofit Category, upper levelearthquake.

Example:Service LifeCategoryAnticipatedService LifeASL 10 – 15 yrs60 - 75 yrsASL 215 – 50 yrs25 - 60 yrsASL 3 50 years 25 yrsStep 1: ASL2; site class CAge(if not retrofitted)

BRIDGE IMPORTANCEandd SERVICE LIFEEARTHQUAKESt d dStandardEEssentialti lASL1 ASL2 ASL3 ASL1 ASL2 ASL3Lower LevelUpperpp LevelPL0 PL3 PL3PL0 PL1PL0 PL3 PL3PL1 PL0 PL1 PL2Step 2: Essential bridge; thereforePerformance criteria (UL) PL1

Step 3: S1 0.39g 0 39g and SS 1.11g 1 11gFor site class C:Fv 1 4 and Fa 1.0Fv 1.4Fa 1 0Fv*S1 0.55g and Fa*SS 1.11gandd SHL IV

Step 4: For PL1PL1, SHL IVIV, andSeismic retrofit category is SRC “C”PERFORMANCE LEVELHAZARDLEVELPL0:No ABBIIIABCIVACD

Step 5: Minimum RequirementsSEISMIC RETROFIT nnections,liquefactionB columns,wallswalls,footingsC E

Screening & PrioritizationScreen / prioritizeEvaluateRetrofit

Process forLower Level earthquakeF MaScreening and prioritization QuickQi k screen basedb d on comparisonioff basicb iearthquake load against wind and brakingloads where earthquake load is taken asF FaSS.W SDS.WIf F both Fwind and Fbraking, bridge passesIf F either Fwind or Fbraking, detailedevaluation requiredPrioritization for further evaluation basedon severity of shortfall in strength

Process forLower Level earthquake (cont’d)Detailed evaluation - Step 1 Calculate transverse and longitudinalperiods of bridgeCalculate SaT and and SaLCalculate FT SaTW and FL SaLWIf FT Fwind and FL Fbraking bridge passes,otherwise go to Step 2

Process forLower Level earthquake (cont’d)Detailed evaluation – Step 2 Calculate elastic, unfactored, strengths intransverse and longitudinal directions, FcapTpand FcapLIf FT FcapT and FL FcapLbridgeg passes,p,otherwise retrofit is required for LowerqLevel earthquake

Process forLower Level earthquake (cont’d)Retrofit strategy, approach, measuresStrategy:gy consider ‘do-nothing’g and ‘fullreplacement’ options; identify relevantapproaches (if more than one)Approach: Decide most effectivecombination of techniques (measures) tosatisfy performance requirement (PL3)Measures: Devise retrofit measures usingconventional strength-basedmethodology.

Process for Upper Level EQ

Process forUpper Level earthquakeScreening and prioritizationDetailed evaluationRetrofit strategy and relatedapproaches and measures

Screening and prioritizationPurpose is to screen an existinginventory of bridges for seismicdeficiencies and prioritize the inventoryfor seismic retrofitting based onvulnerability hazard,vulnerability,hazard and non-structuralnon structuralfactorsSi methodsh d are expectedd to bebScreeningquick and conservative; bridges that‘fail’ are passed to a second level ofscreening i.e. ‘detailed evaluation’

Factors consideredStructural vulnerabilitySeismic and geotechnical hazardsOther ImportanceNetwork redundancyAgeg and physicalp ycondition

Screening and prioritization Three methods: Indices Method (FHWA 1995) Indices used for vulnerable components andhazards and combined for single rating. Expected Damage Method (new) Compares severity of damage includingeconomic loss. Seismic Risk Assessment Method (new)() uses network models and fragility functionsrank is based on direct and indirect losses, usesREDARS softwareft

Evaluation of PerformanceScreen / prioritizeEvaluateRetrofit

Methods of evaluationIn general, all evaluation methodsiinvolve:l Demand analysisyCapacity assessmentCalculation of a capacity / demand ratioeither for each critical component in a bridge or for bridge as a complete system

Methods of evaluation (cont’d)Three categoriescategories, six methods:I. No demand analysis1.MethodM th d A (capacity(it checksh k maded forf seatst anddconnections- 10% to 25% vertical reaction)2 Method B (capacity checks made for seats2.connections, columns, and footings- 25%vertical reaction)II. Component C/D evaluation3. Method C ((elastic analysis:yuniform loadmethod, multimode spectral analysis;prescriptive rules given for calculation ofcomponent capacity)