seismic retrofit of bridges - a short coursemceer.buffalo.edu/research/highwayprj/workshops/... ·...

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MULTIDISCIPLINARY CENTER FOR EARTHQUAKE ENGINEERING RESEARCH

Seismic Retrofit of BridgesSeismic Retrofit of Bridges-- A Short CourseA Short Course

Presented by Ian BuckleGeoffrey Martin, and

Richard Nutt

Tilting of pier 12, Ejian bridge, Chi-chi Earthquake, Taiwan, September 1999.

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Span collapse due to pier translation, Ejian bridge, Chi-chi Earthquake, Taiwan, September 1999

Collapse of Shi-wei bridge due to liquefaction, Chi-chi Earthquake, Taiwan, September 1999

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Failure of shear-critical columns in Tong-tou bridge, Chi-chi Earthquake, Taiwan, September 1999

Shear failure in pier of Wu-shi bridge, Chi-chi Earthquake, Taiwan, September 1999

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Seismic retrofitting manual for highway bridges

1983: Seismic Retrofitting Guidelines for Highway Bridges (FHWA Report 83/007)

1995: Seismic Retrofitting Manual for Highway Bridges (FHWA Report 94-052)

2004: Seismic Retrofitting Manual for Highway Structures (FHWA Report …)

Part 1: BridgesPart 2: Tunnels, walls, slopes, culverts..

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Seismic retrofitting manual for bridges continued

Part 1: BridgesFirst Draft completed Dec 2000Review completed Feb 2001[NCHRP 12-49 completed Nov 2001]Second Draft completed Sept 2002Third Draft completed Nov 2003Fourth Draft completed Feb 2005Final ‘Working Draft’ Dec 2005

Pilot Short Course on Seismic Retrofit of Bridges

Historical perspectiveRetrofit philosophy and processSeismic and geotechnical hazardsScreening and retrofitting of bridges in Categories A and BScreening, evaluating and retrofitting of bridges in Categories C and D

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Course Text

Seismic Retrofitting Manual for Highway Structures, Part 1: Bridges.

prepared for Federal Highway Administration by Multidisciplinary Center for Earthquake Engineering Research

Text Authors (alphabetic order)

Ian Buckle*Ian FriedlandJohn ManderGeoffrey Martin*Richard Nutt*Maurice Power

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Course instructors

Ian BuckleProfessor of Civil Engineering, University of Nevada Reno

Geoff Martin Professor of Civil Engineering, University of Southern California

Rick NuttStructural Engineering Consultant, Sacramento, CA

MULTIDISCIPLINARY CENTER FOR EARTHQUAKE ENGINEERING RESEARCH

Seismic Retrofit of BridgesSeismic Retrofit of Bridges-- An OverviewAn Overview

Presented by Ian BuckleCivil and Environmental Engineering

University of Nevada Reno

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Content

Philosophy and processScreening a bridge inventoryEvaluation of bridge performanceRetrofit strategies for deficient bridges

Philosophy and process

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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, or

waterways• In the lowest seismic zone (with an exception)

Screen / prioritize

IsBridgeExempt

?

Evaluate

RetrofitNext bridge

No

Fail

FailReview

Yes

Pass

Pass

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Performance-based retrofit__________________________________

Explicit attempt to satisfy public expectations of bridge performance for earthquakes ranging from small to large… for example:

Earthquake

LargeIntermediateSmallPerformance

√Closed for repairs

√√Limited access

√√No interruption

12

34 0

1

2

0

2

4

6

8

10Relative Effort

Hazard level

Performance Level

0-2 2-4 4-6

6-8 8-10

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Performance-based retrofit

Application of performance-based design to bridge retrofitting

two earthquake levels (Lower Level, Upper Level)two bridge types (standard, essential)three service life categories (ASL1,-2,-3)two performance levels (life safety, operational)

HL1 HL2 HL3 HL4

PL0PL1

PL20123456789

10Relative Effort

Hazard Level

Performance Level

PL0 PL1 PL2

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Seismic retrofit categories

Seismic Retrofit Categories, SRC, are used to recommend minimum levels of

screeningevaluation, andretrofitting

If these minima are satisfied, the required performance levels will be satisfied.

SRCs are similar to Seismic Performance Categories (SPC) used in new design

Bridge Importance

AnticipatedService Life, ASL

Spectral Accelerations,

Ss and S1

Soil Factors, Fa and Fv

PERFORMANCE LEVEL, PL

SEISMIC HAZARDLEVEL, SHL

SEISMIC RETROFIT CATEGORY, SRC

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Bridge Importance



Ss and S1





Bridge importance

A bridge is essential if it satisfies one or more of the following:

Provides access for emergency vehicles and is required for secondary life safetyWould result in major social and / or economic loss if collapsed or was closedRequired for security / defenseCrosses an essential route

All other bridges are standard

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Bridge Importance



Ss and S1





Service life categories (ASL)

< 25 yrs

25 - 60 yrs

60 - 75 yrs

Age (if not

rehabilitated)

>50 yearsASL 3

15 – 50 yrsASL 2

0 – 15 yrsASL 1

Anticipated Service Life

Service Life Category

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Bridge Importance



Ss and S1





Performance levels for bridge retrofitting

PL3PL3PL0PL3PL3PL0Lower Level

ASL3ASL2ASL1ASL3ASL2ASL1

EssentialStandard

BRIDGE IMPORTANCE and SERVICE LIFE

EARTHQUAKE

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Performance levels: PL0 and PL3

PL0: No minimum performance specified.

PL3: Fully Operational: No collapse, no damage, no interruption to traffic flow. No repair required.


PL2PL1PL0PL1PL1PL0Upper Level


EssentialStandard


EARTHQUAKE

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Performance levels: PL1 and PL2

PL1: Life-safety: No collapse and life-safety preserved but damage will be severe particularly after UL event. Service is significantly disrupted. Bridge may need replacement after UL event.

PL2: Operational: No collapse, life-safety preserved, damage is minor, almost immediate access for emergency vehicles, repairs feasible but with restrictions on traffic flow.

Upper and lower level earthquakes

Lower Level earthquake (LL): 100-year return period (50% probability of exceedance in 75 years)Upper Level earthquake (UL): 1000-year return period (7% probability of exceedance in 75 years)

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PL2PL1PL0PL1PL1PL0Upper Level

PL3PL3PL0PL3PL3PL0Lower Level


EssentialStandard


EARTHQUAKE

Bridge Importance



Ss and S1





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USGS hazard maps__________________________________

Spectral accelerations, Ss and S1______________________________________________________

SDS = Fa. Ss and SD1 = Fv.S1

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Bridge Importance



Ss and S1





Soil factor, Fa__________________________________

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Soil factor, Fv__________________________________

Site classes: A – F__________________________________

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Bridge Importance



Ss and S1





Seismic hazard levels: I - IV____________________________________

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Geotechnical hazards

Soil liquefactionSoil settlementSurface fault ruptureFlooding

Bridge Importance



Ss and S1





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Seismic retrofit category__________________________________

DCAIV

CBAIII

BBAII

BAAI

PL2: Operational

PL1:Life-safety

PL0: No min.

PERFORMANCE LEVELHAZARD LEVEL

Minimum requirements

Minimum retrofit requirements determined by seismic retrofit category (SRC)

A: retrofitting not requiredB: seats + connections +liquefactionC: seats + connections + columns +

footings + liquefactionD: seats + connections + columns +

footings + abutments + liquefaction

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Minimum requirements continued________________________________

C/D1/D2/EB/C/D1/D2A1/A2NREvaluation Methods

C + abutments

B + columns, walls, footings

Seats, connections,liquefaction

NRScreening/Retrofitting

DCBA

SEISMIC RETROFIT CATEGORYACTION

Example:Given: Essential bridge, 30-year service

life, Salt Lake City (84112), dense soils (vs=1000 ft/sec). Find seismic retrofit category, upper level earthquake.

Step 1: ASL2; site class C (T1-1, 1-3)

Step 2: Essential bridge; thereforePerformance criteria (UL) = PL1 (T1-2)

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Example continued

Step 3: (T1-4, 1-5, 1-6)For zip code 84112, SS=1.11 and S1=0.39For site class C, and SS , S1: Fa=1.0 and Fv=1.4Hence Fa.SS=1.11 and Fv.S1=0.55 and SHL = IV (T1-5)For PL1, SHL = IV, and Seismic retrofit category is C (T1-6)

Information required for performance-based retrofitting __________________________________

Seismic Hazard

• Ground motions• Site effects

Bridge Inventory

Performance Objectives

Anticipated Service Life

Seismic retrofit category

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Screen / prioritize

IsBridgeExempt

?

Evaluate

RetrofitNext bridge

No

Fail

FailReview

Yes

Pass

Pass

Retrofit process for two earthquake levels

Two-step process1. Screen, evaluate, retrofit for performance required during Lower Level earthquake2. Screen, evaluate, retrofit, for performance required during Upper Level earthquake

Performance required Lower Level: PL3Upper Level: PL1, PL2

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Exempt?

• Are near end of service life (<15 years remaining life)

• Temporary (expected life less than15 years)

• Closed, but not crossing activeroads, rail lines or waterways

Exempt?

LOWER LEVEL EARTHQUAKE

1. Screen

2. Evaluate Pass

3. Retrofit

Next bridge

No

Yes

UPPER LEVEL EARTHQUAKE

1. Screen

2. Evaluate Pass

3. Retrofit

SRC = A?Yes

No

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Process for Lower Level earthquake

Screening and prioritizationQuick screen based on comparison of basic earthquake load against wind and braking loads where earthquake load is taken as

F = FaSS.W = SDS.WIf F < both Fwind and Fbraking, bridge passesIf F > either Fwind or Fbraking, detailed evaluation requiredPrioritization for further evaluation based on severity of shortfall in strength

Process for Lower Level earthquake continued

Detailed evaluation - Step 1Calculate transverse and longitudinal periods of bridge (ULM, MSM…)Calculate SaT and and SaL

Calculate FT = SaT.W and FL = SaL.WIf FT < Fwind and FL < Fbraking bridge passes, otherwise go to Step 2

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Detailed evaluation – Step 2Calculate elastic, unfactored, strengths in transverse and longitudinal directions, FcapTand FcapL

If FT < FcapT and FL <FcapLbridge passes, otherwise retrofit is required for Lower Level earthquake


Retrofit strategy, approach, measuresStrategy: consider ‘do-nothing’ and ‘full-

replacement’ options; identify relevant approaches (if more than one)

Approach: Decide most effective combination of techniques (measures) to satisfy performance requirement (PL3)

Measures: Devise retrofit measures… using conventional strength-based methodology.

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Exempt?

LOWER LEVEL EARTHQUAKE

1. Screen

2. Evaluate Pass

3. Retrofit

Next bridge

No

Yes

UPPER LEVEL EARTHQUAKE

1. Screen

2. Evaluate Pass

3. Retrofit

SRC = A?Yes

No

Process for Upper Level earthquake

Screening and prioritization

Detailed evaluation

Retrofit strategy and related approaches and measures

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Screening & Prioritization

Screen / prioritize

Evaluate

Retrofit


Purpose is to screen an existing inventory of bridges for seismic deficiencies and prioritize the inventory for seismic retrofitting based on vulnerability, hazard, and non-structural factorsScreening methods are expected to be quick and conservative; bridges that ‘fail’ are passed to a second level of screening i.e. ‘detailed evaluation’

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Factors considered

Structural vulnerabilitySeismic and geotechnical hazardsOther

ImportanceNetwork redundancyAge and physical condition


P=f (R, importance, non-seismic andsocioeconomic factors)

where: P = assigned priorityR = bridge rating (or rank)

based on hazard and vulnerability

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Three methods:Indices Method (FHWA 1995)Expected Damage Method (new, uses bridge fragility functions, rank is based on direct losses due to damage)Seismic Risk Assessment Method (new, uses network models and fragility functions rank is based on direct and indirect losses, uses REDARS software)

Detailed Evaluation

Screen / prioritize

Evaluate

Retrofit

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Detailed evaluation

Methods of Evaluation

Geotechnical Modeling

Structural Modeling

Methods of evaluation

In general, all evaluation methods involve (figure 1-13):

Demand analysisCapacity assessmentCalculation of a capacity / demand ratio either

for each critical component in a bridge or for bridge as a complete system

Exceptions exist

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Methods of evaluation continued

Three categories, six methods:I. No demand analysis

1.Method A1/A2 (capacity checks made for seats and connections)

2. Method B (capacity checks made for seatsconnections, columns, and footings)

II. Component C/D evaluation3. Method C (elastic analysis: uniform load

method, multimode spectral analysis;prescriptive rules given for calculation of component capacity)

Methods of evaluation continued

III. Structure C/D evaluation4. Method D1 (capacity-spectrum method:

elastic analysis for demands, simplified models for calculation of capacity;

5. Method D2 (pushover method: elastic analysis for demands, nonlinear static analysis used for calculation of pier capacity)

6. Method E (nonlinear time history analysis for calculation of both demand and capacity)

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Geotechnical modeling

Geotechnical Modeling and Capacity Assessment

Foundation ModelingShallow footingsPiles and pile groupsAbutments

Ground Displacement DemandsSettlement Liquefaction Induced Lateral Spreads

Structural modeling

Load pathModeling recommendationsCombination of seismic forcesMember strength capacitiesMember deformation capacities

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Load path

Identify clear load path for lateral loads: Deck slab and connectors (studs)Cross frames (diaphragms)Longitudinal beams (girders)Bearings and anchoragesPier (cap beam, columns, walls)Abutments and foundations (back wall, footing, piles)Soils

Structural modeling recommendations

Distribution of massDistribution of stiffness and strengthDampingIn-span Hinges

SubstructuresSuperstructures

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Combination of seismic forces

Loading in 2- or 3-orthogonal directions:SRSS Rule100-40% Rule

Response quantities in biaxial design:SRSS Rule100-40% Rule

Member strength capacities

Flexural and shear strength of reinforced concrete columns and beams

Expected flexural strength Flexural overstrengthFlexural strength of columns with lap-splices in plastic hinge zonesInitial shear strengthFinal shear strength

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Strength capacities continued

Shear strength of reinforced concrete beam-column joints

Maximum beam-column joint strengthCracked beam-column joint strength

Member deformation capacities

Plastic curvature & hinge rotationsDeformation-based limit states

Compression failure of confined and unconfined concreteBuckling longitudinal barsTensile fracture longitudinal barsLow-cycle fatigue longitudinal barsFailure in lap-splice zone

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Retrofit Strategies, Approaches, and Measures

Screen / prioritize

Evaluate

Retrofit

Retrofit strategies, approaches, and measures

Retrofit Measure: a device or technique such as a restrainer, column jacket, stone column..Retrofit Approach: One or more measures used together to achieve an improvement in performance such as strengthening using restrainers and jackets…

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Retrofit strategies, approaches and measures continued

Retrofit Strategy (one of the following):

One or more approaches used together to achieve desired level of improvement in performance such as strengthening and site remediation. Partial or full replacementDo-nothing (retrofitting not justified)

Retrofit approaches

Approaches: one or more measures to achieve:

StrengtheningDisplacement capacity enhancementForce limitationResponse modificationSite remediationPartial replacementDamage acceptance or control

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Retrofit matrix

For a given seismic deficiency, matrix identifies possible approaches,and for each approach, matrix recommends possible measures for consideration

Partial replacement

Site remediation

Response modificationStrengthening

Retrofit matrix: approaches/measures_________________________________

Measures (techniques)

Approaches

Def

icie

ncie

s

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10.3.111.4.210.3.410.3.210.4.2

Unstable footings

Partial replacement

Site remediation

Response modificationStrengthening

Retrofit approachDeficiency

Retrofit matrix: approaches/measures_________________________________

Measures

Retrofit measures

Superstructure measures:RestrainersSeat width extensions, catcher blocksContinuous simple spansBearing side-bar restraints, shear keys, stoppersIsolation bearings and energy dissipators, including ductile-end-diaphragms

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Retrofit measures continued

Substructure measures Column jacketing, using steel, fiber composites, or concrete shellsInfill wallsColumn replacements

Retrofit measures for foundations and hazardous sites

Retrofit Measures for Abutments, Footings and FoundationsHazardous sites including

near active faults unstable slopes liquefiable sites.

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Summary

Performance-based philosophy (methodology):two earthquake levels (Lower Level, Upper Level)two bridge types (standard, essential)three service life categories (ASL1,-2,-3)two performance levels (life safety, operational)

Three-stage process for each earthquake level:screening, evaluation, and retrofit

Summary continued

Seismic Retrofit Categories, SRC, are used to recommend minimum levels of

screeningevaluation, andretrofitting

SRCs are equivalent to Seismic Performance Categories (SPC) used in new designSRCs are based on hazard level and desired performance level

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Summary continued

Three screening methodsSix evaluation methodsRetrofit phase divided into three steps

Decide strategySelect approach (7)Design and install component retrofit measures (25)

Summary continued

Step 1. For Lower Level earthquake: screen, evaluate, retrofit (controlled by service loads such as wind and braking…)

Step 2. For Upper Level earthquake: Calculate seismic retrofit categoryScreen and prioritize

For bridges that do not pass screen:Conduct detailed analysis for demand and evaluate capacityDecide retrofit strategy, select approach, and design & install retrofit measures

seismic retrofit of bridges - a short coursemceer.buffalo.edu/research/highwayprj/workshops/... ·...

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