Seismic Design and Retrofit D t ili F d t lDetailing Fundamentals
Reginald DesRochesf CProfessor and Associate Chair
Georgia Institute of Technology
Learning OutcomesList the types of detailing that increase a bridges displacement capacity.bridges displacement capacity.List the types of detailing to enhance a bridges ductilitybridges ductility. For displacement based design explain the
l ti hi b t th SDC t drelationship between the SDC, expected concrete strain and required detailing. List typical retrofit details along with their seismic performance benefit.
Presentation OutlineVulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
Vulnerabilities of Typical BridgesS t tSuperstructure
Brittle Steel BearingsInadequate Seat WidthsInadequate Seat Widths
ColumnsInsufficient Lap SplicesInadequate Transverse Reinforcement
Limited ductilityLimited ductilityLow shear strength
Footings with Inadequate R i f tReinforcementLiquefiable Soils Common
Presentation Outline Vulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
Unseating at Expansion Joints
Northridge Earthquake 1994
Superstructure Expansion Joint
Pre-1971 Typical Post-1971 Typical Expansion Joint
6-inch hinge seat widthExpansion Joint
24 inch hinge seat width
Minimum Seat Width – SDC A2(1 )(4 0 20 ) 12"kSN H +
= + Δ + >(4 0.20 ) 124000ot hN H= + Δ + >
Δot = movement due to pre-stress shortening, creep, shrinkage, and thermal expansion.p, g , pHh = Largest column height w/in flexible frame (clear height – ft)frame (clear height ft).Sk = angle of skew of support (degrees)
Minimum Seat Width – SDC B, C, D, ,2(1 )(4 1 65 ) ( ) 12"kSN in+
= + Δ + Δ >(4 1.65 ) ( ) 124000ot eqN in= + Δ + Δ >
Δ t d t t h t iΔot = movement due to prestress shortening, creep, shrinkage, and thermal expansion.Δeq = seismic displacement demand of long period frame on one side of exp joint.Sk = angle of skew of support (degrees)
Minimum Seat Width – N
Minimum Seat Width – N
Presentation Outline Vulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
Improved Bridge Seismic Details
COLUMMNSS
Column VulnerabilitiesColumn Vulnerabilities
Typical Pre 1971 Caltrans & Pre 1990 CSUS Details
Non-ductile details have four main structural
CSUS Details
four main structural problems:
1- Lack of Confinement.
COL (#4 @ 12” ties)
2-Inadequate Lap Splice at the Base of the Column
LUM at the Base of the Column
3- No footing Top Steel Rebar Mat
MNS
4- Inadequate Rebar Development into the SuperstructureSuperstructure
New Column Details
COOLUMMN
Ductility/ Confinement/ Continuity
Effect of Confinement
COOLUUMNS
Tied vs. Spiral Columns
Compression Reinforcement
Confinement (SDC B, C and D)
All longitudinal bars should be enclosed by g yhoops or spirals.
#3 for # 9 longitudinal bars or smaller#5 for #10 longitudinal bars or larger#5 for bundled bars
Th i f h i h ll dThe spacing of hoops or ties shall not exceed the least dimension of the compression member or 12 inchesor 12 inches
Confinement (SDC C and D)Hoops and ties shall be arrangedHoops and ties shall be arranged
with corner ties having minimum 135 degree angles.g gNo longitudinal bars shall be further
than 6 inches clear on each side along the tie.Ties shall be located vertically not
more than half a tie spacing above the footing or other support and not
th h lf ti i b lmore than half a tie spacing below the lowest horizontal reinforcement in the supported memberthe supported member.
Requirements for Ductile DesignMaximum axial load shall not be greater than
0.20f’ceAg.Area of longitudinal reinforcement for
compression members shall not exceed 0.04Ag.Mi i L it di l R i f tMinimum Longitudinal Reinforcement
0.007Ag for columns in SDC B, C0 01A for columns in SDC C D0.01Ag for columns in SDC C, D0.0025 for Pier Walls in SDC B, C0 005 for Pier Walls in SDC D0.005 for Pier Walls in SDC D
Inadequate Lap Splice
Splicing of Longitudinal Reinforcement in Columns (SDC C or D)
Splicing of longitudinal reinforcement shall be outside of plastic hinging zone for SDC C and D.
For SDC D, mechanical couplers must be used for splicing.
Development Length (SDC C and D)Column longitudinal reinforcement shall be extended
into footings and cap beams as close as possible to gopposite face of footing or cap beam.
Minimum anchorage length into cap beams should be 24dbl.
Confinement (SDC C and D)The maximum spacing forThe maximum spacing for
lateral reinforcement in the plastic end region shall notplastic end region shall not exceed the smallest of the following:
1/5 least dimension (Columns), 6 times the nominal diameter of
longitudinal reinforcement6 i h f i l h6 inches for single hoop
Confinement (SDC C and D)The lateral reinforcement shallThe lateral reinforcement shall
extend into footing to the beginning of the longitudinal barbeginning of the longitudinal bar bend above the bottom mat.
The longitudinal steel shall extend a distance to ensure adequate development for plastic hinge into bent cap
FProblems with Flared ColumnsFLA Strong flare createsRED
Strong flare creates shorter, stiffer columnFailure at the base ofD
C
Failure at the base of the flare is not desirable and does not agree with C
OL
gthe design assumptions
UMNN
Improved Bridge Seismic DetailsDuctilityConfinementContinuityFlare isolation
Improved Bridge Seismic DetailsFlared ColumnsFlared Columns
Improved Footing Details
Presentation Outline Vulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
Seismic RetrofittingSeismic Retrofitting Fundamentals -Fundamentals Superstructure
Seat ExtendersGoal: Provide additional support lengthRelatively inexpensive and easyRelatively inexpensive and easyConsider out-of-phase motion to determine
t t d l thseat extender length
Seat Extenders – Box Girder Bridges
Seat Extenders – Simply Supported BridgesBridges
Seat Extenders – Simply Supported Bridges Steel BracketBridges – Steel Bracket
Seat Extenders – Simply Supported Bridges Steel BracketBridges – Steel Bracket
Seat Extenders - Steel Beams
Restrainer Cables and BarsGoal: Limit movement at hinge or support adjacent girders should unseating occuradjacent girders should unseating occurRelatively inexpensive and easyC id t f h ti t d t iConsider out of phase motion to determine restrainer size and length
Restrainer Cables and Bars–Design ProcedureDesign Procedure
DesRoches and Fenves, 2000,
Restrainers - Design Procedure2 2
1 2 12 1 2
( )
2
( )eqD D D D D
D D
ρ= + +
−( )( 1) ( ) ( )
( )
( )( ) eq j r
r j r j meff r jeq j
D DK K K K
D+ = + +
1effK K
μ=
0.951 0.05 μμ
− −
effμ
ξ ξπ
= +DesRoches and Fenves, 2000
Restrainers– Multi-Step Procedure
Nr = KrDr/(fyAr)Nr KrDr/(fyAr)
Restrainers– Single Step Procedure1 2( )STEP d STEP i i d
2
1 2( )
0.500.50ff
STEP and STEP same as iterative procedure
K K η⎡ ⎤−= +⎢ ⎥mod
1 2
0.50r effK K
K KK
η+⎢ ⎥
⎣ ⎦
=mod
1 2( )eff
r
KK K
Dμ
η
=+
=0eqD
K DN
η =
r rr
y r
Nf A
=
Restrainer Cables and Bars–Properties and ModelsProperties and Models
Restrainer Cables – Box Girder BridgesBridges
Restrainer Cables – Simply Supported BridgesSupported Bridges
Cables Looped Over Bent-Cap Straight CablesCables Looped Over Bent Cap Straight Cables
Restrainer Cables – Simply Supported BridgesSupported Bridges
Simply Supported Concrete Girder Bridge Connection Details
Restrainer Cables – Simply Supported BridgesSupported Bridges
Simply Supported Steel Girder Bridge Connection Details
Restrainer Cables – Simply Supported BridgesSupported Bridges
Connection Details
Restrainer Bars – Simply Supported BridgesBridges
Restrainer Bars – Simply Supported BridgesBridges
Restrainers – Vertical Tie Downs
Bumpers or StoppersGoal: Limit movement at hinge or supportGoal: Limit movement at hinge or support adjacent girdersRelatively inexpensive and easyRelatively inexpensive and easy
Bumpers or Stoppers
Shear Keys and Keeper PlatesLimit transverse a s e sedisplacement of deck relative to bent
Shear Key
beamDesigned with shearColoumb Friction Model in Designed with shear friction approach based on Priestley
Coloumb Friction Model in Transverse Direction
based on Priestley et al. (1999)
Limited Vsk<1/2Vcol
Damage due to transverse movemento e e t
0.31m
Shear Keys and Keeper Plates
Presentation Outline Vulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
S i i R t fittiSeismic Retrofitting Fundamentals – SubstructureFundamentals Substructure “Detailing for Ductility”
ColumnsGoal: Improve
Shear strength deformationShear strength, deformationDuctility capacityLap SpliceLap Splice.
Approaches include:St l J k tSteel JacketConcrete JacketPre-stressed High Strength CablesComposite Jackets
Column Jacketingg
Connection DetailsConnection Details
Column Steel Jacketing
Confinement of plastic hingeConfinement of plastic hinge region
Increased compressive t th d lti t t istrength and ultimate strain
Enhanced ductility capacity Improved bond transfer andImproved bond transfer and lap splice performanceIncreased shear strengthIncreased shear strength
Steel Jacketed Column
COLLUMMNS
Pre 1971 Column Steel Jacketed Column
Modeling of Steel Jacketed ColumnIncreased compressive t th dstrength and
ultimate strain in confined concreteconfined concrete20-40% increase in column stiffnessin column stiffness for full height jacket (testing byjacket (testing by Priestley et al.)
Steel Jacket Retrofit
Concrete Overlay Jacket
Partial Height Jacket
Column Wrap - Composites
I-80 Salt Lake City
•Low Installation Cost•Lightweight
I 80 Salt Lake City
•Lightweight
I-57 Illinois
Cable Column Wrap
Bent CapspGoal: Improve
Shear strengthShear strengthDuctility capacityFlexural StrengthFlexural Strength
Approaches include:St l J k tSteel JacketConcrete JacketPre-stressed High Strength Cables
Bent Cap Retrofit - Prestressingp g
Connection DetailsConnection Details
Bent Cap Retrofit - Prestressingp g
Connection DetailsConnection Details
Bent Cap Retrofit – Concrete OverlayOverlay
Connection DetailsConnection Details
Bent Cap Retrofit – Steel Jacketp
Connection DetailsConnection Details
Bent Cap Retrofit – Steel Platesp
Connection DetailsConnection Details
Seismic Retrofitting F d t l I l tiFundamentals - Isolation
Presentation Outline Vulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
IsolationGoal
Replace Vulnerable Brittle BearingsReplace Vulnerable Brittle BearingsProtect (Isolate) Vulnerable Substructure Change Vibration ModeChange Vibration Mode
Vulnerable Bearingsg
Vulnerable Bearingsg
Damage to steel and elastomeric bearingsg g
Seismic Response Modification Devices SRMD
Isolation Devices used to reduce
SRMD
used to reduce forces transmitted to the substructure systemDamping Devices used to reduce displacements
Seismic Response Modification Devices SRMD
Isolation Devices used to reduce
SRMD
used to reduce forces transmitted to the substructure systemDamping Devices used to reduce displacements
Isolation - BasicsSa
T1 T2
Period TPeriod, Tacceleration response spectrum
SdSd
Period, Tdisplacement response spectrum
Effect of Damping lower damping
S
higher damping
T1 T2Sa
acceleration response spectrumPeriod, T
Sd
Period, Tdisplacement response spectrump p p
Types of SRMDsypIsolation Devices
Elastomeric BearingsElastomeric BearingsLead-rubber bearingsF i ti P d l B iFriction Pendulum BearingsSliding Bearings
Damping DevicesHysteretic Dampers (Lead Bar)Fluid Viscous DampingVisco-elastic Damper
Isolation – Typesyp
Elastomeric Sliding Friction PendulumElastomeric Sliding Friction Pendulum
Elastomeric Bearings
L d Fill dLead Filled Elastomeric Bearing
Elastomeric Bearing
Friction Pendulum Bearings
Friction – PendulumIsolation Bearing
Friction Pendulum Bearingg
Fluid Viscous Damper & Lock-up DeviceDevice
Dampers
Presentation Outline Vulnerabilities of Typical BridgesVulnerabilities of Typical BridgesSeismic Detailing Fundamentals
Support Lengthpp gDetailing for Ductility
ColumnsFootingsoo gs
Seismic Retrofit FundamentalsSuperstructure
Seat Extenders Restrainers Shear Keys StoppersSeat Extenders, Restrainers, Shear Keys, StoppersSubstructure
Column Jacketing, Bent CapI l tiIsolationApplication of Fragility Curves for Seismic Retrofit
Bridge Fragility CurvesExpert OpinionEmpiricalFragility = P[D≥ds|IM=y]
AnalyticalNon-linear time history approachhistory approach
[ ] 1CDPfp ≥= Cf
Very few studies developing fragilities p g gfor retrofittedretrofittedbridges
Methodology for Fragility Development
Bridge Vulnerability
Given Seismic Event
What is the probable bridge performance overbridge performance over a range of potential EQ
intensities?
Bridge Fragility CurvesMSSS SteelSSS Stee
88%
40%
25%
8%
Nine Bridge Classes Considered
Bridge Fragility Curves - ComparisonsNine Bridge Classes Considered
Typical Bridge Classes & RetrofitsSteel Steel
Jacket
Shear Key
Restrainer Cable
Shear Key
Cable
Elastomeric Bearing
Seat Extender
Motivation: Probable Performance
Gi S i i E tGiven Seismic Event
What is the impact ofWhat is the impact of retrofit on the probable bridge performance over a range of potential EQa range of potential EQ
intensities?
Approach: Bridge Fragility CurvesB id F ilit CBridge Fragility Curve
Fragility = P[D≥ds|IM=y]
Assess bridge performanceEvaluate & select retrofit measuresRegional seismic risk analysesPerformance-based retrofitPerformance based retrofit
Retrofitted MSSS Steel Fragility
Applications of Retrofitted B id F ilit CBridge Fragility Curves
Retrofit Selection based on Median Value ImprovementpPerformance-Based Retrofit in Support of Seismic Retrofit Manual Dual-LevelSeismic Retrofit Manual Dual Level EvaluationCost Benefit AnalysesCost-Benefit AnalysesRegional Seismic Risk Assessment
Performance-Based RetrofitPerformance Based Retrofit
Performance-Based Retrofit
Identify viable retrofit ystrategy based on performance pobjectives
Objective: jLimit slight damage (design level pga=0.7g)
Performance-Based RetrofitIdentify viable retrofit ystrategy based on performance pobjectives
Objective: jLimit slight damage (design level pga=0.7g)P[Slight|PGA=0.7]
AB 1.0 RC 1 0RC 1.0 EB 0.8SE 1.0 SJ 1.0
0.7g
Performance-Based RetrofitIdentify viable retrofit ystrategy based on performance objectivesp j
Objective: Limit slight damage (design level pga=0.7g)
Objective:Avoid complete damage (design level pga=0.7g)
Performance-Based RetrofitIdentify viable retrofit ystrategy based on performance objectives P[Comp|PGA=0.7]
AB 0 30p j
Objective: Limit slight damage
AB 0.30 RC 0.20 EB 0.18 SE 0.12SJ 0.21
(design level pga=0.7g)Objective:
Avoid complete damage (design level pga=0.7g)
Retrofitted Bridge Fragility CurvesRetrofitted Bridge Fragility Curves
Retrofitted Bridge Fragility CurvesRetrofitted Bridge Fragility Curves
Retrofitted Bridge Fragility CurvesRetrofitted Bridge Fragility Curves
Retrofitted Bridge Fragility CurvesRetrofitted Bridge Fragility Curves