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Retrofit Measuresffor
Substructure ComponentsSubstructure Components
Derrell Manceaux, P.E.
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Purposep
To describe typical retrofit measures for: Piers and columns Cap beams Column to cap beam joints Column to cap beam joints
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Retrofit Methods
Column replacement Supplemental columnspp Shear & flexure strengthening
I t f d tilit Improvement of ductility Steel jacketing Active confinement w/ prestressing wire Composite fiber/epoxy jacketp p y j Reinforced concrete jacket
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Column replacementp
Replace when: Damaged column Flexure is greatly insufficient Space limits other measures
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Column replacementp Construction issues Very expensive Longitudinal steel must be anchored into capg p
Headed reinforcement may be considered Provide pressure grout at top of columnp g p
Top of column Near plastic hinge
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Column replacementp Column anchorage-(headed bars)
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Column replacementp Supplemental column Must ensure existing column continue to carry vertical Must ensure existing column continue to carry vertical
loads.
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Flexural strengtheningg g Not desirable in many casesy Shear capacity increases which attracts force to
the pier cap and foundationp p Desirable case Increase strength at lap splice to drive hinging to Increase strength at lap splice to drive hinging to
new locationTwo common methods: Two common methods: Concrete overlay (additional reinforcement) Adding reinforcement with steel shell
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Flexural strengtheningg g Concrete overlay Concrete alone adds strength due to moment arm Long steel adds additional strength Must provide shear transfer with dowels Must provide confinement-(hoops)p ( p )
ties
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Column strengtheningg g Adding reinforcement with steel shell
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Column strengtheningg g Adding reinforcement with steel shell
M t h t f Must ensure shear transfer Provide development length into footing/cap
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Column strengtheningg g Adding reinforcement with steel shell
D l t f t l h ll t l ti t i Development of steel shell to ensure plastic strain
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Ductility and shear improvementy p
Low ductility occurs from using starter bars at base of column
Concrete splits due to radial stresses from bars trying to pull outbars trying to pull out
Proper confinement may stop splice failure Extend confinement to 75% of maximum
moment or column diameter
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Ductility and shear improvementy p
Failure path of an unconfined column Spalling begins at strain =0.005g g Ties began carrying more stress at this point and
fail Confinement is lost and concrete core crushes Vertical steel buckles Vertical steel buckles Conc core fractures and loses axial, flexural and
shear capacityshear capacity
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Ductility and shear improvementy p Steel jacketingStee jac et g Use 2 half shells and weld Fill void with grout Fill void with grout Confinement is provided AFTER expansion of
conc occursconc occurs Place 1-2” gap at end of jacket and supporting
membermember R factors=4-6 may be used
Stiff i 10 15% A l ith Stiffness increases 10-15%-Analyze with new stiffness
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Ductility and shear improvementy p
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Ductility and shear improvementy p Shell for rectangular columns
Rectangular shells do not have membrane action like circular shells-They act like bending members
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Steel Shell Retrofit Example p(for splice) Provide data Perform simplified dynamic analysisp y y Conduct shell design
P f d i h k Perform design check Deflection capacity Shear capacity
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Structural configurationg
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Required data (additional)q ( ) Superstructure weight W 1000 kip
Ground acceleration FvS1 .6 g
Concrete strength
I ff 6 3 ft4
f' 5 ksi ksi
Effective inertia
Nominal moment capacity
I eff 6.3 ft
M n 3317 k ft K ft Nominal moment capacity
Nominal shear capacity
M n 3317 k ft
V N 110.6k
K ft
p y (corresponding shear)
Concrete cover cover 2 in
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Simplified dynamic analysisp y y
St t l tiff k3 E I eff
k 406 426 k Structural stiffness k c
L3 k c 406.426 k
ft
W 2 Mass m W
g m 31.056 k s2
ft
Structural period T 2 mk c
T 1.737 sc
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Simplified dynamic analysis p y y(cont’d)
S FvS1 Spec acc S a
FvS1T
S a 0.345 g
EQ shear V EQ S a W V EQ 345.454 kip4
EQ moment M EQ V EQ L M EQ 10363.61 k ft
EQ deflection EQV EQk c
EQ 0.85 ft
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Shell designg
FBD
fl .3 ksi
Confinement stress
2 f t f l DD2 f s t f l Dfs=stress induced in jacket=29 ksi at strain =0.001fl=confinement stress=300 psi
D
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Shell designg
Sh ll thi k D Shell thickness Use 10mm=3/8 in for
handling
t f lD
2 f s t 0.25 in in
handling Ductility factor R eff
M EQM n
R eff 3.12
4 OK
RdemandRdemand
<4 OK
l 25 L l 7 5 ft Plastic hinge zone l p .25 L l p 7.5 ft
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Design checkg
Perform moment curvature analysis to determine rotational capacity
Conduct pushover analysis to determine rotational demandrotational demand
Verify capacity is greater than demandF d fl ti For deflection
For shear
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Design checkg
Perform moment curvature analysis with shell and existing reinforcing
Plastic rotation capacity p 0.0591 rad L x (Øu – Øy)
Ultimate strain cu 0.03211 Ultimate strain
Deflection at yield
cu
y 4 in Deflection at yield y 4 in
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Design checkg Perform pushover analysis
Target displacement from elastic analysis
T 1.5 EQ T 1.27 in ft
Plastic displacement
T EQ
1 17 i94 ft
Plastic rotation demand
p T y p 1.17 in
p
.94 ft
Plastic rotation demand
Check rotation demand vs capacity
p pL
p 0.04 rad 0.03 rad
Check rotation demand vs capacity
p 0.04 rad p 0.0591 rad<0.03 rad
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Design checkg
Retrofitted column can handle twice the plastic rotation.
Actual required thickness would be about 0 16 in but constructability issues for thin0.16 in but constructability issues for thin shell
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Design checkg
Shear check outside plastic hinge region Target shear
Sh it
V p 1.5 V N V p 165.9 kip
Shear capacity
V c A g f' c 2 .8 V c 174 02 kipV c A g f c1000 V c 174.02 kip
OK
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Prestress Wrappingpp g Better system for confinement (active system) Little effect on flexure strength stiffness Little effect on flexure strength, stiffness High losses due to friction
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Prestress Wrappingpp g
Prestress wrapping with wedges Machine pulls strand
away from pier and wedges are placed to establish tension
Minimum losses No data on rectangular g
systems
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Prestress Wrappingpp g
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Prestress Wrappingpp g
Semi-circular reinforcing bars (same effect as prestress)
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Ductility and shear improvementy p
Semi-circular reinforcing bars
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Ductility and shear improvementy p Composite fiber jacketing High strength glass (E-glass) Carbon Aramid fibers
Increases ductility/shear and not flexure strength Increases ductility/shear and not flexure strength Passive system is preferred method
( ti t d t ) (active systems creep and rupture) Loses strength due to moisture absorption R=4 for most columns
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Infill shear wall
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Infill shear wall Increases shear capacity Prevent hinges from forming (transverse)g g ( ) Must act composite with existing members (dowels)
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Vertical capacity preservationp y p Type “P” retrofit
Flexure strength not increased due to expansion material Flexure strength not increased due to expansion material Rubble is retained in shell to give vertical support
V t ff ti Very cost effective
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Limitation of column forces
Isolation bearings Flexural strength reductiong
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Limitation of column forces
Sierra Point Overhead
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Limitation of column forces Isolation
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Limitation of column forces
Not suited for isolation Bridges on soft soil (acc increases)g ( ) Long period structures (little is gained) Extreme seismicity (large deflections) Extreme seismicity (large deflections)
Period shift
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Limitation of column forces Flexure strength reduction
R d l ti t d h d d Reduces plastic moment and shear demand Must improve ductility capacity Consider making hinge on frames
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Concrete pier wall retrofitp Stiff and strong in transverse direction
E f d ti l d Ensure foundation can carry load Retrofit usually not required unless starter bars exist
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Concrete pier wall retrofitp
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Bent cap configurationsp g Pinned connections have minimum stress
Integral caps have significant longitudinal Integral caps have significant longitudinal forces
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Bent cap configurationsp g Outrigger piers have high torsional stresses
due to longitudinal forcesdue to longitudinal forces
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Pier cap/joint replacementp j p
Consider if column is being replaced Consider if column is being replaced Difficult Expensive
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Bent cap strengtheningp g g Add bolster to side
M t t it t id fl d h Must act composite to provide flexure and shear Dowels help ensure composite action by shear friction Add extra reinforcing and prestress
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Bent cap strengtheningp g g Integral caps more difficult to retrofit Add additional bars in deck for neg moment Add additional bars in deck for neg moment Add bolsters to lower side of cap for positive moment
P t i b t i l Prestressing may be most economical
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Reduction in pier cap forcesp p Link beam retrofit
Creates new critical section below link Creates new critical section below link Limits shear in column to value just below link Must carefully locate beam to ensure cap remains Must carefully locate beam to ensure cap remains
elastic
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Strengthening of column and g gbeam joints Integral bent cap retrofit
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Strengthening of column and g gbeam joints Knee joint retrofit
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Pier and column limit states
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Pier and column limit states
Øy ØuØy Øu
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Summaryy
To describe typical retrofit measures for: Piers and columns Cap beams Column to cap beam joints Column to cap beam joints
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What questions do you have?