Retrofit Sub

8/11/2019 Retrofit Sub

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Retrofit Measures

or

Derrell Manceaux, P.E.



Pur ose

To describe typical retrofit measures for:

Piers and columns Cap beams



Retrofit Methods

Column replacement

Su lemental columns Shear & flexure strengthening

mprovemen o uc y Steel jacketing

Active confinement w/ prestressing wire

Com osite fiber/e ox acket

Reinforced concrete jacket



Column re lacement

Replace when: Damaged column

Flexure is greatly insufficient

Space limits other measures



Column re lacement

Column anchorage-(headed bars)



Column re lacement Supplemental column

loads.



Flexural stren thenin

Not desirable in many cases

Shear capacity increases which attracts force to

the pier cap and foundation

Desirable case

new location

Concrete overlay (additional reinforcement) Adding reinforcement with steel shell



Flexural stren thenin

Concrete overlay

Concrete alone adds strength due to moment arm

Long steel adds additional strength Must provide shear transfer with dowels

Must rovide confinement- hoo s

ties



Column stren thenin Adding reinforcement with steel shell




us ensure s ear rans er

Provide development length into footing/cap




eve opmen o s ee s e o ensure p as c s ra n



Ductilit and shear im rovement

Low ductility occurs from using starter bars at

base of column Concrete splits due to radial stresses from

Proper confinement may stop splice failure

Extend confinement to 75% of maximum

moment or column diameter




Failure path of an unconfined column

Spalling begins at strain =0.005 Ties began carrying more stress at this point and

fail

Confinement is lost and concrete core crushes

Conc core fractures and loses axial, flexural and




Steel acketin

Use 2 half shells and weld

Confinement is provided AFTER expansion of

Place 1-2” gap at end of jacket and supporting

R factors=4-6 may be used t ness ncreases - - na yze w t new

stiffness




Shell for rectangular columns

Rectangular shells do not have membrane action

like circular shells-They act like bending members



Steel Shell Retrofit Exam le

(for splice) Provide data

Perform sim lified d namic anal sis Conduct shell design

er orm es gn c ec Deflection capacity

Shear capacity



Structural confi uration



Re uired data additional Superstructure weight W 1000 ki

Ground acceleration FvS1 .6 g

Concrete strength

4

f' 5 ksi ksi

Effective inertia e.

Nominal shear capacity V N 110.6k

(corresponding shear)

Concrete cover cover 2 in



Sim lified d namic anal sis

3 E I eff k ruc ura s ness c

L3

c .ft

Mass m g m 31.056 k sft

Structural periodT 2

m

k c

T 1.737



Sim lified d namic anal sis

(cont’d) Spec acc a

T S a 0.345 g

EQ shear V EQ S a W V EQ 345.454 ki

EQ moment M EQ V EQ M EQ 10363.61 k f

EQ deflection EQ

V EQ

k c

EQ 0.85 ft



Shell desi n

FBD

fl .3 ksi

Confinement stress

s

fs=stress induced in jacket=29 ksi at strain =0.001

fl=confinement stress=300 psi



Shell desi n

e c ness

Use 10mm=3/8 in fort f l

2 f t 0.25 i in

Ductility factor R eff M EQ

M n R eff 3.12Rdemand

Rdemand

<

Plastic hinge zone p . p .



Desi n check

Perform moment curvature analysis to

determine rotational capacity Conduct pushover analysis to determine

Verify capacity is greater than demand

or e ect on

For shear



Desi n check

Perform moment curvature analysis with shell

and existing reinforcing

Plastic rotation capacity p 0.0591 ra

L x (Øu – Øy)

0.03211



Desi n check

Perform pushover analysis

Target displacement from elastic analysis

1.5 T 1.27 i ft

Plastic displacement

p T p. n .

Check rotation demand vs ca acit

pL

p 0.04 rad 0.03 rad

p 0.04 rad p 0.0591 ra<0.03 rad



Desi n check

Retrofitted column can handle twice the

plastic rotation. Actual required thickness would be about

.

shell



Desi n check

Shear check outside plastic hinge region

Target shear

V p 1.5 V V p 165.9 ki

ear capac y

'2 .8

1000

.

OK



Prestress Wra in Better system for confinement (active system)

,

High losses due to friction



Prestress Wra in

Prestress wrapping with

wedges Machine pulls strand

away from pier and

wedges are placed toestablish tension

Minimum losses

No data on rectan ularsystems



Prestress Wra in

P t W i



Prestress Wra in

Semi-circular reinforcing bars (same effect as

prestress)

D tilit d h i t




Semi-circular reinforcing bars

D tilit d h i t




Composite fiber jacketing High strength glass (E-glass)

Carbon Aramid fibers

Passive system is preferred method

ac ve sys ems creep an rup ure

Loses strength due to moisture absorption R=4 for most columns

I fill h ll



Infill shear wall

I fill h ll



Infill shear wall Increases shear capacity Prevent hin es from formin transverse

Must act composite with existing members (dowels)

V ti l it ti



Vertical ca acit reservation Type “P” retrofit

Rubble is retained in shell to give vertical support

ery cos e ec ve

Li it ti f l f



Limitation of column forces

Isolation bearings

Flexural stren th reduction





Sierra Point Overhead




Limitation of column forces Isolation





Not suited for isolation

Bridges on soft soil (acc increases) Long period structures (little is gained)

Period shift




Limitation of column forces Flexure strength reduction

e uces p as c momen an s ear eman

Must improve ductility capacity

Consider making hinge on frames

Concrete ier wall retrofit



Concrete ier wall retrofit Stiff and strong in transverse direction

nsure oun a on can carry oa

Retrofit usually not required unless starter bars exist





Bent ca confi urations



Bent ca confi urations Pinned connections have minimum stress

forces

Bent ca confi urations



Bent ca confi urations Outrigger piers have high torsional stresses

Pier ca / oint re lacement



Pier ca / oint re lacement

Difficult

Expensive

Bent ca stren thenin



Bent ca stren thenin Add bolster to side

us ac compos e o prov e exure an s ear

Dowels help ensure composite action by shear friction

Add extra reinforcing and prestress

Bent ca stren thenin



Bent ca stren thenin Integral caps more difficult to retrofit

Add bolsters to lower side of cap for positive moment

res ress ng may e mos econom ca

Reduction in ier ca forces



Reduction in ier ca forces Link beam retrofit

Limits shear in column to value just below link

elastic

Stren thenin of column and




beam joints Integral bent cap retrofit





beam joints Knee joint retrofit

Pier and column limit states



e a d co u t states

Summar



To describe typical retrofit measures for:

Piers and columns Cap beams



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