07 Desroches

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Seismic Design and Retrofit

eta ng un amenta s

Reginald DesRochesProfessor and Associate Chair

Georgia Institute of Technology



Learning Outcomes

List the types of detailing that increase a

List the types of detailing to enhance a

.

For displacement based design explain there a ons p e ween e , expec e

concrete strain and required detailing.

List typical retrofit details along with theirseismic performance benefit.



Presentation Outline

Seismic Detailing Fundamentals Su ort Len th

Detailing for Ductility Columns

Footin s

Seismic Retrofit Fundamentals Superstructure

, , ,

Substructure Column Jacketing, Bent Cap

so a on Application of Fragility Curves for Seismic Retrofit



Vulnerabilities of Typical Bridges upers ruc ure

Brittle Steel Bearings

Columns Insufficient Lap Splices

Inadequate Transverse

Reinforcement

Low shear strength

Footings with Inadequate

e n orcemen Liquefiable Soils Common






Footin s


, , ,





Unseating at Expansion Joints

Northridge Earthquake 1994



Superstructure Expansion Joint

Pre-1971 Typical Post-1971 Typical

Expansion Joint6-inch hinge seat width

Expansion Joint24 inch hinge seat width



Minimum Seat Width – SDC A2(1 )

"k S +

.4000ot h

Δot = movement due to pre-stress shortening,

cree shrinka e and thermal ex ansion.

Hh = Largest column height w/in flexible

– . Sk = angle of skew of support (degrees)



Minimum Seat Width – SDC B, C, D2(1 )

"k S +

.

4000ot eq

ot = movemen ue o pres ress s or en ng,

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






Footin s

Seismic Retrofit Fundamentals

Superstructure , , ,





Improved Bridge Seismic Details

C

O

LU

N



Typical Pre 1971 Caltrans & Pre 1990

Non-ductile details have

four main structural

problems: 1- Lack of Confinement.

C

O

(#4 @ 12” ties)

2-Inadequate Lap Splice

U

M

3- No footing Top Steel

Rebar Mat

N

S

4- Inadequate RebarDevelopment into the



New Column Details

C

L

U

N

Ductility/ Confinement/ Continuity



Effect of Confinement

C

L

M

NS



Tied vs. Spiral Columns



Compression Reinforcement



Confinement (SDC B, C and D)

All lon itudinal bars should be enclosed b

hoops or spirals.#3 for # 9 longitudinal bars or smaller

#5 for #10 longitudinal bars or larger

#5 for bundled barsThe spacing o hoops or ties shall not exceed

the least dimension of the compression member



Confinement (SDC C and D)

with corner ties having minimum 135

de ree an les.

No longitudinal bars shall be furtherthan 6 inches clear on each side

along the tie.

Ties shall be located vertically notmore than half a tie spacing above

the footing or other support and not

more an a a e spac ng e owthe lowest horizontal reinforcement in

.



Requirements for Ductile Design Maximum axial load shall not be greater than

0.20f’ce Ag.

Area of longitudinal reinforcement for

compression members shall not exceed 0.04Ag.

n mum ong tu na e n orcement

0.007Ag

for columns in SDC B, C

. g ,

0.0025 for Pier Walls in SDC B, C

.



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

opposite face of footing or cap beam.

Minimum anchorage length into cap beams should

be 24dbl.




lateral reinforcement in the

exceed the smallest of thefollowing:

1/5 least dimension (Columns), 6 times the nominal diameter of

longitudinal reinforcement

nc es or s ng e oop




extend into footing to the

bend above the bottom mat.

The longitudinal steel shall

extend a distance to ensureadequate development for

plastic hinge into bent cap



Problems with Flared Columns

L

A

R

E

shorter, stiffer column

the flare is not desirable

and does not a ree with

O

L

the design assumptions

UM




Ductility

Confinement

Continuity Flare isolation






Improved Footing Details






Footin s

Seismic Retrofit Fundamentals

Superstructure , , ,





-

Superstructure



Seat Extenders Goal: Provide additional support length

Consider out-of-phase motion to determine

sea ex en er eng



Seat Extenders – Box Girder Bridges



Seat Extenders – Simply Supported




–




–



Seat Extenders - Steel Beams



Restrainer Cables and Bars Goal: Limit movement at hinge or support

Relatively inexpensive and easy ons er ou o p ase mo on o e erm ne

restrainer size and length

R t i C bl d B



Restrainer Cables and Bars–

DesRoches and Fenves 2000

R t i D i P d



Restrainers - Design Procedure2 2

1 2 12 1 22eq

D D D D D ρ = + +

−

( 1) ( ) ( )( )

( ) eq r

r j r j meff r jeq j

K K K K D+

= + +

1eff

K K

μ

=

0.951 0.05 μ − −

eff ξ ξ π = + DesRoches and Fenves, 2000



Restrainers– Multi-Step Procedure

=

R t i Si l St P d



Restrainers– Single Step Procedure

20.50

an same as terat ve proce ure

η ⎡ ⎤−=

mod

1 2

r e

K K

η ⎣ ⎦

mod

1 2( )eff

r

K K

D

μ +

0eq D

K D

=

r r

r y r f A=

Restrainer Cables and Bars



Restrainer Cables and Bars–

Restrainer Cables Box Girder



Restrainer Cables – Box Girder

Restrainer Cables Simply



Restrainer Cables – Simply

-





Simply Supported Concrete Girder Bridge Connection Details





Simply Supported Steel Girder Bridge Connection Details





Connection Details

Restrainer Bars – Simply Supported








Restrainers Vertical Tie Downs



Restrainers – Vertical Tie Downs

Bumpers or Stoppers



Bumpers or Stoppers

adjacent girders

Bumpers or Stoppers



Bumpers or Stoppers

Shear Keys and Keeper Plates



y p Limit transverse

displacement of

deck relative to bent

Shear Key

beam

friction approachTransverse Direction

et al. (1999)

Limited Vsk<1/2Vcol

amage ue o ransverse



movement

0.31ｍ










Detailing for Ductility Columns Footin s


, , ,


so a on

Application of Fragility Curves for Seismic Retrofit



e sm c e ro ng –

“ Detailing for Ductility”

Columns



Columns Goal: Improve

,

Ductility capacity .

Approaches include:

ee ac e

Concrete Jacket

Pre-stressed High Strength Cables Composite Jackets

Column Jacketin



Column Jacketin

Column Steel Jacketing



Column Steel Jacketing

region

Increased compressivestrength and ultimate strain

Enhanced ductility capacity

lap splice performance

Steel Jacketed Column



Steel Jacketed Column

C

O

U

N

S

Pre 1971 Column Steel Jacketed Column

Modeling of Steel Jacketed Column



g

Increasedcompressives reng an

ultimate strain in

20-40% increase

for full height

Priestley et al.)

Steel Jacket Retrofit



Steel Jacket Retrofit

Concrete Overlay Jacket



Partial Height Jacket

Column Wrap - Composites



p p

-

•Low Installation Cost

I-57 Illinois

Cable Column Wrap



Bent Ca s



Bent Ca s Goal: Improve

Ductility capacity

Approaches include:

ee ac e

Concrete Jacket

Pre-stressed High Strength Cables

Bent Ca Retrofit - Prestressin



Bent Ca Retrofit Prestressin

Bent Ca Retrofit - Prestressin



Bent Ca Retrofit Prestressin

Bent Cap Retrofit – Concrete



Bent Ca Retrofit – Steel Jacket



Bent Ca Retrofit Steel Jacket

Bent Ca Retrofit – Steel Plates





Seismic Retrofitting

un amenta s - so at on







, , ,


so a on


Isolation



Goal

Protect (Isolate) Vulnerable Substructure

Vulnerable Bearin s



Vulnerable Bearin s



Dama e to steel and elastomeric bearin s

Seismic Response Modification Devices



Isolation Devices

forces transmitted tothe substructure

system

Damping Devices

used to reduce

displacements

Seismic Response Modification Devices



Isolation Devices

forces transmitted to

the substructure

system

Damping Devices

used to reduce

displacements

Isolation - Basics



Sa

T1 T2

,acceleration response spectrum

Period, Tdisplacement response spectrum

Effect of Dampinglower damping



higher damping

T1 T2a

acceleration response spectrumPeriod, T

Sd

Period, Tdis lacement res onse s ectrum

T es of SRMDs



Isolation Devices

Lead-rubber bearings r c on en u um ear ngs

Sliding Bearings

Damping Devices

Hysteretic Dampers (Lead Bar)

Fluid Viscous Damping Visco-elastic Damper

Isolation – T es



Elastomeric Bearings



ea e

Elastomeric BearingElastomeric Bearing

Friction Pendulum Bearings



Friction – Pendulum

Isolation Bearing

Friction Pendulum Bearin



Fluid Viscous Damper & Lock-up



Dampers







, , , Substructure

Column Jacketing, Bent Cap

so a on


Bridge Fragility Curves

E t O i i



Expert Opinion Empirical

Fragility = P[D≥ds|IM=y]

Analytical

Non-linear time

C

D

P

f

≥=C

Very few studies

developing fragilities

for retrofittedretrofitted

bridges

Methodology for Fragility Development



Bridge Vulnerability



Given Seismic Event

What is the probable

a range of potential EQintensities?

Bridge Fragility CurvesMSSS Steel



88%

40%

25%

8%

Bridge Fragility Curves - Comparisons



Typical Bridge Classes & Retrofits

Jacket



Jacket

Restrainera e

ElastomericBearing

Seat Extender

Motivation: Probable Performance



ven e sm c ven

retrofit on the probablebridge performance over

intensities?

Approach: Bridge Fragility Curvesr ge rag y urve



Fragility = P[D≥ds|IM=y]

Assess bridge performance

Evaluate & select retrofit measures

Regional seismic risk analyses -

Retrofitted MSSS Steel Fragility



Applications of Retrofitted

r ge rag y urves



r ge rag y urves

Retrofit Selection based on Median Value

Im rovement

Performance-Based Retrofit in Support of

- Evaluation

-

Regional Seismic Risk Assessment

-



Performance-Based Retrofit



Identif viable retrofit

strategy based on

performanceobjectives

Ob ective:

Limit slight damage

(design level pga=0.7g)





strategy based on

performanceobjectives

Ob ective:

Limit slight damage

(design level pga=0.7g)P[Slight|PGA=0.7]

AB 1.0

.

EB 0.8

SE 1.0SJ 1.0

0.7g





strategy based on

performance objectives Objective:

Limit slight damage


Objective:

Avoid complete damage






strategy based on

performance objectives P[Comp|PGA=0.7]

Objective:

Limit slight damage

.

RC 0.20

EB 0.18

SE 0.12

SJ 0.21


Objective:

Avoid complete damage










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