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

Footin s

Seismic Retrofit Fundamentals

Superstructure , , ,

Improved Bridge Seismic Details

Typical Pre 1971 Caltrans & Pre 1990

Non-ductile details have

four main structural

problems: 1- Lack of Confinement.

(#4 @ 12” ties)

2-Inadequate Lap Splice

3- No footing Top Steel

Rebar Mat

4- Inadequate RebarDevelopment into the

New Column Details

Ductility/ Confinement/ Continuity

Effect of Confinement

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

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

shorter, stiffer column

the flare is not desirable

and does not a ree with

the design assumptions

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+

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

an same as terat ve proce ure

η ⎡ ⎤−=

η ⎣ ⎦

1 2( )eff

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

adjacent girders

Bumpers or Stoppers

Shear Keys and Keeper Plates

y p Limit transverse

displacement of

deck relative to bent

Shear Key

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 Steel Jacketing

region

Increased compressivestrength and ultimate strain

Enhanced ductility capacity

lap splice performance

Steel Jacketed Column

Pre 1971 Column Steel Jacketed Column

Modeling of Steel Jacketed Column

Increasedcompressives reng an

ultimate strain in

20-40% increase

for full height

Priestley et al.)

Steel Jacket Retrofit

Concrete Overlay Jacket

Partial Height Jacket

Column Wrap - Composites

•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

Protect (Isolate) Vulnerable Substructure

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

,acceleration response spectrum

Period, Tdisplacement response spectrum

Effect of Dampinglower damping

higher damping

T1 T2a

acceleration response spectrumPeriod, T

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

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

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

Bridge Fragility Curves - Comparisons

Typical Bridge Classes & Retrofits

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

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

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

07 Desroches

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