ROCKING ROCKING ISOLISOLATION ATION OF BRIDGESOF BRIDGES

DDimitraimitra SSAKELLARAKIAKELLARAKI

TTOKYOOKYO IINSTITUTE OFNSTITUTE OF TTECHNOLOGYECHNOLOGY

GGRADUATERADUATE DDEPARTMENT OF EPARTMENT OF CCIVIL IVIL EENGINEERINGNGINEERING

INTRODUCTIONINTRODUCTION

Direct bridge foundations are Direct bridge foundations are traditionallytraditionally designed by designed by minimizing minimizing upliftuplift from the underlying ground to prevent from the underlying ground to prevent overturning of the bridgeoverturning of the bridge

HOWEVERHOWEVER……

Post earthquake inspection of piers has Post earthquake inspection of piers has revealed cracks indicating revealed cracks indicating rockingrockingduring the earthquakeduring the earthquake

Analytical studies show that rocking of footing Analytical studies show that rocking of footing decreasesdecreases the ductility demand at the pierthe ductility demand at the pier

ADDITIONALLYADDITIONALLY……

EXPERIMENTAL STUDY OF ROCKINGEXPERIMENTAL STUDY OF ROCKING

An experimental steel model consisting of a An experimental steel model consisting of a top plate, top plate, aacolumn column and aand a bottom platebottom plate representing a representing a deckdeck--pierpier--footing systemfooting system is developed. is developed. A rubber blockA rubber block simulates simulates the ground. the ground.

The bridge model is subjected to The bridge model is subjected to shake table tests.shake table tests.

Sliding of the footing is prevented. Sliding of the footing is prevented. Only rockingOnly rocking is is allowed to happen.allowed to happen.

EXPERIMENTAL MODEL (1)EXPERIMENTAL MODEL (1)

Shake tableShake table

Deck Deck

ColumnColumn

FootingFooting

EXPERIMENTAL MODEL (2)EXPERIMENTAL MODEL (2)

RubberRubber

Ball bearingsBall bearings

ColumnColumn

FootingFooting

Steel smooth plate

Steel smooth plate

Direction of shaking

Direction of shaking

Axis of ro

tation

Axis of ro

tation

EXPERIMENTAL INVESTIGATIONEXPERIMENTAL INVESTIGATION

Increase of modelIncrease of model’’s natural period, rotation of the footing, s natural period, rotation of the footing, deck displacement, deck acceleration, flexural deformation deck displacement, deck acceleration, flexural deformation of the columnof the column

effect of deck masseffect of deck masseffect of footing sizeeffect of footing size

effect of column stiffnesseffect of column stiffnesseffect of ground stiffnesseffect of ground stiffness

PARAMETERS CONSIDEREDPARAMETERS CONSIDERED

Seismic excitations selected from: Kobe Earthquake (Japan, 1995)Seismic excitations selected from: Kobe Earthquake (Japan, 1995)Duzse Earthquake (Turkey, 1999), Niigata Chuetsu Earthquake Duzse Earthquake (Turkey, 1999), Niigata Chuetsu Earthquake (Japan 2004), Northridge Earthquake (USA, 1994)(Japan 2004), Northridge Earthquake (USA, 1994)

MAIN POINTS OF INTERESTMAIN POINTS OF INTEREST

MEASURING DIRECTIONSMEASURING DIRECTIONS

-1

0

1

2

3

4

4 8 12 16 20 24 28

mm

time (s)

uplift

compression

-60-40-20

0204060

4 8 12 16 20 24 28

mm

time (s)

deck displacement

-0.4

-0.2

0

0.2

0.4

4 8 12 16 20 24 28

G

time (s)

deck acceleration

-0.1

-0.05

0

0.05

0.1

4 8 12 16 20 24 28

G

time (s)

uplift

compression

-1

0

1

2

3

4

4 8 12 16 20 24 28

mm

time (s)

uplift

compression-0.15-0.1

-0.050

0.050.1

0.15

4 8 12 16 20 24 28

G

time (s)

uplift

compression

EXPERIMENTAL PROCEDURE (1)EXPERIMENTAL PROCEDURE (1)

Free Oscillation TestsFree Oscillation Tests

Horizontal displacementHorizontal displacementat the deckat the deck

-80

-40

0

40

80

0 2 4 6 8 10 12 14Dis

plac

emen

t (m

m)

time (s) 0

5

10

15

0 2 4 6 8 10 12 14Dis

plac

emen

t (m

m)

time (s)

left edgeright edge

Vertical displacement Vertical displacement at the footingat the footing’’s edgess edges

relationship between natural period T and footingrelationship between natural period T and footing’’s uplifts uplift

equivalent viscous damping ratio equivalent viscous damping ratio

INFORMATION OBTAINEDINFORMATION OBTAINED

EXPERIMENTAL PROCEDURE (2)EXPERIMENTAL PROCEDURE (2)

Shake Table Tests : Kobe JMA 000 30%Shake Table Tests : Kobe JMA 000 30%

EXPERIMENTAL PROCEDURE (3)EXPERIMENTAL PROCEDURE (3)Shake Table TestsShake Table Tests

Horizontal displacementHorizontal displacementat the deckat the deck

Vertical displacement Vertical displacement at the footingat the footing’’s edgess edges

Kobe JMA 000 20%Kobe JMA 000 20%

Niigata 019EW 20%Niigata 019EW 20%

-50

-25

0

25

50

4 8 12 16 20Dis

plac

emen

t (m

m)

time (s)-20246

4 8 12 16 20Dis

plac

emen

t (m

m)

time (s)

left edgeright edge

-100

-50

0

50

100

5 10 15 20 25Dis

plac

emen

t (m

m)

time (s) 0

8

16

24

5 10 15 20 25Dis

plac

emen

t (m

m)

time (s)

left edgeright edge

EXPERIMENTAL PROCEDURE (4)EXPERIMENTAL PROCEDURE (4)

When rocking of bridge pier footings When rocking of bridge pier footings takes place:takes place:

Natural period increases leading to an apparent seismic Natural period increases leading to an apparent seismic isolation effect isolation effect

Flexural deformation of the pier decreases significantlyFlexural deformation of the pier decreases significantly

However, deck displacement produced by footingHowever, deck displacement produced by footing’’s s rotation results in relatively increased overall deck rotation results in relatively increased overall deck displacement displacement

Observations based on experimental resultsObservations based on experimental results

EXPERIMENTAL PROCEDURE (5)EXPERIMENTAL PROCEDURE (5)

Large vertical acceleration develops at the footingLarge vertical acceleration develops at the footing’’s s edges during impact with ground indicating danger of edges during impact with ground indicating danger of soil yielding soil yielding

However impact of footing with ground is an extra However impact of footing with ground is an extra energy dissipation mechanism which contributes energy dissipation mechanism which contributes considerably to attenuation of pierconsiderably to attenuation of pier’’s dynamic responses dynamic response

Observations based on experimental resultsObservations based on experimental results

EXPERIMENTAL PROCEDURE (6)EXPERIMENTAL PROCEDURE (6)Regarding the parameters affecting rocking:Regarding the parameters affecting rocking:

effect of deck mass : as deck mass increases larger effect of deck mass : as deck mass increases larger uplift at the footinguplift at the footing’’s edges developss edges develops

effect of footing size : as footing section decreases effect of footing size : as footing section decreases larger uplift at the footinglarger uplift at the footing’’s edges developss edges develops

effect of column height : uplift at footingeffect of column height : uplift at footing’’s edges s edges decreases drastically for short columnsdecreases drastically for short columns

effect of ground stiffness : depending on the ground effect of ground stiffness : depending on the ground motion, stiffer ground generates smaller or larger upliftmotion, stiffer ground generates smaller or larger uplift

ANALYTICAL PROCEDUREANALYTICAL PROCEDURE

A discrete model idealizes the model bridge: A discrete model idealizes the model bridge: Linear beam elements for the columnLinear beam elements for the columnRigid elements for the footingRigid elements for the footingSpring elements for footingSpring elements for footing’’s separation and contact with rubbers separation and contact with rubber

Impact spring Impact spring elementselements

Deck massDeck mass

Footing massFooting mass

Compression

DvIS

kSV

kkSS : stiffness of spring element: stiffness of spring elementDDvvISIS : initial displacement due to : initial displacement due to the dead weight the dead weight

Tension

f

Dv

CORRELATION WITH EXPERIMENTAL DATA(1)CORRELATION WITH EXPERIMENTAL DATA(1)

Model response without upliftModel response without uplift

LINEAR CASELINEAR CASE

Horizontal displacement at the deckHorizontal displacement at the deck

Horizontal acceleration at the deckHorizontal acceleration at the deck

-20

-10

0

10

20

8 12 16 20 24Dis

plac

emen

t (m

m)

time (s)

ExperimentAnalysis

-0.3

-0.15

0

0.15

0.3

8 12 16 20 24

Acc

eler

atio

n (G

)

time (s)

Experiment

-0.3

-0.15

0

0.15

0.3

8 12 16 20 24

Acc

eler

atio

n (G

)

time (s)

Analysis

CORRELATION WITH EXPERIMENTAL DATA(2)CORRELATION WITH EXPERIMENTAL DATA(2)

Horizontal displacementHorizontal displacementat the deckat the deck

Vertical displacementVertical displacementat the left edge of the footingat the left edge of the footing

Horizontal accelerationHorizontal accelerationat the deckat the deck

Vertical displacementVertical displacementat the right edge of the footingat the right edge of the footing

Model response with upliftModel response with uplift

-60

-30

0

30

60

4 8 12 16 20 24Dis

plac

emen

t (m

m)

time (s)

AnalysisExperiment

-0.8

-0.4

0

0.4

0.8

4 8 12 16 20 24

Acc

eler

atio

n (G

)

time (s)

AnalysisExperiment

0

5

10

4 8 12 16 20 24Dis

plac

emen

t (m

m)

time (s)

AnalysisExperiment

0

5

10

4 8 12 16 20 24Dis

plac

emen

t (m

m)

time (s)

AnalysisExperiment

CONCLUSIONS OF ANALYTICAL PROCEDURECONCLUSIONS OF ANALYTICAL PROCEDURE

Analysis gives very good correlation between Analysis gives very good correlation between experimental and computed response if uplift obtains experimental and computed response if uplift obtains relatively small or medium valuesrelatively small or medium values

If uplift with relatively large values occurs If uplift with relatively large values occurs conventional analytical tools cannot simulate accurately conventional analytical tools cannot simulate accurately increase of period due to rocking response, leading to increase of period due to rocking response, leading to underestimation of footingunderestimation of footing’’s uplift and deck displacement. s uplift and deck displacement. Software taking PSoftware taking P--δδ effect into account must be used.effect into account must be used.

THANK YOU FOR YOUR KIND ATTENTIONTHANK YOU FOR YOUR KIND ATTENTION