Performance of Horizontally Curved Highway Bridge with Seismic Isolation

Presented by: Ahmed Abdel-Mohti

Authors: Eric V. Monzon, California State University – SacramentoIan G. Buckle, University of Nevada – Reno Ahmad M. Itani, University of Nevada – Reno

1

Outline

FHWA Curved Bridge Studies at UNR

Seismically Isolated Curved Bridge

▪ Curved Bridge Response

▪ Experimental Results

▪ Isolator Instability

▪ Conclusions

2

Bridge Geometry

Prototype Model

Total length 362.5 ft 145 ft

Span lengths 105 ft – 152.5 ft – 105 ft 42 ft – 61 ft – 42 ft

Centerline radius

200 ft 80 ft

Total width 30 ft 12 ft

Girder spacing 11.25 ft 4.5 ft

Column height 19.17 ft 7.67 ft

Column diameter

5 ft 2 ft3

Curved Bridge Model Geometry

R = 80 ft at bridge c.l.Subtended angle = 104o (1.8 rad)

Abutment 1(A1)

Bent 2(B2)

Bent 3(B3)

Abutment 4(A4)

4

L = 145 ft

12 ft

22 ft

Total Model Weight = 167 kipsSuperstructure = 107 kipsSubstructure = 60 kips

Weight of model:

5

Experiments

1. Conventional bridge (1040, steel bearings, 24 inch diam. columns, sacrificial shear keys at abutments)

2. Conventional bridge with live load (6 trucks)

3. Fully isolated bridge with 12 LRB isolators

4. Hybrid isolated bridge with 6 LRB isolators and ductile cross frames

5. Abutment pounding (nonlinear backfill)

6. Rocking columns

6

Construction of Bridge Model

7

Seismic Isolators

Abutment isolator

Lead-rubber isolator

8

Lead Rubber Bearings

Parameters Abutment LRB Pier LRB

Shear modulus, G 55 psi 55 psi

Bonded diameter, B 7.5 in. 9.0 in.

Layer thickness, tr 0.25 in. 0.25 in.

Total rubber thickness, Tr

2.75 in. 2.75 in.

Total Height, H 7.0 in. 7.0 in.

Lead core diameter, dL

1.25 in. 1.5 in.

Stiffness, Kd 0.86 kip/in 1.24 kip/in

Characteristic strength, Qd

1.41 kip 2.03 kip

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Performance of Column in Seismically Isolated Bridge

0

50

100

150

200

250

300

350

400

450

500

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030

Res

ult

ant

Mo

men

t (k

ip-f

t)

Resultant Curvature (rad/in)

Bent 2 Moment-Curvature

SYL

ELC

HACH

Section_Analysis_M-Phi

Idealized_M-Phi

10

3 x Design EQ

Design EQ

Superstructure Displacement

0

25.4

50.8

76.2

101.6

127

152.4

0

1

2

3

4

5

6

0 25 50 75 100 125 150

Dis

pla

cem

ent

(mm

)

Dis

pla

cem

ent

(in

)

% Design EQ

Bridge with Seismic Isolation

Bridge with Conventional Details

Due to damage in the columns, in addition to effects of in-plane torsional rotation of the curved bridge, the conventional bridge experienced higher displacement at EQ levels beyond the Design EQ.

Column Shear Forces

0

10

20

30

40

50

60

0 25 50 75 100 125 150

She

ar (

kip

s)

% Design EQ

Total Resultant Shear: Pier 2

0

10

20

30

40

50

60

0 25 50 75 100 125 150

% Design EQ

Total Resultant Shear: Pier 3

Conventional Bridge

With seismic isolation

Conventional Bridge

With seismic isolation

The seismic isolation reduced the column base shear by about 50% at Design EQ level.

Due to bridge curvature, the supports experienced unequal base shears resulting in more damage at one support and less damage at the other. This, in turn, resulted in unequal stiffness of the columns which further increases the effect of in-plane torsional rotation.

Column Damage at 1.5 Design EQ (MCE)

Pier 2Bridge with

Seismic Isolation

Pier 3Bridge with

Conventional Details

Pier 3Bridge with

Seismic Isolation

Pier 2Bridge with

Conventional Details

Column Damage at 3x Design EQ

Pier 2Bridge with

Seismic Isolation

Pier 3Bridge with

Conventional Details

Pier 3Bridge with

Seismic Isolation

Pier 2Bridge with

Conventional Details

Isolator Deformation during instability

21

3

456

78

9

1011

12

15

At 3x Design EQ, due to excessive displacements, instability occurred at the isolators at one of the abutments.

However, global instability did not occur because the isolators at other supports remained stable. This, in addition to inertial loading in the bridge, the unstable isolators were able to return to original position at the end of the excitation.

No residual displacement was observed.

Isolator Instability: 300% DE SYL

16

Video in real time

Abutment Isolator at 300% Design Earthquake

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Video in Slow-Motion

Concluding Remarks

• Essentially elastic behavior of columns can be achieved in a seismically isolated bridge even at earthquake levels equal to three times the design.

• In a continuous curved bridge with multiple spans, instability of isolators at one or two supports does not lead to bridge collapse.

• Lead-rubber isolators are highly resilient seismic protective systems. Extreme deformation and instability in these isolators has minimal effect on their stiffness.

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