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TRANSCRIPT
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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
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Outline
FHWA Curved Bridge Studies at UNR
Seismically Isolated Curved Bridge
▪ Curved Bridge Response
▪ Experimental Results
▪ Isolator Instability
▪ Conclusions
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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
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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)
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L = 145 ft
12 ft
22 ft
Total Model Weight = 167 kipsSuperstructure = 107 kipsSubstructure = 60 kips
Weight of model:
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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
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Construction of Bridge Model
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Seismic Isolators
Abutment isolator
Lead-rubber isolator
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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
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3 x Design EQ
Design EQ
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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.
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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.
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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
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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
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Isolator Deformation during instability
21
3
456
78
9
1011
12
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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.
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Isolator Instability: 300% DE SYL
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Video in real time
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Abutment Isolator at 300% Design Earthquake
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Video in Slow-Motion
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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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