Seismic design andretrofit of bridges in Japan
Ryoichi [email protected]
Eight-Japan Engineering Consultants Inc.
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--Contents—
1. Brief history of seismic design
2. Characteristics of seismic design
3. Seismic retrofit of existing structures
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1. Brief history of seismic design
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Huge earthquake
Seismic damages in bridges
Improvement of specification
Huge earthquake
Lessons learned from former large earthquakes are reflected to up-to-date seismic design specifications.
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1923 Kanto earthquake (M7.9)1926 Drafted structural details of road
structureSeismic design started.
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1964 Niigata earthquake (M7.5)1971 Guide specifications on seismic
design of bridges1) Natural period dependent lateral
seismic coefficient2) Liquefaction assessment3) Unseating prevention devices
have been included.
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1995 Kobe earthquake (M7.2)1996 Specifications for highway bridges,
Part V seismic design (revised)Strong design earthquakes are introduced.
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2011 Tohoku earthquake (M9.0)2012 Specifications for highway bridges,
Part V seismic design (revised)Strength of plate boundary type earthquakes have increased.
ResponseAcceleration(gal, cm/s2)
Natural Period (second)
700
1400
2. Characteristics of seismic design
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-- Key points of seismic design --
(1)Two different levels of earthquake ground motions
(2)Performance-based design
(3)Capacity design
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Key point 1:Two Different Levels of Earthquake Ground Motions
Level1 earthquake ground motion:Moderate/Frequent Earthquake
Level2 earthquake ground motion:Strong/Rare Earthquake
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Level1 earthquake ground motiondepends on 3-types of ground conditions
Natural Period (second)
ResponseAcceleration(gal, cm/s2)
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Ground types:typeⅠ -> rock, diluvialtypeⅡ -> intermediatetypeⅢ -> soft, alluvial
Level 2 earthquake ground motionType Ⅰ Type Ⅱ
ResponseAcceleration(gal, cm/s2)
Natural Period (second)
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TypeⅠ: plate boundary type earthquake withlarge magnitude
TypeⅡ: inland direct strike type earthquake
Key point2 : Performance-Based DesignThree seismic performance levels:
Performance level1:Keeping sound functions
Performance level2:Limited damages, quickly repairable
Performance level3:No critical damages
Static and dynamic nonlinear analyses are used to verify seismic performance
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Seismic performance level is chosen according to the importance of the structure.
Earthquake
Ground Motions
Class A
Bridges
Class B
Bridges
Level 1Seismic
Performance Level1
Level 2
Type I Seismic
Performance
Level3
Seismic
Performance
Level2Type II
Low← Importance →High
Stro
ng
←Ea
rth
qu
ake
→ W
eak
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Key point 3 : Capacity design・ Predict mechanism of fracture・ Designate sacrificed members(piers, easy to find damages and repair)・Maintain ductility by forming plastic
hinges in the sacrificed members
16Plastic hinge
Plastic hinge
3. Seismic retrofit of existing structures
Approaches to retrofit:(1)Reinforcement(2)Reduction in Seismic Excitation(3)Vibration Energy Absorption (4)Fail safe system
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Shear failure of RC piers(1995 Kobe)
(1)Reinforcement
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Weak points of bridges -> piers, bearings
Damage at cut off point
Re-bar cut off
Broken bearings(2011 Tohoku)
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Reinforcement of piers and bearings
Concrete wrapping Steel brackets and connecting
pins to protect bearings
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Wrapping with various materials
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RC Steel plate FRP
Most rational method is selected according to cost, workability, and easiness of maintenance.
(2) Reduction in Seismic Excitation
Rubber Bearing
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Prolonging natural periodby using base isolation devices
(2) Reduction in Seismic Excitation
Added Rubber Bearing
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Existing steel bearings are replaced with rubber bearings.
25(Reference: Technical Note of PWRI No.4288)
(3)Vibration Energy Absorption (damper)
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Viscous damper
Inelastic damper(Buckling-Restrained Brace, BRB)
(3)Vibration Energy Absorption (damper)
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Water pipe bridge
(3)Vibration Energy Absorption (damper)
(3)Vibration Energy Absorption (damper)
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Reinforcement of utility poles by using dampers
(3)Vibration Energy Absorption (damper)
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Damaged utility poles in 2011 Tohoku earthquake
Key point:Keep the connecting point stable
(3)Vibration Energy Absorption (damper)
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Bracket must be stronger than damper
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(3)Vibration Energy Absorption (damper)
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Unseated girder in 1995 Kobe earthquake
(4)Fail safe system
It’s very difficult to estimate seismic force accurately.
Fail safe systems are required to save bridgesagainst unanticipated situations.
Unseating prevention system1)Seating length2)Unseating prevention structure3)Structures limiting excessive displacement
Steel bracket to expand seating length
Unseating prevention structure(steel cable)
EX) Damaged bridge in 2003 North miyagi earthquake
About 30cm gap
Superstructure was almost unseated…
Steel cable
Steel cables prevent the superstructurefrom unseating !!!
Fail safe system is very important to avoid critical situation against unpredicted strong earthquake.
-- Summary --
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1. Brief history of seismic design:Specifications have improved after huge earthquakes.
2. Characteristics of seismic design:1) Strong earthquake ground motion2) Performance based design3) Capacity design
-- Summary --
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3. Retrofit of existing structures:Four approaches:1) Reinforcement2) Seismic isolation3) Energy absorption using dampers4) Fail safe system
Thank youfor your kind attention.
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