Seismic design andretrofit of bridges in Japan

Ryoichi FUJITAfujita-ryo@ej-hds.co.jp

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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