seismic design of bridges ice istructe part1

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Eur Ing Athanasios Bistolas, Member of Technical Chamber of Greece

Principal Engineer

Seismic Design of Bridges-An Overview

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The devastation

Christchurch2010

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The devastation

Sandiago,Chile 2010

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Risk to Structures

Risk=Seismic Hazard x Vulnerability

DSHA or PSHA bySeismologistsMajority depends on

nature. Human Beings areobservers.

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Risk to Structures

Seismicity and Seismic Hazard Evaluation

Deterministic ApproachIts objective is to DETERMINE theMAXIMUM earthquake shaking.

Advantage- Easy to apply

Disadvantages

-Conservative

-Does not treat uncertainty well

-No info on the likelihood of the controlearthquake

-No info on the level of shaking duringthe lifespan of the structure

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Risk to Structures

Seismicity and Seismic Hazard Evaluation

Probabilistic ApproachRectifies a number of problemsinherent in DSHA by quantifyinguncertainty and the probability ofearthquake occurrence. Its objective is

to DETERMINE the MOST probablelevel of earthquake shaking during thelifetime o a structure on the particularsite.

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Risk to Structures

Risk=Seismic Hazard x Vulnerability

DSHA or PSHA bySeismologistsMajority depends on

nature. Human Beings areobservers.

Depends on engineers.Set of rules to address theseismic hazard. The main

purpose of the Antiseismicrules in Codes of Practice

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The Past-1920s-70s

• Seismic Loads 2-10% of dead loads

• Loads applied as static• Elastic Design-Stiffness based on gross section properties

• No account of the dynamic behaviour of structures

• No differentiation of response due to various soil conditions

• No account of inelastic behaviour and ductility

RESULT?

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The Past-1920s-70sThe wrong approach which lead to poor performance 

• Seismic Loads 2-10% of dead loads

• Loads applied as static• Elastic Design-Stiffness based on gross section properties

• No account of the dynamic behaviour of structures

• No differentiation of response due to various soil conditions

• No account of inelastic behaviour and ductility

RESULT?

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Elastic ApproachWhere have we got it wrong? 

Vertical Loads  Transverse Response  Combined Response 

Specified Rebar

ActualStrength

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Displacement damages

Unseating 

San Fernando,1971 Kobe, 1995

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Displacement damages

Unseating-Skewed spans 

Chile, 2010

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Displacement damages

Unseating - Use of Restrainers and shock absorbers 

Cable restrainers and buffer stops

Shock absorbers

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Displacement/Foundation damages

Dynamic Amplification-Soft soils

Costa Rica,1990

Liquefaction

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Displacement/Foundation damages

Lateral Spread 

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Displacement/Foundation damages

Liquefaction and Lateral Spread-Soil Improvements 

Stone Columns Compaction Grouting

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Displacement damages

Pounding 

Northridge, 1994 Loma Prieta,1989

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Displacement/Foundation damages

Abutment Slumping 

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Displacement/Foundation damages

Abutment Slumping 

Costa Rica earthquake,1990

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Column failures

Very common type of failure with

dramatic consequences for the

majority of the cases. Mainly due to:

•Inadequate Flexural Strength•Undependable Flexural Strength

•Premature Termination of Laps

•Shear Brittle Failures

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Column Failures

Undependable flexural strength 

Loma Prieta, 1989 Kobe, 1995