17 earthquake engineering

8/11/2019 17 Earthquake Engineering

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Earthquake Engineering

Civil & Environmental EngineeringUniversity College London


Dr Tiziana Rossetto


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Introduction

In the past 3 centuries over 3 million people have dieddue to earthquakes and earthquake related disasters.

The economic losses estimated for the period 1929-1950

are in excess of US$10 billion.

2/3 of the earths crust is seismically active, which means

that about 1,000,000,000 people are living in areas ofthe world that are prone to earthquakes.


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Introduction

For more developed countries the economic loss due to

an earthquake can be enormous even if the death toll isfairly low (Coburn and Spence 1992):

e.g. Kobe Earthquake (Ms 7.0, Japan, 1995) killed

5,420 people but caused US$ 150 billion economic loss


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cost of

damage perfatality


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Introduction

But cost is relative:

e.g. Managua earthquake (Ms 6.1, Nicaragua, 1972),caused 10,000 deaths and US$ 2 billion economic loss.

..but This loss = 40% of GNP

For more developed countries the economic loss due to

an earthquake can be enormous even if the death toll isfairly low (Coburn and Spence 1992):

e.g. Kobe Earthquake (Ms 7.0, Japan, 1995) killed

5,420 people but caused US$ 150 billion economic loss


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Introduction

What do we know?

Earthquakes cannot be prevented nor accuratelypredicted.

It is not ground shaking itself that causes life andeconomic loss but the collapse or damage of buildingsand infrastructure that are too weak to resist the groundshaking.


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What is Earthquake Engineering?

The application of civil engineering toreduce life and economic losses due toearthquakes,

(i.e to mitigate seismic risk)


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What is Seismic Risk from theengineers perspective?

the probability of losses occurring due to

earthquakes within the lifetime of astructure; these losses can includehuman lives, social and economic

disruption as well as material damage.


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What is Seismic Risk?

RISK = SEISMIC HAZARD x VULNERABILITY


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Probability of a potentially damagingearthquake effect occurring at the site ofplanned construction within its design life.


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What earthquake effects cause damage?

Ground shaking

Bhuj E/q, India 2001Loma Prieta, CA 1989


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Ground shaking



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Ground shaking

Surface rupture

Chi Chi E/q, Taiwan

Hector Mines E/q, USA



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Ground shaking

Surface rupture

Landslides



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Ground shaking

Surface rupture

Landslides

Liquefaction



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Ground shaking

Surface rupture

Landslides

Liquefaction

What earthquake effects causedamage?


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Ground shaking

Surface rupture

Landslides

LiquefactionTsunamis



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is the probability of the occurrence of

damage in a building when exposed to aparticular earthquake effect.


Probability of a potentially damagingearthquake effect occurring at the site ofplanned construction within its design life.


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DETERMINED BY MAN: CAN BE

REDUCED THROUGH SEISMICDESIGN


DETERMINED BY NATURE:CANNOT BE REDUCED


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Seismic Hazard and Seismic Design

Design Problem:An earthquake usually constitutes themost severe loading to which most civil engineeringstructures might possibly be subjected, and yet in mostparts of the world, even those that are highly seismic, there

is a possibility that an earthquake may not occur during thelife of the structure.

Note: Lateral loads imposed by winds = 1-3% of building weight.Lateral loads due to earthquakes = 25-30% of building weight.


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Design Problem:An earthquake usually constitutes themost severe loading to which most civil engineeringstructures might possibly be subjected, and yet in mostparts of the world, even those that are highly seismic, there

is a possibility that an earthquake may not occur during thelife of the structure.

Assessment Problem: Most buildings that exist have not

been designed to seismic codes. Can they withstand thetype of earthquake that might happen in their location?What will be the damage incurred if an earthquake doesoccur?


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Hence, engineers require seismic hazardassessments to provide not only a description of

the likely seismic loads (ground shaking) to beexperienced by an engineering structure, but alsoto attach probabilities of occurrence to these

earthquake loads.


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How do earthquake measurementstranslate into earthquake design loads?

Seismograms can be used directly as input to complicated

dynamic inelastic time history finite element analyses ofimportant buildings.


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How do earthquake measurementstranslate into earthquake design loads?Seismograms can be used directly as input to complicated

dynamic inelastic time history finite element analyses ofimportant buildings.


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How do earthquake measurementstranslate into earthquake design loads?

Seismograms used directly as input to complicateddynamic inelastic time history finite element analyses ofimportant buildings.

Earthquake Acceleration Spectra and Peak GroundAcceleration.

Spectral modal analysis methodsEquivalent lateral force methods


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Computation of Elastic ResponseSpectrum

Single degree

of freedom

systems

Plot maximum

response acceleration

vs fundamental period

of SDOF


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Elastic and Inelastic Spectra


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Design Spectra (EC8)

~2.5.ag

Shape will be different

for different soilconditions. i.e. site

amplification effects areconsidered


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Code-type zonation map


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T1 T2 T3

T1T2


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Equivalent Lateral Load

V = C.W ( ~ F=m.a)

I = Importance factor

Sa(T1) = spectral acceleration

for fundamental period, includessite effects

q = behaviour factor, represents

structures ability to dissipate energy

( )qSI

g

TSaC ...1=

V


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Conventional Seismic DesignNormal building life assumed to = 50years

Ultimate limit state design: Design a building to resist anearthquake with a return period of 475yrs (i.e. the designloads will have a 10% probability of being exceeded in the

structures life).

If a structure is very important (i.e. the consequences of its

damage are severe) these loads will be increased: e.g.Nuclear structures designed to resist a 10,000 year returnperiod event.


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Conventional Seismic DesignNormal building life assumed to = 50years

Ultimate limit state design: Design a building to resist anearthquake with a return period of 475yrs (i.e. the designloads will have a 10% probability of being exceeded in the

structures life).If we design for the seismic loads to be resisted bythe building without damage (i.e. for the building toreact elastically) the cost of construction would beprohibitive

So we design buildings to be damaged underearthquake loading


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Designing for Controlled DamageWe design buildings to be damaged and react inelastically:

meaning they dissipate the same energy, but will undergogreater deformation during the event

y

e

u

Displacement ()

Pi

Pe

Equal EnergyRule

Force (P)


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Designing for Controlled DamageWe design buildings to be damaged and react inelastically:meaning they dissipate the same energy, but will undergogreater deformation during the event

The location of the damaged areas is controlled to avoid

catastrophic failure.

Simple design rules andcapacity design


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Simple Design Criteria

Choose an adequate lateral load resisting system


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Adequate Lateral Load Resisting Systems

Concentrically

braced frame

Eccentrically

braced frames

Moment resistingframe:

RC up to 5 floors

Steel up to 2 floors

fuse


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Maintain regularity in plan and elevation (of stiffness andmass distribution)


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Regularity in Plan

U i it C ll L d


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Regularity in Elevation

University College London


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MassIrregularity

StiffnessIrregularity



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Soft-storey Failure



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Soft-storey Failure



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Soft-storey Failure



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Civil & Environmental Engineeringy g




Ensure connection between structural elements

Connecting and anchoring of reinforcement Using appropriate materials



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Civil & Environmental Engineering

Poor Connection

Ci il & E i t l E i iUniversity College London


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Premature shear failure throughinappropriate detailing

Fukae Viaduct, Kobe 1995

Ci il & E i t l E i iUniversity College London


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Premature shear failure throughinappropriate detailing

Fukae Viaduct, Kobe 1995



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Poor Connection



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Avoid designing in locations of stress concentration



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Local Stress Points

Shortcolumn



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Short Column Failure

Shortcolumn



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C & o e ta g ee g





Avoid designing in locations of stress concentration Consider dynamic response in determining spacing

between buildings, so as to avoid pounding



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g g

Pounding



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g g

Pounding



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Simple Design Criteria Choose an adequate lateral load resisting system





Adopt capacity design concepts to control the failuremode



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Capacity Design



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Bad Design



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Bad Design



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Bad Design



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Capacity Design



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Simple Design Criteria Choose an adequate lateral load resisting system





Adopt capacity design concepts to control the failuremode

Be aware of damage potential on non-structural elements


Damage due to Non Structural


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Damage due to Non-StructuralElements



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Base-Isolation



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Base-Isolation



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Final RemarksDamage to buildings is the greatest cause of life loss, directdamage and business interruption in an earthquake.

New buildings should be designed considering earthquakeloading with the consequence of their damage in mind

during the design process.

Assess our existing structures for their earthquakeresistance

Predict the likely loss and intervene with structuralstrengthening if the risk is too high.



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Evening Seminar7th February 2006 at 6pm

The October 8, Pakistan Earthquake:Effects and Relief

Presentation by EEFIT Team and Oxfam

Venue: Chadwick Lecture Theatre, Dept of Civil and

Environmental Engineering, UCL

Open to All

Register your interest with me: [email protected]

17 earthquake engineering

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