Unit 1 Pre-exercise lecture on how earthquakes damage structures
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Transcript of Unit 1 Pre-exercise lecture on how earthquakes damage structures
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This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the Directorate for Education and Human Resources (DUE-1245025). Questions, contact education-AT-unavco.org
IMAGING ACTIVE TECTONICS UNIT 1HOW EARTHQUAKES DAMAGE
STRUCTURES
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OUTLINE
• How earthquakes cause damage• How might that damage affect society?• What do we need to know to prepare?
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DAMAGE!
In what ways do earthquakes cause damage to structures, and why?
Think on your own for a minute, and then confer with your neighbor for a minute, and come up with a list.
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DAMAGE!strain!
shaking• resonance• toppling• swaying
liquefaction• differential movement• sinking
landsliding• differential movement• burial
fault offset• differential movement
secondary effects• fire• flooding
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NEWTON’S SECOND LAW AND INERTIA
F = m aforce = mass x acceleration
Inertial Response
Building atRest
Ground Motion
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NEWTON’S SECOND LAW AND INERTIA
D’Alembert’s principle: the inertial force on a mass resisting acceleration is of equal magnitude and in the opposite direction.
Inertial Response
Building atRest
Ground Motion
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YIELD STRAIN
Inertial forces cause shear strains in a structure and its components. If any element exceeds its yield strain, it will be permanently damaged.
Lateralload
Yield Point
Ductile
Brittle
Deflection
ElasticRange
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TOPPLING AND ROTATION
One response of a structure to lateral inertial forces is to rotate like an inverted pendulum. If it is not secured at its base, it may topple.
Free-standing
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TOPPLING AND ROTATION
On the other hand, if it is secured at its base, it must absorb the inertial force internally, possibly leading to yielding at its base.
Free-standing Fixed-base
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SWAYING AND RESONANCE
Structures have their own natural frequencies, related to their dimensions and stiffness. If they are shaken at those frequencies, they will resonate.
Download video from:
http://www.iris.edu/hq/programs/%20education_and_outreach/videos#P
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SITE AMPLIFICATION
Seismic waves traveling from stiff rock to softer material (e.g. from basement to a sedimentary basin) typically experience an increase in shaking amplitude. Why?
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SITE AMPLIFICATION
Conservation of kinetic energy requires that amplitude of shaking increase when waves travel into less stiff materials. This can strongly affect the likelihood of building damage.
small x large = large x smallamplitude stiffness amplitude stiffness
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LIQUEFACTION
Water saturated
Loose sediments
Sand injected into overlying area upon shaking
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LIQUEFACTION
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LIQUEFACTION
Above: Christchurch, New Zealand sand boils (2011 earthquake)
Right: Christchurch, New Zealand road affected by liquefaction
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BUILDING DAMAGE FROM SHAKING
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LANDSLIDES
Above: Wenchuan, China, 2008
Right: El Salvador, 2001
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FAULT OFFSET
Above: Horizontal offset from 1906 San Francisco earthquake (Marin County)
Right: Vertical offset from 1999 Taiwan earthquake
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TRANSPORTATION CORRIDOR DAMAGE
Above: South Napa, CA earthquake 2014
Right: Northridge, CA earthquake 1994
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SECONDARY HAZARDS
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SAN FRANCISCO, 1906
The worst damage in San Francisco was caused by the fire following the earthquake, not the earthquake itself. (Why?)
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Port of Seattle
Major highway
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EARTHQUAKE PROBLEMS!
In what ways might earthquake damage to structures (buildings and infrastructure) affect society?
Think on your own for a minute, and then confer with your neighbor for a minute, and come up with a list.
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RISK VS HAZARD
Informally, people use these terms interchangeably, but in the natural disasters field, they have specific, different definitions.
Hazard refers to the probability of a particular disaster–e.g. earthquake hazard = the likelihood of a given size of earthquake occurring. Risk refers to the likely implications of such a disaster, and usually is quantified in dollars–e.g. uninsured earthquake risk in LA is ~ $100 billion.
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PREPARING FOR EARTHQUAKE HAZARDS
What information might we want to have in order to characterize and prepare for future earthquake risk and hazard?
Think on your own for a minute, and then confer with your neighbor for a minute, and come up with a list.