unit 1 pre-exercise lecture on how earthquakes damage structures
TRANSCRIPT
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
OUTLINE
• How earthquakes cause damage• How might that damage affect society?• What do we need to know to prepare?
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.
DAMAGE!strain!
shaking• resonance• toppling• swaying
liquefaction• differential movement• sinking
landsliding• differential movement• burial
fault offset• differential movement
secondary effects• fire• flooding
NEWTON’S SECOND LAW AND INERTIA
F = m aforce = mass x acceleration
Inertial Response
Building atRest
Ground Motion
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
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
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
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
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
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?
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
LIQUEFACTION
Water saturated
Loose sediments
Sand injected into overlying area upon shaking
LIQUEFACTION
LIQUEFACTION
Above: Christchurch, New Zealand sand boils (2011 earthquake)
Right: Christchurch, New Zealand road affected by liquefaction
BUILDING DAMAGE FROM SHAKING
LANDSLIDES
Above: Wenchuan, China, 2008
Right: El Salvador, 2001
FAULT OFFSET
Above: Horizontal offset from 1906 San Francisco earthquake (Marin County)
Right: Vertical offset from 1999 Taiwan earthquake
TRANSPORTATION CORRIDOR DAMAGE
Above: South Napa, CA earthquake 2014
Right: Northridge, CA earthquake 1994
SECONDARY HAZARDS
SAN FRANCISCO, 1906
The worst damage in San Francisco was caused by the fire following the earthquake, not the earthquake itself. (Why?)
Port of Seattle
Major highway
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.
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.
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.