seismic risk and performance - ait solutionssolutions.ait.ac.th › wp-content › uploads › 2016...
TRANSCRIPT
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Seminar on
Earthquake Resilient Design for School Buildings
Naveed Anwar, PhD
Post-earthquake School Reconstruction Project
Seismic Risk and PerformanceDay-1Session 1
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•Earthquakes cause disasters!• Why do they cause disasters?
• Understanding the Risk
•Can such disasters be minimized?• How can we reduce the consequences of such disasters
• Understanding the Response and Performance
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Seismic Risk and Performance
Understanding the nature and
source of earthquakes
Understanding Risk they pose
Understanding Performance required
from structures to reduce the risk to acceptable level
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Why do Earthquake Cause Disasters?
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What is a disaster
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Climate Change
Environmental Sustainability
Population Growth
Urbanization and Un-planned
development
Low Quality of Built Environment
Lack of Resources for Communities
Lack of post-event management and recovery, and re-bound capacity
Natural Phenomena
Disaster Hazard Exposure Vulnerability
Increased Consequences
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Seismic Risk
Seismic Risk = Seismic Hazard x Seismic Vulnerability
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What is Seismic Hazard
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10Source: Murty (2004)
For Hazard
Earthquake
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Arrival of Seismic Waves at a Site
Source: Murty (2004)
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Reducing illumination with distancefrom an electric bulb
Effected by “Medium in between”
Clear, FogReflection, Abortion
Source: Murty (2004)
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Seismic hazard Maps
Seismic hazard map of Asia, from the Global Seismic Hazard Assessment Program (GSHAP)
http://www.seismo.ethz.ch/static/gshap/
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What is Earthquake and its causes
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Where do earthquakes occur ?
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Tectonic Plates
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Tectonic Plates
Theory of Continental Drift
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18The Indian Plate is moving north into the Eurasian Plate
Source of Earthquakes in Nepal
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History : First recorded Earthquake
• A violent earthquake struck on June 7, 1255 AD.
• The earthquake toppled many houses and temples.
• It killed 30% of the population of Kathmandu, including King Abhaya Malla.
• The magnitude of the earthquake is said to be around 7.7 in Richter scale.
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How is Seismic Hazard measured
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Measuring Seismic Hazard
• Based on Magnitude (Energy released)• Richter scale (1 to 10)
• Based on Intensity of shaking• Modified Marcelle (MM) scale (1 to 12)
• Based on ground movement characteristics• Ground acceleration (fraction of g)
• Based on Response to ground shaking• Response acceleration (short and long period)
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Earthquake Magnitude
• The magnitude is a number that characterizes the relative size of an earthquake.
• Magnitude is based on measurement of the maximum motion recorded by a seismograph. • local magnitude (ML), commonly referred to as "Richter magnitude“
• surface-wave magnitude (Ms
• body-wave magnitude (Mb
• moment magnitude (Mw)
• Scales 1-3 have limited range and applicability. The moment magnitude (Mw) scale, based on the concept of seismic moment, is uniformly applicable to all sizes of earthquakes
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Seismic hazard Zones (NBC 105:1994)
90% Nepal’s land area underModerate-to-Severe Seismic Hazard
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Seismic hazard map of Nepal showing peak ground acceleration (PGA) on bedrock in gals for 500 years return period
(Wijeyewickrema et al. 2011).
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Using Response levels as indicator of Hazard
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Using Response Spectrums as Measure of Hazard
• Adjust Maximum Considered Earthquake (MCE) values of Ssand S1 for local site effects• SMs = Fa x Ss• SM1 = Fv x S1
• Calculate the spectral design values• SDS = 2/3 x SMS• SD1 = 2/3 x SM1
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Using Levels of Earthquakes
Terminology used to define earthquake
Maximum Credible Earthquake (MCE)
Maximum Design Earthquake (MDE)
Safe Shutdown Earthquake (SSE)
Contingency Level Earthquake (CLE)
Ductility Level Earthquake (DLE)
Operating Basis Earthquake (OBE)
Maximum Probable Earthquake (MPE)
Strength Level Earthquake (SLE)
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Maximum Credible Earthquake (MCE)
• Earthquake associated with specific seismotectonic structures, source areas or provinces that would cause the most severe vibratory ground motion or foundation dislocation capable of being produced at the site under the currently known tectonic framework
• Determined by judgment based on all known regional and local geological and seismological data
• Little regard is given to its probability of occurrence, which may vary from less than a hundred to several tens of thousands of years
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What is vulnerability
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Seismic Vulnerability
• The weaknesses in the location and structure that will be exploited by the earthquakes
• Site Vulnerability• Soil type and profile that may amplify the hazard
• Structural Vulnerability• The factors that will increase the seismic demands and or reduce capacity
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What is exposure
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Exposure to Seismic Hazard
• The people, property, assets, infrastructure that could be effected by the damage caused by earthquake
• No exposure > No disaster• Earthquake in a desert causes no disaster• Same earthquake in a crowded city is disastrous• Unoccupied building has no exposure to life, but exposure to assets still
present
• Nepal Earthquake of 2015 less disastorus due to reduced exposure• Schools closed• Day time, on a holiday
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How can we reduce Consequences of Earthquake?
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What needs to be Done
Minimize Disaster Consequences
Reduce Risk to Disaster(and manage consequences due to disaster)
Reduce Vulnerability to match Acceptable Risk
Define Acceptable Risk
Determine the Hazard
(R = V x H)
Difficult to reduce Hazard
Difficult to reduce Exposure
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Nepal is exposed to High Seismic risk
Risk = Vulnerability X Hazard
Consequences - Disaster
(Death-Dollars-Downtime)
High Vulnerability High HazardHigh Risk
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Hazard-Vulnerability-Risk-Consequences
Structural Displacement
Load
ing
Seve
rity
Resta
urant
Resta
urant
Resta
urant
Haz
ard
Vulnerability
Consequences
School
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Restaurant Restaurant
Resta
uran
t
Operational (O) Immediate Occupancy (IO) Life Safety (LS) Collapse Prevention (CP)
0 % Damage or Loss 99 %
Ref: FEMA 451 B
CasualtiesLowest Highest
Rehab Cost to Restore after eventLowest Highest
Downtime for RehabLowest Highest
School School
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Progression of Structural Design
Limits on the allowable stresses to achieve in-direct FOS
Explicit consideration of partial FOS.
Formulation of limit state design principles.
Formulation of ultimate strength.
The recognition of the difference between brittle and ductile failure.
The introduction of capacity based design approaches.
Performance based design and more explicit linkage between demand and performance.
Risk integrated based design, and a more and holistic approach towards consequence based engineering.
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Seismic Design Approaches
Code Based Design
Performance Based Seismic Design
Consequences and Risk Based Design
Resilience Based Design
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How do structures respond to Earthquakes
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Earthquake effects are Different
• Earthquake is different from all other loads• It is not an applied external force
• Earthquake effects are generated by the structure itself in response to ground shaking• Basically depends on stiffness and mass distribution
• Can be controlled by damping, ductility and energy dissipation mechanisms
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4343
Concept of 100% g (1g)
Most loads
Earthquake
FFKuuCuM NL
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Earthquake Inertial Forces
Source: Murty, (2004)
Effect of Inertia in a building when
shaken at its base
Flow of seismic inertia forces
through all structural
components
Inertia force and
relative motion within a
building
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Building Behavior during Earthquakes
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Schematic Representation of Seismic Forces
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Response Depends on the Structure and Soil
Different Buildings Respond Differently to Same Ground Vibration
Source: Murty (2004)
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Vulnerabilities in Structures
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Typical Vulnerabilities
• Irregularity of mass and stiffness
• Lack of stiffness
• Lack of strength
• Lack of ductility
• Weak failure mechanisms
• Lack of damping
• Lack of Stability
• Lack of energy dissipation mechanisms
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Buildings have unequal vertical members;
they cause the building to twist about a
vertical axis
Source: Murty (2004)
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Sudden deviations in load transfer path along the height lead to poor
performance of buildings.
Source: Murty (2004)
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How can we reduce vulnerability
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Basic approaches
• Systematically check and remove the vulnerabilities• During Basic Planning
• During Structural System
• During Structural Design
• During Structural Detailing
• During Construction
• During maintenance
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Plan shapes of buildings
Preferable Not Preferable
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Simple plan shape buildings do well during earthquakes
Source: Murty (2004)
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Buildings with one of their overall sizes much larger or much smaller than the other two, do not perform well during earthquakes
Source: Murty (2004)
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Identical vertical members placed uniformly in plan of building cause all points on the floor to move by the same amount.
Source: Murty (2004)
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Vertical members of buildings that
move more horizontally sustain
more damagePounding can occur between
adjoining buildings due to
horizontal vibrations of the twobuildings
Source: Murty (2004)
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Open ground storey building - assumptions made in current design practice are
not consistent with the actual structure.
Source: Murty (2004)
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How can we measure and control response
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Determining Response and Controlling it
• Use proper models
• Use proper hazard estimation
• Use appropriate analysis techniques
• Determine key response indicators
• Check response indicators against prescribed limits
• Modify structural properties to reduce demand or increase capacity
• Generally conform to building codes and guidelines
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Next Presentations
• Design Criteria for establishing what is to be done and how design is to be carried out
• Structural Analysis and Design to determine response and control it for new buildings
• Retrofitting to improve structures that have already been built
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Useful Reference
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