The document provides an overview of seismology and earthquake-resistant building planning. It discusses key topics such as:
1) Seismology is defined as the science of earthquakes and elastic waves.
2) The internal structure of the Earth consists of a crust, mantle, outer core, and inner core. Convective currents in the mantle cause tectonic plates to move.
3) Earthquakes are caused by the buildup and sudden release of stresses along fault lines within the Earth. Different types of boundaries exist between tectonic plates.
4) Important considerations for making buildings earthquake resistant include having a regular configuration, ductile elements, quality control measures, and potentially using base isolation
2. Seismology
The term ‘Seismology’ is derived from Greek word
Seismo, which means earthquake and logos means
science; hence the Seismology is Science of
Earthquakes
Seismology can be defined in two ways:
1. The science of earthquakes and the physics of the
earth’s interior
2. The science of elastic wave (seismic waves)
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1) Crust: thikness~5 to 40km
Light materials (e.g basalts and granites)
2) Mantle: thickness ~2900km
Has ability to flow outer core materials
3) Outer Core: thickness ~2200km
In Liquid form
4) Inner Core: radius ~1290km
solid and consists of heavy metals
(e.g., nickel and iron)
INSIDE THE EARTH
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Local
Convective
Currents in the Mantle
Major seven Tectonic Plates on the Earth’s surface
The convective flows of Mantle material cause the Crust and some portion of
the Mantle, to slide on the hot molten outer core. This sliding of Earth’s mass
takes place in pieces called Tectonic Plates.
8. Fault
A fault is nothing but a crack or weak zone inside the Earth. When two blocks of rock
or two plates rub against each other along a fault, they don’t just slide smoothly.
As the tectonic forces continue to prevail, the plate margins exhibit deformation as
seen in terms of bending, compression, tension and friction. The rocks eventually
break giving rise to an earthquake, because of building of stresses beyond the
limiting elastic strength of the rock.
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There are three types of inter-plate interactions are the and
boundaries
1) convergent
2) divergent
3) transform
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How the ground shakes?
Large strain energy released during an earthquake travels as seismic waves in
all directions through the Earth’s layers, reflecting and refracting at each
interface.
These waves are of two types
Body waves
1. P-waves
2. S-waves
Surface waves
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BASIC TERMINOLOGY
Focus: The point on the fault where slip
starts.
Epicenter: The point vertically above this
on
the surface of the Earth.
Focal Depth: The depth of focus from the
epicenter.
Epicentral distance: Distance from
epicenter to any point of interest
Aftershocks and Foreshocks :
Those occurring before the big one are
called
13. Magnitude Vs Intensity
The magnitude of an earthquake is
determined instrumentally and is more
objective measure of its size
Intensity of an earthquake is a subjective
parameter based on assessment of visible
effects. It depends on factors other than
the actual size of the earthquake
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MAGNITUDE :
INTENSITY
Magnitude is a quantitative measure
Intensity is an indicator of the severity
of the actual size of the earthquake.
of shaking generated at a given location
Measured by Richter Scale
Measured by Mercalli scale
Denoted by M(number) i.e. M8 or
severity of shaking is much higher near the
M7.7
epicenter than farther away.
Same at every places like M7
Intensity is varies at each and every place.
19. IS 1893:2002
More than 60 % area is
earthquake prone.
Zone V
%
12
Zone IV
Zone III
Fig. courtesy: nicee
18 %
26 %
Zone II
%
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44
20. The Vulnerability Profile - India
59% of land mass prone to earthquakes
40 million hectares (8%) of landmass prone to floods
8000 Km long coastline with two cyclone seasons
Hilly regions vulnerable to
avalanches/landslides/Hailstorms/cloudburst
68% of the total area susceptible to drought
Different types of manmade Hazards
Tsunami threat
1 million houses damaged annually + human, economic,
social and other losses
21. Hazard, vulnerability & disaster
Disaster = F (Hazard, Vulnerability)
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22. Ingredients of Risk
HxV-C=R
Hazard x vulnerability – capacity = risk
H - potential threat to humans and their welfare
V - exposure and susceptibility to loss of life or dignity
C - available and potential resources
R - probability of disaster occurrence
-
Capacity “resources, means and strengths which exist in
households and communities and which enable them to cope with,
withstand, prepare for, prevent, mitigate or quickly recover from a
disaster”
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23. Earthquake Do Not Kill People
Improperly Designed Structures Do!
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24. • The structure is to resist minor earthquake without
damage.
• The structure is to resist moderate and frequently
occurring earthquakes without any structural
damage, but minor cracks are permissible during
earthquakes
• The structure shouldn’t collapse under severe
earthquake.
25. Planning Parameters for EQRB
• Planning should be based on seismic IS codes i.e. IS
1893-2002, IS 13920-1993
• The base soil should be strong and compacted.
• The zone should be free from seismological hazards.
• Important heavy structures like dams, nuclear power plant
etc. should be planned for higher level of earthquake
protection.
• The weight of building should be as less as possible.
• Building height and width ratio should be maintained.
• Reinforced structure should be planned.
• All parts of buildings like columns, beams, roofs should be
well connected properly
26. • Shear walls, ductility of buildings with greater quality
should be provided for more safety.
• Avoid corners, soft stories at ground floor, short column.
• Good materials, modern engineering technologies,
skilled engineers and labors, fund and construction
methods should be maintained.
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What are the seismic effect on structures?
1.
Inertia force in structures:
Earthquake causes shaking of the
ground. So a building resting on it will
experience motion at its base. From
Newton’s First Law of Motion, even
though the base of the building moves
with the ground, the roof has a tendency
to stay in its original position. But since
Effect of Inertia in a building
when
shaken at its base
the walls and columns are connected to
it, they drag the roof along with them.
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Modes and patterns of failure of Buildings
(1) Soft/weak stories
A soft or weak storey is created when the lateral stiffness and/or
strength of a storey is markedly more flexible than the floors
above and below.
Soft story
This often occurs at the ground
floor when it is left open for
parking, a shop front, or other
reasons.
Most of the deformation is
concentrates at this level and
results in large rotation demand
in columns.
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A typical soft storey
collapse
In this case, after the
first storey failed, this
added force of impact at
each
floor
subsequent
collapse.
Typical soft storey collapse in Bhuj
caused
stories
to
35. IMPORTANT CONSIDERATIONS TO MAKE
A BUILDING EARTHQUAKE RESISTANT
1. Configuration
2. Ductility
3. Quality control
4. Base Isolation
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36. 1. Configuration
A terminally ill patient , however
effective the medication, may
eventually die.
Similarly, a badly configured building
Cannot be engineered for an improved
performance beyond a certain limit.
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37.
38. Regular Configuration
• Regular configuration is seismically ideal. These
configurations have low heights to base ratio,
symmetrical plane, uniform section and elevation
and thus have balanced resistance.
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These configurations
would have maximum
torsional resistance due
to location of shear walls
and bracings. Uniform
floor heights, short spans
and direct load path play
a significant role in
seismic resistance of the
building.
39. Irregular Configuration
Buildings with irregular configuration
Buildings with abrupt changes in
lateral resistance
Buildings with abrupt changes in
lateral stiffness
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45. Ductility
Let us first understand how different materials behave.
Consider white chalk used to write on blackboards and steel pins with solid
heads used to hold sheets of paper together. Yes… a chalk breaks easily!!
On the contrary, a steel pin allows it to be bent back-and-forth. Engineers define
the property that allows steel pins to bend back-and-forth by large amounts, as
ductility; chalk is a brittle material.
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46. The currently adopted performance criteria in the earthquake codes
are the following:
i. The structure should resist moderate intensity of earthquake
shaking without structural damage.
ii. The structure should be able to resist exceptionally large intensity
of earthquake shaking without collapse.
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47. The strength of brittle construction
materials, like masonry and
concrete, is highly sensitive to the
1. quality of construction materials
2. workmanship
3. supervision
4. construction methods
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48. Quality control
special care is needed in construction to ensure
that the elements meant to be ductile are indeed
provided with features that give adequate
ductility.
Thus, strict adherence to prescribed standards of
construction materials and construction
processes is essential in assuring an earthquakeresistant building.
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49. Elements of good quality control
1.Regular testing of construction
materials at qualified laboratories (at
site or away)
2. Periodic training of workmen at
professional training houses, and
3. On-site evaluation of the technical
work
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50. IS CODES
IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design
of Structures (5th Revision)
IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant
Design and Construction of Buildings (2nd Revision)
IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake
Resistance of Earthen Buildings
IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake
Resistance of Low Strength Masonry Buildings
IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of
Reinforced Concrete Structures Subjected to Seismic Forces
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53. While Hazards Are Inevitable, Each Hazard Need Not Convert
Into A Disaster… As What Comes In Between Is
The Culture of Safety And Prevention
Let us Work Together to Build a Culture of Prevention !
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The Earth and its Interior
Long time ago, a large collection of material masses coalesced to form the Earth. A large amount of heat was generated by this fusion, and slowly as the Earth cooled down, the heavier and denser materials sank to the center and the lighter ones rose to the top. The differentiated Earth consists of the Inner Core (radius ~1290km), the Outer Core (thickness ~2200km), the Mantle (thickness ~2900km) and the Crust (thickness ~5 to 40km). Figure 1 shows these layers. The Inner Core is solid and consists of heavy metals (e.g., nickel and iron), while the Crust consists of light materials (e.g., basalts and granites).
Basic Difference: Magnitude versus Intensity
Magnitude of an earthquake is a measure of its size. For instance, one can measure the size of an earthquake by the amount of strain energy released by the fault rupture. This means that the magnitude of the earthquake is a single value for a given earthquake. On the other hand, intensity is an indicator of the severity of shaking generated at a given location. Clearly, the severity of shaking is much higher near the epicenter than farther away. Thus, during the same earthquake of a certain magnitude, different locations experience different levels of intensity ( e.g., Figure 14).
To elaborate this distinction, consider the analogy of an electric bulb (Figure 15). The illumination at a location near a 100-Watt bulb is higher than that farther away from it. While the bulb releases 100 Watts of energy, the intensity of light (or illumination, measured in lumens) at a location depends on the wattage of the bulb and its distance from the bulb. Here, the size of the bulb (100-Watt) is like the magnitude of an earthquake, and the illumination at a location like the intensity of shaking at that location.
Suggestion: Can student think of any other analogies. e.g. ripples formed by dropping a stone into a pond………?
Magnitude and Intensity in Seismic Design
One often asks: Can my building withstand a magnitude 7.0 earthquake? But, the M7.0 earthquake causes different shaking intensities at different locations, and the damage induced in buildings at these locations is different. Thus, it is particular levels of intensity of shaking that buildings and structures are designed to resist, and not so much the magnitude. Buildings are designed as per the intensity, since intensity can vary place to place, for a given magnitude.
The peak ground acceleration (PGA), i.e., maximum acceleration experienced by the ground during shaking, is one way of quantifying the severity of the ground shaking. Approximate empirical correlations are available between the MM intensities and the PGA that may be experienced. For instance, during the 2001 Bhuj earthquake, the area enclosed by the isoseismal VIII is thought to have experienced a PGA of about 0.25-0.30g. Now strong ground motion records from seismic instruments are relied upon to quantify destructive ground shaking. These records are critical for cost-effective earthquake-resistant design.
Seismic Zones of India
The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zone map is required so that buildings and other structures located in different regions can be designed to withstand different level of ground shaking. The seismic zone map of 1984 subdivided India into five zones – I, II, III, IV and V (Figure 12). The maximum Modified Mercalli (MM) intensity of seismic shaking expected in these zones were V or less, VI, VII, VIII, and IX and higher, respectively. Parts of Himalayan boundary in the north and northeast, and the Kachchh area in the west were classified as zone V.
The seismic zone maps are revised from time to time as more understanding is gained on the geology, the seismotectonics and the seismic activity in the country. For instance, the Koyna earthquake of 1967 occurred in an area classified in zone I as per map of 1966. The 1970 version (same as Figure 12) of code upgraded the area around Koyna to zone IV. The Killari (Latur) earthquake of 1993 occurred in zone I. The current Indian seismic zone map (Figure 13) places this area in zone III. The zone map now has only four seismic zones – II, III, IV and V. The areas falling in seismic zone I in the 1984 map were merged with those of seismic zone II. Also, the seismic zone map in the peninsular region is modified; Madras now comes under seismic zone III as against zone II in 1984 map.
The national Seismic Zone Map presents a large-scale view of the seismic zones in the country. Local variations in soil type and geology cannot be represented at that scale. Therefore, for important projects, such as a major dam or a nuclear power plant, the seismic hazard is evaluated specifically for that site. Also, for the purposes of urban planning, metropolitan areas are microzoned. Seismic microzonation accounts for local variations in geology, local soil profile, etc.
Suggestion: Ask the students to indicate in which zone their birthplace is located.
Buildings with one of their overall sizes much larger or much smaller than the other two, or very large buildings, do not perform well during earthquakes.
Base Isolation
The concept of base isolation is explained through an example building resting on frictionless rollers (Figure 6). When the ground shakes, the rollers roll freely, but the building above does not move. It remains stationary. No force is transferred to the building due to shaking of the ground; simply, the building does not experience the earthquake.
Unfortunately, under wind load the building will move and impact against the end of the pit.
Seismic isolation is a relatively recent and evolving technology. It has been in increased use since the 1980s, and has been well evaluated and reviewed internationally. Base isolation has now been used in numerous buildings in countries like Italy, Japan, New Zealand, and USA. Base isolation is also useful for retrofitting important buildings (like hospitals and historic buildings). By now, over 1000 buildings across the world have been equipped with seismic base isolation. In India, base isolation technique was first demonstrated after the 1993 Killari (Maharashtra) Earthquake [EERI, 1999]. Two single storey buildings (one school building and another shopping complex building) in newly relocated Killari town were built with rubber base isolators resting on hard ground. Both were brick masonry buildings with concrete roof. After the 2001 Bhuj (Gujarat) earthquake, the four-storey Bhuj Hospital building was built with the base isolation technique (Figure 8).