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Earthquake and Possible Solutions Using Dampers Technology
Mahdi hosseini1, Mohammad Yousef Dastajani Farahani2 ,Prof.N.V.Ramana Rao3
1 Post Graduate Student, Dept. of Civil Engineering, Jawaharlal Nehru Technological University
Hyderabad (JNTUH), Hyderabad, Andhra Pradesh, IndiaEmail: [email protected]
2 Post Graduate Student, Dept. of Civil Engineering, Jawaharlal Nehru
Technological University Hyderabad (JNTUH), Hyderabad, Andhra Pradesh, India
Email: [email protected]
3 Professor, Dept. of Civil Engineering, Jawaharlal Nehru Technological University Hyderabad
(JNTUH), Hyderabad, Andhra Pradesh, IndiaEmail: [email protected]
ABSTRACT
Earthquake is a natural phenomenon occurs due to the disturbance in tectonic plates. Tectonic
plates are the pieces of the earth’s crust and the upper most mantles. The thickness of the plates
is of around 100km and consist principles materials like oceanic Crust and continental crust.
Earthquakes occur on faults. A fault is a thin zone of crushed rock separating blocks of the
earth's crust. When an earthquake occurs on one of these faults, the rock on one side of the fault
slips with respect to the other. Faults can be centimeters to thousands of kilometers long. The
fault surface can be vertical, horizontal, or at some angle to the surface of the earth. Faults can
extend deep into the earth and may or may not extend up to the earth's surface. A fault can be
defined as the displacement of once connected blocks of rock along a fault plane. This can occur
in any direction with the blocks moving away from each other. Faults occur from both tensional
and compressional forces. Dampers can be installed in the structural frame of a building to
absorb some of the energy going into the building from the shaking ground during an earthquake.
The dampers reduce the energy available for shaking the building.
Key words: Earthquake, Tectonic Plates, Fault, Crust, Dampers
BACKGROUND
HISTORY'S 10 WORST EARTHQUAKES
The Daily Beast’s rundown of the most powerful, deadliest ear thquakes ever .
The death toll continues to climb as Japan recovers from the most powerful quake in its recorded
history and the towering tsunami that followed. Japan’s Kyodo News agency said between 200
and 300 bodies have been found on a beach in Sendai, the population center nearest the quake’s
epicenter, and another 110 people have been confirmed dead elsewhere. But the agency said the
death toll will likely surpass 1,000. Read more and view photos of the devastation plus view
shocking video from Japan's disaster zone. Here The Daily Beast’s rundown of the most
powerful, deadliest earthquakes ever.
Shaanxi Earthquake
The Shaanxi earthquake—also known as the Hua County earthquake—is the deadliest Quake to
date, resulting in approximately 830,000 deaths. On the morning of Jan. 23, 1556, it destroyed a
520-mile-wide area in China, killing 60 percent of the population in Some of the 97 affected
counties. One witness writes, “Mountains and rivers changed places and roads were destroyed. In
some places, the ground suddenly rose up and formed new hills, or it sank abruptly and became
new valleys.” Because a majority of civilians were living in yaodongs, or artificial caves in loess
cliffs, fatalities reached an all-time high as the caves collapsed, killing those inside. Modern
estimates predict the magnitude was around 8.0, not a record high, but the earthquake still ranks
third on the list of deadliest natural disasters in history.
Tangshan Earthquake
Although some say there were early warnings of the Tangshan earthquake, it hit Chinese
civilians unexpectedly at 3:42 a.m. on July 28, 1976, shaking people from their beds and leveling
the entire city in a matter of seconds. The 7.8-magnitude quake killed more than 240,000 people,
leaving survivors without access to water, food, or electricity. Relief workers also caused an
accidental traffic jam on the only drivable road, and although 80 percent of those stuck under the
rubble were saved, a 7.1-magnitude aftershock struck the afternoon of the 28th, killing many
more and cutting off access to those trying to provide aid, making it one of the deadliest quakes
of the 20th century.
Haiyuan Earthquake
The Haiyuan earthquake hit Dec. 16, 1920, killing more than 73,000 in China’s Haiyuan County
and approximately 127,000 in surrounding areas. The 7.8-magnitude quake—reported as 8.5
magnitudes by Chinese news sources—caused nearly all of the houses to collapse in Longde and
Huining, with damages in seven provinces and regions, including dammed rivers, landslides, and
severe cracks in the ground. Seiches were even observed in various lakes and fjords in Norway.
Aftershocks from the earthquake occurred as long as three years later, but the effects did not
come close to the severity of the first.
Aleppo Earthquake
Set in a nest of fault lines in northern Syria, Aleppo—now known as Halab—was hit with an 8.5-
magnitude earthquake in 1138, jolting areas as far as 200 miles away from the city. The most
damage was seen in Harem, where crusaders had built a large citadel that was crumbled below
the castle, killing 600 castle guards at the time. Although residents of Aleppo were warned by
foreshocks and some fled to the countryside, the quake was much Larger than anticipated, and
the city and all homes surrounding it were brought to the ground.
Indian Ocean Earthquake
Underwater earthquakes are believed to be the most dangerous because they can create tsunamis
and tidal waves, which is exactly what happened on Dec. 26, 2004, when the Indian Ocean
earthquake wreaked havoc on India, Indonesia, Sri Lanka, and Thailand—and beyond. With a
magnitude of between 9.1 and 9.3, this earthquake is the second largest ever recorded, and it also
had the longest duration, lasting between eight and 10 minutes. Devastating tsunamis hit land
masses bordering the ocean, prompting a widespread humanitarian response. Initially, reports
said the quake killed approximately 100,000, but later calculations showed it resulted in more
than 230,000 deaths.
Damghan Earthquake
In 856, in the area we now know as Iran, an earthquake of 8.0 magnitudes hit the capital city of
Damghan, destroying the city, countryside, and nearly every village within 200 miles of the
epicenter. Situated between two major tectonic plates, Iran is an area of frequent earthquake
activity, but residents of Damghan were unprepared for a temblor of this magnitude. The quake
resulted in approximately 200,000 deaths.
Ardabil Earthquake
Another Iranian earthquake hit Feb. 28, 1997, when the 15-second quake rippled through
northern Iran, with deaths tallying up to 150,000. There was severe damage to roads and
electrical power lines, and all communications and water distribution became near impossible,
leaving the city of Ardabil in a state of desperation. Hospitals overflowed with patients, and even
as it tried to recover, the area was hit with nearly 350 aftershocks, the highest recorded at 5.2 on
the Richter scale.
Hokkaido Earthquake
In 1730, an 8.3-magnitude earthquake hit Japan’s second largest island, Hokkaido, causing
landslides, power outages, road damage, and a tsunami causing 137,000 fatalities. The island was
struck by a similar, though not as intense, earthquake in 2003.
Ashgabat Earthquake
The 7.3-magnitude earthquake that hit Turkmenistan’s capital, Ashgabat, in 1948 tore much of
the city down, collapsing almost all of its brick buildings, heavily damaging concrete structures,
and derailing freight trains with effects felt across the border in the Darreh Gaz region of Iran.
The Turkmen government has upwardly revised the official death toll from 110,000 to 176,000;
the quake also killed the mother of future dictator Saparmurat Niyazov and resulted in his
placement in a Soviet orphanage, an important component of the former leader’s self-mythology.
Great Kanto Earthquake
The Great Kanto Earthquake of 1923 devastated Tokyo and Yokohama, causing huge fires and
resulting in as many as 142,000 deaths, but it may be best remembered for its horrific aftermath,
when rumors that Koreans were looting businesses and poisoning wells led to the deaths of an
estimated 2,500 non-Japanese immigrants. The Japanese government has heavily funded disaster
preparation ever since, holding “Disaster Prevention Day” on Sept. 1, the Kanto quake’s
anniversary.
INTRODUCTION
Structure of the Earth
The Earth is an oblate spheroid. It is composed of a number of different layers as determined by
deep drilling and seismic evidence (Figure 3.1). These layers are:
The core which is approximately 7000 kilometers in diameter (3500kilometers in radius)
and is located at the Earth's center.
The mantle which surrounds the core and has a thickness of 2900 kilometers.
The crust floats on top of the mantle. It is composed of basalt rich oceanic crust and
granitic rich continental crust.
Fig 3.1 Layers beneath the Earth's surface
The core is a layer rich in iron and nickel that is composed of two layers: the inner and outer
cores. The inner core is theorized to be solid with a density of about 13 grams per cubic
centimeter and a radius of about 1220 kilometers. The outer core is liquid and has a density of
about 11 grams per cubic centimeter. It surrounds the inner core and has an average thickness of
about 2250 kilometers.
The mantle is almost 2900 kilometers thick and comprises about 83% of the Earth's volume. It is
composed of several different layers. The upper mantle exists from the base of the crust
downward to a depth of about 670 kilometers. This region of the Earth's interior is thought to be
composed of peridotite, an ultramafic rock made up of the minerals olivine and pyroxene. The
top layer of the upper mantle, 100 to 200 kilometers below surface, is called the asthenosphere.
Scientific studies suggest that this layer has physical properties that are different from the rest of
the upper mantle. The rocks in this upper portion of the mantle are more rigid and brittle because
of cooler temperatures and lower pressures. Below the upper mantle is the lower mantle that
extends from 670 to 2900 kilometers below the Earth's surface. This layer is hot and plastic. The
higher pressure in this layer causes the formation of minerals that are different from those of the
upper mantle.
The lithosphere is a layer that includes the crust and the upper most portion of the mantle (Figure
3.2). This layer is about 100 kilometers thick and has the ability to glide over the rest of the
upper mantle. Because of increasing temperature and pressure, deeper portions of the lithosphere
are capable of plastic flow over geologic time. The lithosphere is also the zone of earthquakes,
building, volcanoes, and continental drift.
The topmost part of the lithosphere consists of crust. This material is cool, rigid, and brittle. Two
types of crust can be identified: oceanic crust and continental crust (Figure 3.2). Both of these
types of crust are less dense than the rock found in the underlying upper mantle layer. Ocean
crust is thin and measures between 5 to 10 kilometers thick. It is also composed of basalt and has
a density of about 3.0 grams per cubic centimeter.
The continental crust is 20 to 70 kilometers thick and composed mainly of lighter granite (Figure
3.2). The density of continental crust is about 2.7 grams per cubic centimeter. It is thinnest in
areas like the Rift Valleys of East Africa and in an area known as the Basin and Range Province
in the western United States (centered in Nevada this area is about 1500 kilometers wide and
runs about 4000 kilometers North/South). Continental crust is thickest beneath mountain ranges
and extends into the mantle. Both of these crust types are composed of numerous tectonic plates
that float on top of the mantle. Convection currents within the mantle cause these plates to move
slowly across the asthenosphere.
Fig 3.2 Structure of the Earth's crust and top most layer of the upper mantle
TECTONIC PLATES
PRIMARY PLATES
These seven plates comprise the bulk of the continents and the Pacific Ocean.
(i) African Plate (ii) Antarctic Plate (iii)Eurasian Plate (iv)Pacific Plate
(v)Indo-Australian Plate (vi)North American Plate (vii)South American Plate
SECONDARY PLATES
These are generally not shown in the map as these propagate only for lesser areas when
compared with the major tectonic plates.
(i)Arabian Plate (ii)Caribbean Plate (iii) Cocos Plate (iv)Indian Plate
(v)Juan de Fuca Plate (vi)Nazca Plate (vii)Philippine Sea Plate (viii)Scotia Plate
Major no of tectonic plates are located fortunately under Japan so often Japan is effected by
earthquakes.
PROPAGATION OF AN EARTHQUAKE
Due to the disturbances caused in the lithosphere i.e. in the movement of tectonic plates large
amount of energy is released in the earth crust which turns into Seismic waves. The seismic
activity refers to the type, magnitude and the size of the earthquake Earthquakes are measured
using observations from seismometers. The moment magnitude is the most common scale on
which earthquakes larger than approximately 5 are reported for the entire globe. The more
numerous earthquakes smaller than magnitude 5 reported by national seismological observatories
are measured mostly on the local magnitude scale, also referred to as the Richter scale. These
two scales are numerically similar over their range of validity. To sum it up briefly an earthquake
phenomenon can be explained as follows Due to the uneven disturbances caused in the tectonic
plates large amount of energy is emerged into the earth’s surface and its interior parts thus this
energy is completely transferred into nearest areas from the focus with appropriate deviation.
Earthquakes that happen in one place can be detected by seismographs in other. The waves from
the epicenter of a major earthquake propagate outward as surface waves. In the case of
compression waves, the energy released in this fashion radiates from the focus under the
epicenter and travels all the way through the globe. Seismologists, who study the distribution of
these waves, can map such propagation. The further away a point is on Earth from the focus of a
quake, the longer the time it will take for a seismograph station at that point to detect the far-
away quake. Compression waves are also named "P-waves" or "pressure waves." Due to the
nature of Earth's interior, "P-waves" can be refracted and reflected as they encounter differing
density layers between the core and the mantle. As a result, seismographs in some areas on the
other side of the world opposite the epicenter of a major earthquake would record nothing. Such
an area on the Earth's surface is called a "shadow zone."
A seismic shadow zone is an area of the Earth's surface where seismographs cannot detect
an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs,
seismic waves radiate out spherically from the earthquake's focus. The primary seismic
waves are refracted by the liquid outer core of the Earth and are not detected between 104° and
140° (between approximately 11,570 and 15,570 km or 7,190 and 9,670 mi) from the epicenter
Fig 1.1 Earthquake
SEISMIC WAVES
Seismic waves consist of S-waves and P-waves.
PRIMARY WAVES (P-waves)
Primary waves are compression waves that are longitudinal in nature. P waves are pressure
waves that travel faster than other waves through the earth to arrive at seismograph stations first,
hence the name "Primary". These waves can travel through any type of material, including fluids,
and can travel at nearly twice the speed of S waves. In air, they take the form of sound waves;
hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and
about 5000 m/s in granite.
SECONDARY WAVES (S-waves)
Secondary waves are shear waves that are transverse in nature. Following an earthquake event,
S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground
perpendicular to the direction of propagation. Depending on the preoperational direction, the
wave can take on different surface characteristics; for example, in the case of horizontally
polarized S waves, the ground moves alternately to one side and then the other. S-waves can
travel only through solids, as fluids (liquids and gases) do not support shear stresses. S-waves are
slower than P-waves, and speeds are typically around 60% of that of P-waves in any given
material.
There are also some other waves which important in measuring the intensity of earthquake but
not so dangerous compared to s and p waves, They are as follows:
SURFACE WAVES
Seismic surface waves travel along the Earth's surface. They are called surface waves, as they
diminish as they get further from the surface. Their velocity is lower than those of seismic body
waves (P and S).
RAYLEIGH WAVES
Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions
that are similar to those of waves on the surface of water.
LOVE WAVES
Love waves are horizontally polarized shear waves (SH waves), existing only in the presence of
a semi-infinite medium overlain by an upper layer of finite thickness. They usually travel
slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest
amplitude.
STONELEY WAVES
A Stoneley wave is a type of large amplitude Rayleigh wave that propagates along a solid-fluid
boundary or under specific conditions also along solid-solid boundary. They can be generated
along the walls of a fluid-filled borehole.
FIG 1.2 WAVES
THE MOMENT MAGNITUDE SCALE
Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates for large
magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred
because it works over a wider range of earthquake sizes and is applicable globally. The moment
magnitude scale is based on the total moment release of the earthquake. Moment is a product of
the distance a fault moved and the force required to move it. It is derived from modeling
recordings of the earthquake at multiple stations. Moment magnitude estimates are about the
same as Richter magnitudes for small to large earthquakes. But only the moment magnitude
scale is capable of measuring M8 (read ‘magnitude 8’) and greater events accurately.Magnitudes
are based on a logarithmic scale (base 10). What this means is that for each whole number you
go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph
goes up ten times. Using this scale, magnitude 5 earthquakes would result in ten times the level
of ground shaking as a Magnitude 4 earthquakes (and 32 times as much as energy would be
released). To give you an idea how these numbers can add up, think of it in terms of the energy
released by Explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6
ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons
of TNT. Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too
small to be felt by most people. Magnitude scales can be used to describe earthquakes so small
that they are expressed in negative numbers. The scale also has no upper limit, so it can describe
earthquakes of unimaginable and (so far) un-experienced intensity, such as magnitude 10.0 and
beyond.
DAMAGES CAUSED BY EARHTQUAKES
The effects of an earthquake are strongest in a broad zone surrounding the epicenter. Surface
ground cracking associated with faults that reach the surface often occurs, with horizontal and
vertical displacements of several yards common. Such movement does not have to occur during
a major earthquake; slight periodic movements called fault creep can be accompanied by micro
earthquakes too small to be felt. The extent of earthquake vibration and subsequent damage to a
region is partly dependent on characteristics of the ground. For example, earthquake vibrations
last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly
compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs in
densely populated urban areas where structures are not built to withstand intense shaking. There,
L waves can produce destructive vibrations in buildings and break water and gas lines, starting
uncontrollable fires. Damage and loss of life sustained during an earthquake result from falling
structures and flying glass and objects. Flexible structures built on bedrock are generally more
resistant to earthquake damage than rigid structures built on loose soil. In certain areas, an
earthquake can trigger mudslides, which slip down mountain slopes and can bury habitations
below. A submarine earthquake can cause a tsunami, a series of damaging waves that ripple
outward from the earthquake epicenter and inundate Coastal cities
Fig 1.6 Damage of earthquake
GLOBAL DISTRIBUTION OF EARTHQUAKES
According to a moderate estimate about 30,000 earthquakes occur every year. But most of these
are so slight that we cannot feel them. There is no visible damage from them. But every year
there are some earthquakes of great intensity and magnitude. If one of these occurs in a densely
populated region, there is damage and destruction enough to draw people's attention all the world
over.Every year hundreds of earthquakes pass unnoticed because they occur in areas where there
is no possibility of any loss of human life and damage to property.
Earthquakes have a definite distribution pattern. There are three major belts in the world
which are frequented by earthquakes of varying intensities. These belts are as under:
1. The Circum-Pacific Belt
2. The Mid-Atlantic Belt
3. The Mid-Continental Belt
1. The Circum-Pacific Belt:
This belt is located around the coast of the Pacific Ocean. In this belt the earthquakes originate
mostly beneath the ocean floor near the coast. The Circum- Pacific Belt represents the
convergent plate boundaries where the most widespread and intense earthquakes occur. This belt
runs from Alaska to Kurile, Japan, Mariana and the Philippine trenches. Beyond this, it
bifurcates into two branches, one branch going towards the Indonesian trench and the other
towards the Kermac-Tonga trench to the northwest of Newzealand spreading completely over the
belt.This belt is located on the western side of the Pacific Ocean. On the eastern side of the
Pacific Ocean, the earthquake belt runs parallel to the west coast of North America and moves on
towards the South along the Peru and Chile trench lying on the west coast of South America.This
belt has about 66 percent of the total earthquake that are recorded in the world. Most of the
earthquakes occurring in this belt are shallow ones with their focus about 25 km deep.It may be
pointed out that these belts being the zones of convergent plate boundaries (the subduction
zones) are isostatically very unstable, Japan alone experience about 1500 earthquakes per year.
2. The Mid-Atlantic Belt:
This belt is characterized by the sea floor spreading which is the main cause of the occurrence of
earthquakes in it. This earthquake belt runs along the mid- oceanic ridges and the other ridges in
the Atlantic Ocean. In this belt most of the earthquakes are of moderate to mild intensity. Their
foci are generally less than 70 km deep. Since the divergent plates in this belt move in opposite
directions and there is splitting as well, transform faults and fractures are created. All this
becomes the causative factor for the occurrence of shallow focus earthquakes of moderate
intensity. The sea floor spreading is the main cause for the occurrence of earthquakes in this belt.
3. The Mid-Continental Belt:
This belt extends along the young folded Alpine mountain system of Europe, North Africa,
through Asia Minor, Caucasia, Iran, Afghanistan and Pakistan to the Himalayan mountain
system. This belt continues further to include Tibet, the Pamir’s and the mountains of Tine Shan
etc. The young folded mountain systems of Myanmar, China and eastern Siberia fall in this belt.
This belt happens to be the subduction zone of continental plates. It is in this belt that the African
as well as Indian plates sub-duct below the Eurasian plate. This mid- Continental belt is
characterized by experiencing about 20 per cent of the earthquakes in the world. This belt
records earthquakes of shallow and intermediate origin. However, it is true that sometimes
earthquakes of great violence occur in this belt. This belt forms a great circle approximately east
and west around the earth, through the Mediterranean, Southern Asia, Indonesia and the East
Indies, where the great majority of recorded shocks occur. It may be pointed out that more than
50 percent of all earthquakes are associated with the young folded mountains which are said to
be still growing. The Andes, Himalayas and Coast Ranges of the United States are the specific
examples. It is worthwhile to remember that this girdle of young fold mountains has no
correspondence with the line of active volcanoes like the Circum-Pacific earthquake zone. There
are some regions on the earth's surface which are relatively immune from violent and vigorous
earthquakes. This is so because diastrophism and volcanism are either absent or only moderately
active. But the infrequent occurrence of minor shocks in such regions is not ruled out. Such
shocks may occur due to local causes like the subterranean Movement of imprisoned gases or
liquids. The most glaring example of the occurrence of minor earthquakes in quite unexpected
Places are the Koyna earthquake which shook Koynanagar on September 13 and 14, 1967.It is
believed that the earthquake in this stable area was caused due to the building of a 103 m-high
concrete dam across the Koyna River which impounded a huge volume of water to form an
artificial lake. The magnitude of the earthquake was 6.5 on the Richter scale. Then again on
December 11, 1967, the most disastrous earthquake occurred in the same area which affected the
whole of western Maharashtra. The zone of maximum intensity of the shock and its epicenter
was in the vicinity of Koynanagar.The death toll rose to 1000 people, and a large number of
people were injured. Its impact was felt as far north as Ujjain and as far South as Bangalore.
Other towns like Surat, Ahmadabad, Broach and Hyderabad also felt the shock. Since there was
no record of earthquakes in this particular region before 1962, it was thought that Koyna
earthquake was caused due to the hydrostatic pressure exerted by the reservoir. But the recent
investigation does not support this view. Now, the geologists are of the opinion that the shock in
this region was the result of tectonic movement along a north-south axis of weakness in the
underlying rocks buried below the Deccan trap.
There are several different kinds of faults. These faults are named according to the type of stress
that acts on the rock and by the nature of the movement of the rock blocks either side of the fault
plane. Normal faults occur when tensional forces act in opposite directions and cause one slab of
the rock to be displaced up and the other slab down (Figure 3.3).
Fig 3.3 Normal faults
Reverse faults develop when compressional forces exist (Figure 3.4). Compression causes one
block to be pushed up and over the other block.
Fig 3.4 Reverse faults
A graben fault is produced when tensional stresses result in the subsidence of a block of rock. On
a large scale these features are known as Rift Valleys (Figure 3.5).
Fig 3.5 graben fault
A horst fault is the development of two reverse faults causing a block of rock to be
pushed(Figure 3.6)
.
Fig 3.6 horst fault
Figure 10l-8: Location of some of the major faults on the Earth.
POSSIBLE SOLUTIONS
How to reduce Earthquake Effects on Buildings?
Adding Dampers
Dampers can be installed in the structural frame of a building to absorb some of the energy going
into the building from the shaking ground during an earthquake. The dampers reduce the energy
available for shaking the building. This means that the building deforms less, so the chance of
damage is reduced.
Why Earthquake Effects are to be Reduced
Conventional seismic design attempts to make buildings that do not collapse under strong
earthquake shaking, but may sustain damage to non-structural elements (like glass facades) and
to some structural members in the building. This may render the building non-functional after the
earthquake, which may be problematic in some structures, like hospitals, which need to remain
functional in the aftermath of the earthquake. Special techniques are required to design buildings
such that they remain practically undamaged even in a severe earthquake. Buildings with such
improved seismic performance usually cost more than normal buildings do. However, this cost is
justified through improved earthquake performance. Two basic technologies are used to protect
buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic
Dampers. The idea behind base isolation is to detach (isolate) the building from the ground in
such a way that earthquake motions are not transmitted up through the building, or at least
greatly reduced. Seismic dampers are special devices introduced in the building to absorb the
energy provided by the ground motion to the building(much like the way shock absorbers in
motor vehicles absorb the impacts due to undulations of the road).
Base Isolation
The concept of base isolation is explained through an example building resting on frictionless
rollers(Figure 3. 7a). When the ground shakes, the rollers freely roll, but the building above does
not move. Thus, no force is transferred to the building due to shaking of the ground; simply, the
building does not experience the earthquake. Now, if the same building is rested on flexible pads
that offer resistance against lateral movements (Figure 3. 7b), then some effect of the ground
shaking will be transferred to the building above. If the flexible pads are properly chosen, the
forces induced by ground shaking can be a few times smaller than that experienced by the
building built directly on ground, namely a fixed base building(Figure 3. 7c). The flexible pads
are called base-isolators, whereas the structures protected by means of these devices are called
base-isolated buildings. The main feature of the base isolation technology isthat it introduces
flexibility in the structure. As a result, a robust medium-rise masonry or reinforced concrete
building becomes extremely flexible. The isolators are often designed to absorb energy and thus
add damping to the system. This helps in further reducing the seismic response of the building.
Several commercial brands of base isolators are available in the market, and many of them look
like large rubber pads, although there are other types that are based on sliding of one part of the
building relative to the other. A careful study is required to identify the most suitable type of
device for a particular building. Also, base isolation is not suitable for all buildings. Most
suitable candidates for base-isolation are low to medium-rise buildings rested on hard soil
underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.
Fig 3.7 Building on flexible supports shakes lesser – this technique is called Base Isolation.
Seismic Dampers
Another approach for controlling seismic damage in buildings and improving their seismic
performance is by installing seismic dampers in place of structural elements, such as diagonal
braces. These dampers act like the hydraulic shock absorbers in cars – much of the sudden jerks
are absorbed in the hydraulic fluids and only little is transmitted above to the chassis of the car.
When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the
motion of the building. Dampers were used since 1960s to protect tall buildings against wind
effects. However, it was only since 1990s, that they were used to protect buildings against
earthquake effects. Commonly used types of seismic dampers include viscous dampers(energy is
absorbed by silicone-based fluid passing between piston-cylinder arrangement), friction
dampers(energy is absorbed by surfaces with friction between them rubbing against each other),
and yielding dampers (energy is absorbed by metallic components that yield) (Figure 3.8).
Fig 3.8 Seismic Energy Dissipation Devices – each device is suitable for a certain building.
Friction Dampers
Friction dampers are designed to have moving parts that will slide over each other during a
strong earthquake. When the parts slide over each other, they create friction which uses some of
the energy from the earthquake that goes into the building.
Fig 3.20 Pall Friction Damper
This is a Pall Friction Damper installed in the Webster Library of Concordia University in
Montreal, Canada. The damper is connected to the center of some bracing. The damper is made
up from a set of steel plates, with slotted holes in them, and they are bolted together. At high
enough forces, the plates can slide over each other creating friction. The plates are specially
treated to increase the friction between them.
Viscous fluid dampers
Viscous fluid dampers are similar to shock absorbers in a car. They consist of a closed cylinder
containing a viscous fluid like oil. A piston rod is connected to a piston head with small holes in
it. The piston can move in and out of the cylinder. As it does this, the oil is forced to flow
through holes in the piston head causing friction. When the damper is installed in a building, the
friction converts some of the earthquake energy going into the moving building into heat energy.
The damper is usually installed as part of a building’s bracing system using single diagonals. As
the building sways to and fro, the piston is forced in and out of the cylinder.
Why Use Viscous Dampers?
Viscous Dampers dramatically decrease earthquake induced motion . .
(i) Less displacement . . . over 50% reduction in drift in many cases
(ii) Decreased base shear and inter-story shear, up to 40%
(iii) Much lower “g” forces in the structure. Equipment keeps working and people are not
injured
(iv) Reduced displacements and forces can mean less steel and concrete. This offsets the damper
cost and can sometimes even reduce overall cost.
CONCLUSION
When a structure vibrating. An earthquake can be resolved in any vibrating. An earthquake can
be resolved in any three mutually perpendicular directions-the two horizontal directions
(longitudinal and transverse displacement) and the vertical direction (rotation).This motion
causes the structure to vibrate or shake in all three directions; the predominant direction of
shaking is horizontal. All the structures are designed for the combined effects of gravity loads
and seismic loads to verify that adequate vertical and lateral strength and stiffness are achieved
to satisfy the structural performance and acceptable deformation levels prescribed in the
governing building code. Because of the inherent factor of safety used in the design
specifications, most structures tend to be adequately protected against vertical shaking. Vertical
acceleration should also be considered in structures with large spans, those in which stability for
design, or for overall stability analysis of structures.
In general, most earthquake code provisions implicitly require that structures be able to resist:
1. Minor earthquakes without any damage.
2.Moderate earthquakes with negligible structural damage and some non-structural damage.
3.Major earthquakes with some structural and non-structural damage but without collapse.
The structure is expected to undergo fairly large deformations by yielding in some structural
members.
To avoid collapse during a major earthquake, members must be ductile enough to absorb and
dissipate energy by post-elastic deformation. Redundancy in the structural system permits
redistribution of internal forces in the event of the failure of key elements, when the element or
system forces yields to fails, the lateral forces can be redistributed to a secondary system to
prevent progressive primary failure.
Earthquake motion causes vibration of the structure leading to inertia forces. Thus a structure
must be able to safely transmit the horizontal and the vertical inertia forces generated in the
super structure through the foundation to the ground. Hence, for most of the ordinary structures,
earthquake-resistant design requires ensuring that the structure has adequate lateral load
carrying capacity. Seismic codes will guide a designer to safely design the structure for its
intended purpose.
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