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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, India Email: [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, India Email: [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 Dail y Beast ’s rundown of t he most powerful , deadl i est eart hquakes 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 8.5magnitude 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 faraway 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 3. Major earthquakes with negligible structural damage and some non-structural damage. 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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