CHAPTER 1

ENGINEERING SEISMOLOGY

1.1. SEISMOLOGY

Seismologyis the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake environmental effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes (such as explosions). A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology.

1.2. ELASTIC REBOUND THEORY

The elastic rebound theory is an explanation for how energy is spread during earthquakes. As rocks on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original shape.

In geology, the elastic rebound theory was the first theory to satisfactorily explain earthquakes. Previously it was thought that ruptures of the surface were the result of strong ground shaking rather than the converse suggested by this theory.

1.2.1. EXPLAINATION OF ELASTIC REBOUND THEORY

Fig.1.1

If a road is built across the fault as in the figure panel Time 1, it is perpendicular to the fault trace at the point E, where the fault is locked. The far field plate motions (large arrows) cause the rocks in the region of the locked fault to accrue elastic deformation, figure panel Time 2. The deformation builds at the rate of a few centimetres per year, over a time period of many years. When the accumulated strain is great enough to overcome the strength of the rocks, an earthquake occurs. During the earthquake, the portions of the rock around the fault that were locked and had not moved 'spring' back, relieving the displacement in a few seconds that the plates moved over the entire inter seismic period (D1 and D2 in Time 3). The time period between Time 1 and Time 2 could be months to hundreds of years, while the change from Time 2 to Time 3 is seconds. Like an elastic band, the more the rocks are strained the more elastic energy is stored and the greater potential for an event. The stored energy is released during the rupture partly as heat, partly in damaging the rock, and partly as elastic waves. Modern measurements using GPS largely support Reids theory as the basis of seismic movement, though actual events are often more complicated.

1.3.CONTINENTAL DRIFT THEORY (PLATE TECTONICS)

According to the theory of continental drift, the world was made up of a single continent through most of geologic time. That continent eventually separated and drifted apart; forming into the seven continents we have today. The first comprehensive theory of continental drift was suggested by the German meteorologist Alfred Wegener in 1912. The hypothesis asserts that the continents consist of lighter rocks that rest on heavier crustal material similar to the manner in which icebergs float on water. Wegener contended that the relative positions of the continents are not rigidly fixed but are slowly moving at a rate of about one yard per century.According to the generally accepted plate-tectonics theory, scientists believe that Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called plate-boundary volcanoes.

The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates, are dotted with many active volcanoes that form the so-called Ring of Fire. The Ring provides excellent examples of plate-boundary volcanoes, including Mount St. Helens.

However, some active volcanoes are not associated with plate boundaries, and many of these so-called intra-plate volcanoes form roughly linear chains in the interior of some oceanic plates. The Hawaiian Islands provide perhaps the best example of an intra-plate volcanic chain, developed by the northwest-moving Pacific plate passing over an inferred hot spot that initiates the magma-generation and volcano-formation process.

Fig.1.2: Plate tectonics

This figure shows the boundaries of lithosphere plates that are active at present. The double lines indicate zones of spreading from which plates are moving apart. The lines with barbs show zones of under thrusting (subduction), where one plate is sliding beneath another. The barbs on the lines indicate the overriding plate. The single line defines a strike-slip fault along which plates are sliding horizontally past one another. The stippled areas indicate a part of a continent, exclusive of that along a plate boundary, which is undergoing active extensional, compressional, or strike-slip faulting.

1.4. ROLE OF ASTHENOSPHERE IN PLATE TECTONICS

The asthenosphere is the highly viscous, mechanically weak and ductility deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 80 and 200 km ( 50 and 124 miles) below the surface. The Lithosphere-Asthenosphere boundary is usually referred to as LAB. The asthenosphere is generally solid although some of its regions could be melted (e.g. below mid-ocean ridge). The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. For some regions, asthenosphere could extend as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt.

The asthenosphere is now thought to play a critical role in the movement of plates across the face of Earth's surface. According to plate tectonic theory, the lithosphere consists of a relatively small number of very large slabs of rocky material. These plates tend to be about 60 mi (100 km) thick and in most instances many thousands of miles wide. They are thought to be very rigid themselves but capable of being moved on top of the asthenosphere. The collision of plates with each other, their lateral sliding past each other and their separation from each other are thought to be responsible for major geologic features and events such as volcanoes, lava flows, mountain building, and deep crustal faults and rifts.

In order for plate tectonic theory to make any sense, some mechanism must be available for permitting the flow of plates. That mechanism is the semi-fluid character of the asthenosphere itself. Some observers have described the asthenosphere as the 'lubricating oil' that permits the movement of plates in the lithosphere. Others view the asthenosphere as the driving force or means of conveyance for the plates.

Geologists have now developed theories to explain the changes that take place in the asthenosphere when plates begin to diverge from or converge toward each other. For example, suppose that a region of weakness has developed in the lithosphere. In that case, the pressure exerted on the asthenosphere beneath it is reduced, melting begins to occur, and asthenospheric materials begin to flow upward. If the lithosphere has not actually broken, those asthenospheric materials cool as they approach Earth's surface and eventually become part of the lithosphere itself. On the other hand, suppose that a break in the lithosphere has actually occurred. In that case, the asthenospheric materials may escape through that break and flow outward before they have cooled. Depending on the temperature and pressure in the region, that outflow of material (magma) may occur rather violently, as in a volcano, or more moderately, as in a lave flow. Both these cases produce crustal plate divergence, or spreading apart. Pressure on the asthenosphere may also be reduced in zones of divergence, where two plates are separating from each other. Again, this reduction in pressure may allow asthenospheric materials in the asthenosphere to begin melting and to flow upward. If the two overlying plates have actually separated, asthenospheric material may flow through the separation and form a new section of lithosphere.

In zones of convergence, where two plates are moving toward each other, asthenospheric materials may also be exposed to increased pressure and begin to flow downward. In this case, the lighter of the colliding plates slides upward and over the heavier of the plates, which dives down into the asthenosphere. Since the heavier lithospheric material is more rigid than the material in the asthenosphere, the latter is pushed outward and upward. During this movement of plates, material of the down going plate is heated in the asthenosphere, melting occurs, and molten materials flow upward to Earth's surface. Mountain building is the result of continental collision in such situations, and great mountain chains like the Urals, Appalachian, and Himalayas have been formed in such a fashion. When oceanic plates meet one another, island arcs (e.g., Japan or the Aleutians) are formed. Great ocean trenches occur in places of plate convergence. In any one of the examples cited here, the asthenosphere supplies new material to replace lithospheric materials that have been displaced by some other tectonic or geologic mechanism.

1.5. TYPES OF BOUNDARIES

Divergent boundaries

Convergent boundaries

Transform boundaries

DIVERGENT BOUNDARIES

A divergent boundary occurs when two tectonic plates move away from each other. Along these boundaries, lava spews from long fissures and geysers spurt superheated water. Frequent earthquakes strike along the rift. Beneath the rift, magmamolten rockrises fromthe mantle. It oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates. Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor. Thus at divergent boundaries, oceanic crust, made of basalt, is created.

2 .CONVERGENT BOUNDARIES

When two plates come together, it is known as a convergent boundary. The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench. A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench. Powerful earthquake shake a wide area on both sides of the boundary.

TYPES OF CONVERGENT BOUNDARIES

Oceanic-Continental convergence:Here oceanic plate and continental plate converge.Ocean bed undergoes subduction due to greater density than the continental plate, when they converge. It leads to the formation of trenches.

Oceanic-Oceanic convergences:Here two oceanic plates undergo convergence. The older plate gets subducted under the younger one. This type of convergence causes volcanoes.

Continental-Continental convergence:Here two continental plates undergo convergence. Both of them being light, neither subducts. This leads to the formation of mountains.

3. TRANSFORM BOUNDARIES

Two plates sliding past each other forms a transform plate boundary. Natural or human-made structures that cross a transform boundary are offsetsplit into pieces and carried in opposite directions. Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon. As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone. In contrast to convergent and divergent boundaries, no magma is formed. Thus, crust is cracked and broken at transform margins, but is not created or destroyed.

1.6. SEISMIC WAVES

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of an earthquake, explosion, or a volcano that gives out low-frequency acoustic energy. Many other natural and anthropogenic sources create low-amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer. They can be classified into:1) Body waves2) Surface waves

Body waves

Body waves travel through the interior of the Earth. They create ray paths refracted by the varying density and modulus (stiffness) of the Earth's interior. The density and modulus, in turn, vary according to temperature, composition, and phase. This effect resembles the refraction of light waves. They can be classified into:

a) Primary Wavesb) Secondary Waves

Primary waves

Primary waves (P-waves) are compressional 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.

b) Secondary wavesSecondary waves (S-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 direction of propagation, 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.

2) Surface wavesSeismic surface waves travel along the Earth's surface. They can be classified as a form of mechanical surface waves. They are called surface waves, as they diminish as they get further from the surface. They travel more slowly than seismic body waves (P and S). In large earthquakes, surface waves can have amplitude of several centimetres. Major type of surface waves is:a) Raleigh wavesb) Love waves

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 (note, however, that the associated particle motion at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves, roughly 90% of the velocity of S waves for typical homogeneous elastic media. In the layered medium (like the crust and upper mantle) the velocity of the Rayleigh waves depends on their frequency and wavelength.

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 are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest amplitude.

Fig.1.3: Types of seismic waves

1.7. MEASUREMENT OF EARTHQUAKE

MAGNITUDE:

It is a measure of earthquake size and is determined from the logarithm of the maximum displacement or amplitude of the earthquake signal as seen on the seismogram, with a correction for the distance between the focus and the seismometer. This is necessary as the closer the seismometer is to the earthquake, the larger the amplitude on the seismogram, irrespective of the size or magnitude of the event. Since the measurement can be made from P, S or surface waves, several different scales exist, all of which are logarithmic because of the large range of earthquake energies (for example a magnitude 6 ML is 30 times larger, in terms of energy than a magnitude 5 ML). The Richter local magnitude (ML) is defined to be used for 'local' earthquakes up to 600 km away.

Surface wave magnitude (Ms) is based on the maximum amplitude of the surface wave having a period of 20 + 2 s. It is used for observations near the earthquake epicentre where the surface wave is larger than the body wave. This scale applies to any epicentral distance or type of seismograph.

Body wave magnitude (Mb) is calculated from the body waves (P, PP, S) and is usually used at larger distance from the earthquake epicentre (P-wave attenuation is less than surface waves, with distance). It can be used for any earthquake of any depth.

Moment magnitude (Mw) is considered the best scale to use for larger earthquakes as the Ms saturates at about magnitude 8. Moment magnitude is measured over the broad range of frequencies present in the earthquake wave spectrum rather than the single frequency sample that the other magnitude scales use.

INTENSITY

Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment.

Types of intensity measuring scales:Modified Mercalli Intensity (MMI)

Medvedev-Spoonheuer-Karnik

Given below is a table describing earthquakes of various magnitude and their corresponding intensities.

1.8. ISOSEISMAL MAP

In seismology, an isoseismal map is used to show lines of equal felt seismic intensity, generally measured on the Modified Mercalli scale. Such maps help to identify earthquake epicentres, particularly where no instrumental records exist, such as for historical earthquakes. They also contain important information on ground conditions at particular locations, the underlying geology, and radiation pattern of the seismic waves and the response of different types of buildings. They form an important part of the macro seismic approach, i.e. that part of seismology dealing with non-instrumental data. The shape and size of the isoseismal regions can be used to help determine the magnitude, focal depth and focal mechanism of an earthquake.

Fig.1.4: Isoseismic Map

CHAPTER 2

MAJOR EARTHQUAKES CAUSES AND DAMAGES

Introduction

Rocks are made of elastic material, and so elastic strain energy is stored in them during the deformations that occur due to the gigantic tectonic plate actions that occur in the Earth. But, the material contained in rocks is also very brittle. Thus, when the rocks along a weak region in the Earths Crust reach their strength, a sudden movement takes place there ; opposite sides of the fault (a crack in the rocks where movement has taken place) suddenly slip and release the large elastic strain energy stored in the interface rocks. For example, the energy released during the 2001 Bhuj (India) earthquake is about 400 times (or more) that released by the 1945 Atom Bomb dropped on Hiroshima!!

The sudden slip at the fault causes the earthquake.... a violent shaking of the Earth when large elastic strain energy released spreads out through seismic waves that travel through the body and along the surface of the Earth. And, after the earthquake is over, the process of strain build-up at this modified interface between the rocks starts all over again. Earth scientists know this as the Elastic Rebound Theory. The material points at the fault over which slip occurs usually constitute an oblong three-dimensional volume, with its long dimension often running into tens of kilometres.

Most earthquakes in the world occur along the boundaries of the tectonic plates and are called Inter- plate Earthquakes (e.g., 1897 Assam (India) earthquake). A number of earthquakes also occur within the plate itself away from the plate boundaries (e.g., 1993 Latur (India) earthquake); these are called Intra-plate Earthquakes. In both types of earthquakes, the slip generated at the fault during earthquakes is along both vertical and horizontal directions (called Dip Slip) and lateral directions (called Strike Slip).

Fig.2.1: Types of tectonic plate movements

Fig.2.2: Types of faults

Given below is the description of the causes and the damage caused by few major earthquakes in the world.

2.1. GORKHA EARTHQUAKE (NEPAL EARTHQUAKE)

The April 2015 Nepal earthquake (also known as the Gorkha earthquake) killed more than 8,000 people and injured more than 19,000. It occurred at 11:56 NST on 25 April, with a magnitude of 7.8Mwor 8.1 Msand a maximum Mercalli Intensity of IX (Intense). Its epicentre was the village of Barpak, Gorkha district, and its hypocentre was at a depth of approximately 15 km (9.3 mi).It was the worst natural disaster to strike Nepal since the 1934 NepalBihar earthquake.

The earthquake triggered an avalanche on Mount Everest, killing at least 19, making it the deadliest day on the mountain in history. It triggered another huge avalanche in the Langtang valley, where 250 people were reported missing.

Hundreds of thousands of people were made homeless with entire villages flattened, across many districts of the country. Centuries-old buildings were destroyed at UNESCO World Heritage sites in the Kathmandu Valley, including some at the Kathmandu Durbar Square, the Patan Durbar Square, the Bhaktapur Durbar Square, the Changu Narayan Temple and the Swayambhunath Stupa. Geophysicists and other experts had warned for decades that Nepal was vulnerable to a deadly earthquake, particularly because of its geology, urbanization, and architecture.

Fig.2.3: Location of the earthquake (2015)

GEOLOGY OF THE NEPAL EARTHQUAKE

The temblor was caused by a sudden thrust, or release of built-up stress, along the major fault line where the Indian Plate, carrying India, is slowly diving underneath the Eurasian Plate, carrying much of Europe and Asia. Kathmandu, situated on a block of crust approximately 120 km (74 miles) wide and 60 km (37 miles) long, reportedly shifted 3 m (10 ft.) to the south in just 30 seconds.

Fig.2.4: M6+ earthquakes in the Himalayan region between 1900- 2014

Fig.2.5: Movement of the Indian plate towards the Eurasian plate

Fig.2.6

Intensity

The intensity in Kathmandu was IX (Violent).Tremors were felt in the neighbouring Indian states of Bihar, Uttar Pradesh, Assam, West Bengal, Sikkim, Jharkhand, Uttarakhand in the Indian capital region around New Delhi and as far south as Karnataka. Many buildings were brought down in Bihar. Minor cracks in the walls of houses were reported in Odisha. Minor quakes were registered as far as Kochi in the southern state of Kerala. The intensity in Patna was V (Moderate).The intensity was IV (Light) in Dhaka, Bangladesh. The earthquake was also experienced across south western China, ranging from the Tibet Autonomous Region to Chengdu, which is 1,900 km (1,200 mi) away from the epicentre. Tremors were felt in Pakistan and Bhutan.

Fig.2.7: Intensity of the earthquake felt in different parts of Nepal (Mercalli Scale)

Fig.2.8: Graph obtained from the GEOFON Station, Kabul Damage

Thousands of houses were destroyed across many districts of the country, with entire villages flattened, especially those near the epicentre. The Tribhuvan International Airport serving Kathmandu was closed immediately after the quake, but was re-opened later in the day for relief operations and, later, for some commercial flights. Several temples, including Kasthamandap, Panchtale temple, the top levels of the nine-story Basantapur Durbar, the Dasa Avtar temple and two dewals located behind the Shiva Parvati temple were demolished by the quake. Some other monuments, including the Kumari Temple and the Taleju Bhawani Temple partially collapsed.

Fig.2.9: Collapse of one of the load bearing structures, Dharahara Tower in Kathmandu

Load bearing structures were typically built prior to the 1970s, and have low resistance to earthquake. The bricks are stiff and have no way to either pull the structure in the direction opposite of the sway or be ductile enough to allow for small movement in the structure. Load bearing structures exhibit instantaneous failure and fall like a pack of cards. In the recent Nepal earthquake, most structures that fell within seconds of the earthquake were load bearing structures such as the Dharahara Tower in Kathmandu.

2.2. YUSHU EARTHQUAKE, CHINA

The 2010 Yushu earthquake struck on April 14 and registered a magnitude of 6.9Mw or 7.1Ms. It originated in Yushu, Qinghai, China, at 7:49 am local time. 2,698 people have been confirmed dead, 270 missing and 12,135 injured of which 1,434 are severely injured. The epicentre was located in Rima village, Upper Laxiu Township of Yushu County, in remote and rugged terrain, near the border of Tibet Autonomous Region.

Fig.2.10: Location of the earthquake in China

Geology

Qinghai lies in the north eastern part of the Tibetan Plateau, which formed due to the on-going collision of the Indian Plate with the Eurasian Plate. The main deformation in this area is crustal shortening, but there is also a component of left lateral strike-slip faulting on major westeast trending structures such as the Kunlun and Altyn Tagh fault systems that accommodate south eastward translation of the Tibetan area.

Fig.2.11: Five major faults in China

Damage

In Qinghai, building damage was reported with no casualties in the counties of Zadoi, Nangqn, and Qumarleb of Yushu Prefecture.At least 11 schools were destroyed in the earthquake. Over 85% of buildings in Gyegu, mostly of wood-earth construction, were destroyed, leaving hundreds trapped and thousands homeless. The Changu Dam, some 15 km upstream from the Yushu County seat (apparently at 325440N 970250E), was damaged by the earthquake.

Fig.2.12: Cleft in the grassland due to the earthquake

Observations on Damaged Light Concrete Block Confined Masonry (LCBCM) Structures

Due to the high altitude and corresponding lower air pressure of the local climate condition, its hard to manufacture the clay bricks with enough strength fit for seismic resistant building construction. So, the light concrete block becomes the main substitute construction materials to the clay brick which is very popular even in other seismic areas of China. And the light concrete block (sometime with hollows) confined masonry (LCBCM) structure becomes the most popular local structural type. In the 2010 Yushu Ms7.1 earthquake event, most of this kind of building structures without seismic design and construction were severely damaged or collapsed due to the lack of seismic restrain measures and unreasonable construction. For those seismically designed and constructed LCBCM structures, they showed advantages of seismic design. The seismically designed 2-story LCBCM building showed better performance than those 4-story ones. And the residential buildings with relative smaller rooms showed higher seismic resistance than those office building and school buildings with relative larger rooms. Typically, the damage mostly concentrated in the walls of first story with lower shear capacity not enough to satisfy the earthquake demand.

2.3. Haiti Earthquake

Fig.2.13: Location of the Haiti Earthquake

TheEnriquilloPlantain Garden fault zone(EPGFZ or EPGZ) is a system of coaxial left lateral-moving strike slip faults which runs along the southern side of the island of Hispaniola, where Haiti and the Dominican Republic are located. The EPGFZ is named for Lake Enriquillo in the Dominican Republic where the fault zone emerges, and extends across the southern portion of Hispaniola through the Caribbean to the region of the Plantain Garden River in Jamaica.

Geology

The magnitude 7.0 Mwearthquake occurred inland, on 12 January 2010 at 16:53, approximately 25 km (16 mi) WSW from Port-au-Prince at a depth of 13 km (8.1 mi) on blind thrust faults associated with the Enriquillo-Plantain Garden fault system. There is no evidence of surface rupture and based on seismological, geological and ground deformation data it is thought that the earthquake did not involve significant lateral slip on the main Enriquillo fault. Strong shaking associated with intensity IX on the Modified Mercalli scale (MM) was recorded in Port-au-Prince and its suburbs. It was also felt in several surrounding countries and regions, including Cuba (MM III in Guantnamo), Jamaica (MM II in Kingston), Venezuela (MM II in Caracas), Puerto Rico (MM IIIII in San Juan), and the bordering Dominican Republic (MM III in Santo Domingo). According to estimates from the United States Geological Survey, approximately 3.5 million people lived in the area that experienced shaking intensity of MM VII to X, a range that can cause moderate to very heavy damage even to earthquake-resistant structures. Shaking damage was more severe than other quakes of similar magnitude due to the shallow depth of the quake.

The quake occurred in the vicinity of the northern boundary where the Caribbean tectonic plate shifts eastwards by about 20 mm (0.79 in) per year in relation to the North American plate. The strike-slip fault system in the region has two branches in Haiti, the Septentrional-Oriente fault in the north and the Enriquillo-Plantain Garden fault in the south; both itslocation and focal mechanism suggested that the January 2010 quake was caused by a rupture of the Enriquillo-Plantain Garden fault, which had been locked for 250 years, gathering stress. However, a study published in May 2010 suggested that the rupture process may have involved slip on multiple blind thrust faults with only minor, deep, lateral slip along or near the main EnriquilloPlantain Garden fault zone, suggesting that the event only partially relieved centuries of accumulated left-lateral strain on a small part of the plate-boundary system. The rupture was roughly 65 km (40 mi) long with mean slip of 1.8 metres (5 ft. 11 in). Preliminary analysis of the slip distribution found amplitudes of up to about 4 m (13 ft.) using ground motion records from all over the world.

There are two major faults along Hispaniola, the island shared by Haiti and the Dominican Republic. This earthquake occurred on the southern fault, the Enriquillo-Plantain Garden fault system.

There hasn't been a major quake on this system for about 200 years. That means stress has been building up there for quite some time. When the strain finally grew too large, rock along the fault failed, and released a huge burst of energy in less than a minute.Aftershocks are common after large quakes, and they continue for days, weeks and even longer though they become less frequent as time passes.

Fig.2.14: Fault zones near Haiti

Fig.2.15: Enriquillo-Plantain Garden Fault

Cause for the damage to the buildings

The walls of 90% of Haitian buildings are constructed with cement, earth, clisse (sticks, twigs and branches), bricks or stone. Contractors and builders often cut corners in construction, reducing costs by using easily available building materials such as limestone dust and unrefined sand, which produce a cheaper but weaker concrete. The vulnerabilities of these buildings were exacerbated by being constructed along hills and slopes, without proper foundations.

Not only are qualified Engineers, Architects and Contractors not readily available to most construction projects, most small commercial type construction will have other major construction problems. While the sand and gravel may not be building grade the water used to mix the concrete on the job site will probably come out of the closest canal or stream available, and will be contaminated. The purity of the sand is questionable and the gravel for aggregate is ungraded and sometimes contains rocks that are too large to be used. There is generally no use of sieves to grade the aggregate. The concrete is usually mixed on the ground and the consistency from one batch to the next is non-existent. Reinforcement bars used that are not deformed but are smooth which will not allow the concrete to bond with the reinforcement.

The only reinforcement generally in the masonry walls consists of concrete columns which are placed within the thickness of the concrete block and are usually 8 x 8. The masonry block infill walls also are not tied to the columns which allow the walls to lack integrity to act together. Often the walls at the corners are also not bonded together with either concrete block or reinforcing.

Add to these construction problems the fact that most buildings in Haiti have concrete roofs. This heavier weight works against the walls when the earth moves laterally since the roofs will have the tendency to keep moving while the rest of the building is moving back to itsoriginal position. Most of the structures that were seen ,the columns failed at the top and the bottom and the roof or second floor slabs pancaked on the floors below. Most of the structures observed, had little damage all had light weight roof structures.

Fig.2.16: Damage due to improper mixing of concrete and lack of adhesion between the steel rods and the cement.

Fig.2.17: Majority population living in slums close to the tectonically active region.Houses mostly built with inferior material and heavy roofs with weak walls and columns.

2.4. TOHOKU EARTHQUAKE (JAPAN)

The 2011 earthquake off the Pacific coast of Thoku was a magnitude 9.0 (Mw) undersea mega thrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday 11 March 2011, with the epicentre approximately 70 kilometres (43 mi) east of the Oshika Peninsula of Thoku and the hypocentre at an underwater depth of approximately 30 km (19 mi). The earthquake is also often referred to in Japan as the Great East Japan earthquake and also known as the 2011 Tohoku earthquake, and the 3.11 earthquake. It was the most powerful earthquake ever recorded to have hit Japan, and the fourth most powerful earthquake in the world since modern record-keeping began in 1900.The earthquake triggered powerful tsunami waves that reached heights of up to 40.5 metres (133 ft.) The earthquake moved Honshu (the main island of Japan) 2.4 m (8 ft.) east and shifted the Earth on its axis by estimates of between 10 cm (4 in) and 25 cm (10 in),and generated sound waves detected by the low-orbiting GOCE satellite.

Fig.2.18: Location of earthquake in Japan

Fig.2.19: A seismogram recorded in Massachusetts, USGeology

This earthquake was a repetition of the mega thrust mechanism of the earlier 869 Sanriku earthquake, which had an estimated magnitude of 8.3, and also created a large tsunami. This earthquake occurred where the Pacific Plate is subducting under the plate beneath northern Honshu; The Pacific plate, which moves at a rate of 8 to 9 cm (3.1 to 3.5 in) per year, dips under Honshu's underlying plate building large amounts of elastic energy. This motion pushes the upper plate down until the accumulated stress causes a seismic slip-rupture event. The break caused the sea floor to rise by several meters. A quake of this magnitude usually has a rupture length of at least 500 km (300 mi) and generally requires a long, relatively straight fault surface. Because the plate boundary and subduction zone in the area of the Honshu rupture is not very straight, it is unusual for the magnitude of its earthquake to exceed 8.5; the magnitude of this earthquake was a surprise to some seismologists.

This energy of the seismic waves from the earthquake was surface energy (Me) 1.9 0.51017 joules,[62] which is nearly double that of the 9.2-magnitude 2004 Indian Ocean earthquake and tsunami that killed 280,000 people. If harnessed, the seismic energy from this earthquake would power a city the size of Los Angeles for an entire year.

The largest fault slip ever recorded produced the devastating 2011 Japan tsunami.Experts calculate the faultor the boundary between two tectonic platesin the Japan Trench slipped by as much as 164 feet (50 meters). Other similarly large magnitude earthquakes, including the 9.1 Sumatra events in 2004, resulted in a 66-to-82 foot (20-to-25 meter) slip in the fault.

Fig.2.20

Fig.2.21: Fault lines

Damage

The degree and extent of damage caused by the earthquake and resulting tsunami were enormous, with most of the damage being caused by the tsunami. Japan's National Police Agency said on 3 April 2011, that 45,700 buildings were destroyed and 144,300 were damaged by the quake and tsunami. The damaged buildings included 29,500 structures in Miyagi Prefecture, 12,500 in Iwate Prefecture and 2,400 in Fukushima Prefecture. The north eastern ports of Hachinohe, Sendai, Ishinomaki and Onahama were destroyed, while the Port of Chiba (which serves the hydrocarbon industry) and Japan's ninth-largest container port at Kashima were also affected, though less severely.

Fig.2.22: Aerial view of Minato, devasted by both the earthquake & subsequent Tsunami

2.5. GUJARAT EARTHQUAKE

The 2001 Gujarat earthquake occurred on 26 January; India's 52nd Republic Day, at 08:46 AM local time and lasted for over 42 seconds. The epicentre was about 9 km south-southwest of the village of Chobari in Bhachau Taluka of Kutch District of Gujarat, India. The earthquake reached 7.7 on the moment magnitude scale and had a maximum felt intensity of X (Intense) on the Mercalli intensity scale. The earthquake killed around 20,000 people (including 18 in Southeast Pakistan), injured another 167,000 and destroyed nearly 400,000 homes

Geology

Gujarat lies about 400 km from the plate boundary between the Indian Plate and the Eurasian Plate, but the current tectonics is still governed by the effects of the continuing continental collision along this boundary. During the break-up of Gondwana in the Jurassic, this area was affected by rifting with a roughly west-east trend. During the collision with Eurasia the areahas undergone shortening, involving both reactivation of the original rift faults and development of new low-angle thrust faults. The related folding has formed a series of ranges, particularly in central Kutch. The focal mechanism of most earthquakes is consistent with reverse faulting on reactivated rift faults. The pattern of uplift and subsidence associated with the 1819 Rann of Kutch earthquake is consistent with reactivation of such a fault. The 2001 Gujarat earthquake was caused by movement on a previously unknown south-dipping fault, trending parallel to the inferred rift structures.

Fig.2.23: Continental collision between the Indian and the Eurasian plates.

Fig.2.24Damage

Most residential buildings in Ahmedabad suffered some type of damage during the earthquake. Much of the damage was in the form of cracking of infill wall panels at the ground floor level. However, nearly one hundred residential buildings collapsed during January 26, 2001 event. Since for the ground motion experienced in the city, buildings with sound design and construction should not have experienced any structural damage (although some non-structural damage may be expected), the damage appears to be due to a combination of factors. Based on the post-earthquake field investigation, following appear to be the technical reasons for the observed damage.

Soft-Story System:A large number of residential buildings in the city have open ground floors leading to soft first story. Besides the elevator core, there are few walls, if any to provide lateral resistance. The upper floor frames are usually filled with un-reinforced brick masonry forming a very stiff lateral load resisting system. Most of the collapses and significant damage occurred in this type of soft-story buildings. It is well known from observations after past earthquakes in California and as well as after the recent Turkey earthquake that this type of building construction is highly vulnerable to earthquakes. Nearly all the deformation occurs in the columns in the soft-story, with rest of the building going for a ride during the earthquake. If these columns are not designed to accommodate these large deformations, they may fail leading to catastrophic failure of the entire building, as was the case in most of the buildings that collapsed in the city.

Soil Conditions:The localized soil conditions also contributed to the collapse of many buildings. A thick alluvial deposit along the Sabarmati River underlies the City of Ahmedabad. Although a cursory analysis of location of building collapses would indicate no particular pattern, a careful analysis reveals that most of the buildings that collapsed lie along the old path of Sabarmati River. This becomes apparent when location of collapsed buildings are plotted on the city map and compared with the satellite image of the city. Note that the path of most of the building that collapsed in areas west of the Sabarmati River are closely aligned with the old path of the river, visible in the satellite image as a small loop of faint thick white line just west of the present river path. The south, southeast of the city, especially the Mani Nagar area, where additional collapses were observed falls between two lakes, indicating the presence of either poor soil conditions or possibly construction on non-engineered fills.

2.6. SUMATRA EARTHQUAKE

On December 26, 2004, at 07:58:50 local time, a powerful earthquake, moment magnitude (MW) 9.2, occurred in the Indian Ocean. The Sumatra-Andaman earthquake was one of the three largest earthquakes ever recorded. The fault rupture propagated 1,300 to 1,600 kilometres northwest for about 10 minutes along the boundary between the Indo-Australian plate and the Eurasian plate, from northwest Sumatra to the Nicobar Islands and to the Andaman Islands. The hypocentre, the point where the fault rupture originated, was 10 kilometres deep. The faulting spread up dip and down dip from 18 to 25 meters on a low-angle thrust fault plane dipping about 10 degrees northeast. The Indo-Australian plate moved northeast relative to the Eurasian plate.

Geology

The epicentre of the earthquake was about 250 kilometres off the west coast of Aceh Province. Strong to violent shaking in Aceh Province reportedly lasted five to six minutes. Banda Aceh was the only major city that experienced earthquake-shaking damage. One- to two-story, traditional, concrete-frame and wood-frame buildings survived well and were largely undamaged by the strong ground shaking. However, because the earthquake occurred a significant distance offshore, the resulting long-period ground motions caused serious damage to, or the collapse of, buildings more than three stories high.

The fault rupture uplifted the ocean floor, releasing the most destructive series of tsunami waves in recorded history. The waves spread throughout the Indian Ocean, causing damage in the coastal communities of 12 countries. By far, the most damaging effects were sustained by Aceh Province, where three devastating waves struck the western shore within about 30 minutes. The tsunami waves ranged from 4 to 39 meters high and destroyed more than 250 coastal communities.Residential neighbourhoods and fishing villages in coastal areas were entirely devastated, and houses were swept inland or out to sea. The traditional construction that had resisted shaking damage could not resist the tsunami forces and most were obliterated.

Fig.2.25lli

Fig.2.26

Fig.2.27

Damage

Most well designed and well-constructed buildings and industrial facilities that had withstood the earthquake shaking also withstood the tsunami waves and suffered only minor damage. For example, the La Farge Cement Plant, a well-designed and well-constructed steel-frame series of industrial structures about 20 kilometres southwest of Banda Aceh, did not experience structural damage from the strong shaking and was not damaged by the tsunami waves, which, as documented by stadia-rod, reached a wave-flow height of 38.9 meters nearby. Several one- and two-story administrative buildings and machine shops were smashed by waves carrying nearly empty large oil-storage tanks. The impact of the waves caused non-structural damage to some of the buildings. For example, metal siding was stripped from the steel-frame buildings up to the height of the waves.

Roads and Bridges:Roads and bridges were devastated by the force of the tsunami waves. Many bridges were swept off their supports, and connecting earth embankments were significantly scoured, disabling the transportation network for hundreds of kilometres along the west coast of Aceh Province. Hundreds of bridges were picked up and swept inland by the tsunami waves, some more than a kilometre. The extensive damage to bridges severely constrained rescue and relief efforts, as the bridges had been vital links to population centres in the region. Many of the bridges on the coastal road to Meulaboh were destroyed and washed away, and sections of the road disappeared, which isolated many small communities. Survivors could be reached only by boat or helicopter. In addition, the destruction of the bridges resulted in the disruption of the electric distribution system at bridge crossings.

Liquefaction:Although earlier reconnaissances reported no evidence of liquefaction, earthquakes of this magnitude and duration commonly cause liquefaction in coastal areas. During a reconnaissance by helicopter, we observed extensive liquefaction in near-shore beach deposits for at least 150 kilometres along the Aceh coast, from south of Meulaboh to north of Calang.

CHAPTER 3

EFFECT OF STRUCTURAL IRREGULARITIES ON RC BUILDINGS DURING EARTHQUAKE

3.1 VERTICAL IRREGULARITIES

Irregular buildings make up a large portion of the urban infrastructure. The presence of irregularities can be due to architectural, functional, and economical constraints. The main objective of this research is to improve the understanding of the seismic behaviour of building structures with vertical irregularities. This is done by quantifying the effects of vertical irregularities in mass, stiffness, or strength on seismic demands. The seismic demands investigated are elastic story shears, overturning moments, and drifts as well as inelastic drift and energy dissipation demands.

Types of vertical irregularities

1) Soft story (stiffness discontinuity):Exists in a story where the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above.

Fig.3.1

2) Weak story (strength discontinuity)Exists in a story where the strength is less than 80% of that in the story above.

Fig.3.2

3) Geometric Irregularities:These exist where the horizontal dimension of the lateral-force-resisting system is more than 130% of that in an adjacent story. (Penthouses excluded).

Fig.3.3

4) Mass Irregularity:These exist where the effective mass of any story is more than 150% of the affective mass of the adjacent story. (A lighter roof mass excepted).

Fig.3.45) Vertical Discontinuities:These exist where shear walls or rigid infill walls or frame elements are not continuous to the foundations, thus threatening to impart large overturning forces onto columns.

3.2. HORIZONTAL IRREGULARITIES

Torsional Irregularity for systems- diaphragms not flexible. "Torsional irregularity shall be considered to exist when the maximum story drift, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.2 times the average of the story drifts at the two ends of the structure."

Fig.3.5

Re-entrant Corners- both projections of the structure beyond a re-entrant corner are greater than 15 per cent of the plan dimension of the structure in the given direction.

Fig.3.6

Diaphragm Discontinuity- cut-out or opens areas greater than 50 per cent of the gross enclosed diaphragm area or changes in effective diaphragm stiffness of more than 50 per cent from one story to the next.

Fig.3.7

Nonparallel Systems- not parallel or symmetric about the major orthogonal axes.

Fig.3.8

CHAPTER 4

REPAIR AND RETROFITTING

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centres, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries (US, Japan etc.) and late 1970s for many other parts of the world (Turkey, China etc.), many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research works has been carried out

Strategies

Seismic retrofit (or rehabilitation) strategies have been developed in the past few decades following the introduction of new seismic provisions and the availability of advanced materials (e.g. fibre-reinforced polymers (FRP), fibre reinforced concrete and high strength steel).Retrofit strategies are different from retrofit techniques, where the former is the basic approach to achieve an overall retrofit performance objective, such as increasing strength, increasing deformability, reducing deformation demands while the latter is the technical methods to achieve that strategy, for example FRP jacketing.

Increasing the global capacity (strengthening): This is typically done by the addition of cross braces or new structural walls. Reduction of the seismic demand by means of supplementary damping and or use of base isolation systems. Increasing the local capacity of structural elements. This strategy recognises the inherent capacity within the existing structures, and therefore adopts a more cost-effective approach to selectively upgrade local capacity (deformation/ductility, strength or stiffness) of individual structural components.

Selective weakening retrofit:This is a counter intuitive strategy to change the inelastic mechanism of the structure, while recognising the inherent capacity of the structure. Allowing sliding connections such as passageway bridges to accommodate additional movement between seismically independent structures.

Techniques

Base isolatorsBase isolation is a collection of structural elements of a building that should substantially decouple [disambiguation needed] the building's structure from the shaking ground thus protecting the building's integrity and enhancing its seismic performance. This earthquake engineering technology, which is a kind of seismic vibration control, can be applied both to a newly designed building and to seismic upgrading of existing structures. Normally, excavations are made around the building and the building is separated from the foundations. Steel or reinforced concrete beams replace the connections to the foundations, while under these, the isolating pads, or base isolators, replace the material removed. While the baseisolation tends to restrict transmission of the ground motion to the building, it also keeps the building positioned properly over the foundation. Careful attention to detail is required where the building interfaces with the ground, especially at entrances, stairways and ramps, to ensure sufficient relative motion of those structural elements.

There are two basic types of isolation systems. The system that has been adopted most widely in recent years is typified by the use of elastomeric bearings, the elastomer made of either natural rubber or neoprene. In this approach, the building or structure is decoupled from the horizontal components of the earthquake ground motion by interposing a layer with low horizontal stiffness between the structure and the foundation. This layer gives the structure a fundamental frequency that is much lower than its fixed-base frequency and also much lower than the predominant frequencies of the ground motion. The first dynamic mode of the isolated structure involves deformation only in the isolation system, the structure above being to all intents and purposes rigid. The higher modes that will produce deformation in the structure are orthogonal to the first mode and consequently also to the ground motion. These higher modes do not participate in the motion, so that if there is high energy in the ground motion at these higher frequencies, this energy cannot be transmitted into the structure. The isolation system does not absorb the earthquake energy, but rather deflects it through the dynamics of the system. This type of isolation works when the system is linear and even when undamped; however, some damping is beneficial to suppress any possible resonance at the isolation frequency.

The second basic type of isolation system is typified by the sliding system. This works by limiting the transfer of shear across the isolation interface. Many sliding systems have been proposed and some have been used. In China there are at least three buildings on sliding systems that use specially selected sand at the sliding interface. A type of isolation containing a lead-bronze plate sliding on stainless steel with an elastomeric bearing has been used for a nuclear power plant in South Africa. The friction-pendulum system is a sliding system using a special interfacial material sliding on stainless steel and has been used for several projects in the United States, both new and retrofit construction.

Fig.4.1: Elastomeric Base Isolator

Fig.4.2: Sliding Dampers

Friction DampersFriction 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.

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.

Fig.4.3:Friction Dampers

Supplementary dampersSupplementary dampers absorb the energy of motion and convert it to heat, thus "damping" resonant effects in structures that are rigidly attached to the ground. In addition to adding energy dissipation capacity to the structure, supplementary damping can reduce the displacement and acceleration demand within the structures. In some cases, the threat of damage does not come from the initial shock itself, but rather from the periodic resonant motion of the structure that repeated ground motion induces. In the practical sense, supplementary dampers act similarly to Shock absorbers used in automotive suspensions.

Fluid viscous dampersFluid viscous damping is a way to add energy dissipation to the lateral system of a building structure. A fluid viscous damper dissipates energy by pushing fluid through an orifice, producing a damping pressure which creates a force. These damping forces are 90 degrees out of phase with the displacement driven forces in the structure. This means that the damping force does not significantly increase the seismic loads for a comparable degree of structural deformation.The addition of fluid viscous dampers to a structure can provide damping as high as 30% of critical, and sometimes even more. This provides a significant decrease in earthquake excitation. The addition of fluid dampers to a structure can reduce horizontal floor accelerations and lateral deformations by 50% and sometimes more.

Fluid Viscous Dampers DescriptionThe fluid viscous damper for structures, is similar in action to the shock absorber on an automobile, but operates at a much higher force level. Structural dampers are significantly larger than automotive dampers, and are constructed of stainless steel and other extremely durable materials as required to furnish a life of at least 40 years. The damping fluid is silicone oil, which is inert, non-flammable, non-toxic, and stable for extremely long periods of time. The seals in the fluid viscous damper use a patented high technology design based on aerospace research, and provide totally leak free service. This design has been proven through rigorous testing and has been in use for over 40 years in both military and commercial applications.

Fig.4.4: Fluid Viscous Dampers

Yielding DampersAnother 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.

Fig.4.5: Types of Dampers

Tuned mass dampersTuned mass dampers (TMD) employ movable weights on some sort of springs. These are typically employed to reduce wind sway in very tall, light buildings. Similar designs may be employed to impart earthquake resistance in eight to ten story buildings that are prone to destructive earthquake induced resonances.

Adhoc addition of structural support/reinforcementThe most common form of seismic retrofit to lower buildings is adding strength to the existing structure to resist seismic forces. The strengthening may be limited to connections between existing building elements or it may involve adding primary resisting elements such as walls or frames, particularly in the lower stories.

Typical Retrofit Scenario & Solution

Soft-story failureIn many buildings the ground level is designed for different uses than the upper levels. Low rise residential structures may be built over parking garages which have large doors on one side. Hotels may have tall ground floors to allow for a grand entrance or ballrooms. Office buildings may have stores in the ground floor which desire continuous windows for display.

Traditional seismic design assumes that the lower stories of a building are stronger than the upper stories and where this is not the caseif the lower story is less strong than the upper structurethe structure will not respond to earthquakes in the expected fashion. Using modern design methods, it is possible to take a weak story into account. Several failures of this type in one large apartment complex caused most of the fatalities in the 1994 Northridge earthquake.

Typically, where this type of problem is found, the weak story is reinforced to make it stronger than the floors above by adding shear walls or moment frames. Moment frames consisting of inverted U bents are useful in preserving lower story garage access, while a lower cost solution may be to use shear walls or trusses in several locations, which partially reduce the usefulness for automobile parking but still allow the space to be used for other storage.

Beam-column joint connectionsBeam-column joint connections are a common structural weakness in dealing with seismic retrofitting. Prior to the introduction of modern seismic codes in early 1970s, beam-column joints were typically non-engineered or designed. Laboratory testing has confirmed the seismic vulnerability of these poorly detailed and under-designed connections. Failure of beam-column joint connections can typically lead to catastrophic collapse of a frame-building, as often observed in recent earthquakes.

For reinforced concrete beam-column joints - various retrofit solutions have been proposed and tested in the past 20 years. Philosophically, the various seismic retrofit strategies discussed above can be implemented for reinforced concrete joints. Concrete or steel jacketing has been a popular retrofit technique until the advent of composite materials such as Carbon fibre-reinforced polymer (FRP). Composite materials such as carbon FRP and aramic FRP have been extensively tested for use in seismic retrofit with some success. One novel technique includes the use of selective weakening of the beam and added external post-tensioning to the joint in order to achieve flexural hinging in the beam, which is more desirable in terms of seismic design.

Shear failure within floor diaphragmFloors in wooden buildings are usually constructed upon relatively deep spans of wood, called joists, covered with a diagonal wood planking or plywood to form a subfloor upon which the finish floor surface is laid. In many structures these are all aligned in the same direction. To prevent the beams from tipping over onto their side, blocking is used at each end, and for additional stiffness, blocking or diagonal wood or metal bracing may be placed between beams at one or more points in their spans. At the outer edge it is typical to use a single depth of blocking and a perimeter beam overall.If the blocking or nailing is inadequate, each beam can be laid flat by the shear forces applied to the building. In this position they lack most of their original strength and the structure may further collapse. As part of a retrofit the blocking may be doubled, especially at the outer edges of the building. It may be appropriate to add additional nails between the sill plate of the perimeter wall erected upon the floor diaphragm, although this will require exposing the sill plate by removing interior plaster or exterior siding. As the sill plate may be quite old and dry and substantial nails must be used, it may be necessary to pre-drill a hole for the nail in the old wood to avoid splitting. When the wall is opened for this purpose it may also be appropriate to tie vertical wall elements into the foundation using specialty connectors and bolts glued with epoxy cement into holes drilled in the foundation.Sliding off foundation and "cripple wall" failure

Fig.4.6: House slid of foundation.

Single or two story wood-frame domestic structures built on a perimeter or slab foundation are relatively safe in an earthquake, but in many structures built before 1950 the sill plate that sits between the concrete foundation and the floor diaphragm (perimeter foundation) or stud wall (slab foundation) may not be sufficiently bolted in. Additionally, older attachments (without substantial corrosion-proofing) may have corroded to a point of weakness. A sideways shock can slide the building entirely off of the foundations or slab.

Often such buildings, especially if constructed on a moderate slope, are erected on a platform connected to a perimeter foundation through low stud-walls called "cripple wall" or pin-up. This low wall structure itself may fail in shear or in its connections to itself at the corners, leading to the building moving diagonally and collapsing the low walls. The likelihood of failure of the pin-up can be reduced by ensuring that the corners are well reinforced in shear and that the shear panels are well connected to each other through the corner posts. This requires structural grade sheet plywood, often treated for rot resistance. This grade of plywood is made without interior unfilled knots and with more, thinner layers than common plywood. New buildings designed to resist earthquakes will typically use OSB (oriented strand board), sometimes with metal joins between panels, and with well attached stucco covering to enhance its performance. In many modern tract homes, especially those built upon expansive (clay) soil the building is constructed upon a single and relatively thick monolithic slab, kept in one piece by high tensile rods that are stressed after the slab has set. This post stressing places the concrete under compression - a condition under which it is extremely strong in bending and so will not crack under adverse soil conditions.

Reinforced concrete column burst

Fig.4.7: Jacketed and grouted column on left, unmodified on right

Reinforced concrete columns typically contain large diameter vertical rebar (reinforcing bars) arranged in a ring, surrounded by lighter-gauge hoops of rebar. Upon analysis of failures due to earthquakes, it has been realized that the weakness was not in the vertical bars, but rather in inadequate strength and quantity of hoops. Once the integrity of the hoops is breached, the vertical rebar can flex outward, stressing the central column of concrete. The concrete then simply crumbles into small pieces, now unconstrained by the surrounding rebar. In new construction a greater amount of hoop-like structures are used.

One simple retrofit is to surround the column with a jacket of steel plates formed and welded into a single cylinder. The space between the jacket and the column is then filled with concrete, a process called grouting. Where soil or structure conditions require such additional modification, additional pilings may be driven near the column base and concrete pads linking the pilings to the pylon are fabricated at or below ground level. In the example shown not all columns needed to be modified to gain sufficient seismic resistance for the conditions expected. (This location is about a mile from the Hayward Fault Zone.)

1.8. GUIDELINES FROM IS 13935: 1993 FOR REPAIR AND RETROFITTING

GENERAL PRINCIPLES AND CONCEPTS

Non-structural/Architectural Repairs:

The buildings affected by earthquake may suffer both non-structural and structural damages.Non-structural repairs may cover the damages to civil and electrical items including the services in the building. Repairs to non-structural components need to be taken up after the structural repairs are carried out. Care should be taken about the connection details of architectural components to the main structural components to ensure their stability.

Non-structural and architectural components get easily affected/ dislocated during the earthquake. These repairs involve one or more of the following:Patching up of defects such as cracks and fall of plaster;

Repairing doors, windows, replacement of glass panes;

Checking and repairing electric conduits/wiring;

Checking and repairing gas pipes, water pipes and plumbing services.

Re-building non-structural walls, smoke chimneys, parapet walls, etc;

Re-plastering of walls as required;

Rearranging disturbed roofing tiles;

h) Relaying cracked flooring at ground level; andi) Redecoration - white washing, painting etc.

The architectural repairs as stated above do not restore the original structural strength ofstructural components in the building and anyattempt to carry out only repairs to architectural/non-structural elements neglecting therequired structural repairs may have seriousimplications on the safety of the building. Thedamage would be more severe in the event ofthe building being shaken by the similar shockbecause original energy absorption capacityof the building would have been reduced.

2) Structural Repairs:

Prior to taking up of the structural repairs and strengthening measures, it is necessary to conduct detailed damage assessment to determine:The structural condition of the building to decide whether a structure is amendable for repair; whether continued occupation is permitted; to decide the structure as a whole or a part require demolition, if considered dangerous;

If the structure is considered amendable for repair then detailed damage assessment of the individual structuralcomponents (mapping of the crack pattern, distress location; crushed concrete, reinforcement bending/yielding, etc.). Non-destructive testing techniques could be employed to determine the residual strength of the members; and

To work out the details of temporary supporting arrangement of the distressed members so that they do not undergo further distress due to gravity loads.

After the assessment of the damage of individual structural elements, appropriate repair methods are to be carried out component wise depending upon the extent of damage. Therepair may consist of the following:Removal of portions of cracked masonry walls and piers and rebuilding them in richer mortar. Use of non-shrinking mortar will be preferable.

Addition of reinforcing mesh on both faces of the cracked wal1, holding it to the wall through spikes or bolts and then covering it, suitably, with cement mortar or micro-concrete.

Injecting cement or epoxy like material which is strong in tension, into the cracks in walls.

The cracked reinforced cement elements may be repaired by epoxy grouting and could be strengthened by epoxy or polymer mortar application like shotcreting, jacketing, etc.

Seismic Strengthening:

The main purpose of the seismic strengtheningis to upgrade the seismic resistance of adamaged building while repairing so that itbecomes safer under future earthquake occurrences. This work may involve some of thefollowing actions:Increasing the lateral strength in one or both directions by increasing column and wall areas or the number of walls and columns.

Giving unity to the structure, by providing a proper connection between its resisting elements, in such a way that inertia forces generated by the vibration of the building can be transmitted to the members that have the ability to resist them. Typical important aspects are the connections between roofs or floors and walls, between intersecting walls and between walls and foundations.

Eliminating features that are sources of weakness or that produce concentration of stresses in some members. Asymmetrical plan distribution of resisting members, abrupt changes of stiffness from one floor to the other, concentration of large masses and large openings in walls without a proper peripheral reinforcement are examples of defects of this kind.

Avoiding the possibility of brittle modes of failure by proper reinforcement and connection of resisting members.

4) Seismic Retrofitting:

Many existing buildings do not meet the seismic strength requirements of present earthquake codes due to original structural inadequacies and material degradation due to time or alterations carried out during use over the years. Their earthquake resistance can be upgraded to the level of the present day codes by appropriate seismic retrofitting techniques.

Strengthening or Retrofitting vs. Reconstruction:

Replacement of damaged buildings or existing unsafe buildings by reconstruction is, generally, avoided due to a number of reasons, the main ones among them being:Higher cost than that of strengthening or retrofitting,

b) Preservation of historical architecture, andc) Maintaining functional, social and cultural environment.

In most instances, however, the relative cost of retrofitting to reconstruction cost determines the decision. As a thumb rule, if the cost of repair and seismic strengthening is less than about 50 per cent of the reconstruction cost, the retrofitting is adopted. This may also require less working time and much less dislocation in the living style of the population. On the other hand reconstruction may offer the possibility of modernization of the habitat and may be preferred by well-to-do communities.

Cost wise the building construction including the seismic code provisions in the first instance, works out the cheaper in terms of its own safety and that of the occupants. Retrofitting an existing inadequate building may involve as much as 4 to 5 times the initial extra expenditure required on seismic resisting features. Repair and seismic strengthening of a damaged building may even be 5 to 10 times as expensive. It is therefore very much safe as well as cost-effective to construct earthquake resistant buildings at the initial stage itself according to the relevant seismic IS codes.

CHAPTER 5IS 1893(PART 1) : 2002

5.1 DESIGN SPECTRUM

For the purpose of determining seismic forces, the country is classified into four seismic zones. The design horizontal seismic coefficient Ahfor a structure shall be determined by the following expression:

Ah=Z I Sa2 R G

Provided that for any structure with T