Earthquake engineering

A Full Detailed Chapter of Earthquake Engineering for Civil Engineering Studentss

1. Earthquake Engineering Prepared By : Parth Desani - 116470306049

2. INDEX Earthquake Terminology Causes of Earthquake Forecasting of Earthquake Seismic zones of India Classification of Earthquake Factors affecting Earth Plate Magnitude and intensity Effects of earthquake Principles of planning of buildings Masonry construction aspects of earthquake resistance Earthquake resistant features Repairing and Retrofication of earthquake damaged structures Suggestions for construction of new masonry buildings in earthquake sensitive areas I.S. codes for earthquake resistant design

3. 1. Earthquake Terminology Focus : The point within the earth where earthquake rupture starts is called focus or hypocentre . It is the source of elastic waves inside the earth.

4. Epicentre : The point on the earth’s surface vertically above the focus of the earthquake is called epicentre . Focal depth : The depth of the focus from the epicentre is called focal depth. It is an important parameter in determining the damaging potential of earthquake. Most of the damaging earthquake have shallow focus with focal depth less than 70 km. Epicentral distance : The distance from the epicentre to any point if interest in the surface of the earth is called epicentral distance or focal distance.

5. Foreshocks : A number of smaller size earthquake take place before and after a big earthquake. Those occuring before the big one are called foreshocks. Aftershocks : Smaller size earthquakes occuring after the mainshocks are cakked aftershocks. Focal region : Seismic destruction propagates from the focus through a limited region of the surrounding earth’s body, which is called the focal region.

6. Isoseismal line : A contour or a line on a map joining points of equal intensity for a particular earthquake is called isoseismal line. Homoseismal line : The line joining location at which the shock arrives simultaneiusly is known as the homoseismal line. Meisoseismal : The region that suffers the strong shaking and significant damage during earthquake is termed as meisoseismal region. The region surrounding the epicentre is the meisoseimal region. Aseismic : A fault on which no earthquake have been observed is called aseismic .

7. Earthquake : Momentary shaking of the ground or vibration of the ground caused by the slip or by volcanic or magnetic activity in the earth are called earthquakes. Seismology : The science related with study of earthquake and the structure of the earth, is known as seismology. It includes the study of seismic waves, origin, intensity of earthquake, forecasting, etc. Seismicity : The geographic and historical distribution of earthquakes is known as seismicity. Seismograph : A seismograph is an instrument used to measure the vibration of the earth. It records earthquake ground motion in a particular direction as a function of time.

8. Seismogram : It is a record of seismograph in response to ground motion produced by an earthquake. Seismogram is used for the following purpose : To determine epicentre of the earthquake. For obtaining the seismic parameters which are used in design of structures and also for identifying seismic zones. They also helps us in studying seismic waves and their nature which helps in assessing the severity of EQ. Seismoscope : This is a simple siesmograph that records earthquakes ground motion on a paper without time marks. Such instruments provide only the maximum extent of motion during an earthquake. Seismometer : In the most modern seismograph an electric transducer referred to as seismometer, senses the motion and produces an analog electrical signal for subsequent processing.

9. Accelerometer : These are the instruments having electronic transducers that produce an output voltage proportional to ground acceleration during earthquake, i.e. it measures ground acceleration. Accelerogram : The motion of the ground can be described in terms of displacement, velocity or acceleration. The vibration of ground acceleration with time recorded at a point on ground during an earthquake is called an acceleration.

10. Displacement meter : The instrument that measures the displacement of ground is known displacement meter. Seismic gap : A section of a fault that has produced earthquake in the past but is now quite, is called seismic gap. Seismic zone : An area of similar seismic activities. i.e. The Himalaya Zone. Earthquake size : Earthquake size is defined in terms of two things : Magnitude Intensity Earthquake size is a measure of the quantitative and qualitative effects of vibration produced by the earthquake.

11. 2. Causes of Earthquake Momentary shaking of the ground or vibrations or oscillations of the ground caused by the slip or by volcanic or magmatic activity or other sudden changes in the earth are called earthquakes. Seismic Sources Natural sources Man made sources 1. Tectonic earthquakes 1. Controlled sources 2. Volcanic earthquakes i . Chemical explosives 3. Plutonic earthquakes ii. Nuclear explosives 4. Land slides 2. Reservoir induced earthquakes 5. Collapse of cavity 3. Mining induced earthquakes 4. Cultural noise ( Industry, Traffic, etc.)

12. Tectonic earthquake : The sudden release of strain energy by rupture of the rock at plate boundry is the primary cause of the seismic activity around the world. About 90% of all earthquakes result from tectonic events. Volcanic earthquake : Shallow volcanic earthquakes may result from sudden shifting or movement of magma. Plutonic earthquake : Plutonic earthquakes are caused by deep seated changes. Land slides : Massive landslides associated with the volcanic activity produce significant ground motion.

13. 3. Forecasting of Earthquake Natural calamities like heavy rain, flood, cyclone, tsunami, etc. can be predicted in advance. But, no scientific technique is available for prediction of earthquake. The prediction of earthquake is almost impossible. The scientists make prediction of earthquake based on various analysis and assumption. Various predictions are : From the study of location of past earthquake, intensity, time duration, geological condition, etc. Abrupt change in waterl level in ponds, lakes, etc. From the study of changes in ground water level From ground tilting study From the study of main shocks, fore shocks, after shocks. Ground slope changes Increase in volume of rocks, level of random gas level in deep wells and in electric conductivity of rocks. From strange behavior of animals and birds.

14. Latest technique for earthquake forecasting : All the seismograph stations can be connected to on line data collection system and by using computer digital seismograph may be connected to V-set system to warn people few seconds in advance of earthquake. There are three types of earthquake waves, namely P-waves, S-waves and surface waves.

15. P-waves are the fastest having speed 6 km/s, S-waves 3.5 km/s and surface waves 2 km/s. P-waves reach the seismograph station first, while surface waves being slowest reach at last. The surface waves are the most destructing waves. If emergency alarm is attached to the forecasting system, it can warn the people few seconds in advance to escape out of their houses. This type of system is already working in Japan and USA.

16. Strong Ground Motion : Shaking of ground on earth’s surface is a net consequence of motion, vertical and horizontal, caused by seismic waves generated by energy release at each material point within the three dimensional volume that ruptures at the fault. These waves arrives at various instant of time, have different amplitudes and carry different levels of energy. Thus, the motion at any site on ground is random in nature with its amplitude and direction varying randomly with time. Large earthquakes at great distances can produce weak motion that may not damage structures or even felt by humans. But, sensitive instrument can record these. This makes it possible to locate distant earthquake. However, from engineering viewpoint, strong motions that can possibly damage structures are of interest. This can happen with earthquakes in the vicinity or even with large earthquakes at reasonable medium to large distances.

17. Characteristics to strong ground motion : The motion of the ground can be described in terms of displacement, velocity or acceleration. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram . The ground velocity and displacement can be obtained by direct integration of an acceleration. The nature of accelerograms may vary depending on energy released at source, type of slip at fault rupture, geology along the travel path from fault rupture to the earth’s surface, and local soil.

18. These accelerograms carry distinct in formation regarding ground shaking like, Peak amplitude Duration of strong shaking Frequency content Energy content These characteristics of strong motion in the vicinity of causative fault is strongly dependent on the nature of faulting. The motion depends on source parameters such as fault shape, its area, maximum fault dislocation, stress drop and distance of fault plane from ground surface. The elastic properties of the material through which the seismic waves travel, also influence the strong motion. The amplitude of ground acceleration decreases with increasing distance from the causative fault in general.

19. 4. Seismic zones of India

20. Basic Geography and Tectonic features : India lies at the northwestern end of the Indo-Australian plate, which encompasses India, Australia, a major portion of the Indian ocean and other smaller countries. This plate is colliding against the huge Eurasian plate and going under the Eurasian plate; this process of one tectonic plate getting under another is called subduction . A sea Tethys, separated these plates before they collided. Part of the lithosphere, the earth’s crust, is covered by oceans and the rest by the continents. Due to this subduction process, the great Himalayas rising about 2.5 cm per year.

21. Three chief tectonic sub-region of India are the mighty Himalayas along the north, the plains of the Ganges and other rivers, and the Peninsula. The Himalayas consist primarily of sediments accumulated over long geological time in the Tethys. The Indo- Gangetic basin with deep alluvium is a great depression caused by the local of the Himalayas on the continent. The peninsular part of the country consist of ancient rocks deformed in the past Himalayas – like collisions. Before the Himalayan collision, several tens of millions of years ago, lava flowed across the central part of peninsular India leaving layers of basalt rock. Coastal areas like Kachchh show marine deposits testifying to submerged under the sea millions of years ago.

22. Need for seismic zoning : A number of significant earthquakes have occurred in and around India over the past century. Some of these occurred in populated urban areas and hence caused great damage. Many went unnoticed, as they occurred deep under the earth’s surface or in relatively un- inhabitated place. Most earthquake occur along the Himalayan plate boundry but a number of earthquake have also occurred in the Peninsular region. The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zones map is required to identify these regions. Based on the levels of intensities sustained during damaging past earthquakes, the 1970 version of the zone map subdivided India into five zones – I, II, III, IV & V.

23. The maximum Modified Mercalli (MM) intensity of seismic shaking, expected in these zones were V or less, VI, VII, VIII and IX and higher respectively. Parts of Himalayan boundry in the north and northest and the Kachchh area in the west were classified as zone V. The seismic zone maps are revised from time to time as more understanding is gained on the geology, the seismotectonics and the seismic activity in the country. The Indian standards provided the first seismic zone map in 1962, which was later revised in 1967 and again in 1970.

24. The map has been revised again in 2002, and it now has only four seismic zones – II, III, IV, and V. Zone – I has been merged into zone – II. Madras which was earlier in zone – II has been included in zone – III. Revised seismic zone map has been given in IS : 1893 (part – I) – 2002 and reproduced here above fig. Imported cities situated in zone – V : Bhuj , Imphal , Guwahati , Kohima , Tezpur , Mandi , Srinagar, Darbhanga Imported cities situated in zone – IV : Delhi, Amritsar, Ambala , Chandigarh, Patna, Simla , Ludhiyana , Roorkee

25. 5. Classification of Earthquake The various basis of classification of earthquakes are as under : Based on location : Based on location the earthquake are classified as, 1. Interplate earthquake : Most earthquake in the world occur along the boundaries of the tectonic plated and are called interplate earthquakes. About 99 % earthquakes are interplate earthquakes. e.g. 1897 – Assam earthquake 1905 – Kangra (H.P.) 1950 – Assam earthquake

26. 2. Intraplate earthquake : A number of earthquakes also occur within the plate itself away from the plate boundaries, are called intraplate earthquake. About 1% earthquakes are intraplate earthquakes. e.g. 1993 – Lature earthquake 1967 – Koyna earthquake Based on focal depth : Based on focal depth the earthquakes are classified as below : Earthquake Focal depth Shallow earthquake < 70 km Mediam earthquake 70 to 300 km Deep earthquake >300 km

27. c) Based on size or Magnitude : Earthquake are often classified into different groups based on their size as given in table below : Group Magnitude Annual Average Number Great 8 and higher 1 Major 7 – 7.9 18 Strong 6 - 6.9 120 Moderate 5 – 5.9 800 Light 4 – 4.9 6200 (estimated) Minor 3 – 3.9 49000 (estimated) Very minor < 3.9 M2.3 – 1000/day, M1.2 – 8000/day

28. d) Based on epicentral distance : Based on epicentral distance, the earthquakes are classified as under : 1. Local earthquake - < 1° 2. Regional earthquake – 1° – 10° 3. Teleseismic earthquake - > 10°

29. 6. Factors affecting earth plate Internal Factors : Movements of lava below earth crust Vapour pressure Movement of tectonic plates Processes between rocks External factors : Effects of air, water, sunlight Attraction of moon and son on the earth Changes in atmospheric moisture Increase in difference between minimum and maximum atmospheric temperature Atmospheric changes like extreme cold, fog, odd season rainfall, etc. Cyclonic effect Changes in rise and set timings for the sun Irregular monsoons Global warming Changes in sea water level, etc.

30. 7. Magnitude and Intensity Magnitude Intensity Magnitude of an earthquake is a measure of amount of energy released during and earthquake. It is the quantitative measure of the actual size of the earthquake. For a particular earthquake magnitude is same for all the places. It is more precise measure of earthquake. The intensity of earthquake is a measure of the actual ground shaking at a location during an earthquake. It is the qualitative measure of the size if the earthquake. For a particular earthquake intensity of earthquake decreases with distance from the epicenter. It is less precise than magnitude

31. 8. Effects of earthquake Primary effects Secondary effects Primary Effects : Effects related with origin of earthquake are known as primary effects. Primary effects are directly connected with geology and topography. Primary effects are : Change in topography Formation of new hills Change in direction of existing water course Formation of new water course Wrapping of strata Formation of sand dyke Formation of huge cracks in land Change in under ground water level

32. Secondary Effects : Secondary effects are caused due to passage of seismic waves and are associated with ground shaking. Secondary effects are : Destruction of human lives Destruction of multistoried buildings Destruction of dams and bridges Landslides and mudslides Uprooting of trees Psychological effects on human beings Worst effect on communication system Damage to road and railway lines Destruction of telephone and TV tower Huge waves in the sea (Tsunami) Fire by damaging gas lines and snapping electric wires Rupture of damage and levees causing floods Liquefaction of soil and sinking of structure

33. 9. Principles of planning of buildings Building configurations : Configuration requirements of a structure : From a planner point of view, as a precursor to the design analysis, the configuration requirements preferred as far as possible are listed below. The structure should, Be simple and symmetrical. Be not too elongated in plan or elevation. Have uniform and continuous distribution of strength, mass and stiffness, so that centre of mass and centre of stiffness are close to each other. Be without re-entrant corners. Have sufficient ductility. Be preferably without large projections. Be without external elevator shafts and staircase wells as they are undesirable and tend to act on their own in earthquakes. Have horizontal members which form hinges before the vertical members. Have stiffness related to the soil properties.

34. Effects of irregularities : The most important cause of damage of RC building configuration. A building that lacks symmetry and has discontinuity in geometry, mass or load resisting members is called a irregular building. The irregularity in a building result in obstruction to flow of inertia forces and cause a lot of damage to the building. Similarly, a symmetry of buildings causes large torsional moments resulting in damage to the structure. The irregularities in a building are of two types : Vertical Irregularities Horizontal Irregularities

35. Vertical irregularities : Irregularity in stiffness and strength Floating columns Mass irregularities Vertical geometric irregularities Horizontal Irregularities : These irregularities are caused due to : Asymmetrical plan shapes Re-entrant corners Cut – out (Large openings) Non – parallel system

36. 10. Masonry construction aspects of earthquake resistance Plan of building : Shape of building should be easy and symmetry. If the shape is not symmetry earthquake can produces torsion. Place and size should be also symmetry of doors and windows. Earthquake resistivity of a simple rectangular building is more. In plan projection in particular directions should nor be more than L/3 or B/3. Length of block should not more than 3B. Divide large buildings into small blocks by placing 30 to 40 mm separation joints, which can act as expansion joints.

37. Masonry Mortar : Category Range of α h A 0.04 to less than 0.05 B 0.05 to 0.06 (both inclusive ) C More than 0.06 but less than 0.08 D 0.08 to less than 0.12 E More than 0.12 Category Proportion of ingredients A M2 (cement – sand 1:6) or M3 (lime – cinder 1:3) or richer B, C M2 (cement – lime sand 1:2:9 or cement – sand 1:6) or richer D, E H2 (cement – sand 1:4) or M1 (cement – lime sand 1:1:6) or richer

38. Wall dimension and building height : Wall thickness of 1 storey building should net be more than 1 brick (190 mm). In 3 storey building wall thickness of ground floor should be at least 1 ½ brick and wall thickness of top floor should be at least 1 brick. Wall thickness should be more than 1/16 times of total length between two cross walls. The masonry bearing walls can be built up to a maximum of 4 storeys .

39. Wall opening : Opening in wall should be small and at center. Opening from internal corners should be at ¼ height or opening, which should not be less than 60 cm. Opening should not be more than, 1 storey = length of wall between two cross wall 50 % 2 storey = 42 % 3 storey = 33 % Horizontal distance between two openings, should not less than half of height of short opening or 60 cm. Vertical distance should not less than half of width of short opening or 60 cm.

40. When the given criteria about openings are not followed, secure it by placing 2 diameter bars.

41. b1 + b2 + b3 < 0.5 l1 for one storey, 0.42 l1 for two storey, 0.33 l1 for three storey b6 + b7 < 0.5 l2 for one storey, 0.42 l2 for two storey, 0.33 l2 for three storey b4 > 0.5 h2 but not less than 60 cm. b5 > 0.25 h1 but not less than 60 cm. h3 > 60 cm or 0.51 b2 or b9 whichever is more.

42. 11. Earthquake Resistant Features Horizontal Reinforcement in Walls : Horizontal Bands : Horizontal bands are the most important earthquake – resistant feature in masonry buildings. The bands are provided to hold a masonry building as a single unit by typing all the walls together, and are similar to 4 closet belt provided around cardboard boxes. These bands are provided continuous through all the load bearing walls at plinth, lintel, roof and gable level.

43. Plinth band : Where the soil is soft or includes irregular properties, plinth band is required. It also works as D.P.C. Lintel band : This is more important band, which is required in all walls at lintel level. Roof band : Roof band can be provided at the instant below to the R.C.C. roof or floor. Gable band : The triangle portion of gable end of masonry can be considered as gable band.

44. Sections and reinforcement of bands : Grade of concrete should not less than M15. Thickness of should be more than 75 cm. Two longitudinal bars of 8 mm diameter and lateral ties at 6 mm diameter @ 150 mm c/c should be placed.

45. Dowel bars at corners and junctoins : As a suppliment to the bands, steel dowel bars may be used at corners and T – junctions to integrate the box action of walls. Dowel bars are placed in every fourth course, or at about 50 cm intervals and taken into the walls to sufficient length so as to provide full bond strength. As an alternative, strengthening of T – junction and corner can be done by introducing wire mesh. These bars must be laid in 1:3 cement-sand-mortar with a minimum cover of 10mm.

46. Vertical reinforcement in walls :

47. 12. Repairing and Retrofication of Earthquake damaged structure Repair, Restoration and Retrofication : Repair : Service the non-structural damages like cracks and surface finishing after the earthquake is called repairing Restoration : Gain the strength of structures after damaging due to earthquake is called restoration. Restrengthening can be done by using epoxy resin, crack filling by injection, cement mortar, etc. Retrofication : The strengthening process of structure through it structure can resist future earthquakes is called retrofication .

48. Enlistment of checking earthquake damaged structure : Column and beam at corner Column and beam at outside Expanded cantilever, balcony Wall and column of stair and lift Upper storied column, beam and its joints Water tank, partition wall, filler wall Drainage connection, water connection and electricity connection.

49. 13. Suggestions for new construction in earthquake sensitive areas Appoint only licensed engineer for construction work Check the maximum bearing capacity of soil before starting the foundation Take suggestions of experts before start work in fine sand, black cotton soil Prefer the rectangle shape of building Use light weight materials for roofing work Thickness of load transfer wall should not less than 23 cm Use the materials as per IS criteria. If possible choose branded materials and tested materials for work The cement sand mortar proportion should not less than 1:4

50. Use bricks with minimum compressive strength of 35 kg/sq.cm Building should be of 4-5 stories, which height should not be more than 15 m To strength the walls use reinforcement Space for door – window and opening keep less as possible Prefer framed structure in place of load bearing structure Construct shear walls in multi-storied buildings Provide expansion joints in large buildings Provide R.C.C. bands at plinth level, sill level, lintel level and roof level

51. 14. I.S. codes for Earthquake resistant design IS 1893-1984 , criteria for EQ resist design of structures IS 1893-Part I-2002 IS 4326-1993 , EQ resistant design and construction of building IS 13827-1993 , Improving EQ resistance of earthen buildings IS 13828-1993 , Improving EQ resistance of masonry buildings IS 13920-1993 , Ductility, Detailing or Reinforced Concrete subject to seismic forces IS 13295-1993 , Repair and Seismic strengthening of buildings SP 22-1982 , Explantory handbook on codes for EQ engineering