The Earth and its Interior Long time ago, a large collection of material masses coalesced to form the Earth. A large amount of heat was generated by this fusion, and slowly as the Earth cooled down, the heavier and denser materials sank to the center and the lighter ones rose to the top. The differentiated Earth consists of the Inner Core (radius ~1290 km ), the Outer Core (thickness ~2200 km ), the Mantle (thickness ~2900 km ) and the Crust (thickness ~5 to 40 km ). Figure 1 shows these layers. The Inner Core is solid and consists of heavy metals ( e.g., nickel and iron), while the Crust consists of light materials ( e.g., basalts and granites).
Basic Difference: Magnitude versus Intensity Magnitude of an earthquake is a measure of its size. For instance, one can measure the size of an earthquake by the amount of strain energy released by the fault rupture. This means that the magnitude of the earthquake is a single value for a given earthquake. On the other hand, intensity is an indicator of the severity of shaking generated at a given location. Clearly, the severity of shaking is much higher near the epicenter than farther away. Thus, during the same earthquake of a certain magnitude, different locations experience different levels of intensity ( e.g., Figure 14). To elaborate this distinction, consider the analogy of an electric bulb (Figure 15). The illumination at a location near a 100-Watt bulb is higher than that farther away from it. While the bulb releases 100 Watts of energy, the intensity of light (or illumination, measured in lumens ) at a location depends on the wattage of the bulb and its distance from the bulb. Here, the size of the bulb ( 100-Watt ) is like the magnitude of an earthquake, and the illumination at a location like the intensity of shaking at that location. Suggestion: Can student think of any other analogies. e.g. ripples formed by dropping a stone into a pond………? Magnitude and Intensity in Seismic Design One often asks: Can my building withstand a magnitude 7.0 earthquake? But, the M7.0 earthquake causes different shaking intensities at different locations, and the damage induced in buildings at these locations is different. Thus, it is particular levels of intensity of shaking that buildings and structures are designed to resist, and not so much the magnitude. Buildings are designed as per the intensity, since intensity can vary place to place, for a given magnitude. The peak ground acceleration ( PGA ), i.e., maximum acceleration experienced by the ground during shaking, is one way of quantifying the severity of the ground shaking. Approximate empirical correlations are available between the MM intensities and the PGA that may be experienced. For instance, during the 2001 Bhuj earthquake, the area enclosed by the isoseismal VIII is thought to have experienced a PGA of about 0.25-0.30g . Now strong ground motion records from seismic instruments are relied upon to quantify destructive ground shaking. These records are critical for cost-effective earthquake-resistant design.
Seismic Waves Large strain energy released during an earthquake travels as seismic waves in all directions through the Earth’s layers, reflecting and refracting at each interface ( Figure 16). The two types of waves are body and surface waves.
Seismic Zones of India The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zone map is required so that buildings and other structures located in different regions can be designed to withstand different level of ground shaking. The seismic zone map of 1984 subdivided India into five zones – I, II, III, IV and V (Figure 12). The maximum Modified Mercalli (MM) intensity of seismic shaking expected in these zones were V or less , VI , VII , VIII , and IX and higher , respectively. Parts of Himalayan boundary in the north and northeast, and the Kachchh area in the west were classified as zone V. The seismic zone maps are revised from time to time as more understanding is gained on the geology, the seismotectonics and the seismic activity in the country. For instance, the Koyna earthquake of 1967 occurred in an area classified in zone I as per map of 1966. The 1970 version (same as Figure 12) of code upgraded the area around Koyna to zone IV. The Killari (Latur) earthquake of 1993 occurred in zone I. The current Indian seismic zone map (Figure 13) places this area in zone III. The zone map now has only four seismic zones – II, III, IV and V. The areas falling in seismic zone I in the 1984 map were merged with those of seismic zone II. Also, the seismic zone map in the peninsular region is modified; Madras now comes under seismic zone III as against zone II in 1984 map. The national Seismic Zone Map presents a large-scale view of the seismic zones in the country. Local variations in soil type and geology cannot be represented at that scale. Therefore, for important projects, such as a major dam or a nuclear power plant, the seismic hazard is evaluated specifically for that site. Also, for the purposes of urban planning, metropolitan areas are microzoned. Seismic microzonation accounts for local variations in geology, local soil profile, etc. Suggestion: Ask the students to indicate in which zone their birthplace is located.
Prominent Past Earthquakes in India: A number of significant earthquakes occurred in and around India over the past century (Figure 11). Some of these occurred in populated and urbanized areas, and hence caused great damage. Many went unnoticed, as they occurred deep under the Earth’s surface or in relatively un-inhabited places. Some of the damaging and recent earthquakes are listed in Table 1. Most earthquakes occur along the Himalayan plate boundary (these are inter-plate earthquakes), but a number of earthquakes have also occurred in the peninsular region (these are intra-plate earthquakes). Four Great earthquakes (M>8) occurred in a span of 53 years from 1897 to 1950; the January 2001 Bhuj earthquake (M7.7) is almost as large. Each of these caused disasters, but also allowed us to learn about earthquakes and to advance earthquake engineering. For instance, 1819 Cutch Earthquake produced an unprecedented ~3m high uplift of the ground over 100km (called Allah Bund). The 1897 Assam Earthquake caused severe damage up to 500km radial distances; the type of damage sustained led to improvements in the intensity scale from I-X to I-XII. Extensive liquefaction of the ground took place over a length of 300km (called the Slump Belt) during 1934 Bihar-Nepal earthquake in which many buildings and structures went afloat, when the ground liquefied.
Buildings with one of their overall sizes much larger or much smaller than the other two, or very large buildings, do not perform well during earthquakes.
This strategy is often used. It also is helpful in controlling movements caused by shrinkage and temperature effects. The width of separation joints to prevent the separated blocks from pounding into each other during a quake is covered in a following lecture.
Setbacks introduce structural notches. Under the influence of horizontal loads, these notches cause local areas of very high stress in the region of the notch that leads to structural damage.
Base Isolation The concept of base isolation is explained through an example building resting on frictionless rollers (Figure 6). When the ground shakes, the rollers roll freely, but the building above does not move. It remains stationary. No force is transferred to the building due to shaking of the ground; simply, the building does not experience the earthquake . Unfortunately, under wind load the building will move and impact against the end of the pit.
Seismic isolation is a relatively recent and evolving technology. It has been in increased use since the 1980s, and has been well evaluated and reviewed internationally. Base isolation has now been used in numerous buildings in countries like Italy, Japan, New Zealand, and USA. Base isolation is also useful for retrofitting important buildings (like hospitals and historic buildings ). By now, over 1000 buildings across the world have been equipped with seismic base isolation. In India, base isolation technique was first demonstrated after the 1993 Killari (Maharashtra) Earthquake [EERI, 1999]. Two single storey buildings (one school building and another shopping complex building) in newly relocated Killari town were built with rubber base isolators resting on hard ground. Both were brick masonry buildings with concrete roof . After the 2001 Bhuj (Gujarat) earthquake, the four-storey Bhuj Hospital building was built with the base isolation technique (Figure 8).
Earthquake resistant buildings for Architecture students
CT.Lakshmanan Assistant Professor (Selection Grade) School of Architecture & Interior Design SRM University Elementary Seismology Prepared by CT.Lakshmanan
The Vulnerability Profile - India <ul><li>59% of land mass prone to earthquakes </li></ul><ul><li>40 million hectares ( 8% ) of landmass prone to floods </li></ul><ul><li>8000 Km long coastline with two cyclone seasons </li></ul><ul><li>Hilly regions vulnerable to avalanches/ landslides /Hailstorms/cloudburst </li></ul><ul><li>68% of the total area susceptible to drought </li></ul><ul><li>Different types of manmade Hazards </li></ul><ul><li>Tsunami threat </li></ul><ul><li>1 million houses damaged annually + human, economic, social and other losses </li></ul>Prepared by CT.Lakshmanan
Hazard, vulnerability & disaster Disaster = F (Hazard, Vulnerability) Prepared by CT.Lakshmanan
Ingredients of Risk <ul><li>Capacity - “resources, means and strengths which exist in households and communities and which enable them to cope with, withstand, prepare for, prevent, mitigate or quickly recover from a disaster” </li></ul><ul><li>H x V - C = R </li></ul><ul><li>Hazard x vulnerability – capacity = risk </li></ul><ul><ul><li>H - potential threat to humans and their welfare </li></ul></ul><ul><ul><li>V - exposure and susceptibility to loss of life or dignity </li></ul></ul><ul><ul><li>C - available and potential resources </li></ul></ul><ul><ul><li>R - probability of disaster occurrence </li></ul></ul>Prepared by CT.Lakshmanan
Disaster Prevention, Mitigation & preparedness <ul><li>Prevention requires the elimination of risk while mitigation is the reduction of risk .. </li></ul><ul><li>Disaster Preparedness : Forecast and take precautionary measures in advance of an imminent threat. </li></ul>Prepared by CT.Lakshmanan
Seismology <ul><li>The term ‘Seismology’ is derived from Greek word Seismo, which means earthquake and logos means science ; hence the Seismology is Science of Earthquakes </li></ul><ul><li>Seismology can be defined in two ways: </li></ul><ul><li>1. The science of earthquakes and the physics of the earth’s interior </li></ul><ul><li>2. The science of elastic wave (seismic waves) </li></ul>Prepared by CT.Lakshmanan
Source: from internet Prepared by CT.Lakshmanan
Fault <ul><li>A fault is nothing but a crack or weak zone inside the Earth. When two blocks of rock or two plates rub against each other along a fault, they don’t just slide smoothly. </li></ul><ul><li>As the tectonic forces continue to prevail, the plate margins exhibit deformation as seen in terms of bending, compression, tension and friction. The rocks eventually break giving rise to an earthquake , because of building of stresses beyond the limiting elastic strength of the rock. </li></ul>Prepared by CT.Lakshmanan
<ul><li>DEPTH OF FOCUS </li></ul><ul><li>Shallow focus Earthquakes </li></ul><ul><ul><li>< 70 km deep </li></ul></ul><ul><li>Intermediate focus earthquakes </li></ul><ul><ul><li>70 km ~ 300 km </li></ul></ul><ul><li>Deep focus earthquakes </li></ul><ul><ul><li>> 300 km </li></ul></ul>Prepared by CT.Lakshmanan
Magnitude Vs Intensity <ul><li>The magnitude of an earthquake is determined instrumentally and is more objective measure of its size </li></ul><ul><li>Intensity of an earthquake is a subjective parameter based on assessment of visible effects. It depends on factors other than the actual size of the earthquake </li></ul>Prepared by CT.Lakshmanan
M > 8 Great Very great 7 - 7.9 Major Great 6 - 6.9 Strong Moderate 5 - 5.9 Moderate Moderate 4 - 4.9 Light Slight 3 - 3.9 Minor Slight M < 3 Micro earthquake EARTHQUAKE MAGNITUDE CLASS USGS IMD Prepared by CT.Lakshmanan
Magnitude Annual Average No. M > 8 2 7 - 7.9 20 6 - 6.9 100 5 - 5.9 3000 4 - 4.9 15,000 3 - 3.9 >100,000 GLOBAL EARTHQUAKE OCCURRENCE Prepared by CT.Lakshmanan
Earthquake Design Philosophy Prepared by CT.Lakshmanan
<ul><li>IMPORTANT CONSIDERATIONS TO MAKE A BUILDING EARTHQUAKE RESISTANT </li></ul><ul><li>Configuration </li></ul><ul><li>Ductility </li></ul><ul><li>Quality control </li></ul><ul><li>Base Isolation </li></ul><ul><li>Passive Energy Dissipating Devices </li></ul><ul><li>Active Control Systems </li></ul>Prepared by CT.Lakshmanan
A terminally ill patient , however effective the medication, may eventually die. Similarly, a badly configured building Cannot be engineered for an improved performance beyond a certain limit. 1. Configuration Prepared by CT.Lakshmanan
Regular Configuration <ul><li>Regular configuration is seismically ideal. These configurations have low heights to base ratio, symmetrical plane, uniform section and elevation and thus have balanced resistance. </li></ul>These configurations would have maximum torsional resistance due to location of shear walls and bracings. Uniform floor heights, short spans and direct load path play a significant role in seismic resistance of the building. Prepared by CT.Lakshmanan
Irregular Configuration Buildings with irregular configuration Buildings with abrupt changes in lateral resistance Buildings with abrupt changes in lateral stiffness Prepared by CT.Lakshmanan
Discontinuity in diaphragm Stiffness Discontinuity in Diaphragm Stiffness Prepared by CT.Lakshmanan FLEXIBLE DIAPHRAGM R I G I D D I A P H R A G M O P E N Vertical Components of Seismic Resisting System
Out of plane Offsets Shear Wall Out-of-Plane Offset in Shear Wall Non-parallel system Prepared by CT.Lakshmanan Shear walls
Ductility L et us first understand how different materials behave. Consider white chalk used to write on blackboards and steel pins with solid heads used to hold sheets of paper together. Yes… a chalk breaks easily !! On the contrary, a steel pin allows it to be bent back-and-forth . Engineers define the property that allows steel pins to bend back-and-forth by large amounts, as ductility ; chalk is a brittle material. Prepared by CT.Lakshmanan
The currently adopted performance criteria in the earthquake codes are the following: i. The structure should resist moderate intensity of earthquake shaking without structural damage. ii. The structure should be able to resist exceptionally large intensity of earthquake shaking without collapse. Prepared by CT.Lakshmanan
<ul><li>The strength of brittle construction materials, like masonry and concrete, is highly sensitive to the </li></ul><ul><li>quality of construction materials </li></ul><ul><li>workmanship </li></ul><ul><li>3. supervision </li></ul><ul><li>4. construction methods </li></ul>Prepared by CT.Lakshmanan
Quality control special care is needed in construction to ensure that the elements meant to be ductile are indeed provided with features that give adequate ductility. Thus, strict adherence to prescribed standards of construction materials and construction processes is essential in assuring an earthquake-resistant building. Prepared by CT.Lakshmanan
El ements of good quality control. 1. Regular testing of construction materials at qualified laboratories (at site or away) 2. P eriodic training of workmen at professional training houses, and 3. On-site evaluation of the technical work Prepared by CT.Lakshmanan
IS CODES IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision) IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision) IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces Prepared by CT.Lakshmanan
While Hazards Are Inevitable, Each Hazard Need Not Convert Into A Disaster… As What Comes In Between Is The Culture of Safety And Prevention Let us Work Together to Build a Culture of Prevention ! Prepared by CT.Lakshmanan