Independent Natural Phenomenon Of Vibration Biology Essay

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The earthquake, considered as an independent natural phenomenon of vibration of the ground, in very few cases poses a threat to humans, as for instance when it causes major landslides or tidal waves known as tsunamis. The earthquake becomes a dangerous phenomenon only when it is considered in relation to structures. Of course, the problem is the structure under seismic excitation and not the earthquake itself. This is because the structural system is designed basically for gravity loads and not for the horizontal inertia loads that are generated due to ground accelerations during an earthquake. Consequently, the earthquake has become a problem for humans since they started building. Since the early steps of the technological development of mankind the joy of creation was associated with the fear that some superior force would destroy in a few seconds what was built with great effort over a lifetime.

Although destructive, earthquakes are confined to certain geographical areas, seismic zones, and large-scale damage that cause in densely populated areas and the number of deaths are such that they have an impact on the whole world (George G. Penelis and Andreas J. Kappos, Earthquake-resistant concrete structures, E & FN SPON, USA, (1997)).

In the light of the requirements of the present study, the predominant objective of this chapter is to introduce the basic concepts of engineering seismology that have proven to be most valuable in traditional realms of application.

Therefore, and in order to pave the way for the conclusions of the particular work, the appropriate theoretical foundation will be laid including such matters as the nature, causes and effects of earthquakes and their relation to other processes in the earth, the kinds of earthquakes and their intensity and distribution. Herein, in an effort to characterize seismic behaviour by synthesizing between the rigor of the laws of dynamics and their natural aspects, a valuable insight is provided.

2.2 Acquaintance with Earthquakes

An earthquake may be defined technically as a shaking of the earth's surface caused by a sudden disturbance of the elastic equilibrium of the rock masses in or beneath the crust of the earth followed by a rapid release of energy (Macelwane James B., S.J., When the Earth Quakes, Science and Culture Series, The Bruce Publishing Company, Milwaukee, USA, (1947)). At the earth's surface, earthquakes may manifest themselves by a shaking or displacement of the ground and sometimes tsunamis, which may lead to loss of life and destruction of property. These effects range from economical to structural to mental. An earthquake only occurs for a few brief moments; the aftershocks can continue for weeks; the damage can continue for years.

2.2.1 Earthquake Mechanism

From a scientific point of view, an earthquake is a rupture within the earth caused by stress. Earthquakes occur principally by the sudden displacement on faults, when there is a build-up of stress in the crust caused by plate movement at a subduction zone or other fault lines. The earth's crust is part of a collection of well-defined crustal plates that grind past each other, under and over each other and recede from each other. A fracture occurs when the stress increases to beyond the strength of the brittle lithospheric rock. Most of this motion can be explained by the theory of plate tectonics, which explains that an outermost sphere (lithosphere) is divided into a number of relatively rigid plates that collide with, separate from, and translate past one another at their boundaries. The disruption produced at the boundaries between plates results in earthquakes. The origin of earthquakes is ultimately the jostling between moving plates which produces the strain within the lithosphere that must be relieved by earthquakes (http://www.umich.edu/~gs265/society/earthquakes.htm). The world-wide tectonic plates are presented in Figure 2.1.

Figure 2.1: World-wide tectonic plates (University of Twente, 2010). (http://www.itc.nl/Images/News/2010/March/chile_earthquake/tectonic-plates_web.png).

2.2.2 Earthquake Origin

Every day, a serious number of earthquakes are recorded by special instruments, called seismometers that measure motions of the ground. Unfortunately, large earthquakes can destroy anything from the shaking, creating tsunami, dam failure and ground failure such as liquefaction or landslides, which are described later.

Presage of a large earthquake can be considered from foreshocks that occur before the main shock. Besides of foreshocks, smaller quakes generated by adjustments following a major earthquake can be detected, known as aftershocks (Tarbuck, Edward J and Frederick K. Lutgens.  Earth: An Introduction to Physical Geology. Prentice Hall: New Jersey, 1996). Ground motions caused by very distant earthquakes are called teleseisms. Using such ground motion records from around the world, seismologists can identify a point from which the earthquake's seismic waves apparently originated (http://mceer.buffalo.edu/connected_teaching/lessons/aboutEQengineering.pdf). According to the abovementioned, earthquakes arise due to forces within the earth's crust tending to displace one mass of rock relative to another. When these forces reach a critical level, failure in the rock occurs at points of weakness called fault planes and a sudden movement occurs, which gives rise to violent motions at the earth's surface. The failure starts from a point on the fault plane called the focus, and propagates outwards until the forces in the rock mass are dissipated to a level below the failure strength of the rock. The fault plane may be hundreds of kilometers long in large earthquakes and tens of kilometers deep. In a large earthquake the fault plane is likely to break up to the surface, but in smaller events it remains completely buried (Booth Edmund and Key David, Earthquake design practice for buildings, Published by Thomas Telford, 2nd edition, London, (2006)). The location on the surface directly above the focus is known as the epicentre. The description of an earthquake's location is graphically demonstrated in Figure 2.2.

Figure 2.2: Definitions of earthquake sources location

2.3 Seismicity of the World

According to British Columbia Institute of Technology (2009), although earthquakes are unpredictable, there is a pattern to them and they occur in particular regions. There are basically three main earthquake areas: the circum-Pacific seismic belt, the Alpide and the mid-Atlantic Ridge. Of these zones, the first one has the most earthquakes, accounting for 81% of the biggest quakes (ibid), including one in Peru in 1970 which caused 70,000 deaths. The Alpide zone has been home to quakes causing the most damage, including Iran in 1968 (11,000 lives) and Turkey in 1970 and 1971 (1000 lives each time). "The remaining shocks are scattered in various areas of the world. Over 10,000 large earthquakes with magnitude greater than 5.5 recorded in the period 1977-1992 are plotted and illustrated in Figure 2.3 " (http://www.ldeo.columbia.edu/LCSN/Eq/Global/seismicity.html).

Figure 2.3: World Seismicity during 1977-1992. Focal Depth of earthquakes are plotted with colours; green = intermediate depth, red = deep and black = shallow events (http://www.ldeo.columbia.edu/LCSN/Eq/Global/seismicity.html).

2.4 Classification of Earthquakes

In relation to the above general descriptions, earthquakes may be categorized in several ways. An earthquake is said to be natural if the disturbance and the consequent mass movements which give rise to the elastic vibrations or waves are caused by natural processes in the earth. On the other hand, it said to be artificial if the disturbance is caused by man, as through a blast of explosives. Natural earthquakes are perceptible or imperceptible accordingly as the vibrations are felt by human beings, or can be detected only by suitable instruments. They are local, near, or distant according to the geographical location of their source relative to the observer. Perceptible earthquakes are slight, string, violent, or catastrophic according to the intensity of the vibrations and the extent of the damage caused by them. These classifications have been introduced merely for purposes of description and convenience. They are obviously not meant to be exclusive. Hence, the same earthquake will be near to one observer and distant from another. A catastrophic earthquake will grade off to imperceptibility with distance from the source. Natural earthquakes are also classified as shallow, normal, or deep depending on the vertical position of their source in relation to the surface of the earth (Macelwane James B., S.J., When the Earth Quakes, Science and Culture Series, The Bruce Publishing Company, Milwaukee, USA, (1947)). Moreover, and since many phenomena give rise to earthquakes (volcanic activity, explosions, collapse of cave roofs, and so on) a further classification is justified. By far, the most important category from an engineering standpoint is the one of tectonic origin, in other words, the earthquakes associated with large-scale strains in the crust of the earth. This is so because of the frequency of tectonic earthquakes, the energy they liberate, and the extent of areas they affect (Newmark Nathan M. and Rosenblueth Emilio, Fundamentals of Earthquake Engineering, Civil Engineering and Engineering Mechanics Series, Prentice-Hall, Inc., USA, (1971)).

2.4.1 Tectonic Earthquakes

The word "tectonic" derives from the Greek word "τέκτων" which means a builder. Therefore, the earthquakes that involve a sudden deformation of the earth's crust by faulting or warping are by definition structural in character. This class probably also includes most of the shocks of less intensity, for in the majority of slight shocks it can be found that: (1) The area of greatest intensity often lies along a known fault zone; (2) they occur far from any volcano; (3) even in the neighborhood of an active volcano, they are often not correlated with any particular sign of volcanic activity; (4) they often associate themselves in groups in which the center of intensity of successive earthquakes migrates parallel to a fault zone. Furthermore, since rocks are known to be elastic, any sudden slip on a fault plane must generate earthquake waves. Difference in speed and in amplitude or range of mass movement and in the quantity of rock moved by the faulting will account for any observed variation in the overall intensity of the earthquake shocks, from the slightest tremor to the greatest catastrophe. Consequently, although there is generally no doubt regarding the mechanisms that produce tectonic motions due to the indisputable geological processes they are founded on and the Elastic Rebound Theory (Baxter Stefanie J., Earthquake Basics, Special Publication No.23, Delaware Geological Survey, University of Delaware, Newark, Delaware, USA, (2000)) , their character is not so obvious visible evidence at the surface. Movements occurring in a horizontal thrust plane or at a low angle fault would be distinctly of the tectonic type and yet they might not cause relative surface displacements that would be measurable (Macelwane James B., S.J., When the Earth Quakes, Science and Culture Series, The Bruce Publishing Company, Milwaukee, USA, (1947)).

2.4.2 Volcanic Earthquakes

As far as the volcanic earthquakes are concerned, they may be associated with volcanoes in three ways: (1) An earthquake may originate in the neighborhood of an active or dormant volcano; (2) it may occur simultaneously with an eruption; (3) it may be caused by volcanism. Thus, the connection may be geographical, chronological, or genetic. Typically, a volcanic earthquake may be defined as a transient elastic vibration caused by forces originating in the magma chamber and conduits of a volcano. It may be due to an explosion, tension fracture, or fault within the structure of the volcano, and may be produced by the pressure of confined gases or by forces brought into play through the tumescence or withdrawal of lava. These earthquakes are usually of considerable local intensity but of slight total energy. They may do extreme damage on the flank of the volcano or near its crater and yet be nearly imperceptible a few miles away from its base. A tectonic earthquake of the same epicentral intensity would be recorded by seismographs at great distances. Volcanic earthquakes are frequently not recorder by sensitive seismographs in the vicinity (Macelwane James B., S.J., When the Earth Quakes, Science and Culture Series, The Bruce Publishing Company, Milwaukee, USA, (1947)).

2.5 Quantifying Earthquakes

The magnitude and the intensity of an earthquake are terms that were developed in an attempt to evaluate severity of the earthquake phenomenon. Many scientific analyses, with the aid of geodetic measurements, allow the prediction of locations and likelihoods of future earthquakes. Even today, with the development of technology, there is no accurate prediction of the magnitude of an earthquake.

(http://www.statemaster.com/encyclopedia/Earth-quakes)

2.5.1 Magnitude

The size of an earthquake is described in terms of magnitude, which is a measure of the amplitude of a seismic wave and is related to the amount of energy released during an earthquake. In the 1930s Charles Richter developed a magnitude scale (Richter scale) which was an objective way of discriminating between large and small shocks using the seismic wave amplitude recorded by seismographs. The Richter scale was originally set up for local earthquakes that occurred within 100 kilometres (62 miles) of a standardized seismometer. The scale is logarithmic meaning that an increase in magnitude of 1 represents a tenfold amplification of ground motion. For example, the amplitude of the seismic wave associated with a magnitude 6 is 100 times larger than that of a magnitude 4. However, this does not mean that a magnitude 6 is 100 times stronger than a magnitude 4. The amount of energy released or the strength of an earthquake increases by a factor of approximately 32 for every tenfold increase in amplitude; therefore, a magnitude 6 releases approximately 1000 times more energy than a magnitude 4. Since the Richter scale is logarithmic, it is possible for an event to be so small that it has a negative magnitude (Baxter Stefanie J., Earthquake Basics, Special Publication No.23, Delaware Geological Survey, University of Delaware, Newark, Delaware, USA, (2000)).

An alternative method of determining magnitude is to measure the length (duration) of the seismic signal as opposed to the amplitude of the seismic wave. This method involves the measurement of the duration of the signal from the beginning of the signal until it fades to a level considered "background noise." The duration (each millimeter on the seismogram is equivalent to one second) is then compared to a graph developed for the specific seismic station (Baxter Stefanie J., Earthquake Basics, Special Publication No.23, Delaware Geological Survey, University of Delaware, Newark, Delaware, USA, (2000)). An example of this method is presented in Figure 2.4.

Figure 2.4: Duration Magnitude for Delaware Geological Survey seismic station (Baxter Stefanie J., Earthquake Basics, Special Publication No.23, Delaware Geological Survey, University of Delaware, Newark, Delaware, USA, (2000))

Richter's magnitude scale developed in the 1930s is referred to as ML (L standing for local). Several magnitude scales exist, each developed as extensions of the original Richter magnitude scale that are equally as valid as the Richter scale. These include MS (surface wave magnitude), mb (body wave magnitude), and MW (moment magnitude). ML, Mn, MS, and mb are based on instrumental recordings of an earthquake and take into account such factors as ground amplitude, period, and the distance from a station to epicenter. MW is not based on instrumental recordings but rather on a fault's rupture surface and the movement along a fault. Each method will result in a slightly different magnitude for any given event (Baxter Stefanie J., Earthquake Basics, Special Publication No.23, Delaware Geological Survey, University of Delaware, Newark, Delaware, USA, (2000)).

Based on observations since 1990, it can be mentioned that the annually number of earthquakes of magnitude 3 to 4 is about 130,000 whereas the number of earthquakes of magnitudes 5 to 6 is about 1300. "Great earthquakes occur once a year, on average. In the past, however, an earthquake of magnitude larger than 8 was thought to be impressive but up to now the largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960 which had a magnitude (Mw) of 9.5" (Kanamori, H. (1977), The Energy Release in Great Earthquakes, J. Geophys. Res., 82(20), 2981-2987). The Richter magnitude scale can be defined as an open-ended one, although seismologists, taking into consideration the continuous development of seismic measuring techniques, can refine the practical limit (http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php). A table presenting the Richter magnitudes and the associated effects is provided below (please refer to Table 2.1).

Table 2.1: Earthquake Magnitude-Richter Scale (ibid)

Richter Scale

Richter Magnitudes

Effects Near Epicenter

Estimated Number per Year

2.0-2.9

Potentially Perceptible

1,300,000

3.0-3.9

Felt by some

130,000

4.0-4.9

Felt by most

13,000

5.0-5.9

Damaging Shocks

1319

6.0-6.9

Destructive in populous regions

134

7.0-7.9

Major earthquakes; inflict serious damage

15

>8.0

Great earthquakes; cause extensive destruction near epicenter

1

2.5.2 Intensity

By contrast to the definition of magnitude, earthquake intensity describes the effects of the earthquake on the earth's surface, by observing its effects on people and buildings. Unlike magnitude, the intensity of a given earthquake depends on the location at which it is measured; in general, the larger the epicentral distance the lower the intensity. Thus a given magnitude of earthquake will give rise to many different intensities in the region it affects (Booth Edmund and Key David, Earthquake design practice for buildings, Published by Thomas Telford, 2nd edition, London, (2006)). Hence, intensity is a semi-quantitative expression used to describe the effects of ground movement as a function of many variables including the magnitude and depth of an earthquake, distance from the earthquake, local geologic conditions, and local construction practices (Baxter Stefanie J., Earthquake Basics, Special Publication No.23, Delaware Geological Survey, University of Delaware, Newark, Delaware, USA, (2000)).

'At the turn of the 20th century, the Italian seismologist Giuseppe Mercalli introduced a scale to measure the intensity of an earthquake. The Modified Mercalli Scale is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, and is designed by Roman numerals. The scale does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects' (http://www.safe-t-proof.com/html/resources/facts_figures.html#). There is a strong correlation between Richter's scale and Mercalli's scale concerning the intensity of an earthquake. For instance, events of intensities II to III on Mercalli scale are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale. The 12-point scale is presented in Table 2.2.

Modified Mercalli Intensity Scale (MMI)

I

Not felt except by a very few under especially favourable circumstances.

II

Felt only by a few persons at rest, especially on upper floors of buildings.

III

Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. 

IV

During the day felt indoors by many, outdoors by few.

V

Felt by nearly everyone, many awakened. Disturbances of trees, poles and other tall objects sometimes noticed. 

VI

Felt by all; many frightened and run outdoors. Some heavy furniture moved; few instances of fallen plaster or damaged chimneys. Damage slight. 

VII

Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures.

VIII

Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures.

IX

Damage considerable in specially designed structures. Buildings shifted off foundations. Ground cracked conspicuously. 

X

Some well-built wooden structures destroyed. Most masonry and frame structures destroyed. Ground badly cracked. 

XI

Few, if any (masonry) structures remain standing. Bridges destroyed.

XII

Damages total. Waves seen on ground surfaces. Objects thrown upward into air.

Table 2.2: Earthquake Intensity- Modified Mercalli Scale (http://www.umich.edu/~gs265/society/earthquakes.htm)

Whilst the aforementioned scale is the most commonly used in the USA, the Macroseismic Intensity Scale (EMS) is more favoured in Europe, since it relates damage more precisely to the earthquake-resisting qualities of the damaged structures. The Japanese Seismic Intensity Scale is similar in principal but is based on only seven points (Booth Edmund and Key David, Earthquake design practice for buildings, Published by Thomas Telford, 2nd edition, London, (2006)).

The nature of these subjective scales seems undesirable however. Man's reactions to earthquakes depend on numerous factors including previous experience with ground motions. Effects on buildings are contingent on local design and construction practices. Especially objectionable seem clauses in scales that permit assigning an intensity to an earthquake in uninhabited regions in terms of the amplitude of the permanent deformations, slope failures, or relative displacements of the ground because, usually, the area of maximum intensity does not follow surface faults at which slip is noticeable, and slope failures often occur in the absence of earthquakes (Newmark Nathan M. and Rosenblueth Emilio, Fundamentals of Earthquake Engineering, Civil Engineering and Engineering Mechanics Series, Prentice-Hall, Inc., USA, (1971)).

Despite their many shortcomings, subjective intensity scales are an important consideration in areas where no strong-motion instruments have been installed and they afford the only means for interpreting historical information. In order to utilize instrumental data and relate them with the subjective scales, instrumental intensity scales have also been proposed. Those that rest exclusively on the maximum ground acceleration or on the maximum trace of some type of seismograph bear little connection with the destructiveness of the ground motion. A rough correlation of this sort is shown in Figure 2.5 which is probably applicable to typical earthquakes in California, USA (Newmark Nathan M. and Rosenblueth Emilio, Fundamentals of Earthquake Engineering, Civil Engineering and Engineering Mechanics Series, Prentice-Hall, Inc., USA, (1971)).

Figure 2.5: Distance-Intensity for California earthquakes, USA (Newmark Nathan M. and Rosenblueth Emilio, Fundamentals of Earthquake Engineering, Civil Engineering and Engineering Mechanics Series, Prentice-Hall, Inc., USA, (1971)).

Clearly, magnitude and intensity are related to some extent, in that in general larger magnitudes give rise to larger intensities for a given epicentral distance (Booth Edmund and Key David, Earthquake design practice for buildings, Published by Thomas Telford, 2nd edition, London, (2006)).

2.6 Effects of Earthquakes

Earthquakes produce various effects of concern to the inhabitants of seismically active regions with both social and economic consequences. The effects on human beings are surprisingly varied. In addition to the mental stress induced by fear and a feeling of helplessness in the face of overpowering forces, a person is subjected to unpleasant physiological conditions.

Generally, the effects can be classified into two categories, namely the primary and secondary effects. The first type involves immense damage and can cause great loss of life by destroying structures such as buildings, bridges, roads and dams. In slight earthquakes the forces which act leave behind them no visible mark. They betray their presence only by the swaying and rattling, the displacements which they produce for the moment and which immediately cease and disappear with the passing of the shock. In very strong earthquakes the shaking is so severe that damage usually results. Whether a given structure will fail or will go through the earthquake unharmed depends on many factors. First there are the inertia effects of sudden lurches. This property of a material body tends to resist any attempt to start the body moving if it is at rest. Therefore, if the ground gives a sudden lurch in an earthquake all loose objects tend to be left behind because of their inertia. Also, in most earthquakes, the vertical oscillations are of the same order of magnitude as the horizontal vibrations and the forces involved may even surpass the force of gravity. Finally, another important factor is the relation of the tempo of the earthquake shaking to the natural period of the structure. A condition called resonance may occur (Macelwane James B., S.J., When the Earth Quakes, Science and Culture Series, The Bruce Publishing Company, Milwaukee, USA, (1947)). The latter category of effects includes phenomena such as liquefaction, Landsliding and subsidence which shall be discussed at this point.

2.6.1 Liquefaction

The phenomenon known as liquefaction is related to poor soils areas composed of water-saturated fine sands or silts. Under simple, normal static loading, these areas have a reasonable capacity to support a normal building as long as they remain stable. Liquefaction of soils occurs when a material of solid consistency is transformed into a liquid state as a result of increased pore water pressure in between the fine silts or sands. Water-saturated, granular sediments such as silts, sands, and gravels, which are free of clay particles, are more susceptible to liquefaction, specially the looser, finer materials.

Although areas of potential liquefaction may appear sufficient to carry loads, in reality their load-carrying ability is deceptive. Under dynamic shaking as experienced during an earthquake, their soils may be rearranged and in doing so lose the capacity to support the foundation systems above them. In effect, such water-saturated soils become liquefied when shaken, and behave as a dense fluid rather than a solid mass with support capabilities. When this occurs, buildings may sink into the ground, as their foundations are no longer supported by the soils beneath the structural system (Lagorio Henry J., Earthquakes-An Architect's Guide To Nonstructural Seismic Hazards, John Wiley & Sons, Inc., USA, (1990)).

Many examples of this phenomenon exist, an illustration of which is presented in Figure 2.6.

Figure 2.6: The 1964 Niigata earthquake-induced liquefaction (http://articles.architectjaved.com/earthquake_resistant_structures/files/2010/06/Liquefaction_at_Niigata.jpg).

2.6.2 Landsliding

Earthquakes may trigger landslides or other forms of slope instability. Slope failures may occur as a result of the development of excess pore pressures which will reduce the shear strength of the soils or cause loss of strength along bedding or joints in rock materials. Although the majority of such landslides are small, earthquakes have also caused very large slides. In a number of unfortunate cases, earthquake-induced landslides have buried entire towns and villages. More commonly, earthquake-induced landslides cause damage by destroying buildings or disrupting bridges and other constructed facilities. Many earthquake-induced landslides result from liquefaction phenomena, but many others simply represent the failures of slopes that were marginally stable under static conditions [13]. A typical example of landsliding is depicted in Figure 2.7.

Figure 2.7: The 2001 El Salvador earthquake-induced landslide located in a neighbourhood near San Salvador, Santa Tecla (http://nasadaacs.eos.nasa.gov/articles/images/2006_hotspots_landslide.jpg)

3.1 Ground Motions and Seismic Forces

Normally a structure is designed to resist gravity loads in combination with horizontal loads from wind forces. Those forces are transmitted downward through the structure, delivered to the supporting ground and ultimately vertical forces will dominate. On the contrary, earthquake shaking is transmitted from the ground to the structure and horizontal loads developed within the structure by inertial reactions to the shaking will dominate. Therefore, the structure should be designed to support the transient earthquake loads in combination with the existing and relatively constant gravity loads. This demand needs the proportion and detail of all members and connections of the structural system and the consideration of the paths and concentrations of the forces through the system in a very exacting manner with the purpose of ensuring that the development of an overstress or big displacement in a local area will not result in a lie-threatening collapse (Krinitzsky Ellis L., Gould James P., Edinger Peter H., Fundamentals of Earthquake Resistant Construction, Wiley Series of Practical Construction Guides, John Wiley & Sons, Canada, (1993)).

Consequently, it is crucial that the modelling of seismic forces as they move through the structure's components follows a logical force path for the structure to be able to resist them directly without major complexities. In general, two methods are used to determine the equivalent lateral loads to be applied to a building's structural system. The first system, used for low-rise buildings which are located in smaller zones of seismicity is referred to as static lateral force procedure.

The second, called dynamic lateral force procedure, is used in the design of high-rise buildings located in higher zones of seismicity (Lagorio Henry J., Earthquakes-An Architect's Guide to Non-structural Seismic Hazards, John Wiley & Sons, Inc., USA, (1990)).

3.2 Different Structural Shapes for Earthquake Resistance

'The experience of past earthquakes has confirmed the commonsense expectation that buildings which are well-defined, continuous load-paths to the foundation perform much better in earthquakes than structures lacking such features'(Booth Edmund and Key David, Earthquake Design Practice for Buildings, 2nd edition, Thomas Telford Ltd., London, (2006)).

Therefore, structural systems have historically been developed and designed to carry downward, vertically directed gravity load in a large variety of materials, configurations, connections, and details. In general, traditional structures have some fundamental capacity to resist horizontal loads, and this capacity has been used in many structures to carry wind loads. However, it may be required to add special elements, framing, and connections to obtain adequate capacity to resist horizontal loads and transmit them through the framing systems (Krinitzsky Ellis L., Gould James P., Edinger Peter H., Fundamentals of Earthquake Resistant Construction, Wiley Series of Practical Construction Guides, John Wiley & Sons, Canada, (1993)).

Taking into account the lateral forces, which play a very important role in a building, either in vertical or horizontal plane, their presence is the result of the schematic architectural design of the building. In the vertical plane, three kinds of components can resist these forces such as shear walls, braced frames and moment-resistant frames. On the other hand, in the horizontal plane, diaphragms can be used for the same purpose, generally formed by floor and roof planes of the buildings, or horizontal trusses (http://nisee.berkeley.edu/lessons/arnold.html).

These elements, illustrated in Figure 3.1, are basic architectural components. The acquirement of an understanding of how these resistance systems works in response to the forces that the earthquake generates is the scope of this part.

Figure 3.1: Elements used to create earthquake resistant structures (Krinitzsky Ellis L., Gould James P., Edinger Peter H., Fundamentals of Earthquake Resistant Construction, Wiley Series of Practical Construction Guides, John Wiley & Sons, Canada, (1993)).

3.2.1 Shear Walls

Horizontal load resistance in a structure is commonly provided by the entire or portions of its wall system. The load-carrying walls are termed shear walls and have to provide support in every horizontal direction, as shown in Figure 3.1. They will be subjected to combined axial loading from gravity loads and the shear and bending stresses imposed as they transmit lateral earthquake loads vertically through the building framework.

Earthquake loads applied perpendicular to the plane of a wall, perhaps from the weight of the wall or attachments to it, are significant as they will act at the same time as axial, shear, and bending loads are imposed in the plane of the wall. This combination is out-of-plane and in-plane loading must be considered in assessing wall stability.

There is a variety of materials which can be used for shear walls, and the essential dimensions of walls made from similar materials may differ within a given structure. It is important to recognize the differences in flexural rigidity of the walls resulting from dimensional or material differences when creating a structural model, and to account for compatibility at assumed deflections. Consequently, the size and location of shear walls is of vital importance. Plans can be conceived of as collections of resistant elements with varying orientations to resist translational forces, that are placed at varying distances from the centre of rigidity to resist torsional forces [ (Arnold Christopher and Reitherman Robert, Building Configuration and Seismic Design, A Wiley-Inter-science Publication, USA, (1982)) and Krinitzsky Ellis L., Gould James P., Edinger Peter H., Fundamentals of Earthquake Resistant Construction, Wiley Series of Practical Construction Guides, John Wiley & Sons, Canada, (1993)).

Some conceptual aspects of wall location within simple geometric plan forms are presented in Figure 3.2.

Figure 3.2: Shear wall location (Arnold Christopher and Reitherman Robert, Building Configuration and Seismic Design, A Wiley-Inter-science Publication, USA, (1982)).

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