The earthquake, considered an independent natural phenomenon of ground vibration, 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 building construction. The problem is the structure itself 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, described as seismic zones. Nevertheless, they have a serious impact on the whole world due to the large-scale damage along with the number of casualties caused in densely populated areas (Penelis and Kappos, 1997).
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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 this 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, 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, ranging from economic to structural to social effects. An earthquake only occurs for a few brief moments; the aftershocks can last for weeks; the damage can last for years.
2.2.1 Earthquake Mechanism
Earthquakes occur when plates forming the Earth's crust move over and under each other causing a build up of stress which eventually exceeds the capacity of the rock. The rock then fractures which allows movement along a fault line (Brunious and Warner, 1998). The world-wide tectonic plates are presented in Figure 2.1.
Figure 2.1: World-wide tectonic plates (University of Twente, 2010).
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.
Prognostication of a large earthquake can be considered from foreshocks that occur before the main shock. Besides foreshocks, smaller quakes generated by adjustments following a major earthquake can be detected, known as aftershocks (Tarbuck and Lutgens, 2007). 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 originate (MCEER, 2011). According to the above, earthquakes arise due to forces within the earth's crust tending to displace some rock mass. 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 and Key, 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.
Always on Time
Marked to Standard
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 " (Kim, 2010).
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 (Kim, 2010).
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 is 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, strong, 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, 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 and Rosenblueth, 1972).
2.4.1 Tectonic Earthquakes
The word "tectonic" derives from the Greek word "Ï„ÎÎºÏ„Ï‰Î½" 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 the Elastic Rebound Theory, their character is not so obvious visible evidence at the surface (Baxter, 2000). 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, 1947).
2.4.2 Volcanic Earthquakes
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As far as the volcanic earthquakes are concerned, they may be associated with volcanoes in three ways: (1) An earthquake may originate in the neighbourhood 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 recorded by sensitive seismographs in the vicinity (Macelwane, 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, no date).
The size of an earthquake is described in terms of magnitude, which is a measure of the energy released (output in kilowatts) at the source of the earthquake. Magnitude is determined from recorded measurements by seismographs. The Richter scale (developed in 1935) which is actually logarithmic and open-ended scale meaning that every increase or decrease of one magnitude represents a tenfold amplification of ground motion. For example, the amplitude of the seismic wave associated with a magnitude 8 is 100 times larger than that of a magnitude 6. A magnitude 8 releases 1024 times (32 for every tenfold increase in amplitude, so 32x32) more energy than a magnitude 6 (Bellis, 1997).
"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 away a level considered "background noise." The duration (each millimetre on the seismogram is equivalent to one second) is then compared to a graph developed for the specific seismic station" (Baxter, 2000). An example of this method is presented in Figure 2.4.
Figure 2.4: Duration Magnitude Vs Body Wave Magnitude (Mb) for Delaware Geological Survey seismic station (Baxter, 2000).
Based on observations since 1990, it can be mentioned that annually the 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, 1977). 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 (U.S. Geological Survey, 2011). A table presenting the Richter magnitudes and the associated effects is provided below (please refer to Table 2.1).
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, human structures and the natural environment. 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 and Key, 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, 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 comÞosed of 12 increasing levels of intensity that range from imÞerceÞtible shaking to catastroÞhic 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" (U.S. Geological Survey, 2009). 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.
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 and Key, 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 and Rosenblueth, 1972).
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 and Rosenblueth, 1972).
Figure 2.5: Distance-Intensity for California earthquakes, USA (Newmark and Rosenblueth, 1972).
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 and Key, 2006).
2.6 Effects of Earthquakes
Earthquakes Þroduce 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, 1947). The latter category of effects includes phenomena such as liquefaction, landsliding and subsidence which shall be discussed at this point.
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, 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 (Watts, 2007).
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 causeÐ very large sliÐes. In a number of unfortunate cases, earthquake-inÐuceÐ landslides have burieÐ entire towns anÐ villages. More commonly, earthquake-inÐuceÐ landslides cause Ðamage by Ðestroying builÐings or ÐisruÞting briÐges anÐ other constructeÐ facilities. Many earthquake-inÐuceÐ landslides result from liquefaction Þhenomena, but many others simÞly reÞresent the failures of sloÞes that were marginally stable under static conditions (Kramer, 1996). 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 (Edwin, 2006).
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 life-threatening collapse (Krinitzsky, Gould and Edinger, 1992).
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, 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 with continuous load-paths to the foundation perform much better in earthquakes than structures lacking such features' (Booth and Key, 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, Gould and Edinger, 1992).
"Taking into account the lateral forces, which play a very important role in a building, either in the vertical or horizontal plane, their Þresence is the result of the schematic architectural design of the building. In the vertical Þlane, three kinds of comÞonents 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" (Arnold, 1998).
These elements, illustrated in Figure 3.1, are basic architectural components. The acquirement of an understanding of how these resistance systems works in resÞonse to the forces that the earthquake generates is the scope of this part.
Figure 3.1: Elements used to create earthquake resistant structures (Krinitzsky, Gould and Edinger, 1992).
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. Þlans can be conceived of as collections of resistant elements with varying orientations to resist translational forces, which are placed at varying distances from the centre of rigidity to resist torsional forces (Arnold and Reitherman, 1982; Krinitzsky, Gould and Edinger, 1992).
Some conceptual aspects of wall location within simple geometric plan forms are presented in Figure 3.2.
Figure 3.2: Shear wall location (Arnold and Reitherman, 1982).
3.2.2 Braced Frames
Braced frames configured as vertical trusses (please refer to Figure 3.1) may be used in place of shear walls to accept lateral loads and transmit them vertically through the building frame. The connections between the members are fundamentally important, and new codes have included detailing requirements to increase connection strength to the point that ultimate failure of any brace occurs away from the joint. Furthermore, traditional design practice included minimizing eccentricities at connections to reduce moments and shears. However, recent developments include an "eccentric braced frame" that uses eccentricity at a joint to force a segment of the beam to deform plastically in bending away from the joint during an earthquake. The inelastic deformation absorbs and dissipates a much bigger amount of energy than does a concentric system. When proportioned properly, the eccentric system will reduce the potential for abrupt failure of the frame (Krinitzsky, Gould and Edinger, 1992).
3.2.3 Moment-Resistant Frames
The body of the building structure is mainly the one which resists seismic stress and the seismic resistance is focused on joints between columns and beams. These joints accept intense pressure and this is why attention should be given to all structure details. Bodies of buildings constructed either by steel beam or concrete are strong and survive an intense seismic stress. Column frames and beams jointed with screws or with welded joints must present the natural ductility of the material as an advantage which should be ensured with special reinforcements. If it is a steel frame the following should be reinforced:
The column at the joint point with steel strips so that the load can be brought by the opposite vertical steel side of the column.
The horizontal beam with extra steel plate.
Figure 3.3: Beam-Column joint, Moment Resisting-Frame (MCEER, 2011).
Connection of the beam with the column.
In reinforced concrete structures the joints of beams and columns should consist of steel reinforcement so a great degree of ductility can be ensured.
More stirrup cages are used in 1m distance from the joint point in columns and beams.
Extra steel reinforcement in L shape with side length 1m is used at the joints between beams and columns.
Even in cases of distortions and bending, fracture can be avoided if more reinforced concrete is used at the joints between vertical and horizontal elements.
A very good reinforced effect can be achieved in steel and concrete building if architectural details are added which will not only have an aesthetic effect but will also reinforce the joints. In addition, braced structures can be constructed which later will be covered by walls or various coatings which will serve as reinforcement of the joints as well as of the vertical and horizontal elements of the body of the building.
3.2.4 Non - Structural Elements
Moreover, it is important to recognize that non-structural elements may, inadvertently, form part of the lateral resistance system. If rigid enclosure or separation walls are not isolated from the structure by slip joints, they have to be designed as integral parts of the structure. Their location becomes a structural issue. Because of the tremendous rigidity of walls as compared to frames, a small amount of wall in the wrong place can drastically redistribute loads and change the structure's performance. Asymmetrical wall arrangements can overwhelm a symmetrical frame's attempt to respond to lateral forces in a relatively torsion-free manner. Staircases, since they may form diagonal braces, are equally quite rigid and quickly assume a large structural role unless isolated from lateral movements (Arnold and Reitherman, 1982).
3.2.5 Foundation Structures
A vital criterion for the design of foundations of earthquake resisting structures is that the foundation system ought to be capable of supporting the designed gravity loads while maintaining the chosen seismic energy-dissipating mechanisms. In earthquake areas this involves the consideration of the subsequent factors:
"Transmission of horizontal base shears from the structure to the soil.
Provision for earthquake overturning moments (e.g. tension piles).
Liquefaction of the subsoil.
The effects of embedment on seismic response.
Moreover, three basic types of foundation may be referenced, namely piled foundations, discrete pads and continuous rafts" (Baidya, 2006). However, the foundation system in this context includes the reinforced concrete or masonry foundation structure, the piles or caissons, and the supporting soil.
To conceive a reliable foundation system, it is essential that every mechanism by which earthquake-induced structural actions are transmitted to the soil be clearly defined. Subsequently, energy dissipation may be assigned to areas within the superstructure and/or the foundation structure in such a manner that the expected local ductility demands remain within recognized capabilities of the concrete or masonry components selected. It is particularly important to make sure that any damage that may occur in the foundation system does not jeopardize gravity load-carrying capacity.
It is the expected seismic response of the foundation structure that will dictate the necessary detailing of the reinforcement. Where there is no possibility for inelastic deformations to develop during the seismic response, detailing of the reinforcement as for foundation components subjected to gravity and wind induced loads only should be sufficient. However, where during earthquake actions, yielding is intended to happen in some components of the foundation structure, the affected components must be detailed in accordance with appropriate principles to enable them to sustain the imposed ductility demands. Consequently, at the conceptual stage of design, a clear decision must be made concerning the admissibility of inelastic deformations within the foundation system.
Moments and shear forces in the foundation structure may be strongly affected by the distribution of reactive pressure induced in the supporting soil. Therefore, account should be taken of the uncertainties of soil strength and stiffness, particularly under cyclic dynamic actions, by considering a range of possible values of soil properties (Paulay and Priestley, 1992).
3.2.6 Base Isolation
Base isolation is a relatively new development in earthquake resistant design. The principal is to insert a discontinuity at the base of a structure that has relatively low resistance to shear. As earthquake motions are transmitted upward, the effect of the soft discontinuity will be to increase the natural period of the structure and to absorb energy by shear deformation.
Generally, this will reduce the magnitude of the response of the structure to earthquake shaking, particularly if the structure is founded on bedrock. However, it should be noted that if the structure bears in soft soil the base isolation may not achieve a reduction in response, and in some cases might actually increase it. Although base isolation may be effective in reducing response to horizontal shaking, the necessity for vertical stiffness in the structure to resist gravity loads makes isolation impracticable in vertical shaking. A usual base isolation device for installation at a column base is illustrated in Figure 3.4.
Figure 3.4: Lead-rubber seismic isolation bearing (Krinitzsky, Gould and Edinger, 1992).
Bearing is transmitted through rubber and steel plate laminations that are relatively flexible in the horizontal direction, thus achieving the intended damping. The lead plug in the centre will bend during lateral displacements, with highly hysteretic behaviour. The initial stiffness of the plug will limit lateral movements under light loads, while later in time it will enable the structural system to respond well to seismic vibrations.
There are other types of base isolation devices that achieve the desired result of dissipating high frequency energy by permitting controlled sliding displacements on flat or spherical surfaces or through bending of vertical steel plates (Krinitzsky, Gould and Edinger, 1992).
4.1 Detailing of Beams
In order to ensure satisfactory seismic performance, careful detailing of reinforcing bars is essential, and codes of practice provide extensive guidance. Figure 4.1 illustrates typical details for beams.
Figure 4.1: Detailing notes for a ductile beam (a) closed hoop; (b) stirrups with ties; (c) multi-leg hoops for wide beam; and (d) multiple layers of flexural steel (Booth and Key, 2006).
4.2 Detailing of Columns
The typical details for columns are graphically depicted in Figure 4.2, as shown below.
Figure 4.2: Detailing notes for a ductile column: (a) elevation; and (b) sections through column (Booth and Key, 2006).
4.3 Beam-Column Connections
In earthquake-resistant frames the Ðesign of beam-column connections requires as much attention as the Ðesign of the members themselves, since the integrity of the frame may well ÐeÞenÐ on the ÞroÞer Þerformance of such connections. Because of the congestion that may result from too many bars converging within the limiteÐ sÞace of the joint, the requirements for the beam-column connections have to be consiÐereÐ when ÞroÞortioning the columns to a frame. To minimize Þlacement Ðifficulties, an effort shoulÐ be maÐe to keeÞ the amount of longituÐinal reinforcement in the frame members on the low siÐe of the Þermissible range. (Naeim, 2001).
Joints at the ends of beams require special attention due to the fact that the anchorage length for the beam steel on one side of the joint is restricted (please refer to Figure 4.3 and 4.4).
Figure 4.3: Anchorage of flexural steel in beam-column joints (Booth and Key, 2006).
Figure 4.4: continued (Booth and Key, 2006).
4.4 Reinforced concrete damage at the joints
Buildings consisting of frames built from reinforced concrete beams and columns and which are not braced by walls have proved very vulnerable to earthquakes, unless special design and detailing measures are in place to resist earthquakes.
The main points of vulnerability are the following:
beam-column joints (Figure 4.5).
bursting failures in columns (Figure 4.6).
shear failures in columns (Figure 4.7).
anchorage failure of main reinforcing bars in beams and columns (Figure 4.5 and 4.8 further below).
Figure 4.5: Failure of a beam-column joint in Erzincan, Turkey, 1992. The failure of the concrete in the joint and the bursting out of column steel should be noted (Booth and Key, 2006).