D.M.S.S.V.H.COLLEGE OF ENGINEERING MACHILIPATNAM – 521 001 EARTHQUAKE RESISTANT STRUCTURES Submitted by :- K.L.A.V.HARNADH, MOHAMMED AZMUDDIN, B.Tech., III year, B.Tech., III year, Y7CE627, Y7CE615, Harnadh149@gmail.com firstname.lastname@example.org Ph. No. 9959063370 Ph. No. 9700345820. 1.0 INTRODUCTION 2.0 SEISMIC EFFECTS ON STRUCTURES An earthquake is the result of a sudden release of energy in the Earth's crust that 2.1 EARTHQUAKE GROUND creates seismic waves. Earthquakes are MOTION recorded with a seismometer, also known as a seismograph. The moment The seismic waves travel for great magnitude (or the related and mostly distances before finally losing most of obsolete Richter magnitude) of an their energy. At some time after their earthquake is conventionally reported, generation, these seismic waves will with magnitude 3 or lower earthquakes reach the earth's surface, and set it in being mostly imperceptible and motion, which we surprisingly refer to as magnitude 7 causing serious damage earthquake ground motion. When this over large areas. Intensity of shaking is earthquake ground motion occurs measured on the modified Mercalli beneath a building and when it is strong scale. enough, it sets the building in motion, starting with the buildings foundation, At the Earth's surface, earthquakes and transfers the motion throughout the manifest themselves by shaking and rest of building in a very complex way. sometimes displacing the ground. When These motions in turn induce forces a large earthquake epicenter is located which can produce damage. offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can Real earthquake ground motion at a also trigger landslides and occasionally particular building site is vastly more volcanic activity. complicated than the simple wave form. Here it's useful to compare the surface of In its most generic sense, the word ground under an earthquake to the earthquake is used to describe any surface of a small body of water, like a seismic event — whether a natural pond. You can set the surface of a pond phenomenon or an event caused by in motion - by throwing stones into it. humans — that generates seismic waves. The first few stones create a series of Earthquakes are caused mostly by circular waves, which soon being to rupture of geological faults, but also by collide with one another. After a while, volcanic activity, landslides, mine blasts, the collisions, which we term and nuclear experiments. An interference patterns, are being to earthquake's point of initial rupture is predominate over the pattern of circular called its focus or hypocenter. The term waves. Soon the entire surface of water epicenter refers to the point at ground is covered by ripples, and you can no level directly above the hypocenter. longer make out the original wave forms. During an earthquake, the ground vibrates in a similar manner, as waves of different frequencies and amplitude interact with one another. 2.2 BUILDING FREQUENCY AND The relationship between frequency F PERIOD and period T is thus given as The characteristics of earthquake ground T=1/F motions which have the greatest importance for buildings are the This means that a short building with a duration, amplitude (of displacement, high natural frequency also has a short velocity and acceleration) and frequency natural period. Conversely, a very tall of ground motion. building with a low frequency has a long period. Frequency: Frequency is defined as the number of complete cycles of vibration 3.0 PLANNING TOOL OF made by the wave per second ARCHITECTURE FOR EARTHQUAKE Here we can consider a complete vibration to be the same as the distance The behavior of building during between one crest of the wave and the earthquakes depends critically on its next, in other words one full wavelength. overall shape, size and geometry. Hence, Surface ground motion at the building at planning stage itself, architects and site, then, is actually a complex structural engineers must work together superposition of vibration of different to ensure that the unfavorable features frequencies. We should also mention are avoided and a good building that at any given site some frequencies configuration is chosen. If both shape usually predominate and structural system work together to make the structure a marvel. The response of building to the ground motion is as complex as the ground "If we have a poor configuration to start motion itself, yet typically quite with, all the engineer can do is to different. It also begins to vibrate in a provide a band-aid - improve a basically complex manner, and because it is now a poor solution as best as he can. vibratory system, it also posses a Conversely, if we start-off with a good frequency content. However, the configuration and reasonable framing buildings vibrations tend to center system, even a poor engineer cannot around one particular frequency, which harm its ultimate performance too is known as its natural or fundamental much". frequency. 3.1 Size of Buildings The shorter a building is, the higher its natural frequency. The taller the building In tall buildings with large weight-to- is, the lower its natural frequency. base size ratio the horizontal movement of the floors during ground shaking is Period: The natural period is the time it large. In short but very long buildings, takes for the building to make one the damaging effects during earthquake complete vibration shaking are many. And, in buildings with large plan area, the horizontal seismic forces can be excessive to be mid- height of the column of the taller carried by columns and walls. one; this can be very dangerous. 3.2 Horizontal Layout of Buildings Buildings with simple geometry in plan perform well during strong earthquakes. Buildings with re-entrant corners, like U, V, H and + shaped in plan sustain significant damage. The bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts by using a separation joint at the junction. Fig 1 3.5 Building Stiffness and Flexibility 3.3 Vertical Layout of Buidlings The taller a building, the longer its natural period tends to be. But the Earthquake forces developed at height of a building is also related to different floor levels in a building need another important structural to be brought down along the height to characteristic: the building flexibility. the ground by the shortest path, any Taller buildings tend to be more flexible deviation or discontinuity in this load than short buildings. (Only consider a transfer path results in poor performance thin metal rod. If it is very short, it is of building. Buildings with vertical difficulty to bend it in your hand. If the setbacks cause a sudden jump in rod is some what longer, and of the earthquake forces at the level of same diameter, it becomes much easier discontinuity. Buildings that have fewer to bend. Buildings behave similarly) we columns or walls in a particular storey or say that a short building is stiff, while a with unusually tall storey tend to damage taller building is flexible. (Obviously, or collapse which is initiated in that flexibility and stiffness are really just the storey. two sides of the same coin. If something is stiff, it isn't flexible and vice-versa). Figure shows the Displacement of 3.4 Adjacency of Buildings Building according to their Height & Stiffness When two buildings are close to each other, they may pound on . each other during strong shaking. When building heights do not match the roof of the shorter building may pound at the 4.0 EFFECT OF EARTHQUAKE ON REINFORCED CONCRETE STRUCTURES A typical RC building is made of horizontal members (beams and slabs) and vertical members (columns and walls), and supported by foundations that rest on ground. The system comprising of RC frame. The RC frame participates in resting the earthquake forces. Earthquake shaking generates inertia forces in the building, which are proportional to the building mass. Since most of the building mass is present at floor levels, earthquake induced inertia forces primarily develop at the floor levels. These forces travel downwards - through slabs and beams to columns and walls, and then to foundations from where they are dispersed to ground. As inertia forces accumulate downwards from the top of the building, the columns and walls at lower storey experience Fig 2 higher earthquake- induced forces (fig 1) and are therefore designed to be stronger Ductility is the ability to undergo than those in storey distortion or deformation without above. resulting in complete breakage or failure. To see how ductility can 4.1 Role of Floor Slabs and Masonary improve a building's performance during an earthquake,. In response to the ground Floor slabs are horizontal plate like motion, the rod bends but does not elements, which facilitate functional use break. (of course, metals in general are of buildings. Usually, beams and slabs at more ductile than materials such as one storey level are cast together. In stone, brick and concrete) The ductility residential multi-story buildings, of a structure is in fact one of the most thickness of slabs is only about 110- important factors affecting its earthquake 150mm. when beams bend in the vertical performance. One of the primary tasks direction during earthquakes, these thin of an engineer designing a building to be slabs bend along with them .And, when earthquake resistant is to ensure that the beams move with columns in the building will possess enough ductility to horizontal direction, the slab usually withstand the size and types of forces the beams to move together with earthquakes it is likely to experience it. After columns and floors in a RC during its lifetime. building are cast and the concrete hardens, vertical spaces between 5.0 EARTHQUAKE RESISITANCE columns and floors are usually filled-in DESIGN with masonry walls to demarcate a floor into functional spaces (rooms). 5.1 Conventional Approach Normally, these masonry walls, also called infill walls, are not connected to Design depends upon providing surrounding RC columns and beams. the building with strength, stiffness and When columns receive horizontal forces inelastic deformation capacity which are at floor levels, they try to move in great enough to withstand a given horizontal direction, but masonry walls level of earthquake-generated force. tend to resist this movement. Due to their heavy weight and thickness, these This can be accomplished by selection of walls attract rather large horizontal an appropriate structural configuration forces. However, since masonry is a and careful detailing of brittle material, these walls develop structural members, such as beams and cracks once their ability to carry columns, and the connections between horizontal load is exceeded. Thus them. masonry walls is enhanced by mortars of good strength, making proper masonry 5.2 Basic Approach courses, and proper packing of gaps between RC frame and masonry infill Design depends upon underlying walls. more advanced techniques for earthquake resistance is not to strengthen the building, but to reduce the 4.2 Horizontal Earthquake Effects earthquake generated forces acting upon it. Under gravity loads, tension in the beams is at the bottom surface of the This can be accomplished by de- beam in the central location and is at the coupling the structure from seismic top surface at the ends. The level of ground motion it is possible to reduce bending moment due to earthquake the earthquake induced forces in it by loading depends on severity of shaking three ways and can exceed that due to gravity loading. Thus, under strong earthquake - Increase natural period of structures by shaking, the beam ends can develop BaseIsolation. tension on either of the top and bottom faces. Since concrete cannot carry this - Increase damping of system by Energy tension, steel bars are required on both Dissipation Devices. faces of beams to resist reversals of bending moment. - By using Active Control Devices. 6.0 EARTHQUAKE DESIGN b) Under moderate but occasional PHIOSOPHY shaking, the main members may sustain repairable damage, while the other parts Severity of ground shaking at a given that do not carry load may location during an earthquake can be sustain repairable damage. minor, moderate and strong. Thus relatively speaking, minor shaking c) Under strong but rare shaking, the occurs frequently; moderate shaking main members may sustain severe occasionally and strong shaking rarely. damage, but the building should not For instance, on average annually about collapse. 800 earthquakes of magnitude 5.0-5.9 occur in the world while about 18 for Earthquake resistant design is magnitude range 7.0-7.9. So we should therefore concerned about ensuring that design and construct a building to resist the damages in buildings during that rare earthquake shaking that may earthquakes are of acceptable variety, come only once in 500 years or even and also that they occur at the right once in 2000 years, even though the life places and in right amounts. This of the building may be 50 or 100 years? approach of earthquake resistant design is much like the use of electrical fuses in Engineers do not attempt to make houses: to protect the entire electrical earthquake proof buildings that will not wiring and appliances in the house, you get damaged even during the rare but sacrifice some small parts of electrical strong earthquake; such buildings will be circuit, called fuses; these fuses are too robust and also too expensive. easily replaced after the electrical over- Instead the engineering intention is to current. Likewise to save the building make buildings earthquake-resistant; from collapsing you need to allow some such buildings resist the effects of pre-determined parts to undergo the ground shaking, although they may get acceptable type and level of damage. damaged severely but would not collapse during the strong earthquake. Earthquake resistant buildings, Thus, safety of people and contents is particularly their main elements, need to assured in earthquake-resistant be built with ductility in them. Such buildings, and thereby a disaster is buildings have the ability to sway back- avoided. This is a major objective of and-forth during an earthquake, and to seismic design codes throughout the withstand the earthquake effects with world. some damage, but without collapse. 6.1 Design Philosophy 7.0 CONSTRUCTION MATERIALS FOR EARTH QUAKE RESISTANCE a) Under minor but frequent shaking, the main members of the buildings that In India, most non-urban buildings are carry vertical and horizontal forces made in masonry. In the plains, masonry should not be damaged; however is generally made of burnt clay bricks buildings parts that do not carry load and cement mortar. However in hilly may sustain repairable damage. areas, stone masonry with mud mortar is more prevalent. But now a day we are very familiar with R.C.C. buildings, and undergo large elongation before a variety of new composite constructions breaking. Concrete is used with steel materials. reinforcement bars. This composite material is called as reinforced cement 1. Masonary concrete. The amount and location of steel in a member should be such that the Masonry is made up of burnt clay bricks failure of the member is by steel and cement or mud mortar. Masonry can reaching its strength in tension before carry loads that cause compression (i.e. concrete reaches its strength in pressing together) but can hardly take compression. This type of failure is load that causes tension (i.e. pulling ductile failure, and is preferred over a apart). Masonry is a brittle material, failure where concrete fails first in these walls develop cracks once their compression. ability to carry horizontal load is exceeded. Thus infill walls act like 7.1 Earthquake Design Resistant sacrificial fuses in buildings: they Concept develop cracks under severe ground shaking but they share the load of the If two bars of same length and same beams and columns until cracking. cross-sectional area - one made of ductile material and another of a brittle 2. Concrete material. And a pull is applied on both bars until they break, then we notice that Concrete is another material that the ductile bar elongates by a large has been popularly used in building amount before it breaks, while the brittle construction particularly over the last bar breaks suddenly on reaching its four decades. Cement concrete is made maximum strength at a relative small of crushed stone pieces (called elongation. aggregate), sand, cement and water mixed in appropriate proportions. Amongst the materials used in building Concrete is much stronger than masonry construction , steel is ductile, while under compressive loads, but again its masonry and concrete are brittle. behavior in tension is poor. The properties of concrete critically depend The correct building components need on the amount of water used in making to be made ductile. The failure of concrete, too much and too little water columns can affect the stability of both can cause havoc. building, but failure of a beam causes localized effect. Therefore, it is better to 3. Steel make beams to be ductile weak links then columns. This method of designing Steel is used in masonry and RC buildings is called the strong-column concrete buildings as reinforcement bars weak-beam design method. Special of diameter ranging from 6mm to 40mm. design provisions from IS: 13920-1993 reinforcing steel can carry both tensile for RC structures ensures that adequate and compressive loads. Moreover steel is ductility is provided in the members a ductile material. This important where damage is expected. property of ductility enables steel bars to 7.2 Base Isolation for Earthquake 7.3 Traditional Earthquake Mitigation Techniques It is easiest to see the principle at work by referring directly to the most widely used of these advanced techniques, known as base isolation. A base isolated structure is supported by a series of bearing pads, which are placed between the buildings and building foundation. The concept of base isolation is explained through an example building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move. Thus, no force is transferred to the building due to the shaking of the ground; simply, the building does not experience the Fig 3 earthquake. Now, if the same building is rested on the flexible pads that offer 7.4 Base Isolation Technique resistance against lateral movements, then some effect of the ground shaking will be transferred to the building above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure. Fig 4 Due to the flexibility in the structure, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed, to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building. Many of the say that the building is deforming. The base isolators look like large rubber primary cause of earthquake damage to pads, although there are other types that buildings is the deformation which the are based on sliding of one part of the building undergoes as a result of the building relative to other. Also, base inertial forces upon it. isolation is not suitable for all buildings. Mostly low to medium rise buildings 7.6 Spherical Type of Isolation rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation. Lead-rubber bearings are the frequently-used types of base isolation bearings. A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the solid lead "plug". On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction. Fig 5 Spherical sliding isolation systems are 7.5 Working Principle another type of base isolation. The building is supported by bearing pads To get a basic idea of how that have a curved surface and low base isolation works, first examine the friction. During an earthquake the `(fig 3). This shows an earthquake acting building is free to slide on the bearings. on base isolated building and a Since the bearings have a curved conventional, fixed-base, building. As a surface, the building slides both result of an earthquake, the ground horizontally and vertically. The forces beneath each building begins to move. In needed to move the building upwards (fig 3) it is shown moving to left. Each limits the horizontal or lateral forces building responds with movement which which would otherwise cause building tends towards the right. The buildings deformations. Also by adjusting the displacement in the direction opposite radius of the bearings curved surface, the ground motion is actually due to this property can be used to design inertia. The inertia forces acting on a bearings that also lengthen the buildings building are the most important of all period of vibration. those generated during an earthquake. In addition to displacing 8.0 ENERGY DESSIPATING towards right, the un-isolated building is DEVICES FOR EARTHQUAKE also shown to be changing its shape RESISTANCE from a rectangle to a parallelogram. We Another approach for controlling seismic Friction Dampers (energy is absorbed damage in buildings and improving their by surfaces with friction between them seismic performance is by installing rubbing against each other) Seismic Dampers in place of structural elements, such as diagonal braces. These Yielding Dampers (energy is absorbed dampers act like the hydraulic shock by metallic components that yield) absorbers in cars - much of the sudden jerks are absorbed in the hydraulic fluids Viscoelastic dampers (energy is and only little is transmitted above to the absorbed by utilizing the controlled chassis of the car. When seismic energy shearing of solids) is transmitted through them, dampers absorb part of it, and thus damp the Thus by equipping a building with motion of the building. additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building. 8.1 Working Principle Fig 7 The construction of a fluid damper is shown in (fig). It consists of a stainless steel piston with bronze orifice head. It is filled with silicone oil. The piston head utilizes specially shaped passages which alter the flow of the damper fluid and thus alter the resistance Fig 6 characteristics of the damper. Fluid dampers may be designed to behave as a Commonly used types of seismic pure energy dissipater or a spring or as a dampers include: combination of the two. Viscous Dampers (energy is absorbed A fluid viscous damper by silicone-based fluid passing between resembles the common shock absorber piston cylinder arrangement) such as those found in automobiles. The piston transmits energy entering the system to the fluid in the damper, The latest Friction- causing it to move within the damper. ViscoElastic Damper Device (F-VEDD) The movement of the fluid within the combines the advantages of pure damper fluid absorbs this kinetic energy frictional and viscoelastic mechanisms by converting it into heat. In of energy dissipation. This new product automobiles, this means that a shock consists of friction pads and viscoelastic received at the wheel is damped before it polymer pads separated by steel plates as reaches the passengers compartment. In shown below. A prestressed bolt in buildings this can mean that the building combination with disk springs and columns protected by dampers will hardened washers is used for undergo considerably less horizontal maintaining the required clamping force movement and damage during an on the interfaces as in original FDD earthquake. concept. 8.2 Second Type of Energy Dissipation 8.3 Active Control Devices for Devices Earthquake Resistance The innovative methods for After development of passive devices control of seismic vibrations such as such as base isolation and TMD. The frictional and other types of damping next logical steps is to control the action devices are important integral part of of these devices in an optimal manner by seismic isolation systems as they severe an external energy source the resulting as a barrier against the penetration of system is known as active control device seismic energy into the structure. In this system. Active control has been very concept, the dampers suppress the widely used in aerospace structures. In response of the isolated building relative recent years significant progress has to its base. been made on the analytical side of active control for civil engineering The novel friction damper device structures. Also a few models explains consists of three steel plates rotating as shown that there is great promise in against each other in opposite directions. the technology and that one may expect The steel plates are separated by two to see in the foreseeable future several shims of friction pad material producing dynamic "Dynamic Intelligent friction with steel plates. Buildings" the term itself seems to have been joined by the Kajima Corporation When an external force excites a in Japan. In one of their pamphlet the frame structure the girder starts to concept of Active control had been displace horizontally due to this force. explained in every simple manner and it The damper will follow the motion and is worth quoting here. the central plate because of the tensile forces in the bracing elements. When the People standing in swaying applied forces are reversed, the plates train or bus try to maintain balance by will rotate in opposite way. The damper unintentionally bracing their legs or by dissipates energy by means of friction relaying on the mussels of their spine between the sliding surfaces. and stomach. By providing a similar function to a building it can dampen immensely the vibrations when 2) Tendon Control confronted with an earthquake. This is the concept of Dynamic Intelligent Various analytical studies have been Building (DIB). done using tendons for active control. At low excitations, even with the active The philosophy of the past control system off, the tendon will act in conventional a seismic structure is to passive modes by resisting deformations respond passively to an earthquake. In in the structures though resulting tension contrast in the DIB which we propose in the tendon. At higher excitations one the building itself functions actively may switch over to Active mode where against earthquakes and attempts to an actuator applies the required tension control the vibrations. The sensor in tendons. distributed inside and outside of the building transmits information to the 3) Other Methods computer installed in the building which can make analyses and judgment, and as The liquid sloshing during earthquakes if the buildings possess intelligence has assumed significance importance in pertaining to the earthquake amends its view of over flow of petroleum products own structural characteristics minutes by from storage tank in post earthquakes. minute. One of the important consideration with sloshing is that is associated with a very 8.4 Control Force Devices low damping. The wave height was controlled through force applied to the Many ways have been proposed to side wall by a hydraulic actuator. The apply control forces to a structure. Some active control successfully reduced wave of these have been tested in laboratory heights to the level of 6% of those on scaled down models. Some of the without control, for harmonic excitations ideas have been put forward for at sloshing frequency. For earthquake applications of active forces are briefly type excitation the wave heights were described in the following: reduced to 19% level. 1) Active tuned Mass Dampers 9.0Tips to Design (TMD) Is it possible to construct Earthquake- these are in passive mode have been proof buildings? Maybe it is, but the used in a umber of structures as costs can be huge. So, only Nuclear mentioned earlier. Hence active TMD is Power Plants can afford to be a natural extension. In this system 1% Earthquake Proof. For the rest of us, its of the total building mass is directly earthquake resistant buildings, so that we excited by an actuator with no spring can minimize the loss of life and and dash pot. The system has been property, termed as Active Mass Driver (AMD). The experiments indicated that the Earthquakes do not kill people, but building vibrations are reduced about actually people are killed by the collapse 25% by the use of AMD. of badly designed and constructed buildings. But, with the different types of new materials available in our capacity, then it should be inventory, it is feasible to construct an divided into two three smaller earthquake-resistant building. tanks and should be kept at different locations to maintain Some care should be taken while balance of cottage-building. constructing a building in earthquake If the column length is more than prone areas. Special attention should be 12 feet. Then bracing beams given to Structural Design of the should be provided in between structure. Here are some tips for the column at regular intervals. designing safer structures. Bracing beams strengthen a column, and allow construction Building should be of regular of multistoried buildings. shapes. Cylindrical structures The columns should be perform better in high-wing areas connected at each level. Architect should try to design the For strengthening the brick work, building as aerodynamic as a sill or a lintel should be possible. This reduces the effect provided at every 3 feet level and of wind load on tall structures R.C.C. wall should be taken There should no odd shapes in where it is possible. elevation and the whole building Cottage-building should not should be in balance. The centre contain very large and heavy of gravity of buildings should not windows. They are bound to move weaken the structure. Cantilever projections should be The glass used in any structure minimum and their length should should fiber reinforced glass or not be more than 3 to 4 feet. wire glass The span between the columns should be as small as possible. 12.0 CONCLUSION Point loads on load-carrying beams should be avoided. Conventional approach to earthquake The dead loads on the cottage resistant design of buildings depends building should not be increased upon providing the building with unnecessarily. strength, stiffness and inelastic The sunk portions of WC and deformation capacity. But the new bath should be minimum. techniques like Energy Dissipation and Building should be a Reinforced Active Control Devices are a lot more Concrete framed structure. It efficient and better. provides better stability and reliability in Earthquake-prone If we manage to construct our buildings areas. this way, we will be capable to fight the Cottage-building’s foundation Earthquake and preventing the trail of should be placed on hard and loss of life and property that an level ground. Earthquake leaves behind. There should not be very large overhead water tanks than are required. If it has to have larger
"Earth Quake Resistance StrucTures"