JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (JCIET) INNOVATIVE ISOLATION TECHNIQUES FOR SEISMIC FORCES. Ar.Anurakti Yadav LNCT College, Bhopal

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1 JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (JCIET) Journal of Civil Engineering and Technology (JCIET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 1, Issue 1, July-December (2013), pp IAEME: JCIET IAEME INNOVATIVE ISOLATION TECHNIQUES FOR SEISMIC FORCES Ar.Anurakti Yadav LNCT College, Bhopal ABSTRACT Natural disaster, earthquakes, storms, floods, etc regularly hit the world. Among these, the most destructive is earthquake since its occurrence is still unpredictable. Ordinarily the houses are built to withstand only vertical loads and therefore when they are subjected to horizontal stresses produced by earthquake waves they collapse resulting in large-scale destruction. Since earthquake are capable of causing severe damage to environment; physically, ecologically; so it is necessary for an architect or engineers to know regarding earthquake, their occurrence, its harmful effects and precautionary measures that can taken to minimize harms and related factors. Historical evidence tells us that great earthquakes have shaped man s thinking over the millennia. While there is no evidence that the numbers of earthquakes are increasing, the impact in terms of human tragedy and property destroyed is clearly on rise. All this is making mankind more vulnerable than ever to disasters. In other words earthquakes are not killing people, but the buildings created by society are it is not fate but plan and design which are turning destiny s dark hand. Earthquakes the hammer of god, as they have been called is beyond human being s ability and forecast. It is also true that the intensity of an earthquake makes the condition worse. But the scale of reduction can be reduced to a large extent by proper planning and designing of houses and by suitable use of isolation base and seismic dampers. Conventional seismic design attempts to make buildings that do not collapse under strong earthquake shaking but may sustain damage to non-structural elements and some structural members in the building. This may render the building non-functional after the earthquake, which may be problematic in some structures like hospitals, which need to remain functional in the aftermath of the earthquake. Special techniques are required to design buildings such that they remain practically undamaged even in a severe earthquake. Buildings with such improved seismic performance usually cost more than normal buildings do. However, this cost is justified through improved earthquake performance. 57

2 Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. BASE ISOLATION If the flexible pads are properly chosen, the forces induced by ground shaking can be few smaller than that experienced by the building built directly on ground, namely a fixed base isolation. The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building or at least greatly reduced. 58

3 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 earthquake. A base isolated structure is supported by a series of bearing pads, which are placed between the buildings and building foundation. 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 base isolators look like large rubber pads, although there are other types that are based on sliding of one part of the building relative to other. Also, base isolation is not suitable for all buildings. Mostly low to medium rise buildings 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. Response of Base Isolated Buildings The base-isolated building retains its original, rectangular shape. The base isolated building itself escapes the deformation and damage-which implies that the inertial forces acting on the base isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes to as little as. Acceleration is decreased by ¼ of the acceleration of comparable fixed-base buildings because the base isolation system lengthens a buildings period of vibration, the time it takes for a building to rock back and forth and then back again. 59

4 Spherical Sliding Base Isolation Spherical sliding isolation systems are another type of base isolation. The building is supported by bearing pads that have a curved surface and low friction. During an earthquake the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically. The forces needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations. Also by adjusting the radius of the bearings curved surface, this property can be used to design bearings that also lengthen the buildings period of vibration. Base Isolation in India By now, over 1000 buildings across the world have been equipped with seismic base isolation. In India, base isolation technique was first demonstrated after the 1993 Killari (Maharashtra) Earthquake [EERI, 1999]. Two single storey buildings (one school building and another shopping complex building) in newly relocated Killari town were built with rubber base isolators resting on hard ground. Both were brick masonry buildings with concrete roof. After the 2001 Bhuj (Gujarat) earthquake, the four-storey Bhuj Hospital building was built with base isolation technique. Fig. 1 view of basement in Bhuj hospital building SEISMIC DAMPERS 60

5 Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing seismic dampers in place of structural elements, such as diagonal braces. These damper acts like the hydraulic shock absorbers. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building. Dampers were used since 1960s to protect tall buildings against wind effects. However, it was only since 1990s, that they were used to protect buildings against earthquake effects. Seismic dampers are special devices introduced in the buildings to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb due to undulations of the road). Types of seismic dampers 1) Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston cylinder arrangement), 2) Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other), 3) Yielding Dampers (energy is absorbed by metallic components that yield). 4) Viscoelastic Dampers (energy is absorbed by utilizing the controlled shearing of solids). 61

6 Fig. 2 Viscous Damper Viscous damper 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 characteristics of the damper. Fluid dampers may be designed to behave as a pure energy dissipater or a spring or as a combination of the two. A fluid viscous damper resembles the common shock absorber such as those found in automobiles. The piston transmits energy entering the system to the fluid in the damper, causing it to move within the damper. The movement of the fluid within the damper fluid absorbs this kinetic energy by converting it into heat. In automobiles, this means that a shock received at the wheel is damped before it reaches the passengers compartment. In buildings this can mean that the building columns protected by dampers will undergo considerably less horizontal movement and damage during an earthquake. Fig. 3 Friction Damper Friction damper The novel friction damper device consists of three steel plates rotating against each other in opposite directions. The steel plates are separated by two shims of friction pad material producing friction with steel plates. When an external force excites a frame structure the girder starts to displace horizontally due to this force. The damper will follow the motion and the central plate 62

7 Journal of Civil Engineering and Technology (JCIET), ISSN (Print), ISSN (Online) Volume 1, Issue 1, July-December July (2013), IAEME because of the tensile forces rces in the bracing elements. When the applied forces are reversed, the plates will rotate in opposite way. The damper dissipates energy by means of friction between the sliding surfaces. Viscoelastic dampers Viscoelastic Damper Device (F-VEDD) combines the advantages The latest Friction-Viscoelastic of pure frictional and viscoelastic mechanisms of energy dissipation. This new product consists of friction pads and viscoelastic polymer pads separated by steel plates. A prestressed bolt in combination with disk springs springs and hardened washers is used for maintaining the required clamping force on the interfaces as in original FDD concept concept. Seismic dampers in India dampers for seismic control of la gardenia towers Friction-dampers South city, Gurgaon was used in India. La Gardenia housing complex consists of 7 towers of eighteen storeys with two levels of basements. In the chosen structural system, Pall friction-dampers dampers are provided in steel bracing in concrete frames. The use of steel bracing eliminated the need of expensive concrete shear walls and the uses of friction friction-dampers eliminate te the need of dependence on member ductility. Friction-damped Friction damped bracing are located in partitions, around staircases or elevator shaft. Their use provided greater flexibility in space planning because unlike shear walls they do not need to be located conti continuously one over the other. Since friction-damped damped bracing do not carry any gravity load, these do not need to go down through the basements to the foundation. This allows more open space for car parking in the basement. At the ground floor level, the lateral lateral shear from the bracing is transferred through the rigid floor diaphragm to the perimeter retaining walls of the basement. The architects have exposed some friction-dampers friction dampers to view as they add to the aesthetic appearance. A total of 66 friction-dampers friction rs were required to extract sufficient energy to safeguard the structure and its contents from damage. REFERENCE 1) M.S. Shetty, concrete technology. 2) Concrete engineer s handbook. 3) Architectural journals. 4) Internet. 5) Earthquakes and buildings 6) Standard book of engineering 63