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Wednesday, January 11, 2017

BASE ISOLATION AND SEISMIC CONSIDERATION IN EARTHQUAKE RESISTANT STRUCTURES

By Anand Babu
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ABSTRACT 

           The naturally occurring ground movement which eventually goes on creating disasters such as failure of structure and fatality is known as Earthquake. Nowadays a seismic event may also endanger the social-economical stability of large areas due to the complexity of technologically advanced constructions.
           Base Isolation is a very effective way to destroy the damaging component of seismic forces and is one of the most widely implemented and accepted Design considerations for base isolated structures. The main objective of seismic isolation is to shift the fundamental frequency of a structure away from the dominant frequencies of the ground motions. Building is isolated from the ground such that earthquake motions are not transmitted up through the building. It is the main idea behind Base Isolation.
           Seismic isolation is done by supporting the structure on laminated rubber bearings, friction pendulum bearings, roller bearings etc. Rarely soil is also used as a base Isolation media. This paper describes the base isolation techniques and other techniques developed to resist earthquake forces on the structures.
           It has become evident in recent times that base isolation can be very effective in the event of an earthquake. So a case study of recent time describing the use of Base isolators during the reconstruction of District hospital of Bhuj, Gujarat, which had destroyed due to an earthquake is also included in this paper.

CHAPTER 1
INTRODUCTION

          The naturally occurring ground movement which eventually goes on creating disasters such as failure of structure and fatality is known as Earthquake. The energy that is discharged from those seismic activities makes waves, these waves are called as primary waves and secondary waves.These waves cause ground movement transmitted to the structure via foundation. Depending on the intensity of these vibrations, cracks and settlement is caused to the structure.Inertia force is induced in structure because of this earthquake movement. The maximum point at which the structure can deform and come back to its original shape is called as Elastic limit. If building deforms more than its elastic limit, it forms cracks in the structure. However, ductility will induce some acceptable damage to the structure. If more elasticity is introduced to the structure, it may tend to increase the overall cost and decrease the damage by increasing the strength.Earthquakes are unanticipated phenomena if the structure is located in seismic zones. The structural engineer has to step in so as to save lives and cause minimal damage to the structures in times of earthquake. The recent development for anti-seismic designs is base isolation. Base isolation system is the frequently adopted earthquake resistance system. It reduces the effect of ground motion and thus reduce the effect of earthquake on the structure. 
           The field of seismic design is a subject that deals primarily with life safety and uncertainty. For several years now, it has been a quest for structural engineers to design earthquake-proof buildings and bridges. Initially, it has been generally thought that building a massive and stiff construction would make it earthquake resistant. But this stiffness or rigidity of the structural elements would lead eventually to a fragile and sudden failure, all in all not complying with the life safety performance criteria and letting inhabitants no time to react in case of an earthquake.
 Next, the increase of damping, redundancy of buildings, ductility and seismic energy dissipation were taken into consideration and well implemented throughout the years in seismic building codes. Furthermore, a new alternative approach was implemented in earthquake protective systems and base isolation being one of the most common systems nowadays.

CHAPTER 2
 CONCEPT OF BASE ISOLATION

Base isolation has become a traditional concept for structural design of buildings and bridges in high risk areas. By introducing flexible isolation system between the foundation and the structure the system will absorb the shock impact effects of earthquake with the help of its flexibility. This way the seismic energy transmitted to the structure will be reduced to greater extent and the structure will remain stable for a relative period. Rubber bearing and lead rubber bearing are prime factors used to introduce flexibility in the structure. This increased the natural period of the structure and base displacement is more than prearranged limit. Though, base isolation not always liable to work against the strong earthquakes as it may result in larger displacement at the base of the structure. Figure 2.1 shows the performance of building with and without isolation


Fig.2.1. Performance of Building with & without Isolation (source:Dr. R. S. Talikoti, Mr.Vinod R. Thorat www.ijert.org)

Basic principle of base isolation is to differentiate the building from its foundation. During the seismic action, building is unaffected from the ground motion. In other words, even though ground moves aggressively, the building will tend to move ideally as a rigid body rather than collapsing. This reduces the floor hastening and storey gliding and so the building components are less harmed. Any stiff structure will have short period. During the ground movement, amount of acceleration entrusted in the structure is the same as that of ground acceleration that results in zero displacement between the structure and the ground. In other words, ground and structure will move with equal amount.
Base isolation increase the flexibility of the structure and hence increases the period of the structure which is due to the isolators. By introducing base isolation in a structure increases the displacement and eventually decreases the acceleration in the structure as the stiffness of the structure also decreases. Generally, the isolation is placed at the base of the structure, Base isolation protects the building components of the superstructure during earthquakes. So flexible structure will have longer life span.

The principle in base isolation,

To provide horizontally flexible as well as vertically stiff to the building.
To lengthening the natural period of the building.
Damping in the Isolation system reduces the displacement.
It also reduces in the acceleration of the story.
Base isolation system should contain following:-

An elastic mount to add enough vibration periods to the structure to lower down the forces in the structure over.
An energy dissipater or damper to ease the deflection taking place between the structure and the ground.
Introducing the stiffness against the seismic actions and wind loads.

Base Isolation Consideration

Base isolation is required if any circumstances arise of the
following:-

Need to increase the safety of the structure.
Low lateral seismic forces needed.
Any existing building is not capable to withstand any earthquake.
Withstand small earthquakes without any damage.
Structure will not collapse in high level earthquake but some structural and non-structural damage will occur

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