What is Base Isolation?
It is a system that may be defined as a flexible or sliding interface positioned between a structure and its foundation, for the purpose of decoupling the horizontal motions of the ground from the horizontal motions of the structure, thereby reducing earthquake damage to the structure and its contents. Base isolation system absorbs and deflects the energy released from the earthquake before it is transferred to the structure. The term isolation refers to reduced interaction between structure and the ground. Since the seismic isolation system is located under the structure, it is referred as ‘Base isolation’. Base isolation is a passive control system meaning thereby that it does not require any external force or energy for its activation. The base isolators used in this system mitigate the effect of an earthquake by decoupling the components of the buildings from direct contact with the ground essentially isolating the structure from potentially dangerous ground motions. The base-isolation techniques prove to be very effective for the seismic protection of new framed buildings as well as for the seismic retrofitting of existing ones. The concept of separating the structure from the ground to avoid earthquake damage is quite simple to grasp. After all, in an earthquake the ground moves and it is this ground movement which causes most of the damage to structures. An airplane flying over an earthquake is not affected. So, the principle is simple. Separate the structure from the ground. The ground will move but the building will not move.
OBJECTIVES OF SEISMIC ISOLATION SYSTEM
Enhance performance of structures at all hazard levels by:
- Minimizing interruption of use of facility (Immediate Occupancy Performance Level)
- Reducing damaging deformations in structural and non-structural component
- Reducing acceleration response to minimize contents related damage
- Preventing plastic deformation of structural elements
- Protection of Building Frame
- Protection of Non-structural Components & Contents
- Provide for an Operational facility after the Earthquake
- Protection of Life
- Safety of occupants
- Improvement for Safety of Building
PRINCIPLE OF BASE ISOLATION
The fundamental principle of base isolation is to modify the response of the building so that the ground can move below the building without transmitting these motions into the building. A building that is perfectly rigid will have a zero period. When the ground moves the acceleration induced in the structure will be equal to the ground acceleration and there will be zero relative displacement between the structure and the ground. The structure and ground move by same amount. A building that is perfectly flexible will have an infinite period. For this type of structure, when the ground beneath the structure moves there will be zero acceleration induced in the structure and the relative displacement between the structure and ground will be equal to the ground displacement, So in flexible structures the structure will not move, the ground will.
BASIC ELEMENTS OF SEISMIC ISOLATION
Basic elements in seismic isolation systems are:
- A vertical-load carrying device that provides lateral flexibility so that the period of vibration of the total system is lengthened sufficiently to reduce the force response,
- A damper or energy dissipater so that the relative deflections across the flexible mounting can be limited to a practical design level,
- A means of providing rigidity under low (service) load,
- Steel plates, vulcanized rubber, and a lead plug in the center of the design create these functional contrasting directional components.
The benefits of using seismic isolation for earthquake resistant design
- Isolation leads to a simpler structure with much less complicated seismic analysis as compared with conventional structures
- Isolated designs are less sensitive to uncertainties in ground motion
- Minor damage at the design level event means immediate reoccupation
- The performance of the isolators is highly predictable, so they are much more reliable than conventional structural components
- Even in case of larger-than-expected seismic events, damage will concentrate in the isolation system, where elements can be easily substituted to restore the complete functionality of the structure
- Base Isolation minimizes the need for strengthening measures like adding shear walls, frames, and bracing by reducing the earthquake forces imparted to the building.
- Base Isolated building are capable of resisting GSA blasts loads and their ability to move reduces the overall impact of the blast force on the building.