BASE ISOLATION TECHNIQUES

What is the base isolation technique?

Base isolation is a state-of-the-art method in

which the structure (superstructure) is separated

from the base (foundation or substructure) by

introducing a suspension system between the

base and the main structure.

Purpose of Base Isolation

• Wind and Earthquake are the most predominant

loads that demands lateral design of a structure.

Again, earthquake load is not controllable and it

is not practical to design a structure for an

indefinite seismic demand.

• Base isolation takes an opposite approach, i.e. to

reduce the seismic demand instead of increasing

the capacity. Controlling ground motion is

impossible, but we can modify the demand on

structure by preventing/reducing the motions

being transferred to the structure from

foundations.

Response of base isolated & non-isolated structure

Basic requirements

Basic requirements of an isolation system are

– Flexibility– Damping– Resistance to Vertical or other service loads.

Type of Base Isolation Devices

There are Six major types of base isolation devices which are widely adopted for seismic base isolation.

– Elastomeric Bearings– High Damping Bearings– Lead Rubber Bearings– Flat Slider Bearings– Curved Slider Bearings or Pendulum Bearings– Ball & Roller Bearings.

Elastomeric Rubber Bearings

Elastomeric Rubber Bearings

• Bearings formed of horizontal layers of synthetic or natural rubber in thin layers bound between steel plates. These bearings are capable of supporting high vertical loads with very small deformations. These bearings are flexible under lateral loads. Steel plates prevent the rubber layers from bulging. Lead cores are provided to increase damping capacity as plain elastomeric bearings does not provide significant damping. They are usually soft in horizontal direction and hard in vertical direction.

Roller and Ball Bearings

• For isolation applications in machinery isolation, roller and ball bearing are used. It includes cylindrical rollers and balls. It is sufficient to resist service movements and damping depending on the material used.

Springs