Seismic Isolation is a concept in which an isolation bearing is used to isolate the superstructure from ground movement transferring reduced inertia forces by means of very low lateral stiffness isolators compared to main structure. It is governed by a fundamental period shift to higher periods which brings a reduction in forces attracted by the structure but at the cost of increased isolator displacements. However, relative displacement of superstructure, which is primarily responsible for damage in the superstructure, is decreased achieving intended response. If there's a limit on isolator displacement because of neighboring structure of buildings, limited seat width of bridges or due to utilities, increased isolators displacement response can be controlled with the help of added dampers.

Effect of seismic (base) isolation on the response of a structure

Location of Bearings (1) Between superstructure and foundation for the buildings (2) Between deck and piers for bridges

Here is a video demonstrating the effect of base isolation.

www.youtube.com/watch?v=ZqlXp3czrrM

Linear theory of seismic isolation approximate the behavior of seismically isolated structure with 2 degree of freedom system. First degree of freedom is horizontal displacement of isolator and second is the horizontal relative displacement of superstructure. Some important conclusions that can be drawn from linear theory of base isolation are as follows:
As the time period of isolator and superstructure grows apart, that is, superstructure becomes more and more stiffer than isolator, first mode is primarily relative displacement of isolator and second mode for which the participation becomes very less, is the relative deformation of superstructure.

There are different kinds of systems used for seismic isolation:
1. Elastomeric Bearings
2. Sliding Bearings
3. Metallic yielding devices

Elastomeric bearings are made up of alternate layers of rubber and steel. Steel plates used are called shims, and are used as reinforcements to increase the strength and reduce the lateral bulging. Damping is provided by energy dissipation capabilities of rubber. Initially high damping rubber was used to provide the required damping, however high damping rubber had inherent scragging effects. Low damping rubber, made of natural rubber, saw an increased demand in application on account of its availability and ability to predict the behavior more accurately. Development of lead rubber bearings provided an efficient method of achieving damping requirements by energy dissipation in lead. Lead has an excellent energy dissipation quality and it recover its properties even after many cycles of yielding.

• Unordered List ItemHigh damping elastomeric bearings
• Low damping elastomeric bearings
• Lead Rubber Bearings

This video shows a good presentation of an elastomeric bearings.
www.youtube.com/watch?feature=player_embedded&v=LKDu6CCedaA

Rubber has very low stiffness in horizontal direction but is almost incompressible in the vertical compression. So it is good for vertical load transfer from structure to the ground and at the same time provides isolation from horizontal ground motion by having very low shear stiffness. However, rubber does not perform adequately in tension. Modern day rubber bearings, with good quality control, have been able to provide some tension capacity up to a pressure of 3G, where G is the shear modulus of the rubber. After vertical tension reaches 3G, rubber cavitates and loses its capacity. Tension is not a major issue except in near fault areas, where vertical ground motion needs to be considered.
Internal construction of a lead-rubber (LR) bearing (Constantinou, et al., 2007)

Sliding bearings provide isolation by two or more metallic hardware sliding on each other. Damping and energy dissipation is provided by the friction on the sliding surfaces.

• Flat sliding bearings
• Friction pendulum bearings

Flat sliding bearings were mainly developed for non-seismic bridge applications to accommodate thermal and vehicle loading displacements. They provide none or very low horizontal stiffness and hence no restoring force, hence their applications for seismic applications have been very limited.

Friction pendulum bearings, on the other the hand, can provide desired stiffness and restoring force by providing the curvature to the sliding surfaces. They can accommodate very high displacement demands and can work efficiently even in very cold regions as opposed to rubber bearings.

Sectional view of a single friction pendulum bearing (Constantinou et al, 2007)