BASE ISOLATION SYSTEM

ASHOK KUMAR S 07ST03F

INTRODUCTION:-

There are two approaches for structural-level retrofitting:

(1). Conventional methods

-based on increasing the seismic resistance of existing

structure

Ex: shear walls, braced frames or moment resistant frames.

(2). Non-conventional methods

-based on reduction of seismic demands

Ex:- base isolation, dampers.

Definitions An Isolation system is defined as the collection of

isolation units, isolation components and all other

structural elements that transfers force between the

foundation/substructure and superstructure.

An Isolation unit is defined as a device that provides all

the necessary characteristics of the system in an integral

device.

An Isolation component is defined as a device that

provides some of the necessary characteristics of the

system (i,e, flexibility or damping) in a single device.

During a Richter 8.0 Earthquake a seismically isolated

building will behave as if it were experiencing a 5.5

earthquake.

Application of base isolation:-

1st application in New Zealand in 1974.

1st US application in 1984.

1st Japanese application in 1985.

Conventional Structure

The deformation pattern of a conventional structure during an earthquake. Accelerations of the ground are amplified on the higher floors, and the contents are damaged.

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Seismically Isolated Structure

The deformation pattern of an isolated structure during an earthquake. Movement takes place at the level of the isolators. Floor accelerations are low. The building, its occupants and contents are safe.

Suitability of seismic isolation

Earthquake protection of structures using base isolation technique is

generally suitable if the following conditions are fulfilled:

• The subsoil does not produce a predominance of long period ground

motion.

• The structure is fairly squat with sufficiently high column load.

• The site permits horizontal displacements at the base of the order of

200 mm or more.

• Lateral loads due to wind are less than approximately 10% of the

weight of the structure.

BASIC REQUIREMENTS OF SEISMIC ISOLATION SYSTEMS

A practical seismically isolated structure should meet the

fallowing three requirements

• Sufficient horizontal flexibility to increase the structural

period and spectral demands, except for very soft soil sites.

• Sufficient energy dissipation capacity to limit the

displacements across the isolators to a practical level.

• Adequate rigidity to make the isolated building no different

from a fixed-base building under general service loading.

APPLICABILITY OF BASE ISOLATION SYSTEMS

• Most effective

- Structure on Stiff Soil

- Structure with Low Fundamental Period

(Low-Rise Building)

• Least effective

- Structure on Soft Soil

- Structure with High Fundamental Period

(High-Rise Building)

Concept of base isolation

The concept of base isolation is explained through an example building resting on

frictionless rollers (Figure a). 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 shaking of the ground; simply, the building does not experience the earthquake.

Now, if the same building is rested on flexible pads that offer resistance against

lateral movements (Figure b), then some effect of the ground shaking will be

transferred to the building above. If the flexible pads are properly chosen, the

forces induced by ground shaking can be a few times smaller than that

experienced by the building built directly on ground, namely a fixed base building

(Figure c).

Types of Seismic Isolation Bearings Elastomeric Based systems

• Low-Damping Natural or Synthetic Rubber Bearing

• High-Damping Natural Rubber Bearing

• Lead-Rubber Bearing

• (Low damping natural rubber with lead core)

Isolation systems based on Sliding

• Isolator without recentering capacity (Flat Sliding Bearing)

• Isolator with recentering capacity (Spherical Sliding

Bearing)

Elastomeric systems are alternative layers of steel and

elastomers, generally bonded together under high heat

and pressure, to form an integral bearing that is free of

joints. The laminated bearing provides the vertical

stiffness, lateral flexibility and damping characteristics

necessary for seismic isolation.

Sliding systems use two dissimilar materials to form an

interface that permits relative movement between the two

surfaces. Friction acts between the materials and serves to

dissipate energy upon sliding.

ELASTOMERIC-BASED SYSTEMS

Geometry of Elastomeric Bearings

Major Components:

• Rubber Layers: Provide lateral flexibility

• Steel Shims: Provide vertical stiffness to support building

weight while limiting lateral bulging of rubber

• Lead plug: Provides source of energy dissipation

Low Damping Natural or Synthetic Rubber Bearings

Linear behaviour in shear for shear strains up to

and exceeding 100%.

Damping ratio = 2 to 3%

Advantages:

- Simple to manufacture

-Easy to model

- Response not strongly sensitive to rate of

loading, history of loading, temperature, and

aging.

Disadvantage:

-Need supplemental damping system

High-Damping Natural Rubber Bearings• Maximum shear strain = 200 to 350%

– Damping increased by adding extra fine carbon black, oils

or resins, and other proprietary fillers

• Damping ratio = 10 to 20% at shear strains of 100%

• Shear modulus = 50 to 200 psi

Effective Stiffness and Damping depend on:

• Elastomer and fillers

• Contact pressure

• Velocity of loading

• Load history (scragging)

• Temperature

Lead-Rubber BearingsInvented in 1975 in New Zealand and used extensively

in New Zealand, Japan, and the United States.

• Low damping rubber combined with central lead

core.

• Shear modulus = 85 to 100 psi at 100% shear strain

• Maximum shear strain = 125 to 200% (since max.

shear strain is typically less than 200%, variations in

properties are not as significant as for high-damping

rubber bearings)

• Solid lead cylinder is press-fitted into central hole of

elastomeric bearing

ISOLATION SYSTEMS BASED ON SLIDING

• The other approach for increasing flexibility in a structure is to provide a

sliding or friction surface between the foundation and the base of the

structure.

• Sliding bearings consist of an upper and lower bearing plate and an

interposed spherical sliding part. This type of bearing transmits vertical

loads to the sliding surface, obtaining the horizontal displacement. The

friction coefficient between sliding part and bearing plate determines the

dissipation, which results from the relative displacements of the structure

to the subsoil.

• The co-efficient of friction is usually kept as low as practically. However,

it must be sufficiently high to provide a friction force that can sustain

strong winds and minor earthquakes without sliding.

Sliding isolators without recentering

capacity (SI)

• Sliding isolators type SI (= sliding isolator)

without recentering capacity consist of a

horizontal sliding surface, allowing a

displacement and thus dissipating energy

by means of defined friction between both

sliding components and stainless steel.

• One particular problem with a sliding

structure is the residual displacements that

occur after major earthquakes.

SLIDING ISOLATOR WITHOUT RECENTERING

CAPACITY.

Sliding isolator with recentering capacity:-

SLIDING ISOLATOR WITH RECENTERING

CAPACITY

Compared with sliding isolators, sliding isolation

pendula (SIPs) with recentering capacity have a

concave sliding plate.

Due to geometry, each horizontal displacement

results in a vertical movement of the isolator.

Thus a part of kinetic energy is transformed into

potential energy. The potential energy, stored by

the superstructure, which has been pushed to the

top, automatically results in recentering the

bearing into neutral position. The sliding

isolation pendula are excellently suited to isolate

the structure from the subsoil. They remain

horizontally flexible, dissipate energy and

recenter the superstructure into neutral position.

Sliding isolation systems have been successfully used for nuclear power plants, emergency fire water tanks , and other important structures.

Sliding bearing limits the transmission of seismic force to

level that is function of friction coefficient of sliding

interface. This behaviour is interesting for protection of non-

ductile and non-structural components against earthquake

when expected acceleration is more than their strength level.

However there are some negative aspects in seismic behavior

of sliding bearings like lack of restoring force and

transmission of high frequencies. Transmission of high

frequency excitation causes damage in sensitive equipments.

To avoid these undesirable features, sliding bearings are typically

used in combination with a restoring spring. When spring and

slider are used in series (Fig. 1), sliding does not occur for seismic

excitation below a certain threshold, and the isolated structure

responds only in elastic part. This behavior can filter direct and

indirect excitation of high frequency due to stick-slip. However in

strong excitation, this system may result in residual displacement.

When spring and slider are in parallel combination, i.e., Resilient

Sliding Isolation System (Fig. 2) transmission force to equipment

is equal to restoring force of spring plus friction force at sliding

interface. This combination can reduce both transmission of

indirect high frequency excitation and residual displacement.

Fig(1) slider and spring in series Fig(2) slider and spring in parallel

Advantages

• -Isolates Building from ground motion.

• - Minimal repair of superstructure

• -Building can remain serviceable throughout construction.

• -Does not involve major intrusion upon existing superstructure.

Disadvantages

• -Costly, Is challenging to implement in an efficient manner.

• -Costly to connect utilities to building (flexible connections).

• -Must allow for building displacements

CONSTRUCTION STEPS