Base isolation topic as per jntu syllabus for m.tech 1st year structures

Base isolation of structures

Earthquake Protective Systems

Passive Protective Systems

Hybrid Protective Systems

Active Protective Systems

Tuned Mass Damper

Energy Dissipation

Base Isolation

Active Isolation

Semi-Active Isolation

Semi-Active Mass Damping

Active Mass Damping

Active Bracing

Adaptive Control

Earthquake Protective Systems

Base Isolation is the most common System

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Base Isolation falls into general category of Passive Energy Dissipation, which also includes In-Structure Damping.

In-structure damping adds damping devices within the structure to dissipate energy but does not permit base movement

Where isolation and/or energy dissipation devices are powered to provide optimum performance.

Introduction

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.

IntroductionThe 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.

How can the structure be separated from the ground forearthquake loads but still resist gravity?

Ideal separation would be total. Perhaps an air gap, frictionless rollers, a well-oiled sliding surface, sky hooks, magnetic levitation. These all have practical restraints. An air gap would not provide vertical support; a sky-hook needs to hang from something; frictionless rollers, sliders or magnetic levitation would allow the building to move for blocks under a gust of wind.

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

Animation Showing Base Isolation

Fixed Base Isolated Base

Comparison of Acceleration and Displacement for RIGID and FLEXIBLE Structure

Base Isolation Technique

Concept of Base Isolation

Concept of Base Isolation

The concept of base isolation is explained through an example building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move(As shown in Fig (a)). Thus, no force is transferred to the building due to the shaking of the ground; simply, the building does not experience the earthquake.

Concept of Base Isolation

Concept of Base Isolation

Now, if the same building is rested on the flexible pads that offer resistance against lateral movements fig (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 fig (c). The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure.

As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed, to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building. Many of the base isolators look like large rubber pads, although there are other types that are based on sliding of one part of the building relative to other. Also, base isolation is not suitable for all buildings. Mostly low to medium rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.

Concept of Base Isolation

Consideration for Seismic Isolation

The benefits of using seismic isolation and energy dissipation devices (“isolators” for simplicity) for earthquake-resistant design are many:

1. Isolation leads to a simpler structure with much less complicated seismic analysis as compared with conventional structures;

2. Isolated designs are less sensitive to uncertainties in ground motion;

3. Minor damage at the design level event means immediate reoccupation;

4. The performance of the isolators is highly predictable, so they are much more reliable than conventional structural components (e.g. some ductile walls in the Christchurch earthquakes); and finally,

5. 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.

Basic elements of seismic isolation-Isolation Elements

The three basic elements in seismic isolation systems that have been used to date 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, and

A means of providing rigidity under low (service) load .

Seismic- isolation design principle

Factor of Safety = Column Strength/Earthquake force > 1

Factor of Safety = Capacity/Demand > 1

Capacity > Demand

The earthquake causes inertia forces proportional to the product of the building mass and the earthquake ground accelerations. As the ground accelerations increases, the strength of the building, the capacity, must be increased to avoid structural damage.

1. From the above figure:-The top curve shows realistic elastic forces based on a 5%ground response spectrum will be imposed on a non- isolated structure .

2. The spectrum shown is for a rock site if the structure has sufficient elastic strength to resist the level of the load.

3. The lowest curve shows the forces which the uniform building code requires a structure to be designed for.

4.And the second lowest curve shows the probable strength assuming the structure is designed for the UBC(Uniform Building Code) Forces.

5.The probable strength is 1.5 to 2.0 times higher than the design strength because of the design load factors. actual material strength which are greater in practice than those assumed for design, conservatism in structural design, and other factors.

6.The difference between maximum elastic force and the probable yield strength is an appropriate indication of the energy, which must be absorbed by ductility in the structural elements.

Types of Isolators : -

1.SLIDING SYSTEMSA layer with a defined coefficient of friction will limit the accelerations to this value and the forces which can be transmitted will also be limited to the coefficient of friction times the weight.

1.SLIDING SYSTEMS

1.SLIDING SYSTEMS

The building is supported by bearing pads that have a curved surface and low friction. During an earthquake the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically. The forces needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations. Also by adjusting the radius of the bearings curved surface, this property can be used to design bearings that also lengthen the buildings period of vibration.

1.SLIDING SYSTEMS

1.SLIDING SYSTEMS

2.ELASTOMERIC (RUBBER) BEARINGS

Elastomeric bearings are formed of horizontal layers of natural or synthetic rubber in thin layers bonded between steel plates. The steel plates prevent the rubber layers from bulging and so the bearing is able to support higher vertical loads with only small deformations. Under a lateral load the bearing is flexible.

2.ELASTOMERIC (RUBBER) BEARINGS

Lead-rubber bearings are the frequently-used types of base isolation bearings. A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the solid lead “plug”. On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

Eg Lead Rubber Pad

Eg Lead Rubber Pad

Elastomeric Isolators – Lead Core Rubber Bearings

3.SPRINGS

3.SPRINGS

3.SPRINGS

There are some proprietary devices based on steel springs but they are not widely used and their most likely application is for machinery isolation. The main drawback with springs is that most are flexible in both the vertical and the lateral directions. The vertical flexibility will allow a pitching mode of response to occur. Springs alone have little damping and will move excessively under service loads.

4. Roller and Ball Bearings

Rolling devices include cylindrical rollers and ball races. As for springs, they are most commonly used for machinery applications. Depending on the material of the roller or ball bearing the resistance to movement may be sufficient to resist services loads and may generate damping.

Rolling devices include cylindrical rollers and ball races. As for springs, they are most commonlyused for machinery applications. Depending on the material of the roller or ball bearing theresistance to movement may be sufficient to resist services loads and may generate damping.

4. Roller and Ball Bearing Devices

Earthquake Resistance House

Base Isolation House

'Earthquake-resistant' technology enables the building to counter quakes by making its strength and resilience great enough to resist shakings. Although it can protect the building safely, it is ac-companied by a risk that the furniture inside could fall or drop.

The technology called 'seismic isolation structure' which turns destructive seismic shakings into slower and softer ones prevents possible damage. This structure can evade the tremors, taking them in stride and safeguarding the building, not mention the human lives and property

Types of Base Isolators Used-cont.Response of Base Isolated BuildingsThe base-isolated building retains its original, rectangular shape. The base isolated building itself escapes the deformation and damage-which implies that the inertial forces acting on the base isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes to as little as ¼ of the acceleration of comparable fixed-base buildings.

Acceleration is decreased because the base isolation system lengthens a buildings period of vibration, the time it takes for a building to rock back and forth and then back again. And in general, structures with longer periods of vibration tend to reduce acceleration, while those with shorter periods tend to increase or amplify acceleration.

Feasibility of seismic isolationStructures are generally suitable for seismic isolation if the

following conditions exist:-

1.The sub soil does not produce a predominance of long period ground motion.

2.The structure has two stories or more ( or is usually heavy).

3.The site permits horizontal displacements at the base of order 6inches.

4. The structure is fairly squat.

5.Wind lateral loads and other non-earthquake loads are less than approximately 10% of the weight of the structure.Each project must be assessed individually and early in design phase to determine the suitability for seismic isolation.

Seismic-isolation configurations

It Includes:Layout and installation details for the isolation system depends on the site constraints,Type of structureConstruction and other related factors.The following details are provided as an aid in determining the appropriate layouts for particular projects and not intended to restrict the designer in individual cases.1. Bearing location2. Connection details

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