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What is base isolation system? Different types of base isolations used in Building design?
Base isolation system
The base isolation systems are the most extensively used methods which are commonly used to separate the building and ground so that they allow only horizontal moment on the foundation and they are comes under type of the retrofitting of building concepts. The main aim of this system is to decrease the seismic loading action by providing the isolation device in between the end point of foundation to the starting point of the column section.
Introduction to isolation
The seismic loading consideration is most important aspect we need to take in building design. The seismic loading parameters are considered as per the standards which is provided by IS 1893 2016 latest code provisions. As per the IS 1893 standards the seismic loading condition is classified into four zones namely zone II, zone III, zone IV and zone V. The zone II seismic condition is called as low seismic zone and which is having seismic coefficient factor is 0.10, the zone III seismic condition is called as medium seismic zone and which is having seismic coefficient factor is 0.16, The zone IV seismic condition is called as high seismic zone and which is having seismic coefficient factor is 0.24 and finally the zone V seismic condition is called as critical seismic zone and which is having seismic coefficient factor is 0.36.
As we know the loading action rate is more in zone V seismic condition than remaining cases like zone II, zone III, and zone IV due to its high seismic coefficient factor 0.36. Because of high action of loads the defection is obtained in the building for X direction or Y direction. To decrease the deflection of the building various new technologies are introduced by structural engineers which are comes under earth quake resistant building design concepts or it is also called as retrofitting methods. The detailed concepts of retrofitting methods are explained in my previous blog post read with link provided below.
What is retrofitting and types of retrofitting used in building design?
By providing the base isolation devices in between the structure and ground we can decrease the defection values. The following are the types of base isolation systems used in the building design as per the seismic loading considerations.
Types of base isolation systems
The basic base isolation systems are classified in two types
- Rubber base isolation system
- Friction pendulum isolation system
Rubber base system
This is the first isolation system used frequently for the building design. These are made with steel laminated rubber types. The natural and artificial rubber bearings are used in bridge design they are later developed with name as elastomeric bearings. After foundation construction initially the rubber media was made at the end of footing then the column construction was started from the end of the rubber bearing material. Then the rest of the super structure was constructed over that rubber isolator. If any lateral load which comes under the seismic load act on the building it will only isolate in X direction or Y direction. Because of that reason the defection and damage of the building is reduces, so we can make the earth quake resistant building model.
The below specified values are considered for the Lead rubber bearing isolation system in analysis of the building. Which is related to the linear and non linear properties in U1,U2 and U3 directions
- Linear effective stiffness U1 is 15000000
- Non linear effective stiffness U2 and U3 direction is 800
- Non linear stiffness for U2 and U3 direction is 250
- Non linear Yield strength is 80
- Non linear Post Yield strength ratio is 0.1
Friction pendulum system
The friction pendulum system is also having similar properties like rubber base isolation. These are most extensively used kinetic system in base isolation. This isolation contains steel globe placed between two concave curved surface or cylindrical membrane. In this isolation system also initially the footing is made below the ground level and friction pendulum is placed above it then the starting point of the column section is provided from the end of the friction pendulum system. These bearings are used in buildings, in spanning and in heavy roof systems and also we can use in mechanical properties of special metals.
The below specified values are considered for the friction pendulum isolation system in analysis of the building. Which contains the linear and non liner properties for the respective U1, U2 and U3 directions.
- Linear effective stiffness U1 is 15000000
- Non linear effective stiffness U2 and U3 direction is 70
- Non linear stiffness for U2 and U3 direction is 1500
- Non linear friction cost slow – 0.03
- Non linear friction cost fast – 0.05
- Non linear rate parameter -40
- Non linear radius of sliding -2.23
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Conclusions of base isolation system and types of base isolations used in building design
Base isolations are the devices which are placed in between the end point of the footing and the starting point of the column section. The basic isolations are two types one is related to the rubber base isolation system and second one is related to the friction pendulum system case. Depending upon the intensity of defection and damage rate of the building the rubber base or friction pendulum isolation systems are used in the earth quake designs.
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