Base isolation is a key technology in earthquake engineering used to protect structures by introducing flexibility through devices called base isolators. It significantly reduces the inertial forces experienced during ground shaking, thus minimizing deformation and damage to buildings. Various types of bearing systems, including lead-rubber and high-damping rubber bearings, are utilized to enhance the effectiveness of base isolation, addressing the specific needs of different structures while ensuring safety and performance.

1. BASE ISOLATION
PRESENTED BY
AGLAIA

2. Base Isolation
● Base isolation is one of the most popular means of protecting a
structure against earthquake forces.
● It is one of most powerful tools of earthquake engineering pertaining
to the passive structural vibration control technologies.
● if the building is rested on the flexible pads that offer resistance
against lateral movements, then some effect of the ground shaking
will be transferred to the building.
● 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.
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.

3. 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.

4. Response of Base Isolated Buildings
● The 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.

6. Spherical sliding isolation system:
● Spherical sliding isolation systems are another type of base
isolation. 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.

7. Types of bearings:
● Lead-rubber bearings
● These 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. Lead is a crystalline material which
changes its structure temporarily, under deformations beyond its yield
point, and regains its original structure and elastic properties as soon as
the deformation is removed by the restoring force in the rubber.
● Note that lead has good fatigue properties for subsequent cycles of
loading beyond its yield point.

8. How it works?
● To get a basic idea of how base isolation works, first examine the above
diagram. This shows earthquake acting on base isolated building and a
conventional, fixed-base and building.
● As a result of an earthquake, the ground beneath each building begins to
move.
● Each building responds with movement which tends towards the right.
● The buildings displacement in the direction opposite the ground motion is
actually due to inertia.
● The inertia forces acting on a building are the most important of all those
generated during an earthquake.
● In addition to displacing towards right, the un-isolated building is also shown
to be changing its shape from a rectangle to a parallelogram.
● We say that the building is deforming.
● The primary cause of earthquake damage to buildings is the deformation
which the building undergoes as a result of the inertial forces upon it.

9. Elastomeric isolation system
● The most popular seismic isolation systems use elastomeric
bearings which consist of thin rubber sheets bonded onto thin steel
plates and combine with an energy dissipation mechanism.
● The rubber sheets are vulcanized and bonded to the thin steel
plates under pressure and heat.
● The inner thin steel plates provide the vertical load capacity and
Stiffness, and prevent lateral bulging of the rubber.In particular the
steel plates laterally constrain the rubber sheets as vertical load is
applied to the elastomeric bearing, providing the vertical stiffness.
● Horizontal flexibility is provided by the shearing deformability of the
rubber sheets which are not restrained from deform in that direction
by the steel plates.
● Thick mounting steel plates are bonded to the bottom and top
surfaces allowing the isolator to be firmly connected to the
foundation below and the superstructure above.

10. ● The energy dissipation mechanism is based either on the plastic
deformation of a metal or on the inherent damping properties of the
rubber.
● In the first case either lead plugs are inserted in the elastomeric
bearings or auxiliary dampers based on deformations of lead or steel
are used.
● Lead rubber bearings (LRBs) and high-damping rubber bearings
(HDRBs) are most useful in seismic isolation since they provide the
following in a single unit:
● Vertical support due to the high vertical stiffness, which is usually
several hundred times the horizontal stiffness .
● Sufficient vertical stiffness is necessary to avoid rocking of the structure.
● Horizontal flexibility which shifts the fundamental frequency of the
structure out of the dangerous for resonance frequency range.
● An energy dissipation mechanism, either via the plastic deformation of
the lead plug or through the inherent damping properties of high
damping Elastomeric isolation system

11. ● The most popular seismic isolation systems use elastomeric
bearings which consist of thin rubber sheets bonded onto thin steel
plates and combine with an energy dissipation mechanism.
● The rubber sheets are vulcanized and bonded to the thin steel
plates under pressure and heat.
The inner thin steel plates provide the vertical load capacity and
Stiffness, and prevent lateral bulging of the rubber.
● In particular the steel plates laterally constrain the rubber sheets as
vertical load is applied to the elastomeric bearing, providing the
vertical stiffness.
● Horizontal flexibility is provided by the shearing deformability of the
rubber sheets which are not restrained from deform in that direction
by the steel plates.
● Thick mounting steel plates are bonded to the bottom and top
surfaces allowing the isolator to be firmly connected to the
foundation below and the superstructure above.

12. ● The energy dissipation mechanism is based either on the plastic deformation of a
metal or on the inherent damping properties of the rubber. In the first case either
lead plugs are inserted in the elastomeric bearings or auxiliary dampers based on
deformations of lead or steel are used.
Lead rubber bearings (LRBs) and high-damping rubber bearings (HDRBs) are most
useful in seismic isolation since they provide the following in a single unit:
Vertical support due to the high vertical stiffness, which is usually several hundred
times the horizontal stiffness .Sufficient vertical stiffness is necessary to avoid
rocking of the structure.
Horizontal flexibility which shifts the fundamental frequency of the structure out of
the dangerous for resonance frequency range.
An energy dissipation mechanism, either via the plastic deformation of the lead
plug or through the inherent damping properties of high damping rubber.
Finally, there are also some systems that use natural rubber bearings (NRBs) with
additional steel or lead damper; in this case energy dissipation results from the
plastic deformations of the damper.
rubber.
Finally, there are also some systems that use natural rubber bearings (NRBs) with
additional steel or lead damper; in this case energy dissipation results from the
plastic deformations of the damper.

13. High-damping rubber bearings (HDRBs)
● This type of bearing consists of thin layers of high damping rubber sandwiched between steel plates.
The same manufacturing methods for vulcanization and bonding that are used for LRBs are also used to
construct HDRBs. The only difference is the composition of the rubber compound, which provides
increased damping.
● High-damping rubber is actually a filled rubber compound with inherent damping properties due to the
addition of special fillers, such as carbon and resins. The addition of fillers increases the inherent damping
properties of rubber without affecting its mechanical properties. When shear stresses are applied to high-
damping rubber, a sliding of molecules generates frictional heat which is a mechanism of energy
dissipation. In unfilled natural rubber, used for LRBs, frictional heat is negligible because the molecular
attraction in physical cross links is very weak. The energy dissipation mechanism of an HDRB is available
for both small and large strains, is constant and is characterized by smooth elliptical hysteresis loops.
Experimental studies of high damping rubber bearings verified the anticipated energy dissipation capacity,
which is, typically, equivalent to about 15% damping ratio of equivalent linear elastic models.
● However, HDRBs may not provide the necessary initial rigidity under service loads and minor lateral
loads, although some initial rigidity is provided by high-damping rubber compounds which exhibit higher
stiffness under small strains. A structure isolated with HDRBs essentially has a constant, large
fundamental period due to the flexibility of the isolation system, which makes the structure vulnerable to
wind action with dominant frequencies close to the fundamental frequency. In addition, the damping and
mechanical properties of the HDRB appear to be temperature dependent while the hysteretic energy
dissipation mechanism of the LRB is not. HDRBs are not as widely used in seismic isolation as LRBs.

14. Hybrid type: lead high-damping rubber
bearing (LHDRBs)
● A hybrid type of lead high-damping rubber bearing (LHDRBs) may consist of layers of high-damping rubber
sandwiched between steel plates and a smaller diameter lead cylinder plug firmly in a hole at its center. This
hybrid bearing may have the advantages of both isolation systems discussed above. The LHDRBs has both an
initial rigidity, due to the presence of the lead plug, and a continuous energy dissipation mechanism, due to the
damping properties of the high-damping rubber. Therefore, the isolation system is expected to perform well in both
weak and extreme earthquakes as well as under minor lateral loads. In addition, the properties of this bearing will
be less dependent on temperature changes and shear strains than those of HDRB.
● Also, the use of LHDRBs allows the reduction of the initial stiffness of the isolation system which is
responsible for higher frequency effects. A final advantage is that the effectiveness of such a device and its
compact size make it suitable for cases where installation space is limited. LHDRBs offer a practical and cost-
effective trade-off between the advantages and limitations of LRBs and HDRBs.
● When practically possible, it may be more effective to place the high-damping rubber bearings with lead plugs
at the perimeter of the building, preferably under the columns situated as far away as possible from the center of
stiffness and mass of the isolated structure. High-damping rubber bearings with no lead plugs may be used under
the internal columns. This configuration will allow lower prior-to-yielding stiffness of the lead plugs and
consequently, a smoother change of the stiffness during yielding and reverse loading. The high initial stiffness and
its sudden changes are responsible for higher mode effects and acceleration increases, which may be avoided by
reducing the initial stiffness.
● In addition, lower initial stiffness will provide a higher degree of isolation at the prior-to-yielding stage. Placing
the bearings with the lead plugs under the external columns away from the center of stiffness will provide higher
resistance against torsion, due to the larger diameter between the points of application of forces and the center of
stiffness of the isolation system. Note that this configuration may be used only when there is a rigid diaphragm at
the isolation level to redistribute the inertia forces to the LHDRBs.

15. Maintenance and Management of the Isolation
System:
● The isolation system must remain operational for the world expected lifetime of the
structure under all possible environmental effects.
● These may corrode the metallic parts of the isolation system and deteriorate the
elastomer; such effects may be reduced by using a protective rubber cover.
● The maintenance of the isolation system, and especially that of the seismic gap,
must be frequent to ensure the vertical loads which they must sustain.
● When the construction of a diaphragm is not possible, the bearings must be
designed in proportion to the magnitude of the lateral forces carried by the members
above them.
● It is difficult to take into account such design issues due to the uncertainties involved;
therefore, the construction of a diaphragm above the isolation level must be anticipated.
Note that here, the existence of a rigid diaphragm is assumed. Finally, the selected
location of the bearings must be such that access to them is enabled for inspection and
possible replacement purposes.

16. Factors which enable the use of base isolation
● Several factors favor the development and practical application of seismic isolation.
First of all there are increased requirements for the performance of structure in severe
earthquakes which are not met by current conventional design philosophy. These are
crucial to structures containing sensitive and expensive equipment vulnerable even to
micro tremors.
● Second, the advance of computer technology and modern structural analysis method
enables the development of reliable software to stimulate the response of structures.
The development of seismic engineering and earthquake engineering to level where
reliable prediction can be made for expected earthquakes, is another important factor.
● In addition, the construction of shaking tables which can stimulate actual earthquake
excitations makes possible the experiment validation of the behavior of seismic isolation
system. The study of other minor loads such as wind load and the reliable
quantifications of their expected intensities and frequency of occurrence, also enables
the use of seismic isolation.
● Finally, development, manufacture and extensive research in the area of structural
material enable the reliable use of modern material for seismic isolation device. The
development of device which decipate energy and provide the restoring force to avoid
permanent displacement also allow the practical implementation of the seismic isolation
concept.

17. CONCLUSION
● We can use base isolation technique to construct the
earthquake resistant building.
● Proper materials and design should be selected to get the best
result.
● The safety of people should be the main aim.

18. For more…. Mail to aglaiaconnect2018@gmail.com