ABSTRACT
The naturally occurring ground
movement which eventually goes on creating disasters such as failure of
structure and fatality is known as Earthquake. Nowadays a seismic event may
also endanger the social-economical stability of large areas due to the
complexity of technologically advanced constructions.
Base Isolation is a very effective
way to destroy the damaging component of seismic forces and is one of the most
widely implemented and accepted Design considerations for base isolated
structures. The main objective of seismic isolation is to shift the fundamental
frequency of a structure away from the dominant frequencies of the ground
motions. Building is isolated from the ground such that earthquake motions are
not transmitted up through the building. It is the main idea behind Base
Isolation.
Seismic isolation is done by
supporting the structure on laminated rubber bearings, friction pendulum bearings,
roller bearings etc. Rarely soil is also used as a base Isolation media. This
paper describes the base isolation techniques and other techniques developed to
resist earthquake forces on the structures.
It has become evident in recent times
that base isolation can be very effective in the event of an earthquake. So a
case study of recent time describing the use of Base isolators during the
reconstruction of District hospital of Bhuj, Gujarat, which had destroyed due
to an earthquake is also included in this paper.
CHAPTER 1
INTRODUCTION
The naturally occurring ground movement
which eventually goes on creating disasters such as failure of structure and
fatality is known as Earthquake. The energy that is discharged from those seismic
activities makes waves, these waves are called as primary waves and secondary
waves.These waves cause ground movement transmitted to the structure via
foundation. Depending on the intensity of these vibrations, cracks and
settlement is caused to the structure.Inertia force is induced in structure
because of this earthquake movement. The maximum point at which the structure
can deform and come back to its original shape is called as Elastic limit. If
building deforms more than its elastic limit, it forms cracks in the structure.
However, ductility will induce some acceptable damage to the structure. If more
elasticity is introduced to the structure, it may tend to increase the overall
cost and decrease the damage by increasing the strength.Earthquakes are
unanticipated phenomena if the structure is located in seismic zones. The
structural engineer has to step in so as to save lives and cause minimal damage
to the structures in times of earthquake. The recent development for
anti-seismic designs is base isolation. Base isolation system is the frequently
adopted earthquake resistance system. It reduces the effect of ground motion
and thus reduce the effect of earthquake on the structure.
The field of seismic design is a
subject that deals primarily with life safety and uncertainty. For several
years now, it has been a quest for structural engineers to design
earthquake-proof buildings and bridges. Initially, it has been generally
thought that building a massive and stiff construction would make it earthquake
resistant. But this stiffness or rigidity of the structural elements would lead
eventually to a fragile and sudden failure, all in all not complying with the
life safety performance criteria and letting inhabitants no time to react in
case of an earthquake.
Next, the increase of damping, redundancy of buildings, ductility and
seismic energy dissipation were taken into consideration and well implemented
throughout the years in seismic building codes. Furthermore, a new alternative
approach was implemented in earthquake protective systems and base isolation
being one of the most common systems nowadays.
CHAPTER 2
CONCEPT OF BASE ISOLATION
Base isolation has become a traditional concept for structural design of
buildings and bridges in high risk areas. By introducing flexible isolation
system between the foundation and the structure the system will absorb the
shock impact effects of earthquake with the help of its flexibility. This way
the seismic energy transmitted to the structure will be reduced to greater
extent and the structure will remain stable for a relative period. Rubber
bearing and lead rubber bearing are prime factors used to introduce flexibility
in the structure. This increased the natural period of the structure and base
displacement is more than prearranged limit. Though, base isolation not always
liable to work against the strong earthquakes as it may result in larger displacement
at the base of the structure. Figure 2.1 shows the performance of building with
and without isolation
Fig.2.1. Performance of Building with & without Isolation (source:Dr.
R. S. Talikoti, Mr.Vinod R. Thorat www.ijert.org)
Basic principle of base isolation is to differentiate the building from its
foundation. During the seismic action, building is unaffected from the ground
motion. In other words, even though ground moves aggressively, the building
will tend to move ideally as a rigid body rather than collapsing. This reduces
the floor hastening and storey gliding and so the building components are less
harmed. Any stiff structure will have short period. During the ground movement,
amount of acceleration entrusted in the structure is the same as that of ground
acceleration that results in zero displacement between the structure and the
ground. In other words, ground and structure will move with equal amount.
Base isolation increase the flexibility of the structure and hence
increases the period of the structure which is due to the isolators. By
introducing base isolation in a structure increases the displacement and
eventually decreases the acceleration in the structure as the stiffness of the
structure also decreases. Generally, the isolation is placed at the base of the
structure, Base isolation protects the building components of the
superstructure during earthquakes. So flexible structure will have longer life
span.
The principle in base isolation,
• To provide horizontally flexible as well as vertically stiff to the
building.
• To lengthening the natural period of the building.
• Damping in the Isolation system reduces the displacement.
• It also reduces in the acceleration of the story.
Base isolation system should contain following:-
• An elastic mount to add enough vibration periods to the structure to
lower down the forces in the structure over.
• An energy dissipater or damper to ease the deflection taking place
between the structure and the ground.
• Introducing the stiffness against the seismic actions and wind loads.
Base Isolation Consideration
Base isolation is required if any circumstances arise of the
following:-
• Need to increase the safety of the structure.
• Low lateral seismic forces needed.
• Any existing building is not capable to withstand any earthquake.
• Withstand small earthquakes without any damage.
• Structure will not collapse in high level earthquake but some structural
and non-structural damage will occur
CHAPTER 3
BASE ISOLATION TECHNIQUES
In traditional seismic design approach, strength of the structure is
suitably adjusted to resist the earthquake forces. In base isolation technique
approach, the structure is essentially decoupled from earthquake ground motions
by providing separate isolation devices between the base of the structure and
its foundation. The main purpose of the base isolation device is to attenuate
the horizontal acceleration transmitted to the superstructure. All the base
isolation systems have certain features in common. They have flexibility and
energy absorbing capacity. The main concept of base isolation is to shift the
fundamental period of the structure out of the range of dominant earthquake
energy frequencies and increasing the energy absorbing capability.
Presently base isolation techniques are mainly categorized into three types
viz. Passive base isolation techniques, Hybrid isolation with semi-active
devices and Hybrid base isolation with passive energy dissipaters. These
different techniques are discussed in short below –
3.1. Passive base isolation techniques
Various passive base isolation techniques are,
3.1.1 Mud layer below the structure
Frank Lloyed Wright was the first person who implemented the idea of base
isolation technique for isolating Imperial Hotel structure in Tokyo, by
providing closely spaced short length piles in 8 feet thick soil layer
underlain by a thickness of mud layer over hard strata. The building survived
an earthquake in 1923.
3.1.2. Flexible first storey
The flexible first storey concept was first proposed by Martel in 1929 and
was further studied by Green in 1935 and Jecobson in 1938 thereby reduce the
loading on upper storey members. However, further studies by Chopra et. al.with
the aid of computers showed that the concept is impractical. Also the recent
earthquakes at Bhuj in India and Kobe in Japan have revealed that most of the
buildings with soft storey have suffered extensive damage.
3.1.3. Roller bearings in foundations
Roller bearing systems proposed for isolation of the structures were having
serious drawback as the rollers were having to and fro motion in particular
direction and earthquake has three directions motion due to which earthquake
forces could not be isolated effectively. Also the main problem was that the
device needed maintenance for keeping in good operation throughout its working
life period. The system was further modified with ball bearing system.
3.1.4. Rubber layer as foundation support
School building in Skopje, Yugoslavia constructed on rubber foundations in
1969, used to bounce and rock forward and backward during earthquake due to
uniform stiffness of rubber in all directions. Also the rubber foundation
bulged under the weight of the building.
3.1.5. Laminated rubber bearing system
Laminated rubber bearings (LRB) (ref. Fig.3.1), which are made of thin
layers of steel plates and rubber built in layers one over the other, have
horizontal flexibility, high vertical stiffness and they can be characterized
by natural frequency and damping constant.
The main advantages of rubber bearing system are -
• Effective isolation is achieved. It will decrease the structural response
to 1/2 -1/8 of the traditional structural response.
• Stable character of isolators over a long working life
• Recovery of the displacement after earthquakes
• Vertical tension capacity is good
• Isolators are insensitive for foundation settlement, which are generally
small in magnitude. It could adjust the structure force by deformation of rubber
bearings when foundation settlement of building happens before or after
earthquakes
• Decreasing the temperature stress in structures by free horizontal
deformation of bearings during large change of temperature around the structure
Fig.3.1. Laminated rubber bearing system - a) Sectional details b)
Schematic diagram c) Force deformation behavior (source: S. J. Patil, G. R.
Reddy Website: www.ijetae.com)
3.1.6 New Zealand bearing system
The system (ref. Fig.3.2), invented in NewZealand in the year 1975, is
improved version of laminated rubber bearing wherein a centrally located lead
core is introduced, which has energy dissipating capacity. The presence of lead
core reduces displacement of the isolator and isolator essentially works as
hysteretic damper device. The device has been extensively used in New Zealand,
Japan and USA. Buildings isolated with these devices performed well during the
1994 North ridge earthquake and 1995 Kobe earthquake.
Fig. 3.2. New Zealand bearing system (a) Sectional details (b) Schematic
diagram (c) Force deformation behavior (source: S. J. Patil, G. R. Reddy
Website: www.ijetae.com )
3.1.7. Resilient – friction base isolation system
Resilient – Friction Base Isolation (R-FBI) system (ref. Fig.3.3) proposed
by Mostaghel and Khodaverdian consists of concentric layers of Teflon coated
plates which will have sliding resistance and a central core of rubber which
will have beneficial effect of resilience of a rubber.
3.1.8. Electric de-France system
Electric De-France (EDF) (ref. Fig. 3.4) system is friction type base
isolation system developed under the auspices of Electric de France in the year
1970. The system is standardized for Nuclear power plants in the region of high
seismicity. The system consists of laminated Neoprene pad topped by a lead
bronze plate, which is in frictional contact with steel plate anchored to the
base raft of the structure. Therefore its cross section is similar to the LRB
system.
The neoprene pad has very low displacement capacity (5 cm approx.) and when
this capacity is exceeded, the sliding element provides the needed movement.
The system does not include any restoring device and hence permanent
displacement could occur. The system has been implemented in nuclear power
plant at Koeberg in South Africa.
Fig. 3.3. Resilient – friction base
a) Sectional details
b) Schematic diagram
c) Force deformation behavior
Fig 3.4. Electric De-France (EDF) isolation system
(a)Schematic diagram
(b)Force deformation behavior
(source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
3.1.9. Sliding resilient- friction system
The design of sliding resilient- friction base isolator (refer fig. 3.5)
was proposed by Su et. al. This isolator is combination of good features of EDF
and R-FBI systems. The upper surface of the R-FBI system is replaced with
friction plates. As a result the structure can slide on its foundation in a
manner similar to that of the EDF base isolator system. For a low level of
seismic excitation, the system behaves as an R-FBI system. The sliding in the
top plates occurs only during high level of ground acceleration, which provides
additional safety against unexpected severe ground motion.
3.1.10. High damping rubber bearing
A blend of high damping rubber is used in these bearings (ref. Fig. 3.6).
The compound, a high damping elastomer, is called KL301 and is manufactured by
the Bridgestone Corporation Limited, Japan. KL301 has a shear modulus of about
4300 kPa at very small strains, which decreases to 650 kPa at 50% strain, to
430 kPa at 100% strain and 340 kPa at 150% strain. The typical bearing made of this
rubber, consists of 20 layers of 2.2 mm thick rubber at 176 mm dia, nineteen
1mm steel shims, and 12 mm top and bottom plates. The design axial pressure is
3.23 MPa. The bearings were designed with flange type end plates to provide
bolted structure and foundation connection.
Fig 3.5. Sliding resilient friction system
(a) Schematic diagram
b) Force deformation behavior
Fig 3.6. High damping rubber bearing
(a) Sectional details
(b) Forcedeformation behavior
(source:S.J.Patil, G.R.Reddy Website: www.ijetae.com)
3.1.11. Pure friction system
A pure friction type base isolator consists of developing frictional force
by providing a sand layer or rollers at the base, which will dissipate the
energy of earthquake force. The system is developed in China for low-rise
structures. The system is useful for wide range of frequency input.
The main advantage of this isolation device is that it is very cheap. The
main problem with the system is that it is unable to recover the displacement
after earthquakes and sand layer is very sensitive for foundation
settlement.
3.1.12. Friction pendulum system
Friction pendulum system (ref. Fig. 3.7) uses geometry and gravity to
achieve the desired seismic isolation. It is based on well-known engineering
principles of pendulum motion. The structure supported by the FPS responds to
the earthquake motions with small pendulum motions. The friction damping
absorbs the earthquake energy. There are variety of friction pendulum system
developed by various researchers such as, variable frequency pendulum isolators
by Pranesh & Sinha, 2000, variable curvature pendulum systems by Tsai et
al, 2003, sliding concave foundation by Hamidi et al., 2003, double concave
friction pendulum system by Fenz and Constantinou, 2006, Triple friction
pendulum bearing, Fenz and Constantinou, 2008. Friction pendulum system is very
efficient and cost effective seismic protection device, which simply alter the
force response characteristics of the structure at base isolation level.
Fig 3.7. Friction Pendulum Base Isolator
(a) Friction pendulum system
(b) Roller pendulum system
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
3.1.13. Spring type systems
Elastomeric and sliding isolation systems are effective in isolating the
structure from horizontal forces. When three dimensional isolation is required,
spring type systems have been used. The spring type system under the brand name
of GERB was developed with large helical steel springs having flexibility both
in horizontal and vertical direction. The vertical frequency of the system was
3 - 5 times the horizontal frequency. The steel springs were used with GERB
visco damper.
The system has been used in two steel framed houses in Santa Monica,
California. These houses were strongly affected by the 1994 Northridge
earthquake. The response of these buildings was monitored and it was not
effective in reducing the accelerations in these buildings due to rocking
motion.
3.1.14. Sleeved pile isolation system
Where foundation soil is very soft up to large depths and provision of pile
foundation is necessary, sleeved pile isolation system is useful from
earthquake considerations. The system consists of providing a casing around the
pile and a gap is maintained between the pile and the casing to accommodate the
sway of the pile under earthquake load. The pile is passed through the soft
soil and is supported and anchored in the rock below.
This system was implemented in the Union house in Auckland, New Zealand in
the year 1983. The building is 12 storeyed tall and is supported on piles
through soft soil for depth of 10m enclosed in steel casing. The period of the
building on the sleeved pile system is 4 seconds.
3.1.15. Rocking systems
Tall slender structures, having heavy mass at the top, will invariably
develop overturning moments which will lead to development of tensions in the
foundations. It is extremely difficult to provide tension capacity in the
foundations when foundations are in weak soil and providing anchors is a costly
affair. As a remedy to this problem, it is possible to allow lifting of columns
or piers from the foundation. This type of partial isolation will reduce the
earthquake loads throughout the structure.
This concept was implemented in a railway bridge on south Rangitikei river
in New Zealand in the year 1972. It has 69m long pier, which has been designed
to lift under the earthquake load. Two large energy dissipating devices that
are based on the elastic-plastic torsion of mild steel bars have been provided
inside each pier. The method is not used again probably due to complexities
involved in analysis and design of the system.
3.1.16. Base isolation using Geo- Synthetic materials
M.K. Yegian and U. Kadakal have developed a technique of isolating the base
of the structures using geo-synthetic material. They have used high strength,
non woven geotextile placed over an ultra high molecular weight polyethelene
(UHMWPE) liner. These two materials have a static friction co-efficient of 0.1
and a dynamic friction coefficient of 0.07. Thus a geo-synthetic material
placed underneath a foundation of a structure and over a liner will allow the
dissipation of earthquake energy in sliding friction. They suggested that the
sliding friction between the two materials should be in the range of 0.05 to
0.15. The authors have suggested arrangements as shown in fig 9 except the
energy dissipating devices.
3.1.17. BS cushion
In 1999 a new kind of base isolator called BS cushion was invented (Chinese
Patent Number ZL99202381.5) in Hangzhou, China. It is Treated Asphalt-Fiber
Seismic Base Isolation Cushion”. The advantage of this kind of isolator is its
low cost and safety while its isolation effect is moderate.
The invention of BS cushion reminds laminated steel-plate rubber bearing.
Fiber and treated asphalt in BS cushion play similar role as of steel-plate and
rubber in laminated rubber bearing respectively. Before 2001 two 7-storey
masonry-concrete residential buildings isolated with BS cushion were built in
Hangzhou, China. One is isolated by replacing some depth of base soil under
mattress foundation with alternative setting of 4 layers of BS cushion and 4
layers of sand. The fundamental period of this building is elongated from 0.3
second to 1 second (0.3s is tested from a similar building and 1s is tested
from this building).
3.2. Hybrid isolation system with semi-active devices
Hybrid isolation system uses both passive isolation systems and semi-active
/ active controlling devices. The Medical Centre of the Italian Navy at Ancona,
Italy, was selected with the aim of analyzing the behavior of a hybrid system
composed by Low Damping Rubber Bearings (LDRBs) acting as passive seismic
isolators, and Magneto-rheological (MR) dampers, acting as semi-active
controlling devices. The analyses showed that significant reduction of the
building accelerations (up to 50%) can be achieved with the hybrid system.
3.3. Hybrid base isolation with passive energy dissipaters
` The energy dissipating devices (ref. Fig. 9 to 16) mainly dissipate the
earthquake energy and thereby reduce the effect of the earthquake on the
structure. These devices can be used at the base of the structure or in
superstructure at appropriate locations. They can be used in combination with
passive base isolation techniques. The different devices developed world over
are shown in Fig. 3.8 to 3.13.
Fig. 3.8. Use of energy dissipating devices at base level
(Source: S. J.Patil, G. R. Reddy Website: www.ijetae.com)
Structure responses can be controlled by using Visco- Elastic dampers
(VEDs), which are made of linear springs and dash pots provided in parallel and
are generally used in bracings of building frame or at ground level.
Fig.3.9.Visco - Elastic Damper
(Source: S. J.Patil, G. R. Reddy Website: www.ijetae.com)
Elasto-Plastic Dampers (EPDs) are made of number of small ‘X’ shaped
plates, which yield at small deformation thereby dissipate high amount of
energy.
Fig. 3.10.Elasto-plastic damper
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
Lead Extrusion dampers (LEDs) work on the principle of extrusion of lead.
It absorbs vibration energy by plastic deformation of the lead, during which
mechanical energy is converted into heat, lead gets heated up and on being
extruded, lead re-crystallizes immediately and recovers its original mechanical
properties before next extrusion
Fig. 3.11. Lead Extrusion damper
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
Tuned Liquid dampers (TLDs) are rigid wall containers filled up to required
height with a liquid (generally water) to match the sloshing frequency of the
liquid with that of the structure. These containers are generally placed on the
top of the structure. The vibration energy is dissipated in the sloshing action
of the liquid.
Fig.3.12. Tuned liquid damper
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
Shape Memory Alloy Dampers (SMADs) made of nickel-titanium (Ni-Ti) alloy
wires has an interesting pseudo-elastic property by which the alloy regains its
initial shape when external load is removed. This property is useful in putting
back the structure to its original shape. Also it can sustain large amount of
inelastic deformation.
Fig. 3.13. SMA damper
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
An un-bonded brace, a technique developed in Japan, consists of developing
a brace which is prevented from buckling by way of providing a metal collar
filled with concrete at the center of brace and a thin layer of viscous
material which allows slip and Poisson’s ratio expansion at the slip surface
provide relative movement between the steel collar and surrounding concrete.
This protects the brace from buckling and allows proper dissipation of energy
in the brace through stable hysteresis loop. A buckling restrained or core
loaded or non-buckling brace developed in IIT, Madras also works on the similar
lines and dissipates the earthquake energy.
Tuned Mass Damper (TMD) is a spring – mass damper device generally
connected to the structure at its top. It has been used as a passive control
device for response reduction of tall buildings.
Examples Of Isolated Structures In Different Countries –
Few examples of isolated structures are William Clayton building, New
Zealand, Medical Centre of the Italian Navy (Sarvesh K. Jain And Shashi K.
Thakkar, 2004, LRB+MRD system), Nam-Han River bridge on the Young-dong
expressway Seoul, Korea (Sun Young Lee, et al., 2004, LDRB+MRD system),
Experimental building at IIT, Guwahati, India [8] etc.
The number of seismically isolated buildings in Japan, Russia, China, USA,
Italy, Armenia, New Zealand were 1600, 500, 458, 100, 27, 14 and 11
respectively up to December 2002, 2003 and every year the number of isolated
structures are increasing.
CHAPTER 4
FUTURE TRENDS IN BASE ISOLATION
Many of the base isolation techniques described above involve the
materials, which are susceptible to deterioration with time. Regular inspection
and maintenance of the system is required. Special measures need to be taken
for fire protection. As such it is desirable to develop such an isolator, which
has a life span equal to the life of a structure, free from effects of
environment and fire. Also it should be free from maintenance. Hence it will be
an ideal case if researchers develop an isolator using materials which are
unaffected by environment or affected by it to very low extent like natural
earth of specific qualities having inherent properties of spring action and
friction.
The equivalent spring constants and damping co-efficient for foundations
resting on soil can be worked out using equations given in the table (4.1).
Equations for damping accounts for material as well as radiation effects.
From the equations which are valid for low strains, it can be seen that the
spring stiffness is more for large size foundations and for greater value of
shear modulus G. Also it is dependent on Poisson’s ratio of soil. Thus the
spring values of the soil can be altered by varying the above parameters by
choosing soil of appropriate properties. In similar way the damping properties
of the soil medium can be altered. Also the effect of damping and isolation can
be obtained by allowing the structure to slide on a soil medium to required
extent. However, in the case of using soft geological materials such as soil,
sand, pulverised granite etc, the material will see large strains.
Table 4.1.Spring constant and damping coefficients
for foundation on homogeneous half space
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
Where -
Ρ = mass density of soil
Vs = Shear wave velocity of soil medium
G = ρ Vs2
ν = Poisson’s ratio of soil medium
R = equivalent radius for rectangular foundation (R= √BL/π for translation
and R= 4√4BL3 / 3π for rocking).
B = width of the foundation perpendicular to the direction of horizontal
excitation
L = length of the foundation in the direction of horizontal
excitation
I0 = total mass moment of inertia of structure and foundation about the
rocking axis at the base
IT = polar mass moment of inertia of structure and foundation
βx, βψ, and βv are constants depending on ratio L/B
C1= 0.5 ; C2 =0.30/(1+ βψ); C3 = 0.8
CHAPTER 5
ADVANTAGES & LIMITATIONS
5.1. Advantages
• Structural Damage is restricted when the
structure is built on a suitable
seismic isolating system.
• Damage to indoor services and facilities
would be of little concern which
would normally affect gas, water or swage
leakage for unfortified structures. The
base Isolation will protect the structure
by preventing plastic deformation of
structural elements, because, the super-structure
demonstrates elastic behaviour during initial and
following excitation of the base.
• Secondary damage and injury as a result of falling furniture would be
restricted. In the other words, the level of safety
is increased significantly when using base
isolation system rather than conventional systems.
• The function of buildings can be ensured during an excitation or even
after a major earthquake as super-structure is designed to remain elastic.
Therefore, plastic deformation of structural
elements can be prevented and the
building is still a safe place to remain and life
can continue as normal.
• Evacuation routes and corridors are normally secured in a base-isolated
building after an earthquake so, horror of
earthquake can be eased and psychological
burden is alleviated.
• Reduction in earthquake input forces, coul
lead to slender structural elements and consequently the considerable
reduction in the whole weight of structure, which givesthe noteworthy reduction
in construction materials and construction costs.
• Considerable safety improvements would reduce disaster management
protocol for such buildings during an earthquake and reduction of repair costs
after an earthquake, seismic isolation can reduce life cycle cost.
5.2. Limitations
Base isolation enables the reduction in earthquake-induced forces by
lengthening the period of vibration of the structure. However, Base isolation
is not suitable for all buildings. Most suitable candidates for base-isolation
are 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.. Period of vibration in building increases with increasing height.
Taller buildings reach a limit at which the natural period is long enough to
attract low earthquake forces without isolation. Therefore, seismic isolation
is most applicable to low and medium rise buildings and becomes less effective
for tall ones. The cut off mainly depends on structural systems or type of
framing system. Cost involved in constructing a new building is higher than the
cost of conventional earthquake resistant structural system. Seismic isolation
bearings are expensive. Due to these economic considerations, even in developed
countries these devices have so far been used for important buildings only. To
enable its use for common buildings, some low cost devices have to be
developed.
CHAPTER 6
CASE STUDY
It has become evident in recent times that base isolation can be very
effective in the event of an earthquake. The cost of installing base isolation
systems has been so great that it is generally only used for emergency centres,
historical buildings, and buildings housing very expensive and sensitive
equipment and are limited to developed nations only and in a developing country
like India, base isolation technique is as good as nonexistent. Having
technological &research institutes in almost every part of a country, still
research in this field is limited to few IITs only. The only instance of base
isolation in India is at district hospital constructed post 2001 Bhuj
earthquake incorporating lead rubber bearing system Cost involved in
constructing a new building is higher than the cost of conventional earthquake
resistant structural system. Seismic isolation bearings are expensive. Due to
these economic considerations, even in developed countries these devices have
so far been used for important buildings only. To enable its use for common
buildings, some low cost devices have to be developed.
Fig. 6.1.Time Period of base isolated & non base isolated on soft &
stiff soil.
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
Soft soil ground condition isn’t suitable for base-isolated structures.
After LRB yield, the structure period corresponding to the equivalent linear
stiffness can be further prolonged. As a result, the natural period just enters
into the range of the predominant period of earthquake wave. It leads to the
acceleration amplification which makes the enlargement of seismic energy
response. Consequently, it should be paid much attention on the design of
absorption and isolation for base-isolated structures under the soft soil
ground condition.
Requirement of tests on prototype bearing of every type increases the cost
of the project. Therefore, development and standardization of testing methods
for evaluating the properties of isolation devices should be formulated.
Fig. 6.2. Rubber bearing provided to Bhuj hospital.
(Source: S. J. Patil, G. R. Reddy
Website: www.ijetae.com)
Fig. 6.3. Indian scenario for base isolation
(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)
But however some low cost isolation devices can be incorporated to ordinary
structures situated in high seismic zones, especially to structures from rural
regions where maximum part of a India’s population dwells. Of course, these low
cost isolation devices, can never met the performance level as those of high
end devices like LBR, FPS etc but can be of great help in minimizing number of
casualties during major seismic events.
Some examples of low cost isolation system include rubber bearings
reinforced with fibre glass mesh instead of steel(this reduces weight as well
as cost of the bearings to great extend.
Fig. 6.4. Scrap tyre pad as low cost isolator device
(Source: Pallavi WamanraoTaywade, Madhuri Narayan Savale)
Scrap rubber tyre pads can also be utilized for isolating a building. Since
the tires are being designed for friction, load transfer between scrap tire
layers would be large enough to keep all layers intact. A minimal slip
generated between the piled layers at high strain rates may even help to
dissipate some extra energy. Steel mesh in tyre can be assumed to provide
vertical rigidity to an extent. Rectangular shaped layers cut from tread
sections of used tires and then piled on top of each other to form Scrap Tire
Pad (STP) can function as an elastomeric pad.
CHAPTER 7
CONCLUSION
Seismic base isolation method has proved to be a reliable method of
earthquake resistant Design. The success of this method is largely attributed
to the development of isolation devices and proper planning. Different types of
isolation devices have been proposed and extensive research has been made on
them.
They can serve the purpose for almost all types of conditions. Adaptable
isolation systems are required to be effective during a wide range of seismic
events. Besides, the existing devices are expensive and to make isolation
feasible for ordinary buildings, it is efforts are required to develop cost
effective devices.
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