ABSTRACT :
After the devastating earthquakes in
Killari (Maharashtra) on 30th sep 1993 and in Bhuj(Gujarat )
on 26th January 2001, major concern is attracted towards earthquake
resistively. In a developing nation like India
Emphasis on theoretical and
experimental studies of base isolated precast frames is necessary in such
cases. Research work is required for feasibility , analytical and experimental
studies for base iso latticed precast
structure before it can be implemented following are a few research guidelines
,
1)
Different
possible isolation systems for precast buildings can be collected and reviewed
for technical merits and demerits constructability and versatility. Of particular importance is development of
seismic design recommendations in codes for base isolated precast structure.
2)
Seismic response
evaluation of base isolated precast structural system for various seismic zones
and site characteristics will be required. The aims are to carry out dynamic.
Based on this appropriate simplified design rules will be generalized
3)
Economic
studies are required to ascertain the cost effectiveness of proposed strategy.
Earthquake
Resistant Structure
• The conventional approach to earthquake
resistant design of a building depends upon providing the building with the
strength stiffness, and in elastic deformation capacity which is good enough to
withstand a given level of an earthquake generated inertia forces. This is
generally a accomplish through the selection of an appropriate structures
configuration and the careful detailing of the structure member such as beam
columns and shear walls and the connections between them and the adequate
foundation for the structure.
• Among the most important advance
technique of earthquake resistance design and construction are
1.
Base
isolation
2.
Energy
dissipation devices
Base isolation:
In recent base isolation
has become and increasingly applies structure design technique for building and
Bridges in highly seismic areas. Many types of structure have been build using
this approach and man y other and design phase or under construction. Most of
the completed building and those under construction use rubber isolation
bearing in some way in the isolation system. In India
1) Lead - rubber bearing isolation system
2) Spherical sliding isolation system
Lead – rubber Bearing isolation system:
1) This method is widely used in recent
years by use of elastomeric bearings.
2) In this building is decoupled form
horizontal component of earthquake by interposing a layer with low horizontal
stiffness between structure of foundation.
3) This layer gives the structure, a
fundamental frequently that is much lower than its fixed base frequency.
4) The isolation system does not absorb the
earthquake energy but rather deflects it through dynamics of the system.
5)
A base isolated structure is
supported lay a series of bearing pads. Placed between building and buildings
foundation.
6)
The lead rubber bearing are among
frequently used type of base isolation bearings. In this layers of rubbers are
sandwiched together with layers of steel. In middle of the bearing is a solid
lead “plug” on top and bottom bearing is fitted with steel plates which are
used to attach bearing with building and foundation.
7)
The bearing is very stiff and
strong in vertical direction.
8)
Inertial forces, which the building
undergoes are proportional to buildings acceleration during ground motion and
it is being observed that during earthquake building acceleration of base
isolated structure is reduce to 1\4 of the acceleration of fixed base building.
9)
Also lead plug damps the
buildings vibration.
10)
The bearing is very stiff and
strong in vertical direction.
11)
Inertial forces, which the building
undergoes are proportional to buildings acceleration during ground motion and
it is being observed that during earthquake building acceleration of base
isolated structure is reduce to 1\4 of the acceleration of fixed base building.
Spherical
sliding isolation system:-
1.
The building is supported by
bearing pads that have a curved surface and low friction.
2.
Since curved surface the building
slides both in horizontal and vertical direction.
3.
By adjusting the radius of
bearing curved surface, it can be used to design bearings that lengthens the
natural period and vibration of building
4.
Lower the friction, lower the
shear transmitted.
5.
Also slider have no restoring
force and so requirements for bearings sites may extremely large as
displacement may be in any direction. It is possible to limit the displacement
by adding restoring force.
6.
It can be done by elastomeric
bearings, which centre the sliders, they also provide torsion and it the
displacement become too layer, and it provide stiffening action.
7.
To stop movement of sliders,
under wind and other non-critical lateral loads, it to provide a breakable
link, “Mechanical base” It will provide a rigidly fixes structure under normal
loading but would fracture under serve loading and allowing isolation to work.
Benefits :-
1)
Following an earthquake. A base
isolated precast structure will require only normal inspection.
2)
Due to system’s simplicity and
construct ability it will provide savings in number of man hours necessarily
compare to construct a conventional moment resisting system.
3)
Provide major increase in safety seismic
regions economically providing protection to occupants and owners.
Energy dissipation devices :-
1)
This system depends upon damping
and energy dissipation.
2)
As it is known, a certain amount
of vibration energy it transferred to building by earthquake. Building themselves
processes an inherent ability to dissipate, this energy.
3)
This capacity of building to
dissipate energy is quite limited.
4)
So lay equipping a building with
additional devices, which have high damping capacity, we can decrease the
seismic energy entering the building and thus decrease damage.
5)
The damping device can be
categorized
a.
friction dampers : They utilize
frictional dampers to dissipate energy
b.
Metallic dampers: They utilize of
deformation of metal elements.
c.
Visco-elastic dampers: They
utilize the forced movement of fluids within the dampers.
If fluid viscous dampers are especially effective in minimizing forces acting on a building column. They reduce drifts, and shear forces without introducing axial column forces which are in phase with column bending moments. It helps to reduce drifts and shear forces of two to three in comparison to the response of model without dampers. Fluid viscous damping devices have been proved to be a reliable and affordable solution to seismic design problems.
If fluid viscous dampers are especially effective in minimizing forces acting on a building column. They reduce drifts, and shear forces without introducing axial column forces which are in phase with column bending moments. It helps to reduce drifts and shear forces of two to three in comparison to the response of model without dampers. Fluid viscous damping devices have been proved to be a reliable and affordable solution to seismic design problems.
The damping action is provided by
the flow of fluid across the piston head. The piston head is made with a
deliberate clearance between the inside of the cylinder and the outside of the
piston head, which forms an annular orifice. The fluid flows through this
orifice at high speed as the damper strokes. The shape of the piston head
determines the damping characteristics. The force/velocity relationship for
this kind of damper can be characterized as F=CV exp n, where F is the output
force in pounds, V is the relative velocity across the damper in inches per
second, C is a constant determined mainly by the damper diameter and the
orifice area, and n is a constant exponent which can be any value from .30 to
1.95. The exact value for n depends upon the shape of the piston head. Values
of n ranging from .3 to 1.0 seem to work best for structural applications. When
the fluid viscous damper strokes in compression, fluid flows from Chamber 2 to
Chamber 1. When the fluid viscous damper strokes in tension, fluid flows from
Chamber 1 to Chamber 2. The high pressure drop across the annular orifice
produces a pressure differential across the piston head, which creates the
damping force.
As the
damping orifice is provided by the annular clearance between the piston head
and the cylinder body, it is possible to provide inherent thermal compensation
by making these two parts from different materials. By choosing materials with
the correct thermal coefficients of expansion, it is possible to make the
variation in the gap compensate for the variation in fluid properties as temperature
changes. Through proper design techniques, variation in damping as small as
plus or minus 15% over a temperature range of +20 degrees F to +120 degrees F
can be obtained. Spherical bearings at each end of the fluid viscous damper
permit the damper to angulate relative to the structure without binding. Figure
shows a detail of these spherical end bearings. These bearings permit rotation
in every direction, which prevents binding in the fluid viscous damper. In some
cases there is enough flexibility in the structure to make it possible to
solidly mount the dampers. A typical installation of this type is shown in
Figure
SPHERICAL
BEARING
Figure shows a typical mounting bracket for fluid viscous dampers.
This bracket consists of a female clevis mounted to a base plate which in turn
welds to the structure. Upon installation the dampers simply slide into the
clevises. Once the dampers are in the clevises, insertion of the pins holds
them in position. Cotter keys then secure the pins in place. The dampers are
always made with enough extra strokes to account for variance in the
hole-to-hole distance in the structure.
MOUNTING BRACKET
Fluid viscous dampers can be designed into both new buildings and
existing structures. As they are relatively small and inconspicuous, they can
be incorporated into a structure without compromising its appearance. This is
especially useful in the refurbishment of historically significant buildings. They
can also be added without significant structural modification in most cases. Fluid
viscous dampers can be installed as diagonal members in several ways, or can
tie into chevron braces. They can also be used as the two elements of a chevron
brace. Typical fluid damper
installations.
Figure shows the incorporation of
a fluid viscous damper into the diagonal element connecting a beam-column joint
at one level of a moment frame structure to the adjacent beam-column joint at the
next level. Connections of this nature can be designed into a new structure, or
could be added to an existing structure. This was done in the case of the Stockton City Hall
Figure shows a detail of this
configuration. It is also possible to use a modification of the chevron brace, in
which both bracing beams contain fluid dampers. In this case both beams connect
to the structure at both ends with spherical bearings and mounting brackets. In
this case one end of the damper has a spherical bearing and the other end has a
flange that attaches rigidly to the diagonal beam.
FLUID VISCOUS DAMPING IN COMPARISON TO BASE ISOLATION:
Both fluid viscous damping and
base isolation have the same objective of significantly decreasing the response
of a structure to earthquake excitation. With both fluid viscous damping and
base isolation it is possible to have a structure remain within the elastic
region, so there is no permanent deformation from a seismic event. Although the
objectives are the same, the two techniques differ greatly in their
implementation.
CASE STUDY: STOCKTON CITY HALL
A study of possible methods of
refurbishment of the Stockton
City Hall Stockton City Hall
CAPACITY VELOCITY LOCATION QUANTITY:
The total force levels required
from the dampers in the East-West direction are the same. The number and size
of the dampers were changed to accommodate architectural constraints. This particular
installation used the split diagonal damper configuration.
CONCLUSIONS:
Hence we have seen different
innovations used in earthquake engineering such as fluid viscous dampers and
base isolation technique. We have also compared fluid viscous dampers and base
isolation technique. Fluid viscous dampers can be added to either a new or an existing
structure to greatly improve structural performance during a seismic event.
They offer an attractive alternative to base isolation in terms of cost and
ease of installation. The base isolation system is more safety and economical. The theme
of planning new economic seismic devices is crucial to apply to modern
techniques of seismic protection with base isolation to the wide field of small
building.
BIBLIOGRAPHY:
· www.ecs.csun.edu
· www.boomenviro.com
· www.civil.columbia.edu
· www.innovations-report.com/html
·
FEMA 356 (2000), “ Prestandard and Commentary
for the seismic Rehabilitation of buildings “’ American Society of
Civil Engineers, USA .
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