Figure : Classification of Seismic Protection Systems.
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ELASTOMERIC BEARINGS
An elastomeric isolation bearing consists of a number of rubber layers
and steel shims, bonded in alternating layers, to produce a vertically
stiff but horizontally flexible isolator. This flexibility lengthens the
fundamental period of the isolated building and reduces the seismic
forces in the superstructure. But this reduction may be accompanied by
large horizontal displacements in the isolators, which, together with
their lateral flexibility, may lead to significant reduction in their
critical axial load Buckle and Kelly 1986; Koh and Kelly 1986; Buckle
and Liu 1994; Nagarajaiah and Ferrell 1999. Elastomeric isolation
bearings are required to be stable at high shear strains, which occur
during strong earthquakes. Hence, rigorous determination of the critical
axial load during design is important [13].
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LOW DAMPING NATURAL OR SYNTHETIC RUBBER BEARING
Elastomeric bearings have been used widely in bridges as bearing pads
between the girder and the supporting structure for many years.
Elastomeric bearings have multiple layers of steel shims and rubber
laminated together under high pressure and heat in a mold. Steel shims
prevent lateral bulging of the rubber when axially loaded. They do not
resist shear forces and do not prevent the horizontal deformation of the
layered rubbers. Therefore, steel shims increase the vertical stiffness
of isolators but do not increase the lateral stiffness of elastomeric
bearings.
Figure 2.5 shows elastomeric bearings subjected to dynamic shear test.
The uneven surface of the elastomeric bearings shown is due to its steel
shims. Generally, elastomeric bearings have low critical damping
resistance, approximately 2% to 3% of critical viscous damping; and
have minimal resistance under service loads. Therefore, elastomeric
bearings need to be improved. The result is a high damping elastomeric
bearings and the lead rubber bearings.
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HIGH DAMPING RUBBER BEARING
As an alternative to elastomeric bearings, high damping rubber bearings
provide critical damping from 10% to 20% at 100% shear strains. The
construction methodology is the same with elastomeric bearings; however,
the damping is increased by adding carbon block and other fillers. In
addition, it has an adequate resistance to service loads. The damping
characteristic is in between hysteretic and viscous. The energy
dissipation is linear and quadratic for hysteretic and viscous,
respectively. The energy absorption capacity help reduced the earthquake
energy transmitted to the superstructure. The load capacity of HDRB can
be computed using the same method as elastomeric bearings. The damping
value can be computed using the equivalent damping ratios for specific
elastomeric compounds.
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LEAD RUBBER BEARING
Lead rubber bearings are elastomeric bearings that contain one or more
lead plugs inserted into their preformed holes. The lead provides
significant stiffness under service loads and low lateral loads as
compare to elastomeric bearings. Figure 2.6 shows the alternating sheet
of steel shims and rubbers circumscribing a lead core. In addition, the
lead serves as energy dissipation mechanism under severe lateral loads.
During high lateral loads, the lead yields and the lateral stiffness of
the LRB is significantly reduced. This increases the duration of the
period of the structure and thereby serves the purpose of base isolation
system. The bearing is cycled into a hysteretic damping as it absorbs
the energy. LRB has a range of damping from 15% to 30% which is a
function of displacement. The LRB are the most common base isolator used
for isolating midrise buildings. It is usually designed and optimized
according to a specific performance based target design. It combines the
stiffness needed for service loads and low lateral loads while providing
the flexibility and damping needed for high lateral loads. A wide array
of damping and stiffness is possible through the use of LRB. As for the
case of HDRB design, the formulas of elastomeric bearings are suitable
for the design of LRB.
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SLIDING BEARING
One of the most popular and effective techniques for seismic isolation
is through the use of sliding isolation devices. The sliding systems
perform very well under a variety of severe earthquake loading and are
very effective in reducing the large levels of the superstructure’s
acceleration. These isolators are characterized by insensitivity to the
frequency content of earthquake excitation. This is due to tendency of
sliding system to reduce and spread the earthquake energy over a wide
range of frequencies. The sliding isolation systems have found
application in both buildings and bridges. The advantages of sliding
isolation systems as compared to conventional rubber bearings are (i)
frictional base isolation system is effective for a wide range of
frequency input, (ii) since the frictional force is developed at the
base, it is proportional to the mass of the structure and the center of
mass and center of resistance of the sliding support coincides.
Consequently, the torsional effects produced by the asymmetric building
are diminished.
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FLAT SLIDING BEARING
Flat sliding bearings consist of PTFE (Teflon) disc that slides on a
stainless-steel plate. They provide a perfectly plastic hysteresis
shape, and adequate stiffness under service loads with high damping
properties. In addition, the coefficient of friction is a function of
both pressure and velocity of sliding. It provides the resistance under
service loads. However, it must be combined with other bearings (i.e.
HDRBs, LRBs) because it has no capability to return to its initial
position. Figure 2.7 shows an assembly of flat sliding bearing, which
has the stainless-steel plate supporting the circular disc. There is no
other part in the assembly that shows that it has the capability to
return to its initial position. A modified version of flat sliding
bearing that has self-restoring force is the friction pendulum bearings.
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SPHERICAL SLIDING BEARING
The friction pendulum bearings have the same properties as the flat
sliding bearings. However, the sliding surface is concave in shape
rather than flat as shown in Figure 2.8. The hemisphere at the center of
the concave surface is the pendulum slider. The spherical concave
surface provides a restoring force to the pendulum slider to return to
its initial position. Varying the radius of the concave surface varies
the stiffness of the friction pendulum bearings. In addition, once the
coefficient of friction is overcome the lateral movement of the mass is
accompanied by a vertical movement of the mass because of the curved
shape of the slider. As for the case of LRB, the friction pendulum
bearings have a wide array of damping and stiffness design capabilities.
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PASSIVE ENERGY DISSIPATION SYSTEM
Passive energy dissipation systems encompass a range of materials and
devices for enhancing damping, stiffness and strength, and can be used
both for natural hazard mitigation and for rehabilitation of aging or
deficient structures. Some structures have very low damping on the order
of 1% of critical damping and consequently experience large amplitudes
of vibration even for moderately strong earthquakes [14]. Methods of
increasing the energy dissipation capacity are very effective in
reducing the amplitudes of vibration In recent years, serious efforts
have been undertaken to develop the concept of energy dissipation or
supplemental damping into a workable technology and a number of these
devices have been installed in structures throughout the world [7,
15]. In general, they are all characterized by a capability to enhance
energy dissipation in the structural systems to which they are
installed. This may be achieved either by conversion of kinetic energy
to heat, or by transferring of energy among vibrating modes. The first
method includes devices that operate on principles such as frictional
sliding, yielding of metals, phase transformation in metals, deformation
of viscoelastic solids or fluids, and fluid orificing. The latter method
includes supplemental oscillators, which act as dynamic vibration
absorbers.