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Seismic behavior of Isolated Bridges Post Posted: Mon May 10, 2010 12:54 am 
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Introduction to Seismic behavior of Isolated Bridges


Seismic isolation is an old design idea, proposing the decoupling of a structure or part of it, or even of equipment placed in the structure, from the damaging effects of ground accelerations.
One of the goals of the seismic isolation is to shift the fundamental frequency of a structure
away from the dominant frequencies of earthquake ground motion and fundamental frequency
of the fixed base superstructure. The other purpose of an isolation system is to provide an
additional means of energy dissipation, thereby reducing the transmitted acceleration into the
superstructure. This innovative design approach aims mainly at the isolation of a structure from
the supporting ground, generally in the horizontal direction, in order to reduce the transmission
of the earthquake motion to the structure. A variety of isolation devices including elastomeTric
bearings (with and without lead core), frictional/sliding bearings and roller bearings have been
developed and used practically for seismic design of buildings during last 20 years in many
new buildings in countries like USA, Japan, UK, Italy, New Zealand etc. The detailed review of
earlier and recent works on base isolation systems and their applications to buildings had been
widely reported by Kelly (1986), Buckle and Mayes (1990) and Jangid and Datta (1995).
Bridges are lifeline structures. They act, as an important link in surface transportation network
and failure of bridges during a seismic event will seriously hamper the relief and rehabilitation
work. There are many cases of damage of bridges in the past earthquakes all over the world.
Due to their structural simplicity, bridges are particularly vulnerable to damage and even
collapse when subjected to earthquakes. The fundamental period of vibration of a majority of
bridges is in the range of 0.2 to 1.2 second. In this range, the structural response is high because
it is close to the predominant periods of earthquake-induced ground motions. For very rigid
structures like normal bridges with short piers and abutments the time period is often extremely
small. For such structures the response is almost the same as the ground acceleration. The
seismic forces on the bridges can be reduced if the fundamental period of the bridge is
lengthened or the energy dissipating capability is increased. Therefore, the seismic isolation is a
promising alternative for earthquake-resistant design of bridges. Figure 1 shows a typical
isolated multi-span continuous deck bridge in which special isolation devices are used in place
of conventional bridge bearings. These bearings protect the substructure by restricting the
transmission of horizontal acceleration and dissipating the seismic energy through damping.
Considerable efforts have been made in the past two decades to develop improved seismic
isolation design procedure for new bridges and comprehensive retrofit guidelines for existing
bridges. The suitability of a particular arrangement and type of isolation system will depend on
many factors including the span, number of continuous spans, and seismicity of the region,
frequencies of vibration of the relatively severe components of the earthquake, maintenance and
replacement facilities.

Seismically continuous span bridge isolated bridge


An updated state-of-the-art-review on seismically isolated bridges against earthquake excitation
is presented herein. The review briefly covers the characteristics of base isolation devices as
such, but puts most emphasis on the theoretical and parametric studies conducted to understand
the behaviour of seismically isolated bridges with an indication of their range of applicability
and some assessment of their development as backed by the research. The systems presented
here are passive control systems but the work related to active and hybrid control of bridges is
also summarized. The results of some important experimental tests are also included.

Seismic isolation systems


There are two basic types of isolation systems i.e. elastomeric bearings and sliding bearings.
The elastomeric bearings with low horizontal stiffness shift fundamental time period of the
structure to avoid resonance with the excitations. The sliding isolation system is based on the
concept of sliding friction. An isolation system should be able to support a structure while
providing additional horizontal flexibility and energy dissipation. The three functions could be
concentrated into a single device or could be provided by means of different components.
Various parameters to be considered in the choice of an isolation system, apart from its general
ability of shifting the vibration period and adding damping to the structure are: (i) deformability
under frequent quasi-static load (i.e. initial stiffness), (ii) yielding force and displacement, (iii)
capacity of self-centring after deformation and (iv) the vertical stiffness.


Elastometric Bearings


The laminated rubber bearing (LRB) is most commonly used base isolation system. The basic
components of LRB system are steel and rubber plates built in the alternate layers as shown in
Figure 2(a). The dominant features of LRB system are the parallel action of linear spring and
damping. Generally, the LRB system exhibits high-damping capacity, horizontal flexibility and
high vertical stiffness. The damping constant of the system varies considerably with the strain
Abutment
Isolation
system or
bearings
Pier
Deck
Rock
line

level of the bearing (generally of the order of 10 percent). The system operates by decoupling
the structure from the horizontal components of earthquake ground motion by interposing a
layer of low horizontal stiffness between structure and foundation. The isolation effects in this
type of system are produced not by absorbing the earthquake energy but by deflecting through
the dynamics of the system (Kelly, 1997). These devices can be manufactured easily and are
quite resistant to environmental effects. Usually, there is a large difference in damping of a
system and the structure and the isolation system, which makes the system non-classically
damped. This will lead to coupling of the equations of the motion and to analyse the system
correctly complex model analysis is required (Tsai and Kelly, 1993).
(a) LRB System
(b) Lead-rubber bearing.
Figure 2. Elastomeric isolation bearings.
The second category of elastomeric bearings is lead-rubber bearings (Robinson, 1982) as shown
in Figure 2(b). This system provides the combined features of vertical load support, horizontal
flexibility, restoring force and damping in a single unit. These bearings are similar to the
laminated rubber bearing but a central lead core is used to provide an additional means of
energy dissipation. These bearings are widely used in New Zealand and also referred as N-Z
system. The energy absorbing capacity by the lead core reduces the lateral displacements of the
isolator. Generally, the lead yields at a relatively low stress of about 10 MPa in shear and
behaves approximately as an elasto-plastic solid. The interrelated simultaneous process of
recovery, recrystallization and grain growth is continuously restoring the mechanical properties
of the lead. The lead has good fatigue properties during cyclic loading at plastic strains and is
also readily available at high purity of 99.9 per cent required for its predictable mechanical
properties. The lead-rubber bearings behave essentially as hysteretic damper device and widely
studied in the past by Kelly et al. (1972, 1977) and Skinner et al. (1975).

Sliding Isolation Systems


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 characterised 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 centre of mass and centre of resistance of the
sliding support coincides. Consequently, the torsional effects produced by the asymmetric
building are diminished.
The simplest sliding isolation system is the pure friction (P-F) system. In this system a sliding
joint separates the superstructure and the substructure. It has been developed for low rise
housing in China (Li, 1984). The use of layer of sand or roller in the foundation of the building
is the example of P-F base isolator. The P-F type base isolator is essentially based on the
mechanism of sliding friction. The horizontal frictional force offers resistance to motion and
dissipates energy. Under normal conditions of ambient vibrations and small magnitude
earthquakes, the system acts like a fixed base system due to the static frictional force. For large
earthquake the static value of frictional force is overcome and sliding occurs thereby reducing
the accelerations. There has been a significant amount of research work on the performance of
P-F system in the past by Westermo and Udwadia (1983), Mostaghel and Tanbakuchi (1983),
Younis and Tadjbakhsh (1984) and Jangid (1996).
Mostaghel and Khodaverdian (1987) proposed the resilient-friction base isolation (R-FBI)
system as shown in Figure 3(a). This base isolator consists of concentric layers of Tefloncoated
plates that are in friction contact with each other and contains a central core of rubber. It
combines the beneficial effect of friction damping with that of resiliency of rubber. The rubber
core distributes the sliding displacement and velocity along the height of the R-FBI bearing.
They do not carry any vertical loads and are vulcanised to the sliding ring. The system provides
isolation through the parallel action of friction, damping and restoring force.
The concept of sliding bearings is also combined with the concept of a pendulum type response,
obtaining a conceptually interesting seismic isolation system known as a friction pendulum
system (FPS) (Zayas et al., 1990) as shown in Figure 3(b). In FPS, the isolation is achieved by
means of an articulated slider on spherical, concave chrome surface. The slider is faced with a
bearing material which when in contact with the polished chrome surface, results in a maximum

sliding friction coefficient of the order of 0.1 or less at high velocity of sliding and a minimum
friction coefficient of the order of 0.05 or less for very low velocities of sliding.
(a) R-FBI system
(b) FPS System

Sliding type isolation systems


The dependency of coefficient of friction on velocity is a characteristic of Teflon-type materials
(Mokha et al., 1990). The system acts like a fuse that is activated only when the earthquake
forces overcome the static value of friction. Once set in motion, the bearing develops a lateral
force equal to the combination of the mobilised frictional force and the restoring force that
develops as a result of the induced rising of the structure along the spherical surface. If the
friction is neglected, the equation of motion of the system is similar to the equation of motion
of a pendulum, with equal mass and length equal to the radius of curvature of the spherical
surface. The seismic isolation is achieved by shifting the natural period of the structure. The
natural period is controlled by selection of the radius of curvature of the concave surface. The
enclosing cylinder of the isolator provides a lateral displacement restraint and protects the
interior components from environmental contamination. The displacement restraint provided by
the cylinder provides a safety measure in case of lateral forces exceeding the design values.
Lin and Hone (1993) have proposed a new system of free circular rolling rods located between
the base and the foundation. The most attractive feature of this type of isolator is their low
value of rolling friction coefficient, which allows a very low earthquake force to be transmitted
to the superstructure. However, such a system suffers from re-entering capability, resulting in
large peak and residual displacements. To overcome this Jangid and Londhe (1998) proposed
that the shape of rolling rods should be elliptical rather then circular. The low value of the
rolling friction coefficient ensures the transmission of a limited earthquake force into the
superstructure and the eccentricity of the elliptical rolling rods provides a restoring force that
reduces peak base displacements and brings the structure back to its original position.
An important friction type base isolator is a system developed under the auspices of “Electric
de France” (EDF) (Gueraud et. al., 1985). This system is standardized for nuclear power plants
in region of high seismicity. The base raft of the power plant is supported by the isolators that
are in turn supported by a foundation raft built directly on the ground. The main isolator of the
EDF consists of laminated (steel reinforced) neoprene pad topped by lead-bronze plate that is in
friction contact with steel plate anchored to the base raft of the structure. The friction surfaces
are designed to have a coefficient of friction of 0.2 during the service life of the base isolation
system. The EDF base isolator essentially uses elastomeric bearing and friction plate in series.
An attractive feature of EDF isolator is that for lower amplitude ground excitation the lateral
flexibility of neoprene pad provides base isolation and at high level of excitation sliding will
occur which provides additional protection. This dual isolation technique was intended for
small earthquakes where the deformations are concentrated only in the bearings. However, for
larger earthquakes the bronze and steel plates are used to slide and dissipate seismic energy.
The slip plates have been designed with a friction coefficient equal to 0.2 and to maintain this
for the lifetime of the plant.

Initiating and Limiting Devices


Depending on properties of isolating systems it may be necessary to design initiating or limiting
devices (Priestley et al., 1996). The first case applies to system that would be too flexible under
non-seismic load (e.g. wind or traffic). Any of various types of knock off shear keys will solve
the problem, obviously implying some local damage under earthquake forces. Limiting devices
are required to avoid excessive displacement in the isolators in the case of a low probability,
extreme seismic event. Some kind of isolation/dissipation devices (e.g. some dampers) shows
significant strain hardening when the displacement increases beyond a certain level and
generally do not need limiting devices. In other cases, such as lead /rubber bearings that might
become unstable under excessive deformations, a limit to the displacements could be obtained
with rigid stoppers or with deformable buffers, in case there are concerns on the response of the
strUcture under impact loads. Steel tapered beams or stiff rubber buffers could be used to this
purpose. In all cases the structure will be subjected to higher then expected forces, and there
will be some ductility demand in the piers.

Seismic isolation of bridges


In bridges, the base isolation devices can rather easily incorporated by replacing the
conventional bridge bearings by isolation bearings. Base isolation bearings serves the dual
purpose of providing for thermal movement as well as protecting the bridge from dynamic loads
by increasing the fundamental period and dissipating the seismic energy by hysteretic damping.
In order to demonstrate the effectiveness of seismic isolation a three-span continuous deck
bridge made of reinforced concrete is considered. The properties of the bridge deck and piers
are given in Table 1.
Table 1: Properties of the bridge deck and piers
Properties Deck Piers
Cross-sectional area (m2) 3.57 4.09
Moment of inertia as (m4) 2.08 0.64
Young’s modules of elasticity (m2) 20.67109 20.67109
Mass density (kg/m3) 2.4103 2.4103
Length/height (m) 3@30 = 90 8
These properties correspond to the bridge studied by Wang et al. (1998) using a sliding
isolation system. The bridge is modeled as shown in Figure 4(a) as a discrete model. It is to be
noted that the bridge is also modeled as shown in Figure 4(b) in the past in which the deck is
assumed to be rigid. The fundamental time period of the piers is about 0.1 sec and the
corresponding time period of the non-isolated bridge works out to be 0.5 sec in both
longitudinal and transverse directions. The damping in the deck and piers is taken as 5% of the
critical in all modes of vibration. In addition, the number of elements considered in the bridge
deck and piers are 10 and 5, respectively. Response quantities of interest for the bridge system
under consideration (in both longitudinal and transverse directions) are the base shear in the
piers and the relative displacement of the elastomeric bearings at the abutment. The pier base
shear is directly proportional to the forces exerted in the bridge system due to earthquake
ground motion. On the other hand, the relative displacements of the isolation bearing are crucial
from the design point of view of isolation system and separation joints at the abutment level.

Mathematical modeling of isolated bridges


The time variation of the base shear in the pier and relative displacement of the
bearings of the bridge isolated by the LRB, N-Z and FPS is shown. The LRB system is
designed to provide isolation period of 2 sec (based on rigid deck and pier condition) and 10
percent damping ratio. The isolation period for the N-Z and the FPS system is taken as 2.5 sec.
The yield strength of the N-Z system is taken as 5 percent of deck weight and the friction
coefficient of FPS system is considered as 0.05. The system is subjected to Kobe, 1995
earthquake ground motion in the longitudinal and transverse directions. The base shear in the
piers is significantly reduced (about 80 to 90%) for the isolated system as compared to the nonisolated
system in the both directions of the bridge. This indicates that the isolation systems are
quite effective in reducing the earthquake response of the bridge system. The maximum peak
displacement of the bearing is 32.87, 27.65 and 31.50 for LRB, N-Z and FPS system,
respectively in the longitudinal direction of the bridge.

Analytical Study


There had been several analytical studies in the past to demonstrate the effectiveness of seismic
isolation for earthquake-resistant design of bridge. Li (1989) studied the response of a typical
three-span bridge structure with a seismic isolation system consisting of rubber bearings and
hysteretic dissipaters in longitudinal direction. The non-linear equations of motion were derived
for first mode of vibration and the stochastic response to filtered white noise ground
acceleration is determined using equivalent linearization technique. A procedure is also
developed for optimal design of bridge isolation system with hysteretic dampers. It is concluded
that hysteretic damper act most effectively when mounted on a stiff supporting structure, their
effectiveness decreases with increasing flexibility of the supporting structure. It is also
concluded that, larger the value of maximum allowed isolator displacements is used the more
effective is the isolation system.


Wang and Gould (1994) studied the effects of pier uplift on sliding isolated highway bridges
for a symmetrical two-span continuous deck bridge. Pier uplift is found to enhance the
effectiveness of sliding isolation for a highway bridge, especially for piers with inadequate
lateral strength capacity. The elastic behaviour of the pier is possible, while the seismic
performance of a sliding isolated highway bridge is affected minimally by the pier uplift. These
benefits are achieved with a relatively small amount of pier uplifting.
Lam and Davidson (1995) implemented the system identification procedure that minimizes the
differences between the power spectra of the field and simulated data to enable the non-linear
properties of a base isolated bridge to be determined. The accuracy of the method is
demonstrated by comparing the displacement time histories of the recorded and simulated data.
For the bridge used in the case study, it was found that local modes of vibration provided a
major contribution to the base moment of the piers, a feature not directly accounted for in the
design procedure.
Jangid and Banerji (1995) found out the response of bridges isolated by the P-F system in
longitudinal direction. The behaviour of the P-F system is modelled as a rigid-plastic. It is
shown that the P-F devices installed between superstructure and substructure can reduce the
base shear in the piers considerably. The main disadvantage of P-F system is that there is no recentering
force, as a result, there is large peak and residual sliding displacements.
Monti et al. (1995) analysed continuous six-span isolated bridge subjected to the seismic inputs
at various supports to investigate the effects of the spatial variation of earthquake motion.
Bridges of varying stiffness and ductility have been designed, as in current engineering
practice, for synchronous motion with and without isolating devices. The results obtained were
used in assessing the relevance of non-synchronous input motion on the ductility demand in the
piers of conventional bridge structures and on the isolator displacements of isolated bridges. It
was seen that the piers designed for synchronous input remains in the elastic range also in the
case of hardening isolators.
An equivalent linear model for the seismic analysis of base isolated bridges with bi-linear
hysteretic bearing and modifications in the design specifications that can be readily applied by
practising engineers had been suggested (Hwang, 1996; Hwang and Chiou, 1996; Hwang et al.,
1996a, 1996b). In addition, the equivalent damping ratios calculated for a single-degree-offreedom
system subjected to a generated AASHTO design earthquake may be larger than 30%
for a wide fundamental range. As a result, three-dimensional inelastic time-history analysis may
be necessary. As an alternate approach, empirical formulas for the calculation of equivalent
period shift and equivalent damping have been proposed and validated. The system composite
damping ratio is obtained both by classical and non-classical damping assumptions. The
composite-damping ratio formulated in the study is compared with that obtained from the
modal strain energy method. It is observed that the composite damping ratios determined on the
basis of classical damping and non-classical damping are almost identical for seismically
isolated bridges defined in AASHTO (1991).
Mayes (1996) described how the force-reduction and force-redistribution advantages of seismic
isolation could benefit the design and economics of bridges (for both new and retrofitted) in the
central and eastern United States.
Delis (1997) addressed the analysis and design issues of a seismically isolated continuous steel
bridge located in a highly seismic area. Two types of isolation systems are considered namely
the friction bearings and friction bearings with non-linear fluid viscous dampers. A non-linear
3-D time history analysis was performed using several ground-input motions. It is seen that the
earthquake forces are drastically reduced due to bearings. Further, the increased displacement
demands at abutments are accommodated with specially designed expansion joints that allow
large seismic movements in both horizontal directions.
Calvi and Pavese (1997, 1998) presented displacement-based designs approach using a linear
equivalent single degree-of-freedom model. The preliminary design of an isolation system for
existing bridges is based on the definition of a "structure regularity" which all
estimation of whether the response of the real structure will be similar to that predicted in the
preliminary design phase. The efficiency of the approach is also shown in designing the
isolation system for a highly irregular bridge. In addition, an optimisation procedure which has
become a proposal for a design method has been implemented in an efficient software program
and applied to a large number of cases which confirmed the soundness of the principles adopted
in the design philosophy. The design approach assumes that a displacement profile predicted
using a linear equivalent model will be reproduced by the envelope of the maximum
displacement obtained from a series of non-linear analyses, considered to be representative of
the real response.
Tsopelas and Constantinou (1997) studied the bridges with E-shaped steel dampers. The action
of E-shaped dampers is first to provide rigidity against service loads at selected locations and,
second is to yield and dissipate energy in seismic excitation. The behaviour of E-shaped device
is a function of its geometry and material properties. This behaviour is nearly elasto-plastic
with small post-yielding stiffness. It is demonstrated that significant permanent displacements
develop, particularly in earthquakes with shock loading characteristics. It is concluded that
elasto-plastic isolation systems may be very useful when sufficient displacement capacity is
provided and provisions for bridge re-centering or bearing replacement are made.
Anderson and Mahin (1998) postulated a preliminary seismic design method for simple baseisolated
bridges based on conservation of displacement and energy for the long and short-period
ranges of structural response. The approach focuses on estimation of overall displacements of
the deck level of the bridge, which overcome the limitations of equivalent linear idealization
and more explicitly acknowledges the non-linear inelastic behaviour presented in the isolated
systems.
Iemura et al. (1998) shown that when seismic isolation systems are installed, a reasonable
inelastic design method is required. Since the conventional inelastic design method takes into
account bridge piers, it is hard to design seismic isolators that can cope with the interaction
between seismic isolators and bridge piers. Therefore, a design procedure for isolated bridges is
proposed using the relationship between seismic isolation systems and piers with respect to
their energy dissipation.
Li and Xin (1998) used differential equation model for describing the hysteretic restoring force
model of isolation bearings. The Wilson-theta and fourth-order Runge-Kutta methods are
combined to develop a computer program for the non-linear seismic response analyses of
isolation systems for bridges. The example of a three-span continuous bridge with isolation
bearings is investigated and the isolation effects were discussed.


Saiidi et al. (1999) developed a non-linear model for the time-step analysis of bridges subjected
to two orthogonal horizontal components of earthquake motion. Elastomeric isolators with or
without lead cores were used and hysteretic behaviour of the isolators, the columns, abutments
and shear keys was taken in to account. The non-linear analysis showed that, contrary to linear
theory predictions, the use of isolators does not necessarily increase the displacement of the
superstructure. Furthermore, it was also shown that properly designed isolators could reduce
the ductility demand in RC Bridge columns substantially.
Hayashikawa et al. (2000) studied the non-linear behaviour of steel towers of cable-stayed
bridges subjected to major three-dimensional earthquake ground motions. It is shown that the
seismic performance of steel towers with passive control device is effective in reducing the
reaction forces at the tower basements.
Sugiyama (2000) compared dynamic characteristics for a bridge with sliding type isolation
system and a bridge with a LRB type system under earthquake motion. The results showed that,
from the point of view of reduction of the girder acceleration, a sliding type base isolation
system is more effective than a LRB in the case that a stronger earthquake affects the bridge
although the relative displacement between the superstructure and the substructure is
considerably large. It has also been revealed that no significant difference is recognised
between these two types of base isolation systems in the case of a relatively weak earthquake.
Abe et al. (2000) studied the seismic response of three bridges with lead-rubber bearings, high
damping rubber bearings and natural rubber bearings. Their seismic performance is evaluated
through comparison between identified stiffness and damping values from observed records and
predicted values from the loading test on each device. The analysis revealed that the base
isolation effect is present in all bridges, while contribution of the minor friction element can
significantly influence the performance.
An identification algorithm to investigate the dynamic properties of a base-isolated highway
bridge equipped with lead rubber bearing was developed by Tan and Huang (2000). A linear
model was used for the pier while a bi-linear model was used for lead-rubber bearings. It is
concluded that physical parameters obtained through the proposed identification process may
provide a basis by which various warning systems for an isolated bridge can be established.
Kim et al. (2000) investigated the efficiency of using dissipating restrainers at expansion joints
for preventing collapse of highway bridges in the event of an earthquake. The restrainers
consist of a non-linear viscous damper and an elastic spring connected in parallel or in series.
Two-dimensional finite element analysis using bilinear hysteretic models for bridge
substructure joints and non-linear gap elements for expansion joints is performed on bridges
with one or two expansion joints. The analytical study demonstrates that the energy dissipating
restrainers are effective in reducing the relative opening displacements and impact forces due to
pounding at the expansion joints, without significantly increasing ductility demands in the
bridge substructures.
Wilde et al. (2000) studied the seismic response of bridges with LRB and shape memory alloys
(SMA) which provides stiff connection between the pier and the deck for small external
loading. The proposed isolation system utilizes the different responses of the SMA at different
levels of strain to control the displacements at various excitation levels. At the same time the
hysteresis of the SMA is used to increase the energy dissipation capacity.
A combination of helical springs and fluid dampers is proposed as isolation and energy
dissipation devices for bridges subjected to earthquake loads by Parvin and Ma (2001). Vertical
helical springs are placed between the superstructure and substructure as bearings and isolation
devices to support the bridge and to eliminate or minimize the damage due to earthquake loads.
Since helical springs provide stiffness in any direction, a multidirectional seismic isolation
system is achieved which includes isolation in the vertical direction. To reduce the response of
displacement, non-linear fluid dampers are introduced as energy dissipation devices. Time
history analysis studies conducted show that the proposed bridge system is sufficiently flexible
to reduce the response of acceleration.

PARAMETRIC STUDY


Turkington et al. (1989c) conducted a parametric study of bridges seismically isolated by leadrubber
bearings. It is concluded that the LRB combined with elastomeric bearings provide an
effective means of distributing the response forces between piers and abutments. It is also
concluded that a characteristic of earthquake records affects the performance of LRB. Vibratory
earthquake records generally results in greater amount of additional damping than do impulsive
earthquakes and larger magnitude earthquakes generally results in greater period shifts.
Thakkar and Maheshwari (199 ) studied a seismic response of a base isolated bridge by varying
different parameters such as soil stiffness, embedment depth, hydrodynamic pressure and
earthquake response spectrum. Elastomeric bearing is seen to be effective in reducing seismic
response of substructure on rocky sites. The increase in embedment depth also causes reduction
in bending moments and shear forces in the substructure. The use of elastomeric bearing in
place of rocker-roller bearing is seen to be beneficial from seismic considerations.
Eftekhari and Zadeh (1996) discussed the effects of isolators and their locations on the dynamic
behaviour of isolated bridges. The behaviour was analysed of materials and isolators assumed
to be linear to study the effect of variation of the different isolator parameters such as stiffness
of the deck, piers and elastomeric bearings. In addition, the non-linear behaviour of isolators
was also investigated.
Adachi et al. (1998) conducted analytical study on seismic behaviour of seismic isolator and
RC bridge column system. A parametric study using a 2-DOF system was carried out to
propose an equivalent 1-DOF using equal energy principle. Dolce (1998) studied the seismic
behaviour of a railway bridge equipped with seismic isolation/dissipation devices. A typical
bridge of the new Italian high-speed railway line under construction having elasto-plastic
behaviour is considered and the parameters characterizing the response of such devices
(threshold force, post-yield stiffness) are varied. The actual advantages of seismic isolation in
comparison with conventional design of railway bridges are highlighted and some indications
on the optimal choice of the behavioural parameters are also obtained.
Tongaonkar and Jangid (1998) investigated the seismic response of bridges with a sliding
isolation system between the superstructure and substructure. Frictional force of the isolation
system is assumed to have ideal Coulomb-friction characteristics. In addition, a linear restoring
force is also provided by the isolation system. Seismic response of the isolated bridge system in
both longitudinal and transverse directions is obtained by solving the non-linear equations of
motion (non-linearity due to sliding system) in the incremental form using Newmark's method.
The system is subjected to real earthquake ground motion in both horizontal directions and the
effects of isolation parameters on the peak response of isolated bridge were investigated.
Reinhorn et al. (1998) examined the effects of variation of the ratio of isolator and pier yield
characteristics on the response of isolated bridges. It has been recognized that, due to low
redundancy and domination of the deck mode of vibration, isolated bridges are extremely
sensitive to the characteristics of the ground motion. After yielding, the stiffness properties and
the periods of the deck-bridge system may be entirely dominated by the secondary stiffness of
the isolators because of the larger mismatch with the support stiffness. This study investigated
the sensitivity of bridge response to small variations of the post-yield stiffness of the isolation
system. Since the deck contributes most of the mass of the bridge, analytical models for design
tend to diminish the attention to the masses of the piers and to the vibration modes they induce.
However, in deck isolated bridges the local modes of tall massive columns may contribute
substantially to the drift demand of the respective isolator-pier systems. The response
implications of neglecting the column mass in modelling of bridges are also studied.
Kawashima and Shoji (1998) presented analysis on the interaction with emphasis on the yield
force level of the isolator device. It was found from the analysis that the post-yield stiffness and
the yield force level of the device are important to predict the non-linear response of the pier.
The device with zero post-yield stiffness develops the hysteretic response at the device and
when the force of the device with positive yield–stiffness increases to the yield force of the
pier, significant hysteretic behaviour of the pier occurs.
Tongaonkar and Jangid (2000) investigated the effectiveness of elastomeric bearings for
seismic isolation of bridges. A parametric study is conducted to investigate the effects of
bearing parameters (such as stiffness and damping characteristics) on the effectiveness of
isolation for the bridge system. It is shown that the elastomeric bearings are quite effective in
reducing the seismic response of bridges. Further, the effectiveness of the elastomeric bearings
is significantly influenced by the stiffness and damping properties. In addition, a comparison of
the response of an isolated bridge with a linear isolation system is carried out with the
corresponding non-linear model of the elastomeric bearings. The results of the two models were
found to be comparable.
Koh et al. (2000) developed a method to evaluate the cost effectiveness of seismic isolation for
bridges in low and moderate seismic regions, for calculating the minimum life-cycle cost of
seismically isolated bridges under specific acceleration levels and soil conditions. Input ground
motion is modelled as a spectral density function compatible with a response spectrum for
combination of acceleration coefficient and site coefficient. Failure probability is calculated by
spectrum analysis based on random vibration theories to simplify repetitive calculations in the
minimization procedure. The results show that seismic isolation is more cost effective in low
and moderate seismic regions than in high seismic regions. The correlation was weak between
soil types and the cost effectiveness of the seismic isolation system in low and moderate
seismic regions, but was strong in high seismic regions.
Chaudhary et al. (2000) proposed identification of system parameters from seismic
accelerations recorded on a base-isolated bridge to examine the performance of various
components of bridges. The study proposed a two-step system identification method for
identifying structural parameters from strong-motion records. The first step entails identification of complex modal parameters of a non-classically damped base-isolated bridgepier-
pile-foundation system for which necessary theoretical formulations are first derived. In
the second step, a global search scheme is introduced to identify the structural parameters of the
system corresponding to the identified modal parameters. The proposed system identification
method was applied to two base-isolated bridges such that the recorded responses at the pier
cap and girder are successfully recreated for one main shock and four aftershocks of the 199
Kobe earthquake and modal and structural parameters are identified. Performance of the baseisolation
system is evaluated by comparing the physical properties of the bearings, determined
from experimental data, with the identified values and is found to be satisfactory in both
bridges.
Sawada et al. (2000) studied the non-linear interaction between pier and isolator in seismically
isolated bridges subjected to extreme earthquakes. Isolated bridges with reinforced concrete
piers and lead-rubber bearings are modelled as a 2-DOF system and investigation focused on
the effects of the primary structural parameters such as yield strength ratio on the displacement
ductility of both pier and isolator. To identify the range of yielding strength ratio as all the
restrictions on some typical non-linear responses of isolated bridges under severe earthquakes
are satisfied, a practical procedure using the contour diagrams for those responses is introduced.
Ceravolo et al. (2000) examined the possible uses and advantages of seismic isolation at the
base of girder bridges. The attention was focused in particular on the feasibility of adopting a
system based on abutment of the horizontal stiffness of foundation piles, as it was obtained by
piles partly un-confined by surrounding soil. Eight bridges with standard characteristics,
representing a sufficiently wide class of girder bridges were subjected to real seismic excitation
and a parametric analysis was conducted with the aim evaluating the optimal characteristics of
the isolation and applicability of the technique.
Liao et al. (2000) studied the parameters that may affect the bridge responses with respect to
base shear reduction and displacement amplification of a highway bridge subjected to near-fault
ground motions recorded during the Chi-Chi earthquake. Chaudhary et al. (2001) studied the
performance of various components of bridge system using identification of system parameters
with the help of multiple sets of records made on base-isolated Yama-age bridge in Japan. The
effectiveness of base isolation and effects of soil-structure interaction on the overall
performance were investigated by comparing the identified and physical parameters.
Franchin et al. (2001) discussed three aspects related to the method of analysis for linear or
linearized isolated bridge namely (i) classical modal analysis, using real modes and the
diagonal terms of the modal damping matrices, still provide a fully acceptable approximation,
(ii) parametric study conducted shown that none of the linearized expressions in current use
gives satisfactory results for both the displacement and the force responses, a requirement for a
reliable design of an isolated bridge and (iii) a rational, approximate procedure for equivalent
damping applicable to all types of structures with non-proportional damping, which in the case
of bridges can be shown to reduce to the expression provided in the Japanese bridge design
guidelines.

Experimental Study


A number of experimental studies have been reported on the load deformation behaviour of
different isolation devices and response characteristics of structural models isolated by different
isolators in the past. Kelly et al. (198 ) and Buckle and Kelly (1986) studied quarter-scale
models of straight and skewed bridge decks mounted on plain and lead-filled elastomeric
bearings subjected to earthquake ground motion using the shaking table. The deck response was
compared to determine the effectiveness of mechanical energy dissipaters in base isolation
systems and the mode of failure of base-isolated bridges. The control of translational and
motions were studied to determine if the seismic performance of skew bridges, in particular,
can be improved. A simple analysis of the limit state of an isolation bearing is described and
results of tests carried out to verify the analysis are presented.

Two analysis models for high damping rubber bearings were proposed based on shaking table
tests of a seismically isolated bridge deck by Hwang and Ku (1997). These analysis models are
established using the modified Gauss-Newton system identification method and the fractional
derivative Kelvin model based on sinusoidal test results. The test produced a maximum shear
strain in the bearing of approximately 100 percent. Two existing equivalent linear models
specified by the AASHTO and the Public Works Research Institute of the Japanese Ministry of
Construction are also characterized using sinusoidal test results. The predicted seismic
responses of the test structure by the proposed models and the two equivalent linear models
were compared with the measured responses. It is concluded that the proposed models can
predict the seismic responses of the bearing better than the two equivalent linear models. For
practical applications, the fractional derivative Kelvin model is implemented into an interaction
procedure adopted by the current design practice. An evaluation of the convergence of iteration
and the accuracy of prediction is conducted.
Dolce and Marnetto (1998) showed the main experimental results on the mechanical behaviour
of some selected shape memory elements. The conceptual design of a family of energy
dissipating devices having full re-centering and high-energy dissipation capabilities, as well
high resistance to large strain cycle fatigue and great durability, had described. The
experimental behaviour of the devices is shown by the results of cyclic tests on the devices and
of shaking table tests on scaled frame models. Finally, the applicability to bridges is
demonstrated by an example.
Ando et al. (1998) conducted forced and free vibration tests on Ohito Viaduct Bridge 2,
seismically isolated by lead-rubber bearings, to study the behaviour of the bridge. The resonant
frequencies found to depend significantly on exciting force because of amplitude dependence of
equivalent stiffness isolator. It is also reported that stiffness of isolator depends strongly on
displacement amplitude even in linear range. Ogawa et al. (1998) conducted experimental
studies to investigate an isolation system consisted of PTFE bearings and rubber springs. The
system was found to be efficient in reducing the displacement response of the isolated structure,
which utilizes frictional energy dissipation and rubber springs.
Robson and Harik (1998) performed dynamic testing on a highly skewed, three-span,
seismically isolated, pre-stressed concrete slab-on-girder bridge. The pullback, quick-release
method of testing was used, a first for this bridge type. A simple new quick-release mechanism
was employed with relatively low lateral test force. Also, a new method of attaching the
pullback cable to the bridge was implemented. After testing, a three-dimensional finite element
model of the bridge was created. An optimisation program was used to refine, or calibrate, the
model to match experimentally determined natural frequencies and mode shapes. This
automated, systematic optimisation of model parameters produced an accurate analytical
representation of the bridge.
Pinto et al. (1998) described large-scale pseudo-dynamic tests on seismically isolated bridges
carried out at the ELSA Laboratory. Two alternative solutions were adopted, one solution with
isolation/dissipation (I/D) devices over the abutments and over all piers, and another solution
with I/D devices over the central short pier only, where the demands concentrated for the
conventionally designed structure. The dissipation of energy was totally concentrated in the I/D
devices and the bridge piers were completely protected. It is, seen that the fully isolated bridge
requires significant clearances at the abutments in order to accommodate the dynamic displacements in those zones. Also, it is seen that partial isolation is more economical (one
isolator against five for the full isolation solution), the partial isolation exploits in a more
effective way the deformation capacity of the lateral piers.
Adachi et al. (2000) conducted shake table tests to study the non-linear seismic response
behaviour of a seismic isolator and a reinforced concrete bridge column system. The global
system response was dominated by the primary mode even if the non-linear behaviour was
found at both the seismic isolators and the column. The simulation results using ordinary
models of the seismic isolators and the column can express the test results quite well and the
global response can be simulated using an equivalent 1-DOF model with proper damping
factor.
Identification of system parameters with the help of records made on base-isolated bridges
during earthquakes provides an excellent opportunity to study the performance of the various
components of such bridge systems. Chaudhary et al. (2001) examined the soil-structure
interaction (SSI) effects in base-isolated bridges by comparing the identified and physical
stiffness of the substructure components. It is found that SSI is relatively pronounced in bridges
founded in weaker soils and is more strongly related to the ratio of pier flexural stiffness and
horizontal foundation stiffness than soil shear modulus alone. However, substantial reduction in
shear modules is observed for moderate seismic excitation and this effect should be taken into
account while computing foundation impedance.


Application of Seismic Isolation for Bridges


The seismic isolation had been successfully used several countries using the elastomeric and
sliding bearings. A total of 2 isolated bridges namely in Iceland, 49 in New Zealand, 12 in
Japan, 21 in the United States and 168 in Italy - had been built (Priestley et al., 1996). Leadrubber
bearings were used in three Italian bridges and in 66 of the other 87 bridges, showing
clearly that this is the preferred choice except in Italy, where more frequently, some dampers
(in most cases a steel damper) is coupled with traditional sliding support. Seismic isolation and
energy dissipation devices can also be used in retrofitting the bridges (Buckle and Mayes, 1989;
Penzien, 2001). These are used to replace vulnerable support bearings by which the bridgessystem’s
flexibility can be increased considerably, lengthening the fundamental periods
resulting in reduced horizontal seismic forces but increasing superstructure displacements.
Seismic isolation systems such as lead-rubber bearing and FPS are most commonly used in
retrofitting existing bridges.
Kelly et al. (1984) studied the retrofit of an existing freeway overpass undertaken to improve
earthquake performance via the installation of lead-rubber bearings between the superstructure
and the supporting columns. Before the retrofit, the columns of the bridge were capable of
resisting approximately one-quarter of the design site earthquake but the lead-rubber bearings
are shown to improve this performance.
Parducci and Mezzi (1992) discussed great number of highway bridges in Italy provided with
seismic isolating devices. As a typical example of the criteria for the design of such structures,
the bridges of a new highway are described. They are composed of various continuous multispan
sections. The seismic isolating systems have been designed on the basis of the dissipating
behaviour of the elasto-plastic restrainers. The general considerations concerning the design of
optimum structural configuration suggested by the use of seismic isolation were also discussed.
Matson and Buckland (199 ) presented the experience gained in the seismic evaluation and
retrofit of major bridges on the west coasts of Canada and the United States. These includes
Golden Gate Bridge South Approach retrofitted with isolation bearings, Granville Bridge, an
eight-lane truss bridge was retrofitted with cable restrainers and rubber bumpers. Also, in the
Burrard Bridge truss was fitted with lead-core base isolation bearings. These case histories
provide examples of seismic retrofitting by means of reducing the forces by de-tuning,
absorbing energy, providing ductility, limiting travel and strengthening.
The Sacramento River Bridge at Rio Vista, California had been retrofitted using seismic
isolation (Abbas et al., 1996). The retrofit of the bridge consists of seismic isolation of the
bridge deck in the steel truss approach spans, and use of passive energy dissipaters at the tower
column base connections. Extensive non-linear dynamic time history analyses were also
performed to evaluate the performance of the isolation and energy dissipation systems.
The seismic upgrading of a motorway overpass bridge, situated in Chalastra just outside
Thessaloniki, Greece, 130-m-long, six-span pre-stressed bridge of the Gerber type resting on
five-reinforced concrete piers and two reinforced concrete abutments was done. The retrofitting
was carried out by replacing the existing steel bearings with elastomeric ones of high damping
( %) each with varying height each, so that the required resistance in the system pier-cup-piles
for gravity loading and earthquake would be less than the available one (Penelis et al., 1988).
A seismic retrofit strategy was developed for the Poplar Street Bridge over the Mississippi
River at St. Louis for the Missouri Department of Transportation (Capron, 1999). The 660 m
long structure consists of two parallel five-span continuous roadways with an orthotropic steel
plate deck and variable depth steel box girder. The seismic evaluation considered three levels of
design earthquakes and identified deficiencies in the bearings, reinforcement splices in the
columns and piers, and one foundation. The retrofit strategy included adding force transmitters
or dampers to the existing expansion bearing piers, adding transverse shear blocks to the beam
seats etc.
The I-40 Mississippi River bridge, was built in the late 1960s has a total length of .3 km
situated at the southeastern edge of the New Madrid seismic zone. The Arkansas and Tennessee
Departments of Transportation conducted retrofitting of the bridge where existing bearings
were replaced by friction pendulum bearings. The overall construction cost and the extent of
retrofit work was significantly reduced by switching from the existing bearings to the isolation
bearings (Imbsen et al., 1999).
A high-damping rubber bearing had been used in a large-scale pedestrian bridge spanning over
railway lines in the Shizouka City (Higashi-Shizouka pedestrian bridge) for more flexibility of
the structure and greatly reducing the power of earthquakes and, consequently, enhances
resistance against shock (Iwata et al., 2000). The safety of the bridge structure was confirmed
through non-linear dynamic analysis, as well as through a hybrid earthquake-loading test
(pseudo-dynamic test).
A six-span continuous pre-stressed concrete twin cell box girder bridge has an overall length of
244.8 m, Yama-age bridge situated on National Highway number 294 in Japan Tochigi, is the
first bridge in Japan, which is base-isolated with high damping rubber bearings (Chaudhary
2001). This bridge performed very well during 199 Kobe earthquake.


Active and Hybrid Control Strategy for Bridges


Apart from the passive control of the bridge structures, there had been studies for active and
hybrid control strategies for better earthquake protection bridges. Nagarajaiah et al. (1992)
developed a control algorithm for friction controllable sliding isolation system for bridges
including the effects of stick-slip phases. The developed algorithm is used to verify the
accuracy of the algorithm with continuous sliding assumption and to establish its limits.
Comparisons with experimental results were presented and effects of stick-slip phases on the
response were also evaluated.
Reinhorn et al. (1993) presented three control algorithms for the hybrid system applied to
bridges. Two of these algorithms are verified experimentally, and the third is verified was
analytical model. The results show that the hybrid system is capable of significantly improving
the seismic response of the bridges. Yang et al. (1993) presented a method for controlling
seismically excited bridges by using variable dampers. A simulation study using a continuous
girder bridge is conducted to examine the effectiveness of the control algorithm in reducing the
absolute acceleration of the bridge girder and the relative displacement between the girder and
the supports. Simulation results indicate that the performance of the control method is
excellent.
Yang et al. (1994, 199 ) presented control methods for hybrid protective systems for bridges.
The control methods are based on the theory of variable structure system or sliding mode
control. Simulation results demonstrate that the control methods are robust with respect to
system parametric uncertainties and performance is quite remarkable. Sensitivity studies are
conducted to evaluate the effectiveness of hybrid protective systems and passive sliding
isolators for reducing the response of seismic-excited bridge structures.
Fideliu (1998) presented classical optimal control strategy with full known state for seismic
response control of cable-stayed bridge. The parameters for control strategy are proposed based
on energetic interpretation for the optimisation index and applied to a three-dimensional cable
stayed bridge, equipped with many active devices. The results had shown excellent
performance, validating the proposed strategy. Symans and Kelly (1998) investigated the
effectiveness of a hybrid system containing semi-active dampers through an analytical and
computational study of the seismic response of a bridge. The results show that such a system
may prevent or significantly reduce the structural damage during an earthquake.

Conclusion


. The review on the literature on the subject reveals that works on the following areas are still
inadequate and deserve attention of future research for more understanding of the subject and
for providing definite guidelines for design
. Investigations of effectiveness of seismic isolation for skew bridges and bridges curved in
plan and elevation.
. Effect of special correlation of earthquake ground motion on the response of seismically
isolated bridges.
. Earthquake response of bridges with sliding systems and lead-rubber bearing with soil
structure interaction.
. Earthquake response of seismically isolated regular bridges considering soil-water structure
interaction.
. Analysis and feasibility of semi-active control devices for a seismic design of bridge

 
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Re: Seismic behavior of Isolated Bridges Post Posted: Wed Mar 09, 2011 10:53 am 
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Its informative one also provide more information on topic.

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