20-10-2012, 02:18 PM
Seismic behavior of isolated bridges: A-state-of-the-art review
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Introduction
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 elastomeric
bearings (with and without lead core), frictional/sliding bearings and roller bearings have been
developed and used practically for aseismic 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).
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.
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).