Page 192 - Dynamic Loading and Design of Structures

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amount of horizontal displacement of the isolated structure and an efficient way to do this is
by increasing its damping (refer to Figure 4.8 for the effect of damping on seismic response
spectra). This type of structural response control is referred to as passive control.
Currently used isolation systems are based on the concept of flexible supports which can
either remain essentially elastic (linear isolation) or enter the inelastic range (non-linear
isolation) upon exceeding a certain level of horizontal shear (Skinner et al., 1993). The basic
elements included in a seismic isolation system are:
●Horizontally flexible supporting devices (isolators) located either between the structure and
its foundation or at a higher level in the structure; in buildings the flexible supports are
commonly located at the superstructure-foundation interface, whereas in bridges they are
located at the top of the piers and abutments.
●A supplemental damping device (or energy dissipator) for reducing the relative horizontal
displacement between the superstructure and substructure (i.e. the portion of the structure
below the isolators).
●Some means for controlling displacements at service levels of lateral loading (i.e. wind
loading and SLS or smaller earthquake loading).

Today there are many types of isolators including, among others, rubber (elastomeric)
bearings, roller bearings, sliding plates, rocking structures, cable supports, sleeved piles,
helical springs, and air cushions. Detailed descriptions of the various isolating devices can be
found in the massive literature available, which includes two recently published books dealing
exclusively with this topic (Skinner et al., 1993; Naeim and Kelly, 1999) and chapters on
seismic isolation included in books of broader scope (Booth, 1994; Hu et al., 1996; Priestley
et al., 1996).
Supplemental damping devices can be of different types, including
●Hysteretic dampers, wherein energy dissipation is taking place by yielding of metals such as
lead and mild steel, which have hysteresis loops very close to elastoplastic. A popular
isolator that incorporates a damping device is the lead-rubber bearing, shown in Figure
4.18, which is an elastomeric bearing (layers of rubber reinforced with thin steel plates to
increase the vertical stiffness) with a lead core which provides both damping (after yield)
and resistance to service lateral loads.
●Viscous dampers, such as the oil dampers commonly used in the motor industry, but also
newer devices such as shear panels containing high viscocity fluids that have recently been
developed in Japan. These mechanical devices are separate from the isolators.
●Frictional dampers based on the concept of friction between different materials, for
instance stainless steel and PTFE (Teflon). Such systems have a number of advantages, but
(unlike the previous ones) they need to be supplemented by a restoring force mechanism
(i.e. a means for returning the isolated