208 Seismic analysis & design
2 SEISMIC ISOLATION
The main objective of a seismic isolation system is to increase the natural period of a structure.
However, rather than simply increasing the natural period to a high value, an efficient seismic
design also considers how energy dissipation capability can be increased and how lateral
forces can be distributed to as many substructures as possible. Bridges, and viaducts in par-
ticular, are ideal candidates for the adoption of such a seismic isolation approach due to the
ability to distribute lateral forces among multiple supports, and thanks also to the general
ease of installation and inspection of isolation devices.
An isolation system placed between the bridge superstructure and its supporting substruc-
ture is generally capable of increasing both flexibility and energy dissipation. Flexibility in the
horizontal plane will lower the frequency of the bridge, decreasing earthquake-induced accelera-
tion, while the energy-dissipating capacity of the seismic isolators will considerably reduce the
damaging energy exerted to the bridge piers. Moreover, when isolation devices are installed at
the tops of a bridge’s piers, the lateral force from the superstructure during a seismic event can be
distributed among all piers, avoiding the concentration of lateral forces at specific locations.
Among the different seismic isolation devices available, elastomeric isolators have found
wide application in bridge structures. This is due to their simplicity and their combining of
isolation and energy dissipation functions in a single compact unit. They provide a high level of
damping—a crucial aspect of seismic protection—to minimise the seismic energy flow to the
superstructure and to limit the horizontal displacements of the isolators (Mendez et al., 2011).
A particularly popular type of elastomeric isolator among seismic engineers is described
below: the Lead Rubber Bearing.
3 LEAD RUBBER BEARINGS (LRB)
As shown by Figures 3 to 8, a Lead Rubber Bearing (LRB) is similar to a normal elastomeric
bearing with anchor plates, but made using specially chosen elastomer and with a lead plug at
Figure 3. 3D illustration of a multi-directional
LRB, designed to allow movement in any hori-
Figure 4. Cut-out view of a multi-directional
LRB, showing the lead core at the centre of the
Figure 5. 3D illustration of a guided LRB,
designed to allow movements along one axis and
resist transverse forces.
Figure 6. Section through a guided LRB, show-
ing guide-bars at each side of the elastomeric
pad, and the pad’s lead core.
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