Page 193 - Dynamic Loading and Design of Structures

Page 168

Figure 4.18 Two commonly used isolating devices: (a) the lead—rubber bearing; (b) the friction
pendulum system.

structure to its initial position after a strong earthquake). An efficient system in this
category is the friction pendulum (Figure 4.18b), wherein the sliding surface of the
bearing is concave, hence the restoring force is provided by the horizontal component
of the weight of the structure itself.

Control of displacements under service horizontal loading can be obtained in several ways.
Specially manufactured elastomers have a high rigidity at low strains, typically three to four
times that at higher strains, and so do the aforementioned lead-rubber bearings. Alternatively,
fuse-type sacrificial elements such as steel pins can be used, designed to fail at a desirable
level of lateral loading; these elements should be replaced after each earthquake motion
exceeding that level.
In a seismically isolated structure the largest part of the lateral displacement takes place at
the location of the isolators. So long as this displacement can take place, the drifts in the
superstructure can remain very low, hence damage to both structural and non-structural
elements is minimal. Failure of such a system can occur due to rollover (instability by falling
over) of the bearings at large displacements,