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Figure 8.34g. Lead-rubber bearing (LRB) for an interior column of a five-story steel framed building; approximate dimensions.

Standard holes for bolts

Figure 8.34g. Lead-rubber bearing (LRB) for an interior column of a five-story steel framed building; approximate dimensions.

and produces hysteretic behavior that is stable over many cycles. Unlike mild steel, lead recrystallizes at normal temperature (about 20°C), so that repeated yielding does not cause fatigue failure. Lead-rubber bearings generally exhibit characteristic strength that ensures rigidity under service loads.

High-damping rubber bearings are made of specially compounded rubber that exhibits effective damping between 0.10 and 0.20 of critical. The increase in effective damping of high-damping rubber is achieved by the addition of chemical compounds that may also affect other mechanical properties of rubber.

Scragging is the process of subjecting an elastomeric bearing to one or more cycles of large amplitude displacement. The scragging process modifies the molecular structure of the elastomer and results in more stable hysteresis at strain levels lower than that to which elastomer was scragged. Although it is usually assumed that the scragged properties of an elastomer remain unchanged with time, recent studies suggest that partial recovery of unscra-gged properties is likely. The extent of this recovery is dependent on the elastomer compound.

8.7.2.2. Sliding Isolators

Sliding isolaters with either a flat or a single-curvature spherical sliding surface are typically made of PTFE or PTFE-based composites in contact with polished stainless steel. The shape of the sliding surface allows large contact areas that, depending on the materials used, are loaded to average bearing pressures in the range of 1015 to 10150 psi (7 to 70 Mpa).

Sliding isolaters tend to limit the transmission of force to an isolated structure to a predetermined level. While this is desirable, the lack of significant restoring force can result in significant variations in the peak displacement response, and can result in permanent offset displacements. To avoid these undesirable features, sliding isolators are typically used in combination with a restoring force mechanism.

Figure 8.34h. Sliding bearing; friction-pendulum system (FPS). An FPS consists of an articulated slider that glides on a polished spherical concave chrome surface. Whereas in elastomeric base-isolated buildings PA effects are equally distributed between superstructure and foundation, in sliding base-isolated buildings the entire PA effect can be accommodated in either the superstructure or the foundation, depending on whether the spherical surface is attached to the foundation or the superstructure.

Figure 8.34h. Sliding bearing; friction-pendulum system (FPS). An FPS consists of an articulated slider that glides on a polished spherical concave chrome surface. Whereas in elastomeric base-isolated buildings PA effects are equally distributed between superstructure and foundation, in sliding base-isolated buildings the entire PA effect can be accommodated in either the superstructure or the foundation, depending on whether the spherical surface is attached to the foundation or the superstructure.

Combined elastomeric-sliding isolation systems have been used in buildings in the United States. Japanese engineers have also used elastomeric bearings in combination with mild steel elements designed to yield in strong earthquakes and enhance the energy dissipation capability of the isolation systems.

Details of a spherical sliding system commonly referred to as a friction-pendulum system (FPS) are shown in Figs. 8.34h, 8.34i, 8.34j(1), 8.34j(2), and 8.34j(3). Figure 8.34k shows a schematic of base-isolation devices acting in conjuction with viscoelastic dampers.

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