Mechanical Properties of Seismic Isolation Systems

A seismic isolation system is the collection of all individual seismic isolators and may be composed entirely of one type of seismic isolator, a combination of different types of seismic isolators, or a combination of seismic isolators acting in parallel with energy dissipation devices (i.e., a hybrid system).

The most popular devices for seismic isolation in the United States may be classified as either elastomeric or sliding. Examples of elastomeric isolators include high-damping rubber bearings (HDR), low-damping rubber bearings (RB), or low-damping rubber bearings with a lead core (LRB). Sliding isolators include flat assemblies or those with a curved surface, such as the friction-pendulum system (FPS).

8.7.2.1. Elastomeric Isolators

Elastomeric bearings represent a common means for introducing flexibility into structure. They consist of thin layers of natural rubber that are vulcanized and bonded to steel plates (see Fig. 8.34e). Natural rubber exhibits a complex mechanical behavior that can be described simply as a combination of viscoelastic and hysteretic behavior. Low-damping natural rubber bearings exhibit essentially linearly elastic and linearly viscous behavior at large shear strains. The effective damping is typically less than or equal to 0.07 for shear strains in the range of 0 to 2.0.

Lead-rubber bearings are generally constructed of low-damping natural rubber with a preformed central hole into which a lead core is press-fitted (see Figs. 8.34f and 8.34g). Under lateral deformation, the lead core deforms in almost pure shear, yields at low levels of stress (approximately 1160-1450 psi (8 to 10 Mpa) in shear at normal temperature),

Figure 8.34e. High-damping rubber bearing, made by bonding sheets of rubber to thin steel plates. The steel plate increases vertical compressive stiffness of the unit while maintaining the desired low lateral stiffness.

Figure 8.34f. Installation of a lead-rubber bearing under interior columns. A lead-rubber bearing (LRB) consists of one or more lead plugs inserted into holes preformed in low-damping rubber bearings. The lead core provides for energy dissipation by deforming plastically at a stress of 1500 psi (10 Mpa).

Figure 8.34f. Installation of a lead-rubber bearing under interior columns. A lead-rubber bearing (LRB) consists of one or more lead plugs inserted into holes preformed in low-damping rubber bearings. The lead core provides for energy dissipation by deforming plastically at a stress of 1500 psi (10 Mpa).

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