Seismic Isolation

Seismic isolation is a viable design strategy that has been used for seismic rehabilitation of existing buildings and in the design of a number of new buildings. In general, this system will be applicable to the rehabilitation and design of buildings whose owners desire superior earthquake performance and can afford the special costs associated with the design, fabrication, and installation of seismic isolators. The concepts are relatively new and sophisticated, and require more extensive design and detailed analysis than do most conventional schemes. In California, peer review of these new concepts is required for all designs that use seismic isolation.

Conceptually, isolation reduces response of the superstructure by "decoupling" the building from seismic ground motions. Typical isolation systems reduce seismic forces transmitted to the superstructure by lengthening the period of the building and adding some amount of damping. Added damping is an inherent property of most isolators, but may also be provided by supplemental energy dissipation devices installed across the isolation interface. Under favorable conditions, the isolation system reduces drift in the superstructure by a factor of at least two—and sometimes by as much as factor of five— from that which would occur if the building were not isolated. Accelerations are also reduced in the structure, although the amount of reduction depends on the force-deflection characteristics of the isolators and may not be as significant as the reduction of drift.

Reduction of drift in the superstructure protects structural components and elements as well as nonstructural components sensitive to drift-induced damage. Reduction of acceleration protects nonstructural components that are sensitive to acceleration-induced damage.

To understand the design principles for base-isolated buildings, consider Fig. 8.34a, which shows four distinct response curves A, B, C, and D. Let us examine the design of a building, say, some five stories tall, with a fixed-base fundamental period of 0.6 sec. Curve A, the lowest, shows lateral design forces resulting from loads prescribed in building codes such as IBC 2003 and ASCE 7-02. Curve B, the second lowest, represents the probable strength of the structure. This strength is generally greater than the design strength because of several factors. Chief among them are: 1) actual material strengths are almost always higher than those assumed in design; 2) use of load factors typically overestimates the actual loads imposed on the structure; 3) some conservatism is used in sizing of structural members; 4) designs are often based on drift limits; and 5) members are designed to have at least some ductility. It is estimated that the probable strength of a structure designed to code-level forces is about 1.5 to 2.0 times larger than the design strength.

Curve D at top shows the forces our fixed-base building would experience if it were to remain elastic for the entire duration of a design earthquake. However, in earthquake-resistant design, it is assumed that the lateral-force-resisting system will make

Base Isolation Diagram

Figure 8.34a. Design concept for base-isolated buildings: Top curve D shows the forces in the structure if it were to remain elastic during an earthquake. The conventional design approach is to build ductility into the structure to absorb the difference in forces between B and D. By providing seismic isolation, the maximum force experienced by the building, curve C, is reduced to its probable strength, curve B.

Figure 8.34a. Design concept for base-isolated buildings: Top curve D shows the forces in the structure if it were to remain elastic during an earthquake. The conventional design approach is to build ductility into the structure to absorb the difference in forces between B and D. By providing seismic isolation, the maximum force experienced by the building, curve C, is reduced to its probable strength, curve B.

excursions well into the nonlinear inelastic capacities of the structural materials. Therefore, typical buildings are designed to resist only a fraction of the full linear elastic demands of major earthquakes. Heavy reliance is placed on special prescribed details that are presumed to provide ductility for the extreme nonlinear inelastic demands. The difference between the linear elastic demand, Curve D, and the probable capacity of the building, Curve B, conceptually represent the magnitude of energy dissipation expected of the structure.

Let us compare this to the energy dissipation required of the building, if it is seismi-cally isolated. The elastic forces experienced by a seismically isolated building are significantly reduced for two reasons. First, the flexibility of the base isolators shifts the period of the building toward the low end of the spectrum. For instance, our example building with a fixed-base period of 0.6 sec would probably now have a period in the neighborhood of, say, 2 to 2.5 sec. The drop in the elastic design force, as seen in the graph, is considerable.

The second factor contributing to the reduction in force level is the additional damping provided by the dampers. Depending on the type of base isolater and supplemental viscous damper (if any) chosen for the building, the damping may increase from a generally assumed value of 5% of critical to as much as 20% or more. Together, these two factors help to reduce the ductility demand expected of the structure during a large seismic event. In fact, it is quite likely that our base-isolated structure may never be pushed beyond its elastic limit. In other words, in the 2.0- to 2.5-sec-period range, the probable strength of the building is very nearly the same as the maximum unreduced elastic demand. Therefore,

Roof Reduced roof acceleration = 0.8 to 1.2 g acceleration = 0.5 to 0.3 g

Roof Reduced roof acceleration = 0.8 to 1.2 g acceleration = 0.5 to 0.3 g

Seismic Isolation Base Cabinets

Figure 8.34 b. Comparison of response of a fixed-base and a base-isolated building: (1) fixed-base; (2) base-isolated. Base isolation typically reduces roof acceleration of low-rise buildings by about 60 to 80%.

Figure 8.34 b. Comparison of response of a fixed-base and a base-isolated building: (1) fixed-base; (2) base-isolated. Base isolation typically reduces roof acceleration of low-rise buildings by about 60 to 80%.

the building need not take excursions into nonlinear inelastic range, and can remain elastic for the entire duration of a design earthquake.

In simple terms, seismic isolation involves placing a building on isolators that have great flexibility in the horizontal plane (Fig. 8.34b). The system consists of:

• A flexible mounting to increase the building period which, in turn, reduces seismic forces in the structure above.

• A damper or energy dissipater to reduce relative deflections between a building and the ground it rests upon.

• A mounting that is sufficiently rigid to control the building lateral deflection during minor earthquakes and wind storms.

To decrease base shear, flexibility can be introduced into the building by many devices, including elastomeric bearings, rollers, sliding plates, cable suspension, sleeved piles, and rocking foundations. However, decrease in base shear due to lengthening of a building's period comes at a price; the flexibility at the base gives rise to large relative displacements across the flexible mount. Hence, the necessity of providing additional damping at the base isolation level.

While a flexible mounting is required to isolate a building from seismic loads, its flexibility under frequently occurring wind and minor earth tremors is undesirable. Therefore, the device at the base must be stiff enough at these loads, such that the building's response is as if it were on a fixed base.

Generally one isolator per column is used. However, more than one isolator may be required in certain buildings. For isolation of shear walls, one or more isolators are used at each end, and if the wall is long, isolators may be placed along its entire length, the spacing depending upon the spanning ability of the wall between the isolators.

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