Seismically Isolated Structures ASCE 702 Design Provisions

The procedures and limitations for the design of seismically isolated structures is determined considering zoning, site characteristics, vertical acceleration, cracked section properties of concrete and masonry members, seismic use group, configuration, structural system, and height. Both the lateral force-resisting system and the isolation system must be designed to resist the deformations and stresses produced by the effects of ground motions. The stability of the vertical load-carrying elements of the isolation system must be verified by analysis and tested for lateral seismic displacement equal to the total maximum displacement. All portions of the structure, including the structure above the isolation system, must be assigned a seismic use group based on ASCE 7-02 provisions with an occupancy importance factor taken as 1.0 regardless of its seismic use group categorization. Each structure must be designated as being regular or irregular on the basis of the structural configuration above the isolation system.

Three procedures are permissible: static analysis, response spectrum analysis, and time-history analysis. The static analysis procedure is generally used to start the design

Figure 8.34i. Friction pendulum system details. The PA effect in this arrangement, where A = earthquake-induced displcement, is accounted for in the design of the superstructure. If the spherical plate is attached to the foundation, the PA effect is accounted for in the design of the foundation.

process and to calculate benchmark values for key design parameters (displacement and base shear) evaluated using either response spectrum or time-history analysis procedures.

The static analysis procedure is straightforward. However, the procedure cannot be used when the spectral demands cannot be adequately characterized using the assumed spectral shape. Typically this occurs for:

Figure 8.34j(1). Installation details of FPS bearing under existing interior columns.

Figure 8.34j(2). FPS bearing. (Photograph courtesy of Anoop Mokha, Ph.D., S.E., Vice President, Earthquake Protection Systems, Vallejo, CA.)

Figure 8.34j(3). FPS bearing in new construction.

Figure 8.34k. Base isolator operating in concert with a viscous damper.

1. Isolated buildings located in the near field.

2. Isolated buildings on soft soil sites.

3. Long-period isolated buildings (beyond the constant velocity domain).

Further, the static procedure cannot be used for nonregular superstructures or for highly nonlinear isolation systems.

Response spectrum analysis is permitted for the design of all isolated buildings except for those buildings located on very soft soil sites (for which site-specific spectra should be established), buildings supported by highly nonlinear isolation systems for which the assumptions implicit in the definitions of effective stiffness and damping break down, or buildings located in the very near field of major active faults where response spectrum analysis may not capture pulse effects adequately.

Time-history analysis is the default analysis procedure: it must be used when the restrictions set forth on static and response spectrum analysis cannot be satisfied, and may be used for the analysis of any isolated building. Arguably the most detailed of the analysis procedures, the results of time-history analysis must be carefully reviewed to avoid any gross design errors.

8.7.3.1. Equivalent Lateral Force Procedure

This procedure is permitted when the following restrictions are met:

1. The structure is located at a site with S1 less than or equal to 0.60 g.

2. The structure is located on a class A, B, C, or D site.

3. The structure above the isolation interfaces is less than or equal to four stories or 65 ft (19.8 in.) in height.

4. The effective period of the isolated structure at maximum displacement Tm is less than or equal to 3.0 sec.

5. The effective period of the isolated structure at the design displacement Tp is greater than three times the elastic, fixed-base period of the structure above the isolation system.

6. The structure above the isolation system is of regular configuration.

7. The isolation system meets all the following criteria:

a. The effective stiffness of the isolation system at the design displacement is greater than one-third of the effective stiffness at 20% of the design displacement.

b. The isolation system is capable of producing a restoring force such that the lateral force at the total design displacement DT is at least 0.025w greater than the lateral force at 50% of the total design displacement.

c. The isolation system has force-deflection properties that are independent of the rate of loading.

d. The isolation system has force-deflection properties that are independent of vertical load and bilateral load.

e. The isolation system does not limit maximum considered earthquake displacement to less than SM1/SD1 times the total design displacement.

8.7.3.1.1 Lateral Displacements. There are as many as six definable displacements in base isolation terminology. Three of these are defined in Fig. 8.34l, while the others, related to certain prescribed formulas, are explained in the text. Design Displacement. The isolation system must be designed and constructed to withstand design lateral earthquake displacements DD, calculated to occur in the direction of each of the main horizontal axes of the structure in accordance with the following equation:

D = gSD1TD

Dtm = same as DrD but calculated for maximum considered earthquake, MCE

Dm = total design displacement (DT + torsional component) at corners of building due to DBE

Dd = design displacement at the center of rigidity of building due to design basis earthquake, DBE

Dd = design displacement at the center of rigidity of building due to design basis earthquake, DBE

Figure 8.34l. Isolator displacement terminology. Note: MCE = earthquake corresponding to 2% probability of exceedence in a 50-year period (2500-year return period); DBE = earthquake corresponding to 10% probability of exceedence in a 50-year period (475-year return period).

where

Dd = design displacement of the isolation system g = acceleration of gravity

SD1 = design 5% damped spectral acceleration at 1-sec period

TD = effective period of seismically isolated structure in seconds at the design displacement in the direction under consideration BD = numerical coefficient related to the effective damping of the isolation system at the design displacement, DD as set forth in Table 9.13.3.3.1

Effective Period at Design Displacement. The effective period of the isolated structure at design displacement TD shall be determined using the deformational characteristics of the isolation system in accordance with the following equation:

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