The maximum considered earthquake shall be used to

times the design earthquake response spectrum given

calculate the total maximum displacement of the

in Figure 3-2.

isolation system.

(b)

Time histories.

Pairs of appropriate

(c) Response-spectrum analysis. Response-

horizontal ground-motion time-history components

spectrum analysis shall be performed using a modal

shall be selected and scaled from not less than three

damping value for the fundamental mode in the

recorded events. Appropriate time histories shall be

direction of interest not greater than the effective

based on recorded events with magnitudes, fault

damping of the isolation system or 30 percent of

distances, and source mechanisms that are consistent

critical, whichever is less. Modal damping values for

with those that control the design earthquake (or

higher modes shall be selected consistent with those

maximum considered earthquake).

Where three

appropriate for response spectrum analysis of the

appropriate recorded ground-motion time-history

structure above the isolation system with a fixed

pairs are not available, appropriate simulated ground-

base. Response-spectrum analysis used to determine

motion time-history pairs are permitted to be used to

the total design displacement and the total maximum

make up the total number required. For each pair of

displacement shall include simultaneous excitation of

horizontal ground-motion components, the square

the model by 100 percent of the most critical

root sum of the squares of the 5 percent damped

direction of ground motion, and 30 percent of the

spectrum of the scaled, horizontal components shall

ground motion on the orthogonal axis.

The

be constructed. The motions shall be scaled such that

maximum displacement of the isolation system shall

the average value of the square-root-sum-of-the

be calculated as the vectorial sum of the two

squares spectra does not fall below 1.3 times the 5

orthogonal displacements. The design shear at any

percent damped spectrum of the design earthquake

story shall not be less than the story shear obtained

(or maximum considered earthquake) by more than

using Equation 8-9 and a value of *V*S taken as that

10 percent for periods from 0.5*T*D seconds to 1.25 *T*M

equal to the base shear obtained from the response-

seconds.

spectrum analysis in the direction of interest.

(5) Analytical procedure.

(d)

Time-history analysis.

Time-history

analysis shall be performed with at least three

(a) General. Response-spectrum and time-

appropriate

pairs

of

horizontal

time-history

history analyses shall be performed in accordance

components as defined in Paragraph 8-3f(4)(b). Each

with Section 5.4 of FEMA 302, and the requirements

pair of time histories shall be applied simultaneously

of the following Paragraphs.

to the model considering the most disadvantageous

location of mass eccentricity.

The maximum

(b)

Input earthquake.

The design

displacement of the isolation system shall be

earthquake shall be used to calculate the total design

calculated from the vectorial sum of the two

displacement of the isolation system and the lateral

orthogonal components at each time step.

The

forces and displacements of the isolated structure.

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