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Structural Concrete - Hassoun

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20.2 Seismic Design Category 789<br />

Table 20.1<br />

Risk<br />

Category<br />

(Continued)<br />

Nature of Occupancy<br />

Exceed maximum allowable quantities per control area as given in Table<br />

307.1(2) or per outdoor control area in accordance with the International<br />

Fire Code and are sufficient to pose a threat to the public if released<br />

• Aviation control towers, air traffic control centers, and emergency aircraft<br />

hangers<br />

• Buildings and other structures having critical national defense functions<br />

• Water storage facilities and pump structures required to maintain<br />

water pressure for fire suppression<br />

Seismic<br />

Importance<br />

Factor, I e<br />

The parameters S s and S 1 shall be determined from 0.2- and 1-s spectral response acceleration<br />

shown in Figs. 20.1a and 20.1b. Wheres 1 ≤ 0.04 and S s ≤ 0.15, the structure is permitted to be<br />

assigned seismic design category A.<br />

The values of F a and F v are determined from Tables 20.2 and 20.3 and are dependent on the<br />

mapped spectral values (S S and S 1 ) and the site class as can be determined in Table 20.4 in accordance<br />

with Chapter 20 of ASCE 7-10. Where the soil properties are not known in sufficient detail<br />

to determine the site classes, site class D shall be used unless the building official or geotechnical<br />

data determines site class E or F soils are present at the site (IBC 2012, Section 1613.3.2).<br />

20.2.3 Design Response Spectrum<br />

Design response spectrum is used to determine the design spectral response acceleration for a given<br />

structure (i.e., given period of vibration). After calculating design response acceleration coefficients<br />

S DS and S D1 from Section 20.2.2, the design response spectrum curve shown in Figure 20.3 (ASCE<br />

7-10, Section 11.4.5) should be constructed as follows:<br />

1. For periods T ≤ T 0 , the design spectral response acceleration, S a , shall be determined as<br />

(<br />

S a = S DS 0.4 + 0.6 T )<br />

(20.3)<br />

T o<br />

where<br />

T 0 = 0.2 S D1<br />

(20.4)<br />

S DS<br />

and T is the fundamental period of the structure (in seconds) determined in Section 20.3.1<br />

(Eq. 20.16).<br />

2. For periods T 0 ≤ T ≤ T s , the design spectral response acceleration, S a , shall be determined as<br />

S a = S DS (20.5)<br />

where<br />

T s = S D1<br />

(20.6)<br />

S DS

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