Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 294

Determination of the Seismic Design Category SDC

(1) Based on the values of S s and S 1 and site class D, determine the site coefficients F a and F v from
ASCE Tables 11.4-1 and 114.2. Notice that linear interpolation is performed.

F a = 1.59
F v = 2.32

(2) Calculate the adjusted maximum considered earthquake spectral response acceleration:

S MS = F a S s = 1.59 0.26 = 0.41g

S M1 = F v S 1 = 2.32 0.12 = 0.28g

(3) Calculate the design earthquake spectral response accelerations:

S DS = 2/3 S MS = 2/3(0.41) = 0.28g
S D1 = 2/3 S M1 = 2/3(0.28) = 0.19g

(4) For S DS = 0.28g ASCE Table 11.6-1 shows that the SDC is B

For S D1 = 0.19g ASCE Table 11.6-2 shows that the SDC is C

Use the more severe of the two:

SDC = C

Considering the exception in ASCE 11.6 (see Section 11.2) check the building SDC:

T s = S D1 /S DS = 0.19/0.28 = 0.68 seconds
Building T (see below) = 0.45 < 0.8(T s ) = 0.8(0.68) = 0.54 seconds O.K.

The SDC can be determined based on Table 11.6-1 and the building assigned SDC B. For illustrative
purposes only this building example will continue using SDC C.

Table 11-1 shows that the building frame system with ordinary shear wall can be used for SDC C.

Fundamental Period

The approximate natural period of the structure can be calculated as follows:

T = C t (h n ) x

For building frame system C t = 0.02 and x = 0.75

T = 0.02(63) 0.75 = 0.45 second (also T can be obtained from Fig. 11-8)