Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 279

(5) Structures exempt from seismic design requirements
(6) The types of lateral force analysis that must be performed
The SDC is a function of two parameters 1) the Occupancy Category I, II, or III (ASCE 11.5), which depends
on the occupancy and use of the building, and 2) the design spectral response acceleration. The following
procedure summarizes the determination of the SDC:
(1) From 2009 IBC Figures 1613.5(1) through 1613.5(14), determine the maximum considered earthquake
spectral response acceleration at short period (0.2 second) S s and at one second and S 1 . These values can
alternatively be obtained from the website of the United States Geological Survey (USGS) by inputting the
zip code of the building location.
(2) Depending on the soil properties at the site, determine the site class definition (A, B, C, D, E, or F), from
1613.5.2. In the absence of sufficient details on site soil properties it is allowed to assume site class D
unless the building official specifies higher site class (E or F).
(3) Based on the magnitude of the maximum considered earthquake spectral response accelerations (S s and S 1 )
from step 1 and the site class from step 2 determine the site coefficients F a and F v from Tables 1613.5.3(1)
and 1613.5.3(2). Figures 11-4 and 11-5 can be used to determine F a and F v
(4) Calculate the adjusted (for the site class effects) maximum considered earthquake spectral response
acceleration S MS and S M1 as follows:
S MS = F a S s
S M1 = F v S 1
(5) Calculate the five percent damped design spectral response acceleration at short period S DS , and at one
second period S D1 (1613.5.4):
S DS = 2/3 S MS
S D1 = 2/3 S M1
(6) SDC (A, B, C, D, E, or F) is determined based on the Occupancy Category (I, II, or III) as defined in 1602
and the short period design spectral response accelerations S DS . Another seismic design category is
determined based on the occupancy category and S D1 (2009 IBC Table 1613.5.6(1) and Table 1613.5.6(2)).
The more critical of the two categories shall be used. Figure 11-6 sumaries how to determine the seismic
design category based on the design response accelerations S D1 and S DS
The 2009 IBC allows the SDC to be determined based on S DS alone (from table 1613.5.6(1)) provided that all
the following conditions apply (1613.6.1):
a) The approximate fundamental period of the structure, T (see Section 11.6.1 for T calculations), in each
of the two orthogonal directions is less than 0.8 T s where T s = S D1 /S DS.
b) The seismic response coefficient, C s (see Section 11.6.1) is determined based on S DS.