Page 200 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch04.qxd 9/29/05 5:09 PM Page 4.28

BUILDING CODES, LOADS, AND FIRE PROTECTION*

4.28 CHAPTER FOUR

TABLE 4.14 Values of F a as a Function of Site Class and Mapped Short-Period Maximum Considered
Earthquake Spectral Acceleration
Mapped maximum considered earthquake
spectral response acceleration at short periods
Site class S S ≤ 0.25 S S = 0.5 S S = 0.75 S S = 1.0 S S ≥ 1.25
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.2 1.2 1.1 1.0 1.0
D 1.6 1.4 1.2 1.1 1.0
E 2.5 1.7 1.2 0.9 0.9
F a a a a a
Note: Use straight-line interpolation for intermediate values of S S .
a
Site-specific geotechnical investigation and dynamic site response analyses shall be performed except that for structures with
periods of vibration equal to less than 0.5 s., values of F a for liquefiable soils may be assumed equal to the values for the site
class determined without regard to liquefaction in Step 3 of Sec. 9.4.1.2.2.
Source: From Minimum Design Loads for Buildings and Other Structures, SEI/ASCE 7-02, American Society of Civil
Engineers, Reston, Va., with permission.
overstrength, and energy dissipation the seismic force-resisting structural system possesses. Thus,
for more ductile systems with larger R, the equivalent lateral seismic design force will be lower than
for more seismically vulnerable systems with a smaller R factor. The most conservative, lower-bound
R is 1.0, which corresponds to a pure linear-elastic response and the highest possible seismic forces.
The seismic design forces determined in this manner are intended to be used directly with LRFD
(strength or limit states) design, but can be reduced for application in allowable strength design,
where prescribed in codes and standards.
SEI/ASCE 7-02 defines the seismic base shear coefficient C s in units of acceleration of gravity g
in accordance with the following equations:

C = S DS (4.8)
s
RI / s
where S DS = design spectral response acceleration in the short-period range, g
R = response modification factor for structure (see SEI/ASCE 7-02)
I s = occupancy importance factor for seismic use group (Table 4.12)

TABLE 4.15 Values of F v as a Function of Site Class and Mapped 1-s Period Maximum Considered
Earthquake Spectral Acceleration

Mapped maximum considered earthquake
spectral response acceleration at 1-s periods
Site class S 1 ≤ 0.1 S 1 = 0.2 S 1 = 0.3 S 1 = 0.4 S 1 ≥ 0.5
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.7 1.6 1.5 1.4 1.3
D 2.4 2.0 1.8 1.6 1.5
E 3.5 3.2 2.8 2.4 2.4
F a a a a a
Note: Use straight-line interpolation for intermediate values of S 1 .
a
Site-specific geotechnical investigation and dynamic site response analyses shall be performed except that for structures with
periods of vibration equal to less than 0.5 s; values of F v for liquefiable soils may be assumed equal to the values for the site class
determined without regard to liquefaction in Step 3 of Sec. 9.4.1.2.2.
Source: From Minimum Design Loads for Buildings and Other Structures, SEI/ASCE 7-02, American Society of Civil
Engineers, Reston, Va., with permission.

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