30 November 1998
FEMA 302 AND OTHER STANDARD GUIDANCE FOR COLD-FORMED STEEL SEISMIC DESIGN
C1. INTRODUCTION. Seismic use groups (FEMA 302, 1.3) are used to determine occupancy
importance factors (FEMA 302, Table 1.4). Seismic use group III is for the most critical facilities as
defined in FEMA 302, 1.3. This table is reproduced below:
Table C-1 Occupancy Importance
Seismic Use Group
TI 809-04 uses enhanced performance objectives to define seismic design forces for more critical
facilities rather than the occupancy importance factors presented here.
C2. DEFINING GROUND MOTION. Seismic ground motions shall be defined according to FEMA 302
Chapter 4 and TI 809-04 Chapter 3. This paragraph defines ground motions for the maximum
considered earthquake ground motions derived from Maps 1 through 24 (FEMA 302, Chapter 4).
Spectral response acceleration at short periods, SS and at 1 second, S1 are obtained from Maps 1
through 24 of FEMA 302. For most regions of the nation, the maximum considered earthquake ground
motion is defined with a uniform likelihood of exceedance of 2 percent in 50 years (2500 year return
Site classifications shall be determined based on soil type (A through F), which may be based on shear
wave velocity, νs average blow counts from standard penetration resistance test N or unconfined shear
strength, su of the soil (FEMA 302, 4.1.2). From the site classifications, values of site coefficients (Fa
and Fv) are determined for the mapped spectral response acceleration values (FEMA 302, Table 126.96.36.199a
and Table 188.8.131.52b). These tables are reproduced below:
Table C-2a Values of Fa as a Function of Site Class and Mapped Short-Period
Maximum Considered Earthquake Spectral Acceleration
Short Period Maximum Considered Response Spectral Acceleration
SS ≤ 0.25
SS ≥ 1.25
SS = 0.50
SS = 0.75
SS = 1.00
1997 NEHRP, Part 2: Commentary (FEMA 303), Chapter 4 Ground Motion, p. 37.
Defined in ASTM D1536-84.
a indicates that a site-specific geotechnical investigation and dynamic site response analyses shall be performed.