30 November 1998
The effects of gravity load and seismic forces shall be combined as follows to account for diagonal strap
overstrength, when the effect of gravity and seismic loads counteract each other, by combining
Equations C-9 and C-15:
(0.9 - 0.2S DS )D + Ω 0 QE
For both expressions in Equations C-17 and C-18, the total horizontal force is Ω 0QE. Every other term in
these equations represent vertical loads. The shear panel systems should be analyzed based on the
most critical load combination defined by either Equation C-17 or C-18. Each panel component
(including all connections), other than the diagonal strap, should be designed based on these loads.
C6. EQUIVALENT LATERAL FORCE PROCEDURE. FEMA 302 and TI 809-04 present two methods
for defining the structural response: the Equivalent Lateral Force Procedure (FEMA 302, 5.3) and the
Modal Analysis Procedure (FEMA 302, 5.4). Paragraphs C7 through C9 presents the determination of
base shear, period and vertical distribution of lateral forces using the Equivalent Lateral Force
Procedure. Only this method is presented because of its simplicity and recognizing that typical cold-
formed steel structures will likely be low rise construction so that first mode response will dominate the
seismic response of the structures. However if deemed beneficial the modal analysis approach
presented in FEMA 302 and TI 809-04 (Chapter 3-2.c.(2)) could be used.
C7. SEISMIC BASE SHEAR. Using the Equivalent Lateral Force Procedure, the seismic base shear, V
in a given direction shall be determined according to the following equation (FEMA 302, 5.3.2):
V = Cs W
Cs = the seismic response coefficient.
W = the total dead load and applicable portions of other loads (see FEMA 302, 5.3.2).
The seismic response coefficient, Cs shall be determined according to the following equation:
The value for Cs is calculated according to Equation C-20; and need not exceed the following:
but shall not be less than:
Cs = 0.1SD1I
nor shall it be taken as less than the following equation for Seismic Design Categories E and F: