30 November 1998
Mtr = ∑ ρiQ si = ∑ ρi2k siθ
Equation 3-3 shows that the torsional resistance from each panel is proportional to ρi2ksi. The total
torsional moment resistance, Mtr, is set equal to the Mt and the additional shear force due to torsion, Qsi is
calculated using Equations 3-1 and 3-3. Note that the torsional rotation, θ in these equations does not
need to be solved for and can be treated as a constant. Also the panel shear stiffness, ksi, is not needed
if all the panels can be assumed to be equal or if their relative stiffness can be determined.
6. COLD FORMED STEEL SEISMIC REQUIREMENTS
a. Wind and Earthquake Loads. The requirements of the 1996 AISI2, Section A5.1.3, shall be
modified as follows (FEMA 302, 8.5.1): "A4.4 Wind or Earthquake Loads where load combinations
specified by the applicable code include wind loads, the resulting forces are permitted to be multiplied by
0.75. Seismic load combinations shall be as determined by these provisions."
b. Boundary Members, Chords and Collectors. All boundary members, chords, and collectors shall
be designed to transmit the specified induced axial forces (FEMA 302, 8.6.1). Connections for diagonal
straps-to-column and columns-to-anchors and shear panel anchorage, and collectors shall have
adequate strength to account for the effects of material overstrength as indicated in this guidance. The
pullout resistance of screws shall not be used to resist seismic forces (FEMA 302, 8.6.2).
c. Shear Panel Anchors. Shear panels shall be anchored such that the bottom and top tracks are not
required to resist uplift forces by bending of the track or track web (FEMA 302, 8.6.3). Both flanges of
studs shall be braced to prevent lateral torsional buckling.
d. Pretension of Diagonal Straps. Provision shall be made for pretensioning or other methods of
installation of tension-only diagonal straps, to guard against loose straps (FEMA 302, 8.6.4).
e. All Steel Design. The guidance of FEMA 302, 8.6.5 for shear walls shall not be used and
configurations with plywood sheathing or oriented strand board are not permitted. The following guidance
shall be used in place of the FEMA 302 guidance in section 8.6.5.
Shear panel design shall be based on the cold-formed steel shear design guidance presented here. This
design requires that shear panels be adequately anchored at their top and bottoms to a floor diaphragm.
Shear panels in the two orthogonal directions must be anchored to the same diaphragm at each floor
level to tie the two orthogonal lateral load-resisting systems together. Shear panels above the ground
floor must have shear panels in the same bay and direction at every floor level below them.
Using the following guidance, the diagonal straps are sized to resist the total horizontal loads at each floor
level as defined in Equations C-12 and C-13, based on trial shear panel locations and aspect ratios.
Then the greater loads defined in Equations C-17 and C-18 are used to size the shear panel columns.
Finally the panel connections and anchors are designed based on the guidance that follows.
Specification for the Design of Cold-Formed Steel Structural Members, American Iron and Steel Institute (AISI), 1996.