Cold-formed steel strap braced wall

One story of a wall braced by an X of flat straps, working in tension only: wall shear, chord studs, strap connection, hold-down force and drift. To AISI S240-20 B5.3 and AISI S400-20 E3, with AISI S100-16 for the members.

Inputs of the strap braced wall #

• Wall: length w between the chords, height h. Anchor offset: S240 Eq. B5.3.4.2-1 takes L with the anchor offsets. Enter 0 where the anchor is at the chord.
• Straps: on both sides or one, width, thickness, Fy and Fu. One diagonal works in each direction.
• Seismic design by AISI S400 E3: on by default. S240 A1.2.2 requires S400 in seismic design categories D, E and F, and wherever R is not taken as 3. Turning it off treats E as a plain load; whether you may is your determination.
• Connection method: the three Methods of S400 E3.4.1(a). Method 2 is checked. Methods 1 and 3 are your statement, and the sheet takes them as given.
• Ry and Rt: from Table A3.2-1 by Fy, or entered from tests (A3.2.1).
• Wall finish strength vfinish: S400 Eq. E3.3.3-1 takes at least 0.2 Vn / w. Enter 0 to use that.
• Overstrength Ω0 and Cd: prefilled at 2 and 3.5, from ASCE 7-22 Table 12.2-1 line A.19, flat strap bearing wall systems (R = 4). The S400-20 User Note calls it line A.18, from an earlier edition. Change them if your system differs.
• SDS: for Ev in the seismic combinations.
• Strap connection: screws (size, number at each end, number across the strap for the net section, gusset), or a strength you entered for a weld or a tested connection.
• Chord studs: SSMA designation, how many in each chord, Fy, Kx, bracing, punched or unpunched. Say which web you have. Standard SSMA studs are punched, and punched is prefilled. It changes Ae, the moment at shallow depths, and distortional buckling. The sheet states it.
• Distortional buckling: computed, or restrained by your determination.
• Moment from single-sided straps: on by default for straps on one side.
• B2: the story's sidesway multiplier, from your own analysis. 1.0 is prefilled. It acts only when the chord has a moment.
• Track, top of the hold-down: for a narrow wall's frame action.
• Hold-down: available strength and movement, from the product's report.
• Drift: wind factor and limit, Cd, Ie and allowable drift ratio are all your entries.
• Loads: by load type: in-plane shear V at strength level, gravity compression P on each chord, and the chord force from the stories above.

Auto-design varies the strap thickness and width, the chord studs of the given depth and their count, and the screws at each strap end. The wall, the loads, the steel and the entered hold-down are kept.

Checks performed #

Limitations #

• Minor-axis bending of the chord is a lower bound of our own. S100 gives no effective section for a C bent about its minor axis in a form we could apply directly, so all compressed flats are taken at the extreme fiber stress, with the flange's compressed zone and the lips as unstiffened when the tips are in compression. It errs low. F2.1.2 is applied with CTF = 1.0.
• Under ASD the expected strength is taken in full. The Standard shows no ASD reduction in what we read. This errs on the safe side.
• The frame analysis takes the Commentary's closed form. Its deflection is not used for drift, as the Commentary says.
• Drift uses the truss flexibility plus the hold-down movement you entered. Frame action is left out.
• Chord studs add as individual members, with no composite action and no built-up strength.
• Not covered: the track's connections and the collectors (the force is reported), the hold-down and anchor products, gussets as members, reduced-width fuses, strap screws at interior studs, and foundations.
• Not checked, for you to confirm: pretensioning, track and shear device details of S400 E3.4.1(c), (d), and the limits of E3.2.3 with masonry or concrete walls. Results list them.

Verification examples #

Verification status: 19 values compared in 11 examples (5 published, 6 hand calculations from the cited equations): 19 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.

No published worked example of a strap braced wall is in the free library. The AISI Framing Design Guide D110 and the shear wall guide D113 are paid and would give them. Everything below is what we could check without them, and most of it is hand calculation. No published value checks the design method as a whole.

• Published values: Ry and Rt from S400 Table A3.2-1 at four strengths; the chord tension strength from the gross area SSMA prints for 600S162-54.
• Hand calculations, from the cited equations: the wall shear, the expected strength factor, the capacity protected connection force both ways, the chord forces, the frame action equations and the drift.
• Not verified by any outside value: the minor-axis strength and the combined chord check of a narrow wall.