Cold-formed steel diaphragm
A roof or floor diaphragm of wood structural panels screwed to cold-formed steel joists, spanning between two shear wall lines under a uniform in-plane load. To AISI S240-20 B5.4.2 for wind (and seismic outside S400), or AISI S400-20 F2 where the seismic force-resisting system is designed by S400, with AISI S100-16 for the chords.
Inputs of the diaphragm #
- Standard: S240 B5.4.2 (φ 0.65, Ω 2.00) or S400 F2 (φ 0.60, Ω 2.50). Both print the same table of nominal unit shears.
- Sheathing: Structural I or other graded panels, 3/8, 7/16 or 15/32 in., blocked with the screw spacing at the boundary and continuous edges (the other edges follow the table), or unblocked in the configuration that applies. The panel material (OSB or plywood) enters only the deflection.
- Load duration: the table values are for short-term load (wind, seismic); S240 B5.4.2.2 multiplies them by 0.75 for normal and 0.67 for permanent load.
- Diaphragm: length L perpendicular to the load and depth b parallel to it; the in-plane line loads by type (W, E), nominal. For seismic, enter the diaphragm design force (ASCE 7-22 12.10).
- Framing and requirements: the joist designation and spacing, the sheathing screw size and field spacing are checked against the listed requirements. The others (steel grades, track, framing screws, panel standards, panel widths, blocking, strap blocking, and in S400 the edge distance) are your confirmation. In S400, a diaphragm that supports masonry or concrete walls must be blocked (F2.2.2), and R over 3 brings F2.5.
- Chords and collector: the chord member (an SSMA designation), how many per side, Fy and the length between braces for compression; the shear wall length along the support line (shorter than b makes a collector) and, for seismic, the overstrength factor Ω0 for the collector (S400 F2.3).
- Deflection: under the W or E loads at their nominal value, with the sheathing shear modulus G (about 177,300 psi for 7/16 in. 24/16 OSB from the S240 Commentary, prefilled), the chord splice term Σ Δci Xi, and a limit (0 reports the deflection only).
Auto-design varies the panel thickness, the edge screw spacing (blocked) and the chord members. The standard, panel grade, blocking, the diaphragm's size, the loads and the framing are kept.
Checks performed #
| Check | Basis |
|---|---|
| Aspect ratio, joist thickness and shape, screw size, field screw spacing, joist spacing | S240 B5.4.2.1 / S400 F2.4.1.1 |
| Other listed requirements, confirmed | S240 B5.4.2.1 / S400 F2.4.1.1 |
| Blocking with masonry or concrete walls | S400 F2.2.2 |
| Unit shear v = w L / (2 b) against the table value | S240 B5.4.2.2, B5.4.2.3 / S400 F2.4 |
| Chord tension and compression, M / b with M = w L² / 8 | S100 D2, E2, E3 |
| Collector tension and compression | S100 D2, E2, E3; S400 F2.3 |
| Deflection, blocked and unblocked | S240 Eqs. B5.4.2.4-1, B5.4.2.4-5 |
Limitations #
- A simple-span diaphragm under a uniform load, between two support lines. Openings, cantilevered or continuous diaphragms and point loads are not modelled.
- The collector is checked with the chord member's strength.
- The shear walls, the transfer of the shear into them, chord splice connections and the joists' gravity design are not part of this calculation.
- Steel deck diaphragms (AISI S310) are a separate design.
Verification examples #
Verification status: 23 values compared in 9 examples (6 published, 3 hand calculations from the cited equations): 23 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
The nominal unit shears are published values: cells of S240-20 Table B5.4.2.2-1 and S400-20 Table F2.4-1, read back through the available strength. The unit shear, chord force and deflection are hand calculations of the cited equations. No published worked example of a cold-formed steel diaphragm is in the free library; AISI D110 would give one.