Cold-formed steel ceiling framing
A ceiling joist (an SSMA C-section) or a hat-channel furring member, over one to four equal spans between hangers or supports, under the ceiling's dead load, mechanical loads, any live load and the vertical seismic force. To AISI S100-16 (with S2-20; AISI S220-20 sends nonstructural members to S100), with ASCE 7-22 13.3.1.6 and the load combinations.
Inputs of the ceiling framing #
- Member: a C-section by SSMA designation or custom dimensions, or a hat channel by its depth, crown width (the face under the board), flange width on each side and design thickness. Fy.
- Top flange brace spacing (C): the spacing of bridging or blocking that braces the compression flange in the spans; 0 takes it as braced. Your call. Over interior supports the bottom flange is in compression and is taken as braced by the board.
- Hat braced: the hat's lateral-torsional buckling is taken as prevented by the board screwed to the crown and the flanges screwed at each support. You confirm it; this type has no other method for a hat.
- Spans: the span between supports, the number of equal continuous spans, and the member spacing (the tributary width). ASTM C754 spacing tables are yours to apply.
- Loads: area loads (D or L) and point loads (fixtures, equipment) by type; self-weight. Vertical seismic: Ev = 0.2 SDS D (ASCE 7-22 13.3.1.6 with 12.4.2.2) as the load type E, up and down, in the seismic combinations.
- Hangers: diameter and tensile strength of the wire or rod. Deflection: the total load limit (L/360 prefilled, IBC Table 1604.3).
Auto-design varies the C-section (SSMA designations, lightest first); Fy when unlocked. A hat channel or a custom section is kept.
Checks performed #
| Check | Basis |
|---|---|
| Flexure of a C: lateral-torsional buckling over the brace spacing (Cb = 1.0) and local buckling | S100 F2, F3 |
| Flexure of a hat: effective section (flanges k = 0.43, crown k = 4, webs under gradient), Fn = Fy | S100 F3, Appendix 1 1.1, 1.1.2, 1.2.1 |
| Shear (the two webs of a hat) | S100 G2 |
| Bending and shear | S100 H2 |
| Hanger tension, 0.75 Fu Ab, φ 0.75, Ω 2.00 | AISC 360-22 J3.7, applied to the hanger |
| Hangers stay in tension | ASCE 7-22 13.3.1.6 (advisory) |
| Total load deflection, effective inertia at the service moment | IBC Table 1604.3 (limit entered), S100 L1 |
Limitations #
- The hat's properties are computed on its square-corner centerline: corner radii are left out, and the webs are taken vertical.
- Web crippling is not checked: the reactions are taken through hangers, clips or screws, not by bearing.
- The hanger strength rule is the threaded-rod rule, conservative for an unthreaded wire; product or code values for wire may be higher.
- The lateral bracing of a suspended ceiling (ASCE 7-22 13.5.6), the attachments to the structure above and ASTM C754 prescriptive limits are not part of this calculation.
Verification examples #
Verification status: 8 values compared in 4 examples (0 published, 4 hand calculations from the cited equations): 8 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: not yet recorded for this module.
No published ceiling framing example is in the free library. The values are hand calculations: the hat's gross properties on the stated model, the statics of one and two spans, the vertical seismic combination and the hanger strength.