Built-up cold-formed steel member
Two lipped C-sections joined back to back or toe to toe (a box), as a column, a beam or both: the member strengths of the pair and the connections that make it act as one. To AISI S100-16 (with S2-20) I1.1 and I1.2, with Chapters E, F, G and H.
Inputs of the built-up member #
- C-section: an SSMA designation or custom dimensions, and Fy.
- Built-up section: back to back (webs in contact) or a box (lips in contact). S100 gives no torsional or lateral-torsional buckling method for a box of two open sections: the box needs your confirmation that its connections close it, and torsion is then left out.
- Length and loads: a simple span with an axial load (compression positive) and a uniform transverse load bending the strong axis, by load type.
- Bracing: Kx, Ky, Kt, the weak-axis and torsional brace spacing, a compression flange braced throughout or not, Cb (1.0 is conservative), distortional buckling with kφ.
- Connections: the intermediate connector spacing a, the screw, the screws at each tie, the end connection (screws in a line, or a weld), the distance g between the screw rows of a back-to-back web (0 takes D − 2 in.), Fu (0 takes the ASTM A1003 minimum).
- Deflection limit: span over n, entered.
Auto-design varies the C-section (lightest first) and the connector spacing. Length, loads, bracing, the pack and Fy (unless unlocked) are kept.
Checks performed #
| Check | Basis |
|---|---|
| Compression with the modified slenderness | S100 E2, E3, Eq. I1.2-1 |
| Connector spacing a/ri at most half the governing slenderness | S100 I1.2(a) |
| End connectors at most 4 diameters apart over 1.5 times the width | S100 I1.2(b) |
| Each intermediate tie carries 2.5 % of the available axial strength | S100 I1.2(c), J4.3 |
| Distortional buckling in compression and bending | S100 E4, F4 (one C) |
| Bending | S100 F2 (the doubly symmetric pair), F3 |
| Connection spacing in bending, back to back | S100 I1.1, Eq. I1.1-1, J4.4.1 |
| Shear, bending and shear | S100 G2, H2 |
| Compression and bending | S100 H1.2 with B1 (C1.2.1.1, Cm = 1.0) |
| Deflection | IBC Table 1604.3 (limit entered), S100 L1 |
Limitations #
- Simple span, uniform transverse load about the strong axis. Weak-axis bending, end moments and point loads are not modelled.
- Distortional buckling is that of one C. S100 does not credit the lips in contact in a box.
- The box's torsional and lateral-torsional buckling are excluded on your confirmation only.
- Bearing (web crippling) and the member's end connections to the structure are not designed.
- I1.1 in bending takes one screw at each connection for Ts (pull-out only; pull-over and screw tension are not computed).
Verification examples #
Verification status: 7 values compared in 2 examples (0 published, 2 hand calculations from the cited equations): 7 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 worked example of a built-up cold-formed member is in the free library. The values are hand calculations from the cited equations, on the gross properties of one C that the stud type verifies against the SSMA tables: the factored loads, the modified slenderness, the end connector count, the L/6 limit and the box's moment of inertia. AISI D100 examples would replace them.