Cold-Formed Steel Design
Section strength of single C and Z purlins, girts and studs in bending: effective-width flexure, shear, web crippling and their interactions to AISI S100-16 or AS/NZS 4600.
Compression elements are reduced by the Winter equation of AISI S100-16 Appendix 1 §1.1.1, with the web stress gradient of §1.1.2 (including its rule for h₀/b₀ > 4) and the simple-lip edge stiffener of §1.3. Flexure is M_n = S_e(F_n)·F_n, with F_n = F_y where the sheeted flange is in compression. Shear follows §G2.1 or AS/NZS 4600 cl. 3.3.4.1, web crippling §G5.1 at every support of a simple, two-span or three-span model, bending with shear §H2, and bending with web crippling EN 1993-1-3 Eq. (6.28c) at an interior support.
The section
D, B and L are outside dimensions and R is the inside bend radius. Catalogue sections are from Lysaght LYT0063, with R = 5 mm as marked on its section diagrams.
Code and span
Loads
Normal stress in the effective section
The checks
—
| The checks | Clause | Demand | Capacity | Utilisation |
|---|
The sheeted flange is assumed continuously restrained. Lateral-torsional and distortional buckling are not computed for either flange; where the free flange is in compression, flexure is reported as not covered unless a reduced stress is entered.
Load combinations
| # | Load combinations | w (kN/m) | M (kN·m) | V (kN) |
|---|
Effective section
Capacities
Web crippling rows used
The lightest section that passes
| Section | kg/m | S_e cm³ | Worst utilisation | Governing |
|---|
Press "Search the catalogue" to run every size of this section type at the span and loading above, each in its published grade.
Questions engineers ask
Why do AISI and AS/NZS give different answers for the same section?
Three reasons. AISI S100-16 works at E = 203 000 MPa and AS/NZS 4600 at 200 000 MPa, which moves every elastic buckling stress; the shear provisions are different equations, AS/NZS cl. 3.3.4.1 has a yield plateau of 0.64 f_y d₁ t where AISI §G2.1 uses 0.6 A_w F_y; and AS/NZS takes φ_b = 0.95 for a stiffened compression flange but 0.90 for an unstiffened one. The code selector switches between them, and the load combinations change with it.
Is this the Direct Strength Method?
No. This is the effective width method of AISI S100-16 Appendix 1. If your section has been analysed for its elastic buckling loads in a finite-strip program, the Direct Strength Method can use that information; this tool does not.
Which deflection limit does the purlin check use?
IBC Table 1604.3, with its footnotes for metal roofing (L/150 under live load) and metal siding (L/90 under wind), plus its roof and exterior-wall rows. The limit is an input: pick the row that matches what you are building. Deflection is computed under service loads with the effective second moment at F_y, which is conservative.
Does it check the member, or only the section?
The section, at the actions you give it: flexure of the effective section, shear, web crippling at the supports, bending with shear, and bending with web crippling at an interior support. Lateral-torsional and distortional buckling are not computed. Where a load combination puts the unsheeted flange in compression (uplift on a purlin, suction on a girt, or the support region of a continuous member) flexure is reported as not covered unless you enter a reduced stress F_n from such a check. Bridging, anchorage and laps are not designed.
Which sections and spans can it design?
Single C and Z sections, lipped or plain, bent about the axis parallel to their flanges; hat sections, angles and back-to-back channels are not offered. The member is a single simple span, or two or three equal continuous spans under uniform load, with the support moments, reactions and shears from statics. In stud mode the member is checked in bending only; axial load is not checked.
Does it check the sheeting screws and the sag rods?
No capacity is checked for either. The tool reports the tension per sheeting screw from the governing uplift or suction combination, and the factored tension in the top sag rod, for comparison with the fastener manufacturer's values and for a separate rod design.
What is the option "AISI S100-16 LRFD resistance with EN 1990 combinations"?
It is not an EN 1993-1-3 design. The member is checked with the AISI S100-16 LRFD resistances and loaded with the EN 1990 Eq. (6.10) combinations. The one Eurocode equation the tool uses is EN 1993-1-3 Eq. (6.28c), the linear bending and web crippling interaction, which it applies under every code because the AISI §H3 interaction is not implemented.
Where do the catalogue sections come from?
From the Lysaght LYT0063 Zeds and Cees guide (2024): depths, flanges, lips, thicknesses, masses and grades as printed, with the 5 mm inside radius marked on its section diagrams. Zeds are computed at the mean flange width, which is the guide's own basis. Every dimension can be edited.
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