Cold-Formed Steel (DSM)

A single Direct Strength Method core engine serves three national standards:AS/NZS 4600:2018, AISI S100-16, and CSA S136-16. The implemented DSM member curve family is shared, while each code path sets its own resistance factors (φ), shear coefficients, tension rupture factors, material handling, prequalification clauses, and displayed references.

Release status: gated beta. The solver path is available to administrators and staging users. The scope and fail-closed limitations below define what it currently checks.

Supported Standards

Resistance Factors

CSA S136 general Chapter F member bending and the implemented I6.2.1 one-flange-through-fastened path both use φ_b=0.90. The I6.2.1 equation and applicability limits are handled separately from ordinary DSM member bending.

Country-Specific Routing

• AS/NZS 4600:2018: limit-state resistance factors, automatic Table 7.1 prequalification checks, Clause 1.6.3(c) φ=0.80 for affected DSM capacities outside Table 7.1, the G550 thin-steel reduction for t<0.9 mm, and V_y=0.60 A_wf_y for the Clause 7.2.3.2 unstiffened-web shear path.
• AISI S100-16: LRFD output only; ASD allowable-strength output (R_n/Ω) is not implemented. Sections outside B4.1 automatically use the B4.2(a)/A1.2(c) rational-analysis factor φ=0.80 for affected DSM capacities; B4.2(b) testing is needed only to retain a higher Chapter E-H factor. Uses V_y=0.60 A_wF_y, D2/D3 tension factors, and A3 material applicability warnings.
• CSA S136-16: LSD output only. Uses the joint S100/S136 core chapters with Canada Appendix B mandatory for Canadian applications. Sections outside B4.1 automatically use the B4.2(a)/A1.2(c) rational-analysis factor φ=0.75 for affected DSM capacities. General factors include φ_c=0.80 and φ_v=0.80; no AS/NZS G550 reduction is applied.
• Other jurisdictions, including India: CFS catalog sections remain available for structural analysis, but no CFS design engine is assigned. Automatic selection returns no design code, and direct attempts to run AS/NZS 4600, AISI S100, or CSA S136 fail closed as OUT_OF_SCOPE.

Checks Implemented

Member Capacity (ULS)

• DSM Compression: Global (flexural/torsional/flexural-torsional), local, and distortional buckling per AS4600 7.2.1 / AISI and CSA E2-E4.
• DSM Bending: member and section global/local/distortional capacities per AS/NZS 4600 Clause 7.2.2 / AISI and CSA F2-F4, using section-library finite-strip buckling data. The AS/NZS Chapter 7 path does not require user-entered Z_e.
• Shear: ordinary unstiffened-web shear only, using A_w=ht, k_v=5.34, and the three-branch curve in AS/NZS 4600 Clause 7.2.3.2 / AISI and CSA G2.1. User or section-library shear-area overrides do not alter this web capacity. Spacing alone never activates reinforced-web strength. G3 / Cl 3.3.4.2 punchout capacity can be published after the hole shape, d_h, non-circular L_h, and the explicit centered-hole/spacing/corner-radius confirmation are supplied and every code dimensional limit passes. Solid-web shear remains independent.
• Tension: AS4600 uses gross yielding and fracture per 3.2. AISI/CSA use D2 gross yielding and D3 rupture. Net tensile area An can come from the member input or section data; otherwise An=Ag is assumed and a non-governing advisory states that connection holes are not modelled.
• Web crippling / local bearing: Optional design-tab inputs (reaction R*, bearing length N, bearing station, end distance, one/two-flange, fastened to support). Same unified equation for all three codes: AS/NZS 4600 Cl 3.3.6.2 Tables 3.3.6.2(A–D); AISI S100-16 G5 Tables G5-1–G5-4 (LRFD φ_w); CSA S136 G5 with the Canada LSD φ_w column. Demand is the user R*, not member shear. Row name: φRb (Web Crippling). PDF handcalcs cite G5 / 3.3.6.2 with table coefficients and φP_n.

Interactions (ULS)

• AS4600 N+M: First-order demands use the Clause 3.5.1(1) amplified member interaction and Clause 3.5.1(2) section interaction. P-Delta second-order demands use the Chapter 7 DSM member interaction in Clause 7.2.4 and are not amplified again.
• AS4600 axial-free biaxial bending: When N*=0, the member row uses the corresponding reduction of Clause 3.5.2(1). It is suppressed whenever compression or tension is present; those cases use Clauses 3.5.1 and 7.2.5 respectively.
• AISI/CSA H1.2: Compression plus biaxial bending and flexure-only component interaction rows use the supplied required strengths. Those actions must satisfy the applicable Chapter C analysis requirements; a first-order run does not certify Chapter C model qualification, and no separate legacy amplified row is emitted.
• AS4600 T+M: Chapter 7 DSM member and full-section tension-fibre interactions per Clause 7.2.5.
• AISI/CSA H1.1: Two-branch tension plus major-axis bending interaction using D2/D3 tension strength and Chapter F bending capacities.
• M+V interaction: the unstiffened-web circular interaction is implemented per AS/NZS 4600 Clause 7.2.3.5 / AISI and CSA H2. The separate transverse-stiffened-web interaction is not implemented.
• Bending + web crippling: AS/NZS 4600 Cl 3.3.7(a) for supported lipped C/Z sections and Cl 3.3.7(b) for supported back-to-back C sections use M_s=Z_ef_y from the Cl 3.3.2 effective section. AISI S100 H3-1/H3-2 remains LRFD with its local-only M_nℓo; CSA S136 H3-1/H3-2 remains LSD on the same North American basis. For every code, M* is interpolated at the entered bearing station. Row name: M+R Interaction (Web Crippling). Nested-Z H3-3 / Cl 3.3.7(c) is not implemented.

Serviceability (SLS)

• Effective I_eff: per AS/NZS 4600 Clause 7.1.4 / AISI-CSA L2: the complete DSM nominal flexural capacity is recalculated with M_y replaced by service moment M in every equation, then I_eff=I_g(M_n/M)≤I_g. Default limit L/250.

Special Features

• CUFSM integration: reads pre-computed finite strip buckling loads from section library
• DSM data requirement: compression and major-axis bending require validated Nol, Nod, Mol, and Mod elastic buckling inputs from CUFSM or another finite-strip source. Missing DSM buckling data is reported as OUT_OF_SCOPE instead of using empirical fallback strengths.
• Back-to-back sections: CeeBackToBack and SupaCeeBackToBack use full built-up-section gross, elastic, and DSM properties. Per-web shear and web-crippling checks account for the physical web count separately.
• Sheeting restraint: AS/NZS 4600 Cl 3.3.3.4 is available for upright edge-stiffened C/Z members when the AS configuration, attached physical tension flange, applicability confirmation, and every clause limit are established. Its Chapter 3 strength φ_bRZ_ef_y, using the Cl 3.3.2 effective section, replaces unrestrained Cl 7.2.2 member bending; Chapter 7 section, shear, and bearing checks remain. The implemented member path is 2D major-axis bending only: any nonzero minor-axis moment or tension makes the member OUT_OF_SCOPE, with no small-demand tolerance. No Cl 3.5.2 or 7.2.5 member formula is inferred. Sheeting-supported compression remains a manual warning. AISI/CSA I6.2.1 remains unchanged: M_n=R M_nℓo, with M_nℓo from F3 using M_ne=M_y and F4 excluded. The AS configuration values are not taken from the North American I6.2.1 table.
• AS/NZS G550 thin steel: AS4600 auto-reduces f_y/f_u per Cl 1.5.1.1(b): min(90%, 495 MPa) for t≥0.6mm, min(75%, 410 MPa) for t<0.6mm. AISI and CSA paths do not apply this AS/NZS grade-table reduction.

Supported Section Types

• Cee (lipped channel)
• Zed (lipped Z-section)
• TopHat
• CeeBackToBack / SupaCeeBackToBack
• PlainChannel (unlipped channel, including unlipped Cee metadata normalized by the engine)

Current bundled CFS libraries contain five CFS families: Cee, Zed, TopHat, CeeBackToBack, and PlainChannel. The engine also accepts the SupaCeeBackToBack token for imported or future library data.

Limitations

• Transverse-stiffened shear: not implemented. Legacy/imported stiffener-spacing data is ignored for capacity, the ordinary unstiffened-web row remains at k_v=5.34, and a blocking OUT_OF_SCOPE row is emitted. The engine does not yet collect and verify the reinforced-web curve, a≤2h, stiffener inertia/area, material/type, attachment, or the stiffened M+V interaction required by AS/NZS 7.2.3.3/3.3.8.3 and AISI/CSA G2.2/G4.
• Transverse/minor-axis shear: fails closed. The reviewed provisions are web-shear provisions; a local-axis shear area does not by itself justify applying them to flange-direction shear. No φV_t or minor-axis M+V capacity is published.
• AS/NZS Clause 3 effective-section scope: the current Z_e/M_s helper is limited to upright, uniform-thickness, edge-stiffened C/Z geometry with the supported perpendicular-lip model; the Cl 3.3.7 built-up branch sums two identical back-to-back C components. Hats, plain channels, sideways members, nested Z sections, and geometry outside the helper's applicability fail closed for these Chapter 3 rows. The numerical Z_e regressions are not a substitute for validation against an independent published effective-section example.
• Net tensile area An: AS4600 fracture and AISI/CSA D3 rupture can use an explicit member An or section-data An. When neither is supplied, the member check assumes An=Ag and emits a non-governing holes-not-modelled advisory; selecting "Confirm Ag" records that assumption explicitly and removes the advisory. The engine does not derive bolt-hole paths, stagger, slots, or shear lag from connection geometry.
• TopHat major-axis member LTB: reported as not checked until trusted shear-center, asymmetry, stress-side sign, and polar-radius constants are available for the F2.1.2-style path.
• Back-to-back major-axis member LTB: uses the supplied open-section St-Venant torsion constant J. Composite or closed-cell torsional action through screws, welds, bridging, or other shear-transfer connections is not modeled; verify member LTB separately where that action is relied upon.
• Minor-axis member LTB and biaxial member interaction: 769 bundled sections (634 Cees and 135 lipped Zeds) carry signed principal-axis Appendix 2 elastic-LTB curves. The curves use the catalog's validated A, I, J, and I_w with geometry-derived shear-centre and monosymmetry terms; HH7121 agrees with the independent mesh-converged finite-strip curve within 1.5%. The member's k_LTL selects M_cre, then the engine applies F2-F4 and H1.2. Curves span max(1.0 m, 5D) through 30 m and are never extrapolated. For PlainChannel, TopHat, back-to-back, SupaCee/SupaZed, one Cee, and 49 lipped Zed records, this member capacity and dependent H1/Clause 3.5 biaxial member interactions are omitted because the family is deferred or the geometry/property audit exceeded its gate. The omission is documented here and in Design Notes but does not set the overall result to OUT_OF_SCOPE; verify these member limit states separately.
• G3 web holes: the design tab accepts circular or non-circular shape, d_h, and L_h for non-circular holes. The user must explicitly certify that the hole is centered at web mid-depth, clear spacing meets the code minimum, and non-circular corner radii are at least 2t. Reduced capacity remains OUT_OF_SCOPE if confirmation is off or any code dimensional gate fails; d_h alone never publishes reduced capacity.
• Sheeting-supported compression: AS4600 Cl 3.4.3 / AISI and CSA I6 compression-with-sheeting checks are manual-warning rows because the member model does not yet carry span and sheeting stiffness inputs needed for those provisions.
• Restrained warping torsion: torsion stress recovery uses section stress points where available, but restrained warping torsion normal/shear terms are not recovered by the frame/member result path.
• Bending plus torsion H4: no numeric H4 row is emitted because restrained-warping stresses are not available from the six-DOF frame result path.
• Missing DSM buckling data: compression and major-axis bending rows are not production capacity checks without validated elastic buckling inputs. The design tab reports that condition as OUT_OF_SCOPE.
• No plastic bending reserve: conservative; bending capacity is capped without post-yield bending gains.
• Member-by-member design checks: continuous-member effects are not inferred automatically.
• G6 / Cl 3.3.6.3 holes at the bearing: not implemented. If punchout depth and bearing (R* + N) are both set, φRb is OUT_OF_SCOPE so an unreduced solid-web capacity is not published. Punchout alone still drives G3 shear reduction only.
• AISI/CSA fastened ITF 2.5h band: for channel/C and Z with flanges fastened under two-flange interior loading, G5-2/G5-3 require x_end ≥ 2.5h. Between 1.5h and 2.5h the check is OUT_OF_SCOPE (AS 3.3.6.2 does not apply this note).
• Nested-Z Cl 3.3.7(c) / H3-3: not implemented (no nested-Z section type).
• AISI G5 overhang boost (G5-2/G5-3): not implemented.
• AISI ASD allowable-strength checks: not implemented. AISI S100 currently reports LRFD strengths using φ factors, not ASD strengths divided by Ω safety factors; CSA reports LSD factors and AS4600 reports limit-state φ factors.

Verification

The displayed benchmark table verifies the shared DSM equations primarily against the AISI Direct Strength Method Design Guide (CF06-1, 2006). Additional checks cover Bentley STAAD.Pro cases, first-principles calculations, country-factor routing, and an offline cross-software replay of 133 members and eight independently exported section definitions through the current AISI wrapper. A five-case DS1/CO1 detail gate matches DD1101, EE2501, and FF3501 within 3%; preserves GG6101 as a documented comparison-versus-AISI G2.1 web-area discrepancy; and closes HH7121 using an independently mesh-converged Appendix 2 principal-axis global curve. HH7121 gives 5263.349 versus the comparison result of 5059.570 because the saved comparison model uses its analytical global option; the difference is retained rather than tuning the numerical result to that model. The replay is a diagnostic cross-software comparison rather than a blanket equivalence claim. These checks do not constitute independent end-to-end AS/NZS 4600 and CSA S136 worked-example validation. View benchmarks →