path: /blog/cold-formed-steel-design-guide/ canonical: https://steelcalculator.app/blog/cold-formed-steel-design-guide/ meta_title: 'Cold-Formed Steel Design Guide -- AISI S100 Effective Width & Buckling (2026)' meta_description: 'Complete cold-formed steel design guide per AISI S100-16: C and Z sections, effective width method, distortional buckling, purlin and girt design, and worked examples.' robots: 'index,follow' lastmod: '2026-05-20' schema_file: 'schema/blog_cold-formed-steel-design-guide.json' FAQPage: '@type': 'FAQPage' mainEntity: - '@type': 'Question' 'name': 'What is the difference between cold-formed steel design and hot-rolled steel design?' 'acceptedAnswer': '@type': 'Answer' 'text': 'Cold-formed steel design uses thin-gauge sections (typically 0.5-6 mm thick) formed by rolling or pressing sheet steel at room temperature. The governing standard is AISI S100 (North America), which uses the effective width method to account for local buckling of thin elements. Hot-rolled design uses AISC 360 for thicker sections where local buckling of individual plate elements is less critical. Cold-formed members also require distortional buckling checks that do not exist in hot-rolled design.' - '@type': 'Question' 'name': 'What is the effective width method in cold-formed steel design?' 'acceptedAnswer': '@type': 'Answer' 'text': 'The effective width method per AISI S100 reduces the gross width of slender compression elements to an effective width that accounts for local buckling. When a thin plate element buckles locally, the post-buckling stress redistributes to the stiffer edge-supported regions. The effective width calculation depends on the plate slenderness ratio (lambda), the edge support conditions (stiffened vs unstiffened), and the stress gradient across the element.' - '@type': 'Question' 'name': 'What is distortional buckling and why does it govern cold-formed steel design?' 'acceptedAnswer': '@type': 'Answer' 'text': 'Distortional buckling is a mode where the compression flange and the edge stiffener (lip) rotate about the flange-web junction without the web deforming. It governs C and Z sections with intermediate lip lengths and is checked per AISI S100 Section C3.1.4. Distortional buckling often controls the design of purlin and girt sections in the 150-300 mm depth range, and the check is unique to cold-formed steel -- hot-rolled standards do not include it.' - '@type': 'Question' 'name': 'Are C sections and Z sections designed the same way?' 'acceptedAnswer': '@type': 'Answer' 'text': 'No. Z sections have different torsional-flexural buckling behaviour due to their unsymmetrical cross-section. Under gravity loading on a sloping roof, Z sections experience a torsional component that C sections (which are singly-symmetric) do not. Z sections also have different lip stiffener effectiveness and distortional buckling characteristics. The AISI S100 effective width provisions apply to both, but the section properties (Ix, Iy, J, Cw) and buckling modes differ significantly.'

Cold-Formed Steel Design Guide -- AISI S100 Effective Width, Distortional Buckling & Worked Examples

Cold-formed steel (CFS) members carry the roof and wall loads of virtually every modern metal building, most framed low-rise and mid-rise construction, and an expanding share of structural framing. The design methodology for CFS is fundamentally different from hot-rolled steel: thin elements buckle locally long before the section reaches yield, and the design standard -- AISI S100 -- uses the effective width concept to account for this post-buckling strength. This guide explains the essential theory, walks through worked examples for C and Z sections, and compares AISI S100 methodology with AISC 360 for hot-rolled design.

PRELIMINARY -- NOT FOR CONSTRUCTION. All examples are educational references only. Every structural design must be independently verified by a licensed Professional Engineer. Consult the current edition of AISI S100 for the governing provisions applicable to your project.

What is cold-formed steel?

Cold-formed steel is manufactured from steel sheet or strip by roll-forming, press-braking, or stamping at ambient temperature. The forming process introduces cold work that increases the yield strength in corners (the virgin steel Fy may be 345 MPa in the flat, but the corner Fy can exceed 400 MPa after forming). Typical CFS products include:

Key distinction: Cold-formed sections have width-to-thickness ratios typically 30-200, compared to 5-30 for hot-rolled W-shapes. This slenderness is what drives the fundamentally different design approach.

AISI S100 vs AISC 360 -- why different standards

AISC 360 governs hot-rolled structural steel (W, S, HP, M, C, L shapes). AISI S100 governs cold-formed structural members. Mixing the two is incorrect and unconservative for CFS:

Safety note: Using AISC 360 for cold-formed C and Z sections misses the effective width reduction, distortional buckling, web crippling under concentrated loads, and the AISI S100-16 interaction equations. This can produce dangerously unconservative results for thin-gauge members.

The effective width method -- core of CFS design

When a thin compression plate element buckles at a stress below yield, it does not lose all strength. The post-buckling stress redistributes to the stiffer edge-supported portions of the plate. The effective width method replaces the actual plate width with a reduced "effective" width that carries the applied stress at yield:

Uniform compression element (stiffened, both edges supported):

lambda = (1.052 / sqrt(k)) _ (w/t) _ sqrt(Fy/E)

where k = 4.0 for stiffened elements, w = flat width, t = thickness.

For lambda <= 0.673: be = w (fully effective)

For lambda > 0.673: be = (rho) * w, where rho = (1 - 0.22/lambda) / lambda

The reduction factor rho penalises slender elements: if lambda = 1.5, rho = 0.569 -- the element is only 57% effective. This reduction directly reduces Ae, Ie, Se, and all section capacities.

Effective width worked example: C20015 purlin in bending

A C20015 lipped channel purlin (depth 200 mm, flange 63 mm, lip 15 mm, thickness 1.5 mm). Steel grade G450 (Fy = 450 MPa, Fu = 500 MPa). E = 203,000 MPa.

Compression flange (stiffened element with one edge stiffened by lip):

Flange flat width wf = 63 - 2 * (1.5 + 1.5) = 63 - 6 = 57 mm.

w/t = 57 / 1.5 = 38.0. k = 4.0 (stiffened under uniform compression).

lambda = (1.052 / sqrt(4.0)) _ 38.0 _ sqrt(450/203000) = 0.526 _ 38.0 _ sqrt(0.002217) = 0.526 _ 38.0 _ 0.04709 = 0.941.

lambda = 0.941 > 0.673, so flange is NOT fully effective.

rho = (1 - 0.22/0.941) / 0.941 = (1 - 0.234) / 0.941 = 0.766 / 0.941 = 0.814.

Effective flange width be = 0.814 * 57 = 46.4 mm (vs gross 57 mm).

Web under bending stress gradient:

Web flat width h = 200 - 2 * (1.5 + 1.5) = 194 mm.

h/t = 194 / 1.5 = 129.3. Stress gradient psi = -1.0 (pure bending, compression at flange, tension at other flange edge). k = 4 + 2*(1+psi)^3 + 2*(1+psi) = 4 + 0 + 0 = 24.0.

lambda = (1.052 / sqrt(24.0)) _ 129.3 _ 0.04709 = 0.2147 _ 129.3 _ 0.04709 = 1.307.

rho = (1 - 0.22/1.307) / 1.307 = (1 - 0.168) / 1.307 = 0.832 / 1.307 = 0.637.

Effective web depth he = 0.637 * 194 = 123.6 mm, located at the compression portion of the web.

Result: The effective section modulus Se is approximately 62% of the gross Sg. A hot-rolled calculation that ignores effective width would overestimate capacity by ~60%.

Distortional buckling -- the second failure mode

Distortional buckling involves rotation of the compression flange and lip about the flange-web junction. The web remains largely undeformed; the mode is intermediate between local buckling (plate elements) and lateral-torsional buckling (full member). AISI S100 Section C3.1.4 provides the elastic distortional buckling stress Fd:

Fd = (k*d * pi^2 _ E) / (12 _ (1 - mu^2) _ (w/t)^2)

where k_d depends on the flange-lip geometry and the rotational stiffness provided by the web.

Distortional buckling typically governs C and Z sections with intermediate lip lengths (10-25 mm for typical 1.5-2.0 mm thickness). For very short lips, local buckling governs. For very long lips, the section approaches a stiffened element and distortional buckling becomes less critical. The worst case is usually a lip length of approximately 0.25-0.35 times the flange width.

Key distortional buckling parameters

C vs Z sections -- selecting the right shape

C sections (lipped channels) are singly-symmetric and best suited for applications where the shear centre does not create significant torsional loading. Z sections are point-symmetric (rotate 180 degrees to match) and are the dominant choice for purlins because:

1. Overlap at laps: Z sections nest inside each other at lap splices, creating a continuous double-section at supports
2. Gravity load alignment: On sloping roofs, the principal axes of Z sections are closer to the vertical/horizontal than C sections, reducing the torsional component
3. Manufacturing efficiency: Z sections can be nested for shipping and storage

Design rule of thumb for purlin selection:

• Roof slope < 10 degrees: C sections acceptable
• Roof slope 10-30 degrees: Z sections preferred
• Roof slope > 30 degrees: Z sections required (torsional component on C sections becomes excessive)
• Multi-span with laps: Z sections strongly preferred (lap splices transfer moment across supports)

Purlin and girt design -- practical considerations

Purlin design checks (gravity + wind uplift)

Per AISI S100, a cold-formed purlin must satisfy at minimum:

1. Flexure about major axis: Mx / (phi_b * Mnx) <= 1.0 with Se based on effective width at the stress level in the compression flange
2. Flexure about minor axis: My / (phi_b * Mny) <= 1.0 from eccentricity of loading and roof slope
3. Combined bending check: Interaction equation per Section C5.2
4. Distortional buckling: Checked at the stress level including gravity and wind uplift
5. Shear: V / (phi_v * Vn) at supports
6. Web crippling: Under concentrated reaction at supports (4 loading conditions per Table C3.4.1-1)
7. Combined bending + web crippling: Section C3.5
8. Deflection: L/180 for total load, L/360 for live load in roof applications
9. Wind uplift reversal: Bottom flange in compression -- lateral-torsional buckling of the unrestrained bottom flange

Bridging and bracing

Cold-formed purlins and girts require discrete bracing to restrain lateral-torsional buckling. Bridging lines (typically 1/3 and 2/3 of span for multi-span) provide lateral and torsional restraint:

• No bridging: Lb = full span; LTB often governs
• One row at mid-span: Lb = span/2; approximately 4x increase in Mn for LTB-controlled sections
• Two rows at third points: Lb = span/3; approximately 9x increase over unbraced

Bridging design is per AISI S100 Section D3.2.1, requiring the bridging to resist 2% of the compression flange force as a lateral restraint force.

Screwed and bolted connections in cold-formed steel

Cold-formed steel connections differ from hot-rolled connections in three important ways:

1. Bearing and tilting: Thin material (t < 3 mm) fails by bearing with piling (material builds up ahead of the fastener) and tilting of the fastener, not the shear rupture of the bolt shank that governs hot-rolled connections
2. Screw connections: Self-drilling screws are the most common CFS fastener; AISI S100 Section E4 provides capacity equations for screw shear, pull-out, pull-over, and tension
3. Tear-out and edge distance: Minimum edge distances are larger relative to thickness (1.5d for screws vs 1.25d for bolts in hot-rolled)

Screw shear worked example

No. 12 self-drilling screw (d = 5.5 mm) through 1.5 mm G450 sheet (Fu = 500 MPa).

Tilting and bearing: Pns = 4.2 _ (t^3 _ d)^0.5 _ Fu = 4.2 _ (1.5^3 _ 5.5)^0.5 _ 500 = 4.2 _ (3.375 _ 5.5)^0.5 _ 500 = 4.2 _ 4.31 * 500 = 9,050 N = 9.05 kN.

phi _ Pns = 0.50 _ 9.05 = 4.53 kN per screw (LRFD).

For comparison, an M12 bolt in hot-rolled 10 mm plate: phi _ Rn_shear = 0.75 _ 0.625 _ 800 _ 84.3 / 1000 = 31.6 kN -- approximately 7x the cold-formed screw capacity.

Multi-region code comparison

AS/NZS 4600 adopted the AISI S100 provisions almost verbatim through the harmonisation project. Engineers designing cold-formed steel in Australia and New Zealand can use AISI S100 methodology with very minor regional adjustments (material standards, load factors).

Common design mistakes in cold-formed steel

1. Using AISC 360 for C/Z sections: AISC 360 does not include effective width reductions or distortional buckling checks. Capacity ratios from AISC 360 can be 40-60% unconservative for thin CFS members.
2. Ignoring distortional buckling entirely: Even when effective width is calculated correctly, distortional buckling can govern at lower stress levels. Always check Section C3.1.4.
3. Using gross section modulus Se for deflection: Deflection should use the effective moment of inertia Ie at the service load stress level, not Ig. Ie/Ig ratios of 0.6-0.8 are common.
4. Forgetting wind uplift reversal: The bottom flange in uplift has no sheathing restraint. LTB of the bottom flange under uplift often governs purlin size selection.
5. Inadequate bridging: One row of bridging at mid-span for a 9 m purlin still leaves Lb = 4.5 m, which may govern LTB for shallow sections.
6. Minimum edge distance violations: Self-drilling screws require 1.5d edge distance; field conditions often result in closer edge distances that reduce capacity.
7. Ignoring web crippling under concentrated reactions: Thin CFS webs are vulnerable to crippling at bearing points. The unfactored reaction rarely exceeds 5-10 kN per web, making stiffened bearing details essential for heavily loaded purlins.
8. Mix-and-match phi factors: AISI S100 uses different phi factors than AISC 360 for connections (0.50 vs 0.75 for screws vs bolts). Applying AISC bolt phi factors to screw connections in CFS is unconservative.

Cold-formed steel in modern construction

Cold-formed steel is experiencing rapid adoption driven by:

• Sustainability: CFS is 100% recyclable with high recycled content. Lighter members mean lower embodied carbon per square metre of floor area.
• Prefabrication: CFS panels, trusses, and modular units can be factory-assembled with automated roll-forming and screw-fastening lines, reducing site labour.
• Mid-rise construction: CFS load-bearing wall systems now reach 6-10 storeys in North America under IBC 2024 with properly engineered lateral systems.
• BIM integration: CFS manufacturers provide BIM objects with accurate section properties, streamlining coordination with architectural and MEP models.

The AISI S100 standard continues to evolve. The 2022 edition (with S3-24 supplement) adds provisions for advanced high-strength steels (Fy up to 690 MPa), improved distortional buckling formulations, and expanded Direct Strength Method (DSM) coverage as an alternative to the effective width method.

Frequently Asked Questions

When should I use cold-formed steel vs hot-rolled steel? Use cold-formed for light to moderate loads where member depth is limited and standard sizes are available (purlins, girts, studs, joists, trusses up to ~12 m span). Use hot-rolled for heavy loads, long spans, and situations requiring high ductility (moment frames in seismic regions).

Is cold-formed steel allowed in seismic design? Yes. AISI S400 (North American Standard for Seismic Design of Cold-Formed Steel Structural Systems) and AISI S240 (general CFS framing) provide comprehensive seismic provisions. CFS strap-braced walls and CFS shear walls are prequalified lateral force-resisting systems in ASCE 7-22 Table 12.2-1.

Can I weld cold-formed steel? Yes, but with limitations. AISI S100 Section E2 covers welding of CFS. Thin material (t < 2.5 mm) is difficult to weld without burn-through. Resistance welding (spot welding) and arc spot welding (puddle welds) are more common than fillet welds in CFS construction.

How do I calculate the effective section properties for a C section with a lip stiffener? Follow AISI S100 Appendix 1 (Section 1.2): (1) determine if the lip qualifies as an adequate edge stiffener, (2) compute the buckling coefficient k for the stiffened flange, (3) calculate lambda and rho for the flange, (4) compute effective width, (5) repeat for the web under its stress gradient, (6) assemble the effective cross-section, (7) compute Ie and Se. The iterative nature (stress-dependent effective width) means section properties change with applied load -- most design software iterates to convergence.

Run These Calculations

Purlin & Girt Calculator -- Multi-span C/Z purlin design with lap splices, bridging, and wind uplift per regional codes.

Beam Capacity Calculator -- Hot-rolled beam flexure, shear, LTB, and deflection checks per AISC 360.

Column Capacity Calculator -- Axial compression and buckling per AISC 360 and AS 4100.

Bolted Connection Calculator -- Bearing-type and slip-critical bolt groups with shear, bearing, tension, and block shear.

Further reading

• How to Read Steel Section Tables -- W, HSS, C, L Properties Decoded
• Moment of Inertia Calculator for Steel Sections -- Ix, Iy, J
• Steel Beam Deflection Guide -- Formulas, Limits & Calculator
• Steel Roof Truss Design -- Types, Load Paths & Worked Example
• Steel Frame Analysis Tutorial -- Portal Method to Matrix Stiffness
• Steel Design Spreadsheet Problems -- Why Calculators Are Better
• AISC 360 Design Examples -- Beam, Column & Connection Worked Solutions
• Steel Fy and Fu Reference -- Yield and Tensile Strength by Grade
• AISI S100 Code Notes -- Key Clauses and Design Workflow
• CFS Purlin Verification Example -- Hand Calculation Walkthrough

Educational reference only. All cold-formed steel designs must be independently verified by a licensed Professional Engineer. Verify all designs against the current edition of AISI S100 (or the governing standard in your jurisdiction). Results are PRELIMINARY -- NOT FOR CONSTRUCTION.