CSA S136:24 Design Framework

CSA S136:24 is the Canadian standard for cold-formed steel structural members, superseding CSA S136-19. It adopts the AISI S100 North American Specification with Canadian-specific modifications for:

• CSA G40.21 steel grades (230G, 350G, 350W-CF)
• Canadian loading requirements (NBCC 2020)
• Metric design tables consistent with the CISC Handbook
• Additional provisions for cold climates (low-temperature ductility)

Cold-Formed Steel vs Hot-Rolled Steel

Effective Width Method — Local Buckling

Principle

Cold-formed steel sections have high width-to-thickness ratios that cause local buckling of individual plate elements before the section reaches yield. Rather than treating local buckling as a failure, the effective width method accounts for post-buckling strength reserve.

Per CSA S136:24 Clause B2.1, the effective width of a uniformly compressed stiffened element is:

b_e = w when lambda <= 0.673** **b_e = rho x w when lambda > 0.673

Where: rho = (1 - 0.22/lambda) / lambda <= 1.0 lambda = sqrt(f / F_cr) = (1.052/sqrt(k)) x (w/t) x sqrt(f/E)

Worked Example — C-Section Flange

Given: 350G C-section, w = 60 mm flange flat width, t = 1.52 mm, f = 350 MPa, k = 4.0 (stiffened by web and lip).

lambda = (1.052/sqrt(4.0)) x (60/1.52) x sqrt(350/203,000) lambda = 0.526 x 39.47 x 0.04154 lambda = 0.862 > 0.673 => post-buckling reduction required

rho = (1 - 0.22/0.862) / 0.862 = 0.745 / 0.862 = 0.864

b_e = 0.864 x 60 = 51.8 mm

Effective flange width = 51.8 mm (14% reduction from gross).

Distortional Buckling — Clause D6

Distortional buckling involves rotation of the flange-lip assembly about the flange-web junction. Unlike local buckling (individual plate elements), distortional buckling affects the cross-sectional geometry as a whole.

Per CSA S136:24 Clause D6.1.2, the nominal distortional buckling strength is:

P_nd = A_g x F_n where F_n = F_y for lambda_d <= 0.561** **F_n = (1 - 0.25/lambda_d^0.6) x F_y / lambda_d^0.6 for lambda_d > 0.561

Where lambda_d = sqrt(F_y / F_crd) and F_crd is the elastic distortional buckling stress determined by finite strip analysis, the CUFSM program, or direct strength method equations.

Practical Consideration

For typical CFS purlins (C200 to C300 sections at 1.2-3.0 mm thickness), distortional buckling is often the governing limit state when the section has intermediate lips. Increasing the lip stiffener length is the most effective way to improve distortional buckling capacity — each 5 mm increase in lip length can increase F_crd by 15-25%.

Canadian CFS Section Profiles

Common C-Sections (Canam CSSMA Catalog)

Common Z-Sections

Z-sections are preferred for purlins because the principal axes align better with the roof slope, reducing twisting under gravity load.

Bridging and Bracing — Clause D3.2

Bridging Requirements

Cold-formed steel purlins and girts are thin-walled and laterally unstable under load. Bridging (rows of lateral bracing) restrains lateral-torsional buckling and provides load sharing between adjacent members:

Bridging Types

Bridging Design Force

Per CSA S136 Clause D3.2.2, the bridging must resist:

F_br = 0.02 x P_f (compression flange force)

For a C250 purlin at 2400 mm spacing carrying 0.75 kPa gravity load over 7500 mm span:

P_f = (0.75 x 2.4 x 7.5) x (7.5/8 x 1000) / 250 = moment/depth approx. F_br = 0.02 x P_f = 0.02 x 33,750 = 675 N per bridging connection.

Combined Bending and Web Crippling

CFS members are susceptible to web crippling at supports where concentrated reactions bear on thin webs. Per CSA S136:24 Clause C3.4:

P_n = C x t^2 x F_y x sin theta x (1 - C_R x sqrt(R/t)) x (1 + C_N x sqrt(N/t)) x (1 - C_H x sqrt(H/t))

Where the coefficients C, C_R, C_N, C_H depend on the loading condition (end-one-flange, interior-one-flange, end-two-flange, interior-two-flange). The CISC Handbook provides tabulated web crippling capacities for standard C and Z sections.

For combined bending and web crippling:

(M_f/M_n)^2 + (R_f/R_n) <= 1.35

This elliptical interaction permits up to 35% overstress when both bending and reaction are present simultaneously, reflecting the beneficial effect of the bending tension flange stiffening the web.

Screw Fastener Design

Self-drilling screws are the primary fastener for CFS construction in Canada. Per CSA S136 Clause E4:

Screw shear strength: V_n = 0.6 x F_u x A_s (with reduction for thin sheet tilting/bearing)

Screw tension strength: T_n = P_ss (pull-out) or P_ov (pull-over), whichever is less

For 12-gauge (2.66 mm) sheet with No. 12-14 self-drilling screws:

• Shear: approximately 3.5-5.0 kN per screw
• Tension (pull-out): approximately 2.0-4.0 kN per screw (varies with sheet thickness)
• Screw spacing: minimum 3d centre-to-centre, 1.5d edge distance

Frequently Asked Questions

How does the CSA S136 effective width method differ from CSA S16 section classification?

CSA S136 uses the effective width method — a continuous post-buckling strength model where plate elements are reduced to an effective width b_e that carries the full yield stress. CSA S16 uses section classification (Class 1-4) — a discrete approach where Class 4 sections are treated with effective area but Class 1-3 sections use gross section properties. The effective width method is more refined for thin elements (w/t > 25) but more computationally intensive. CSA S136 applies to thicknesses 0.46-6.35 mm; CSA S16 applies to thicknesses >= 3 mm.

What bridging spacing is required for Canadian CFS purlins per CSA S136:24?

For spans up to 4500 mm, no bridging is required. One row of bridging at mid-span for spans 4500-7500 mm. Two rows at third points for spans 7500-12000 mm. Three rows at quarter points for spans exceeding 12000 mm. Bridging must resist 2% of the compression flange force and must be continuous between all supported members in the row. End bays require bay bridging (full-span angle) to transfer accumulated lateral loads to the primary framing.

When does distortional buckling govern over local buckling in CFS members?

Distortional buckling typically governs when the section has intermediate-depth lips (lip/web ratio 0.15-0.25) and moderate flange width-to-thickness ratios (30-60). For sections with lip/web ratio > 0.30 (deep lips), local buckling of the stiffened flange is usually critical. For sections with lip/web ratio < 0.10 (shallow or no lips), the flange is unstiffened and local buckling governs. Direct strength method (DSM) software such as CUFSM can determine the governing mode for any section geometry.

What Canadian CFS section catalogs are available for designers?

The CSSMA (Canadian Sheet Steel Manufacturers Association) publishes the CSSMA Product Catalog with standard C and Z sections from 150 mm to 300 mm depth at various thicknesses. Canam, Bailey Metal Products, and Vicwest all publish section property tables aligned with CSSMA standard profiles. The CISC Handbook includes cold-formed section property tables. For proprietary profiles (sigma sections, deep deck), manufacturer catalogs provide tested capacities per CSA S136.

Related Pages

• Canadian Cold-Formed Steel Reference
• CSA S16 Beam Design Guide — Hot-Rolled Sections
• Canadian Steel Grades — G40.21 Carbon Steel
• Canadian Steel Fy/Fu Values
• Beam Deflection Calculator — Free Online Tool
• Steel Column Capacity Calculator
• All Canadian Steel Design References

Design Resources

Calculator tools

• Beam Capacity Calculator
• Beam Serviceability Limits Calculator
• Column Buckling Calculator
• Wind Load Calculator — NBCC 2020

Design guides

• CSA S16 Column Design Guide
• Canadian Bolted Connection Design
• Canadian Welding Procedure — CSA W59
• AISI Cold-Formed Steel Design Guide
• EN 1993-1-3 Cold-Formed Steel Guide

This page is for educational reference only. Cold-formed steel design per CSA S136:24 (AISI S100 North American Specification with Canadian appendix). All section properties must be confirmed from manufacturer catalogs. All results are PRELIMINARY — NOT FOR CONSTRUCTION. All structural designs must be independently verified and sealed by a licensed Professional Engineer.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.