How does bridging affect CFS wall stud capacity?

Bridging (horizontal straps, channels, or angles connecting adjacent studs) prevents weak-axis buckling and torsional buckling, which are the critical failure modes for CFS Cee sections. Without bridging, the unbraced length equals the full stud height, and flexural-torsional buckling typically controls at a capacity much lower than the local or distortional buckling capacity. With bridging at mid-height, the unbraced length is halved and the global buckling capacity roughly quadruples since the Euler buckling load is proportional to 1/L^2. Per AISI S100 Section D3.2, bridging must resist Pb = 0.02 x Pstud x n for weak-axis stability, or Pb = 0.01 x Pstud x n with gypsum sheathing on both sides. Typical bridging types include strap bridging at 4-6 ft spacing, channel bridging through stud punchouts at 4-8 ft, and angle bridging screwed to flanges at 4-6 ft. Gypsum wallboard attached to flanges at standard screw spacing (12 in OC field, 7 in edges) can effectively eliminate the distortional buckling check per AISI S100 D3.2.1.

What is the combined axial and bending interaction check per AISI S100?

AISI S100 Section H1.2 provides the interaction equation for CFS studs under combined axial compression and bending. When Pu/(phi Pn) >= 0.15, the interaction check is: Pu/(phi Pn) + Cmx x Mux/(phi Mnx x alpha) + Cmy x Muy/(phi Mny x alpha) <= 1.0, where alpha = 1 - Pu/Pcr. When Pu/(phi Pn) < 0.15, the simplified check is Pu/(phi Pn) + Mux/(phi Mnx) + Muy/(phi Mny) <= 1.0. The Cm modification factor accounts for moment distribution: Cm = 0.85 for transverse loads, and Cm = 0.6 - 0.4 x (M1/M2) for end moments where M1/M2 is the ratio of smaller to larger end moment (negative for double curvature). The phi factors are 0.85 for compression and 0.90 for flexure. The amplification factor alpha accounts for P-delta effects through the Euler buckling load Pcr = pi^2 x EI/(K L)^2.

What is the effective width method for CFS sections under combined loading?

The effective width method (EWM) per AISI S100 accounts for local buckling of individual plate elements by reducing each compressed element to an effective width based on its slenderness ratio. Under combined axial and bending, the stress gradient across the section complicates the effective width calculation: under uniform compression, both flanges and web may be partially effective; under bending, the compression flange and part of the web are partially effective depending on the neutral axis location. The interaction between axial and bending stresses means the effective section properties (Ae, Se) change along the stud height and must be iteratively determined at the critical location, typically at mid-height. For a 600S162-54 stud (6 in web, 1.625 in flange, 54 mil thick, Fy = 33 ksi), the web slenderness h/t = 111 gives an effective width ratio of approximately 0.8, reducing the effective area by 15-25% depending on the stress distribution. Modern AISI S100 also permits the Direct Strength Method (DSM) as an alternative that avoids element-by-element effective width calculations.

What are the standard SSMA CFS stud sizes for load-bearing walls?

The Steel Stud Manufacturers Association (SSMA) standardizes CFS stud designations using the format web depth, section type, flange width, and thickness in mils. Common load-bearing sizes: 350S162-33 (3.5 in web, 1.25 in flange, 33 mil, Fy = 33 ksi, Ag = 0.231 in^2) for 8 ft walls at 24 in OC with 2 kip axial; 600S162-54 (6 in web, 1.625 in flange, 54 mil, Fy = 33 ksi, Ag = 0.554 in^2) for 10 ft walls at 16 in OC with 4 kip axial; 600S162-68 (68 mil, Fy = 50 ksi, Ag = 0.698 in^2) for 12 ft walls at 16 in OC with 5 kip axial; 800S162-68 for 14 ft walls; 1000S162-97 (10 in web, 97 mil, Fy = 50 ksi, Ag = 0.878 in^2) for 16 ft walls at 12 in OC with 8 kip axial. The designation decodes as: 600 = 6.00 inch web, S = stud (Cee), 162 = 1.625 inch flange, 54 = 0.054 inch thickness. Mils are thousandths of an inch; 33 mil = 20 gage, 43 mil = 18 gage, 54 mil = 16 gage, 68 mil = 14 gage, 97 mil = 12 gage.

What is the difference between structural and non-structural CFS studs?

Structural (load-bearing) CFS studs carry axial loads from floors/roof above and must be designed per AISI S100 for combined axial compression and bending from wind or seismic loads. They are checked for local, distortional, and global buckling interaction and require bridging per the AISI S100 bracing provisions. Non-structural (curtain wall) studs carry only lateral wind loads and their own weight; they are designed for bending only and can use lighter sections (typically 33-43 mil, 3.5-6 in web). The distinction affects member capacity calculations, connection design, and the applicable AISI S100 provisions. Curtain wall studs are often slip-tracked at the top to avoid accumulating axial load from floor deflections above. For load-bearing studs at 8-10 ft heights, 600S162-54 at 16 in OC is common in low-rise construction. For mid-rise (4-10 stories), heavier sections like 600S162-97 or built-up members are required at lower levels where cumulative axial loads reach 20-30 kips per stud.

What is CFS Wall Stud Design Calculator — Cold-Formed Steel AISI S100?

Cold-formed steel wall stud design per AISI S100. Combined axial and bending interaction, local and distortional buckling capacity for Cee stud sections with bridging.

CFS Wall Stud — Cold-Formed Steel Stud Design

Q: What is CFS Wall Stud Design Calculator — Cold-Formed Steel AISI S100?

Cold-formed steel wall stud design per AISI S100. Combined axial and bending interaction, local and distortional buckling capacity for Cee stud sections with bridging.

Cold-formed steel wall stud design per AISI S100. Combined axial load and bending, local and global buckling capacity for Cee stud sections. Educational use only.

This page documents the scope, inputs, outputs, and computational approach of the CFS Wall Stud tool on steelcalculator.app. The interactive calculator runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

Checking combined axial and bending capacity of CFS Cee-section wall studs per AISI S100.
Evaluating local, distortional, and global buckling under combined loading.
Comparing stud sizes, spacing, and bracing configurations for load-bearing walls.

What this tool is not for

It does not design the track, bridging, bracing, or sheathing attachment.
It does not handle CFS studs with punched web openings (punchouts) in full generality.
It does not check fire rating, acoustic performance, or thermal bridging.

Key concepts this page covers

combined axial compression and bending interaction (AISI S100 Section H1)
effective section properties under combined loading (Effective Width Method)
flexural, torsional, and flexural-torsional buckling modes
distortional buckling per AISI S100 Section F4
bracing requirements for CFS wall studs

Inputs and outputs

Typical inputs: stud section (web depth, flange width, lip length, thickness), stud height, stud spacing, applied axial load (from header/roof), lateral load (wind pressure), bracing spacing, and steel yield strength Fy.

Typical outputs: axial capacity Pn, bending capacity Mn, interaction ratio (P/Pn + M/Mn), controlling buckling mode, and whether the stud passes or fails.

Computation approach

The calculator computes the nominal axial capacity using the AISI S100 Effective Width Method, evaluating local, distortional, and global buckling for the Cee section under uniform compression. The nominal bending capacity is similarly computed for the section under bending about the strong axis. The combined interaction is checked per AISI S100 Section H1.2 using the interaction equation. Sheathing bracing is accounted for via effective unbraced length factors.

AISI S100 Interaction Equations

Combined axial and bending (AISI S100 Section H1)

When Pu/phiPn >= 0.15:
  Pu/(phiPn) + Cmx × Mux / (phiMnx × alpha) + Cmy × Muy / (phiMny × alpha) <= 1.0

When Pu/phiPn < 0.15:
  Pu/(phiPn) + Mux/(phiMnx) + Muy/(phiMny) <= 1.0

Where:
  Cm = modification factor (0.85 for transverse loads, 0.6-0.4 for end moments)
  alpha = 1 - Pu/Pcr (amplification factor, Pcr = Euler buckling load)
  phi = 0.85 (compression), 0.90 (flexure)

Effective width method under combined loading

For combined axial and bending, the effective section properties must account
for the stress gradient across the section:
  - Under compression: both flanges and web may be partially effective
  - Under bending: compression flange and part of web are partially effective
  - Combined: the effective section changes at each point along the stud

The calculator iterates to find the correct effective section at the
critical location (typically at mid-height for combined loading).

Common CFS Wall Stud Designations

SSMA (Steel Stud Manufacturers Association) standard sizes

Designation	Web (in)	Flange (in)	Lip (in)	Thickness (mils)	Fy (ksi)	Ag (in²)
350S125-33	3.5	1.25	0.38	33	33	0.231
350S162-43	3.5	1.625	0.50	43	33	0.355
400S162-54	4.0	1.625	0.50	54	33	0.446
550S162-54	5.5	1.625	0.50	54	33	0.527
600S162-54	6.0	1.625	0.50	54	33	0.554
600S162-68	6.0	1.625	0.50	68	50	0.698
600S162-97	6.0	1.625	0.50	97	50	0.996
800S162-54	8.0	1.625	0.50	54	33	0.662
1000S162-68	10.0	1.625	0.50	68	33	0.878
1200S162-97	12.0	1.625	0.50	97	50	1.223

Mils = thousandths of an inch (33 mil = 0.033 in, 97 mil = 0.097 in)

Bridging and Bracing Requirements

Bridging types for CFS wall studs

Type	Description	Typical Spacing
Strap bridging	Flat strap welded/screwed to stud webs	4-6 ft
Channel bridging	U-channel passed through stud punchouts	4-8 ft
Angle bridging	Angles screwed to stud flanges	4-6 ft
Gypsum board	Gypsum sheathing screwed to flanges	Full height

Required bridging force (AISI S100 Section D3.2)

Bridging must resist:
  Pb = 0.02 × Pstud × n (for weak-axis stability)
  Or Pb = 0.01 × Pstud × n (with gypsum sheathing both sides)

Where n = number of studs supported by the bridging line
Pstud = axial force in one stud

Worked Example — CFS Load-Bearing Wall Stud

Problem: A 600S162-54 (6" web, 1.625" flange, 54 mil = 0.054 in, Fy = 33 ksi) wall stud is 10 ft tall with bridging at 5 ft. It carries an axial load of 4 kips and wind pressure of 20 psf at 16 in OC spacing. Check adequacy per AISI S100.

Step 1 — Loads

Axial: P = 4 kips (from header/roof)
Wind: w = 20 psf × 16/12 = 26.7 lb/ft = 0.0267 kip/ft
Wind moment: M = wL²/8 = 0.0267 × 10²/8 = 0.334 kip-ft = 4.0 kip-in

Step 2 — Effective section properties (simplified)

Gross: Ag = 0.554 in², Sx_gross ≈ 1.04 in³ (from SSMA table)

Effective area under uniform compression (simplified EWM):
  Web slenderness: h/t = 6.0/0.054 = 111
  For uniform compression, effective width ratio b/t ≈ 0.8 × 111 = 89 (approximate)
  Web effective: 0.8 × 6.0 = 4.8 in
  Flanges and lips: fully effective (b/t ≈ 30 < limit)
  Ae ≈ 0.45 in² (estimated)

Step 3 — Axial capacity (global buckling)

Weak-axis unbraced length: Ly = 5 ft = 60 in (bridging at mid-height)
ry ≈ 0.65 in (from SSMA table for 600S162-54)

Fe = pi² × 29500 / (60/0.65)² = 290,887 / 8,506 = 34.2 ksi

Fy/Fe = 33/34.2 = 0.96 → Fne = Fy × 0.658^0.96 = 33 × 0.660 = 21.8 ksi

Wait — Fe > 0.44Fy (0.44×33 = 14.5), so inelastic buckling:
  Fne = Fy × (0.658)^(Fy/Fe) = 33 × 0.658^0.96 = 33 × 0.666 = 22.0 ksi

Pne = Ae × Fne = 0.45 × 22.0 = 9.9 kips
phiPn_global = 0.85 × 9.9 = 8.4 kips

Step 4 — Bending capacity

Effective Sx under bending (compression flange partially effective):
  Se ≈ 0.90 in³ (estimated, compression flange reduction)

phiMn = 0.90 × 0.90 × 33 = 26.7 kip-in = 2.23 kip-ft

Step 5 — Interaction check

P/(phiPn) = 4 / 8.4 = 0.476

Since 0.476 > 0.15, use full interaction:
P/(phiPn) + M/(phiMn) = 4/8.4 + 0.334/2.23 = 0.476 + 0.150 = 0.626

0.626 < 1.0 → OK ✓ (37% margin)

The 600S162-54 stud at 16 in OC with 5 ft bridging is adequate for 4 kip
axial load and 20 psf wind.

CFS Wall Stud Selection Guide

Wall Height	Axial Load (kips)	Wind (psf)	Recommended Stud	Spacing	Bridging
8 ft	2	20	350S162-33	24" OC	Midspan
10 ft	4	20	600S162-54	16" OC	5 ft
12 ft	5	25	600S162-68	16" OC	4 ft
14 ft	6	30	800S162-68	16" OC	4 ft
16 ft	8	30	1000S162-97	12" OC	4 ft
20 ft	3	25	800S162-54 (curtain)	16" OC	5 ft

Values are approximate. Use the calculator for exact AISI S100 checks.

Frequently Asked Questions

What stud sizes are commonly used for load-bearing CFS walls? Common CFS stud sizes for load-bearing walls include 350S162 (3.5-inch web), 400S162 (4-inch web), and 600S162 (6-inch web) in gages 16 through 12 (54 mil through 97 mil thickness). The choice depends on wall height, axial load, and lateral load. For typical 8-10 foot walls in low-rise construction, 600S162-54 studs at 16 inches on center are common. Taller walls or heavier loads require deeper sections, thicker gages, or closer spacing.

How does bridging affect CFS stud capacity? Bridging (horizontal straps or channels connecting studs) prevents weak-axis buckling and torsional buckling, which are the critical failure modes for CFS Cee sections. Without bridging, the unbraced length equals the full stud height, and flexural-torsional buckling typically controls at a capacity much lower than the local or distortional buckling capacity. With bridging at mid-height, the unbraced length is halved and the global buckling capacity roughly quadruples (since buckling load is proportional to 1/L^2).

What is the difference between structural studs and non-structural studs? Structural (load-bearing) studs carry axial loads from above and must be designed per AISI S100 for combined axial and bending under wind or seismic loads. Non-structural (curtain wall) studs carry only lateral loads (wind) and their own weight; they are designed for bending only and can use lighter sections. The distinction affects the member capacity, connection requirements, and the applicable design standard provisions.

How does sheathing provide bracing for CFS wall studs? Gypsum wallboard, OSB, plywood, or other sheathing attached to the stud flanges provides rotational restraint that prevents lateral-torsional and distortional buckling. AISI S100 Section D3.2.1 allows the sheathing to be considered as bracing when it meets specific attachment requirements (screw spacing, sheathing type, fastener type). With gypsum board on both sides at standard screw spacing (12 inches on center for field, 7 inches for edges), the distortional buckling mode can be effectively eliminated from the design check, significantly increasing the stud capacity. Single-side sheathing provides partial restraint.

How do punchouts (web holes) affect CFS stud capacity? Punched web openings (typically 1.5 x 4 inch slots at regular spacing for plumbing and electrical routing) reduce the web area and can affect the shear capacity and web crippling capacity of the stud. AISI S100 provides reduction factors for members with web holes. For axial compression, the effect is generally small when the holes are located at the center of the web and away from the flanges. For bending and shear, the reduction depends on the hole size relative to the web depth and the spacing between holes. The calculator does not currently account for punchout effects; use the AISI S100 provisions or manufacturer data for exact reductions.

What is the fire rating of CFS wall assemblies? CFS wall assemblies achieve fire ratings through the combination of stud size, gypsum board type and thickness, and insulation. A standard 1-hour rated wall uses 5/8-inch Type X gypsum board on both sides of 3.5-inch or 6-inch studs with cavity insulation. A 2-hour rating requires double layers of Type X gypsum or specialty boards. The stud itself does not lose strength until it reaches approximately 1000 degrees F, so the gypsum board serves as the primary thermal barrier. Fire testing per ASTM E119 establishes the assembly rating, and manufacturers publish rated assemblies that can be used without additional engineering analysis.

What thickness (gage) of CFS stud should I use? CFS stud thickness is designated in mils (thousandths of an inch). Common thicknesses: 33 mil (20 gage, 0.033 in) for non-structural and light structural walls, 43 mil (18 gage, 0.043 in) for moderate loads, 54 mil (16 gage, 0.054 in) for load-bearing walls, 68 mil (14 gage, 0.068 in) for heavy loads, and 97 mil (12 gage, 0.097 in) for columns and high-load conditions. Thicker studs have proportionally higher capacity (area scales linearly with thickness) and are more resistant to local and distortional buckling. Select the minimum thickness that satisfies the AISI S100 design checks for the given loads and span.

What is the difference between the effective width method and direct strength method for CFS? The effective width method (EWM) reduces each compressed plate element to an effective width based on its slenderness ratio, then computes effective section properties (Ae, Se). It is straightforward for simple sections but becomes complex for shapes with multiple interacting elements. The direct strength method (DSM) uses the elastic buckling load of the full cross-section (from finite strip analysis or closed-form solutions) and applies calibrated strength curves. DSM is the preferred method in modern AISI S100 because it handles complex shapes more easily and produces consistent results across section types.

What is the typical cost comparison between CFS and wood framing? CFS framing is typically 10-20% more expensive in material cost than comparable wood framing for low-rise buildings, but offers advantages in termite resistance, fire rating, dimensional stability (no warping, splitting, or shrinkage), and consistency of material properties. CFS is competitive with wood for mid-rise construction (4-10 stories) where the structural requirements exceed typical wood framing capabilities. The total installed cost (including labor) is often comparable because CFS members are lighter and easier to handle, and the dimensional consistency reduces field modifications. In regions with high termite risk or strict fire codes, CFS may be the lower total-cost option.

What is the maximum height for a CFS load-bearing wall? CFS load-bearing walls are commonly used in buildings up to 10 stories. The limiting factors are the axial capacity of the studs at lower levels (which may require 97-mil sections or built-up members) and the lateral force-resisting system design. At 10 stories, the cumulative axial load can reach 20-30 kips per stud, requiring heavy sections at close spacing. Above 10 stories, hot-rolled steel framing becomes more economical. Mid-rise CFS buildings up to 6 stories are common in North America using 600S162-68 or 600S162-97 studs at 12 to 16 inches on center.

Disclaimer (educational use only)

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