Cold-Formed Steel Design — AISI S100, Effective Width & Buckling Modes

Cold-formed steel (CFS) members are manufactured by bending thin steel sheet (typically 18 to 54 mil / 0.018" to 0.054") at room temperature into C-shapes, Z-shapes, tracks, angles, and hat sections. Unlike hot-rolled structural steel governed by AISC 360, CFS design follows AISI S100-16 (North American Specification for Cold-Formed Steel) and its specialized framing standard AISI S240. The thin elements of CFS sections create buckling modes that do not occur in hot-rolled shapes, making the design methodology fundamentally different.

Key CFS section types

Common cold-formed steel section properties

Table 1 — C-studs (CS members), ASTM A1003 Gr 50, Fy = 50 ksi

C-stud designation format: XXXSYYY-ZZ where XXX = out-to-out depth (x100), YYY = flange width (x100), ZZ = thickness (mil, x1000). Example: 600S162-54 = 6.00" depth, 1.625" flange, 0.054" thickness.

Table 2 — Track sections (T members), ASTM A1003 Gr 50, Fy = 50 ksi

Track designation format: XXXTYYY-ZZ where XXX = out-to-out depth (x100), YYY = flange width (x100), ZZ = thickness (mil). Tracks have unstiffened (unlipped) flanges.

Table 3 — Channel sections (back-to-back or single), ASTM A36/A572

Standard structural channels for backing, jamb studs, and miscellaneous framing.

Three buckling modes in CFS

The fundamental difference between CFS and hot-rolled design is that CFS elements are slender enough to buckle locally before the full section yields. AISI S100 addresses three distinct buckling modes:

1. Local buckling — individual flat elements (flanges, webs, lips) buckle in short half-waves between restraint points. Controlled by the effective width method (AISI S100 Section 1.1) where the effective width be of a compression element is:

be = rho x w    where rho = (1 - 0.22/lambda) / lambda <= 1.0
lambda = (1.052/sqrt(k)) x (w/t) x sqrt(f/E)

k = plate buckling coefficient (4.0 for stiffened elements, 0.43 for unstiffened edges).

2. Distortional buckling — the compression flange and lip rotate as a unit about the flange-web junction. This mode has a half-wavelength between local and global buckling (typically 200–800 mm). AISI S100 Section 1.4 provides the distortional buckling stress Fd based on the elastic distortional buckling stress Fcrd.

3. Global buckling — the entire cross-section buckles in flexural, torsional, or flexural-torsional mode. For C-studs loaded in compression, flexural-torsional buckling almost always governs over pure flexural buckling because the shear center does not coincide with the centroid.

Design standard comparison

Cold-formed steel design is governed by different standards around the world. While the underlying mechanics are the same, the formulations, safety factors, and permitted materials differ.

Key differences in effective width method

The effective width method (EWM) is shared across all three standards but implemented differently:

• AISI S100: Uses the classic Winter equation with lambda calculated from the plate slenderness. The plate buckling coefficient k varies by boundary condition: k = 4.0 for stiffened elements (webs), k = 0.43 for unstiffened elements (free edges), and k = 0.5 to 4.0 for edge-stiffened elements depending on the lip stiffness.
• AS/NZS 4600: Adopts the identical Winter equation with the same k values. The main practical difference is that AS/NZS 4600 permits the Direct Strength Method as a standalone approach for all limit states, whereas AISI treats DSM as an optional alternative to EWM.
• EN 1993-1-3: Uses a modified effective width approach where the plate slenderness is calculated relative to the yield strength divided by a stress ratio. The edge stiffener design uses an iterative procedure to find the effective stiffness of the lip, which can converge to a lip that is effectively unstiffened if the lip is too short.

Effective width method overview

How local buckling reduces effective section properties

When a thin flat element in compression is loaded, it does not develop uniform stress across its full width. The center of the plate buckles first, shedding load toward the supported edges where the boundary conditions restrain displacement. The effective width method models this behavior by treating only the edges of the plate as effective — the central portion is assumed to carry no stress.

For a C-stud web in bending:

• The compression portion of the web is reduced from its full height to an effective height based on the stress level.
• The effective flange in compression is similarly reduced.
• The tension flange and tension web remain fully effective.
• These reduced widths are used to calculate the effective moment of inertia Ie and effective section modulus Se, which are always less than the gross values.

The reduction is stress-dependent: at low stresses (far from Fy), the effective width is close to the full width. As the stress approaches Fy, lambda increases and the effective width shrinks. This means the same section has different effective properties at different load levels.

Compact vs noncompact vs slender elements in cold-formed

In hot-rolled steel (AISC 360 Table B4.1a/b), elements are classified as compact, noncompact, or slender based on width-to-thickness ratios. Cold-formed steel uses a different framework because essentially all CFS elements are slender — the width-to-thickness ratios are far too high to qualify as compact.

Instead of the three-tier system, AISI S100 applies the effective width method to every element. The key threshold is:

• lambda <= 0.673: The element is fully effective (rho = 1.0). The entire width contributes to section properties. This is analogous to a "compact" element.
• lambda > 0.673: The element is partially effective (rho < 1.0). The effective width be = rho x w is less than the full width. This is analogous to "slender" — the element buckles locally before reaching yield stress.

There is no intermediate "noncompact" category in CFS. The transition from fully effective to partially effective occurs at lambda = 0.673 for all stiffened elements, which corresponds to approximately (w/t) = 40 for Fy = 50 ksi with k = 4.0.

Edge stiffeners and intermediate stiffeners per AISI S100

AISI S100 classifies compression elements into three categories based on edge support:

• Stiffened elements — both longitudinal edges supported (e.g., web between two flanges). k = 4.0. These have the highest post-buckling strength because both edges restrain out-of-plane displacement.
• Unstiffened elements — one longitudinal edge free, one supported (e.g., track flange projecting beyond the web). k = 0.43. These have the lowest buckling stress — the free edge offers no restraint against out-of-plane movement.
• Edge-stiffened elements — one edge free and one edge supported, with a stiffener lip at the free edge (e.g., C-stud flange with return lip). The effective k depends on the stiffener geometry and ranges from 0.5 to 4.0.

The edge stiffener (lip) on a C-stud flange is designed to act as an elastic rotational restraint that elevates the flange's local buckling stress. AISI S100 Section 1.2 establishes two requirements for an edge stiffener to be considered fully effective:

1. Moment of inertia: Is >= Ia, where Is is the moment of inertia of the lip about its own centroidal axis and Ia is the required stiffener inertia calculated from the flange flat width (w), the stress level (f), and the modulus of elasticity. Ia increases with w/t ratio and stress — deeper flanges at higher stress require stiffer lips.
2. Lip flat width ratio: d/w >= 0.8, where d is the lip flat width (excluding corner radius) and w is the flange flat width. This ensures the lip itself has adequate plate stability to provide edge restraint. If d/w < 0.8, the lip is treated as an unstiffened element and the flange k drops to 0.43.

For a standard 1.625 in C-stud flange with 0.5 in lip: d/w = 0.5/1.625 = 0.31 < 0.8, so the lip alone does NOT meet the flat-width ratio criterion. However, the lip combined with the flange corner radius provides additional restraint, and the AISI S100 procedure accounts for this through the Is/Ia ratio. When Is/Ia >= 1.0, the flange is fully stiffened (k = 4.0). When Is/Ia is between 0.5 and 1.0, the flange k is reduced proportionally. When Is/Ia < 0.5, the lip is ineffective and the flange is treated as unstiffened (k = 0.43).

Intermediate stiffeners are used in hat sections, multi-web panels, and deep C-studs. AISI S100 permits multiple intermediate stiffeners in a single flat compression element. The stiffened element is subdivided into sub-elements, each spanning between adjacent stiffeners. Each sub-element then has its own effective width calculation based on the stiffener spacing. The intermediate stiffener itself must be checked for local buckling: its flat width-to-thickness ratio must satisfy w_st/t <= 0.42 x sqrt(Es/Fy) for the stiffener to be fully effective. If exceeded, the effective stiffener area is reduced proportionally, which in turn reduces the effective k for the adjacent sub-elements — an interdependent design check.

Worked example — effective width calculation for a C-stud in bending

Given: 600S162-54 C-stud in pure bending about the x-axis. Fy = 50 ksi, web height w = 5.60 in (net of corner radii), t = 0.056 in, k = 24.0 for web in bending (both edges restrained by flanges, non-uniform compression).

Step 1 — Calculate lambda:

lambda = (1.052/sqrt(k)) x (w/t) x sqrt(f/E)
lambda = (1.052/sqrt(24)) x (5.60/0.056) x sqrt(50/29500)
lambda = 0.2146 x 100.0 x 0.04119
lambda = 0.884

Step 2 — Calculate reduction factor rho:

Since lambda = 0.884 > 0.673, the web is partially effective.

rho = (1 - 0.22/lambda) / lambda
rho = (1 - 0.22/0.884) / 0.884
rho = (1 - 0.249) / 0.884
rho = 0.751 / 0.884
rho = 0.849

Step 3 — Effective width of web:

be = rho x w = 0.849 x 5.60 = 4.75 in (effective)

The web loses 0.85 in of effective width (15.1% reduction). The effective Ix and Sx are recalculated using the reduced compression zone of the web and similarly reduced compression flange. For this section, Ieff drops from approximately 11.1 in4 (gross) to about 9.8 in4 (effective) — a 12% reduction in moment capacity.

Worked example — stud wall design for 10-ft floor-to-floor height

Given: Load-bearing CFS stud wall, floor-to-floor height = 10 ft. Studs at 16" on center. Axial dead load = 0.85 kip/ft of wall, axial live load = 1.10 kip/ft of wall. Wind pressure = 25 psf (out-of-plane). Sheathing: 5/8" gypsum board each side, fastened at 12" on center. Select a stud size.

Step 1 — Factored loads per stud (16" spacing):

Axial: Pu = 1.2(0.85) + 1.6(1.10) = 1.02 + 1.76 = 2.78 kips per foot of wall
Per stud (16" spacing): Pu = 2.78 x (16/12) = 3.71 kips
Wind: Wu = 1.0 x 0.025 x (16/12) = 0.033 kip/ft per stud

Step 2 — Unbraced length:

Sheathing braces the studs at 12" fastener spacing for weak-axis buckling. Strong-axis (x-axis) is unbraced for the full 10 ft height.

KxLx = 1.0 x 120 = 120 in (strong axis)
KyLy = 1.0 x 12 = 12 in (weak axis, sheathing braced)
KtLt = 1.0 x 120 = 120 in (torsional)

Step 3 — Try 600S162-54 (Fy = 50 ksi):

Gross properties: Ag = 0.840 in2, Ix = 5.18 in4, Iy = 0.596 in4, rx = 2.48 in, ry = 0.842 in, J = 0.000815 in4, Cw = 2.47 in6, x-bar = 0.566 in, ro = 2.56 in.

Step 4 — Global buckling (flexural-torsional):

Fe,x = pi^2 x 29500 / (120/2.48)^2 = 124.5 ksi
Fe,y = pi^2 x 29500 / (12/0.842)^2 = 1,433 ksi  (sheathing bracing effective)
Fe,t = (1/(Ag x ro^2)) x [GJ + pi^2 x E x Cw / (120)^2]
     = (1/(0.840 x 2.56^2)) x [11,300 x 0.000815 + pi^2 x 29500 x 2.47 / 14400]
     = 0.1816 x [9.21 + 49.8]
     = 10.7 ksi

Flexural-torsional buckling stress:

1/Fe,FT = 1/(2 x Fe,t) x [1 + Fe,t/Fe,x - sqrt((1 - Fe,t/Fe,x)^2 + 4 x (0.566/2.56)^2 x Fe,t/Fe,x)]

For this geometry, Fe,FT typically falls between Fe,t and Fe,x. Using the simplified approximation for singly-symmetric sections:

Fe,FT ~ 10.7 ksi (torsional mode controls for this unbraced length)

Step 5 — Nominal buckling stress:

lambda_c = sqrt(Fy / Fe,FT) = sqrt(50/10.7) = 2.16 > 1.5

For lambda_c > 1.5: Fn = 0.877 x Fe,FT = 0.877 x 10.7 = 9.38 ksi

Step 6 — Effective area at f = Fn = 9.38 ksi:

At this relatively low stress level, most elements remain fully effective. Ae is approximately 0.78 in2 (web has minor reduction at flange-web junction).

Step 7 — Axial capacity:

phi_c x Pn = 0.85 x 9.38 x 0.78 = 6.22 kips

Required: Pu = 3.71 kips. 3.71 < 6.22 — axial capacity is adequate.

Step 8 — Combined axial and bending (beam-column check):

Wind moment: Mu = Wu x L^2 / 8 = 0.033 x 10^2 / 8 = 0.41 kip-ft = 4.95 kip-in.

Effective Sx at combined stress level (f = Pu/Ae + Mu/Se) requires iteration. At the combined stress, Se_eff is approximately 3.0 in3.

Pu/(phi_c x Pn) + Mu/(phi_b x Mn) = 3.71/6.22 + 4.95/(0.95 x 3.0 x 9.38)
= 0.597 + 4.95/26.73
= 0.597 + 0.185
= 0.782 < 1.0  -- OK

Result: 600S162-54 at 16" on center is adequate for this 10-ft load-bearing wall with 25 psf wind.

Worked example — CFS stud axial capacity

Given: 600S162-54 stud (6" deep, 1-5/8" flange, 54 mil = 0.054"), Fy = 50 ksi, KL = 10 ft (braced at midheight by sheathing, effective KL = 5 ft for weak axis).

Step 1 — Section properties (from AISI S200 tables): Ag = 0.840 in^2, Ix = 5.18 in4, Iy = 0.596 in4, rx = 2.48 in, ry = 0.842 in, J = 0.000815 in4, Cw = 2.47 in6.

Step 2 — Global buckling (flexural-torsional): Fe,x = pi^2 x E / (KxLx/rx)^2 = pi^2 x 29500 / (120/2.48)^2 = 124.5 ksi (strong axis) Fe,y = pi^2 x E / (KyLy/ry)^2 = pi^2 x 29500 / (60/0.842)^2 = 57.3 ksi (weak axis) Fe,t = [GJ + pi^2ECw/(KtLt)^2] / (Ag x ro^2) — torsional buckling stress.

Flexural-torsional: Fe,FT controls for singly-symmetric C-sections. Typically Fe,FT ~ 35–45 ksi for this configuration.

Step 3 — Nominal axial capacity: Fn = 0.658^(Fy/Fe,FT) x Fy (if lambda_c <= 1.5) with phi_c = 0.85 (LRFD). Assuming Fe,FT = 40 ksi: Fn = 0.658^(50/40) x 50 = 0.658^1.25 x 50 = 0.583 x 50 = 29.2 ksi.

Step 4 — Effective area: Local and distortional buckling reduce the effective area. Ae ~ 0.72 in^2 (typical reduction for 54-mil studs at this stress level).

phi_c x Pn = 0.85 x 29.2 x 0.72 = 17.9 kips per stud.

AISI S240 structural framing requirements

AISI S240 (North American Standard for Cold-Formed Steel Structural Framing) governs the design of CFS framing systems for walls, floors, and roofs. It covers both prescriptive (pre-engineered) and engineered design approaches:

Sheathing bracing: CFS studs and joists rely on sheathing for lateral bracing per AISI S240 Section C4. Minimum fastener spacing is 12 in on center along panel edges and 24 in on center in the field. Gypsum board provides approximately 50-100 lb/ft of lateral bracing force capacity per fastener row. OSB (7/16 in thickness) provides 150-250 lb/ft. When sheathing is omitted from one face, the unbraced length for weak-axis buckling reverts to the full stud height, which can reduce axial capacity by 50-70%.

Web holes and cutouts: AISI S240 Section C3 permits web holes up to 1.5 in diameter without reduction in capacity, provided the hole center is at least 10 in from the support face. Standard industry punch-out patterns (1.5 in x 4.5 in elongated holes at 24 in spacing) require a web hole reduction factor applied to flexural and shear capacity. The hole depth must not exceed 70% of the web depth. Multiple holes in the same web panel are evaluated cumulatively — the sum of hole widths in a panel must not exceed 1.5 times the hole depth. Holes located in bearing zones require reinforcement per AISI S240 Section C3.3.

Bearing stiffeners: At concentrated loads and reactions, AISI S240 requires web crippling checks per AISI S100 Section G5. The four web crippling load cases (one-flange loading / two-flange loading, each with end or interior conditions) all apply to CFS framing. When the factored reaction exceeds the web crippling strength, bearing stiffeners (track sections or cut stud sections) must be installed. Industry punch-out patterns reduce web crippling capacity by 30-40% compared to solid web sections.

Floor vibration: AISI S240 requires floor systems to meet a minimum fundamental frequency of 8 Hz for ordinary occupancy or 15 Hz for sensitive equipment areas. The frequency is estimated as f = 0.18 x sqrt(g/delta_D) where delta_D is the dead-load deflection in inches. This requirement often governs for long-span CFS floor joists (20 ft+) when strength criteria alone would permit a lighter section. A typical 600S162-54 joist at 16 in on center spanning 22 ft under 15 psf dead load deflects approximately 1.0 in, giving f = 0.18 x sqrt(386/1.0) = 3.5 Hz — well below the 8 Hz threshold. To meet the vibration criterion, a deeper section (800S162-54 or greater) or reduced spacing is required.

Common mistakes engineers make

1. Treating CFS studs like hot-rolled columns. AISC 360 column curves assume the full gross section yields before buckling. CFS elements buckle locally at stresses well below Fy, so the effective area is always less than the gross area. Using Ag instead of Ae overestimates capacity by 15–30%.

2. Ignoring flexural-torsional buckling for C-studs. Because the shear center of a C-section is outside the web, compression loads always trigger flexural-torsional buckling before weak-axis flexural buckling. Using weak-axis Euler buckling alone overestimates the critical load.

3. Assuming sheathing bracing is always effective. Gypsum board and OSB sheathing provide lateral and rotational bracing to studs, but only if the fastener spacing, edge distance, and panel stiffness meet AISI S240 requirements. Bracing capacity must be explicitly checked — it is not automatic.

4. Using Fy = 33 ksi for all CFS. Modern structural studs commonly use 50 ksi material (ASTM A1003 Structural Grade 50). Specifying 33 ksi when the project requires 50 ksi — or vice versa — leads to either overweight designs or under-capacity members.

5. Neglecting distortional buckling for sections with short lips. The AISI S100 DSM distortional check can govern for sections with lip lengths less than the optimal value. A lip that is too short provides insufficient edge stiffening, while a lip that is too long buckles as an unstiffened element. The optimal lip length is approximately 0.25 to 0.33 times the flange width.

Cold-formed steel connection design

CFS connections are fundamentally different from hot-rolled steel connections. The thin base metal (0.018-0.101 in) limits fastener type, installation method, and load capacity:

Screw connections (AISI S100 Chapter E): Self-drilling screws are the primary fastener for CFS. Screws are classified by diameter (No. 6, No. 8, No. 10, No. 12, No. 14) and drill point (Drill Point 2 for total stack thickness 0.035-0.100 in, Drill Point 3 for 0.100-0.175 in). Design capacities per screw:

• Shear: Pns = 4.2 x (t2^3 x d)^0.5 x Fu2 — based on tilting and bearing in the connected ply, lower of sheet bearing and screw shear
• Pull-out (threads pulling from the far side): Pot = 0.85 x tc x d x Fu2 — where tc is the thickness of the ply engaging the threads and d is the screw diameter
• Pull-over (screw head pulling through the near sheet): Pnot = 1.5 x t1 x Fu1 x d_w — where d_w is the washer or screw head diameter

Minimum edge distance per AISI S100 Section E2: 1.5d for sheared edges, 3d for rolled edges. Minimum center-to-center spacing: 3d. Connections with three or more screws in a line must account for the reduced capacity of the middle fastener (less uniform load distribution). Connections exposed to cyclic loading (seismic or wind) must use screws with minimum edge distance increased by 50%.

Welded connections (AISI S100 Chapter F): Arc spot welds (puddle welds) and arc seam welds are permitted for CFS assemblies. For arc spot welds: nominal shear strength Pn is the lesser of the weld shear (0.7 x pi x d_e^2 / 4 x Fxx) and the base metal tear-out (per Section F1 equations based on sheet thickness and edge distance). Minimum sheet thickness for arc welding is 0.028 in. Resistance spot welding is permitted for thicknesses up to 0.125 in. Welding of galvanized steel requires reduced currents or grinding of the coating at the weld location (zinc vapor can cause porosity in the weld metal).

Bolted connections (AISI S100 Chapter E): ASTM A307 or A325 bolts in standard holes (nominal bolt diameter + 1/16 in). Bearing and tear-out follow the same limit states as AISC 360 but with CFS-specific resistance factors and the net section reduction at the bolt line. Washers are required under both head and nut when the connected ply thickness is less than the bolt diameter. Minimum edge distance: 1.5 x d. Net section capacity: Pn = Ae x Fu, where Ae accounts for both the hole reduction and the shear lag effect. For thin plies connected by a single bolt, shear lag can reduce the net section efficiency to 50-60%.

Connection design must always verify the net section at the fastener line. A single row of No. 10 screws at 1 in spacing removes 10-15% of the gross section area, but the reduced effective width around each fastener hole (due to local stress concentrations in the thin sheet) can further diminish capacity by 20-30%. For this reason, bearing-type connections (where fasteners bear directly on the connected ply) are preferred over slip-critical connections in CFS.

FAQ

What is the difference between cold-formed steel and hot-rolled steel?

Cold-formed steel (CFS) is made by bending thin sheet steel at room temperature into structural shapes. Hot-rolled steel is formed at elevated temperatures (above 1,700 degrees F), producing thicker members with different material properties. CFS members are thinner (0.018" to 0.25"), lighter, and governed by AISI S100 rather than AISC 360. The key design difference is that CFS elements are slender enough to buckle locally before yielding, requiring the effective width method.

What is the Direct Strength Method (DSM) in AISI S100?

The Direct Strength Method is an alternative to the effective width method that uses the full section properties (gross, not effective) and compares elastic buckling stresses (local, distortional, global) obtained from a finite strip analysis against the yield stress. DSM avoids the need to calculate effective widths element-by-element and is particularly useful for complex cross-sections where manual effective width calculations are impractical. It is codified in AISI S100 Appendix 1.

At what thickness does cold-formed steel require protective coating?

Most CFS framing members are made from galvanized steel (ASTM A653 with G40 to G90 coating) or galvannealed steel. AISI S220 (Residential Prescriptive) and AISI S240 (Structural Framing) require a minimum G40 coating for interior dry service conditions and G60 or heavier for exterior or damp locations. Uncoated CFS (bare steel) is not permitted for structural framing applications.

How far can a cold-formed steel floor joist span?

CFS floor joist spans depend on the section depth, thickness, and loading. Typical spans using C-shaped joists at 16" or 24" on center range from 12 ft to 30 ft. A 600S162-54 joist at 16" on center can span approximately 16–18 ft for typical residential floor loads (40 psf live + 12 psf dead). For longer spans, deeper sections (8" or 10") or back-to-back configurations are used. Span tables are provided in AISI S210 (Floor and Roof System Design).

What are the standard thicknesses for cold-formed steel framing?

CFS framing uses "mil" designations (1 mil = 0.001 inch). The standard thicknesses in North America are 18 mil (0.018"), 27 mil (0.027"), 30 mil (0.030"), 33 mil (0.033"), 43 mil (0.043"), 54 mil (0.054"), 68 mil (0.068"), and 97 mil (0.097"). The design thickness is 0.95 times the minimum thickness per ASTM A1003. Structural bearing walls typically use 54 mil or heavier. Non-structural partitions may use 33 mil or lighter members.

Can cold-formed steel be used for lateral force-resisting systems?

Yes. AISI S213 (Standard for Cold-Formed Steel Framing — Lateral Design) covers CFS shear walls (sheathed with gypsum, OSB, or steel sheet) and CFS strap-braced walls. Shear wall capacities range from approximately 100 to 1,000 plf depending on sheathing type, fastener schedule, and stud thickness. These systems are permitted in Seismic Design Categories A through F, with height limits varying by system type.

What are the requirements for screw connections in cold-formed steel framing?

Self-drilling screws are the primary fastener for CFS construction. AISI S100 Chapter E governs screw connection design: shear capacity Pns = 4.2 x (t2^3 x d)^0.5 x Fu2 (where t2 = thickness of ply engaging threads, d = screw diameter, Fu2 = tensile strength of the ply engaging threads), pull-out capacity Pot = 0.85 x tc x d x Fu2, and pull-over capacity Pnot = 1.5 x t1 x Fu1 x d_w. Minimum edge distance is 1.5d for sheared edges or 3d for rolled edges. Minimum center-to-center spacing is 3d. Screw diameters range from No. 6 (0.138 in) to No. 14 (0.242 in). The drill point must match the total stack thickness — Drill Point 2 for stacks up to 0.100 in, Drill Point 3 for 0.100-0.175 in. Washers are required when either connected ply is thinner than 0.033 in to prevent the screw head from pulling through the sheet. Screw shear governs for thin-to-thick connections (screw fails before sheet bearing), while bearing governs for thick-to-thick connections (sheet tears out before the screw shears).

What is the optimal lip length for a C-stud edge stiffener?

The lip (edge stiffener) on a C-stud flange must satisfy two conditions per AISI S100 Section 1.2: (1) the lip moment of inertia Is must equal or exceed the required stiffener inertia Ia (calculated from flange flat width, stress level, and modulus of elasticity); (2) the lip flat width ratio d/w must be at least 0.8 for the lip to be fully effective. Optimal lip length is approximately 0.25 to 0.33 times the flange width. For a 1.625 in flange, this gives an optimal lip of 0.41 to 0.54 in. Standard C-studs with 0.5 in lips (d/w = 0.31) are near this optimum for 1.625 in flanges. When Is/Ia >= 1.0, the flange is fully stiffened with k = 4.0. When Is/Ia is between 0.5 and 1.0, the flange k is reduced proportionally. If Is/Ia < 0.5, the lip is ineffective and the flange is treated as unstiffened with k = 0.43, reducing the compression member capacity by 40-60%. A lip that exceeds 0.4 times the flange width may itself buckle locally, so longer lips are not always better — the optimal range balances edge restraint against local lip stability.

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Related references

• How to Verify Calculations
• Steel Tension Members
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• Column Design Guide
• Steel Material Properties
• Steel Section Types
• Load Combinations (ASCE 7)
• Beam Design Guide
• Steel Connection Design

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard (AISI S100, AS/NZS 4600, or EN 1993-1-3) and project specification before use. Cold-formed steel design involves iterative effective width calculations that are sensitive to assumed stress levels, boundary conditions, and fastener details. The worked examples on this page use representative section properties; actual values must be obtained from the manufacturer's certified section property tables or calculated per the applicable code. The site operator disclaims liability for any loss arising from the use of this information.

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