Cold-Formed Steel Connection Design Guide — AISI S100

Cold-formed steel (CFS) connections behave differently from hot-rolled steel connections. The thin sheet steel — typically 0.5 mm to 3 mm thick — introduces failure modes that rarely govern in heavy structural steel: tilting and bearing in screw connections, pull-out of the fastener from the supporting member, and localized buckling at the connection. The governing design standard in North America for CFS connections is AISI S100 — North American Specification for the Design of Cold-Formed Steel Structural Members — with connection provisions concentrated in Chapter J.

This guide covers the four primary CFS connection types: self-drilling screws, bolts in bearing, arc spot (puddle) welds, and power-actuated fasteners. Each has distinct capacity equations, spacing limits, and practical installation requirements. A worked example walks through a typical CFS ledger-to-track screw connection, and the FAQ section addresses the most common questions engineers ask about thin-sheet connection design.

PRELIMINARY — NOT FOR CONSTRUCTION. This guide is for educational reference. All connection designs must be independently verified by a licensed Professional Engineer.

Connection types in AISI S100

Self-drilling screws (Sections J4.3, J4.4)

Self-drilling screws are the workhorse of CFS framing. They drill their own hole and form their own threads in a single operation — no pre-drilling, no nuts. AISI S100 Section J4 covers two distinct limit states: shear in the screw itself and bearing/tilting in the connected sheet steel.

Screw shear capacity (Section J4.3.1):

The nominal shear strength per screw P_ns depends on the screw diameter, material, and whether the shear plane passes through the threaded or unthreaded portion:

P_ns = 0.75 × F_u_screw × A_b   (threads excluded from shear plane)
P_ns = 0.67 × F_u_screw × A_b   (threads included in shear plane)

Where F_u_screw is the manufacturer's reported ultimate tensile strength (typically 1500 MPa for #10 and #12 screws) and A_b is the nominal body area. The resistance factor φ = 0.50 for screw shear — notably lower than the φ = 0.75 used for hot-rolled bolts — reflecting greater uncertainty in thin-sheet connection behavior.

Bearing and tilting capacity (Section J4.3.2):

When the screw bears against the thin sheet, the sheet itself can fail either by the screw tilting (the hole elongates and the screw rotates) or by pure bearing (the sheet material crushes ahead of the screw). The nominal bearing/tilting strength per screw for each connected sheet:

P_nbt = C × m_f × d_screw × t × F_u_steel

Where:

• C = bearing coefficient (from Table J4.3.2-1, depends on d/t ratio)
• m_f = modification factor for connection type (1.0 for double shear, lower for single shear)
• d_screw = nominal screw diameter
• t = base steel thickness of the thinner connected part
• F_u_steel = tensile strength of the connected sheet

For typical #10 screws (d = 4.8 mm) in 1.2 mm G60 steel (F_u = 380 MPa), the bearing capacity is approximately 2–3 kN per screw, and shear capacity is approximately 3–4 kN per screw, making bearing the governing limit state in most CFS connections.

Screw pull-out and pull-over (Section J4.4)

When screws carry tension — such as in ledger-to-track connections where the jamb stud pulls away from the track — two additional limit states govern:

Pull-out (Section J4.4.1): The screw threads strip out of the supporting member:

P_not = 0.85 × t_c × d_screw × F_u  (for t_c < 1.0 mm)
P_not = 1.25 × t_c × d_screw × F_u  (for t_c ≥ 1.0 mm and finer threads)

Where t_c is the thickness of the member into which the screw threads engage. This is why the screw must be long enough to fully engage the supporting steel — if the screw tip barely penetrates, pull-out governs dramatically.

Pull-over (Section J4.4.2): The screw head pulls through the thin sheet:

P_nov = 1.5 × t × d_w × F_u

Where d_w is the washer or head diameter, and t is the sheet thickness. Pull-over governs when the screw head or washer diameter is small relative to the sheet thickness.

Bolts in bearing (Section J4.5)

CFS-to-CFS bolted connections use standard bolts (A307, A325) with nuts and washers on both sides. The failure modes parallel hot-rolled steel bearing design but with two key differences:

1. Bearing factor C: The bearing coefficient in AISI S100 differs from AISC 360 because thin sheets can buckle locally near the bolt hole. The C factor drops sharply when the end distance or bolt spacing is tight.
2. Shear lag in net section: Because CFS members are often singly-symmetric (C-shapes, Z-shapes) and connected through only one flange or web, shear lag (U factor) can reduce the net section fracture capacity by 30–50%.

The edge distance minimums for bolted CFS connections are:

• End distance ≥ 1.5 × d_hole (center of hole to end of sheet in direction of force)
• Spacing ≥ 3.0 × d_hole (center-to-center along the force direction)

These are tighter than AISC bolt spacing minimums because thin sheet buckling adjacent to bolt holes is sensitive to edge restraint.

Arc spot welds (puddle welds) — Section J4.6

Arc spot welds — also called puddle welds — are made by burning through the top sheet to fuse it to the supporting sheet below. They are commonly used to attach steel deck to joists or purlins. The nominal shear strength of an arc spot weld:

P_n = 0.625 × (π/4) × d²_visible × F_xx   (weld metal shear)
P_n = 2.20 × t × d_a × F_u                  (sheet tear-out around weld)

Where d_visible is the visible diameter of the weld (typically 12–16 mm), d_a is the average diameter of the fused zone, and F_xx is the electrode classification strength. The sheet tear-out limit almost always governs for thin deck applications.

Connection spacing and edge distance limits

AISI S100 Table J3.1.1-1 establishes minimum edge distances and spacings that are substantially tighter than AISC 360 requirements because thin-sheet localized buckling is sensitive to restraint. The critical distances for screw connections are:

For a #10 screw (d = 4.8 mm), the minimum end distance is 7.2 mm and the minimum spacing is 14.4 mm. In practice, fabricators specify 12 mm edge distance and 25 mm spacing for constructability.

Corrosion and dissimilar metal considerations

CFS framing is protected by a zinc coating (galvanized, typically G60 or G90 per ASTM A653). When bolts, screws, or welds penetrate this coating, the exposed steel at the hole or weld zone becomes a corrosion initiation point. Two design responses:

1. Screws and bolts must be corrosion-resistant: Carbon steel screws must have a coating meeting ASTM B633 or ASTM F1941. Stainless steel screws (Type 304 or 316) are used in high-corrosion environments.
2. Welds damage the zinc coating: Arc spot welds vaporize the zinc coating around the weld zone. Touch-up paint (zinc-rich primer per ASTM A780) is required on the welded side after welding.

For exterior CFS connections or connections in high-humidity environments (swimming pools, coastal), specify stainless steel fasteners and limit arc spot welds to interior applications only.

Worked example: CFS ledger-to-track screw connection

A 1.2 mm thick G60 galvanized steel ledger track (F_y = 345 MPa, F_u = 380 MPa) is screw-fastened to a 1.5 mm thick CFS jamb stud. The ledger carries a 4.0 kN factored tension load (dead + wind uplift) distributed across four #10-16 self-drilling screws (d = 4.8 mm, F_u_screw = 1500 MPa, washer head diameter d_w = 9.5 mm). Check the screw group capacity.

Step 1 — Screw pull-out (tension on ledger):

The screw threads engage the 1.5 mm jamb stud. Since t_c = 1.5 mm ≥ 1.0 mm:

P_not = 1.25 × 1.5 × 4.8 × 380 = 1.25 × 2,736 = 3,420 N/screw
Design P_not = φ × P_not = 0.50 × 3,420 = 1,710 N/screw

Step 2 — Pull-over (screw head through 1.2 mm ledger):

P_nov = 1.5 × 1.2 × 9.5 × 380 = 1.5 × 4,332 = 6,498 N/screw
Design P_nov = φ × P_nov = 0.50 × 6,498 = 3,249 N/screw

Pull-out governs at 1,710 N/screw. For 4 screws: total pull-out capacity = 4 × 1,710 = 6,840 N = 6.84 kN > 4.0 kN demand. OK.

Step 3 — Screw shear (if lateral load is also present):

Per screw (threads included in shear plane):

P_ns = 0.67 × 1,500 × (π/4) × 4.8² = 0.67 × 1,500 × 18.1 = 18,190 N/screw
Design P_ns = 0.50 × 18,190 = 9,095 N/screw

For a combined tension + shear check, AISI S100 Section J4.5.3 requires an interaction equation:

(T_d / T_n)^2 + (V_d / V_n)^2 ≤ 1.0

If each screw sees 1.0 kN tension and 0.5 kN shear: (1.0/1.71)² + (0.5/9.1)² = 0.342 + 0.003 = 0.345 ≤ 1.0. OK.

Step 4 — Confirm screw spacing:

4 screws at 150 mm spacing on center: 150 mm >> 14.4 mm minimum spacing. 25 mm edge distance >> 7.2 mm minimum. All detailing limits satisfied.

Key takeaways

• CFS screw connections are governed by bearing/tilting in the thin sheet, not by screw shear. The bearing coefficient C from AISI S100 Table J4.3.2-1 accounts for the d/t ratio of the screw diameter to sheet thickness, and for thin sheets (t < 1.5 mm) with #10 or #12 screws, C is typically between 2.5 and 3.5.

• Screw pull-out is the governing limit state for tension connections. The pull-out strength depends directly on the thickness of the member the screw threads engage, not on the screw tensile strength. A #10 screw in 1.0 mm sheet has approximately half the pull-out capacity of the same screw in 2.0 mm sheet.

• Edge distance and spacing minimums are tighter than hot-rolled steel practice because thin sheet buckling adjacent to the connection is sensitive to restraint. For #10 screws, minimum end distance is 3.0 × d_screw (14.4 mm) — roughly half the equivalent 1.5 × d_bolt minimum for hot-rolled bolts.

• Arc spot welds (puddle welds) are common for deck attachment but the sheet tear-out limit almost always governs over weld shear. The fused zone diameter d_a determines the tear-out perimeter, and small puddle welds (< 10 mm visible diameter) have very low capacity in thin sheet.

FAQ

Why is the resistance factor φ lower for CFS connections than for hot-rolled?

The φ = 0.50 for screw shear and bearing in AISI S100 reflects greater uncertainty in thin-sheet connection behavior. CFS connections are more sensitive to installation variability (screw torque, hole alignment, sheet flatness) than hot-rolled bolted connections. Additionally, the statistical basis for the AISI resistance factors comes from a smaller test database than the extensively validated AISC bolt database.

When should I use screws vs. bolts in CFS connections?

Screws are the default for CFS-to-CFS connections up to 6 mm total sheet thickness. They're faster to install (no nuts, no holes to align) and the factory-applied drill point eliminates pre-drilling. Bolts (with nuts and washers) are used when: (1) the connection must be removable, (2) the total sheet thickness exceeds the screw's drilling capacity (typically > 3 mm per ply), or (3) the connection sees cyclic or seismic loading where the nut provides positive clamping.

How many screws do I need for a typical CFS wall bottom track?

A 3 m (10 ft) wall section with 400 mm (16 in) stud spacing has 8 stud-to-track connections. For a bottom track resisting 2.0 kN/m wind load at the base, the total shear demand is 6.0 kN. With #10 screws providing 2.5 kN bearing capacity per screw, two screws per stud (one each flange) provides 2 × 8 × 2.5 = 40 kN capacity — far exceeding the demand. One screw per flange is typically adequate; two are used for redundancy and erection stability.

Can I weld galvanized steel without removing the zinc coating?

No. AWS D1.3 — the Structural Welding Code for Sheet Steel — requires zinc coating removal within 25 mm of the weld zone. Failure to remove the zinc results in zinc vapor entrapment in the weld puddle, causing porosity and reduced strength. After welding, apply zinc-rich paint (ASTM A780) to restore corrosion protection. For thin CFS (t ≤ 1 mm), arc spot welds burn through the coating during welding, but touch-up paint is still required afterward.