9.2 Light-Gauge Metal Framing
Key Takeaways
- Cold-formed steel framing follows IBC Chapter 22 and AISI S100/S240 (structural) or S220 (nonstructural).
- The universal designator gives web depth (1/100 in), member type letter, flange (1/100 in), and thickness in mils.
- Thickness is specified in mils, not gauge: 33 mil ≈ 20 ga, 43 mil ≈ 18 ga, 54 mil ≈ 16 ga.
- Members are joined with self-drilling self-tapping screws, never field-nailed; exterior uses G90 galvanizing.
- Studs lay out at 16 or 24 in o.c.; stud count = (length/spacing) + 1 plus openings and corners.
9.2 Light-Gauge Metal Framing
Cold-formed steel (CFS), also called light-gauge metal framing, falls under CSI Division 05 40 00. Design references are IBC Chapter 22 and the American Iron and Steel Institute (AISI) S100 North American Specification, plus AISI S240 for the standard. CFS is roll-formed from sheet steel rather than hot-rolled, so it behaves and is detailed very differently from structural steel.
The Universal Designator System
CFS members use a four-part code, e.g. 362S162-54:
| Segment | Meaning | Example |
|---|---|---|
| First 3 digits | Web depth in 1/100 inch | 362 = 3.625 in |
| Letter | Member type | S = stud, T = track, U = channel, F = furring |
| Next 3 digits | Flange width in 1/100 in | 162 = 1.625 in |
| Last 2 digits | Base steel thickness in mils | 54 = 54 mils |
So 362S162-54 is a 3-5/8-inch stud, 1-5/8-inch flange, 54-mil (≈16 gauge) thickness. Trap: depth and flange are in hundredths of an inch, not gauge.
Gauge-to-Mil Conversion
Modern CFS is specified by mil (thousandths of an inch) of base metal thickness, but the field still talks gauge. Memorize the common equivalents:
| Mils | Approx. gauge | Base thickness |
|---|---|---|
| 18 | 25 ga | 0.0179 in |
| 27 | 22 ga | 0.0269 in |
| 33 | 20 ga | 0.0329 in |
| 43 | 18 ga | 0.0428 in |
| 54 | 16 ga | 0.0538 in |
| 68 | 14 ga | 0.0677 in |
The color coding (AISI): a painted ID strip identifies thickness on the member.
Structural vs. Nonstructural
- Nonstructural (interior partition) studs are typically EQ (equivalent) gauge or thin 25/20-gauge, carrying only drywall and lateral wind/partition loads. Governed by AISI S220.
- Structural / load-bearing studs are 33-mil and heavier, carry axial and bending loads, and follow AISI S240/S100. Galvanized coating is commonly G60 (interior) or G90 (exterior/high-humidity), where G90 = 0.90 oz zinc per square foot total both sides.
Spacing, Takeoff, and Bridging
Studs are spaced 16 inches or 24 inches on center (o.c.). Stud count for a wall = (wall length in inches / spacing) + 1, then add studs for openings, corners, and intersections.
Worked takeoff: a 40-foot partition at 16 in o.c. = (480 / 16) + 1 = 31 studs, before adding ~2 per corner and king/jack studs at each door. Track is ordered as top + bottom = 2 x 40 = 80 linear feet.
Bridging/bracing rows control stud rotation and weak-axis buckling and are required at intervals set by the design — commonly one row at mid-height for partition walls and multiple rows for tall load-bearing walls. Cold-rolled channel (CRC) through the stud knockouts plus clip angles, or proprietary flat-strap bracing, satisfies this. Deflection track (slotted or double-track) at the head of non-load-bearing partitions allows the structure above to deflect without loading the studs — omitting it cracks drywall, a frequent callback.
Connections and Common Traps
CFS is joined with self-drilling, self-tapping screws (e.g. #8, #10, #12) — not welded in the field on light members, and never nailed. Bridging/bracing rows control stud rotation and are required at intervals per the design (commonly mid-height for shorter walls).
Exam traps:
- Mils, not gauge, is the code unit; convert correctly (33 mil ≈ 20 ga).
- Track (T) has no return lip; stud (S) has a stiffening lip — do not interchange in a takeoff.
- Fire-rated assemblies require listed screws/spacing; substituting fasteners voids the UL listing.
- A stud must seat fully into the track with a code-limited gap (typically 1/8 in max) at the bearing end, and screws are placed each side of the flange.
- EQ-gauge studs are thinner steel marketed as equivalent to a heavier gauge for nonstructural use only — never substitute EQ for a load-bearing 33-mil-plus member.
A wall framing schedule specifies '600S162-43' studs. What is the web depth and base steel thickness?
For a 32-foot nonstructural partition framed at 24 inches on center, approximately how many vertical studs are required before adding for openings and corners?
Quick Reference
For any CFS question first decide structural vs. nonstructural (AISI S240 vs. S220), then decode the designator, then convert mil to gauge if needed. Fasten with self-tapping screws, protect exterior members with G90 galvanizing, and lay out at 16 or 24 in o.c. Get these four moves right and most Division 05 40 00 questions become arithmetic.
Gauge, Designations, and Connections
Cold-formed steel studs are identified by a SSMA designation like 362S162-43: web depth in 1/100 in (362 = 3-5/8 in), S = stud, flange width (162 = 1-5/8 in), and mil thickness (43 mil ≈ 18 gauge). Lower gauge number = thicker steel. Members connect with self-drilling/self-tapping screws (not nails); structural (load-bearing) studs carry building loads while non-structural (drywall) studs only support finishes. Track (U-channel) caps the top and bottom of stud walls.
Common Exam Traps
- Trap: Higher gauge number = thicker steel. It is the reverse — 18 ga is thicker than 25 ga.
- Trap: Nailing metal studs. Use screws.
- Trap: Using drywall (non-structural) studs in a load-bearing wall.
- Trap: Misreading the designation — the first number is web depth in 1/100 in.
Between 18-gauge and 25-gauge steel studs, which is thicker and better suited to a load-bearing wall?