What is the difference between a purlin and a wall girt?

A purlin supports roof sheeting and transfers gravity and wind loads to primary framing. A girt supports wall cladding and transfers wind pressure and suction to columns. Both are cold-formed C- or Z-sections, but girts experience load reversal from wind on windward and leeward walls, requiring checks for both pressure and suction conditions. Girts also resist cladding self-weight as minor-axis bending simultaneously with major-axis wind bending.

Why is the effective width method critical for cold-formed steel girts?

Cold-formed C-sections like C15012 have slender elements with width-to-thickness ratios far exceeding hot-rolled limits. The C15012 web has h/t = 118 versus the stiffened element limit of 31.1 at Fy = 50 ksi. The effective width method per AISI S100 B2 reduces the gross section properties by calculating effective widths for each buckled element. For the C15012, the effective section modulus is approximately 80 percent of the gross value. Neglecting this reduction overestimates capacity by 25 percent or more.

When does combined bending control wind girt design?

Combined bending controls when cladding self-weight produces significant minor-axis bending simultaneously with wind suction. For C20024 at 4 ft spacing and 25 ft span, the major-axis utilisation was 84.4 percent and minor-axis utilisation was 58.3 percent, producing a combined interaction of 1.427 that exceeded 1.0. The C25024 at 4 ft spacing barely satisfied the interaction at 99.6 percent utilisation. For lightweight single-skin cladding, minor-axis bending may be negligible, but for insulated metal panels or heavy cladding, it is often the controlling check.

Wind Girt Design Worked Example — C15012 Cold-Formed Suction Combined Bending

AISI S100 / ASCE 7-22 Cold-Formed Wind Girt Design — C15012 Worked Example

Complete step-by-step design of a cold-formed C-section wall girt per AISI S100-16 (2020 Ed.). Covers wind pressure and suction per ASCE 7-22, combined bending (major axis wind + minor axis cladding weight), bridging design for the compression flange, and serviceability deflection limits.

Related pages: Cold-Formed Steel Guide | Wind Load Design Guide | ASCE 7 Wind Load Worked Example

Problem Statement

Design a horizontal wall girt in a pre-engineered metal building in Exposure B (suburban terrain). Building: 100 ft x 60 ft x 20 ft eave height. Girts span 25 ft horizontally between main frame columns, supporting 26-gauge through-fastened ribbed metal wall panels.

Design data:

Girt: C15012 (C6x2.0) cold-formed C-section, ASTM A653 Grade 50
t = 0.048 in. (18-gauge), Fy = 50 ksi, Fu = 65 ksi
Span: L = 25 ft, tributary height: h = 5 ft
Cladding: 1.8 psf, insulation: 0.5 psf
Basic wind speed: V = 115 mph, Exposure B
Deflection limit: L/240 for wind load
Girt orientation: open side facing interior, outer flange continuous with wall cladding

Step 1 — Section Properties (C15012)

Property	Value	Unit
Ag	0.504	in^2
Ixx	2.83	in^4
Sxx (gross)	0.943	in^3
Iyy	0.176	in^4
Syy	0.098	in^3
rx	2.37	in.
ry	0.591	in.
x0	0.90	in.

Effective width analysis (AISI S100 B2): Web h/t = 118 >> 31.1 limit — web is slender. Flange w/t = 35.8, rho = 0.924 (flange partially effective). Web under pure bending: k = 24, rho = 0.756.

Sxx_eff approx = 0.80 x 0.943 = 0.754 in^3. Mnx = 37.7 kip-in., phi x Mnx = 33.9 kip-in.

Step 2 — Wind Load (ASCE 7-22 Chapter 30 Part 2)

Velocity pressure: qh = 0.00256 x 0.70 x 115^2 = 23.7 psf

Zone 4 (interior wall), effective area 125 ft^2: p_net (suction) = 23.7 x (-0.95 - 0.18) = -26.8 psf

Corner Zone 5: p_net (suction) = 23.7 x (-1.40 - 0.18) = -37.4 psf

Factored suction load: wu = 0.9 x (1.8 x 5 + 1.71) + 1.0 x (-26.8 x 5) = 0.9 x 10.71 - 134 = -124.4 plf = 0.124 klf

Mx* = 0.124 x 25^2/8 = 9.69 kip-ft = 116.3 kip-in. >> 33.9 kip-in. — section grossly undersized! (343% utilisation)

Step 3 — Redesign Iterations

Try C20019 at 4 ft c/c: wu = 0.098 klf, Mx* = 91.9 kip-in. Sxx_eff = 1.91 in^3, phi x Mnx = 86.0 kip-in. Still over: 107%.

Try C20024 at 3.5 ft c/c or C25024 at 4 ft c/c with combined bending check:

For C25024 at 4 ft c/c: Mx* = 91.9 kip-in., phi x Mnx = 144 kip-in. — 63.8%

My* (cladding weight) = 0.0112 x 25^2/8 x 12 = 10.5 kip-in. phi x Mny = 29.3 kip-in. — 35.8%

Combined bending: 91.9/144 + 10.5/29.3 = 0.638 + 0.358 = 0.996 < 1.0 — OK.

Step 4 — Bridging Requirements

Under wind suction, the inner (unclad) flange is in compression. Bridging per AISI S100 D3.2:

P_br = 0.02 x Mx* / (d x phi) = 0.02 x 91,900 / (10.0 x 0.75) = 245 lb per bridging point — negligible for standard sag rods.

Minimum 2 bridging rows at L/3 = 8.33 ft spacing.

Step 5 — Deflection Check

Wind service load: w = 26.8 x 4 = 107.2 plf. For C25024 (Ixx = 18 in^4): delta = 1.78 in. > L/240 = 1.25 in. — FAIL.

Try C30024 (Ixx = 32 in^4): delta = 1.00 in. < 1.25 in. — OK.

Summary

Limit State	C15012 at 5 ft	C20024 at 4 ft	C25024 at 4 ft	C30024 at 4 ft
Mx*/Mnx	343% (FAIL)	84.4%	63.8%	~45%
My*/Mny	—	58.3%	35.8%	~25%
Combined bending	FAIL	142.7% (FAIL)	99.6%	~70%
Deflection L/240	—	—	142% (FAIL)	80%

Conclusion: Wind girt design for a 25 ft span at Exposure B, 115 mph, requires careful iteration through multiple limit states. The C15012 is far too light. The C30024 at 4 ft spacing satisfies all AISI S100 and ASCE 7-22 requirements. Corner zone girts require separate design for higher suction coefficients (GCp = -1.40).