Steel Purlins & Girts — Z vs C Sections, Sag Rod Bracing & Span Tables

Purlins and girts are secondary framing members in metal building systems. Purlins support the roof sheeting and span between primary frames (rafters or trusses); girts support the wall sheeting and span between columns. Both are typically cold-formed Z-sections or C-sections made from high-strength galvanized sheet steel (ASTM A653 Gr 55, Fy = 55 ksi). Their design is governed by AISI S100-16 (cold-formed steel) rather than AISC 360.

Z-section vs C-section

Key Z-section issue: Because the principal axes are inclined, a gravity load on a Z-purlin creates both downslope (weak-axis) and normal (strong-axis) bending components. The downslope component must be resisted by bridging, sheeting restraint, or sag rods. Without these, the purlin rolls and deflects laterally.

Common purlin sizes

Design method — purlin flexural capacity

Per AISI S100-16, the flexural capacity of a Z-purlin is:

phi_b × Mn = phi_b × Se × Fy    (for yielding-controlled sections)

Where phi_b = 0.90 (LRFD) and Se = effective section modulus accounting for local buckling of compressed elements. For most standard purlin sizes at Fy = 55 ksi, local buckling of the compression flange lip reduces Se below the full Sx. Distortional buckling may further reduce capacity for deeper sections with small lips.

Lateral-torsional buckling is typically prevented by the roof sheeting on the compression flange (through-fastened or standing seam). Through-fastened sheeting provides full lateral bracing of the connected flange per AISI S100 Section I6.4.1. Standing seam sheeting provides partial bracing that must be quantified by the base test method (AISI S100 Section I6.2).

Worked example — roof purlin design

Given: Z10x3-14ga purlin (Fy = 55 ksi), simple span 25 ft, tributary width 5 ft, roof dead load = 5 psf, roof live load = 20 psf, roof slope = 1:12. Through-fastened metal sheeting on compression (top) flange.

Step 1 — Loading: wD = 5 × 5 = 25 plf. wL = 20 × 5 = 100 plf. wu = 1.2 × 25 + 1.6 × 100 = 190 plf = 0.190 klf. Mu = 0.190 × 25² / 8 = 14.84 kip-ft = 178 kip-in.

Step 2 — Effective section modulus: For Z10x3-14ga with through-fastened sheeting: Se ≈ 3.35 in³ (reduced from Sx = 3.63 due to effective width reduction in compression flange and lip).

Step 3 — Flexural capacity: phi_b × Mn = 0.90 × 3.35 × 55 = 165.8 kip-in.

178 > 165.8 — FAILS. Increase to Z10x3-12ga: Se ≈ 4.70 in³. phi_b × Mn = 0.90 × 4.70 × 55 = 232.7 kip-in > 178 — OK.

Step 4 — Deflection check (L/180 for roof live load): delta_allow = 25 × 12 / 180 = 1.67 in. delta_L = 5 × wL × L^4 / (384 × E × I) = 5 × 0.100 × (300)^4 / (384 × 29500 × 24.8) = 5 × 0.100 × 8.1 × 10^9 / (2.81 × 10^8) = 4.05 × 10^9 / 2.81 × 10^8 = 14.4 in — FAILS badly.

This illustrates a critical purlin design reality: deflection almost always governs over strength for simple spans above 20 ft. The solution is to use continuous spans (lapped at supports), which reduce midspan deflection by 60-75%.

Sag rods and bridging

Sag rods are tension rods (typically 1/2" or 5/8" diameter) that span between adjacent purlins at one or more points along the span to resist the downslope component of loading and provide lateral bracing.

Sag rod spacing rules of thumb:

• One row at midspan for spans up to 25 ft
• Two rows at third points for spans 25–35 ft
• Three rows at quarter points for spans over 35 ft

Bridging (diagonal angles connecting adjacent purlins) is an alternative to sag rods that provides both lateral bracing and torsional restraint. Required per AISI S100 Section C2 when sheeting bracing alone is insufficient (common for standing seam roofs).

Continuous span design

Most metal building purlins are designed as 2-span, 3-span, or multi-span continuous members with lapped connections at interior supports. Lapping (overlapping the top and bottom Z-sections by 2–4 ft at the support) creates a built-up section with approximately double the moment of inertia at the high-moment support region.

Benefits of continuous spans:

• Interior support negative moment = wL²/10 (vs wL²/8 for simple spans)
• Midspan positive moment = wL²/14 to wL²/16 (depending on lap length)
• Deflection reduced by 60–75% compared to simple spans
• Total purlin weight reduced by 20–30% for the same span and load

Code comparison

AISI S100-16 (USA/Canada): Primary standard for CFS purlins. Effective width method or Direct Strength Method (DSM). Through-fastened sheeting bracing per Section I6.4.1. Standing seam bracing per base test method (Section I6.2). phi_b = 0.90 for flexure.

AS/NZS 4600:2018 (Australia/NZ): Closely aligned with AISI S100. Purlin design per Section 3.3 (flexural members). Through-fastened sheeting restraint provisions. Australian practice commonly uses Lysaght or Stramit proprietary purlin systems with manufacturer-published span tables based on AS/NZS 4600.

EN 1993-1-3 (Eurocode 3): Cold-formed members per Part 1-3. Effective cross-section properties per Section 5.5. Lateral restraint from sheeting per EN 1993-1-3 Section 10. Eurocode uses a more complex interaction formula for combined bending and axial force in purlins on sloped roofs, explicitly accounting for the biaxial bending in Z-sections.

Common mistakes engineers make

1. Designing Z-purlins as simple spans when continuous spans are feasible. Simple span Z-purlins are grossly inefficient for spans above 20 ft — deflection governs and requires oversized sections. Continuous spans with laps at supports are standard practice in metal building design.

2. Ignoring the downslope bending component in Z-sections. On a 4:12 roof slope, approximately 15% of the gravity load acts as a downslope (weak-axis) force. Without sag rods or bridging, this force causes the purlin to twist and deflect laterally, reducing strong-axis capacity and causing visible deformation.

3. Assuming through-fastened sheeting always provides full bracing. Through-fastened (screw-attached) sheeting braces the connected flange effectively, but standing seam sheeting provides only partial restraint because the clips allow the panel to slide. Using full bracing capacity with standing seam sheeting unconservatively overestimates purlin capacity by 15–30%.

4. Using AISC 360 for purlin design. Purlins are cold-formed members — their thin elements buckle locally and distortionally in ways that AISC 360 does not address. Using AISC column curves or beam flexure equations for CFS purlins produces incorrect capacities. AISI S100 must be used.

Run this calculation

• Section Properties Database
• Beam Calculator

Related references

• How to Verify Calculations
• roof load combinations
• portal frame primary members
• cold-formed steel design
• steel member weight calculator
• Cold Formed Sections
• Roof Framing

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 and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.