Should I use Z-sections or C-sections for roof purlins?

Z-sections are the default choice for roof purlins in metal building systems because of their superior lapping capability in continuous spans. Z-sections nest together at laps (the flanges are offset in opposite directions, so one Z slides inside the next), creating a semi-rigid continuous connection without bolts through the overlap. This lapping reduces midspan deflection by 60-75% compared to simple spans of the same section, which is critical because deflection almost always governs purlin design for spans over 20 ft. The principal axes of a Z-section are inclined approximately 17° from the web, so gravity load produces both strong-axis (normal to roof) and weak-axis (downslope) bending — sag rods or bridging must resist the downslope component. C-sections have principal axes aligned with the web (simpler bending behavior) but cannot nest for lapping. C-sections require specialized splice clips or sleeves for continuous span action, adding fabrication cost and erection time. C-sections are used for: (a) wall girts where nesting is not needed because girts span between columns with simple supports and biaxial bending from wind plus wall dead load; (b) simple-span purlin applications under 20 ft where deflection doesn't govern and lapping provides no significant benefit; (c) projects where the architect prefers the appearance of C-section (flat web, flanges on same side). For a 25 ft purlin span with roof live load of 20 psf and 5 ft tributary width: a simple-span Z10x3-14ga deflects ~14 inches under live load alone (L/21, failing the L/180 limit by factor 8.6×). The same purlin in a lapped continuous 3-span configuration deflects approximately 3.5 inches (L/86), still failing but dramatically better — the fix is deeper purlins (Z12x3.5) or closer spacing.

What is the effective section modulus and why does it differ from Sx?

Cold-formed steel sections have thin elements (thickness typically 0.075" to 0.105" for 12-14 ga purlins) that are susceptible to local buckling before the full cross-section reaches yield. AISI S100-16 accounts for this through the effective width method: the compression flange and portions of the web are reduced to their effective widths (be) based on the plate buckling coefficient k, the width-to-thickness ratio w/t, and the applied stress. The effective section modulus Se = I_effective / (distance to extreme fiber) is less than the full elastic Sx = Ix / y_max because portions of the cross-section buckle and become ineffective. For Z10x3-14ga at Fy = 55 ksi: Sx (full section) ≈ 3.63 in³, Se (effective) ≈ 3.35 in³ — about 8% reduction. For Z12x3.5-14ga: Sx ≈ 4.83 in³, Se ≈ 4.35 in³ — 10% reduction (deeper web is more slender). The reduction increases for: higher Fy (same w/t but buckling occurs before Fy is reached — the applied stress in the effective width formula is limited by Fcr < Fy), thinner gauge (higher w/t), and sections with small lip stiffeners (distortional buckling of the compression flange + lip reduces capacity further). AISI S100 also checks distortional buckling (the flange + lip rotate about the flange-web junction, a mode unique to cold-formed shapes with edge stiffeners) per Section I2.2. For standard Z and C purlin sizes with ASTM A653 SS Grade 55, distortional buckling typically reduces capacity by 5-15% below the local-buckling-only value for depths above 10", making it a critical check for deeper purlins. The combined local + distortional effective section modulus is used for design: φMn = φ × Se_combined × Fy, φ = 0.90 (LRFD).

How are sag rods and bridging designed for purlin systems?

Sag rods resist the downslope (weak-axis) component of purlin loading and provide lateral bracing to the purlin's compression flange. For a roof with slope θ: downslope load = w × sin θ. For a 1:12 slope (θ = 4.76°): sin θ = 0.083, so the downslope component is about 8% of the gravity load — modest but not negligible for spans over 25 ft. Sag rod force = (w × sin θ) × tributary_width × sag_rod_spacing. For purlins at 5 ft o.c., w = 25 psf (dead + live), sag rods at midspan (12.5 ft from support for 25 ft span): rod force = (25×0.083) × 5 × 12.5 = 2.075 × 5 × 12.5 = 129.7 lb — small, a 1/2" rod (6.4 kip capacity) is more than adequate. Standard sag rod arrangement: 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 35-45 ft. Bridging (horizontal U-channel or angle members running perpendicular to the purlins, connecting all purlins in the bay) provides lateral-torsional buckling restraint and distributes sag rod forces to adjacent purlins. Without bridging, each purlin's sag rod force must be resisted by the roof sheeting diaphragm, which is not designed for this concentrated load. SJI/AISI bridging details: bridging rows must align with sag rod rows; bridging must connect to the top and bottom flanges (to restrain torsional rotation); bridging must be continuous across the full bay width and anchored at each end to a fixed point (building frame or braced bay). For standing seam roofs where the sheeting does not provide full bracing of the purlin flange, bridging serves the additional role of flange bracing — the bridging spacing (along the purlin) replaces the unbraced length Lb in the LTB check.

How does through-fastened vs standing seam roofing affect purlin design?

Through-fastened (screw-down) metal roofing provides positive attachment of the roof panel to the purlin flange at every flute (typically 6-12" o.c.), creating a diaphragm that continuously braces the purlin's top flange against lateral movement. AISI S100 Section I6.4.1 recognizes through-fastened sheeting as providing full lateral bracing of the attached flange, permitting k = 1.0 (braced length = 0 for that flange). The purlin's flexural capacity is then governed by the unbraced length of the bottom flange (in compression under uplift) and local/distortional buckling. Through-fastened roofing is the standard for pre-engineered metal buildings. Standing seam roofing uses concealed clips that attach to the purlin flange, allowing the panels to thermally expand and contract without exposed fasteners. The clips provide only partial lateral bracing — the clip stiffness (determined by AISI base test method per S100 Section I6.2) is typically 10-30% of a through-fastened attachment. Standing seam roofs require more conservative purlin design: (a) the unbraced length for LTB is increased (typically taken as the full span for simply supported, or the distance between bridging rows); (b) the roof diaphragm shear capacity is lower (fewer fasteners per square foot), affecting the building's lateral load path; (c) wind uplift resistance relies on clip spacing and clip uplift capacity (per ASTM E1592 testing), with clip spacing often governing the purlin spacing. A common design compromise: use through-fastened roofing for low-slope industrial buildings (cost-optimized), and standing seam for architectural buildings where roof appearance, thermal movement, and long-term weathertightness justify the premium and more conservative purlin design.

What wind uplift provisions apply to purlin and girt design?

Wind uplift reverses the purlin bending direction, putting the bottom flange in compression (roof purlins) or the outer flange (wall girts). For cold-formed steel sections, the compression flange under uplift may be the unstiffened lip edge, which has lower buckling resistance than the stiffened web-flange junction (which is in compression under gravity). The design must check: (1) Flexural capacity with the effective section modulus computed for uplift bending — Se may be different from gravity-load Se because the compression element's boundary conditions differ. (2) Lateral-torsional buckling with the LTB modification factor Cb for the uplift moment diagram (typically a uniform or trapezoidal negative moment, Cb = 1.0-1.3 depending on end fixity). (3) Connection of the purlin to the rafter for net uplift — wind suction can exceed the purlin's dead load hold-down. A typical purlin clip (1/8" bent plate with two bolts to the rafter) must resist the net uplift per ASCE 7 combo 7 (0.9D + 1.0W). For Z10x3-14ga at 5 ft spacing and 25 ft span with wind uplift of -35 psf (C&C corner zone): net uplift = 0.9 × 5 psf (dead) + 1.0 × (−35 psf) = 4.5 − 35.0 = −30.5 psf × 5 ft × 25 ft = −3,813 lb per purlin. The clip and two-bolt connection must resist 3,813 lb in tension. (4) Purlins in metal buildings typically use a 'purlin clip' (bent plate) that provides a bolted connection but is flexible enough to allow simple-span rotation of the purlin end. Under uplift, this clip must transfer the uplift reaction while the purlin's effective unbraced length is the full span (the clip provides no rotational restraint). For continuous lapped purlins, the lap splice detail must also transfer the moment reversal from gravity (positive moment at lap) to uplift (negative moment at lap), which may change the required lap length and bolt pattern.

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

Q: How are sag rods and bridging designed for purlin systems?

Sag rods resist the downslope (weak-axis) component of purlin loading and provide lateral bracing to the purlin's compression flange. For a roof with slope θ: downslope load = w × sin θ. For a 1:12 slope (θ = 4.76°): sin θ = 0.083, so the downslope component is about 8% of the gravity load — modest but not negligible for spans over 25 ft. Sag rod force = (w × sin θ) × tributary_width × sag_rod_spacing. For purlins at 5 ft o.c., w = 25 psf (dead + live), sag rods at midspan (12.5 ft from support for 25 ft span): rod force = (25×0.083) × 5 × 12.5 = 2.075 × 5 × 12.5 = 129.7 lb — small, a 1/2" rod (6.4 kip capacity) is more than adequate. Standard sag rod arrangement: 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 35-45 ft. Bridging (horizontal U-channel or angle members running perpendicular to the purlins, connecting all purlins in the bay) provides lateral-torsional buckling restraint and distributes sag rod forces to adjacent purlins. Without bridging, each purlin's sag rod force must be resisted by the roof sheeting diaphragm, which is not designed for this concentrated load. SJI/AISI bridging details: bridging rows must align with sag rod rows; bridging must connect to the top and bottom flanges (to restrain torsional rotation); bridging must be continuous across the full bay width and anchored at each end to a fixed point (building frame or braced bay). For standing seam roofs where the sheeting does not provide full bracing of the purlin flange, bridging serves the additional role of flange bracing — the bridging spacing (along the purlin) replaces the unbraced length Lb in the LTB check.

Q: How does through-fastened vs standing seam roofing affect purlin design?

Through-fastened (screw-down) metal roofing provides positive attachment of the roof panel to the purlin flange at every flute (typically 6-12" o.c.), creating a diaphragm that continuously braces the purlin's top flange against lateral movement. AISI S100 Section I6.4.1 recognizes through-fastened sheeting as providing full lateral bracing of the attached flange, permitting k = 1.0 (braced length = 0 for that flange). The purlin's flexural capacity is then governed by the unbraced length of the bottom flange (in compression under uplift) and local/distortional buckling. Through-fastened roofing is the standard for pre-engineered metal buildings. Standing seam roofing uses concealed clips that attach to the purlin flange, allowing the panels to thermally expand and contract without exposed fasteners. The clips provide only partial lateral bracing — the clip stiffness (determined by AISI base test method per S100 Section I6.2) is typically 10-30% of a through-fastened attachment. Standing seam roofs require more conservative purlin design: (a) the unbraced length for LTB is increased (typically taken as the full span for simply supported, or the distance between bridging rows); (b) the roof diaphragm shear capacity is lower (fewer fasteners per square foot), affecting the building's lateral load path; (c) wind uplift resistance relies on clip spacing and clip uplift capacity (per ASTM E1592 testing), with clip spacing often governing the purlin spacing. A common design compromise: use through-fastened roofing for low-slope industrial buildings (cost-optimized), and standing seam for architectural buildings where roof appearance, thermal movement, and long-term weathertightness justify the premium and more conservative purlin design.

Q: What wind uplift provisions apply to purlin and girt design?

Wind uplift reverses the purlin bending direction, putting the bottom flange in compression (roof purlins) or the outer flange (wall girts). For cold-formed steel sections, the compression flange under uplift may be the unstiffened lip edge, which has lower buckling resistance than the stiffened web-flange junction (which is in compression under gravity). The design must check: (1) Flexural capacity with the effective section modulus computed for uplift bending — Se may be different from gravity-load Se because the compression element's boundary conditions differ. (2) Lateral-torsional buckling with the LTB modification factor Cb for the uplift moment diagram (typically a uniform or trapezoidal negative moment, Cb = 1.0-1.3 depending on end fixity). (3) Connection of the purlin to the rafter for net uplift — wind suction can exceed the purlin's dead load hold-down. A typical purlin clip (1/8" bent plate with two bolts to the rafter) must resist the net uplift per ASCE 7 combo 7 (0.9D + 1.0W). For Z10x3-14ga at 5 ft spacing and 25 ft span with wind uplift of -35 psf (C&C corner zone): net uplift = 0.9 × 5 psf (dead) + 1.0 × (−35 psf) = 4.5 − 35.0 = −30.5 psf × 5 ft × 25 ft = −3,813 lb per purlin. The clip and two-bolt connection must resist 3,813 lb in tension. (4) Purlins in metal buildings typically use a 'purlin clip' (bent plate) that provides a bolted connection but is flexible enough to allow simple-span rotation of the purlin end. Under uplift, this clip must transfer the uplift reaction while the purlin's effective unbraced length is the full span (the clip provides no rotational restraint). For continuous lapped purlins, the lap splice detail must also transfer the moment reversal from gravity (positive moment at lap) to uplift (negative moment at lap), which may change the required lap length and bolt pattern.

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

| Feature — Z-section — C-section | | ----------------------- — ----------------------------------------------------------------------------- — -------------------------------------------------------- | | Shape — Equal flanges angled in opposite directions — Standard channel shape | | Biaxial bending — Principal axes inclined ~17 degrees to web — bends in both axes under gravity — Principal axes align with web — simpler bending behavior | | Continuous span lapping — Excellent — Z's nest together at laps — Poor — C's cannot nest, require specialized clips | | Shear center — Away from web, outside section — Away from web, outside section | | Lateral stability — Requires bridging/sag rods — Requires bridging/sag rods | | Common use — Roof purlins (lapped continuous spans) — Wall girts, simple span purlins |

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

| Designation — Depth (in) — Flange (in) — Thickness (ga/mil) — Weight (plf) — Ix (in4) — Sx (in3) | | ------------ — ---------- — ----------- — ------------------ — ------------ — -------- — -------- | | Z8x2.5-14ga — 8.0 — 2.5 — 14 ga (0.075") — 3.14 — 10.1 — 2.53 | | Z8x2.5-12ga — 8.0 — 2.5 — 12 ga (0.105") — 4.37 — 13.7 — 3.43 | | Z10x3-14ga — 10.0 — 3.0 — 14 ga (0.075") — 3.78 — 18.1 — 3.63 | | Z10x3-12ga — 10.0 — 3.0 — 12 ga (0.105") — 5.28 — 24.8 — 4.96 | | Z12x3.5-14ga — 12.0 — 3.5 — 14 ga (0.075") — 4.55 — 29.0 — 4.83 | | Z12x3.5-12ga — 12.0 — 3.5 — 12 ga (0.105") — 6.35 — 40.0 — 6.67 |

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

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.
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.
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%.
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.

Cold-formed steel purlin design per AISI S100 — detailed procedure

The AISI S100 specification provides two methods for determining the flexural capacity of cold-formed steel purlins:

Effective Width Method (EWM) — AISI S100 Section B2:

Calculate full (gross) section properties (Ix, Sx, Zx)
Determine the effective width of each compression element using the plate buckling coefficient k and the slenderness ratio lambda = 1.052 × sqrt(f/b/t)
Reduce the compression flange and lip to their effective widths
Recalculate section properties using the effective section (Se = effective section modulus)
phi × Mn = phi × Se × Fy (with appropriate checks for local buckling interaction)

Direct Strength Method (DSM) — AISI S100 Appendix 2:

Calculate full section properties
Determine elastic buckling loads for each mode using a finite strip analysis (e.g., CUFSM software): Mcr (global), Ml (local), Md (distortional)
Calculate nominal capacity for each mode using DSM formulas:
- Global: Mn = Mcr (if lambda <= 0.60) or 1 - 0.15(My/Mcr)^0.4^0.4 × Mcr^0.6
- Local: Mn = My (if lambda_l <= 0.776) or 1 - 0.15(Mcr/Ml)^0.4^0.4 × Ml^0.6
- Distortional: Mn = My (if lambda_d <= 0.673) or 1 - 0.25(Mcr/Md)^0.6^0.6 × Md^0.4
phi × Mn = min(phi × Mn_global, phi × Mn_local, phi × Mn_distortional)

The DSM is increasingly preferred because it captures all three buckling modes explicitly and does not require iterative effective width calculations.

Z-section vs C-section detailed comparison

Property	Z-Section	C-Section
Flange orientation	Equal flanges, opposite direction	One flange has lip, channel shape
Principal axis inclination	~17° from web (biaxial bending under load)	Aligned with web (simpler analysis)
Nesting at laps	Excellent — flanges interlock	Poor — flanges do not nest naturally
Section efficiency	Higher (symmetric about strong axis)	Lower (asymmetric section)
Shear center location	Outside section, eccentric from web	Outside section, eccentric from web
Torsional stiffness	Moderate	Moderate
Optimal use	Roof purlins (continuous, lapped)	Wall girts (simple span), short purlins
Connection at supports	Nest in pairs at laps, creating built-up section	Individual clips or brackets needed
Economy for long spans	Better (lap creates composite action)	Worse (no lapping benefit)
Manufacturer availability	Wide (Butler, Varco-Pruden, Nucor)	Wide (all manufacturers)

Purlin span tables by roof load

Approximate maximum simple spans for Z-section purlins (Fy = 55 ksi) with through-fastened sheeting, L/180 deflection limit under live load:

Purlin Section	15 psf LL	20 psf LL	25 psf LL	30 psf LL
Z8x2.5-14ga	21 ft	19 ft	17 ft	16 ft
Z8x2.5-12ga	25 ft	23 ft	21 ft	20 ft
Z10x3.0-14ga	26 ft	24 ft	22 ft	20 ft
Z10x3.0-12ga	31 ft	28 ft	26 ft	24 ft
Z12x3.5-14ga	31 ft	28 ft	26 ft	24 ft
Z12x3.5-12ga	36 ft	33 ft	31 ft	29 ft

For continuous spans (2-span or 3-span), the above spans can be increased by approximately 30-40% because the continuous moment distribution reduces both the maximum moment and deflection.

Sag rod and bracing requirements

Sag rods serve two purposes: they resist the downslope component of gravity loading on sloped purlins, and they provide lateral bracing to the purlin compression flange (or tension flange for uplift).

Sag rod design loads: For a roof slope of angle theta, the downslope force per purlin per sag rod:

F_downslope = w_gravity × sin(theta) × tributary_width × sag_rod_spacing / number_of_purlins_between_anchorage

Typical sag rod sizes:

1/2" diameter rod for purlin spans up to 25 ft and slopes up to 4:12
5/8" diameter rod for spans 25-35 ft or slopes up to 6:12
3/4" diameter rod for spans over 35 ft or slopes over 6:12

Sag rod anchorage: Sag rods must be anchored at the ridge or eave to transfer the accumulated downslope forces to the primary structure. The ridge purlin is typically the anchor point, with all sag rods spanning downslope to it. The ridge connection must resist the total downslope force from all purlins on one side of the roof.

Bridging requirements: Diagonal bridging (angles or channels) is required in addition to sag rods when:

Standing seam roofing is used (reduced diaphragm stiffness)
Purlin spans exceed 30 ft (simple span) or 40 ft (continuous)
The roof slope exceeds 4:12
Heavy concentrated loads are present (HVAC units, rooftop equipment)

Girt design for wind loads

Girts are the wall-framing equivalent of purlins, spanning between columns to support wall cladding. They are designed for wind pressure and suction (out-of-plane bending) and are typically C-sections for simplicity.

Wind load determination per ASCE 7-22:

Wind pressure q = 0.00256 × Kz × Kzt × Kd × V² × G × Cp (analytical procedure)
For a typical metal building wall: Cp = +0.80 (pressure) or -0.70 / -1.0 (suction at corners)
The controlling load case is often wind suction (uplift on the girt, compression flange unbraced)

Girt design example: Given: 8" C-section girt (C8x2.5-14ga), 25 ft span, 6 ft tributary width, wind pressure = 20 psf. Mu = 1.0 × 20 × 6 × 25² / 8 = 9,375 lb-ft = 9.38 kip-ft = 112.5 kip-in. Required Se = 112.5 / (0.90 × 55) = 2.27 in³. C8x2.5-14ga Se (effective) ≈ 2.0 in³ — slightly short. Use 12ga or reduce span.

Key girt design considerations:

Wind suction reverses the compression flange from the sheeting-connected side to the interior side, requiring bridging or brace points for the unbraced flange
Girts at building corners and edges experience higher wind suction (Cp = -1.4 per ASCE 7 Fig. 30.4-1 for Zone 5)
Deflection limits are often L/120 for metal wall panels and L/180 for masonry veneer

Bridging and tie requirements

Requirement	Specification	Purpose
Flange brace (purlin)	At each support and at midspan	Prevent lateral buckling of bottom flange
Anti-roll device	At each rafter for Z-purlins	Prevent rotation at supports
Flange brace (girt)	At midspan for suction loading	Brace interior flange under uplift
Sag rod spacing	Per span length table above	Resist downslope forces, provide lateral bracing
Diagonal bridging	As required per AISI S100 Section C2	Torsional restraint at discrete points
Eave strut tie	At eave connecting purlins to column	Transfer horizontal thrust
Ridge tie	At ridge connecting purlins across peak	Prevent ridge separation under load

Typical purlin/girt sizes by span

Span (ft)	Typical Purlin (roof, 20 psf LL)	Typical Girt (wall, 20 psf wind)
15	Z8x2.5-14ga	C6x2.0-14ga
20	Z8x2.5-12ga	C8x2.5-14ga
25	Z10x3.0-14ga	C8x2.5-12ga
30	Z10x3.0-12ga	C10x3.0-12ga
35	Z12x3.5-12ga	C10x3.0-10ga
40	Z12x3.5-10ga (continuous)	C12x3.5-10ga (continuous)

These are preliminary selections for typical metal building loads. Final sizing requires a full AISI S100 check including local buckling, distortional buckling, and combined loading effects.

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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.

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Frequently Asked Questions

What is the recommended design procedure for this structural element?

The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyze the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.

How do different design codes compare for this calculation?

AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.