How are cold-formed steel girts designed for wind pressure?

Cold-formed steel girts are horizontal members spanning between columns that support wall cladding and resist wind pressure. Design follows AISI S100 for cold-formed steel, considering both strength and deflection. For a typical girt spanning 25 ft with 5 ft tributary width under 22.4 psf wind pressure, the line load is w = 22.4 × 5/12 = 9.33 lb/in = 0.112 kip/ft. Maximum moment for simple span is Mu = wL²/8 = 0.112 × 25²/8 = 8.75 kip-ft = 105 kip-in. Required section modulus Sreq = Mu/(φb × Fy) = 105/(0.90 × 50) = 2.33 in³. Common sections include 8Z2.5×105 (Z-purlin at 14 gauge, Sx=3.54 in³, spans to 22 ft), 10Z3.25×105 (Sx=5.30 in³, spans to 27 ft), 12Z3.25×105 at 12 gauge (Sx=7.60 in³, spans to 32 ft). Deflection limits for metal panel cladding are L/120 per IBC Table 1604.3. Z-sections are preferred over C-sections for girts because nesting geometry allows continuous lapping over supports.

When are expansion joints required in steel buildings?

Expansion joints are typically required when steel building length exceeds 300-400 ft without a break, though the exact threshold depends on the annual temperature range and cladding type. Steel expands at α = 6.5 × 10⁻⁶ per °F (11.7 × 10⁻⁶ per °C). Thermal movement ΔL = α × L × ΔT. For 300 ft of building with 80°F temperature range: ΔL = 6.5×10⁻⁶ × 300 × 12 × 80 = 1.88 inches. For 500 ft with 100°F range: ΔL = 3.90 inches. The joint must accommodate calculated movement plus construction tolerance — typical joint widths are 1.5 in. at 200 ft, 2 in. at 300 ft, 3 in. at 400 ft, 4 in. at 500 ft, and 6 in. at 600 ft. Cladding connections must allow thermal sliding: slotted holes in clip angles with slot length at least 1.5× the calculated movement prevent binding. Expansion joints must pass through the entire building envelope including cladding, roofing, and interior finishes. Fixed base columns develop higher thermal restraint forces than pinned bases and may require closer joint spacing.

What deflection limits apply to cladding support members?

Cladding deflection limits vary significantly by material type, per IBC Table 1604.3 and industry standards. Metal panels (standing seam and corrugated) allow L/90 to L/120 — the most lenient, enabling 25-30 ft girt spans. Glass curtain wall per AAMA CW-DG-1 requires L/175, limiting typical girt spans to 15-20 ft. Precast concrete panels per PCI Design Handbook require L/240 (20-25 ft spans). EIFS per EIMA guidelines requires L/240 (20-25 ft spans). Fiber cement panels require L/240 per manufacturer specifications. Brick veneer on steel studs imposes the strictest limit at L/600 to L/720 per BIA Technical Note 28B, which reduces practical span to 10-12 ft and often requires heavier hot-rolled sections instead of cold-formed. Stone veneer also requires L/600 per marble institute standards. The stiffness of the support member, not strength, typically governs girt and purlin design — most cold-formed sections have adequate bending capacity but deflection controls span limits for brittle cladding materials.

How are purlins sized for roof loads in steel buildings?

Purlins span between rafters or trusses, supporting roof cladding. Sizing depends on span, roof load, purlin spacing, and deflection limits (typically L/150 for metal roofing). For 20 psf roof load at 5 ft purlin spacing: 20 ft span uses 8Z2.5×105 (14ga), 25 ft uses 10Z3.25×105 (14ga), 30 ft uses 12Z3.25×105 (12ga). For heavier 30 psf load at 5 ft spacing: 25 ft uses 10Z3.25×105 (12ga, one gage heavier), 30 ft uses 12Z3.25×105 (10ga). Reducing purlin spacing to 4 ft extends span capability — a 12Z3.25×105 (10ga) reaches 35 ft at 25 psf with 4 ft spacing. Cold-formed Z-sections dominate over hot-rolled channels due to higher strength-to-weight ratio (55 ksi yield for cold-formed vs 50 ksi for hot-rolled), nesting geometry allowing continuous lapped splices, and lower cost. Standing seam roof systems exert uplift on purlins — the wind suction case typically controls for light roof dead loads (2-3 psf seam roof) where 0.9D + 1.0W produces net uplift.

What is thermal bridging and how is it addressed in steel building envelopes?

Thermal bridging occurs where steel members penetrate the building's thermal insulation envelope, creating a low-resistance heat flow path. Steel has thermal conductivity approximately 300-400 times that of typical insulation (45 W/m·K for steel vs 0.035 W/m·K for mineral wool), so even small penetrations cause disproportionate heat loss. A steel girt passing through wall insulation can reduce the effective R-value of the wall assembly by 30-50%. Mitigation strategies include: continuous exterior insulation (placing insulation outside the girt line eliminates thermal bridges entirely — the most effective approach), thermal break shims (high-density plastic or fiberglass strips between the girt and cladding, typically 1/4 to 1/2 inch thick), slotted or intermittent connections (reducing the steel cross-section in the thermal path), and insulated metal panel systems with integral foam cores that span between girts and provide a continuous thermal break. ASHRAE 90.1 and the IECC require thermal bridging to be accounted for in building envelope calculations. For cold climates (ASHRAE Climate Zones 6-8), continuous exterior insulation is essentially mandatory for steel-framed buildings to meet energy code requirements.

Steel Building Envelope — Cladding, Girts, and Thermal Movement

The building envelope is the interface between the structural steel frame and the exterior environment. Structural engineers must design secondary steel (girts, purlins, cladding supports) for wind pressure, thermal expansion, and deflection compatibility with the primary frame. Poor envelope detailing is a leading cause of water infiltration and cladding damage.

Girt design for wind pressure

Girts are horizontal members (typically cold-formed C or Z sections, or light hot-rolled channels) that span between columns and support wall cladding. They act as simple or continuous beams loaded by wind pressure perpendicular to the wall.

Common girt sections and capacities

Section	Type	Depth (in.)	Fy (ksi)	Sx (in^3)	Ix (in^4)	Max Span at 20 psf (ft)	Weight (lb/ft)
8Z2.5x105	Z (14ga)	8	55	3.54	13.8	22	3.47
10Z3.25x105	Z (14ga)	10	55	5.30	25.2	27	3.47
12Z3.25x105	Z (12ga)	12	55	7.60	42.5	32	4.62
C8x11.5	Hot-rolled	8	50	8.14	32.6	34	11.5
C10x15.3	Hot-rolled	10	50	12.6	60.0	39	15.3
C12x20.7	Hot-rolled	12	50	17.5	101	44	20.7

Max span assumes simple span, L/120 deflection limit, metal panel cladding, 5 ft tributary width.

Worked example — girt sizing

Given: Wall girt spanning 25 ft between columns. Wind pressure per ASCE 7: q_z * G * C_p = 22.4 psf (net positive) and 15.8 psf (net negative suction). Girt spacing = 5 ft on center. Tributary width = 5 ft.

Step 1 — Line load on girt: w_positive = 22.4 * 5 / 12 = 9.33 lb/in. = 0.112 kip/ft w_negative = 15.8 * 5 / 12 = 6.58 lb/in. = 0.079 kip/ft

Step 2 — Maximum moment (simple span): M_u = w * L^2 / 8 = 0.112 * 25^2 / 8 = 8.75 kip-ft = 105 kip-in.

Step 3 — Required section modulus (assuming compact section, phi_b = 0.90, Fy = 50 ksi): S_req = M_u / (phi_b * Fy) = 105 / (0.90 * 50) = 2.33 in.^3

A C8x11.5 channel (S_x = 8.14 in.^3) or a 10 in. x 2.5 in. Z-purlin at 14 gauge (S_x approximately 3.5 in.^3 per AISI S100) would be adequate. The cold-formed Z-section is more common due to lower cost and nesting efficiency.

Step 4 — Deflection check (L/120 for metal panel cladding): delta_allow = 25 * 12 / 120 = 2.50 in.

For the C8x11.5 (I_x = 32.6 in.^4): delta = 5 * w * L^4 / (384 * E * I) = 5 * 0.00933 * 300^4 / (384 * 29000 * 32.6) = 1.05 in. < 2.50 in. (OK)

Purlin design for roof loads

Purlins span between rafters or trusses, supporting the roof cladding and transferring loads to the primary frame. Cold-formed Z-sections are the most common choice due to their nesting ability and high strength-to-weight ratio.

Purlin sizing by span and load

Span (ft)	Roof Load (psf)	Purlin Spacing (ft)	Recommended Section	Deflection Check
20	20	5	8Z2.5x105 (14ga)	L/150 OK
25	20	5	10Z3.25x105 (14ga)	L/150 OK
25	30	5	10Z3.25x105 (12ga)	L/150 OK
30	20	5	12Z3.25x105 (12ga)	L/150 OK
30	30	5	12Z3.25x105 (10ga)	L/150 OK
35	25	4	12Z3.25x105 (10ga)	L/150 OK

Thermal expansion and movement joints

Steel expands at a coefficient of alpha = 6.5 x 10^-6 per degree F (11.7 x 10^-6 per degree C).

Thermal movement by building length

Building Length (ft)	Temperature Range (deg F)	Thermal Movement (in.)	Joint Required?	Typical Joint Width (in.)
100	60	0.47	No	N/A
200	60	0.94	Marginal	1.5
300	80	1.88	Yes	2.0
400	80	2.50	Yes	3.0
500	100	3.90	Yes	4.0
600	120	5.61	Yes	6.0

Expansion joints are typically required when the building length exceeds 300-400 ft without a joint. The joint must accommodate at least the calculated movement plus construction tolerance.

Cladding connections must allow thermal sliding. Slotted holes in clip angles are common — the slot length should be at least 1.5 times the calculated thermal movement to avoid binding.

Cladding support deflection limits

Deflection limits for cladding support members depend on the cladding type:

Cladding type	Deflection limit	Source	Typical Girt Span
Metal panels (standing seam)	L/120	IBC Table 1604.3	25-30 ft
Brick veneer on steel studs	L/600 to L/720	BIA Technical Note 28B	10-12 ft
Glass curtain wall	L/175 (AAMA)	AAMA CW-DG-1	15-20 ft
Precast concrete panels	L/240	PCI Design Handbook	20-25 ft
EIFS (exterior insulation finish)	L/240	EIMA guidelines	20-25 ft
Metal wall panels (corrugated)	L/90 to L/120	Manufacturer specs	25-30 ft
Stone veneer	L/600	BIA / marble institute	10-12 ft
Fiber cement panels	L/240	Manufacturer specs	20-25 ft

These limits are often more restrictive than the primary-structure deflection limits and frequently govern the design of girts, studs, and mullion framing.

Metal roofing types and spans

Roofing Type	Min. Slope	Span Capability	Typical Gauge	Weight (psf)	Cost Index
Standing seam (structural)	1/4:12	5 ft purlin spacing	24 ga	1.0-1.5	3
Standing seam (architectural)	3:12	2.5 ft deck	24 ga	0.8-1.2	2
Corrugated (exposed fastener)	3:12	2.5 ft spacing	26 ga	0.6-1.0	1
Insulated metal panel (IMP)	1/2:12	6-12 ft span	22 ga + foam	2.5-4.0	4
Metal shingles	4:12	Solid deck	26 ga	0.5-0.8	2.5

Standing seam roofing with concealed clips is the standard for commercial steel buildings. Structural standing seam panels span directly between purlins without a deck substrate.

Curtain wall mullion design

Curtain wall mullions are vertical or horizontal aluminum or steel members that support glazing. Structural engineers often design steel backup mullions for wide spans or heavy glazing loads.

Mullion sizing by span and wind pressure

Span (ft)	Wind Pressure (psf)	Mullion Type	Size	Weight (lb/ft)
10	25	Aluminum	2.5x6 tube	2.1
12	30	Steel tube	HSS4x2x3/16	5.9
15	30	Steel tube	HSS5x3x3/16	7.1
18	35	Steel tube	HSS6x3x1/4	11.6
20	40	Steel tube	HSS6x4x1/4	13.5
24	40	Steel box	Built-up 8x4	18.0

Deflection limit: L/175 for single glazing, L/240 for insulating glass units (IGU).

Thermal bridging mitigation

Steel is highly conductive (k = 45 W/m-K). Thermal bridges at cladding attachments, shelf angles, and steel framing penetrating the insulation layer create significant heat loss and condensation risk.

Detail	Thermal Bridge Heat Loss	Mitigation	Cost Impact
Steel girt through insulation	30-50% of wall R-value	Thermal break clips (offset brackets)	+15-25%
Shelf angle at floor line	20-30% of wall R-value	Stainless steel or FRP shims	+5-10%
Steel stud at parapet	40-60% of wall R-value	Continuous insulation outside studs	+10-20%
Column outside insulation line	50-70% of wall R-value	Thermal break at column connection	+10-15%

Thermal break clips (such as Schöck Isokorb or Armatherm) reduce heat flow by 70-80% at steel connections. Required by ASHRAE 90.1 and IECC for commercial buildings in most climate zones.

Code comparison for envelope design

Aspect	IBC / ASCE 7	EN 1991 / EN 1993	AS/NZS 1170	NBC Canada
Wind pressure on cladding	Components & cladding (C&C) pressures, ASCE 7 Ch. 30	EN 1991-1-4 zones A-E, c_pe values	AS/NZS 1170.2 local pressure factors	NBC + CSA S16 Annex
Thermal expansion coeff.	AISC Manual Table 17-12	EN 1993-1-2 Table 3.1 (12 x 10^-6/C)	AS 4100 Table 1.4 (11.7 x 10^-6/C)	CSA S16 Cl. 7.3 (12 x 10^-6/C)
CFS girt design	AISI S100	EN 1993-1-3	AS/NZS 4600	CSA S136
Movement joint spacing	No code mandate (200-400 ft guideline)	EN 1993 recommends 50-80 m	No specific clause	No specific clause
Deflection limits	IBC Table 1604.3	EN 1993-1-1 Table 7.2	AS 4100 Table B1	NBCC 4.1.8.13
Thermal bridging	ASHRAE 90.1, IECC	EN 1745 thermal performance	NCC Section J	NBC Section 5.4

Key clause references

ASCE 7-22 Chapter 30 — Components and cladding wind pressures (C&C)
AISI S100-22 — Cold-formed steel girt and purlin design
IBC Table 1604.3 — Deflection limits for building components
AISC 360-22 Section L — Serviceability design considerations
EN 1991-1-4 Section 7 — Wind pressure on surfaces and cladding
AAMA CW-DG-1 — Curtain wall design guide, deflection criteria
ASHRAE 90.1 — Thermal bridging requirements

Common mistakes

Using MWFRS wind pressures for cladding design instead of C&C pressures — components and cladding pressures from ASCE 7 Chapter 30 are significantly higher than MWFRS pressures at building corners and edges (GC_p can reach -2.8 at corner zones). Girts in these zones need upsizing.
Neglecting bi-axial bending in girt flanges — girts mounted on sloped walls or carrying the self-weight of the cladding plus wind perpendicular to the wall experience bending about both axes. The weak-axis component can be significant for C-sections and must be checked per AISI S100 C3.3.
Assuming fixed-end conditions at column supports — girts are typically bolted through clip angles with two bolts, providing minimal rotational restraint. Design as simple spans unless the connection is demonstrably moment-resisting.
Ignoring differential movement between primary frame and cladding — the primary steel frame shortens under gravity load (column axial shortening) while the cladding hangs from upper connections. In tall buildings this differential can exceed 1 in. and must be accommodated with slotted connections.
Not providing movement joints in long buildings — thermal expansion of 2-3 inches in a 300 ft building can cause cladding buckling, window breakage, and water infiltration if not accommodated with expansion joints.

Frequently asked questions

What is the difference between C&C and MWFRS wind pressures? MWFRS (Main Wind Force Resisting System) pressures apply to the primary structural frame. C&C (Components and Cladding) pressures apply to individual cladding elements and their attachments. C&C pressures are higher, especially at corners and edges.

How far apart should expansion joints be? For steel-framed buildings, expansion joints are typically needed every 300-400 ft. The joint width must accommodate the calculated thermal movement plus construction tolerance (usually 2-4 inches total).

Should I use Z-sections or C-sections for girts? Z-sections are more common for girts because they nest efficiently for shipping and can be lapped at supports for continuity. C-sections are simpler to connect at ends but provide less efficient material use for continuous spans.

What is thermal bridging in steel buildings? Steel members that penetrate the building insulation layer create thermal bridges, conducting heat directly through the wall. This can reduce the effective wall R-value by 30-50%. Thermal break clips or continuous exterior insulation mitigate this.

What deflection limit should I use for metal wall panels? L/120 for standing seam and corrugated metal panels. L/240 for insulated metal panels (IMP). More restrictive limits apply to brittle claddings like brick (L/600) or stone (L/600).

How do I accommodate column shortening in cladding connections? Use slotted holes in clip angles, with the slot length at least 1.5 times the calculated differential shortening. In buildings over 20 stories, the differential can exceed 1 inch and must be explicitly detailed.

Building envelope components — detailed reference

Envelope components table

The building envelope for a steel-framed structure consists of multiple integrated systems. Each component must be designed for both structural performance and environmental separation.

Component	Function	Typical Material	Design Standard	Connection to Steel Frame
Cladding (wall)	Weather barrier, aesthetics	Metal panel, precast, brick veneer	ASCE 7 Ch. 30 (C&C)	Girt or stud backup
Cladding (roof)	Weather barrier, insulation	Standing seam, corrugated, IMP	ASCE 7 Ch. 30 (C&C)	Purlin or deck
Girts	Horizontal wall support	CFS Z or C, hot-rolled channel	AISI S100 / AISC 360	Bolted to column clips
Purlins	Horizontal roof support	CFS Z-section	AISI S100	Bolted to rafter clips
Curtain wall mullions	Glazing support	Aluminum or steel tube	AAMA CW-DG-1	Steel backup angles
Flashing and trim	Water management	Sheet metal (copper, aluminum, steel)	SMACNA	Mechanically fastened
Weatherproofing	Air/water barrier	Self-adhered membrane, fluid-applied	ASTM E2357, E331	Adhered to sheathing
Insulation	Thermal resistance	Rigid foam, mineral wool, fiberglass	ASHRAE 90.1	Between girts/studs
Vapor barrier	Moisture control	Polyethylene sheet, smart membrane	ASHRAE 160	Interior face of insulation

Cladding types and weight comparison

Cladding Type	Weight (psf)	Panel Span Capability	Thermal Value	Acoustic Rating	Relative Cost	Service Life
Corrugated metal (exposed fastener)	0.8-1.2	2.5-4 ft	Low (R-1 to R-2)	STC 20-25	1.0x	20-30 yr
Standing seam metal	1.0-1.5	4-6 ft	Low (R-1 to R-2)	STC 20-25	2.0x	30-40 yr
Insulated metal panel (IMP)	2.5-4.0	6-12 ft	High (R-6 to R-32)	STC 25-30	3.0x	30-40 yr
Precast concrete	50-80	20-30 ft	Moderate (R-3 to R-8)	STC 50+	4.0x	50-100 yr
Brick veneer	40-50	N/A (supported)	Moderate (R-1 to R-2)	STC 45+	3.5x	50-100 yr
Stone veneer	15-25	N/A (supported)	Low (R-0.5 to R-1)	STC 45+	5.0x	50-100 yr
EIFS (stucco)	2-4	N/A (substrate)	Moderate (R-4 to R-10)	STC 25-30	2.0x	20-30 yr
Glass curtain wall	6-12	4-6 ft (typical lite)	Low (R-1 to R-3 for IGU)	STC 25-35	5.0x	25-40 yr
Fiber cement panel	3-5	2-4 ft	Low (R-0.5 to R-1)	STC 25-30	1.5x	25-40 yr

Thermal performance of envelope assemblies

Assembly	R-Value (ft^2-hr-BTU/in.)	U-Factor	Notes
Metal panel only (no insulation)	R-1 to R-2	0.50-1.00	Not code-compliant for most climate zones
Metal panel + 2 in. fiberglass	R-7 to R-8	0.13-0.14	Minimum for Climate Zone 3-4
IMP (3 in. foam core)	R-21 to R-24	0.04-0.05	Good for Climate Zones 4-6
IMP (4 in. foam core)	R-28 to R-32	0.03-0.04	Good for Climate Zones 5-7
Steel stud + cavity insulation + continuous exterior	R-13 to R-21	0.05-0.08	ASHRAE 90.1 compliant with thermal break
Brick veneer + 2 in. continuous insulation + steel stud	R-15 to R-20	0.05-0.07	High mass, good thermal lag

Moisture control strategies

Strategy	Mechanism	Where Used	Detailing Requirement
Vapor barrier (interior)	Blocks warm-side vapor diffusion	Cold climates (CZ 4-8)	Continuous, sealed at penetrations
Vapor retarder (smart)	Variable perm rating	Mixed climates (CZ 3-5)	Seams sealed, compatible with insulation
Air barrier	Blocks air leakage (carries moisture)	All climates	Continuous around entire envelope
Rain screen	Pressure-equalized cavity drains water	All exterior walls	Cavity min. 3/4 in., vented top and bottom
Drainage plane	Directs water down and out	Behind all cladding	WRB lapped shingle-style
Flashing	Directs water out at openings	Windows, doors, base	End dams, weeps at base

Interface details — steel frame to cladding

Critical interface points where water infiltration commonly occurs:

Interface	Failure Mode	Proper Detail
Girt-to-column connection	Thermal bridge + moisture	Thermal break clip + closed-cell sealant
Base of wall	Water pooling, capillary rise	Base flashing with weep holes, minimum 6 in. above grade
Window head/sill	Water penetration around framing	Subsill flashing with end dams, head flashing with drip edge
Roof-to-wall transition	Wind-driven rain entry	Counter-flashing over wall flashing, continuous seal
Expansion joint	Water and air leakage	Preformed joint cover with backer rod and sealant
Penetration (pipe, duct)	Unsealed gap	Pipe flashing boot, sealant, and weep
Parapet top	Cap flashing failure	Coping with cleat, continuous sealant, drip edge both sides

Steel stud backup wall design

When cladding is brick veneer, EIFS, or fiber cement over steel stud backup, the stud wall must be designed for both structural loads and deflection compatibility.

Parameter	Typical Requirement	Source
Stud size	3-5/8 in. to 8 in. CFS	Based on span and load
Stud gauge	20 ga to 14 ga (33-54 mil)	Design per AISI S100
Stud spacing	16 in. or 24 in. o.c.	Architectural/structural
Deflection limit	L/360 (brick), L/240 (EIFS)	BIA / EIMA guidelines
Bridging/bracing	At 4 ft o.c. max	AISI S100 Section B1
Header design	Deflection head track	Allows slab deflection
Out-of-plane wind	C&C pressures (ASCE 7 Ch. 30)	Individual stud design
Axial load	Limited ( veneer support only)	AISI S100 combined check

Steel stud walls are designed as non-structural backup for the cladding. The primary steel frame carries all gravity and lateral loads. The studs span vertically between floor tracks and are designed for wind pressure and their own dead load only.

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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.