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.

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: wpositive = 22.4 * 5 / 12 = 9.33 lb/in. = 0.112 kip/ft wnegative = 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)

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). For a 300 ft long building with a 120 degree F temperature range:

Delta*L = alpha * L _ Delta_T = 6.5e-6 _ 300 _ 12 * 120 = 2.81 in.

Expansion joints are typically required when the building length exceeds 300-400 ft without a joint, depending on the framing system and cladding type. The joint must accommodate at least the calculated movement plus a 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
Metal panels (standing seam)	L/120	IBC Table 1604.3
Brick veneer on steel studs	L/600 to L/720	BIA Technical Note 28B
Glass curtain wall	L/175 (AAMA)	AAMA CW-DG-1
Precast concrete panels	L/240	PCI Design Handbook
EIFS (exterior insulation finish)	L/240	EIMA guidelines

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

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

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

Topic-specific pitfalls

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.

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Related references

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.