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Curtain Wall Wind Load & Deflection Calculator Façade Engineering
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Curtain Wall Wind Load & Deflection Calculator

Determine required aluminum mullion moment of inertia (Ix), allowable deflection limits (L/175 and L/240), and bending stress under ASCE 7 wind pressures.

Anchor floor-to-floor span
Glass bay tributary width
Components & Cladding (C&C)
Moment of inertia from extrusion catalog
Required Moment of Inertia (Ix Req)
10.74 in⁴
Selected section PASSES deflection check (Ix = 12.50 in⁴)
Actual Deflection
0.707 Inches
L / 204 Ratio
Allowable Deflection
0.750 Inches
Cap: 0.75" at glass edge
Total Tributary Wind Force: 2,100 lbs / Mullion
Distributed Uniform Load (w): 14.58 lbs / linear inch
Maximum Bending Moment (M): 37,800 in-lbs (3,150 ft-lbs)
Estimated Minimum Extrusion Depth: ∼6.0" - 7.0" Profile
Structural Safety Ratio: 1.16 (16% Reserve)
Evaluated according to AAMA TIR-A11 & ASTM E1300 curtain wall design standards.

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Curtain Wall Engineering: Wind Loads & Mullion Deflection

Unitized and stick-built curtain walls must safely transfer lateral wind pressures from building façades back into the primary floor slabs without breaking glass or causing silicone seal failures. Sizing vertical aluminum mullions is governed almost entirely by serviceability deflection limits rather than yield stress, because excessive bowing pinches insulated glass units (IGUs) against aluminum framing pockets.

Beam Bending Governing Equations

  1. Uniform Distributed Line Load (w):
    w (lb/in) = [ Wind Pressure (PSF) × Mullion Spacing (ft) ] / 12
  2. Allowable Deflection (Δ_allow):
    Per AAMA and international building codes (IBC):
    For spans ≤ 13.5 ft (162 inches): Δ_allow = Span / 175 (capped at 0.75" or 19 mm to preserve glass bite).
    For spans > 13.5 ft: Δ_allow = Span / 240 + 0.25".
  3. Required Moment of Inertia (Ix_req):
    For a simply supported vertical beam:
    Δ = (5 × w × L^4) / (384 × E × Ix)
    Rearranging for required stiffness:
    Ix_req = (5 × w × L^4) / (384 × E × Δ_allow)
    Where E = 10,100,000 psi for 6063-T6 architectural aluminum.
  4. Maximum Bending Moment (M):
    M = (w × L^2) / 8 (in-lbs)

Frequently Asked Questions

Why is curtain wall mullion deflection capped at 0.75 inches (19 mm)?

Even on very tall 20-foot spans where L/175 allows 1.37 inches of bow, building codes enforce an absolute 0.75-inch cap. Excessive center-span deflection pulls glass edges out of the glazing pocket (loss of glass bite) or causes corner point-loading that shatters tempered glass.

How does steel reinforcement inside an aluminum mullion help?

Structural steel has a modulus of elasticity (E = 29.0 × 10⁶ psi) nearly three times stiffer than aluminum (E = 10.1 × 10⁶ psi). Inserting a steel bar or tube into the hollow core of an aluminum mullion quadruples its stiffness without requiring a deeper, more expensive exterior extrusion.

What is the difference between positive and negative wind pressure on a façade?

Positive wind pressure pushes directly inward against the windward face of the building. Negative wind pressure (suction) pulls outward on the leeward face and along building corners due to aerodynamic vortex separation. Negative corner suction pressures are often 50% to 100% higher than positive pressures.

What alloy is standard for curtain wall extrusions?

6063-T6 aluminum is the universal architectural industry standard due to its excellent surface finish for anodizing and powder coating, good corrosion resistance, and adequate yield strength (Fy = 25 ksi).