Load Collector: Mullion Capacity for Hurricane Windows
Mullions are the unsung heroes of your curtain wall. They collect wind pressure from every glass panel they touch and channel it to the building structure. Undersize them and your whole wall system fails.
Key factors that determine whether your mullion will survive hurricane winds.
Section Properties
Moment of inertia (I) determines how much a mullion will deflect. Larger, deeper sections have exponentially higher I values. A 4-inch deep mullion has 8x the stiffness of a 2-inch deep mullion.
Deflection Limits
Standard limit is L/175 where L is the span. For an 8-foot mullion, max deflection is 0.55 inches. Excessive deflection allows glass to pop out of glazing pockets and causes seal failures.
Stress Check
Bending stress must stay below allowable values - typically 15-20 ksi for aluminum alloys. The formula: fb = M x c / I, where M is moment, c is distance to extreme fiber, and I is moment of inertia.
Anchor Design
Anchors must transfer full reaction force to the structure. Each end sees half the total load. Steel clips with multiple bolts are typical, sized for shear and tension in the fasteners.
Mullion Design FAQs
Common questions about mullion structural capacity in hurricane zones
What is mullion tributary width and why does it matter?
Mullion tributary width is the total width of glass panels that a mullion supports - typically half the width of each adjacent panel. For example, if a mullion sits between two 4-foot wide glass panels, its tributary width is 4 feet. This width determines how much wind load the mullion must carry.
How do I calculate the load on a mullion?
Mullion load equals: Wind Pressure (PSF) x Tributary Width (ft) x Mullion Span (ft). For a mullion between two 4-foot panels, spanning 8 feet, at 75 PSF: Load = 75 x 4 x 8 = 2,400 lbs total, or 300 PLF distributed along the length.
What is the deflection limit for mullions?
Standard mullion deflection limit is L/175, where L is the span. For an 8-foot (96 inch) mullion, maximum deflection is 96/175 = 0.55 inches. Some specifications require L/240 for better glass edge protection.
When do mullions need steel reinforcement?
Mullions need steel reinforcement when standard aluminum sections cannot meet strength or deflection requirements. This commonly occurs with spans over 8-10 feet, wind pressures above 80 PSF, or wide tributary widths over 5 feet.
How are mullion anchors designed for hurricane loads?
Mullion anchors must transfer the full wind reaction force to the structure. Design includes: reaction force calculation at top and bottom, shear and tension capacity of fasteners, base plate or clip design, and connection to the building structure.
Calculate Your Mullion Wind Loads
Get precise wind pressure values for your curtain wall system. ASCE 7-22 compliant calculations for Miami-Dade HVHZ permit approval.