What is a mullion in a window or curtain wall?

A mullion is a vertical or horizontal structural member that divides and supports glass panels in windows, storefronts, or curtain walls. Mullions transfer wind loads from the glass to the building structure. They must be strong enough to resist bending (deflection) under design wind pressure.

How much can a mullion bend under wind load?

Building codes typically limit mullion deflection to L/175 of the span length (where L is the mullion length). For a 10-foot mullion, maximum allowable deflection would be about 0.69 inches. Excessive deflection can crack glass, break seals, and cause leaks.

What affects mullion wind resistance?

Key factors include: 1) Moment of inertia - the cross-sectional shape and size, 2) Material - aluminum, steel, or fiberglass, 3) Span length - longer mullions deflect more, 4) Tributary area - how much glass load the mullion carries, and 5) Connection details at top and bottom.

Mullion Wind Resistance - Frame Bending Under Load

Why Mullion Design Matters

A mullion is like the spine of a window system. It carries the wind load from the glass panels and transfers it to the building structure at the top and bottom. When wind pushes on the glass, the mullion bends - and this bending (deflection) must be carefully controlled.

Too much deflection can crack the glass, break the weather seals, and cause water leaks. Building codes limit deflection to L/175 of the span length. For a 10-foot mullion, that's about 0.69 inches of maximum allowable bow.

Moment of Inertia

The cross-section shape matters more than total material. A deep, hollow rectangular section resists bending better than a solid square of the same weight.

Span Length

Deflection increases with the FOURTH power of span. Double the span = 16x the deflection! This is why tall storefront windows need beefier mullions.

Tributary Area

Each mullion carries load from half the glass on each side. Wider glass panels = more load on the mullion = more deflection.

Material Choice

Aluminum is common but steel or fiberglass may be needed for high-load situations. Steel is 3x stiffer than aluminum for the same size.

In Monroe County with 180 mph design wind speeds, mullion engineering is critical. The combination of high wind pressure and the desire for large glass openings creates challenging engineering requirements. That's why many storefront and curtain wall systems require structural engineering review.

Mullions Under Pressure

Why Mullion Design Matters

Moment of Inertia

Span Length

Tributary Area

Material Choice

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