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Structural Glazing Mullion Wind Load Deflection Calculator

Check a window/curtain wall mullion's calculated wind-load deflection against the standard L/175 glazing deflection limit.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
9.84 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Calculated mullion deflection

0.18 in

High confidence

PASSES — calculated deflection (4.52 mm) is within the L/175 allowable limit of 17.14 mm.

L/175 allowable limit
0.67 in

Running these inputs gives 4.52 mm as the calculated mullion deflection. L/175 allowable limit carries the most weight in this calculation, at 17.1 mm. Note that moment of inertia i (×10⁶ mm⁴) sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 0.178 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • AAMA TIR-11 / ASTM E1300: mullion deflection under uniform wind load, treated as a simply-supported beam, Δ = 5wL⁴/(384EI); standard allowable deflection limit for mullions supporting glass is L/175 (for spans up to about 4.1m/13'-6"), keeping frame deflection small enough that the glass doesn't lose edge support or over-stress

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Frequently asked questions

Why is the deflection limit L/175 instead of a fixed number?
The allowable deflection scales with the span so that longer mullions are allowed proportionally more movement — L/175 keeps the deflection-to-span ratio consistent, which is what actually protects the glass from losing edge bearing or being over-stressed, per AAMA TIR-11.
What happens if the mullion fails this deflection check?
Excess mullion deflection can reduce the glass's edge bearing in its pocket, transfer unintended stress into the glass, or affect weatherseal performance. The fix is typically a deeper/stiffer mullion section (higher moment of inertia) or a shorter span (added intermediate support).
Does this deflection limit apply to all mullion spans?
The L/175 limit given here applies to spans up to about 4.1 m (13'-6"); longer spans may be governed by a different code-specified limit or project-specific criteria, so verify against your applicable code and project specification for spans beyond that range.