Facade · worked example · 3 pages

Facade panel and supporting mullion

A glazed panel spans between vertical mullions; each mullion spans floor to floor and hands the wind load to slab fixings. From a specified design wind pressure this sheet follows that load path: tributary width, mullion bending and deflection, and the fixing reactions. The pressure is a GIVEN here — deriving it for a real site is a wind assessment to AS/NZS 1170.2, outside this example.

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What this calculation covers

Givens

Specified design wind pressure
Mullion span (floor to floor)
Mullion spacing = panel width
Mullion second moment (catalogue)
Mullion section modulus (catalogue)
Aluminium elastic modulus
Limit stress, 6063-T6 (example)
Capacity factor (example)

Load path: panel to mullion

Each mullion carries half a panel each side — one full panel width of wind.

Tributary area per mullion span
Line load on one mullion
Total wind force per mullion span

Mullion bending and deflection

Midspan bending moment
Design bending capacity
Mullion strength adequate
Midspan deflection
Facade deflection limit L/175
Deflection within limit

Fixing loads

Reaction at each floor fixing
Force per anchor (two per bracket)

A shallower mullion for comparison

Deflection for a mullion with second moment I
Shallower section (I = 4×10⁶ mm⁴)
Shallower mullion passes?

Results summary

At 2 kPa each mullion carries 10.8 kN of wind: 4.86 kN·m of bending (60% of capacity) and 15.6 mm of deflection against a 20.6 mm L/175 limit. Each floor bracket must resist 5.4 kN. The shallower 4×10⁶ mm⁴ mullion deflects 23.4 mm and fails serviceability — stiffness, not strength, sizes the section.

Try changing…

Widen the spacing to 1.8 m — load, moment and deflection all rise a fifth; deflection nears its limit.
Raise the pressure to 2.5 kPa (a suction peak) — deflection reaches 19.5 mm, still inside L/175.
Stretch the span to 4.0 m — deflection grows with L⁴ (×1.52 for +11%) and the check fails.

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