Framing Systems and Structural Performance
This domain tests how frames, mullions and anchors are sized and proven against wind and movement. Expect questions on converting wind pressure to design pressure, applying deflection limits, understanding test pressure versus design pressure, and recognising when movement accommodation governs detailing.
From wind load to design pressure
ASCE 7 provides velocity pressure and coefficients; the design professional converts these into a design pressure in psf for each fenestration area. NAFS then requires the product to be rated at or above that value in an appropriate performance class. Pressures are positive (inward) and negative (outward); negative pressures usually govern because glass and frames behave differently in suction.
Deflection and structural testing
- ASTM E330 is the uniform static load test. NAFS structural testing runs at 1.5 times the design pressure.
- A common serviceability limit for mullions and framing is L/175 of the span. Glass deflection is checked separately under ASTM E1300.
- Anchorage, embed and fastener capacity must exceed the reactions the mullions deliver, including eccentricities.
- Movement: thermal expansion of long aluminum runs, live-load and wind drift, and seismic interstorey drift (AAMA 501.4 static drift test) must be accommodated by slotted connections and expansion joints.
Worked example
Mullions at 5 ft on centre spanning 8 ft, design pressure 40 psf. Uniform load = 40 psf x 5 ft = 200 lb/ft. Convert: 16.7 lb/in over a 96 in span. Deflection limit L/175 = 0.55 in. Using delta = 5wL^4/(384EI) with E = 10.1 x 10^6 psi for aluminum, the required moment of inertia is about 3.3 in^4, so a mullion with I of at least that value passes. The structural test for this assembly would run at 1.5 x 40 = 60 psf.
Traps that cost marks
- Confusing test pressure (1.5 x DP) with design pressure when reading a report.
- Checking only glass or only framing; both must pass, and the weaker one governs the rating.
- Forgetting negative pressure governs many cladding designs.
- Detailing rigid anchors that restrain thermal and drift movement, producing cracked glass or failed seals.
- Ignoring dead load and glass weight in head and sill design.