Curtain Wall Mullion Deflection and Required Stiffness

Which is what sizes aluminum framing far more often than strength does. A mullion is a simple beam spanning floor to floor, loaded by the wind pressure on the glass either side of it, so the line load is the pressure times the tributary width -- half a bay each way. Aluminum's elastic modulus is about 10 million psi, a third of steel's, which is why aluminum framing is deflection-governed almost without exception: it has plenty of strength and not much stiffness. The deflection limit is not about comfort. Glass is rigid and its sealant joints have limited movement capacity, so a frame that deflects too far either breaks the glass or opens the seal and leaks, and the industry limit is span over 175 for framing supporting glass on ordinary spans, switching to span over 240 plus a quarter inch on longer ones -- because on a very long span a pure ratio would allow a deflection larger than any sealant can accommodate. Note how hard the span drives it: deflection goes as the span to the FOURTH while the allowance goes as the span, so required stiffness scales as the CUBE. A 10 ft span at 5 ft tributary width and 30 psf needs 4.92 in^4; take the same wall to a 13 ft span and it needs 10.8 in^4, more than double for a 30% longer span, and more still past 13 ft 6 in where the limit tightens. Floor-to-floor height is the single most consequential input in a curtain wall's framing cost. A screen, never a stamp: the glazing system manufacturer's published section properties, the project's own wind design, AAMA's requirements in full, and the engineer of record govern.

line_load_pli = wind_pressure_psf x tributary_width_ft / 12; allowable_deflection_in = span/175 up to a 162 in span, otherwise span/240 + 0.25; required_i_in4 = 5 w L^4 / (384 E delta); moment_in_lb = w L^2 / 8.

Simple-span uniform-load deflection applied to a curtain wall mullion whose line load is the design wind pressure times the tributary width. The deflection limit is AAMA's for framing supporting glass -- L/175 up to a 13 ft 6 in span, L/240 plus a quarter inch beyond it -- cited by name and not reproduced. A screen, never a stamp; the glazing system manufacturer's published section properties, the project's wind design, and the engineer of record govern.

The deflection relation is public mechanics of materials; the wind pressure, span, and tributary width come from the project, and the limit rule is cited to its standard rather than mirrored.

Estimate. AHJ and licensed professional govern.

Field names used by the API: span_in, tributary_width_ft, wind_pressure_psf, modulus_psi, allowable_stress_psi, line_load_plf, allowable_deflection_in, required_i_in4, moment_in_lb

• Deflection limit AAMA L/175 for framing supporting glass up to a 13 ft 6 in span; L/240 + 1/4 in beyond itAAMA curtain wall requirements
• Simple span, uniform load floor-to-floor mullion pinned at both ends; a continuous or spliced mullion is a different modelmechanics of materials
• Stiffness scales as L cubed deflection goes as L^4 and the allowance as L, so span dominates the section sizemechanics of materials