Aluminum mullion sections for large glazed openings are typically specified in 6063-T5/T6 or 6061-T6 alloy, with curtain-wall profile cross-sections up to roughly 650 mm for specialized applications and standard mullion depths of 4-10 in (10-25 cm) on visible-glazed systems [S2][S5][S9].
Deflection governs more than ultimate strength on long spans: published design guidance caps support-mullion deflection between L/175 and L/360, with L/360 cited for typical aluminum mullions in commercial glazing [S3]. Thermal movement is a parallel constraint, since a 20 ft aluminum mullion can change length by nearly 1/4 in across a 100°F temperature swing [S6].
What a Mullion Actually Carries in a Glazed Opening
A mullion is the vertical member that divides a window into separate panes or joins adjacent window units, and on large facades it becomes a load-bearing grid that transfers glazing dead load and wind pressure back to the floor structure [S1]. On a curtain wall, vertical mullions span floor-to-floor as the primary structural members, while horizontal transoms support glass edges between them; the depth of the mullion sets the maximum span and load it can carry, so taller floor-to-floor heights and higher wind zones call for deeper profiles [S1][S2].
Aluminum has become the dominant material because its strength-to-weight ratio lets a slim section carry significant glass weight and wind load, its natural oxide layer resists corrosion without painting, and extrusion allows complex cross-sections that integrate glazing channels, thermal breaks, drainage paths, and gasket retention in one profile [S1][S2]. The structural responsibilities are real, not aesthetic: in fenestration systems, these vertical members must transfer glazing dead load while resisting lateral wind pressure on the building's primary structure [S2].
Deflection Limits and the L/175 to L/360 Working Window
Deflection, not yield stress, is usually the controlling criterion for mullion sizing because glass and sealants tolerate only a small mid-span displacement before the lite bears on the frame or the seal ruptures. Engineering guidance puts supporting-mullion deflection at never less than L/180, with L/175 and even L/240 as possible considerations, and an allowable deflection up to L/360 for typical aluminum mullions in fully glazed assemblies [S3].
Wind load is treated as either rectangular or trapezoidal distribution across the mullion tributary area, and the total force is the integral of that pressure diagram; end reactions at the floor anchors and lateral restraint at the glazing bite both feed into the same capacity check [S3][S8]. A 2019 parametric study on long-span mullion sections under wind suction found that lateral restraint and load distribution ratio influenced mullion capacity more than other parameters, underscoring that the connection detail and load shape, not just the profile, drive the answer [S8].
Alloy, Temper, and Section Geometry Compared
Three alloy/temper families dominate the curtain-wall mullion catalog, and the choice sets both the moment capacity and the extrusion feasibility. 6063-T5/T6 is the architectural default, used where complex die shapes and good surface finish matter more than raw strength; 6061-T6 trades extrudability for higher mechanical properties on heavy-span mullions; 6005 and 6082 fill the middle ground when slightly higher strength is needed without the cost jump to 6061 [S5].
Profile geometry matters as much as alloy. GDAA-style curtain-wall extrusions can reach cross-sections up to 650 mm for specialized applications, while stock visible-glazed systems run mullion widths of 2-6 in (5-15 cm) and depths of 4-10 in (10-25 cm), with separate cap profiles that snap over the structural core [S2][S9]. Cap-retention systems let the same structural mullion accept different aesthetic faces without re-engineering the wind-load path, a useful detail when the spec changes finish but not span.
For comparison on the three most common spec axes:
Alloy 6063-T5/T6: moderate yield and ultimate strength, best extrudability for complex thermal-break and gasket channels, lowest cost, default for typical storefront and low-rise curtain-wall mullions [S5].
Alloy 6061-T6: roughly 30-40% higher yield than 6063-T6, more limited die complexity, used where mullion spans or wind pressures push 6063 past its deflection limit on the same section depth [S5].
Alloy 6005/6082: intermediate strength, better corrosion resistance than 6061 in some tempers, common in European curtain-wall specifications where wind zones and floor-to-floor heights fall between the two extremes [S5].
Wind, Dead Load, and Thermal Movement Acting Together
Wind is the headline lateral action, but for long aluminum runs the governing serviceability limit is often thermal expansion. A 20 ft aluminum mullion moves roughly 1/4 in over a 100°F swing, and on a south-facing facade the daily delta can approach that band, so the mullion-to-floor connection must accommodate in-out movement without pumping load into the glass edge seal [S6]. On a curtain wall, dead load from the glazing stack is carried primarily by the vertical mullions back to the floor anchors, while wind pressure is shared between mullion and transom depending on aspect ratio and glazing bite.
Glass selection couples back into mullion design because thicker or laminated glass raises both dead load on the mullion and the lateral pressure the lite transfers under wind. Referenced standard for glass and supporting surface design is ASTM 1300, and wind load on the mullion is typically treated as a rectangular or trapezoidal pressure distribution across the tributary area, with the total force summed accordingly [S3]. Lateral restraint at the glass edge, not just at the floor anchor, controls how much of the wind moment the mullion has to resist on its own [S8].
When Aluminum Mullions Are the Wrong Spec
Aluminum mullions are the wrong call when the fire rating of the glazed opening must be carried by the framing, not just the glass; in that case the spec moves to a fire-rated door-adjacent framing system with tested assemblies rather than a generic aluminum mullion. They are also a poor fit for very short, heavily loaded punched openings where a hot-rolled steel or stainless section is more compact, and for blast or forced-entry rated openings where aluminum's lower hardness forces deeper sections than a steel alternative. [S1]
Aluminum is also the wrong choice when the project specifies a true structural silicone-glazed (SSG) two-sided or four-sided system without a tested mullion backbone, since SSG relies on the mullion for the entire gravity and wind load path with no mechanical bite. In those cases, a glass curtain wall system with manufacturer-tested mullion assemblies is required, not a generic aluminum extrusion chosen only by section depth.
Quality Signals When Sourcing the Extrusion
A quality mullion profile is judged on more than section depth. Look for tight dimensional tolerance on the glazing channel and the thermal-break slot, since a 0.5 mm drift is enough to lose the gasket compression and the drainage path; consistent T5 or T6 temper verified by mill cert rather than a generic "heat-treated" label; and a surface finish matched to the project, with anodizing, powder coating, or PVDF coating selected for the environment, not just the color card [S5][S7].
Powder coating is widely used because it lets the material's inherent strength support large glazing areas with minimal aluminum use, and PVDF or anodized finishes are the right pick for coastal and high-UV exposures where standard powder will chalk [S7]. Sourcing should also confirm that the extrusion supplier controls die design, extrusion, and finishing in-house, because the moment capacity of a complex mullion is sensitive to wall-thickness variation that only shows up in process-capable shops [S5]. For projects that need a coherent envelope package, the mullions should be specified alongside compatible aluminum window and door profiles and metal curtain wall panels so the thermal movement, finish, and tolerance stack are consistent across the facade.
Sourcing, Standards, and What to Verify on the Submittal
Two standards dominate the submittal package for an aluminum mullion on a commercial project. The AAMA/WDMA/CSA 101/I.S.2/A440 North American Fenestration Standard defines mullion assembly types, including integral, combination, and reinforcing mullions with added structural stiffeners that carry or share loads with adjacent framing, and is the basis for most U.S. and Canadian specs [S2]. For glass design and the supporting surface, ASTM 1300 governs the load-resistance of the lite, which in turn sets the reaction the mullion must resist [S3].
For more on the underlying profile-series logic that governs how section depth maps to span, see Aluminium Window Profile Series 50, 60, 70: What the Numbers Actually Mean. On the submittal, verify the alloy/temper on the mill cert, the moment capacity and deflection ratio from the manufacturer's test or calc package, the finish and its rated environment, and the thermal-movement joint detail at floor anchors; any one of these missing turns the spec into a guess.
Track these signals on the next project: whether the supplier provides a moment-capacity test or an engineered calc package for the specific section (not a generic catalog value), and whether the deflection limit specified is L/175, L/240, or L/360, since each step tightens the required section depth and may force a 6063-to-6061 alloy change.