How Are Curtain Wall Mullions Sized?

Curtain wall mullion sizing is an important part of designing a safe, efficient, and visually consistent building facade. The right mullion size must support the specified wind loads, control deflection, accommodate the glazing system, and meet the project's architectural requirements.

However, there is no single mullion size that works for every curtain wall. A profile suitable for a low-rise commercial building may not provide sufficient stiffness for a taller facade with larger glass panels.

For architects, developers, contractors, and procurement teams, understanding the sizing process helps identify the right aluminum profile and avoid unnecessary material costs or performance problems.

What Determines Curtain Wall Mullion Sizing?

Curtain wall mullion sizing relies on several key design factors, not just floor height:

• Mullion Span: The span is the distance between structural support points. Longer spans increase bending, which may require a deeper profile, reinforcement, or adjusted supports.
• Design Wind Loads: Wind pressure is a primary input and varies based on building height, location, and codes. Profiles must be checked for specific inward and outward pressures, never using generic values from other projects.
• Glass Width (Tributary Width): The glass panel width determines the load transferred to the mullion. For internal mullions, this is typically half the distance to adjacent mullions. Edge and corner mullions require separate calculations due to different load distributions.
• Deflection Criteria: Mullions need adequate stiffness, not just strength. Excessive bending can compromise glass, seals, and weatherproofing. Allowable limits are set by building codes and industry standards like AAMA TIR-A11-15.
• Glass Weight & Other Loads: The system must account for the actual load path, including glass dead load, building movement, and thermal expansion. Engineers must identify the specific forces acting on each member rather than assuming uniform loads.

Curtain Wall Dimensions: Width, Depth, Thickness, and Span

Mullion dimensions describe different features of the frame. A narrow visible face does not necessarily mean a shallow or lightweight structural section. PRANCE’ s guide to curtain wall components explains why elevation drawings alone do not reveal framing depth, reinforcement, or connections.

Curtain Wall Mullion Design Loads: What Must Be Included?

Wind pressure is often the governing load for mullion design. Glass and opaque panels receive wind pressure or suction and transfer it through the framing and anchors to the building structure.

The design pressure must reflect the project location, elevation, exposure, and facade zone. Corner areas may experience higher suction than central wall areas, so a single pressure should not automatically be applied to every elevation.

Other relevant actions can include curtain wall self-weight, seismic effects, and loads associated with building-maintenance equipment. Their relevance and load paths depend on the selected system. The STRUCTURE Magazine design guide identifies these as common considerations alongside wind.

Glass weight also needs a defined support path. Setting blocks transfer the glass weight to its supporting framing; transom connections, mullions, and anchors must then be checked for the loads assigned to them.

How Is Curtain Wall Mullion Span Calculated?

The mullion span is determined by the distance between its effective supports and the structural behavior of the selected framing system.

In a simplified floor-to-floor arrangement, a vertical mullion may be modeled as a beam spanning between support points at successive floors. Other arrangements may use continuous members, different bracket conditions, or specialized unitized connections.

Before calculating a curtain wall mullion span, confirm:

• The positions of each structural support
• The components are simply supported, continuous, or cantilevered
• Whether multi-span or diverse load conditions need to be considered
• The ability of the connecting parts to transfer forces and accommodate displacements

These details matter because the same physical mullion length can produce different structural behavior under different support conditions.

Select the Right Curtain Wall Mullion Depth

Mullion depth selection is primarily a question of achieving the required stiffness and structural performance while meeting architectural and construction constraints.

A deeper profile can increase bending stiffness when the cross-section is designed effectively. However, depth alone does not establish whether a mullion is adequate.

The selection process should consider:

• Required section properties about the relevant bending axis.
• Design wind pressure and mullion span.
• Deflection criteria.
• Glass dimensions and edge-support requirements.
• Connection and anchorage conditions.
• Thermal movement and adjacent interfaces.
• Architectural sightlines and available installation space.

The engineer should compare calculated demands against the manufacturer's documented section properties and the relevant design criteria.

A larger profile is not automatically the best solution. Depending on the project, a different extrusion geometry, reinforcement, support arrangement, or mullion spacing may provide a more efficient design.

For projects using aluminum framing and architectural glass, the PRANCE aluminum curtain wall system is a relevant product reference when reviewing facade system options. Actual profile dimensions and structural properties should be confirmed through project-specific technical documentation.

Case Study: Hidden-Frame Glass Curtain Wall for a Hospital in Saudi Arabia

PRANCE's published Saudi Arabia Hospital Project includes a hidden-frame glass curtain wall with an aluminum profile listed as 70 × 140 mm and a profile thickness of 3 mm. The glazing configuration is recorded as 6 mm Low-E glass, a 12 mm air space, and 6 mm glass. The overall project area is listed as 15,000 m².

This example demonstrates how mullion dimensions and glazing specifications are documented together as part of a facade system. The profile depth, glass build-up, framing arrangement, and supporting connections must be assessed as a coordinated design.

Importantly, the published dimensions do not establish the project's design wind pressure, calculated mullion span, section moment of inertia, or allowable deflection. These values should not be inferred from the profile dimensions alone.

How PRANCE Product Dimensions Should Be Used？ 

PRANCE’ s glass curtain wall product page lists 6063/T5 and 6063/T6 aluminum profiles. It also lists minimum product wall thicknesses of 2.5 mm for uprights and 2.0 mm for beams, with specific thicknesses determined by the project. 

These data provide a basic introduction to the product. The final selection, however, still requires consideration of the actual extrusion geometry, material properties, structural calculations, and connection details.

Why Anchors and Building Movement Affect Mullion Sizing？

A suitable mullion cannot compensate for an inadequate anchor or supporting slab. The design must communicate anchor reactions, connection eccentricities, and the loads transferred to the primary structure.

Building movement also needs coordination. Slab-edge deflection, creep, construction tolerances, thermal movement, and wind or seismic drift can affect joint clearances and facade connections. A larger profile alone does not resolve these movement requirements.

Conclusion

Curtain wall mullions are sized through coordinated checks of loads, support span, section properties, deflection, and connections. The objective is not simply to choose a deeper extrusion, but to select a framing assembly that works with the glass, anchors, joints, and building structure.

For developers and procurement teams, the most useful approach is to request documented section properties, structural calculations, and performance evidence before approving the proposed mullion.​

FAQS

1.How do you calculate curtain wall mullion size?

The process typically involves establishing the design loads and support conditions, calculating the resulting structural effects, and checking strength, deflection, and system compatibility.

2.What affects curtain wall mullion depth selection?

The main considerations include span, design wind pressure, tributary width, section properties, allowable deflection, connection details, and architectural sightlines. Glass weight and other system-specific loads must also be considered where applicable.

3.What is the allowable deflection for a curtain wall mullion?

The allowable deflection depends on the applicable code, technical standard, project specification, and glazing system. Ratios such as L/175 appear in some North American guidance, but no single ratio should be assumed to apply to every project.

4.Does a taller building always need deeper mullions?

Not necessarily. Greater height can lead to higher design wind pressures, but the required profile also depends on exposure, facade geometry, support spacing, panel width, and the selected system.

5.Can a mullion profile from another project be reused?

Not really. The new project's wind loads, spans, glass specifications, connections, deflection requirements, and governing codes must be checked.