Mullion sizing in Broward County is not a catalog lookup — it is a structural calculation governed by ASCE 7-22 component and cladding pressures, AAMA deflection standards, and the Florida Building Code. A 10-foot mullion in C&C Zone 5 at 170 MPH needs roughly 6.8 in4 of moment of inertia to stay within L/175 deflection limits. This page walks through the engineering, the alloy choices, and the numbers that determine whether your storefront passes plan review.
Adjust span, tributary width, and wind pressure to see how required Ix changes in real time. The cross-section visualization scales to show the relative mullion size needed, and the graph plots the Ix-span relationship for your parameters.
This animation shows the exaggerated deflection curve of a simply-supported mullion under uniform wind pressure. The blue curve represents elastic deformation, and the dashed line marks the L/175 allowable limit. Deflection governs mullion sizing far more often than bending stress in storefront applications.
Most structural engineers discover early in storefront design that bending stress almost never controls mullion selection in high-wind zones. The reason lies in the mathematics: deflection for a uniformly loaded beam is proportional to the span raised to the fourth power (L4), while bending moment is proportional to L2. This quartic relationship means that every foot of additional span amplifies deflection demands exponentially relative to stress demands.
In Broward County, where ASCE 7-22 prescribes 170 MPH ultimate design wind speed for Risk Category II buildings, C&C pressures at Zone 5 corners can reach -62.4 psf for small effective wind areas. When you apply this pressure over a 5-foot tributary width to a 12-foot mullion, the required moment of inertia jumps to approximately 11.7 in4 — demanding a fairly substantial aluminum section even though the bending stress at that Ix level remains well below the 15 ksi allowable for 6063-T5.
Broward County plan reviewers pay close attention to effective wind area (EWA) calculations on storefront submittals. The EWA equals the span multiplied by the tributary width but cannot be less than the span divided by three. For a 10-foot mullion at 5-foot spacing, EWA = 50 sq ft. As EWA increases, the GCp coefficient from ASCE 7-22 Figure 30.3-1 decreases — meaning lower design pressure per square foot.
However, the total load on the mullion increases because the tributary area is larger. The net effect typically favors wider spacing for economy, but wide-module storefronts introduce their own challenges: larger glass lites need thicker glass to resist wind pressure independently, and the heavier dead load from the glass itself adds to the mullion's gravity loading.
Choosing the right aluminum alloy affects both structural capacity and project economics. Both alloys share the same elastic modulus (10,100 ksi), so deflection performance is identical for equal sections — but the 60% higher allowable stress of 6061-T6 reduces required section modulus when bending strength governs the design.
The industry-standard extrusion alloy. Excellent corrosion resistance, smooth surface finish, and readily available from every major storefront manufacturer. Suitable for most moderate-span applications in Broward County where deflection governs.
The structural-grade alloy for demanding applications. When bending stress governs on long spans or extreme pressures, 6061-T6 allows thinner walls and more compact profiles. Required more frequently in Broward County Zone 5 corner conditions exceeding 12-foot spans.
This table shows the minimum Ix required to satisfy L/175 deflection for various mullion spans and tributary widths at Zone 4 and Zone 5 pressures. Values assume Exposure C, Risk Category II, simply-supported condition. For thermal-break mullions, increase Ix by 15-25% to account for derated composite section properties.
| Mullion Span | Trib. Width | Zone 4 (-48 psf) | Zone 5 (-62 psf) | Max Defl. (L/175) | Sx Required (6063-T5) |
|---|---|---|---|---|---|
| 8 ft (96 in) | 4 ft | 1.9 in4 | 2.5 in4 | 0.549 in | 1.54 in3 |
| 8 ft (96 in) | 5 ft | 2.4 in4 | 3.1 in4 | 0.549 in | 1.93 in3 |
| 10 ft (120 in) | 4 ft | 4.0 in4 | 5.2 in4 | 0.686 in | 2.48 in3 |
| 10 ft (120 in) | 5 ft | 5.0 in4 | 6.8 in4 | 0.686 in | 3.10 in3 |
| 12 ft (144 in) | 4 ft | 6.9 in4 | 8.9 in4 | 0.823 in | 3.58 in3 |
| 12 ft (144 in) | 5 ft | 8.6 in4 | 11.7 in4 | 0.823 in | 4.46 in3 |
| 14 ft (168 in) | 5 ft | 13.7 in4 | 18.3 in4 | 0.960 in | 6.07 in3 |
| 16 ft (192 in) | 5 ft | 20.7 in4 | 27.8 in4 | 1.097 in | 7.94 in3 |
Broward County enforces the Florida Energy Conservation Code, which in Climate Zone 1A requires commercial fenestration assemblies to meet U-factor limits — typically 0.50 BTU/hr-ft2-F or lower for fixed glazing per ASHRAE 90.1. Thermally broken mullions achieve this by interrupting the aluminum conduction path with polyamide (nylon 6,6) or polyurethane isolator strips.
The structural consequence is significant. The thermal break creates a discontinuity in the mullion cross-section that reduces the effective moment of inertia by 10-25%, depending on the break's location relative to the neutral axis and the strip's width. A mullion with 8.0 in4 gross Ix might deliver only 6.4 in4 effective Ix after the thermal break is accounted for.
This deration is not optional engineering judgment — Broward County structural plan reviewers specifically check whether the Ix used in deflection calculations reflects the thermally broken section or the gross aluminum section. Using the wrong value is one of the most common reasons for storefront submittal rejection in Broward County permit offices.
Reputable storefront manufacturers publish both gross and effective section properties for their thermally broken profiles. Kawneer, Oldcastle (YKK AP), Arcadia, and EFCO all provide engineering data with separate Ix values for structural analysis. When these are unavailable, the conservative approach is to apply a 20% reduction factor to the gross Ix — though specific finite element analysis of the composite section can yield more accurate results for critical applications.
Storefront system submittals in Broward County face rigorous structural plan review. Understanding what reviewers check accelerates approvals and prevents costly redesigns.
Submit complete ASCE 7-22 wind load calculations showing V = 170 MPH, exposure category, topographic factor, all GCp values for each C&C zone, and internal pressure coefficients. The calculation must clearly identify which zones apply to each mullion location on the building facade. Broward County reviewers reject calculations that do not specify the building's mean roof height and dimensions used to determine the Zone 5 boundary distance "a."
Provide deflection and stress calculations for every unique mullion condition — field mullions, corner mullions, head/sill conditions, and any spliced spans. Show the actual Ix and Sx of the selected profile versus the required values. If thermal breaks are present, the effective (derated) section properties must be used. Include load diagrams showing the uniform load distribution from tributary width and the resulting moment and deflection diagrams.
All storefront systems must have a current Florida Product Approval (FL number) or Miami-Dade NOA. The approval must cover the installed configuration — framing depth, glass type, glazing method, and tested design pressure. Broward County requires that the system's tested DP rating meets or exceeds the calculated C&C Zone 5 pressure at each mullion location. Field-fabricated storefronts without product approvals require full-scale ASTM E330 chamber testing.
ASCE 7-22 Section 26.2 defines the width "a" that establishes C&C Zone 5 (corner zone) boundaries. For low-rise buildings, "a" is the lesser of 10% of the least horizontal dimension or 0.4 times the mean roof height, but not less than either 4% of the least horizontal dimension or 3 feet.
Consider a typical two-story commercial building in Fort Lauderdale — 60 feet wide, 120 feet long, 28-foot mean roof height. The zone width "a" equals the lesser of: 0.10 x 60 = 6 feet or 0.4 x 28 = 11.2 feet. So a = 6 feet, but checked against minimums: 0.04 x 60 = 2.4 feet and 3 feet — both less than 6, so a = 6 feet controls.
This means every mullion within 6 feet of a building corner must be designed for Zone 5 pressures — roughly 30% higher than Zone 4 field-of-wall pressures. In practice, the first one or two storefront bays at each corner require heavier mullion sections than the rest of the facade.
Savvy designers in Broward County position the storefront module grid so that heavier corner mullions align with the architectural intent. Rather than running identical 5-foot modules across the entire facade and upgrading the corner ones, some projects use wider corner bays (6-7 feet) with reinforced mullions that serve as an architectural expression while accommodating the higher Zone 5 loads.
Another approach uses the effective wind area reduction. If a corner mullion has a 12-foot span and 5-foot tributary width, its EWA is 60 sq ft, which reduces the Zone 5 GCp from -1.8 (at 10 sq ft) to approximately -1.35. This significantly lowers the required Ix compared to a naive calculation using the worst-case -1.8 coefficient.
The required moment of inertia depends on mullion span, tributary width, and C&C wind pressure zone. For a typical 10-foot mullion span with 5-foot tributary width in Broward County's C&C Zone 5 (design wind speed 170 MPH, approximately -62.4 psf negative pressure), the required Ix is roughly 6.8 in4 to satisfy the L/175 deflection limit per ASCE 7-22 and AAMA/WDMA/CSA 101. Longer spans increase the required moment of inertia cubically — a 12-foot span under the same conditions demands approximately 11.7 in4. Use the interactive calculator above to check your specific configuration.
The L/175 deflection limit means a mullion can deflect no more than its unsupported span length divided by 175 under design wind load. For a 120-inch (10-foot) mullion, the maximum allowable deflection is 120/175 = 0.686 inches. This limit comes from AAMA/WDMA/CSA 101/I.S.2/A440 and is the standard for commercial storefront systems. Some specifications require stricter limits — L/240 for certain curtain wall applications or L/360 when adjacent to brittle finishes like stone veneer or tile. Broward County plan reviewers routinely verify this calculation on commercial storefront submittals and will reject any analysis that uses a less restrictive limit without project-specific justification.
6063-T5 aluminum (allowable bending stress ~15 ksi, elastic modulus 10,100 ksi) is the standard alloy for storefront extrusions and works for most moderate-span applications. 6061-T6 aluminum (allowable bending stress ~24 ksi, same elastic modulus) offers 60% higher strength and is preferred for long spans or high wind load conditions. In Broward County with 170 MPH design wind speed, mullion spans beyond 10 feet in C&C Zone 5 often require 6061-T6 to keep cross-section sizes practical. The elastic modulus is identical for both alloys, so deflection performance is the same for equal cross-sections — the advantage of 6061-T6 is in bending stress capacity only.
C&C Zone 5 (corner zones within distance "a" from building edges) experiences significantly higher suction pressures than Zone 4 (field of wall). At 170 MPH in Broward County with Exposure C, Zone 5 negative GCp can reach -1.8 for small effective wind areas (≤10 sq ft), compared to Zone 4 at -1.4. When combined with internal pressure coefficients and velocity pressure, this translates to roughly -62.4 psf in Zone 5 versus -48 psf in Zone 4 — about 30% higher. For a 10-foot mullion with 5-foot tributary, this difference means the Zone 5 mullion needs approximately 6.8 in4 versus 5.0 in4 for Zone 4, a 36% increase in required section properties.
Yes. Thermally broken mullions use a polyamide (nylon 6,6) or polyurethane strip to interrupt the aluminum thermal bridge for energy code compliance. This break reduces the composite section's effective moment of inertia by 10-25% depending on the break's location and width. Engineers must use the effective (derated) Ix — not the gross aluminum Ix — when checking deflection and stress per Broward County submittal requirements. Most major manufacturers publish both gross and effective section properties for their thermally broken profiles. If specific data is unavailable, a 20% reduction factor is a conservative starting point.
Effective wind area for a mullion equals the span length multiplied by the tributary width, but not less than the span length divided by three (per ASCE 7-22). For a 10-foot mullion with 5-foot tributary width, EWA = 10 x 5 = 50 sq ft. Larger effective wind areas reduce the GCp coefficient from ASCE 7-22 Figure 30.3-1, which lowers design pressure. For 50 sq ft EWA in Zone 5, GCp negative drops from -1.8 (at 10 sq ft) to approximately -1.4, reducing design pressure to around -49 psf. This is why accurate EWA calculation is critical — using the overly conservative 10 sq ft assumption when your actual EWA is 50 sq ft wastes money on oversized mullions without improving safety.
Stop guessing at mullion sizes. Our wind load calculator generates precise C&C pressures for every zone on your building facade, so you can size mullions with confidence and breeze through Broward County plan review.
Calculate Storefront Wind Loads