Sizing a rectangular aluminum tube for a framework is not just about choosing a section that “seems big enough.” The normative framework imposed by Eurocode 9 (EN 1999-1-1) structures the approach into several successive checks, and feedback from construction sites post-2020 confirms that neglecting any of these, particularly the service deflection, leads to costly issues.
Local buckling of thin walls in series 6000 alloy
The first check to be conducted concerns local stability. An extruded rectangular tube in 6060 T6 or 6005A T6 alloy has walls whose width/thickness ratio directly determines the classification of the section. Eurocode 9 classifies sections into four categories, from 1 (plastic) to 4 (thin), based on this ratio.
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For a lightweight framework (pergola, facade frame, greenhouse structure), we frequently observe sections of class 3 or 4. This means that the full plastic capacity of the profile cannot be mobilized. The calculation must then be based on elastic resistance, or even on a reduced effective width for class 4 sections.
Applying the method for sizing a rectangular tube without checking this classification amounts to overestimating the load-bearing capacity of the profile, sometimes significantly.
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The National Annexes revised between 2022 and 2024 (notably the French and German NAs) reinforce this requirement by more strictly framing local buckling checks for common extruded hollow profiles.
Service deflection criteria: the true sizing of an aluminum framework
Service deflection governs sizing more often than stress. We recommend starting with this check even before assessing resistance. This may seem counterintuitive for those coming from steel, but aluminum has an elastic modulus about three times lower. At equivalent sections, an aluminum tube deflects three times more than a steel tube.
Deflection limits according to use
The technical guides from professional organizations in construction and aluminum carpentry set varying thresholds:
- L/200 for non-visible secondary structural elements or without fragile infill
- L/300 for frameworks supporting cladding, composite panels, or carpentry elements
- L/500 or stricter for glazed elements or structures where even slight misalignment causes joint cracking
Experience feedback published after 2020 is clear: sizing solely “for stress” frequently leads to frameworks that are legally compliant but functionally deficient. Cracking joints, shifting infills, and a perceived sense of flexibility by users.
Practical calculation of deflection
The classic formula for the deflection of a simply supported beam under uniformly distributed load remains the starting point: f = 5·q·L⁴ / (384·E·I). The determining parameter is the moment of inertia I of the rectangular tube, which depends on the outer dimensions and wall thickness.
For a hollow rectangular tube, the inertia about the strong axis (the one resisting the main bending) differs significantly from the inertia about the weak axis. Orienting the tube with the larger dimension in the direction of the load is obvious, but we regularly observe orientation errors on site.
Bending resistance and global buckling check
Once the deflection criterion is satisfied, the bending resistance check is performed according to the section class determined earlier. For a class 1 or 2 section, the plastic resisting moment applies. For a class 3 section, we limit ourselves to the elastic moment. For a class 4 section, the reduced effective section must be recalculated.
Global buckling (overturning) occurs when the compressed flange of the tube is not laterally supported along its entire length. In a roof or pergola framework, the purlins or intermediate beams play this support role. Their spacing determines the buckling length and thus the critical load.
Eurocode 9 imposes a combined check of bending + normal force as soon as the tube simultaneously carries an axial load (self-weight transmitted in compression in the posts, for example). This interaction reduces the bending capacity and cannot be ignored in a three-dimensional framework.
Alloy and metallurgical state: direct impact on sizing
The choice of alloy is not a commercial detail. The conventional yield strength f₀ varies from one to two times between a 6060 T5 and a 6005A T6. This value directly enters into the calculation of the resisting moment, and thus into the necessary section.
- Alloys from the 6000 series (6060, 6063, 6005A, 6082) are the most common for extruded tubes intended for frameworks
- The T6 state (thermally treated and aged) offers the best mechanical characteristics for a standard extruded profile
- The T5 state (cooled after extrusion and then aged) has lower values but is sufficient for lightly loaded frameworks
Specifying the alloy and metallurgical state at the sizing phase avoids recalculating the entire framework at the time of ordering the profiles.
Connections and actual buckling length
Sizing the tube alone is not enough. The actual rigidity of the framework depends on the support conditions at the nodes. A bolted connection with play behaves like a hinge, while a welded or bolted connection with a plate stiffens the node.
The buckling length adopted in the calculation must reflect the type of connection planned. Using a buckling length equal to the free length between nodes assumes perfect hinges. If the nodes are semi-rigid, the reality lies between fixed and hinged, and a finer modeling (or a corrective coefficient) becomes necessary.
For aluminum frameworks of modest span, the combination of deflection + local buckling + alloy choice covers the vast majority of sizing cases. Cases involving dynamic loads, fatigue, or earthquakes fall under complementary parts of Eurocode 9 and systematically justify the intervention of a structural engineering office.