Round tube
Equal stiffness in every direction and the best shape for torsion and columns. Used for handrails, truss chords, masts and space frames. Supplied by OD and wall, from small drawn tube up to large extruded sizes.
Engineering guide
Structural Aluminium Tubing: How to Select Alloy, Shape and Size
Structural aluminium tubing is hollow round, square or rectangular extrusion used to carry load in frames, trusses, railings, racks and vehicle structures rather than to carry fluid under pressure. The usual grades are 6061-T6, 6063-T5/T6, 6082-T6 and 6005A-T6, made to ASTM B221 or B429 in North America and EN 755 in Europe. This guide covers the alloys, section data, design codes and buying details; Metline supplies all of these tubes worldwide.
- Shapes
- Round, square, rectangular, hex
- Main grades
- 6061 · 6063 · 6082 · 6005A
- Standards
- ASTM B221 / B429 · EN 755
- Design codes
- Aluminum Design Manual · EN 1999
What makes tubing "structural"?
"Structural" describes the job, not a special alloy. A structural tube is selected for bending, buckling and torsional stiffness, connection details and weight, whereas a pressure pipe is selected for burst strength and leak-tightness. The same 6061-T6 can be either; what changes is the standard, the tolerances and the inspection.
ASTM B429 makes this distinction explicit: it covers 6061 and 6063 extruded pipe and tube for structural uses such as highway and bridge rails, fence posts, handrails, sign structures, awning supports and lighting brackets, and it permits tube produced through bridge or porthole dies. It excludes pressure service, for which the buyer should use seamless tube to ASTM B210 or B241.
Closed sections are the most efficient shapes for members loaded in compression or torsion. A square tube has far higher torsional stiffness than an open channel or angle of the same weight, and a round tube buckles evenly in every direction. That is why tubes dominate machine frames, space frames, stage trusses, solar racks, boat T-tops and vehicle subframes.
Alloys and tempers for structural tubing
Values below are typical properties from Aluminum Association and ASM data; design must use the specified minimums of the governing standard, which are lower and depend on wall thickness.
| Alloy / temper | Typical tensile | Typical yield | Strengths | Typical uses |
|---|---|---|---|---|
| 6061-T6 | 310 MPa | 276 MPa | Best all-round strength, weldability and machinability | Machine frames, trailers, truss chords, EV frames, marine T-tops |
| 6063-T6 | 241 MPa | 214 MPa | Superb surface, easy to anodise, lower cost | Railings, display frames, architectural framing, light structures |
| 6063-T5 | 186 MPa | 145 MPa | Press-quenched, economical | Solar racking, non-critical frames, enclosures |
| 6082-T6 | 310 – 340 MPa | 260 – 300 MPa | Highest strength of common 6xxx alloys in thick walls | European structures, scaffolding, cranes, heavy frames |
| 6005A-T6 | ≈ 285 MPa | ≈ 240 MPa | Extrudes complex hollows well | Rail vehicles, bridges, large multi-cell sections |
| 5083-O / H111 | 290 MPa | 145 MPa | No strength loss when welded, seawater resistant | Marine structures, offshore walkways, cryogenic frames |
| 7075-T6 | 572 MPa | 503 MPa | Very high strength; poor weldability | Aerospace, motorsport, drone and high-load members |
| 2024-T3 | 483 MPa | 345 MPa | High strength, good fatigue resistance | Aircraft structures, machined fittings |
Minimum values: for example, ASTM B221 gives 6061-T6 extrusions 260 MPa tensile and 240 MPa yield; EN 755-2 gives 6082-T6 tube up to 5 mm wall 290 / 250 MPa. Check the mill certificate for the exact figures of your lot.
Tube shapes and when to use each
Square tube
Flat faces make joints, drilling and bolting simple. Strong in both axes and very good in torsion. The default for machine bases, frames and racks.
Rectangular tube
More material away from the neutral axis in one direction, so it is efficient for beams loaded mostly in one plane, such as floor joists and trailer cross-members.
Hex and oval tube
Used where appearance, grip or anti-rotation matter: handles, tool frames and decorative structures.
Multi-cell and custom hollows
Extruded with internal webs, slots, screw bosses or flanges to cut assembly steps. Custom dies make sense when volumes justify tooling.
Open sections for comparison
Angles, channels, I-beams and T-bars are cheaper per kg and easier to connect, but much weaker in torsion than a closed tube of equal weight.
Square tube weights and section properties
Values are calculated from geometry with sharp corners and a density of 2.70 g/cm³. Real extrusions have corner radii, so mass and properties are slightly lower; use the supplier drawing for final design. More sizes are in the square tube weight calculator.
| Size (mm) | Wall (mm) | Mass (kg/m) | Area (cm²) | Moment of inertia I (cm⁴) | Section modulus Z (cm³) |
|---|---|---|---|---|---|
| 25.4 × 25.4 | 1.6 | 0.41 | 1.52 | 1.4 | 1.14 |
| 38.1 × 38.1 | 3.2 | 1.21 | 4.47 | 9.1 | 4.80 |
| 50.8 × 50.8 | 3.2 | 1.65 | 6.09 | 23.1 | 9.10 |
| 50.8 × 50.8 | 6.35 | 3.05 | 11.29 | 37.9 | 14.94 |
| 76.2 × 76.2 | 3.2 | 2.52 | 9.34 | 83.1 | 21.82 |
| 76.2 × 76.2 | 6.35 | 4.79 | 17.74 | 145.5 | 38.18 |
| 101.6 × 101.6 | 6.35 | 6.53 | 24.19 | 367.5 | 72.33 |
| 152.4 × 152.4 | 6.35 | 10.02 | 37.10 | 1321.3 | 173.40 |
Two rules of thumb follow from the numbers. First, doubling the outside size of a square tube at the same wall raises its stiffness about seven to eight times while only doubling weight, so go bigger before going thicker. Second, because aluminium is about one third as stiff as steel, an aluminium member replacing steel usually needs a deeper section rather than the same section in a stronger alloy; no aluminium alloy is stiffer than another by any useful margin.
Seamless, porthole and drawn structural tube
| Route | How it is made | Best for | Standard |
|---|---|---|---|
| Porthole (bridge-die) extrusion | Metal splits around a mandrel and re-welds in the die, leaving longitudinal weld seams that are solid-state bonded | Most square, rectangular and custom structural tube; economical in 6063, 6061, 6005A, 6082 | ASTM B221 / B429; EN 755-2 with tolerances to EN 755-8 |
| Seamless extrusion | Hollow billet extruded over a mandrel with no seams | Highly stressed, pressure-adjacent or fatigue-critical parts; 2xxx and 7xxx alloys | ASTM B241 / B221; EN 755-2 with tolerances to EN 755-7 |
| Cold drawn tube | Extruded tube drawn through a die over a plug for tighter size and finer surface | Small diameters, telescoping parts, precision frames | ASTM B210; EN 754 |
| Seam-welded tube | Strip roll-formed and HF welded | Thin-wall, low-cost frames and furniture | Supplier specification; EN 1592 |
For ordinary structures porthole tube is fully acceptable: properly made extrusion welds have strength close to the parent metal, which is why ASTM B429 allows it. Specify seamless when a design code, a pressure duty or your customer requires it.
Design codes, tolerances and inspection
Codes and standards
- North American design
- Aluminum Design Manual (Aluminum Association), Specification for Aluminum Structures
- European design
- EN 1999-1-1 (Eurocode 9); execution to EN 1090-3
- Product standards
- ASTM B221, B429, B241, B210; EN 755-1/-2, EN 754
- Dimensional tolerances
- ANSI H35.2 for ASTM orders; EN 755-7 (seamless), EN 755-8 (porthole), EN 755-9 (profiles)
- Welding
- AWS D1.2 or EN ISO 9606-2 / EN 1090-3; fillers ER4043, ER5356
- Certification
- Mill test certificate to EN 10204 3.1; third-party inspection on request
Local buckling
Thin walls buckle before the material yields. Codes limit width-to-thickness ratios (b/t) for each element; a 100 × 100 × 2 mm tube may be governed by buckling, not strength.
Welded strength
Heat-affected zones of 6xxx T6 tube fall to roughly half strength. Place welds at low-stress points or use bolted or riveted joints in highly stressed members.
Deflection
With a modulus near 69 GPa, check deflection limits early. Many aluminium frames are stiffness-governed.
Galvanic contact
Isolate steel bolts and brackets with washers, sleeves or coatings, especially outdoors and in marine service.
Where structural aluminium tubing is used
| Sector | Typical members | Common choice |
|---|---|---|
| Building and architecture | Railings, canopies, sunroom and facade sub-frames, glass balustrade posts | 6063-T6 / 6061-T6 |
| Industrial automation | Machine frames, conveyor supports, guarding, test rigs | 6061-T6, 6082-T6 square tube |
| Transport | Trailer cross-members, bus frames, EV battery frames, roof racks | 6061-T6, 6082-T6, 6005A-T6 |
| Solar and energy | PV mounting rails, purlins, inverter frames | 6063-T5 / 6005A-T6 |
| Events and display | Stage trusses, exhibition stands, lighting rigs | 6082-T6, 6061-T6 round tube |
| Marine | T-tops, rails, gangways, floating pontoon frames | 6061-T6, 5083 / 5086 |
| Aerospace and motorsport | Roll structures, drone frames, ground support equipment | 7075-T6, 2024-T3, 6061-T6 |
A typical light-industrial machine base uses 50 × 50 × 3 mm or 80 × 80 × 4 mm 6061-T6 square tube, bolted with corner brackets so that the frame keeps full T6 strength. A typical pedestrian handrail uses 42.4 mm or 48.3 mm OD 6063-T6 round tube, chosen for its surface and anodising response rather than strength.
Cutting, bending, joining and finishing
- Cutting: carbide circular saws with lubricant; precision cut-to-length and mitre cuts are routine
- Bending: mandrel bending in T4 or O temper, followed by ageing to T6 if strength is needed; T6 tube bends only to generous radii
- Machining: 6061-T6 and 6082-T6 machine cleanly; 6063 is softer and can be gummy
- Joining: MIG/TIG welding, structural adhesives, rivets, bolts and extruded corner connectors
- Finishing: anodising (clear, colour, hard), powder coating, electrophoretic coating, brushing and polishing
How to order structural tubing
- SpecifyAlloy and temper, shape, outside dimensions, wall, corner radius if critical, length, quantity and standard (ASTM B221 / B429 or EN 755-2).
- Confirm routeSay whether seamless tube is required, or porthole tube is acceptable, and list any tolerance tighter than standard.
- QuoteWe confirm availability, certification and delivery terms (EXW, FOB, CIF or DAP).
- ProcessCut-to-length, drilling, bending and finishing to drawing on request.
- ShipBundled with paper or film interleaving, in wooden crates or on pallets, with an EN 10204 3.1 certificate.
References and standards
- ASTM B429/B429M – Aluminum-Alloy Extruded Structural Pipe and Tube
- ASTM B221 – Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes
- EN 755-2 – Extruded rod/bar, tube and profiles: mechanical properties
- EN 1999-1-1 – Eurocode 9: Design of aluminium structures
- The Aluminum Association – Aluminum Design Manual
Structural aluminium (aluminum) tubing: FAQ
What is the strongest aluminium alloy for structural tubing?
7075-T6 is the strongest common grade, but it is difficult to weld and costs more. For welded structures 6061-T6 and 6082-T6 are the practical high-strength choices.
Is 6063 strong enough for structural use?
For light structures, railings, frames and racks, yes: 6063-T6 has a typical yield strength of about 214 MPa. For heavily loaded or welded members, 6061-T6 or 6082-T6 gives roughly 30 % more strength.
What is the difference between ASTM B221 and B429?
B221 is the general specification for extruded bar, rod, profiles and tube in many alloys. B429 is specific to 6061 and 6063 structural pipe and tube, allows porthole-die tube, and is not for pressure service.
Is porthole extruded tube safe for structures?
Yes. Porthole tube has solid-state extrusion welds that are close to parent-metal strength, and structural standards accept it. Seamless tube is preferred for pressure, severe fatigue or where a specification demands it.
Can aluminium tube replace steel tube in a frame?
Often, with a lighter result. Because aluminium is about one third as stiff, the replacement usually needs a deeper or wider section to match deflection. Check buckling and welded strength as well.
What sizes of structural tube are available?
Round tube from small drawn sizes to large extruded diameters, and square and rectangular tube from about 20 mm up to 150 mm or more, in walls from about 1.2 mm to over 6 mm. Non-standard sizes can be extruded to drawing.
Need structural aluminium tube?
Send your alloy, sizes, quantities and destination. We will confirm availability, certification and a delivered price.
