Aluminium I-Beams

What is an aluminium I-beam?

An I-beam puts most of its material in the flanges, as far from the centre as possible, where it does the most work resisting bending. The web keeps the flanges apart and carries shear. The result is a member that is stiff and strong in one direction for very little weight.

Aluminium I-beams are extruded rather than rolled. That has two practical consequences. First, the shape can be optimised: the Aluminum Association (AA) standard I-beam was designed specifically for aluminium, with parallel flanges, generous fillets and a better strength-to-weight ratio than a copy of a steel shape. Second, custom beams with ribs, slots or bolt channels can be made by building a new die, which is far cheaper than new rolls.

An aluminium I-beam weighs about one third as much as a steel beam of the same size and does not rust, so it is used where weight is paid for every day (trailers, boats, vehicles, mobile structures, cranes and gantries) or where corrosion and maintenance dominate (marine, chemical plant, coastal buildings, platforms and walkways). Its modulus of elasticity is also about one third of steel, so beams are sized for deflection as much as for strength.

Aluminum Association standard I-beam sizes

AA I-beams have parallel inner flange faces of uniform thickness, which makes bolted connections simpler because no tapered washers are needed. Each nominal size is produced in a light and a heavy weight. Values below are nominal dimensions and section properties about the strong (x) axis, calculated for 6061 at a density of 2.70 g/cm³.

Section properties are published in the Aluminum Association Aluminum Design Manual. Use the manual or your design code to check strength and deflection; values here are for selection only.

American Standard aluminium I-beams

American Standard I-beams follow the proportions of the steel S-shape, with tapered inner flange faces (about 16.7 % slope) and a narrower flange than the AA section. They are specified where an aluminium beam must replace a steel S-beam with the same outside dimensions, or match existing connections.

Approximate weights for the lightest section at each depth in 6061; heavier sections exist at several depths. Confirm exact dimensions on the drawing before ordering.

Small, metric and special aluminium beams

Alloys and mechanical properties for I-beams

6xxx values are specified minimums from ASTM B221 and EN 755-2 for typical wall thickness bands; 5xxx values are indicative.

Designing with aluminium I-beams

• Check deflection first. With one third of steel's stiffness, an aluminium beam of equal size deflects about three times as much under the same load. Deeper sections recover stiffness efficiently, because the moment of inertia grows with roughly the cube of depth.
• Use the right code. In North America, design to the Aluminum Association Aluminum Design Manual; in Europe, EN 1999 (Eurocode 9). Steel tables and allowable stresses do not apply.
• Watch lateral buckling. Narrow-flange beams over long unbraced spans can buckle sideways; brace the compression flange or choose an AA or wide-flange section.
• Allow for welding. Welding 6061-T6 reduces strength near the weld to about T4 level. Place welds in low-stress regions or use bolted connections.
• Isolate from steel. In wet environments, separate aluminium beams from carbon steel with isolating pads and use stainless fasteners to avoid galvanic corrosion.
• Allow for expansion. A 10 m aluminium beam grows about 11 mm over a 50 °C temperature change; use slotted connections on long runs.

Lengths, tolerances and inspection

ASTM B308 covers 6061-T6 standard structural profiles (I-beams, channels, angles and tees) produced by extrusion and is often called up alongside B221 on structural drawings. For aerospace and defence orders, AMS-QQ-A-200/8 and /16 are used.

Where aluminium I-beams are used

• Trailers and truck bodies: boat trailer frames, flatbed cross-members and dump body sub-frames
• Marine: deck beams, pontoons, gangways, docks and offshore walkways
• Lifting: portable gantry cranes, hoist beams and lifting frames
• Buildings and bridges: pedestrian bridges, canopies, roof structures and mezzanines
• Industrial plant: platforms, walkways and supports in chemical, water and food plants
• Formwork and scaffolding: shoring beams and stage structures that must be carried by hand
• Rail and transit: under-frames, equipment racks and platform structures

Aluminium I-beam compared with steel

For a beam designed to the same deflection, the aluminium section must be deeper or heavier in proportion than steel, but because aluminium is one third the density, the finished beam is still typically around half the weight. Where depth is fixed and deflection governs, steel may be the more economical choice.

Related profiles and guides

How to order aluminium I-beams

1. SpecifySection family and size (e.g. AA 6 × 4 × 4.03 lb/ft), alloy and temper (e.g. 6061-T6), specification (ASTM B221 / B308, EN 755), length, quantity and any machining.
2. QuoteWe confirm the die (existing or new), alloy, temper, finish, packing and delivery terms (EXW, FOB, CIF or DAP) for your destination.
3. SampleFor new or appearance-critical sections, a short length or finish sample can be supplied for approval before the production run.
4. ProcessBeams are extruded or supplied from mill stock, cut to length, drilled or coped to drawing, and inspected for dimensions and straightness.
5. DocumentEach delivery carries a mill test certificate to EN 10204 3.1 with cast analysis and mechanical results; coating thickness reports and third-party inspection on request.
6. ShipLengths are film-protected, bundled with spacers and packed in cartons, crates or on pallets for sea, air or road freight to your port or door.

Aluminium (aluminum) I-beams: frequently asked questions