Aluminium bridges are built mainly from 6xxx-series extrusions such as 6005A-T6, 6082-T6 and 6061-T6, with 7020-T6 used where higher strength is needed and 5083 plate for welded decks. This guide explains which alloy suits each bridge member, the minimum properties you can design to, the codes that govern the work and what to specify when you buy the profiles.
An aluminium bridge weighs roughly one third of an equivalent steel structure and does not need a paint system to survive outdoors. Those two facts drive almost every aluminium bridge project: the span can be prefabricated, lifted in one piece with a light crane and installed overnight, and the owner avoids decades of repainting.
Aluminium has been used in bridges since the 1930s. The Arvida Bridge over the Saguenay River in Quebec, opened in 1950 with a main arch of about 88 m, was built entirely of aluminium and is still in service after more than seventy years. Today the most common applications are footbridges and cycle bridges, lightweight deck replacements on older steel or movable bridges, military and emergency bridges, pontoon and marina gangways, and secondary parts such as parapets, railings and sign gantries.
Extrusion is what makes aluminium economic for bridges. A die can produce a hollow deck plank with integral stiffeners, interlocking edges and a non-slip top surface in one piece, so the designer places material exactly where it carries load. Steel cannot be shaped this way without extensive welding.
Bridge profiles come almost entirely from the 6xxx (Al-Mg-Si) family, because these alloys extrude well into complex hollow shapes, reach useful strength after ageing and weld with standard MIG procedures. The choice between them is about section shape, strength and how the member is joined.
| Alloy & temper | Typical bridge use | Why it is chosen |
|---|---|---|
| 6005A-T6 | Hollow deck planks, chords, truss members | Extrudes thin-walled hollow sections more easily than 6082 at similar strength; the usual European deck alloy |
| 6082-T6 | Heavy solid and hollow sections, chords, cross-girders | Highest strength of the common 6xxx alloys in thick sections; good corrosion resistance |
| 6061-T6 | Trusses, girders and deck sections (North American practice) | Long service record in US and Canadian bridges; covered by ASTM B221 and the Aluminum Design Manual |
| 6063-T5 / T6 | Railings, parapets, handrails, trims, non-structural cladding | Best surface finish and anodising response, but lower strength, so it is used for secondary members |
| 7020-T6 | Highly loaded members, military bridges | Higher strength and partial natural re-ageing after welding; needs care against stress corrosion |
| 5083-H111 / H321 (plate) | Welded deck plates, gusset plates, marine-exposed parts | Keeps most of its strength after welding and has excellent seawater resistance |
Other 7xxx extrusion alloys such as 7003, 7005 and 7021 appear in some lightweight portable and military bridge designs. They offer more strength but are more sensitive to stress corrosion cracking and to the welding procedure, so they should only be used where the designer specifies the temper, the heat treatment and the corrosion testing explicitly.
Structural design uses the specified minimum values of the product standard, not typical values. The table summarises EN 755-2 minimums for extruded profiles in the tempers most often used on bridges. Values fall slightly with wall thickness, so always check the band that matches your section.
| Alloy / temper | Profile type and wall (mm) | Rm min (MPa) | Rp0.2 min (MPa) | A min (%) |
|---|---|---|---|---|
| 6005A-T6 | Solid, ≤ 5 | 270 | 225 | 8 |
| 6005A-T6 | Solid, > 5 – 10 | 260 | 215 | 8 |
| 6005A-T6 | Solid, > 10 – 25 | 250 | 200 | 8 |
| 6005A-T6 | Hollow, ≤ 5 | 255 | 215 | 8 |
| 6005A-T6 | Hollow, > 5 – 15 | 250 | 200 | 8 |
| 6082-T6 | Solid and hollow, ≤ 5 | 290 | 250 | 8 |
| 6082-T6 | Solid and hollow, > 5 – 25 | 310 | 260 | 10 |
| 6061-T6 | ≤ 5 | 260 | 240 | 8 |
| 6061-T6 | > 5 – 25 | 260 | 240 | 10 |
| 6063-T5 | ≤ 3 | 175 | 130 | 8 |
| 6063-T6 | ≤ 10 | 215 | 170 | 8 |
| 7020-T6 | ≤ 40 | 350 | 290 | 10 |
Summarised from EN 755-2; elongation shown is A (5.65√So). Under ASTM B221, 6061-T6 extrusions carry minimums of 260 MPa (38 ksi) tensile and 240 MPa (35 ksi) yield. The mill certificate for each lot states the tested values.
Multi-void hollow extrusions, typically 200 – 400 mm wide, that interlock side by side and are welded by friction stir or MIG welding or simply clipped and bolted. The top face often carries an extruded ribbed or epoxy-grit anti-slip surface.
I-sections, box sections and large hollow tubes that form the primary load path of girder and truss bridges. Long lengths reduce the number of splices.
Round, square or rectangular hollow sections bolted or welded to the chords. Standard tube sizes keep the cost down.
Channels, I-beams and hat sections that carry the deck between the main girders.
Posts, top rails, infill bars and balusters, often in 6063 or 6005A with an anodised or powder-coated finish.
Lighting columns, sign gantries, cable trays, inspection walkways and maintenance platforms fitted to steel and concrete bridges.
A typical modern footbridge combines two truss or box-girder sides in 6082-T6 with a deck of hollow 6005A-T6 planks spanning between cross-girders. Because the deck plank and the truss can both be extruded to the drawing, the fabricator mostly cuts, drills and joins standard lengths rather than building members up from plate.
| Bridge type | How aluminium is used | Main benefit |
|---|---|---|
| Pedestrian and cycle bridges | Complete superstructure: trusses or girders plus extruded deck | Light enough to lift in one piece; no repainting |
| Highway deck replacement | Orthotropic extruded deck panels fixed to existing steel girders | Lower dead load raises the live-load rating of old bridges |
| Movable bridges (bascule, swing, lift) | Deck and leaf structure | Smaller counterweights and drive machinery |
| Military and emergency bridges | Modular panels and ramps carried by vehicles or by hand | Rapid deployment and reuse |
| Pontoon bridges and gangways | Deck, frames and floats in marine alloys | Seawater resistance and buoyancy |
| Aircraft and boarding bridges | Frames, tunnels and floor structures | Low moving mass and corrosion resistance |
| Suspension and cable-stayed footbridges | Deck and hangers | A lighter deck reduces cable and tower loads |
Large road and rail bridges with long spans are still mainly steel and concrete. Aluminium competes best where its low weight brings a clear system saving, which is why decks and footbridges dominate.
Aluminium is not a drop-in substitute for steel. Its lower stiffness, sensitivity to welding heat and fatigue behaviour must be taken into account from the concept stage.
With about one third of the elastic modulus of steel, an aluminium girder deflects three times as much under the same load if the section is unchanged. Deeper sections, closed boxes and trusses are used to control deflection, which often governs the design more than strength.
A light, flexible footbridge can have natural frequencies in the range excited by walking (about 1.6 – 2.4 Hz vertically). Designers check footfall response to EN 1990 Annex A2, ISO 10137 or national guidance and add depth or damping where needed.
Welding softens 6xxx alloys in a zone around the weld. Eurocode 9 and the Aluminum Design Manual apply reduced strength in this heat-affected zone, so welds are placed in low-stress areas or replaced by bolted, riveted or friction-stir-welded joints.
Fatigue strength of welded details is lower than in steel and controls many deck designs under traffic. EN 1999-1-3 gives detail categories; smooth extruded shapes and friction stir welds improve fatigue performance.
Aluminium expands about twice as much as steel. Bearings and expansion joints must accommodate this, and mixed aluminium-steel structures need slotted connections.
Isolate aluminium from carbon steel, copper and wet concrete with gaskets, sleeves and coatings, and use stainless steel or coated fasteners.
| Standard | Scope |
|---|---|
| EN 1999-1-1 (Eurocode 9) | General rules for the design of aluminium structures, including HAZ reduction factors |
| EN 1999-1-3 | Fatigue design of aluminium structures |
| EN 1090-3 | Technical requirements for the execution (fabrication and erection) of aluminium structures |
| AASHTO LRFD Bridge Design Specifications, Section 7 | Aluminium structures in highway bridges (North America) |
| Aluminum Design Manual (Aluminum Association) | Specification for aluminium structures used in the US and Canada |
| CSA S6 / CSA S157 | Canadian highway bridge code and aluminium design standard |
| EN 755-2 / ASTM B221 | Mechanical properties of extruded bars, tubes and profiles |
| EN 755-9 / EN 12020-2 | Dimensional and form tolerances (straightness, twist, angularity) for profiles |
| AWS D1.2 / EN ISO 15614-2 | Structural welding code and welding procedure qualification for aluminium |
On European projects the fabricator usually works to EN 1090-3 with an execution class set by the designer, which in turn sets the inspection and traceability requirements for the extrusions. Ask for this before ordering, because it decides what certification you need from the material supplier.
6005A-T6 is the most common choice for hollow extruded deck planks because it extrudes thin multi-void sections well at good strength. 6082-T6 is used for heavier sections and 6061-T6 is common in North American practice. Welded plate decks use 5083.
Well-detailed aluminium bridges have lasted many decades with little maintenance; the Arvida Bridge in Canada opened in 1950 and is still carrying traffic. Design life depends mainly on fatigue detailing, drainage and avoiding galvanic contact with other metals.
Yes. Aluminium decks and complete aluminium bridges carry road traffic, but the design is usually governed by deflection and fatigue rather than strength, so sections are deeper than in steel.
Yes, by MIG, TIG or friction stir welding. Welding reduces strength in the heat-affected zone, which codes such as Eurocode 9 and the Aluminum Design Manual account for with reduced design values.
At minimum an EN 10204 3.1 mill test certificate showing chemistry and tensile results for each lot, traceable to the profiles delivered. The fabricator's execution class or project specification may require further tests or third-party inspection.
Not for corrosion protection in most environments. Structural members are often left mill finish, while railings and visible parts are anodised or powder coated for appearance.
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