ice | manuals doi: 10.1680/mobe.34525.0345 Aluminium in bridges CONTENTS P. Tindall Hyder Consulting (UK) Ltd Understanding the unique characteristics of aluminium alloys and exploiting them in ways developed by other industries can produce light, durable and cost-effective bridges. Adopting the concept of placing material where it will be most efficiently used can be described as design in its purest form. Introduction Aluminium is not widely used in the bridge market, partly through ignorance, partly through misconceptions, but largely because designers have never been taught how to use it. Many bridge engineers will be surprised to learn of a highway bridge 153 m long, with a main span arch of 91.5 m, that was built of aluminium in 1950 and in 2008 is still in service (see Figure 1). Aluminium is a material that has unique properties that need to be exploited and worked with. When used correctly, the results are light, durable structures that are costeﬀective. Aluminium is not some kind of funny steel. It has suﬀered badly as a structural material when designers have adopted typical steelwork details. This chapter seeks to point the inquisitive along the path to successful use of the material, with some simple basic facts, allied to references to more comprehensive information. In general text, the term ‘aluminium’ is used, although in reality the structural materials of interest are all alloys of aluminium with small percentages of other elements added. Why aluminium? Why should we consider aluminium? Principally, because it is light and durable, and also because of the wide variety of structural forms and shapes that can be created. These properties have been widely exploited in aerospace, railway carriage and architectural applications; they are also useful for bridgeworks. The low self-weight can be extremely useful for handling during fabrication and construction, as well as in the ﬁnal design. The durability of aluminium alloys is extremely good and is one of the most underestimated virtues of the material. Introduction 345 Why aluminium? 345 Alloys and product forms 346 Design and details 347 Design standards 349 Fabrication 350 Fatigue 350 Fire safety 350 Historic and recent bridges 351 Bridge decks and furniture 352 Corrosion behaviour 353 Coatings and finishes 353 Sustainability 354 Future trends 354 References 355 Further reading 355 As a designer, I have often been confronted with negative perceptions – cost, corrosion, deﬂection and fatigue being the main concerns that abound. Many believe that aluminium alloys will not be ﬁt for a highway bridge. In the main, these perceptions arise from examples of poor design. With the right approach, an aluminium structure will compete with any other material on cost, and will outperform most in service. Price per tonne for the basic material is high compared with steel, but when fabrication, erection and treatment costs are taken into account there is little diﬀerence for the completed structure. Aluminium will often be cheaper than steel or concrete when whole-life costs are calculated. Aluminium alloys are available in a range of strengths, and will meet the most demanding requirements. Pure aluminium is a relatively low-strength material, but alloying with small amounts of other elements can signiﬁcantly increase its strength. Tensile strength of the generally applicable structural alloys is comparable with mild steel, and good low-temperature toughness eliminates concerns about brittle fracture in cold climates. With only a third of the density of steel, the strength-toweight advantage of aluminium is signiﬁcant. The low self-weight is especially beneﬁcial during the fabrication, transport and erection stages of the project, as well as for the design of the completed structure and its supports. Low self-weight is especially relevant for bridge refurbishment, for moving structures, and for reducing inertial forces on elevated structures subject to earthquake. Aluminium can be formed or extruded into simple, complex or bespoke shapes that allow for structural eﬃciency, as well as ease of fabrication. The tonnages involved in most bridge applications make it feasible to create speciﬁc ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. www.icemanuals.com 345 ice | manuals Aluminium in bridges the expense of durability or workability, others oﬀering better formability at the expense of strength. Aluminium alloys are categorised by the main alloying element and use an internationally recognised four-digit reference. The alloys that are of interest to the bridge engineer are described in the following list. Figure 1 The Arvida Bridge over the Saguenay (courtesy of Alcan Aluminium Co. Canada) sections, and so the designer is free to create innovative and eﬃcient solutions. A signiﬁcant reason for using aluminium is its good corrosion resistance. Under most atmospheric conditions, no coating is necessary, as illustrated by aluminium parapet systems that have been in service for many years. Coatings can be applied for aesthetic purposes or for speciﬁc environments (see Figure 2). Alloys and product forms There are many diﬀerent aluminium alloys available, and each of these in diﬀerent tempers or heat treatments, such that the combinations run into hundreds. For the bridge engineer, however, there are only three families of alloys that need to be considered, and a relatively small number of alloys and tempers within each family (see below). These are all alloys that are readily available, have good corrosion and strength characteristics and are easily fabricated. Alternative alloys are available and are used in other industries, some oﬀering much higher strengths at n 5xxx series alloys have magnesium as the main alloying element. These alloys have the best corrosion resistance but are generally only available in sheet or plate form. The alloys have increased strength from work hardening during the rolling process and are available in several diﬀerent degrees of hardness (O denotes the base condition; the letter H followed by two numbers denotes work-hardened material). A common alloy is 5083 H12. Proof strength and ultimate strength increase with the work hardening, but formability decreases. The alloys are readily available and are a good choice if forming a structure from plate materials. n 6xxx series alloys have magnesium and silicon added as the main alloying elements. The 6xxx series alloys are readily extrudable as well as being available in sheet and plate form. These alloys are the most commonly used in structural and architectural applications, principally on account of the forms and shapes that can be created by extrusion. The alloys have their strength increased by heat treatment processes and are available in a range of tempers (indicated by the letter T followed by a number, e.g. 6082 T6). They are readily weldable and give good all-round performance. n 7xxx series alloys have zinc and magnesium as their main alloying elements. The 7xxx alloys are stronger than the 5xxx and 6xxx alloys, and have their strength increased by heat treatment. They are harder to form and are more expensive than other common alloys. They are readily weldable and have good post-weld strength. They are frequently used for military bridges and for mechanical applications, such as cranes, but less so for normal structural applications. The range of alloys, tempers and forms that are available can be confusing to the ﬁrst-time user of aluminium. The very great versatility that allows for eﬃcient structures means that it is necessary to become familiar with processes, lead times and availability, rather than simply choosing a standard section from a table. Aluminium alloy plates and sheets are usually available from stock, particularly for the common alloys 5083, 5754 and 6082. Simple rectangular and circular hollow sections and equal angles are also available in moderate quantities from stock for relatively small section sizes suitable for small footbridges, gantries and the like. For complex extrusions and larger Figure 2 Raalte Verkeesbrug (courtesy of Bayards Aluminium Constructions B.V., the sizes, it is usually necessary to have a Netherlands) speciﬁc order. This is not as daunting 346 www.icemanuals.com ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. ice | manuals Aluminium in bridges Mass: kg/m3 Modulus of elasticity: N/mm2 Poisson’s ratio Coefficient of thermal expansion Melting range: 8C 69 000–88 000 0.3–0.35 16 10ÿ6 /8C to 24 10ÿ6 /8C 475–770 2650–2850 Alloy Condition Product Proof strength: N/mm2 Ultimate strength: N/mm2 HAZ strength factor 1.00 5083 O S, Es, F 125 275 5083 H12 S 250 305 0.90 5754 O S 80 190 1.00 6005 T6 E 200 250 0.66 6061 T6 S,E 240 260 0.67 6063 T6 E 190 220 0.50 6082 T6 S, E, F 260 310 0.60 7020 T6 S, Es 290 350 0.80 S, sheet and plate; Es, simple extrusions; E, extrusions; F, forgings; HAZ, heat-affected zone Table 1 Typical properties for aluminium alloys as may be anticipated by those familiar with lead times on some other materials. The dies used to create the shape of an extrusion can be changed quickly, and quantities as small as 1 t can be ordered. This enables engineers to design their own sections for particular applications. There are also speciﬁc extrusions developed by a number of suppliers that are suitable for forming orthotropic deck panels. Aluminium castings are also available. They are generally cast from diﬀerent alloys to those used for plates and extrusions. The design of castings is highly specialised and is not covered in detail in the current design codes. They can be used if a suitable and rigorous testing regime is established. Design and details Aluminium is a metal that can be cut, welded, formed, bolted and riveted much like steel. The latest design codes are written in the mirror image of the steel design codes, and similar design calculations have to be carried out. Aluminium is, however, a diﬀerent material. It does not behave like steel, and the diﬀerences need to be recognised and exploited to give a good structure. The three most signiﬁcant diﬀerences that aﬀect structural design are the lower value of Young’s modulus, the eﬀect of welding on parent metal strength, and the sheer versatility of shapes available from the extrusion process. Young’s modulus for aluminium at a value of approximately 70 103 N/mm2 is roughly one-third that of steel. The immediate implication is that deﬂections will be greater unless the section modulus is increased by a corresponding factor. This is easily and eﬃciently accomplished by increasing section depth. What is less obvious, is that buckling strength is aﬀected, be it for column buckling, torsional buckling or local buckling, as nearly all forms of allowable buckling stress can be expressed by an equation containing the terms 2 E=2 . The designer who simply emulates a conventional steel design using I beams and channels (in bending or compression) will soon ﬁnd that allowable stresses are particularly low. Hollow sections are, therefore, particularly advantageous in aluminium structures as they are far less susceptible to torsional and lateral torsional modes of buckling. Local buckling of member elements also needs to be considered. This is an area with which the majority of steel or concrete designers will not be familiar, as design codes relevant to those materials impose shape limitations that guarantee suﬃcient stockiness to avoid such failure modes. Provided that this aspect is considered in design, then it does not constitute a problem. It should be observed that open sections in aluminium will often be extruded with bulbs or returns at the outer edges of the sections speciﬁcally to improve compressive performance of the edges (see Figure 3). The versatility of the extrusion process Figure 3 ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Diagram to illustrate typical bulbs and stiffeners www.icemanuals.com 347 ice | manuals Aluminium in bridges Extruded shape creates weld preparation and additional thickness to mitigate effect of HAZ softening Examples of thickening at welded joints (a) B A A C D E C D E B Area subject to HAZ softening Section A–A Section B–B Section C–C Section D–D Section E–E (b) Figure 4 (a) Examples of welded joints (thickening and doublers); (b) examples of staggered and reinforced butt joints to mitigate the effects of HAZ softening allows for local thickening ribs to be readily built into sections, rather than having increasing element thickness. Most of the alloys in common structural use derive some of their strength from work hardening or heat treatment. This enhancement to the basic strength of the alloy is largely nulliﬁed by the welding process, such that the strength of welds and immediate surrounding material can be as little as 50% of the original value. In steel, there is no signiﬁcant diﬀerence in strength at welds, and so designers are used to placing welds where it is easy to do so, or to suit material sizes. With aluminium alloys, greater thought and planning is required to provide an eﬃcient structure. There is also a strong case for considering bolted or riveted joints for many applications. The extent of the heat-aﬀected zone (HAZ), where there is a loss of strength, is governed by the welding process and material thickness. The reduction in strength varies with both the alloy and the level of heat treatment, or work hardening, which has been applied in producing the alloy. Current design codes give comprehensive rules for the extent of the HAZ, and how to account for the loss of strength on member capacity. In general, the eﬀects will usually extend 15–25 mm from any weld. For longitudinal welds, the loss of strength is usually not signiﬁcant and 348 www.icemanuals.com can often be ameliorated by local thickening incorporated in extruded proﬁles. Transverse welds have a greater signiﬁcance, and careful thought as to their location and detailed design can make large diﬀerences, e.g. placement of connections at points of contraﬂexure rather than at areas of high moment; using doublers with longitudinal welds, to supplement transverse welds, etc. The examples of stiﬀeners and welded joints given in Figures 3 and 4 illustrate some of the shapes that can be readily incorporated into extrusions. Other features can be easily accommodated, including aids to ﬁt-up, weld preparations, slots for bolt heads and ribs for attachment of other components, etc. With careful planning and the right mind-set, the possibilities are enormous. These types of feature can lead to signiﬁcant savings in fabrication time and cost. The ease with which extrusions can be produced makes these types of feature realisable, even for one-oﬀ projects. The limits are only governed by the designer’s mind-set! The concept of placing material where it is most eﬃciently used rather than simply selecting a section from a catalogue has to be practised. It can be enormous fun and is design in its purest form. Figure 5 gives examples of extrusion features. ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. ice | manuals Aluminium in bridges and are parameters that can aﬀect the structural design. Good liaison between the designer and welding engineer is required. There are a number of specialist books and publications that give comprehensive details on the analysis and use of aluminium alloys. Many of them are listed at the end of this chapter. Some of the books are quite old and a number are produced in the USA and thus use diﬀerent units and refer to diﬀerent standards. Some are quite theoretical and possibly beyond the need of the average bridge designer; however, the key aspects of material properties and how best to exploit them are fundamental to the eﬃcient use of aluminium and each reference will add to that understanding. Design standards (a) Thread fit Restrained nut connection Adjustable nut connection Adjustable slip fit connection Self-tapping screw (b) Figure 5 Examples of extrusion features Connection design is critical to the eﬃciency and integrity of any structure, and particularly so in aluminium. Mechanical connections are more frequently used in aluminium structures than in steel; this is partly on account of the reduced strength that some alloys exhibit when welded. It is also a reﬂection of the ease with which the material can be formed and drilled to suit mechanical connections. Rivets, bolts and pins are all commonly used. There are also occasions when welding is appropriate. It should be noted that selection of welding process, joint detailing and ﬁller wire selection need the inputs of a skilled welding engineer, The Institution of Structural Engineers produced the ﬁrst recognised UK design code for the structural use of aluminium in 1950. British Standard Code of Practice CP 118 was issued in 1969 and was updated and converted to metric units in 1973. For simple and quick assessment of member sizes for static applications it remains a useful tool using permissible stress rules akin to BS 449. The current UK standard, BS 8118, was published in 1991; this is based on limit state principles and allows higher loads in some elements than does CP 118. There are notable increases in allowable loads for welded joints. Rules for fatigue design changed considerably and those included in CP 118 are no longer considered valid. BS 8118 has two parts: Part 1, which considers design matters; and Part 2, which gives speciﬁcations and requirements for fabrication. Eurocode 9 (BS EN 1999 Parts 1-1 to 1-5) was published in 2007. The National Annexes to Eurocode 9, which will give UK-speciﬁc factors and choices, will be available in 2008. The use of the National Annexes is a prerequisite for use of Eurocode 9 for building control approval in the UK. It is currently anticipated that BS 8118 will be declared obsolete in 2009 or 2010. BS EN 1999 Part 1-1 gives the main structural rules and Part 1-3 covers fatigue. Eurocode 9, in common with other Eurocodes, makes considerable cross-reference to other European standards. In particular, it needs to be read in conjunction with Eurocode 0 for general principles, Eurocode 1 for loading, and EN 1090-3 for execution (fabrication and erection) rules. Many of the Eurocode 9 rules are similar to those in BS 8118, albeit they are more extensive and allow greater reﬁnement. Comparative exercises have shown little diﬀerence in allowable loads for static design of typical members and details. The BSI committee responsible for UK inputs to Eurocode 9 considered that the fatigue rules given in Eurocode 9 Part 1-3 were prone to misinterpretation for some details and potentially unsafe in some instances. The National Annex for Part 1-3 will, therefore, refer to a ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. www.icemanuals.com 349 ice | manuals published document (anticipated as being PD 8118 Part 3) which uses base data previously issued in prENV 1999 Part 2, published in the UK as a Draft for Development in 2000. Eurocode 9 includes several parts that had no previous UK equivalent: Part 1-2 covers design for ﬁre resistance; Parts 1-4 and 1-5 cover cold-formed structural sheeting and shell structures respectively. These parts have limited relevance to the bridge designer. Fabrication In many respects, fabrication of an aluminium structure is similar to that of a steel structure. The components are however, cleaner, lighter and easier to handle. Appropriate design and incorporation of suitable features can simplify many operations and give eﬃciencies. Cutting of aluminium sections and plates can be carried out by a variety of methods. Sawing, using band-saws or circular saws, is particularly easy for straight cuts. The characteristics of the saw blades are similar to those used for woodcutting operations. For cutting of shaped pieces, the water jet process is particularly suitable and gives a clean edge that can generally be left without the need for further machining or treatment. Components can be readily shaped, bent and formed, and press brake forming is ideal for creating angles and corrugated shapes from sheet and thin plate. Welding of the common structural alloys is readily carried out using the gas-shielded MIG (metal inert gas) or TIG (tungsten inert gas) processes. The techniques are similar to welding steel but there are diﬀerences and, interestingly, it is often easier to train a welder from scratch than it is for an experienced steel welder to convert to aluminium. Care needs to be taken to remove the oxide ﬁlm and any contaminants from the fusion faces before welding. It is also important that the welding is carried out in a dry and draught-free environment to ensure the integrity of the gas shielding to the molten weld pool and prevent contamination. Recent developments in the patented friction-stir welding process are showing signiﬁcant advantages over gas-shielded processes, particularly for straightforward, long seam welds, such as would be used in joining extrusions to make orthotropic decks or deep girders. The process has been used extensively in military bridging applications and railway carriage fabrication, and gives high-quality welds with less eﬀect on material properties than from conventional welding. Connections made by riveting or bolting are straightforward. Generally, the diﬀerence between bolt size and hole size is less than for steel; typically holes are 1 mm larger than bolts. Friction grip bolted connections are frequently used for bridging applications. Explosion bonding has been successfully used to join aluminium alloys to steel plates. The resulting composite 350 www.icemanuals.com Aluminium in bridges can be welded to an aluminium structure on one face and to a steel structure on the other side. This technique has been used successfully in the oﬀshore industry to join aluminium modules and topsides to steel base structures. Recent developments in the aerospace and automotive industries use high precision castings for connections and nodes, with extruded members between the castings. Whether such developments will progress to civil engineering structures remains to be seen. Fatigue Aluminium structures that are subjected to ﬂuctuating service loads may be liable to fail by fatigue in a similar manner to steel structures. The methodology and principles given for carrying out fatigue calculations to BS 8118 are similar to those in BS 5400, and so will be familiar to most bridge designers. Fatigue failure usually initiates at a point of high stress concentration associated with abrupt changes in geometry or at welds. Careful attention to detail can therefore make signiﬁcant diﬀerences to fatigue life. For bridge work, it is recommended that the structures are designed on a ‘safe life’ basis, i.e. the members are proportioned such that the predicted cyclic stress levels do not result in any fatigue cracks. Eurocode 9 Part 1-3 uses a similar methodology to BS 8118 for calculating fatigue ‘safe life’. The responsible BSI committee considered that some of the detail categories in Eurocode 9 Part 1-3 were subject to misinterpretation, or could give lives that could only be achieved with unrealistic expectations of internal defects. The UK National Annex, therefore, gives references to diﬀerent detail categories from those in the informative Annex of Eurocode 9 Part 1-3. The Eurocode also allows a damage-tolerant approach to fatigue design, i.e. some cracking is allowed to occur in service, provided that there is a stable, predictable crack growth and there are suitable inspection regimes in place. Such an approach should only be taken in conjunction with the owner of the structure, and is not likely to be suitable for bridgework. In general, the ‘allowable’ fatigue stress is independent of the alloy being used. The environment can have an eﬀect on fatigue life, and Eurocode 9 recommends small reductions in fatigue strength for certain alloys if used in a marine environment. Fire safety The theory of the ﬁre safety of structures in aluminium alloys is governed by the same principles and methods as those used for steel structures. However, most of the aluminium alloys start to lose some strength when held at temperatures above 100 8C, and have lost a signiﬁcant proportion by 350 8C. ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. ice | manuals Aluminium in bridges Contrary to popular opinion, aluminium is classiﬁed as non-combustible, and does not burn. Its high coeﬃcient of thermal conductivity is advantageous in situations with local ﬁres, in that the heat is dissipated rather than rising quickly at the ﬁre location. Applications that need a speciﬁc ﬁre resistance should have insulation applied in a similar manner to steel structures. Eurocode 9 Part 1-2 gives comprehensive rules for determining the ﬁre resistance of aluminium structures in such cases. Historic and recent bridges Highway bridges One of the ﬁrst recorded uses of aluminium for bridgework was for the reconstruction of the bridge deck of Smithﬁeld St Bridge in Pittsburgh in 1933. The saving in weight allowed a signiﬁcant increase in bridge capacity and was a good example of the weight advantage of the material. The bridge continued in service with an aluminium deck for over 60 years. The Massena Bridge built in New York in 1946 incorporated a 30 m span formed of two aluminium alloy riveted plate girders. Four years later, in 1950, the construction of the Arvida Bridge in Canada was completed. With an arch span of 91.5 m and total length of 153 m this was the ﬁrst example of an all-aluminium civilian bridge and is still one of the largest (see Figure 1). It is probably fair to say that these bridges were rather experimental in nature. They were fabricated from a high-strength alloy containing copper, which would not be recommended for structural use today, as the corrosion characteristics are signiﬁcantly inferior to other alloys. Despite that, in 2008 the Arvida Bridge is still in service and maintenance work has been relatively low. Two opening bascule bridges were built of aluminium in the UK during the same period, the ﬁrst being the Hendon Dock Bridge in Sunderland (Figure 6) and the second in Aberdeen. These used a similar copper-based alloy for the deck plate, but used a 6000 series alloy for the riveted truss girders. The weight saving leads to economy in the bearings, machinery and counterweights, and it is surprising that aluminium has not been used more widely in opening bridges. As steel prices continued to increase in the 1950s a number of research programmes in the USA resulted in speciﬁc aluminium bridge systems and extrusions including the Fairchild system and Baronic system. These used girders, formed from aluminium sheeting, acting compositely with concrete decks. A number of these were built, as were several bridges using riveted aluminium plate girders acting compositely with concrete decks. These bridges tended to emulate steel techniques and fell from favour as uneconomic when aluminium prices started increasing in the late 1960s. They have continued to give good service. Figure 6 Hendon Dock Bridge in Sunderland (courtesy of the Stafford Linsley collection) Development in Europe of large extrusions from 6000 series alloys in the 1960s extended the potential application of aluminium in bridges, but unfortunately coincided with the regression in use of aluminium through price increases. A fall in price in the 1990s brought renewed interest and development of extrusion systems. A range of box girders made from welding aluminium extrusions together, complemented by orthotropic decks formed from multivoided hollow extrusions, are available. A number of bridges have been built in Europe between 1998 and 2008 using this technology. Speciﬁc examples include the Forsmo Bridge in Norway, a 39 m long bridge to replace a badly deteriorated steel and concrete structure. The old bridge deck was demolished and the new bridge was lifted in and opened to traﬃc, all in a ﬁve-day period. Several opening bridges have been built in Amsterdam including the Helmond and Riekerhavenburg bridges, made entirely of aluminium and using extruded trapezoidal proﬁles for the deck (Figure 7). The stiﬀ lightweight deck allows overall economy to the structure weight, which is particularly beneﬁcial for opening structures and for erection considerations. Footbridges While relatively few highway bridges have been built from aluminium, its use is far more widespread in long-span footbridges, and for link-spans in marine applications (see Figures 8 and 9). The low self-weight and excellent corrosion resistance are particularly beneﬁcial for these applications. Early examples include the 42 m welded plate girder Letten Bridge in Sweden and 52 m span arched truss footbridge at Pitlochry in Scotland. The recent Lockmeadow footbridge in Maidstone uses a clever arrangement of interlocking extrusions that are held together by stressing bars. ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. www.icemanuals.com 351 ice | manuals Aluminium in bridges (a) Figure 7 (a) The Riekerhaven Bridge and (b) the Westerdoksluis Bridge (courtesy of Bayards Aluminium Constructions B.V., the Netherlands) Military bridges Aluminium is particularly suitable for military bridges, where the need for portability and speed of erection favours materials with a high strength-to-weight ratio. The majority of military bridges built since 1960 have been built from aluminium. Early examples were made using the same high-strength copper-based alloy as the Arvida Bridge in Canada, but since 1980 the materials of choice have been drawn from the 7000 series alloys. These specialist alloys are similar to those generally available for civilian use but can have tensile strengths in excess of 500 N/mm2 . Bridge decks and furniture Several manufacturers have developed large multi-voided hollow extrusions speciﬁcally for forming orthotropic Figure 8 352 (b) bridge decks. These have been used extensively in Sweden for bridge rehabilitation and over 70 bridges have had deteriorated concrete decks replaced by the SAPA bridge deck system. Similar systems developed in other countries including the USA, Germany and Norway have also been used for the decks of new or replacement bridges. The extrusions are typically formed from alloy 6063 in the T6 condition. (See Figure 10.) The weight of these deck systems is between 50 and 70 kg/m2 , which is only about one-tenth that of a typical concrete deck. The aluminium decks have good corrosion resistance which, allied to the low weight, makes them ideal for bridge upgrades, opening bridges and long-span bridges. Many of the Swedish decks have been surfaced with a 6 mm thick acrylic surfacing system to minimise total weight. Others have a 40 mm thick layer of a mastic A typical marine link bridge (photo courtesy of the author) www.icemanuals.com ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. ice | manuals Aluminium in bridges Figure 9 Stokkenbrug (courtesy of Bayards Aluminium Constructions B.V., the Netherlands) asphalt type of surfacing. The close tolerances inherent in the extruded decks allows for good ride quality even with such thin surfacing systems. Aluminium bridge parapets have been used for many years, generally with no paint system or other surface treatment. The good corrosion resistance is highlighted by their excellent performance despite being subject to road pollutants, salt spray, etc. A number of manufacturers have had their systems successfully tested to meet the requirements of EN 1317. Aluminium is also appropriate for other bridge furniture including sign gantries, access walkways and maintenance gantries. Corrosion behaviour The aluminium alloys in general structural use have excellent corrosion resistance, which is attributable to the protective oxide ﬁlm which forms naturally on exposure to air. The ﬁlm is usually invisible, relatively inert and adheres strongly to the metal surface. Once formed, it prevents further oxidation and reforms naturally if damaged. It is thus self-healing. 300 mm 250 mm 100 mm 50 mm Figure 10 Example profiles designed for 1.2 m and 3 m support girder spacing The 5xxx and 6xxx series alloys will develop small pits when exposed to industrial pollution. A layer of corrosion product, which inhibits further corrosion, seals the pits and results in a dull grey weathered appearance for untreated aluminium. The 7xxx series alloys exhibit a layered form of corrosion rather than pitting, and have slightly inferior durability. Bi-metallic corrosion is often perceived as a problem, although in reality it is only of concern in certain circumstances. The oxide ﬁlm on the aluminium alloy acts as a natural barrier and it is only if this breaks down from acidic or alkaline moisture, or through abrasion, that the bi-metallic cell will activate. Thus corrosion between stainless steel and aluminium, which have a large potential diﬀerence, is non-existent in most environments as the contact is between two inert oxide layers. Stainless steel ﬁttings are used on aluminium masts and superstructures of yachts with no separating layers, and give many years of good service. Connections between dissimilar metals that are subject to fretting or relative movement can sometimes corrode rapidly, as the protective oxide ﬁlm is repeatedly worn away. Insulating the dissimilar metals with an inert non-absorbent barrier can prevent reaction. Areas that act as moisture traps are susceptible to deterioration in most materials, and aluminium is no exception. Good detailing should avoid areas that may pond or hold moisture. Contact with cement and certain wood preservatives can cause corrosion of the aluminium. Foundation base-plates and interfaces with concrete decks, abutments, etc. should therefore be protected by painting the aluminium contact surface. Annex D of Eurocode 9 gives guidance on circumstances when protection measures are appropriate. Coatings and finishes A variety of ﬁnishes and coatings are available for use on aluminium. While not generally necessary for corrosion resistance of the whole structure, these may be applied for aesthetic purposes or to provide protection to speciﬁc areas in contact with other materials. Typical ﬁnishes include anodising, painting, baked organic coatings and mechanical treatment. Anodising is a process that thickens the layer of natural oxide protecting the aluminium. The anodised surface is extremely hard, provides good abrasion resistance and maintains its appearance for many years. Anodised ﬁnishes are used extensively for architectural applications such as glazing systems and for domestic products. Diﬀerent anodising solutions and dies can give a variety of colours and surface hardness characteristics. The main restriction on anodising is that it is limited to pieces that will ﬁt into the treatment tanks at the anodiser’s works. ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. www.icemanuals.com 353 ice | manuals Generally this will give a limitation of about 10 m on length and 2 m on width or depth. The ﬁnished colour of an anodised piece will vary with alloy composition. If a uniform appearance is required it is necessary to use the same alloy throughout, and to select a welding wire that behaves similarly. (A variety of welding wires can be used on some alloys and some will anodise to a darker colour than the parent metal.) Most of the standard decorative paint topcoats can be used on aluminium provided that suitable preparatory work has been carried out. The preparation essential to ensure good adhesion of the paint involves an initial degreasing and then either etching with an acid solution or jet cleaning with an abrasive such as corundum. Steelor copper-based abrasive should not be used. Any acid should be washed oﬀ with clean water prior to subsequent treatment. The initial etch or jet cleaning is then followed by the application of a thin wash primer, a vinyl-based resin containing a small amount of phosphoric acid. This primer provides the key for standard paint systems to adhere. Areas that will be in contact with concrete or treated timber should be coated with a bituminous paint after the pre-treatment and primer coating. Paints containing lead, copper or tin should be avoided, as they are detrimental to aluminium. Such compounds are not common in modern paints. Baked organic coatings are popular for architectural applications. These give a thick, hard ﬁnish with excellent weathering properties and a wider range of colours than those available from the anodising process. There is a range of proprietary systems, generally based on a polyvinyl ﬂuoride. The systems are usually applied by spraying using an electrostatic process prior to oven baking. Piece size is limited by the available oven size at the applicators, and these are likely to be similar to those noted above for anodising. The organic coatings and anodised systems may be appropriate for pedestrian parapets and fascias on architecturally inﬂuenced bridges. Mechanical treatment may be used to enhance the appearance of mill ﬁnish aluminium. Sheet and plate can have an embossed ﬁnish, such as stucco, applied by passing through textured rollers. Light blasting with corundum or beads can be used to give a uniform matt ﬁnish and avoid initial high reﬂectivity. Sustainability Aluminium is the third most abundant element in the earth’s crust, occurring mostly as aluminosilicates. It is extracted by electrolysis of alumina from bauxite fused with cryolite. The abundance of raw material and ease of recycling are such that resource depletion will not be an issue. Extraction rates are good, with 4 t of bauxite producing 1 t of aluminium. 354 www.icemanuals.com Aluminium in bridges Various studies have been carried out to consider ecological impacts by applying life-cycle analysis techniques, and comparing aluminium with other materials such as steel for structural components in buildings. Using the cumulative energy demand as a comparator, aluminium structures with an anodised or painted surface have slightly less of an impact than painted or galvanised steel, once recycling is taken into account. If the aluminium is left with no surface treatment, the diﬀerence becomes greater. The studies have generally assumed that the structural elements of the building will not be repainted in their lifetime. Similar studies for bridgework should therefore show an even greater beneﬁt from using aluminium. The electrolytic process used in a primary smelter is energy intensive, and it is not surprising that the majority of primary aluminium is produced using hydroelectric power. Current ﬁgures show that about 95% of aluminium used in the construction industry is recycled. Energy consumption in the recycling process is only about 5% of that used in producing new aluminium. Other advantages accrue from savings in fabrication, transport and handling due to the low self-weight and from labour-saving features that can be incorporated in extrusion design. Future trends Research into structural application of aluminium alloys in civil engineering structures is disappointingly sparse, and most future developments are likely to arise from advances in other applications, such as the aerospace and transport industries that have diﬀerent criteria to address. Traditionally, the use of castings has been discouraged for structural applications, as the quality and brittle characteristics of many castings have been unsuitable. The automotive and aerospace industries have developed the use of high-precision castings that have good ductility, and are increasingly using these in combination with purposedesigned extrusions. Such methods are particularly appropriate for volume production. The technology is likely to be useful in civil engineering, if suﬃcient quantities can justify the development and tooling costs for any speciﬁc project or proprietary system. Development of material technology continues. New processes using ﬁbre-reinforced aluminium and powder metallurgy promise materials that are stronger and stiﬀer than the current alloys in structural use. The increase in stiﬀness will be particularly relevant. Technology for joining materials is an area of research and development that transcends diﬀerent industries quite well. Developments in friction stir welding and adhesive bonding over the past ten years have yet to become ﬁrmly established in the civil engineering industry, and are not covered in detail by the current design codes. They are, ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. ice | manuals Aluminium in bridges however, well established in the manufacture of railway carriages and in military bridging. References British Standards Institution. (1969) BS 449-2. British Standard Speciﬁcation for the Use of Structural Steel in Buildings: Metric Units. BSI, London. British Standards Institution. (1992) BS 8118-1. Structural Use of Aluminium. Part 1: Code of Practice for Design. BSI, London. British Standards Institution. (1992) BS 8118-2. Structural Use of Aluminium. Part 2: Speciﬁcation for Materials, Workmanship and Protection. BSI, London. British Standards Institution. (1998) BS EN 1317-2. Road Restraint Systems, Performance Classes, Impact Test Acceptance Criteria and Test Methods for Safety Barriers. BSI, London. British Standards Institution. (2000) DD ENV 1999-2. Eurocode 9. Design of Aluminium Structures, Part 2. Structures Susceptible to Fatigue. BSI, London. British Standards Institution. (2007) EN 1999-1-1. Eurocode 9. Design of Aluminium Structures. Part 1-1: General Structural Rules. BSI, London. British Standards Institution. (2007) EN 1999-1-2. Eurocode 9. Design of Aluminium Structures. Part 1-2: Structural Fire Design. BSI, London. British Standards Institution. (2007) EN 1999-1-3. Eurocode 9. Design of Aluminium Structures. Part 1-3: Structures Susceptible to Fatigue. BSI, London. British Standards Institution. (2007) EN 1999-1-4. Eurocode 9. Design of Aluminium Structures. Part 1-4: Cold-formed Structural Sheeting. BSI, London. British Standards Institution. (2007) EN 1999-1-5. Eurocode 9. Design of Aluminium Structures. Part 1-5: Shell Structures. BSI, London. British Standards Institution. BS 5400 Part 10. Steel, Concrete and Composite Bridges, Part 10: Code of Practice for Fatigue. BSI, London. British Standards Institution. EN 1091-3. Execution of Steel and Aluminium Structures. Part 3: Technical Requirements for Aluminium Structures. BSI, London. CP 118: 1969 British Code of Practice. (1969) The Structural Use of Aluminium. The Institution of Structural Engineers. (1962) Report on the Structural Use of Aluminium. ISE, London. Further reading Bowen L. P. (1996) Structural Design in Aluminium. Hutchinson, London. Bull J. W. (1994) The Practical Design of Structural Elements in Aluminium. Avebury Technical, Aldershot. Dwight D. (1999) Aluminium Design and Construction. ‘e-book’, E & F N Spon. Kissell J. R. and Ferry R. L. (1995) Aluminium Structures – A Guide to Their Speciﬁcation and Design. Wiley, London. Mandara A. and Mazzolani F. M. (1997) Plastic design of aluminium – concrete composite sections: a simpliﬁed method. Proceedings of the IABSE International Conference on Composite Construction – Conventional and Innovative, Innsbruck, Austria, 16–18 September. Mazzolani F. M. (1994) Aluminium Alloy Structures (2nd edn). Chapman & Hall, London/E and F N Spon, London. Reynolds Metal Company. (1997) Alumadeck – Bridge System – The Technical Guide for Aluminium Decks. Reynolds Metal Company. Soetens F. (1987) Welded connections in aluminium alloy structures. Heron, 32. Soetens F. and Van Straalen I. J. J. (2003) Aluminium bridges, aluminium bridge decks. Proceedings of the European Bridge Engineering Conference on Lightweight Bridge Decks, Rotterdam, Brisk Events. Svensson L. and Petterson L. (1990) Aluminium extrusion bridge rehabilitation system. Proceedings of the 1st International Conference on Bridge Management, University of Surrey. Thomas W. M. (1998) Friction stir welding and related friction process characteristics. Proceedings of INALCO, Cambridge. ICE Manual of Bridge Engineering # 2008 Institution of Civil Engineers Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. www.icemanuals.com 355 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.