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Resurgence of Aluminium in Structural Engineering

(Artculo original)

S. K. Ghaswala, Consulting Engineer, Mumbai

More than half a century ago this author presented a paper on "Basic Concepts of Structural Theory of Aluminium

Alloys" at the 4th Congress of IABSE (International Assoc. of Bridge and Structural Engineering) at London and

Cambridge in August/September 1952. This was followed by another paper on "Some Aspects of the Plastic Design of

Aluminium Structures," published in Vol 16 of IABSE Congress at Zurich, Switzerland in 1956. These two papers

together sought to unify the basic principles for design of aluminium structures and tended to establish this light metal

as a distinctive structural material as was known half a century ago. Since then progress was rather limited except for

its use in 'aircraft structures and to some extent in the framework of railway coaches.

Currently, there has been a resurgence of this metal due to a deeper understanding of its design theory as well as theavailability of a larger variety of alloys with enhanced strength. A synoptic review of these aspects is presented here

by first evaluating the basic differences in steel and aluminum and then briefly highlighting the importance of buckling,

torsion and plasticity, in order to stress the need for designing aluminum differently from steel and not merely copying

it. This is followed by the applicapability or otherwise of aluminum in different types of bridges; jointing techniques

and some interesting and unique applications of aluminium indicating the versatility of this metal. The article ends with

a digress on futuristic trends on how aluminium will shape up and show its radiating potencies in the years to come in

the domain of structural engineering in construction industry.

Differing Properties

The Structural design of aluminium is distinctly different from that

of steel mainly because of its differing physical and mechanical

properties.

The density of aluminium at 2.7, its elastic modulus at 70,000

N/mm2 and its rigidity modulus at 27,000 N/mm2 are all one-third

of steel, while its Poisson's Ratio at 0.33 is nearly the same as

steel. Unlike steel aluminium has a non-linear steress/strain

curve. The linear thermal expansion of aluminium at 23 x 10-6 /C

is nearly twice as large as steel. Aluminium melts at 6000-C as against steel at 15000C, requiring careful consideration
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during welding. Aluminium has a thermal conductivity nearly three times, and electrical conductivity about twice that

of steel. Aluminium resists the ravages of time, temperature, rust, humidity and warping adding to its long life-cycle.

In comparison with steel the cost of machining and shaping aluminum is just l/9th of steel and canthis form a deciding

factor in its selection.

Aluminum, unlike steel, is available in a variety of strengths tempers, mechanical properties as a well as shapes, such

as extrusions like bulb angles and channels, top-hat sections and squat and I sections.

Design Variations

A member subjected to axial tension can be assumed to behave

elastically as long as the maximum stress on the minimum net

section does not exceed the yield strength. Should this happen,

the member will elongate permanently and failure will occur when

stress on the minimum net section reaches the ultimate strength

of aluminium. The elongation required to permit sufficient stress

relief by yielding is generally of the order of 3%, a requirement

met by all structural aluminium alloys. Generally all discontinuities

act as stress-raisers and can be disregarded in static structures,but have to be carefully taken into account where fatigue is of

importance.

In view of aluminium's low density a proved method of design suggested by Reinhold Gitter of Germany (SEI, 4/06)

would be to increase all dimensions, with the exception of width, by a factor of 1.4. This results in a cross section

having a moment of inertia (I) about three times as large, so that a section of the same stiffness (E x I) will have

about 50% weight. In steel only standard sections are available whereas aluminium can individually be designed to

save weight which at times can be as high as 50%. This arises when there are no restrictions in height., and local

buckling is not a design criterion. Another aspect is that if I is increased by a factor of three and the height is

increased only by 1.4, the section modulus increases by 2.14, the stresses in all sections will be virtually half that of

steel section. This gives rise to an important consideration that in aluminium design the engineer should not always

look to higher strengths in alloys, since lower strength alloys like AlMgSl type offer at times better results.

On the lines of open-web steel joists, aluminium joists can offer greater advantage due to their lighter weight and ease

of fabrication. Work carried out by S.M. Hasan (Proc. First East Asian Confon Struct, Eng, Bangkok, vo!2, 1/86)

indicates that the open-web steel joists having a span/depth ratio of 11.i appeared most economical and efficient for

load carrying capacity in the range of 2.4 to 20 M in length and 200 mm to 750 mm in depth. It may be interesting to

investigate this aspect further for such joists in aluminium.

Different countries have differing codes for Safety Factor for aluminium design. Where no particular codes exist or any

special recommendations given, the allowable stresses for simple elastic analysis of static structures may be obtained

for tension and bending using a safety factor of either two on the yield strength or three on the ultimate strength

whichever gives the higher stress. In the case of compression members, a factor of two against yield point may be

used.

Buckling and Torsion

Buckling of aluminium requires special consideration not usually necessary in traditional materials. When the shapes

are standardised for which tables are available, simple checks on eleastic stability suffice as in steel design. With

aluminium the greater flexibility in its manufacturing process and the desire to take full advantage of its low weight,

there is a tendency to adopt thinner sections and special shapes and proportions as in extrusions. Here it becomes

necessary to examine carefully the various possible modes of instability. A detailed discussion is not possible in this

article. However some practical suggestions can be made as under:


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struts have different buckling loads but different weights as well. However, since the elastic modulus ratio of these

metals is o.34; as the slenderness increases, the critical buckling load ratio tends towards this value. Because this

coincides with the weight ratio, two bars of different metals but equal weight will have approximately the same

buckling load.

Bridge Design Concepts

In plate girder bridges much of the web material is utilized very

inefficiently in comparison! with truss type bridges. In steel where

costs are low this may not be a serious consideration but in

aluminium this becomes a major factor and as such the light metal

offers little economy in plate girder bridges. However a different

type of girder known as 'tension-field' beam or the 'Wagner' beam

can prove quite effective. Here the beam has a web of very thin

sheet of aluminium which is purposely made to wrinkle of buckle

under shear stress. The wrinkled web then 'acts as a diagonal

tension member as in ordinary open web trusses thereby creating

a 'tension-field' beam, In cantilever bridges aluminium offers

specific advantages particularly when used in suspension spanssince the light metal reduces stresses in the arms of the cantiliver. In long span suspension bridges with deep

stiffening trusses aluminium offers considerable advantages, provided other factors like overall stiffness, aerodynamic

oscillations and general construction facilities are carefully considered. In cable-stayed bridges the high compressive

forces introduced in the deck by stay cables restricts its increase in the length of the span. According to J. M. Schlenck

of the University of Stuttgart, Germany, the potential limit of suspension bridges could be as high as 18,000 M, when

used with high tensile steel cables. However the limit for cable-stayed bridges would, in our present state of

knowledge, be limited to 1500 M. (J. Mathivat, IABSE Proc, Deauville, 10/94.)

The ultimate tensile strength of commonly used aluminium alloys

varies from 240 N/mm2 to 340 N/mm2, with their 0.2% proof

stress being about 80% of the above figures. However there are

available aluminium alloys of very high strength of 450 and 560

N/mm2, which are currently used in aircraft, which in times to

come could be available for bridge construct-ion. In fact these

alloys can easily compete with high strength steels like PE 415,

FE 500 and FE 550 which have ultimate strength ranging from

450 to 580 N/mm2. In fact in the next generation Tata Steel

hopes to fabricate steel of 1000 N/mm2 yields strength having

an elongation of 50 percent. when in the not-too-distant future

high strength aluminium alloys are developed, aluminium

bridges of unprecedented spans could be achieved easily.

In moveable bridges and in military bridges which have to bequickly dismantled, shifted and reassembled at another location

aluminium offers outstanding advantages, although at present

their limiting span is about 40 M. The use of extruded sections in

combination with the newly developed 'friction stir welding

process, makes aluminium very suitable for deck structures. The

possibility of producing hollow profiles without much welding

increases the bending strength and stiffness in all directions and

also increases the torsional resistance of bridges. The other


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advan-tages are reduction in maintenance costs because of high corrosion resistance of these alloys and the fact that

the low weight of aluminium compared to steel allows higher traffic loads without any extra reinforcement td the

existing structure.

The development of an all-aluminium light weight concrete (AALC) girder for bridges is now being studied at various

European centres and it appears to offer advantages over an all-steel or all-concrete construction. The first full

composite aluminium girder and concrete deck was built in Des Koines, USA as far back as 1958 followed by othersmaller bridges particularly the French suspension bridge in Grosle. Pitiably so far no specifications have been

formulated for designing AAlC bridges and such standardisation is very necessary if such structures are to be built. For

this purpose it is felt that considerable research is very necessary on several fronts such as the use of conventional

theory which has proved useful for steel composite girders; the influence of low value of elastic modulus on the

stiffness and stability of the bridge and lastly the Rheological and thermal influences on the carrying capacity of the

bridge trusses because of the higher thermal expansion of aluminium.

Jointing

Very careful consideration is required for welding of aluminium structures because of its comparatively higher

coefficient of expansion than steel; its non-linear stress-strain curve and its low elastic modulus, resulting in the

softening of the material in the 'heat affected zone'(HAZ) causing buckling.

A variety of methods are available for jointing such as welding, riveting. bolting, adhesive bonding, and a combination

of these Techniques. Essentially for a design connection it is essential to see that the force in the connection caused by

the design load is less than or atleast equal to the design strength of the connection. The major difference between

steel and aluminium welding is that in aluminium, the adhered oxide film on aluminium surface has to be carefully

removed prior to welding and molten metal has to be shielded against oxygen from the atmosphere. As a

generalisation it can be said that in one of the AlMgSiCu alloys, the elastic limit and the ultimate strength are reduced

drastically after welding. Within the range of plate thickness for general engineering applications, the distance from a

failure plane of HAZ to the centre of the weld is about 14 mm and to the toe of the weld about 8 mm. The elastic limit

and ultimate strength in the failure planes of HAZ are about 0.58 and 0,59 times respectively of the parent metal. A

very recent development is friction stir welding (FSW) process where the joint is created by frictional heat.

Considerable work is being done at various centres and particularly in Japan (Jnl of Japan Inst. of Metals, 11/07). FSW

is a solid phase welding process in which the controlling parameters are time, temperature and deformation. To create

a friction stir weld the probe part of the tooling is driven downwards into the joint until the shoulder contacts the work

piece and is then traversed forward. It is the role of the probe to create the plasticized layer and thoroughly stir both

sides of these layers together. During jointing the shoulder remains firmly in contact with the joint and provides both

extra frictional heat and constraint to the flow of the third body of plasticized material. Essentially FSW is a three-

dimensional version of friction surfacing as shown in the diagram. When jointing is done by riveting care should be

taken to see that the rivets of aluminium alloys are not harder than the alloy used for the components. In the case of

field erection, bolts can also be used. Generally it is advisable to use steel bolts which are galvanised, or aluminised or

cadium plated to precent rusting.

Applications

The Hall process of commercial exploitation of aluminium was patented as far back as 1886 but the first notable

application of aluminium was made over a century later in 1993 in the reconstruction of the bridge deck of Smithfield

Street bridge in Pittsburgm, USA. The deteriorated steel and wooden stringers were replaced with a light weight

aluminium deck, which increased the load carrying capacity of the bridge from 4.4 to 16 tonnes. When rock is present

to allow construction of strong abutments an arch bridge becomes very favourable over other types. This was

exhibited in the construction of the world's FIRST all-aluminium bridge at Arvida in Quebec, Canada in 1946. Currently

it is the. LONGEST aluminium bridge in the world and is still in service. The main span of the arch is 88.4 M with &

height of 14.5 M. On both sides of the span there are small multiple arches 6.17 M long which from the approaches.

The concrete deck is 7.2 M wide and is 2125 mm thick with a 62.5 mm thick bituminous wearing surface, and two

footpaths 1.2 M wide. It uses an aluminium alloy designated 2014 - T6, as a result of which there was a weight saving


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of 43.5% over a comparative steel bridge. The bridge is designed to carry a load of 20T truck or a 12T transformer on

a 12T flat bed pulled by an 18T tractor. A number of small bridges have been built since then. Some of these are

ALCOA railroad bridge, New York, 1946; Des Moines Iowa bridge, 1958; two twin moveable bridges in Sunderland and

Aberdeen UK; the Saone River bridge, Montmerle, France, having two suspension spans of 79.9 M each with

aluminium truss girders; a 39 M long Formosa bridge in Norway, 1996; and a single lane floating bridge in the

Netherlands. One of the most unique pedestrian bridges is the 'Bridge of Aspiration'connecting two buildings across

Floral street at 4th floor level between the Royal Ballet School and the Royal Opera House at Convent Garden in U.K.Small in size but extremely complex in geometry the structure is quite elegant and has to be seen to be believed. The

bizzare twisted shape is formed by a series of 23 square frames each rotated about 4o relative to its neighbour so that

the whole set describes a full 90 o turn. The entire bridge was erected at the site in a sigle 3-hour operation in August

2002 at a cost of Euro 1.2 million and opened for service in March 2003.

Braced domes are typical examples of space structures which are ideally

suited for aluminium. Their outstanding feature is that they support

themselves while being built and hence require no expensive scaffolding.

Yet another type is stressed-skin sheet space grid which comprises a

large number of identical 3-dimensional units of triangular shape in thin

aluminium sheets joined to form tetrahedra or pyramids. A valuable

feature of these grids is that their strength depends much more on their

geometrical configuration than in the properties of the alloy used. The use

of aluminium in jib-type cranes is justified on the grounds of its low

weight which could be at times as low as one-third a similar steel jib.

The low density and high strength can also be adventageously utilised in

bridge erection equipment like falsework, shuttering, cradles etc. The

launching truss is another example where it can find use in launching

precast concrete bridge beams. It is an efficient and quick method of

construction particularly where access below the bridge for heavy lifting

equipment is difficult.

Sustainable buildings are now gaining momentum on an international

scale as evinced form the innovative and spectacular Polytechnic of Milanin Poggiofranco in Italy, This dual tower structure has an aluminium

double skin on its northern and eastern facades made out of sandwich

panels. Similiarly in solar power plants as the one in Nevada, USA

.extruded aluminium components with high torsional rigidity and high

recycled content are used extensively. Another use could be in cable-

suspended roofs, first used in A.D-70 in the 189 by 156 M long famous

Roman Colosseum. Although the system was used off and on, no real

advances were made till 1950 when several such structures were built

with steel cables and supports. Since these are fully tension-stressed

structures, it is worth investigating the application of aluminium cables in

such roofs since the lighter weight can enable the span to be increased

considerably or the supports could be made lighter. Among other unique

structures built in aluminium are the 45.38 M high Barcelona Airport

Tower, Spain, with an octagonal footprint of 2.7 M sides; very large space

frames like the Corti Veneta panels on the Verona highway connecting

north Europe with Italy having dimensions of 9 x 44 M positioned at 15 M

elevation. (Japan Inst of Light Metals, Jnl, 7/07). In India aluminium has

been used in various shop fronts, door and window frames, roofing,

curtain walling, handrails and the like. However hardly any applications

have been made in structural work like bridges, steel-type towers, space frames, deployable structures, domes etc.

Interestingly, long back, the coaches of the earstwhile BB&Ci railway (now Western railway) introduced for the first


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time in their electric railways doors and bodywork in aluminium. The various structures built for the 2008 Olympics

held in China have made extensive use of this light metal for stressed components. Even the Olympic Torch with its

flame is in aluminium. It stands 82 cm high and weighs 985 grams, This metal was specifically chosen for its low

weight, ability to resist corrosion and colour discolourisation.

Future Outlook

Aluminium with a proportion of 8% of the earth's crust the world's third most abundant element after Oxygen and

Silicon. Today's known reserves of Bauxite the ore from which aluminium is produces-are sufficient to last for a 1000

years at the current rate of consumption. The secondary metal produced from recycled aluminium requires only about

2.8 Wh/Kg of metal produced, as against a primary production requirement of about 45 Wh/Kg Increasing the

production of recycled aluminium is important from an ecological point of view since producing this metal by recycling

creates only about 4% as much C02 as by primary production. At the end of the structure(s) life-stage aluminium is

100% recyclable and can be used in structures without any loss of strength or quality, rarely found in other materials.

Of an estimated total of over 700 million tonnes of aluminium produced in the world since commercial manufacture

began, about 75% of this bulk is still in productive use. Recycling of post-consumer aluminium now saves an

estimated 84 million tonnes of Greenhouse Gases per year of emissions, roughly equivalent to the amount of

emissions of some 15 million automobiles.

On the basis of the present cost, aluminium is four to five times more expensive and as such has to be designed and

used very carefully so that its area or volume is stressed to its maximum possible extent. This high cost arises mainly

because primary aluminium requires in its smelting process 15 MW per tonne of electricity which is as much as a third

of the overall cost of production. If this process requirement can be reduced by introducing advanced metallurgical

processes, the basic cost of the metal can come down very appreciably. So far none is in sight. However as more and

more aluminium production is envisaged on a large scale, as in the case of Sohar Aluminium which is building in Oman

at Sohar a 2.4 billion dollar smelter of 350,000 tonnes capacity and having the world's longest potline, it is possible

that the price of the metal may fall.

Today's aluminium alloys are nearly 1.5 to 2 times stronger than the early alloys used in bridge and other structures.

New production processes have been developed to compact powdered aluminium into billets, which can be extruded,

forged or rolled into aluminium mill products. These are being mechanically alloyed and reinforced with 'whiskers' and

fibres to produce unique structural properties. In fact fibre-reinforced aluminium is stronger and stiffer than steel and

yet weighs only one-third as much. However at present its cost and reparability have presented it from being used in

major structural applications. A major revolution could be started with the advent of large scale production of hybrid

aluminium structures in which a mix of these different aluminium systems is adopted to maximise the structural

properties and minimise the total life-cycle costs. All these developments can have a major impact on the final design

of aluminium structures in the years ahead.

So far no single common Standard was adopted across Europe for design of aluminium structures. The British, French,

German, Indian and other countries have their own respective Codes. It was in 1995 that the European Committee for

Standardization started its excercise of unifying all Codes, from which has now arisen the Eurocodes, comprising a

suite of 58 parts and sub-parts. The respective countries are required to withdraw their existing Codes and adopt

theses Eurocodes by March 2010. The recently relased Eurocode - 9, Design of Aluminium Structures, is by far the

most extensive and uptodate standard so far compiled. In fact EN-1999-1-1 offers a wide range of alloys with proofstress ranging from 35 to 290 N/mm2. The high strength alloys used in aircraft, which in times to come will be used in

civil structures are ES AW-7075,of 560 N/mm and EN Art - 2024 of 450 N/mm2.

Realising that India cannot afford to lag behind in standardization the Indian Codes are currently under revision and

possibly in times to come will have to be intune with Eurocodes. It may then be possible to design aluminium

structures more economically and efficiently and thereby enlarge its scope and applications in the domain of structural

engineering.

NBMCW September 2009

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