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Građevinar 11/2015
1075
GRAĐEVINAR 67 (2015) 11, 1075-1085
DOI: 10.14256/JCE.1395.2015
Authors:
Assoc.Prof. Davor Skejić, PhD. CE
University of Zagreb
Faculty of Civil Engineering
davors@grad.hr
Prof. Ivica Boko, PhD. CE
University of Split
Faculty of Civil Engineering, Arch. and Geodesy
ivica.boko@gradst.hr
Assoc.Prof. Neno Torić, PhD. CE
University of Split
Faculty of Civil Engineering, Arch. and Geodesy
neno.toric@gradst.hr
Subject review
Davor Skejić, Ivica Boko, Neno Torić
Aluminium as a material for modern structures
The paper offers a systematic outline of aluminium alloys and places emphasis on
their advantages by providing examples of aluminium use in modern structures.
Rapid development of standards for this "new" material enables its wider utilization
although all participants in the construction process, structural engineers in
particular, should be additionally educated in this segment to enable a real increase
in its use.
Key words:
aluminium alloys, structures, standards, design, application
Pregledni rad
Davor Skejić, Ivica Boko, Neno Torić
Aluminij kao materijal za suvremene konstrukcije
U radu se daje sustavan prikaz aluminijskih legura, te se na primjerima primjene
aluminija u suvremenim građevinskim konstrukcijama naglašavaju njegove prednosti.
Ubrzani razvoj normi za ovaj "novi" materijal omogućuje njegovu primjenu, no za
stvarno povećanje te primjene nužno je o tome dodatno educirati sve sudionike
gradnje, a posebno inženjere konstruktore.
Ključne riječi:
aluminijske legure, konstrukcije, norme, projektiranje, primjena
Übersichtsarbeit
Davor Skejić, Ivica Boko, Neno Torić
Aluminium als Material für moderne Tragkonstruktionen
Diese Arbeit gibt eine systematische Darstellung von Aluminiumlegierungen und weist
an Beispielen der Anwendung von Aluminium in modernen Bauwerkskonstruktionen
auf seine Vorteile hin. Die zügige Entwicklung von Normen für dieses "neue" Material
ermöglicht seine Anwendung; dennoch erweist sich für eine weitere Ausdehnung die
Edukation aller Teilnehmer des Bauprozesses und insbesondere der Tragwerksplaner
als notwendig.
Schlüsselwörter:
Aluminiumlegierungen, Tragkonstruktionen, Normen, Entwurf, Anwendung
Aluminium as a material for
modern structures
Primljen / Received: 25.6.2015.
Ispravljen / Corrected: 26.10.2015.
Prihvaćen / Accepted: 20.11.2015.
Dostupno online / Available online: 10.12.2015.
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1. Introduction
Aluminium (Al) is the third most abundant element, subordinate
only to oxygen and silicon, and it makes up 8 % of the Earth"s crust.
It is also the only light metal to find its application in load-bearing
structures in civil engineering. Nowadays, only steel is used more
than aluminium in the field of civil engineering.
In nature, aluminium does not occur as a free metal but as an oxide
mixed with steel, silicon, vanadium and titanium oxides. The ideas
of isolating aluminium started to emerge at the beginning of the
nineteenth century. A Danish scientist, Hans Oersted, managed
to isolate the metal particle of aluminium in 1825, but the first
significant result was achieved in 1827 by a German chemist
Friedrich Wöhler after 20 years of research. In 1854, a French
chemist Henri Sainte-Claire Deville, a professor at the Sorbonne in
Paris, developed a reduction process using sodium. The process,
with further refinements, enabled limited production of expensive
aluminium. Siemens"s discovery of the dynamo machine in 1866
was an important stepping stone toward solving the principles of
electrolysis, which was contemporaneously and independently
patented by Paul Héroult in France, and Charles Martin Hall in the
USA. After a long initial period of technological development, the
time finally came for the use of aluminium alloys in structural design.
However, it is only in the early 1950s that the first structures made
of aluminium alloys were erected in form of prefabricated systems.
At that time, development of such structural applications was
hindered by inadequacy or even a complete absence of standards
and recommendations, all of which made structural design difficult
for engineers, consultant engineers, and supervisory boards.
Over the past several decades, structural behaviour of extruded
and welded members has been in the focus of both theoretical
inquiries and experimental research [1, 2]. Conclusions obtained
so far present a solid basis for modern standardization and it may
reasonably be argued that, at the European level, the limitations
once imposed by the absence of standards and regulations have
been successfully surpassed: starting with the first edition of the
ECCS committee recommendations, published in 1978 by ECCS T2,
presided by F. M. Mazzolani, up until publication of the final version
of Eurocode EC 9 in 2007, by the Technical Committee CEN-TC 250/
SC9, presided by F. M. Mazzolani. This committee is still active and
new amendments and corrections are being continually presented
and considered. Croatian standardization body, the Croatian
Standards Institute, and more specifically its sub-committee HZN-
TO 548/PO 9 (presided by D. Skejić) is also regularly involved in the
discussions and amendments of the standards. Nevertheless, a
lack of information on the potential of aluminium alloys in structural
applications is evident, which is why their comparative advantages
are quite seldom recognised by structural engineers.
Therefore, the main aim of this paper is to provide structural
engineers with knowledge about possibilities offered by the
utilisation of aluminium alloys in modern structural engineering.
Basic properties of aluminium alloys are presented in the paper,
and readers are introduced to a variety of related standards
(HRN) EN 1999 and their applications, which provides an
interesting introduction to a plethora of technical areas related to
aluminium. Small weight, corrosion resistance, and a wide range
of structural shapes, rightfully open the door to a more extensive
use of aluminium in structural design. The authors hope that this
paper will encourage structural engineers to delve into the series
of standards (HRN) EN 1999, recognize positive properties of
aluminium alloys, and start to use them more often in practice.
2. Aluminium alloys for structural applications
2.1. General
The success of aluminium alloys when used as a construction
material, and their possibility of competing with steel, are based
on assumptions related to their physical properties, production
process, and technological features. Aluminium alloys are
normally considered to be economical and, consequently, more
competitive in applications where the following prerequisites
are important, [1, 2]:
A. Low self-weight
Low specific weight of aluminium alloys, amounting to just a
third of steel"s weight, enables and/or facilitates:
-simplification of construction phases;
-transport of fully prefabricated components;
-reduction of load transfer to foundations;
-energy savings either during construction or in subsequent
use;
-reduced need for physical labour.
B. Corrosion resistance
The formation of a protective oxygen film on the surface enables:
-reduction of maintenance costs;
-good performance in corrosion-inducing aggressive
environments.
C. Functionality of structural shape
The extrusion process enables:
-improvement of geometrical characteristics of cross sections
by design of minimum weight shapes presenting at the same
time the highest level of structural efficiency;
-creation of stiff forms without using complex sections, thus
avoiding welding or bolting;
-simple connecting systems between different components,
thus improving joint details;
-combining different functions of structural components,
which results in a more economical and rational cross section.
2.2. Aluminium
Regardless of the fact that aluminium is present on the Earth"s
surface in inexhaustible amounts in the form of numerous oxide and
silicon-based minerals, bauxite has remained, due to its solubility in
alkaline media and high percentage of aluminium (20 % - 30 %), the
main economically and technologically significant raw material for
the production of alumina (Al2O3). Bauxite is a heterogeneous ore that
mainly consists of one or more aluminium hydroxides, sometimes
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Aluminium as a material for modern structures
combined with silicon dioxide, steel oxide, and aluminosilicate
(argentite). Aluminium oxide occurs in bauxite in form of three
hydrated mineral types: hydrargillite, boehmite, and diaspore. Based
on the data provided by the International Aluminium Institute [3],
the world"s annual production of alumina and primary aluminium,
expressed in 103 tonnes, is presented in Figure 1.
Figure 1. World"s annual production of alumina and primary
aluminium expressed in 103 tonnes, [3]
Basic physical properties of aluminium and aluminium alloys are
presented in Table 1. Other beneficial properties of aluminium
and its alloys are:
-2.9 times lighter than steel,
-shows good mechanical properties at low temperatures
(including toughness),
-reflects well light and heat,
-non-toxic and without negative impacts on nature,
-good corrosion resistance due to its natural oxide layer,
-non-magnetic,
-no arcing during processing.
Table 1. Comparison of basic physical properties of pure aluminium
and its alloys with steel
However, in addition to its high production price, there are
some other properties that negatively influence the choice of
aluminium as a building material. The principal ones are its high
deformability (modulus of elasticity is three times lower than
that of steel), susceptibility to stability problems, significant
reduction of load bearing capacity in heat affected zones during
welding, and a relatively high sensitivity to fire.
2.3. Aluminium hardening
In its pure form, aluminium is a metal with a relatively low
strength. The tensile strength of aluminium in its purest form
is about 40 MPa, and a proof strength is about 10 MPa. These
values are too low for the majority of technical applications.
Therefore, aluminium alloys with mechanical properties that
considerably surpass those of the original material have been
developed. Hardening of aluminium can be achieved by alloying,
work hardening, or precipitation. One of the most important
properties of aluminium, and of the majority of its alloys, is
their easy strain deformability and the possibility of achieving
numerous thermal (metallurgical) stages, which consequently
enable development of a wide variety of mechanical properties
for structural applications.
2.3.1. Alloy hardening
Imperfections in the lattice structure can very efficiently be
created by introducing foreign elements in the aluminium matrix.
Up a certain point, their efficiency depends on the difference
between the atomic radii of the foreign element and aluminium.
Magnesium ranks among the elements that are best suited
for meeting strength improvement requirements. That is why
a hundred years ago aluminium-magnesium alloys used to be
the first choice materials for structural aluminium applications.
High strength values have been obtained in alloys containing
up to 10 % of magnesium. However, problems during the cold
and hot working of these alloys, and the less than optimum
corrosion behaviour of alloys with a very high magnesium level,
led to a gradual adoption of alloys with a lower percentage of
magnesium, but with an addition of manganese.
2.3.2. Work hardening
Plastic deformation creates imperfections in the lattice,
generating a significant number of the so-called dislocations,
especially along slip planes. New slip planes continually emerge
with an increase in load and deformation. The mechanical
strength of material increases with an increase in the density
of dislocations. Simultaneously, the ductility decreases until,
finally, the deformation process is stopped. During cold rolling,
this work hardening is performed until cracks begin to occur
in the material, usually at the edges of strips. However, this
kind of hardening process can be reversed by heat treatment.
Depending on temperature, and on the time of exposure to
temperature, what has been gained in the hardening process
can be reversed and returned to its original stage, i.e. the stage
before cold working. Original ductility of material can also
be restored. This heat process of obtaining the so-called "O
temper" material is described as annealing. The cold working
process can be restarted from this soft "O" stage. In industrial
production, this process can be repeated several times to
produce very thin materials from originally thick metal plates.
Physical properties / Metal Aluminium/Aluminium alloys Steel
Melting temperature 660 °C 1425 - 1540 °C
Density at 20°C 2700 kg/m37850 kg/m3
Thermal elongation 23·10-6 °C-1 12·10-6 °C-1
Specific heat ~ 920 J/kg°C ~ 440 J/kg°C
Thermal conductivity ~ 240 W/m°C ~ 54 W/m°C
Modulus of elasticity 70 000 N/mm2210 000 N/mm2
Shear modulus 27 000 N/mm281 000 N/mm2
Poisson's ratio 0,3 0,3
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2.3.3. Precipitation hardening
Precipitation hardening, also called age hardening of metal, is
a type of a heat treatment method consisting of an isolation of
a finely dispersed phase in the base metal structure. The effect
of precipitation hardening was first discovered and practically
applied by Wilm in 1906. The precipitation hardening effects can
be attributed to the fact that one or more suitable elements can
form particles or the so-called intermetallic compounds, either
among each other or together with the aluminium matrix. They
also create lattice imperfections and, depending on the size of
these particles and the uniformity of their distribution, they can
cause a significant increase in strength. The entire process begins
with the heat treatment of the solution, i.e. all alloying elements
are in a solution (a solid solution) that is subsequently quenched
to obtain a uniform distribution of all elements at ambient
temperature. After that, all the elements start to move into the
aluminium matrix. They form intermetallic compounds and grow.
Although this happens at room temperature, the process is more
efficient at elevated temperatures (natural and artificial ageing).
It is important to note that the effects of precipitation hardening
can be reduced if a material is exposed to high temperatures for
short periods of time, or to more moderate temperatures over a
longer period of time. It should also be noted that precipitation
hardened alloys are nowadays dominant in many areas (e.g. in the
production of extruded sections). During (hot) forming processes
they have a low resistance to deformation and, in many cases,
the extraordinary strength is reached afterwards thanks to the
precipitation hardening process.
2.4. Aluminium alloys
Basic families of aluminium alloys with different chemical
composition and mechanical properties are obtained by combining
the primary aluminium with alloying elements. Aluminium alloys
are usually classified according to their production procedure (cast
and wrought alloys) and heat treatment (heat-treatable and non-
heat-treatable), or on the basis of their chemical composition.
In practice, only several elements have proven to be fully effective
as alloying elements for aluminium destined for structural
applications. These are: magnesium (Mg), silicon (Si), manganese
(Mn), copper (Cu), and zinc (Zn). They can be used either individually
or in combinations. Aluminium alloys are classified based on their
principal alloying element, i.e. according to the alloying element
most represented in a particular alloy. The designation of wrought
alloys according to [4 and 5] is presented in Table 2.
The 1xxx series designates pure aluminium with a minimum purity
of 99.00 % and higher. The second number refers to the level of
impurity. If the second number equals 0, the aluminium is not
alloyed and the amount of impurities is within the limits of natural
concentrations. On the other hand, if the second number is different
from 0, it points to the need for conducting a special control of the
level of one or more impurities, or the alloyed element itself. The
last two numbers specify the lowest prescribed percentage of
aluminium above 99.00 % (for instance, number 50 means that
the minimum aluminium content is 99.50 %). Aluminium alloys are
identified using a system in which the first numbers range from 2
to 8, as shown in Table 2. In the designation system for families
of aluminium alloys, the second number specifies the modification
of an alloy. If this number equals 0, it is an original alloy. If the
number is different from 0, it is a modification of an alloy. The last
two numbers have no particular meaning, i.e. they are used only to
distinguish different aluminium alloys in a group.
Table 2. Major families of wrought aluminium alloys
The designation of principal cast aluminium alloys is defined by
standards [6-8]. An outline of the numerical designation system
that is used for main groups of aluminium alloys is given in Table
3. Cast aluminium alloys have a designation starting with letters
EN (the European numerical designation system) after which,
separated by a space, there is a letter A (standing for aluminium).
The letter A is followed by another letter (X) marking the form of
the product, which can be one of the following:
-B: aluminium alloy ingot for remelting,
-C: castings,
-M: master alloys.
The first number in a cast alloy designates the main alloying
element. The second number indicates the group the alloy
belongs to, the third number can be any number, and the fourth
number is always 0. The fifth number is also 0, except in cases
when the alloy is meant for aeronautical applications.
Table 3. Designation system of the most common and main groups of
cast aluminium alloys
2.5 Choice of alloy
Many factors have to be taken into account when selecting an
appropriate alloy for structural applications. For the design of
Main alloying element No. designation Chem. designation
Aluminium (Al) EN AW 1xxx EN AW Al
Copper (Cu) EN AW 2xxx EN AW AlCu
Manganese (Mn) EN AW 3xxx EN AW AlMn
Silicon (Si) EN AW 4xxx EN AW AlSi
Magnesium (Mg) EN AW 5xxx EN AW AlMg
Magnesium & silicon (Mg i Si)EN AW 6xxx EN AW AlMgSi
Zinc (Zn) EN AW 7xxx EN AW AlZn
Other elements (Iron Fe) EN AW 8xxx EN AW AlFe
Main alloying element Numerical designation
Copper (Cu) EN AX 2xxxx
Silicon (Si) EN AX 4xxxx
Magnesium (Mg) EN AX 5xxxx
Zinc (Zn) EN AX 7xxxx
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Aluminium as a material for modern structures
load-bearing aluminium alloy structures, it is not only necessary
to choose the corresponding alloy, but the designer also has to
be familiar with physical properties of those alloys, which are
listed in HR EN 1999-1-1 [9]. The choice of the most suitable
alloys amongst those listed in [9] seems rather difficult. Except
for strength values, which are the most important property for
a structural engineer, alloys are distinguished according to many
other aspects:
-availability in the form of a sheet or/and sections,
-purchase availability,
-decorative anodisable,
-filigreed/multihollow cross sections possible,
-exceptionally good or better welding strength,
-exceptionally good corrosion resistance (for special
applications),
-price,
-bendability/formability (sections),
-foldability (sheets),
-high ductility,
-strength at and under influence of elevated temperatures.
Several of the above-mentioned requirements must normally
be met for each application but the purchase availability and low
cost are always important. Therefore, in the majority of cases,
the choice of an alloy and temper is practically given.
According to [9], aluminium alloys can be classified into three
groups of durability: A, B and C, with each group having lower
durability than the previous one. These classifications are used
to define the need for and the level of required protection. In
structures where more alloys are utilized, including electrodes
in welds, the classification of durability must be in accordance
with the lowest classification.
Wrought heat-treatable alloys, suitable for a base material for
structures, belong to the 6xxx series (EN AW-6082, EN AW-
6061, EN AW-6005A, EN AW-6063, and EN AW-6060) and fall
into the durability class B. In the 7xxx series, the EN AW-7020
alloy is suitable for general construction applications and it falls
into the durability class C.
The wrought non-heat-treatable alloys from the 3xxx, 5xxx, and
8xxx series (EN AW-3004, EN AW-3005, EN AW-3103, EN AW-
5005, EN AW-5052, EN AW-5454, EN AW-5754, EN AW-5083,
and EN AW-8011A) are recommended for structural elements. All
alloys from the 3xxx and 5xxx series belong to the durability class
A, whereas the EN AW-8011A alloy falls into the durability class B.
Six cast alloys, four heat-treatable alloys (EN AC-42100, EN
AC-42200, EN AC-43000, and EN AC-43300) and two non-
heat-treatable alloys (EN AC-44200, and EN AC-51300) are
recommended for structural applications. All cast alloys from
the 4xxxx series belong to the durability class B, whereas the
cast alloys from the 5xxxx series are covered by the durability
class A.
To sum up, the following alloys are most commonly used for
structural applications:
-EN AW-6082 and EN AW-6061 for structures made of
sheets and extruded profiles,
-EN AW-5083 and EN AW-5754 for structures made of
sheets,
-EN AW-6060 and EN AW-6063 for structures made of
extruded profiles.
To be able to select a suitable aluminium alloy, it is essential
to be familiar with the designation system used for their
characteristic stages or tempers. Aluminium stages can be:
F - rough stage of fabrication
O - annealed stage
H - work-hardened stage (e.g. cold-working)
W - tempered nonstabilized stage
T - heat-treated, i.e. temper stage
Other most commonly used temper designations (T) are:
T1 - cooled from hot working and naturally aged,
T2 - cooled from hot working, work-hardened and naturally
aged,
T3 - solution heat-treated, work-hardened and naturally aged,
T4 - solution heat-treated and naturally aged,
T5 - cooled from hot working and artificially aged,
T6 - solution heat-treated and artificially aged,
T7 - solution heat-treated and artificially over-aged,
T8 - solution heat-treated, work-hardened, and artificially
aged,
T9 - solution heat-treated, artificially aged, and work-
hardened.
Additionally, for the design of aluminium structures, it is very
important to be familiar with the designations of semi-finished
products related to a particular aluminium alloy. Therefore, in
HR EN 1999-1-1, [9], next to the reference to the corresponding
norm, the following designations are also given:
SH - sheet (HRN EN 485) EP/H - extruded hollow profile
(HRN EN 755)
ST - strip (HRN EN 485) EP/O - extruded open profile
(HRN EN 755)
PL - plate (HRN EN 485) ER/B - extruded bar (HRN EN 755)
DT - drawn tube (HRN EN 754) ET - extruded tube (HRN EN 755)
EP - extruded profiles (HRN EN 755) FO - forgings (HRN EN 586)
3. Application of aluminium alloys in civil
engineering
3.1. Competitiveness
The best application of aluminium is related to some typical
cases in which benefits are gained by at least one main property
of aluminium: lightness, corrosion resistance, and functionality
[10]. The following structural applications in the area of civil
engineering, best related to the mentioned properties, are:
-Long-span roof structures where live loads are low compared to
dead loads, i.e. reticular spatial structures and geodetic domes
for covering long-span areas (e.g. halls and auditoriums).
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-Structures located in hardly accessible places, away from
fabrication shops, for which unhindered transport and easy
erection are of crucial importance (e.g. transmission towers
transportable by helicopters).
-Structures located in corrosion-inducing or humid environments
(e.g. swimming pool roofs, river bridges, hydraulic structures,
and off-shore superstructures).
-Structures with moving parts (e.g. sewage plant crane bridges
and moving bridges), where lightness means efficient use of
energy.
-Special-purpose structures for which maintenance operations
are especially difficult and should be limited (e.g. masts, lighting
towers, antenna towers, road sign gantries, etc.)
The mentioned applications belong mainly to the field of civil
engineering. It should however be noted that the potential
of aluminium application is much wider and is not limited to
structural engineering only.
3.2. Lightweight structure
When the weight of a structure is crucial, the use of aluminium
can present a valid alternative to steel. Besides, complete
absence of maintenance increases its advantages, especially
for those structures that are located in a humid environment.
Figure 2. Roof structure for Inter-American Exhibition Centre, Sao Paolo,
Brazil – under construction
There are several applications of reticular spatial structures
in South America (Brasil, Colombia, Ecuador). A historical
breakthrough in the area was a spectacular spatial structure
built for the Inter-American Exhibition Centre in Sao Paolo,
Brasil in 1969 (Figure 2). It covers an area of 67.600 m2 with a
60x60m mesh. The height of the structure (layer) is 2.36 m. It
was entirely bolted on the ground and was subsequently erected
to its ultimate level of 14 m by means of 25 cranes located at
the corners of the mesh, in the position of actual supports.
The weight of the structure is 16 kg/m2, it consists of 56820
elements, measuring 300 km in total length. The assembly time
was extremely short: 27 hours; a total of 550,000 bolts were
used for 13,724 nodes. The materials used for the structure
are: aluminium alloys 6063 and 6351, T6 series for cylindrical
bars, Al 99.5 for trapezoid sheets, and galvanized steel bolts for
connections. A very similar case is the International Congress
Centre of Rio de Janeiro, where the same type of mash was used
(60 x 60 m). It covers a total area of 33000 m2 (Figure 3).
Figure 3. International Congress Centre of Rio de Janeiro, Brazil
(model)
Among many different applications of reticulated systems covered
with aluminium sheeting, the most noteworthy are: the roofing of
the sports hall Coliseo General Rumiñahui, Quito, Ecuador (Figure 4)
and The Memorial Pyramid in La Baie, Quebec, Canada (Figure 5),
commemorating a damage caused by a flood in the 1980s.
Figure 4. Coliseo General Rumiñahui, Quito, Ecuador
Figure 5. The Memorial Pyramid in La Baie, Quebec, Canada
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Aluminium as a material for modern structures
Reticular domes present the most challenging application of
aluminium alloys in structural engineering. This concept enables
realization of large-scale facilities (sports arenas, exhibition
centres, congress halls, auditoriums, etc.). These applications
are highly interesting due to their short erection time,
connecting systems used, and remarkable dimensions. The first
applications are: "Dome of Discovery", built in London in 1951 for
the South Bank Exhibition during the Festival of Britain (with
directional reticulated arches, 110 m in diameter, and 24 kg/m2
in weight), and a geodetic dome built in 1959 for covering the
"Palasport" in Paris, using the Kaiser Aluminium system, 61 m in
diameter and 20 m in height. Both structures were prototypes
in their respective fields - the first and the largest.
Figure 6. Spruce Goose Dome, Long Beach, California, USA
Interesting structural systems for aluminium-made geodetic
domes have recently been erected in the USA, where "ad hoc"
systems for roofing industrial plants with ecological purposes,
and for roofing large-span public buildings, were applied. A well-
known example is the Spruce Goose Dome: the largest dome of
this kind in the world, spanning more than 125 m in diameter
(Figure 6). Many geodetic domes are used in industry, e.g. for
coal storage plants (Figure 7).
3.3. Low maintenance requirements
Some special structures are used as a means to support fixed
elements that are positioned at a certain level above the ground.
They can be dominantly horizontal (e.g. gantries for traffic
signs), or vertical (e.g. antennas, high voltange transmission
lines and lightning towers). For structures of this type, it is of
paramount importance to eliminate any sort of maintenance. At
the same time, the extrusion process can enhance geometrical
properties of cross sections, thus enabling a minimum weight
and a maximum structural efficiency. In addition, the light
weight of aluminium enables easy transport and fast assembly
of prefabricated systems, thus offering competitive solutions
when compared to other materials.
Figure 8. Aluminium towers, Naples, Italy
Many high voltage transmission lines have been built in Europe.
Two important aluminium towers were erected in Naples. One of
them is a tower supporting parabolic antennas of the Electrical
Department of Naples, erected in 1986. The reason for choosing
aluminium is, in essence, its light weight (the tower was erected
at the top of an already existing concrete staircase) and its
corrosion resistance property (no maintenance problems). It is
35 m high, from the top of the staircase, and its total height
is 50 m. It consists of a cylinder with an internal diameter of
1800 mm and a 20 mm thick wall. It was fabricated by welding
in workshop, and was separated along the height into three
Figure 7. Aluminium dome for coal storage plant
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parts that were later field-bolted during erection (Figure 8, left).
The second example is the "Information Tower" located near
a football stadium in Naples and equipped with antennas and
screens so that football matches can be viewed from outside of
the stadium (Figure 8, right).
Aluminium properties play a crucial role in hydraulic applications
(e.g. pipelines, tanks). Here aluminium is typically used in
rotating crane bridges for large circular settling pools in
wastewater treatment plants. Basically, its corrosion resistance
property eliminates the need for maintenance of the structure
even when it is placed in a corrosion-inducing environment,
whereas its lightweight property permits energy savings during
operation of the plant (Figure 9).
Figure 9. Wastewater treatment plant pool Po-Sangone, Turin, Italy
It should be emphasized that the main future trend in the
application of aluminium alloys involves the use of such alloys
on offshore structures, Here, aluminium alloys offer noteworthy
benefits including cost reduction, ease of fabrication, and
already proven efficiency under aggressive ambient conditions.
Staircases, mezzanine flooring, access platforms, walkways,
gangways, bridges, towers, and cable ladder systems, can all be
constructed in form of prefabricated units for simple assembly
offshore or at a fabrication yard. Helidecks are also fabricated
from aluminium alloys since the early 1970s, and they are
now completely tested for use in heavy duty conditions, Figure
10. Moreover, they are designed in modules and with bolt
connections, thus enabling easy and quick erection, and simple
handling and shipping. In addition, their use enables up to 70
% reduction in weight with respect to steel, not to mention
compliance with the most stringent safety criteria, and up to 12
% cost reduction.
An entire array of modules for crew quarters and utilities has
recently been developed: from large purpose-built modules to
flexible ones. The modules can be used individually or assembled
in a group to form multi-storey complexes, connected with
central, transverse corridors and staircases.
3.4. Bridges
All types of bridges have so far been constructed using
aluminium alloys. The Arvida Bridge in Quebec, Canada, built
Figure 10. Helidecks
Figure 11. The Arvida Bridge in Quebec, Canada
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Aluminium as a material for modern structures
in 1950, was one of challenging motorway-bridge prototypes
made of aluminium alloys. It was built according to a Maillart"s
scheme. It has a total span of 150 m, an 87 m arch, and its total
weight amounts to 200 tonnes (Figure 11).
A technology of composite structures made of aluminium
beams and concrete decks has also been developed. Concrete-
aluminium composite systems were used for some bridges
erected in the 1960s in the USA and later on in France.
A new, important field of application is that of military bridges
where lightness and corrosion resistance play a fundamental
role. Currently it is possible to cross a 40 m span using
prefabricated elements as they are easily transportable and
easy to erect. The main applications of this kind were realized
in Great Britain, Germany, and Sweden, Figure 12. In Germany,
military bridges are fabricated in form of pre-fabricated units
characterized by simple transport and assembly (Figure 13).
Figure 12. Swedish military bridge
Figure 13. German military bridge set-up
Due to a slow moving load, foot-bridges also rank among
structures where aluminium alloys are successfully utilized.
Additional advantages due to aluminium"s lightness are rather
obvious in the case of moving bridges. Examples of aluminium
foot bridges can be found in France, Germany, the Netherlands,
Italy, and Canada.
3.5. Refurbishment of bridges
A lightweight system for the replacement of damaged concrete
deck structures of bridges has been developed and employed
in Sweden. It is based on an orthotropic plate made of hollow
aluminium extrusions. This solution can in many cases prove
to be very competitive as an alternative to more conventional
solutions. Weight reduction has enabled the use of existing
foundations and supports.
4. Eurocodes and aluminium
The unavoidable complexity of standards for aluminium
structures is essentially caused by the nature of the material
itself, which is considered to be much more "critical" and less
known than steel, thus requiring more complex analyses
and solutions to more sophisticated problems. This is why
the related standard should be educative and informative as
well as normative. The ENV issue of Eurocode 9 "Design of
aluminium structures" (1998) is made of three documents (Part
1.1 "General structural rules", Part 1.2 "Structural fire design",
and Part 2 "Structures susceptible to fatigue"). Upon an explicit
request made by EAA (European Aluminium Association), and
due to a great interest in new fields of application as expressed
by representatives of the aluminium industry, two new issues
were added in the transition phase: "Cold-formed structural
sheeting" and "Shell structures". On the basis of suggestions
collected in the meantime, the transformation from ENV to EN
began in 2001. This phase was concluded in 2005 and the final
version of Eurocode 9 consists of the following five documents:
-Part 1.1: General structural rules
-Part 1.2: Structural fire design
-Part 1.3: Structures susceptible to fatigue
-Part 1.4: Cold-formed structural sheeting
-Part 1.5: Shell structures
Unlike other Eurocodes, Eurocode 9 consists only of one part
that is divided into: one fundamental document, "General
structural rules", and four other individual documents all related
to the fundamental one. Specific types of structures (e.g.
bridges, towers, silos, etc.) are not mentioned, as it is the case
for the steel. Only general topics are given, i.e. topics that are
applicable not only to structural engineering, but also to broader
civil engineering areas, transport industry included.
The preparation of Eurocode 9 was based on the most significant
results achieved in the field of aluminium alloy structures,
without ignoring previous activities conducted within ECCS, and
those related to the revision of outstanding codes, like BS 8118.
The ECCS method for column buckling was, with some minor
editions, also utilised in EC 9. The method is based on the use
of two buckling curves (a and b) which cover extruded profiles
made of heat-treated and work-hardened alloys, respectively
[11]. Generally, the design of beam, column and beam-column
members is performed taking into account specificities of
aluminium alloys. For welded profiles, the reduction effects of
the heat-treated zone are accounted for via the corresponding
reduction factors. This method is based on experimental
evidence, which enabled the characterization of aluminium alloy
members as "industrial bars".
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Davor Skejić, Ivica Boko, Neno Torić
The novelty of Eurocode 9, Part 1.1 "General structural rules",
lies in the fact that, for the first time in an aluminium structural
standard, the analysis of inelastic behaviour begins from the
cross section up to the structure as a whole. A classification of
cross-sections was made on the basis of experimental results,
all of which had been gathered from an "ad hoc" research
project supported by the main representatives of the European
aluminium industry, who provided the material for specimens.
The result was a specification of behavioural classes based on
the slenderness ratio (b/t), according to an approach qualitatively
similar to the one applied to steel, but with a different range
of behaviour, which was based on experimental evidence and
verified through numerical simulations [12].
An evaluation of resistance of cross sections has been introduced
in a unique manner, with a special reference to ultimate-limit
states related to the four classes. For the members belonging
to class 4 (slender sections), the check of the local buckling
effect is performed using a new calculation method based on
the effective thickness concept. Three new buckling curves for
slender cross sections are assessed, including heat-treated and
work-hardened alloys, together with welded and non-welded
shapes [2]. This method presents a starting point for a detailed
treatment of the cold-formed sheeting, as given in Part 1.4
"Cold-formed structural sheeting".
The problem of evaluation of internal forces and moments
is considered by taking into account several models for
material constitutive law, from the simplest ones to the most
sophisticated ones that lead to different levels of approximation.
A global analysis of structural systems in the inelastic range
(plastic, strain hardening) relies on a simple method similar to
the well-known method of plastic hinge, but considers typical
parameters of aluminium alloys such as: absence of yielding
plateau, continuous strain-hardening, and limited ductility of
some alloys [2].
The importance of ductility with regard to the local and global
behaviour of aluminium structures has been highlighted, due
to sometimes poor values of ultimate elongation, and a new
"ad hoc" method for the evaluation of the rotation capacity for
members in bending has been set up [13].
A new classification system for the resistance, stiffness and
ductility of joints has been proposed. A new method for the
resistance assessment of T-stubs has been established and
introduced in Part 1.1 on the basis of experimental evidence
about the monotonic and cyclical testing.
The new Part, 1.5, addressing shell structures, has been compiled
following the same format of the similar document in EC 3.
However, the method relies on the corresponding buckling curves
obtained from empirical evidence on aluminium shells [14].
Structural fire design is a transversal topic for all Eurocodes
dealing with structural materials, and it is comprised in Part
1.2. For aluminium structures, it has been standardized for
the first time according to general structural rules, in which
fire resistance is assessed on the basis of three criteria:
resistance (R), insulation (I) and integrity (E). It is generally
known that aluminium alloys are normally less resistant to
high temperatures compared to steel and reinforced concrete.
Nevertheless, by introducing rational risk assessment methods,
fire scenario analyses can in some cases lead to a more
favourable time-temperature relation. This may give rise to a
more competitive status of aluminium, and thermal properties
of its alloys may have a beneficial effect on temperature
development in structural components [1].
Knowledge about fatigue behaviour of aluminium joints
has been consolidated over the past 30 years [1]. The ECCS
recommendations for the fatigue design of aluminium
structures were published already in 1992. They represented
the fundamental bases for the development of Eurocode 9. It
was decided to develop Part 1.3 in EC 9, Structures susceptible to
fatigue, in a general manner, by giving general rules applicable to
all types of structures exposed to fatiguing loading with respect
to the limit state of fatigue-induced fractures. This part does
not correspond to the similar part for the steel Eurocode, where
only bridges are addressed. Three design methods have been
introduced:
-Safe life design (SLD)
-Damage tolerant design (DTD)
-Design assisted by testing
The following basic groups of detail categories have been
considered:
-non-welded details in wrought and cast alloys,
-members with transverse welded attachments,
-members with longitudinal welded attachments,
-welded joints between members,
-crossing welds/built-up beams,
-mechanically fastened joints,
-adhesively bonded joints.
The use of finite elements and guidance on the assessment by
fracture mechanism was suggested for the stress analysis.
Finally, it should be noted that the importance of the quality
assurance on welding was highlighted with specific reference
to the standards belonging to the group of HRN EN 1090
"Execution of steel and aluminium structures".
5. Conclusion
Although a high price of aluminium alloys remains the main
obstacle to their wider use in construction, the utilisation of
aluminium alloys in structural engineering is undoubtedly
justified. This paper addresses only some cases in which the
use of aluminium alloys in structural engineering constitutes a
practical, advantageous, and often the only practical solution.
Examples of the structures where the priority is given to the
lowest possible weight (large spans, transport, installation)
and the longest durability (corrosion resistance) are presented.
Reasons for using aluminium are usually based on the
properties of its alloys: light weight, easy workability, toughness
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Aluminium as a material for modern structures
at low temperatures, low maintenance costs, durability, and
recyclability. However, the stability and fire resistance of
aluminium structures have not been sufficiently investigated,
and a varied behaviour of a great number of aluminium alloys in
use does not allow simple comparison with steel. A considerable
attention is given to these issues in the most recent first phase of
development of the second generation of European standards.
Despite the current situation with standards, this paper offers
fundaments for an introduction to specific issues for the design
of these still somewhat neglected metal structures, by outlining
basic properties of aluminium alloys and the development of
their respective Eurocodes.
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