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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
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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
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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
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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
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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.
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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
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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
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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.
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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.
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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)
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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.
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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
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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,
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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.
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355
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