Metals in building
Introduction
Metals have been used by humans for over 6000 years. The first metals were simply picked
up off the ground, but in time people learnt to extract metals from their ores. Nowadays the
technology has become quite complex and not only can many metals be extracted from their
ores, but the properties of metals can be modified by various types of finishing processes or
by mixing with other metals to form alloys. For building purposes, most metals are alloys.
The major base metals used are iron, copper, lead, zinc and aluminium. Metals using iron as
their base are called ‘ferrous’ metals while the others are termed ‘nonferrous’. Brass is an
important nonferrous metal used in building, being an alloy of the base metal copper.
Metals in Building, Unit CPCCBS6001A, Ed 1
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Properties of metals
Metals are substances that can either be hammered (the quality called malleability) or drawn
out as wire (the quality called ductility) or melted and formed into shapes in moulds. Most
metals can be polished. All metals are, to greater or lesser degrees, conductors of forms of
energy such as heat and electricity.
Other characteristics possessed by metals may vary considerably from metal to metal. Some
metals (e.g. stainless steel) have good strength qualities, whereas others (e.g. tin) have very
little strength. All metals, however, will lose strength when repeated force is applied to
them—a process known as metal fatigue.
The degree of hardness of a metal will vary according to its natural characteristics (lead and
tin, for example, are soft metals; chromium and nickel are hard) and according to the degree
to which the metal is worked. When a metal is worked at normal temperatures (by being
rolled or forged, for instance) the result will be an increase in its hardness and strength—this
it called work hardening.
Most metals are subject to corrosion, which occurs when the surface of the metal combines
with oxygen in the air to form a coat or crust that is no longer metallic (e.g. rust on iron or
steel). Corrosive liquids and gases can actually eat away metals. (We can see the effect of
salt air or spray on aluminium.) The process of corrosion is usually greatly speeded up by
the action of heat and moisture. Some metals have very low corrosion-resistance, while
others have a good degree of corrosion-resistance. Metals with a high degree of corrosion
resistance (e.g. chromium) are often used either as coatings or in alloys with other metals to
increase their resistance to corrosive agents.
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Working with metals
Forming metals
How metals are formed depends upon the type of metal, the objects being made, and
whether parts made of other metals are also incorporated. The following are some of the
methods used:
•
Casting—where molten metal is poured into moulds and allowed to cool and
harden. Rolling hot or cold metal is rolled between heavy rollers to produce various
bars, strips, sheets or sections of metal.
•
Forging—where hot metal is squeezed into shape, often using mechanical
hammers and suitably shaped dies.
•
Extrusion—where heated metal is forced through a suitably shaped hole in a
hardened steel die to produce continuous solid or hollow sections.
•
Drawing wire—or tubes are pulled through tapered dies to reduce the thickness of
the metal. Normally the metal is cold and the process lessens its strength.
Joining metals
Metals can be joined by a variety of methods, including the following.
•
Mechanical joints: Bolts, screws or rivets are used to join metal components
together.
•
Soldering and brazing: Most metals can be joined using an alloy which is a mixture
of two or more metals that melt at a lower temperature than the melting point of the
metals being joined. Soldering usually refers to tin-lead and lead-silver alloys which
melt below 300°C.
•
Brazing: Gives stronger joints than soldering; however, as it is done at higher
temperatures (over 600°C), brazing cannot be used on metals such as lead which
have low melting points.
Brazing is a process where a molten filler metal is used to join metal parts. The
filler metal has a melting point below that of the metals to be joined. Brazing is
accomplished at temperatures above that of soldering (420°C) and below that of
welding. Heating for brazing may be accomplished by dipping the parts into a bath
of the molten alloy, by heating with torches, furnace heating, or by electrical resist
once.
Brazing materials consist of the following:
Aluminium-silicon : For brazing aluminium.
Copper-phosphorous:
For brazing copper and copper alloys.
Silver
For brazing ferrous metals, copper, and copper alloys.
Copper zinc alloys For brazing ferrous metals, copper, and copper
.
Metals in Building, Unit CPCCBS6001A, Ed 1
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Heat-resisting alloys For brazing ferrous metals.
•
Welding: Most welding involves a metal being heated to a temperature below its
melting point, and the soft metal being hammered together. This traditional
blacksmithing method has been replaced by gas welding (using oxyacetylene or
propane) and arc welding (using an electric arc struck between the work and a
welding rod or a carbon electrode).
Welding is a process of joining metals by applying heat and pressure, with or
without filler material, to produce an actual union through fusion. There are a
number of welding processes.
These include:
Carbon arc
This is an electric arc process wherein a carbon electrode is
used and fusion is produced by heating. Pressure may or
may not be applied, and filler metal may or may not be used.
Electric arc
In this process a metal electrode is used which supplies the
filler material in the weld as well as the heat to produce the
fusion.
Forge welding
In this process fusion is produced by heating the metal in a
forge or furnace and applying pressure or blows to the work.
Fusion welding
Welding without pressure, in which a portion of the base
metal is melted. It is usually accomplished by gas flame or
electric arc heating.
Resistance welding In this process welding is accomplished by placing the work
to be joined under pressure in a machine, then applying an
electric charge through the joint, the resistance of which
produces heat to fuse the joint.
Welding sometimes imparts distortion, brittleness, or changes in strength and ductility at the
joint. To overcome these deficiencies, cold working and annealing are often necessary to
restore the original working characteristics. With nonferrous metals such as aluminium the
factor of colour may have an important bearing upon the choice of the proper welding
process and the proper electrode or filter metal.
•
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Both brazing and welding involve heating the adjacent metal to extremely high
temperatures which allow the metal to flow together and form one continuous unit.
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Heat treating
Heat treating is employed in metal working to induce certain properties or to relieve stresses
and strains after certain metal-working processes have been performed.
Heat-treating processes include:
Annealing
A heating and cooling operation performed on metal in the solid state. The
cooling is done at a relatively slow rate. The process generally results in reducing hardness,
improving machinability, cold working, removing stresses, and altering the ductility and
toughness of metals. The temperature of the operation and the rate of cooling will depend
upon the specific metal and the purpose of the annealing process.
Tempering
A specific heat treatment whereby metal is brought to a desired degree of
hardness or softness. The metal is heated to a particular temperature and then cooled
suddenly in a water or oil bath.
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Surface working
Surface-working processes involve the application of certain operations to the surface of
metals to alter their appearance and include:
blasting
Obtaining a mottled or pebbled surface by means of
cleaning
blasting the surface with sand, grit, or steel shot through a nozzle by air
pressure.
brushing
Producing smooth, satin, bright, or buffed
buffing
finishes by means of wheels on high-speed lathes, by
polishing
belts on sanding wheels, or by disks and wheels on hand buffers and
grinders.
etching
A process of chemical etching the surface by means of acid or alkali solutions
to obtain architectural effects.
grinding
A surface texture obtained by the use of grit in a grinding wheel or disk.
hammering
Metal surfaces can be altered by hammering to obtain the desired degree of
surface alteration.
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Ferrous metals
Ferrous metals are those metals that contain a large amount of iron. The main types of
ferrous metal are:
•
cast iron
•
wrought iron
•
steels
Manufacture
Iron ore, as mined, is a combination of iron and oxygen and various other substances. In this
country most of the ore is obtained from open-cut mines.
The first step in processing the ore is to reduce it to metallic iron (often called ‘pig iron’), a
process carried out in a blast furnace using coke as a fuel and reducing agent. The metallic
iron, at this stage, contains a relatively high proportion of carbon (about 4 per cent).
To make steel, the carbon content of the metallic iron must be lowered to less than 1 per
cent by an oxidation process in the steelmaking furnace. At the same time, the metal is given
whatever special chemical and physical properties may be required by the addition of other
metals. The quantities and timing of the additions of carbon and various other elements are
carefully controlled to make the wide range of irons and steels that are available.
Effects of added elements
Carbon is the principal hardening element in steel. In plain carbon steels, it is used as the
controlling element to regulate physical properties. When the carbon content is increased,
hardness and tensile strength are improved but ductility and weldability are reduced (see
Figure 1).
Figure 1 - Influence of carbon on the properties of ferrous metals
Manganese increases strength and hardness but to a lesser degree than carbon. It also
improves the toughness and abrasion resistance of steel.
Chromium increases hardening ability and tensile strength and improves corrosion and
abrasion resistance. It is usually associated with nickel additions to form ‘stainless steel’.
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By-products and recycling
Blast furnace slag
Blast furnace slag is the waste from the smelting process. It is an important by-product which
can be used for concrete aggregate, road metal and slag wool for insulation.
Steel scrap
This is a major source of metallic iron for steel making. Scrap may either be residue left from
the steelmaking process or purchased from discarded or obsolete constructions. About half
of the crude steel produced annually in the world will eventually be returned to the steelmaking furnaces.
Recycling and waste minimisation should be practised by all sectors of the building industry.
Cast iron
Cast iron is produced by re-melting pig iron with steel and cast iron scrap. The cast iron has
a high carbon content which makes it free-running and, therefore, very suitable for moulding
intricate shapes. Cast iron has been used in the past for the decorative iron lace on buildings
which is often wrongly called ‘wrought iron’.
Cast iron is used for fire grates; for soil waste pipes and ventilating pipes; for drainage
gratings and frames; and for baths and basins (with a vitreous enamel finish).
Wrought iron
This is a low carbon iron which is excellent for forging but cannot be cast, tempered or
welded (by gas or arc). Wrought iron was very popular for decorative finishes (such as
balustrades and balcony railings) in the 1950s but has since lost popularity.
Steels
Steels are produced by removing impurities from pig iron and then accurately adjusting the
quantities of all the ingredients.
Steels are noted for their high strength compared to their production costs, and also for their
poor performance in building fires. Ordinary steels do not resist corrosion well, but special
steels (e.g. stainless steel) are produced today with excellent corrosion resistance.
Steel, an alloy of iron (Fe), is used not only as a part of structural systems in buildings but,
when exposed, as part of the architectural ornamental expression. Exposed structural
systems used include space frames and arches to span large, unobstructed floor areas such
as halls, exhibition spaces, and sports arenas. Other uses include cable-supported
structures, and low buildings where fireproofing is not mandatory by building codes and the
steel frame may be left exposed.
Steel is also used for ornamental metalwork to enhance the appearance of buildings. Some
structures are famed for their beautiful grille work, stairs, railings, sculpture and ornamental
metalwork.
The classification and identification of steel is rather complex and involves, among other
things:
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•
chemical composition
•
method of manufacture
•
mechanical properties
•
heat treatment
•
reference to a recognised standard.
Metals in Building, Unit CPCCBS6001A, Ed 1
© New South Wales Technical and Further Education Commission, 2015
For building purposes, carbon steel and high-strength, low-alloy steel are the primary steels
used. Stainless steel is a steel alloy and is discussed separately later in this unit.
Carbon steel
Classification
Steels are classified as carbon steels when:
•
the amount of carbon does not exceed 2%
•
the amount of iron exceeds 95%.
Carbon steels are likewise classified by grade on the basis of their carbon content. Amounts
of up to 0.8% carbon increase the strength and hardness of carbon steel and decrease the
ductility.
Physical properties
Carbon steel used for building purposes include mild steel, medium steel, and very mild
steel. Of these, the mild steels are the most widely used and are generally carried in stock
by warehouses and fabricators. Mild steels:
•
have a tensile strength in the range of 36,000 to 65,000 psi
•
form easily
•
retain true sections when fabricated.
Shapes
Bar sizes of carbon steel include channels, angles, tees and rolled sections having a
maximum dimension of the cross-section of less than 75 mm.
Structural size shapes are rolled, flanged sections having at least one dimension of the
cross-section of 75 mm or greater.
Pipe and tubing
Pipe and tubing are available for a variety of building purposes. They may be obtained in
standard weight, extra-strong, and double extra-strong.
Sheet and strip
Sheet and strip are used for components of buildings such as:
•
curtain walls and fascias
•
roofs
•
interior walls and partitions
•
doors and windows
•
floors and ceiling systems
•
and signage
Steel sheet and strip are made up of carbon steelshigh-strength, low-alloy steels.
Hot rolled sheet and strip are produced by squeezing hot steel ingots with huge rolls
repeatedly until the desired thickness is reached. Cold-rolled sheets are formed by further
rolling hot-rolled sheets after they have been allowed to cool and have been pickled to
remove scale.
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The most popular steel sheet and strip for are:
•
hot-rolled carbon steel
•
cold-rolled carbon steel
•
high-strength, low alloy weathering steel
•
zinc cured steel
High-strength, low-alloy steel (weathering steel)
Weathering steel is a high-strength, low-alloy steel that is resistant to corrosion from normal
atmospheric exposure and attains a light oxide coating of varying colours that can be
pleasing. The colour of weathered steel can be described as russet brown with an
intermixing of browns, reds, and blues.
In ordinary carbon steel, rust begets rust. The corrosion resistance of high-strength, lowalloy weathering steel depends upon the formation of a dense, stable layer of rust that
prevents oxygen and other contaminants from continued reaction with the base metal. The
corrosion resistance of weathering steel is approximately four times that of carbon steel.
The action of continued wetting and drying of the surface is essential to develop this tight
oxide covering over a period of about three years. An indoor environment or an arid climate
are not conducive to the full development of this oxide coating.
Careful design is essential in the detailing of various building elements utilising weathering
steel. Surfaces must drain properly to permit drying; otherwise normal loose rusting occurs.
Water that drains or drips from the steel must not be allowed to impinge on other materials
that will show rust stains or streaking. Porous materials are vulnerable to such absorption
and discolouration.
To obtain a uniform weathering that is pleasing in appearance, mill scale must be removed
by cleaning, sanding, scraping, or wire brushing. Blast cleaning is perhaps the best
preparation. Oil, grease, and chalk marks must also be removed, or else these areas will not
weather uniformly.
Structural steel
Structural steel products are available in hot rolled sections and cold formed sections.
Hot rolled sections
These are formed while the steel is at elevated temperatures and include the following
profiles:
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Product codes for hot rolled steel sections have a standard format as follows:
XXX YYY ZZ.Z
Where:
XXX is the distance in millimetres from the outside face to outside face of the structural
member. This is useful for construction co-ordination purposes.
YYY is the section type ie UC, UB, CHS, PFC, UA, EA etc.
ZZ is the weight per meter in kilograms of the section. This is useful for construction planning
to calculate the total member weight and equipment and manpower requirements during
erection.
Example: 310 UB 46.2
Typically UBs can be distinguished from UCs by their more rectangular rather than square
profile. Refer to AS 3679 Structural steel - Hot rolled Bars and sections Appendix D for
standard section profiles and dimensions for the common hot rolled structural sections
produced in Australia. This standard also includes information on sampling and testing steel
sections.
Refer to AS 2812:2005 Welding brazing and cutting of metals – Glossary of terms for
background information of welding metals and terminology.
When purchasing structural steel work the following information should be supplied by the
purchaser:
1.
2.
3.
4.
5.
6.
quantity and delivery instructions
steel section type
steel grade
steel dimensions e.g. length and mass per unit length
whether a certificate of compliance or test certificate is required
whether it is the intention of the purchaser to inspect the steel manufacturers works
to inspect the product during manufacture, or select and identify test samples or
witness tests being made.
7. any information concerning processing or end use that the purchaser considers
would assist in manufacturing
8. any exceptions to standard and any special or supplementary requirements such as
a non-destructive testing inspection
What are the components of welding symbols?
Basic welding symbols communicate information about the type, size and position of welds
in welded joints. They are drawn to Australian Standard AS 1100.101-1992
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A welding symbol can be made up of any of these eight elements:
1. a reference line, which is always drawn parallel to the bottom edge of the drawing or
to the base line of the view where it is used.
2. an arrow, which indicates the location of the welded joint. The side nearer the arrow
is known as the arrow side and the further side is called the other side. The arrow
connects to the reference line and welding symbol.
3. welding symbols
4. supplementary symbols, eg R30 means that the hole has a radius of 30mm
5. the dimension of the weld
6. finish symbol, eg C = chipping, G = grinding, M = machining, R = rolling, P = peening
7. a tail
8. specifications and process – references to these are placed in the tail.
Figure 1 Welding specification
AS 1554.1:2011 Structural steel welding states that steel fabrication are managed under a
suitable quality management system generally complying with AS 3834
Refer to AS 1554.1:2011 Structural steel welding Section 4 for information on the
qualifications necessary for welders.
Refer to Figure 1 for common welding symbols. Additional information is available
from AS1100.101
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Figure 2: Weld symbols
1
1
Standard Drawing Symbols Abbreviations Graphical Representation, Oten, 1991
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Cold formed sections
These are formed while the material is cold as distinct from materials that are shaped or
worked while under the effect of heat. Unlike hot rolled sections, cold formed sections have
constant thickness.
Cold formed sections may be formed by:
Rolling in a rolling mill (for material up to 20 mm in thickness), the product being what is
known as ‘cold rolled sections’ (see Figure 2).
Figure 2 - Rolling in a rolling mill
Pressing by means of a press brake (for material up to 20 mm in thickness), the product
being what is known as ‘pressed steel sections’ (see Figure 3).
Figure 3 - Pressing with a press brake
Pressing by means of a swivel bender (for material up to 30 mm in thickness)—the product
being what is known as ‘pressed steel sections’ (see Figure 4).
Figure 4 - Pressing with a swivel bender
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Use of structural steels
Cold rolled sections
These are used for:
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Figure 5 - Single storey construction showing steel floor and wall framing and steel trusses for tiled
roof
Pressed steel
Pressed steel is used for:
•
door and window frames
•
metal trims (such as skirtings)
•
wall panels
Note: Pressed steel sections are limited to the size of the break press; or, with swivel
bending, are able to be produced economically in small quantities.
Alloy steels
Alloy steels contain certain added elements that provide special properties such as ultra-high
strength or resistance to corrosion or heat.
Stainless steel (containing chromium and nickel) is one such steel alloy which, although
much more expensive than mild steel, is being increasingly used in building in a wide variety
of applications because of its durability and low maintenance needs (even under extreme
conditions of atmospheric pollution, as it has excellent resistance to corrosion).
Stainless steel has outstanding structural advantages because its hardness and toughness
allows it to be used in very light sections, thus reducing greatly the weight of finished articles.
Even more importantly, it is less affected by extreme heat, such as in a fire.
Except for very simple cutting or drilling on site, all shaping and fitting of stainless steel must
be done in suitably equipped factories and workshops.
Stainless steel is also used for sanitary ware (e.g. sinks and bench tops).
Metals in Building, Unit CPCCBS6001A, Ed 1
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Prevention of corrosion in steel
Upon exposure to the atmosphere ferrous metals combine with oxygen to form a red oxide
(ie rust). Rust corrodes the metal and eventually wears it away, leaving behind a red
powdery residue. This not only affects the appearance of the metal but substantially reduces
its strength.
One way of making steel rust resistant is by applying one of many protective coatings
available for steel products. These fall roughly into two groups: metallic coatings and nonmetallic coatings. As most require scrupulously clean conditions and special surface
preparation of the steel for successful application, factory application of surface coatings is
preferable. Other methods of corrosion protection include active electrically charged
systems, and passive systems incorporating sacrificial anodes. Of key importance is to
ensure that dissimilar metals do not come into contact ie do not weld stainless steel to mild
steel
Metallic protective coatings
These function by taking advantage of electro-chemical differences between different
metals. In adverse atmospheric conditions it is the surface coating that is sacrificed rather
than the base metal.
A number of methods are used to apply metallic coatings, such as electroplating, spraying
and hot dipping. Metals used to coat the steel include cadmium, zinc, tin, aluminium and
copper.
Zinc aluminium alloy applied by the hot dip process has effectively replaced galvanised steel
in applications such as roofing because of its greatly increased durability.
Non-metallic coatings
These are available in a wide variety of colours and include:
•
paints
•
baked epoxy finishes
•
vinyl coatings
•
bituminous coatings
•
vitreous enamel coatings
Baked epoxy finishes are applied to zinc-aluminium coated steel which is chemically treated
to assist bonding. An epoxy primer and then the final colour coat are baked on separately.
This type of finish is popular for domestic and commercial roofing and wall cladding for
normal conditions.
In marine and polluted industrial conditions steel can be coated with a tough vinyl which is
laminated to the steel substrate. The vinyl coating locks out moisture, making an extremely
corrosion-resistant finish.
Vitreous enamel coatings comprise a layer of glass fused to a properly prepared steel base.
Painting should be considered as a complete system that includes surface preparation, pretreatment to facilitate adhesion, primer, intermediate coat or coats and finish coat. Different
types of steel require different pre-treatments and coatings.
Bituminous coatings are based on bituminous resins such as coal tar or asphalt. The
bituminous resins perform well underground and in contact with water but do not have good
weather durability when exposed to sunlight.
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Nonferrous metals
Most nonferrous metals are more costly to produce than ferrous metals. However, they often
have much better working properties and resistance to corrosion. The more common
nonferrous metals are copper, aluminium, zinc, lead, nickel, tin and cadmium.
Copper
Copper has been in use for at least 10 000 years: nearly 5000 years ago it was being beaten
into sheets, pipes, and other building products.
Copper is a pinkish coloured metal and is easily hammered into sheets. It is much more
expensive than some alternatives but its extreme resistance to corrosion outweighs this
disadvantage in certain applications. Upon exposure to the atmosphere, copper forms a
protective copper oxide coating which is light green in colour.
Uses
Its resistance to corrosion has made it popular for use as water pipes and tanks. It also
conducts electricity very well, hence its use for electrical wiring. Other uses include roofing,
roof plumbing, flashing and damp courses.
Brass
Brass is an alloy of copper and zinc, and is an attractive golden colour.
Uses
Brass is used for plumber’s hardware (e.g. pipe connectors and fittings; taps and outlet
spouts, often chrome finished). Screws, nails, grilles, hinges, door locks and latches and
chains are often made from brass.
Aluminium
Aluminium is a light-weight metal (approximately one-third the weight of iron) and is silverwhite in colour.
Aluminium was introduced as a building material after World War Two in competition with
traditional building metals, such as steel and copper. Probably the major characteristic that
has helped aluminium gain widespread acceptance in the building industry is its suitability for
extrusion production methods. This means that very complicated shapes can be produced
economically.
Uses
Aluminium products are extensively used in the building industry—for domestic windows,
doors and insect screens; for commercial windows and curtain walls for residential and
industrial roofing and rainwater goods; for balustrades and railings and for reflective
insulation.
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Corrosion resistance
One of the most significant properties of aluminium is its excellent resistance to atmospheric
corrosion. On exposure to the atmosphere, a whitish coating of aluminium oxide forms which
then protects the surface from further corrosion. The structural integrity is not impaired as a
result of this process.
Thus, untreated aluminium can be used for roofing, cladding and so on, but where long-term
appearance is important the aluminium should be finished.
Compatibility with other building materials
Corrosion of a metal may be accelerated through contact with another metal of very different
electro-chemical properties especially in the presence of an electrically conductive solution,
such as sea spray or industrially polluted moisture.
Copper, brass and nickel alloys, all have a large potential difference to aluminium and in a
salt solution cause it to rapidly corrode.
Some other building materials are also incompatible with aluminium and direct physical
contact with those materials should be avoided or barriers should be used. Table 1 broadly
indicates the types of barriers suitable for most building construction applications. For more
information refer to CSIRO Notes on the Science of Building 79 – 'Corrosion of metals in
building'.
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Table 1 - Compatibility of aluminium with various building materials
Contact material
Compatibility
Recommended barrier
Stainless steel
18/18 type or 300
series should be
specified
Satisfactory.
Recommended for all
fastenings.
No protective barrier required.
Zinc
Under severe
environments such as
coastal or industrial, zinc
will suffer from
preferential attack.
In severe environments metal
contact surfaces should be coated
with a bitumastic paint.
Galvanised steel
As for zinc.
As for zinc.
Mild steel
Aluminium will corrode in
contact with mild steel in
presence of an
electrolyte.
Coat contact surfaces with
bitumastic paint or yellow zinc
chromate paint.
Lead
Corrosion of the
aluminium will only occur
in marine or severe
industrial environments.
In severe environments separate
contact surfaces with non metallic
spacers or bitumastic paint.
Copper and brass
(including monel
metal)
Attack of the aluminium
surface in contact with
these materials will occur
in most atmospheric
conditions.
Copper and brass must be plated
with nickel and/or chromium;
otherwise use non-metallic
separators.
Concrete,
cement, lime etc,
stone and brick
Wet or ‘green’ products
can cause severe attack
on aluminium.
Surfaces in contact with these
products must be protected by
painting or separating with nonmetallic material. Wash thoroughly
with clean water if contact occurs.
Damp or
unseasoned
timber
Because of their acidic
nature can cause
aluminium to corrode.
Timber must be primed with yellow
zinc chromate undercoat and sealed
with suitable protective paint.
Treated timber
Wood preservatives use
salts of heavy metals
such as mercury or
copper, or certain
chlorides.
Timber should be coated with
caulking compound or mastic.
Hardboard,
plasterboard
The absorption of
moisture into hardboard
or plaster board may give
rise to poultice corrosion.
Seal using suitable primer.
Plastics, rubber
No corrosive effect.
No special treatment required.
Adhesives,
sealants, etc
These should not contain
chlorides in excess of
0.1% and those
containing water soluble
sulphates should be fully
Carefully select adhesives and
sealants compatible with aluminium.
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tested before use.
Finishes for aluminium
Although aluminium is naturally corrosion resistant, various finishes may be applied for
aesthetic reasons. These include textured finishes ranging from a fine satin finish (achieved
by chemical etching) to a scratch-brushed or hammered finish.
Bright finished aluminium can be achieved mechanically or chemically and results in highly
reflective product. To retain the desired appearance, however, the sections should be
anodised immediately.
Anodising is an electro-chemical process which greatly increases the thickness of the
protective oxide film which would naturally form on the surface, thereby increasing the
resistance of the surface to corrosion and damage and enhancing the appearance of the
finished product. Film thicknesses can be specified for different applications.
The oxide film may be artificially coloured. Depending upon the process, however, some
colours may be subject to ultraviolet deterioration and therefore are only suitable for interior
applications.
Paint may be applied to aluminium but factory application is recommended as the process
must be carried out in a dust-free environment and the aluminium surfaces must be pretreated to remove surface contaminations and to provide a key for good adhesion. Powder
coating is now widely used as a finish to aluminium in residential building.
Methods of joining aluminium sections
Most physical joining of aluminium elements is achieved with the use of bolts and nuts,
screws, nails and rivets. For reasons of compatibility, fasteners are normally aluminium alloy,
stainless steel or cadmium-plated steel.
Some modern adhesives such as epoxy and epoxy-PVC types are commonly being used to
produce high-strength joints between aluminium and a great variety of other materials.
Welding is also used to join aluminium. If welded assemblies are subsequently anodised,
some discolouration in the anodised film occurs across the welded zone.
Zinc
Zinc is a soft, greyish metal which can be hammered or rolled into sheets: such sheets have
been used for roofing rainwater goods. Today, zinc’s most important function in the building
industry is as a protective coating on steel.
The zinc coating acts first as a barrier to corrosion. However, should the coating be
scratched or damaged, exposing the steel, the zinc surrounding the damaged part will itself
corrode instead of the steel. Thus by sacrificing the zinc the steel is protected and will not
rust until all available zinc is used.
Zinc-aluminium coating
Research has produced a protective coating for steel which combines zinc and aluminium in
an alloy. It is easily applied, by hot dipping, and holds to the metal better than zinc
galvanising, thus giving much better protection. It is used on sheet steel and cladding.
Lead
Lead is soft and easily worked, but its great density makes it heavy to handle, and thin
sheets and pipes will not even support their own weight.
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Lead has been used for thousands of years: lead water pipes were used by the Romans,
and our word ‘plumber’ comes from the Latin word plumbum meaning lead.
Due to its toxic properties, however, lead is no longer used for water pipes. In the past, it
was used for roofing and roof plumbing, but today its use is limited—although in certain roof
plumbing situations, its weight and malleability still make it a useful and preferred material.
Uses
Lead is used:
•
for flashing and damp coursing
•
for solder (as an alloy with other metals)
•
as sheet lead lining for sound proofing
Nickel
Nickel is a hard, silvery-white, malleable metal. It is resistant to corrosion.
Nickel is used:
•
on steel as a base for chromium plating
•
as a constituent of stainless steel
•
as a nickel alloy (known as ‘Monel metal’)
Tin
Tin is a very costly, soft, weak metal with a low melting point (232°C), but extremely resistant
to corrosion.
Uses
Tin is used:
•
as a coating on sheet steel (tin plate)
•
for solders
Cadmium
Cadmium is a white, malleable metal that looks like tin.
Uses
Cadmium is used:
•
for electroplating steel components (such as screws, latches, handles, locks)
•
as plating on brass plumbing fittings, locks, latches, handles and other such fittings
Chromium
Chromium is well known for its high resistance to corrosion as a plating, and as a constituent
of stainless steels and other corrosion-resistant alloys. It is extremely hard and scratch
resistant.
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Stainless steel
Stainless steel is far harder than mild steel and silvery in appearance. It has wide
applications in commercial buildings and has been used extensively for domestic sinks.
More recently it has been used for bench tops and as a termite barrier where it takes the
form of a very fine mesh which termites cannot penetrate.
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Further applications of metals
Metal frame construction
Domestic and commercial buildings can both be of metal frame construction. This type of
construction is versatile, light, strong, time and labour saving, economical, and stable. Walls,
roofs and floors can all be constructed this way.
The metal frames made from steel are pre-fabricated in the workshop or before being
erected. They can be joined together using rivets, welds, screws or bolts.
Figure 6 - Metal framing for a brick veneer house
Fasteners
The wide range of metal fasteners used to join or fix building materials and components
includes:
•
nails
•
screws
•
bolts, nuts and washers
•
timber connectors and framing anchors
•
masonry anchors
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Nails
Nails are manufactured in a wide variety of sizes, shapes and finishes, according to their
particular use (see Figure 7).
Figure 7 - Some types of nails
Screws
Screws are available in a range of sizes, shapes and coatings for use with wood or masonry.
The four most common types of wood screw are:
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•
countersunk head
•
round head
•
raised head
•
coach screws (see Figure 8)
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Figure 8 - Common types of wood screw
Bolts, nuts and washers
Bolts, nuts and washers are normally made of plain steel, alloy steel or a non-ferrous metal,
and may have a protective metal coating (such as zinc or cadmium). The bolt heads are
usually either dome headed with a square shank; dome headed with a slot; hexagonal or
square headed. The nuts may be square or hexagonal and the washers are flat discs with a
central hole. The two most common types of bolt are:
•
the coach (or cup head) bolt
•
the hexagonal head bolt (see Figure 9).
•
Figure 9 -Two common types of bolt
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Timber connectors and framing anchors
These are used for joining various timber-framing members. They are made from hot-dipped
galvanised steel and are strong and quick to install. Figure 10 illustrates some of them,
together with their methods of fixing.
Figure 10 - Timber-framing anchors
Masonry anchors
Masonry anchors are used in concrete or masonry. A strong fixing is provided by the casing
expanding into the hole as the nut or bolt is tightened. A masonry anchor may be placed into
a mortar joint but is far more effective if placed in the body of the masonry.
The two most common types are:
•
the ‘Loxin’
•
the ‘Dynabolt’ (see Figure 11)
Figure 11 - Types of masonry anchors
Structural steelwork
Shop drawings
The structural integrity of completed steelwork is paramount. Onsite inspections of steel type
and size, weld quality, assembly, bolting etc play an important role in achieving this.
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However where high quality welding is required the QA process begins much earlier and
even prior to fabrication.
On larger projects contracts involving structural steelwork typically ask for preparation and
submission of shop drawings. The purpose of shop drawings is to collect and assemble all
relevant information necessary for fabrication. Shop drawings show all necessary detail to
permit fabrication by offsite personnel. Shop drawings show all dimensions cleats, gusset
plates, brackets, connecting plates, annotations for painting or galvanising, weld sizes etc.,
and include details of end plates and hole locations sizes and dimensions.
Usually shop drawings are prepared by subcontractors for approval by the builder and
relevant consultant, based on the consultants design. The builder/consultant stamps the
shop drawings with a stamp stating that the drawing has been inspected and generally
complies, and that the subcontractor remains responsible for the configuration/design.
Hence the subcontractor is being made responsible for their own design as well as its
fabrication.
With structural steel fabrication often it is necessary to incorporate information from the
architectural drawings, particularly in relation to the structural grid and other dimensions
without provided on the structural drawings by the structural engineer. It may also be
necessary to incorporate into the shop drawings site measurements and adjustments that
may be necessary to complete the design. The shop drawings are usually then submitted for
approval to the design consultants. These are checked for compliance contractual
obligations drawings and details. At the end of the project the final shop drawings are
assembled into a work as executed set of documents and given to the client.
Once approved fabrication can begin.
Welding
Key aspects of weld quality requirements include 2:
1. weld quality and acceptance requirements
2. personal qualification
3. selection, identification and traceability of materials, welds, welding and repairs
4. inspection and testing
5. non-conformance handling to prevent their inadvertent acceptance
Note arc welding differs from fusion welding.
Appropriate quality records include where necessary documentation to:
1. technical review
2. material inspection
3. welding consumable inspection
4. building procedure specifications
5. welder qualifications
6. testing personnel qualifications
7. non-destructive testing
8. destructive testing procedures and reports
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AS 3834:2008 Quality requirements for fusion welding of metallic materials in Part 3
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9. dimension reports
10. repairs
11. non-conformances
Typically welding inspection is controlled by overall QA process, and will require the use of
Inspection and Test Plans, and non-conformance reporting. Welding inspection may include
drawing review, material control, material cutting and shaping, process and operator
qualification, material welding, production process control, dimensional control, coordination,
coating application, dispatch, bolting procedures and preparation of manufacturing
documentation.
Australian standards and project documents set out the qualifications of welders and
inspection personnel. These standards and documents list functions from contract / drawing
review to preparation for dispatch as tasks of a welding inspector.
Structural bolts
Structural steelwork typically incorporates numerous bolted connections. The bolts and nuts
used have identification markers to clearly indicate their class/grade, type and size to ensure
compliance with contractual requirements shown in the project documentation by
supervisors on site.
Of importance is the steel grade in mega Pascals (MPa) which may be specified as the
tensile, yield or proof stress of the steel, and the thread pitch. Standards also set out a
suitable thread length for each bolt or threaded rod that is to be used to accommodate
appropriate clamping requirements. (Refer AS 1559)
When ordering or carryout an inspection all of these aspects must be taken into account.
Relevant Australian Standards
Relevant Australian Standards are
AS 4100 Steel structures
AS 1110 – AS 1112 ISO metric hexagon bolts and screws
AS 1252 High strength steel bolts with associated nuts and washers for structural
engineering
AS 1275 Metric screw threads for fasteners
AS 1559 Fasteners - bolts, nuts and washers for tower construction
Bolting Category System
The following bolting category identification system is based on that used in AS4100:
Category 4.6/S – Commercial bolts used snug tight
Category 8.8/S – High strength structural bolts used snug tight
Category 8.8/TF – High strength structural bolts fully tightened in friction type joints
Category 8.8/TB – High strength structural bolts fully tightened in bearing type joints
This category designation system is derived from the strength class designation of the bolt
and the bolting procedure as follows:
Bolt strength grade mark: This is indicated by the number stamped on the bolt head. For
example with a class 8.8 bolt, the figure before the decimal point is 1/100th of the minimum
Tensile Strength expressed in MPa and the digit after the decimal point indicates the
proportion of Yield Strength to Tensile Strength. Hence a Class 8.8 bolt has a minimum
Tensile Strength of 8 x 100 = 800 MPa, and Yield Strength of 0.8 x 800 = 640 MPa
Bolting procedure This is based on the following supplementary letters:
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S
represents snug tight
TF
represents fully tensioned, friction type joint
TB
represents fully tensioned, bearing type joint
Category 4.6/S refers to commercial bolts of strength class 4.6 tightened snug tight (Snug
tight is the final mode of tightening for bolting categories 4.6/S and 8.8/S, and the first step in
full tensioning for bolting categories 8.8/TF and 8.8/TB).
Category 8.8/S refers to high strength structural bolts of strength class 8.8 used snug tight.
Category 8.8T refers to both categories 8.8/TF and 8.8/TB
Category 8.8/TF refers to high strength structural bolts strength class 8.8 used in friction type
joints, fully tensioned in a controlled manner to the requirements of AS 4100.
Category 8.8/TB refers to high strength structural bolts strength class 8.8 used in bearing
type joints, fully tensioned in a controlled manner to the requirements of AS 4100.
Tightening
Section 15 of AS4100 Steel structures deals with erection matters including recommended
bolt tightening procedures. Acceptable alternate bolt tightening levels include:
Snug tight: application of the full effort of a man on a standard podger spanner, or the point
at which there is a change in note or speed of rotation when a pneumatic impact wrench
begins impacting solidly.
Full tightening/tension: tightened by either the part turn method, or by the direct tension
indicator method. With the part turn method after snug tightening the relative position of the
nut and bolt end is permanently marked with a punch, and then the nut rotated by the
specified additional amount, and crayon used to indicate tightening complete. With the
tension indicator method a load indicating washer is positioned with projections facing the
bolt head, and snug tightened, and then the nut is further tightened until the specified gap is
achieved. When bolt tightening is required a special washer of the same strength as the bolt
must be placed under the nut and the load indicating washer positioned to face this high
strength washer.
Supervision and QA
AS4100 full tightening methods have been shown to be superior to torque tightening
methods which was deleted from the standard.
Other superior methods exist for gauging bolt tightening precision including bolt heating,
ultrasonic measurement, use of mechanical direct tension indication and strain gauges.
In structural joints using either 4.6/S or 8.8/S procedures the site supervisor should ensure
that the correct bolt type and number of bolts have been used in the joint, and appropriate
snug tightening procedures used during erection.
In structural joints using either 8.8/TF or 8.8/TB procedures the site supervisor should
ensure that the correct fasteners and washers have been used and correctly installed, and
that none show physical damage which might indicate they been driven into misaligned
holes. Where the part turn final tightening method has been used nut bolt markings should
be checked. Where load indicating washers have been used for final tightening, gaps should
be inspected required tolerance. Note load indicating washers cannot be reused after
disassembly, and retightening is not permitted.
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Summary
Metals
Metals are widely used in the building industry. Some common metals and their applications
are:
•
steel—framing and cladding materials
•
lead—flashings
•
copper—plumbing pipe and fittings and electrical cable
•
brass—tapware and pipe fittings, door hardware
•
zinc—protective coatings
•
aluminium—window and door framing, roof cladding.
Some metals require protective coating to fulfil their service. Steel and aluminium in
particular have to be protected from corrosion by the elements and generally most metals
should not be allowed to come into contact with each other.
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