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8/10/2019 Lecture 7 Structural Steel 2014
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Use of steel in construction
Dr Rick Chan,Lecturer, SCECE
Brooklyn Bridge, NYPhoto: Ricky Chan
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Lecture Outline
What is steel?
Where do we use steel?
How it is made?
Advantages & disadvantages
Mechanical properties
Comparison with other construction materials
Corrosion
Welding and fire resistance
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The Colosseum in Rome,
ItalySource: http://www.telegraph.co.uk/
The Eiffel Tower, FranceSource:http://www.planetware.com/
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What is steel
Steel may be defined as an alloyof iron and carbon
Tensile strength: 200-500MPa Density: 7850kg/m3
Youngs modulus: 200,000MPa
Shear modulus: 80,000MPa
Poissons ratio: 0.3
Coeff. thermal expansion: 12x10-6/K
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Southbank Footbridge
Photo: Ricky Chan
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Effects of carbon content
An increase in carbon content willIncreases tensile strength
Increases hardness
Reduction in ductility
Increase difficulty in weldingGreater tendency to corrode
Steel used in construction isgenerally lowin carbon content to
ensure duct i l i ty
Transmission towersPhoto: Ricky Chan
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Effects of carbon content
Rollason, Metallurgy forEngineers, Butterworth-Heinemann
El= Elongation
TS = Tensilestrength
BH = BrinellHardness
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Carbon content in Steel (by mass)
Low carbon steel 0.15% C
Mild Steel 0.15-0.25% C
Medium Carbon Steel 0.2-0.5% C
High Carbon Steel 0.5-1.4% C
Common in construction
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Australian standards
There are several Australian Standards which governs chemicalcomposition, test requirement, and geometrical requirements, etc for
structural steel sections
AS/NZS 1163:2009 : Cold-formed structural steel hollow sections
AS/NZS 3679.1:2010 : Structural steel - Hot-rolled bars and sections
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Structural sectionsPhoto: Ricky Chan
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Use of steel in constructionTall Buildings
Empire State Building, NYC
Photo: Ricky Chan
Bank of China, Hong Kong (left)
Photo: Ricky Chan
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Use of steel in constructionFramed buildings
A residential building in JapanPhoto by Ricky Chan
A car park in Japan
Photo by Ricky Chan
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Use of steel in constructionBridges
A plate girder bridge in JapanPhoto by Ricky Chan
A box girder bridge in JapanPhoto by Ricky Chan
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Use of steel in constructionlong span roofs
Hong Kong airport
Photo by Ricky Chan
Kuala Lumpur airport
Photo by Ricky Chan
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Use of steel in constructionlong span roofs
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Osaka airport
Photo by Ricky Chan
Southern Cross Station
Photo by Ricky Chan
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Use of steel in constructionreinforcement steel
Reinforcement bars in a column
Photo by Ricky Chan
Reinforcement bars in beam /
columnPhoto by Ricky Chan
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Earthworks and foundations
Steel pilesPhoto by Ricky Chan
Sheet pilesPhoto by Ricky Chan
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Extraction of metals
Metals occurs in nature as ores, in the form of oxides, sulphides,carbonates, etc.
We need to extract the metals from their compounds which occurs
naturally
But metal tends to revert to their compounds, i.e. it corrodes
To extract it and keep it that way
Extraction metallurgytechniques with extracting the metals from
their compound
Unaided fire can reach about 1100-1200
o
C Copper, lead and tin were produced in pre-historic times
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Extraction of Iron
Extraction of Iron dated back to 1200BC, the Iron Age
Iron has melting point of 1535oC
In 18s and 19s century, forced air blast furnaces wereable to melt iron
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Modern extraction of iron
Limestone, Iron ore and Coke are put together in blastfurnace. The following reactions occur in general
2C(s)+O2(g)2CO(g)Fe2O3(s)+3CO(g)2Fe(l)+ 3CO2(g)
The limestone remove silica in the oreCaC03(s)CaO(s)+ CO2(g)
CaO(s)+ SiO2(s)CaSiO2(l)
see the making of steel from BlueScope Steel
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Effect of cooling rate
The rate of cooling of steel crystal distribution, and affects mechanicalproperties of steel product
Annealing
Steel is cooled slowly in a controlled manner (usually in furnace)
Coarse-grain structure
Higher ductility, easier to machine
Lower yield strength than normalized steels
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Effect of cooling rate
NormalizingSteel is allowed to cool in still air
Fine grain structure
Harder and Higher yield strength than annealing
Quenching
Rapid cooling rate by plugging the steel into water (or iced brine)
Intensely hard but brittle steel
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Hot working
At temperature above re-
crystallization For steel, at
temperature over 910oC Rollingis a common method of forming
structural sections
Exposure to air at high
temperature causes a heavy
film of oxide layer to form on
surfaceRolling
Illston J.M. & Domone P.L.J.
Construction materials, 2001
Hot-rolled I-beams / columns
Photo: Ricky Chan
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Cold working
Because of the cold ductility of metal, they can be shaped below re-
crystallization temperature Yield strength can be increased
Metals sheets
Cold drawn wires
Cold formed steel decks
Steel deck in Westfield
Shopping Centre
Photo: Ricky Chan
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Advantages of steel
High tensile strength (very high yield stress)
Long span structures made possibleGenerally it is the only high tensile strength material commonly
used in construction. Recently the development of carbon fibrebased material replace some steel, but use is very limited.
High compressive strength
High shear strength
e.g. shear resistance of concrete structures relies on steel stirrupsto provide shear resistance
Youngs modulus is high (E=s/e)
Structure built with steel are resistance to deformationE = 200GPa compares to Aluminium's E=75GPa
Ductile
Prevents sudden failure
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Advantages of steel
Durability
Resistance to wear and abrasion
Malleability
Can be rolled or shaped into various shape to enhance structural
efficiency. e.g. an I-beam is most efficient in bending
Alloying
Adding other chemical will change its properties. E.g. Stainless
steel contains chromium, nickel and molybdenum.
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Disadvantages of steel
Large amount of energy used in production (sustainability issue)
International Iron and Steel Institute research shows that theamount of energy required to produce a tonne of steel is less than
half of what it was 35 years ago
Mining of iron ore destroys natural landscape
Slender steel member may buckle under compression
Can be avoided by engineering
Corrosion
Can be delayed by coating / galvanising
Poor fire resistivityCan be improved by fire proofing material
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Comparison with concrete and timber
Material Youngs
Modulus(MPa)
Working
stress(MPa)
Density
(kg/m3)
Stiffness
Efficiency
Strength
efficiency
Energy of
production(MJ/kg)
Energy of
production(MJ/m3)
Energy
per unitstress
A B C D=A/C E=B/C F G=F*C H=G/B
Steel 210,000 160 7800 27 0.02 30 234,000 1500
Concret
e
25,000 8 2400 10 0.003 0.8 1920 240
Timber 11,000 7.5 600 18 0.013 1 600 80
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Mechanical properties
We want to know what is the yield stress of
material, as we want to keep the steel
members within elastic range under designservice loads.
We are also interested in its stiffness
(resistance to deformation) and ductility.
Tensile test on coupona small test piece iscut from steel section and put under tensile
test.AS13912007 Metallic materials
Tensile testing at ambient temperature
Determine: yield stress, ultimate stress,
Youngs modulus, elongation (ductility), etc.
Youngs Modulus = 200GPa for steel
(constant)
Yield / ultimate stress vary with steel grades
Fig 14, AS1391-2007
Photo: Ricky Chan
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Tensile test
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Tensile testPhoto: Ricky Chan
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Stress-strain curve in tensile test
Tensile test carried out by Ricky Chan
Elastic
Plastic
Strain hardening
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Elastic region
Stress is linearly proportional to strain
This is known as Hooks Law
i.e. if we can measure strain, we can calculate stress (E is material
constant)
Strain are measured by attaching strain gauges (by glue) to steels
surface. Changes in electrical resistance is converted to strain.
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EesAttaching a rosette strain
gage to surface of steelPhoto: Ricky Chan
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Stress
Strain
P = Uniaxial tension
L = Length
L0= Initial length
A0= Initial cross-sectional area
e = engineering strain
s=nominal stress
Valid when strain magnitudes not exceed 0.002 (0.2%)
0A
Ps
0
0
L
LLe
Small strain problems
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Large strain problems
For large strain problems, strain should be expressed in natural strain
Total strain = elastic strain + plastic strain
For incremental strain
integration from original length to the current length
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pe eee
L
dL
d e
L
L L
L
L
dLd
0 0
lnee
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Creep
Time-dependent strain when a steady stress is maintained
Creep reduces with decrease in temperature
Significant strains do not normally occur at temperature below 40% of
melting point of metal in K (degree K = oC +273)
Creep would not be expected in ferrous metals at room temperature
But a related phenomenon called relaxationoccurs in cold-workedsteel such as prestressing tendons
Prestressing tendons used in bridge
construction
Photo: Ricky Chan
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The Charpy impact test
Steel becomes brittle in cold temperature
Requirement in Section 10 of AS4100 Brittle fracture based on lowest
one day mean ambient temperature. e.g. Grade 300+ (AS/NZ3679.1 Grade 300) belongs to Steel Type 1
according to Table 10.4.4. It lowest permissible service temperature is -10C for thickness between 6 and 12mm
Charpy impact test is a standardised test on materials toughness.
Toughness is the energy require to fracture (fail) a specimen. (areabeneath stress-strain curve of standard tensile test is toughness underlow-strain rate). Charpy impact test is a high strain rate test.
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Impact strength
Taylor G.D., Materials in Construction,2002
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Brinell hardness test
Proposed by Swedish engineer Johan
Brinell in 1900
Correlates the diameter of an indentation
on a material test piece to a hardness
scale.
Typical tests use a 10mm dia steel ball as
an indenter with 3000kgf (29kN) force
For hard material, a tungsten carbide ball
is used.
Typical values:
Softwood: 1.6
Hardwood: 2.67.0
Aluminium: 15
Mild Steel: 120
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Oxidation of metal
Almost all metals are unstable in oxygen containing atmosphere,exceptions are gold and silver
Metals release electrons and oxygen accept electrons
General equation: M + O MO
Not a major cause of corrosion in buildings as temperature is low
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Oxidation & oxide layer
Taylor G.D., Materials in Construction,2002
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Oxidation of metal
In order for oxidation process to continue, oxygen must have access
to metallic ions below the oxide layer
Properties of oxide layer defines the rate of corrosion
Oxide layer of zinc, chromium, lead and aluminum are so tightly
bounded, oxygen cannot penetrate down and eventually corrosion will
cease
A bridge showing signs of corrosionPhoto: Ricky Chan
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Electrolytic andacidic corrosion
Metals have tendency to dissolve in aqueous solution
The tendency depends on types of metals, temperature
This tendency is measured using a Standard Hydrogen Electrode
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Electrode potentials
Metal Electrode potential (Volts)
Magnesium -2.4
Aluminum -1.7
Zinc -0.76
Chromium -0.65
Iron -0.44
Nickel -0.23
Tin -0.14
Lead -0.12
Hydrogen 0.00
Copper +0.34
Silver +0.80
Gold +1.40
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Acidic Corrosion
In the presence of acid, free hydrogen ions receive electrons to give
hydrogen gas
2H+(aq)+ 2e-(from metal) H2(g)
Reactions occur with metal above hydrogen in the table of electrode
potentials
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Electrolytic Corrosion
Different metals in contact
e.g. Zinc in contact with copper
Electrons flow from zinc (anode) to copper (cathode)
Giving a potential difference of 0.34-(-0.76) = 1.1V
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Pitting Corrosion
Anode and cathode on the same piece of metal
Ferric oxide behaves as a cathode with respect to iron
Taylor G.D., Materials in Construction, 2002
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Pitting Corrosion
Electrolytic corrosion cells may form in a single metal in presence of
moisture
Contributes to most corrosion problems in civil structures / buildings
A heavily rusted bridge in TorontoPhoto: Ricky Chan
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Effects of oxygen
In pure water, corrosion is extremely slow because water only ionizes
slightly
H2O H++ OH-
Fe Fe2++ 2e-
At cathode, 2H+ + 2e- H2 Fe2++ 2(OH)-Fe(OH)2
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Effects of oxygen
But, with oxygen in water, oxygen reacts with electrons to form
hydroxyl ions
2H2O + O2+4e-4(OH)-
Steel corrode quite rapidly
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Protection against corrosion
Impervious coating
By covering the metal with coating (paint, pitch, tar, etc.)
Level of protection depends on thickness of paint
Relatively cheap method
Paint must be applied immediately after manufacture
The film of paint must be intact (unbroken)
Paint workshop for structural steelPhoto: Ricky Chan
C th di t ti
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Cathodic protection
Sacrificial anodes
Simply connect the metal you want to protect to a more reactive
metal
Replacement of sacrificial metal is required
Not commonly used in construction
H t di l i i
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Hot-dip galvanizing
A very common form of protection (but could be costly)
Prepared steel sections are dipped into molten zinc at about 450oC
Zinc, being more reactive than iron, act as a sacrificial anode
50mm to 200mm thick, 100mm common
Zinc reacts with oxygen to form oxide, and because of its oxide is tightlybounded, corrosion will cease
Galvanising factoryPhoto: Ricky Chan
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W ldi
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Welding
The material is heated locally to melting temperature, additional metal
may added to connect two metal components
Structural steel sections are often connected by arc welding
Welding should be performed by skilled workmen to ensure quality
Governed byAS1554Structural steel welding
Welded beam / column
connectionPhoto: Ricky Chan
T f ld
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Types of weld
Fillet Weld Butt Weld
Fi i t f t t l t l
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Fire resistance of structural steel
At about 250oC, yield strength
increases
At about 400oC, strength
decreases rapidly
Some means of fire protection
are needed
Robinson J. T., Architecture and
construction in steel, 1993
50% strength @ 600C
S d fi fi t i l
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Sprayed fire proofing materials
Spray-on fire proofing materials
Photo: Ricky Chan
Further reading
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Further reading
Shielded metal arc welding http://www.youtube.com/watch?v=WaDsmeB5ywM
Flux-cored arc welding http://www.youtube.com/watch?v=Li_pAMrUWSw
Fire protection of structural steel in buildings http://www.pfpa.com.au/docs/Steel%20Fireproofing/Rakic%20-
%20Type%20of%20Fireproofing%20materials.pdf
Galvaniser Association of Australia http://www.gaa.com.au/
http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/http://www.gaa.com.au/