Lecture 7 Structural Steel 2014

8/10/2019 Lecture 7 Structural Steel 2014

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Use of steel in construction

Dr Rick Chan,Lecturer, SCECE

[email protected]

Brooklyn Bridge, NYPhoto: Ricky Chan


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RMIT Universityyyyy School/Department/Area 2

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


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


7/57RMIT Universityyyyy School/Department/Area 7

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


Structural sectionsPhoto: Ricky Chan



Use of steel in constructionTall Buildings

Empire State Building, NYC

Photo: Ricky Chan

Bank of China, Hong Kong (left)

Photo: Ricky Chan



Use of steel in constructionFramed buildings

A residential building in JapanPhoto by Ricky Chan

A car park in Japan

Photo by Ricky Chan



Use of steel in constructionBridges

A plate girder bridge in JapanPhoto by Ricky Chan

A box girder bridge in JapanPhoto by Ricky Chan



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


Osaka airport

Photo by Ricky Chan

Southern Cross Station

Photo by Ricky Chan



Use of steel in constructionreinforcement steel

Reinforcement bars in a column

Photo by Ricky Chan

Reinforcement bars in beam /

columnPhoto by Ricky Chan



Earthworks and foundations

Steel pilesPhoto by Ricky Chan

Sheet pilesPhoto by Ricky Chan



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



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



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


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.


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


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/
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Documents

Lecture 7 Structural Steel 2014