Chapter 2A - Properties of Steel

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Properties of steel



Background of Structural Steel

• Economical production in large volume not available until mid 19th centuryand the introduction of the Bessemer process. Steel became the principalmetallic structural material by 1890.

• Steels consists almost entirely of iron (over 98%) and small quantities of carbon, silicon, manganese, sulfur, phosphorus, and other elements.

• The quantities of carbon affect properties of steel the most.

• Increase of carbon content increases hardness and strength

• Alloy steel – has additional amounts of alloy elements such chronium,vanadium, nickel, manganese, copper, or zirconium.

• The American Society for Testing of Materials (ASTM) specifies exactmaximum percentages of carbon content and other additions for a numberof structural steels. Consult Manual, Part 2, Table 2-1 to 2-3 for availabilityof steel in structural shapes, plate products, and structural fasteners.



ASTM classifications of structural steels

• Carbon steels – A36, A53, A500, A501, A529, A570. Have well-defined yield point. Divided into four categories:

– Low-carbon steel (< 0.15%)

– Mild steel (0.15 to 0.29%, structural carbon steels)

– Medium-carbon steel (0.3 to 0.59%)

– High-carbon steel (0.6 to 1.7%)

• High-Strength Low-Alloy steels – A242, A572, A588, A606, A607,A618, A709

– Well-defined yield point

– Higher strengths and other properties

• Alloy Steels – A514, A709, A852, A913.

– Yield point defined as the stress at 0.2% offset strain

– Low-alloy steels quenched and tempered→ 550 to 760 MPa yield

strengths



Advantages and disadvantages of steel as a

structural material

• Advantages

– High strength per unit of weight→ smaller weight of structures

– Uniformity

– Elasticity

– Long lasting

– Ductility

– Toughness

– Easy connection

–Speed of erection

– Ability to be rolled into various sizes and shapes

– Possible reuse and recyclable



Advantages and disadvantages of steel as a

structural material

• Disadvantages

– Maintenance costs

– Fire protection/Fireproofing costs

– Susceptibility to buckling failure

– Fatigue

– Brittle fracture



Types of Steel

• Three basic types of steel used for structural steel

– Plain Carbon Steel

– Low-alloy steel

– High-alloy “specialty steel”

•The most commonly used is mild steel - ASTM A36

• Typical high strength steel:

• The higher the steel strength, the higher the carbon content and the less

ductile it is.

248 (36 )

400 (58 )

y

u

F MPa ksi

F MPa ksi

290 344 (42 50 )

444 482 (63 70 )

y

u

F MPa ksi

F MPa ksi

ASTM A242

344 (50 )

448 (65 )

y

u

F MPa ksi

F MPa ksi

ASTM A992



Stress-strain curve•

Standard Plain Carbon Steel

)(

)(

Length Original L

n Deformatio L

o

)(

)(

Area A

Load P f

Yield plateauFy

Strain “

”

Stress “f ”

Fu

E

Necking & FractureStrain Hardening

Elastic



What is a Limit State

• When a structure or structural element becomes unfit

for its intended purpose it has reached or exceeded a

limit state

• Two categories of limit states:

– Strength limit states

– Serviceability limit states



Limit States

• Strength Limit States

based on the safety or load carrying capacity ofstructures, and include plastic strengths, buckling,fracture, fatigue, overturning, and so on.

a) Loss of Equilibriumb) Loss of load bearing capacity

c) Spread of local failure

d) Very large deformations

• Serviceability Limit States

a) Excessive deflection

b) Excessive local damage

c) Unwanted vibration



Design Philosophies

• Allowable Stress Design (ASD)

•

Plastic Design (PD)

• Load and Resistance Factor Design (LRFD)



Allowable Stress Design

• Service loads are calculated as expected during service life.

• Linear elastic analysis is performed.

• A factor of safety (FOS) of the material strength is assumed

(usually 3-4)

• Design is satisfactory if (maximum stress < allowable stress)

• Limitations

– Case specific, no guarantee that our design covers all cases

– Arbitrary choice of FOS?!

FOS

StrengthMaterialStressAllowable



Plastic Design

• Service loads are factored by a “load factor”.

• The structure is assumed to fail under these loads, thus, plastic

hinges will form under these loads “Plastic Analysis”.

• The cross section is designed to resist bending moments and

shear forces from the plastic analysis.

• Members are safe as they are designed to fail under these

factored loads while they will only experience service loads.

• Limitations

– No FOS of the material is considered, neglecting the uncertainty in

material strength!

– Arbitrary choice of overall FOS?!



Load and Resistance Factor Design (LRFD)

• LRFD is similar to plastic design in that it performs design with theassumption of failure! - Reliability Based Design

• Service loads are multiplied by load factors (g) and linear elastic

analysis is performed.

• Material strength is reduced by multiplying the nominal materialstrength by a resistance factor (f)

• The design rule is: Load Effect < Resistance

– Where Rn is the nominal strength and Q is the load effect for the ith limit

state

niii R Q fgThis rule shall be attained

for all limit states!!



Load and Resistance Factor Design (LRFD)

• Resistance: Shear, Bending, Axial Forces

• Advantages of LRFD

– Non-case specific, statistical calculations guarantee population

behavior.

– Uniform factor of safety as both load and material factors are tied by

reliability analysis



Probabilistic Basis for LRFD

If we have the probability distribution of the load effect (Q) and the materialresistance (R) then:

• The probability of failure can be represented by observing theprobability of the function (R-Q)

• The probability of failure PF can be represented as the probability thatQ ≥ R:

Probability

of failure



AISC Load combinations

• AISC considers the following load combinations in designn i i i R Q f g

)(5.06.12.12 R or S or LLD r

D 4.11

)8.0(5.0)(6.12.13 W or L Ror S or L D r

S LE D 2.05.00.12.15

6 0.9 (1.6 1.0 ) D Wor E

)(5.05.06.12.14 Ror S or L LW D r

00.175.0 f

i i Q g

n i R f

Dead loads (D)

Live loads (LL)

• Occupancy load(L)

• Roof load (Lr)

• Snow load (S)

• Rain loads (R)

• Trucks andpedestrians

Wind Loads (W)

Earthquakes (E)e.g. f for yield is 0.9 and for bolt shear is 0.75

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Chapter 2A - Properties of Steel

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