Introduction To Steel Design

Moayyad Al Nasra, PhD, PE

Advantages of Steel as Structural Material

• High Strength

• Homogeneous and uniform material

• Elasticity

• Ductility

• Toughness

• Constructability, speed or erection, connectability,

• …Etc.

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Disadvantages of Steel As a Structural Material

• Corrosion

• Fireproofing cost

• Susceptibility to buckling

• Fatigue

• Brittle fracture

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Iron

• The basic constituent of steel is iron.

• Iron is made from iron ores (limonite (brown ore), hematite (red ore), magnetite)

– Hematite is the most common ore which contains about 70 % iron.

• This mined ore is crushed and placed in a blast furnace (developed by Bessemer in 1860) along with:

• Iron ore

• Coke (heated coal in absence of air)

• Limestone (flux that holds impurities)

• Heat (3000 deg F) and “blast” of air

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Iron Products

• Most iron has a high carbon content.

• Wrought iron (very low C content)

– Similar to pure iron, less ductile than steel

• Cast iron (high C content)

– Strong, hard, brittle

• Steel (low C content)

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Steel

• Steel is iron with controlled amounts of C and other alloying agents (manganese, silicon, nickel, aluminum, chromium, molybdenum)

• Example: stainless steel is iron, carbon, chromium, nickel (highly resistant to corrosion)

• One cubic foot of steel/iron weighs 490 lbs (490 lb/cu ft)

• Steel properties are affected by: – Chemical composition

– Heat treatment

– Mechanical work

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Chemical Composition of Steel

(Carbon Content) • In general, with increasing C:

– Yield strength (yield point) increases

– Tensile strength (ultimate strength) increases

– Hardness increases

– Abrasion resistance increases

– Ductility (% elongation) decreases

– Toughness decreases

– impact resistance decreases

– weldability decreases (also affected by manganese)

• NOTE: other alloying agents affect steel properties

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Effect of carbon content on the mechanical properties of steel 8 (C) Al Nasra

Properties of steel • Heat Treatment

– quenching – sudden cooling (produces hard, brittle steel)

– annealing – controlled cooling (produces softer, more ductile steel)

• Mechanical Work

– cold working – working with steel when it’s cold (produces hard, brittle steel)

– hot working – working with steel when it’s hot (produces softer, more ductile steel)

• Mechanical Properties of Steel

– Steel grades indicate its yield strength

– E=29,000 ksi for all steel grades

– Poisson’s ratio = 0.27-0.30

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Typical Stress-strain of steel

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Iron and Steel

• Steel is defines as a combination of iron and

a small amount of carbon (0.15% to 1.7% ),

usually less than 1%.

• Wrought Iron refers to iron with a very low

carbon content (<0.15%).

• Cast Iron refers to a very high carbon

content (>2%)

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Steel Sections

Structural Shapes 12 (C) Al Nasra

Section Identification Examples

• W10X30 is W section approximately 10 in. deep weighing 30 lb/ft

• S24X100 is S section ~24 in deep weighing 100 lb/ft

• M12X10 is miscellaneous section ~12 in deep weighing 10 lb/ft

• C15X50 is a channel section ~15 in deep weighing 50 lb/ft

• L6X6X1 is an equal leg angle, each leg is 6 in. long and 1 in thick.

• HP12X74 is a bearing pile section ~12 in deep, weighing 74 lb/ft.

• MC 18X58 is a miscellaneous channel ~18 in deep , weighing 58 lb/ft,

which can not be classified as a C shape because of its dimensions.

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Section Identification Examples, Cont’d

• HSS14X10X5/8 is a rectangular hollow structural section ~14 in deep, 10 in. wide with 5/8-in wall thickness. It weighs 93.10 lb/ft

• WT9X23 is a tee obtained by splitting a W18X46. This type of section is known as a structural tee.

• PL1/2X10X1 ft 4 in is a plate 1/in thick, 10 in. wide, and 1 ft 4 in. long

• Metric section W920X449 is w section where depth is 920 mm, weighing 449 kg/m which equivalent to W36X302.

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Stress-Strain Relationship

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Points on the Stress-Strain Curve

• A, is zero stress – zero strain

• B’, is proportional limit, end of linear section

• B, elastic limit, end of elastic portion

• C, yield point, increase in strain w/o substantial increase in stress.

• D, ultimate stress, max. stress

• E, breaking point

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Notes

• Steel is commonly produced by 0.2% to 0.3% by weight carbon, 98% iron, rest are silicon, manganese, sulfur, ..etc.

• The increase in carbon percentage, increases strength, and reduces ductility.

• The increase in temperature, reduces strength. In case of fire

– 32 F 100% strength

– 800 F 77% strength

– 1000F 63% strength

– 1200 F 37% strength

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Notes, Cont’d

• Stress – Strain curve for ductile steel in

compression is assumed to be similar to the

tension, (NO BUCKLING)

• Examples of Structural Steel

– A36, is carbon steel Fy=36 ksi, Fu 58-80 ksi

– A572, Gr. 50 is high strength low-alloy, Fy=50ksi, Fu =

65 ksi

– A572, Gr. 60 is high strength low-alloy, Fy is 60 ksi, Fu

75 ksi

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Responsibilities of the Structural

Engineer

• Safety

• Cost

• Constructability

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Load and Resistance Factor Design (LRFD)

VS

Allowable Strength Design (ASD)

• Similarities

– Both design procedures are based on the limit

states design principles, which provide

boundaries of structural usefulness. The limit

state concept is used to describe a condition at

which a structure or part of a structure ceases to

perform its intended function

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Load and Resistance Factor Design (LRFD)

VS

Allowable Strength Design (ASD

• Differences

– LRFD, multiply the service loads by a load

factor (usually >1.0) resulting in factored load

– ASD, the service loads are generally not

multiplied by a load factor

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Load and Resistance Factor Design (LRFD)

VS

Allowable Strength Design (ASD)

• Differences, Cont’d

• LRFD

– Introduced a reduction safety factor such that

– (reduction factor,Φ)

• (nominal”theoretical”strength of a member)

>computed factored force in member, Ru

– ΦRn>Ru

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Load and Resistance Factor Design (LRFD)

VS

Allowable Strength Design (ASD)

• Differences, Cont’d

• ASD

– Introduced a larger safety factor “Ω omega” such that

– (nominal strength of member)/(safety factor=Ω)

>largest computed force in member, Ra

– Rn/Ω>Ra

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LRFD VS ASD

– The relationship between the factor of safety and the

resistance factor is, in general

• Ω=1.5/Φ

– (for example if Φ=0.9, Ω=1.5/0.9=1.67)

• The load factors in the linear combination, ζ “xi”, thus if

we set Qi=one of N service loads in a group

ζi= load factor associated with loads in LRFD

Rn= nominal structural strength

Then for LRFD

ΦRn> Σζi.Qi ( i from 1 to N)

And for ASD

(Rn/Ω) > Σ Qi ( i from 1 to N)

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