Lecture -2 Introduction to Steel Structures

N-W.F.P. University of Engineering and Technology

Peshawar

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Design of Steel Structures CE-411

By: Prof Dr. Akhtar Naeem [email protected]

N-W.F.P. University of Engineering and Technology

Peshawar

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By: Prof Dr. Akhtar Naeem [email protected]

Lecture 02: Introduction to Steel Structures

CE-411: Lecture 02 Prof. Dr Akhtar Naeem Khan

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Topics to be CoveredWhat is Steel?

Mention of Iron in Holy Quran

Steel Making process

Treatments and processes affecting steel properties

Mechanical properties of Structural Steel


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What is Steel?Steel is an alloy in which iron is mixed with

carbon and other elements.

An Alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties.

An Alloy usually has different properties (sometimes significantly different) from those of its components.


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Iron is mentioned in the Holy Quran 6 times:

1. Surah Bani Israil (17:50)

2. Surah Al Kahf ( 18: 96)

3. Surah Al Anbiya.. (21:22)

4. Surah Saba (34:10)

5. Surah Qaf (50:22)

6. Surah Al-Hadid (57:25)

Mention of Iron in Holy Quran

CE-411: Lecture 02 Prof. Dr. Akhtar Naeem Khan 6

Mention of Iron in Holy QuranAl-Hadeed (Sura 57:25)

We have indeed sent Our messengers with clear proofs, and sent down with them the book and the balance, so that people may uphold equity. And we sent down iron in which there is strong power, and benefits for the people; and (We did it) so that ALLAH knows who helps Him and his messengers without seeing (Him). Surely ALLAH is Strong, Mighty.


Steel Making Process

http://www.tatasteel.com/products-and-processes/processes/steel-making-process.asp


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Chemical Reactions in Blast Furnace

Iron (III) Oxide + Carbon Monoxide Iron + Carbon Dioxide

Fe2O3(s) +3 CO(g) 2 Fe(s)+3 CO2(g) (At 1500o C)

Calcium Carbonate Calcium Oxide + Carbon Dioxide

CaCO3(s) CaO(s)+CO2(g)

This oxide helps to remove some of the

acidic impurities from the ore



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Calcium Oxide (g) + Silica (s) Calcium Silicate (l)

CaO(s) + SiO2(s) CaSiO3 (l)

The metal that leaves the Blast Furnace contains between 4%

and 5% Carbon and is brittle. This carbon and other impurities are removed in the next step.


Chemical Reactions in Blast Furnace


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Removal of Impurities by Oxidation

o The Bessemer process named after its Inventor Henry Bessemer who invented the process in 1855

o The key principle is removal of excess carbon and impurities by injection of oxygen through molten iron

o Oxidizing excess carbon and impurities also keeps the metal molten.



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Making Steel Rolled ShapesSteel Making Process


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Treatments and Processes Affecting Steel Properties

o Quenching

Quenching refers to heating steel to below a critical temperature, holding that temperature and then rapidly cooling it in a desirable medium such as air water or oil to obtain desired hardness property.


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o Tempering

Tempering refers to heating steel above a critical temperature, then cooling it rapidly to freeze it in a very hard state followed by rewarming it to an intermediate temperature to give a hardness suitable for the job intended.


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o Annealing

Heating (usually up to 1150o F) followed by cooling of steel in solid state to relieve the residual stresses and to enhance ductility.



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o Killed Steel

It indicates that the steel has been completely deoxidized by the addition of an agent such as silicon or aluminum, before casting, so that there is practically no evolution of gas during solidification. These are characterized by a high degree of chemical homogeneity and freedom from porosity.



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o Rimmed Steel

A low-carbon steel containing sufficient iron oxide to give a continuous evolution of carbon monoxide while the ingot is solidifying. Incomplete oxidation allows the metal at the top of the ingot to remain liquid while solidifying in formation of a bottom and side rim of virtually pure iron of considerable thickness virtually free of voids. Sheet and strip products made from rimmed steel ingots have very good surface quality.



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o Work Hardening

An increase in strength and hardness attained by stressing the steel to cause plastic deformations at lower temperatures.



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Effect of Carbon percentage on Steel Properties

o Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel. Hardness and tensile strength increases as carbon content increases up to about 0.85%.


Type of Steel %age of Carbon

Mild Steel Up to 0.25%

Medium Carbon Steel 0.25% to 0.45%

High Carbon Steel 0.45% to 1.50%

• Adding metals such as nickel, chromium, and tungsten to iron produces a wide range of alloy steels, including stainless steel and high speed steels.

Effect of Carbon percentage on Steel Properties


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Mechanical Properties of Structural Steels

o Most widely used standards for structural materials are American Society for Testing & Materials (ASTM) Standards.

o ASTM specifications for structural steels generally identify the Process by which steel is to be made, chemical composition, and tensile requirements.


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o Yield strength is usually taken to be that stress which leaves the specimen with a permanent set of 0.2% when specimen is unloaded or stress corresponding to 0.5% elongation (strain)



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o Yielding is a discontinuous phenomenon.

o In tension test it begins with sudden appearance in specimen of one or more narrow slip bands called Flow Lines.

o Slip bands are plastic regions separated by completely elastic regions.


Lecture 01 Prof. Dr. Akhtar Naeem Khan

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Stiffness: The resistance of structural component to deformation.

• Material• Length• X-Section

Mechanical properties of structural steel


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Stiffness: Variation with Geometry and Material.



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Stiffness: Variation with Geometry and Material.



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Strength: The max load which a structure or structural component can resist.

Toughness: The ability of a structure or structural component to absorb energy.

Fatigue is a progressive, localized permanent damage under fluctuating repeated stress.



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Material A is more Stiffer but less Tougher than Material C

Material A has more strength than Material C



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o Specification A6 outlines general requirements for Rolled Steel plates, shapes, sheet piling, and Bars for structural use.

o Specification A370 outlines the procedures for Mechanical testing of steel products.

o Standard specimen called Coupons cut from shapes, are used in tensile test to establish properties of material.



Composition & Property ASTM A36 Mild (low-carbon) steel

ASTM A572 Grade 50 steel

MinimumProperties

Ultimate Tensile Strength, psi 58,000 - 79,800 65,000 min

Yield Strength, psi 36,300 50,000 min.

Elongation 20.0% 18% min

Chemistry Iron (Fe) 99% 98%

Carbon (C) 0.26% 0.23%

Manganese (Mn) 0.75% 1.35%

Copper (Cu) 0.2% --

Phosphorus (P) 0.04% max 0.04%

Sulfur (S) 0.05% max 0.05%

Silicon -- 0.4%

Vanadium and Columbium -- 0.02 – 0.15



Comparison of Mechanical Properties Wrought Iron and Steel

Property Wrought Iron Steel

Elastic Modulus

190Gpa

(27Mpsi)

190 – 210 GPa

(27-30 Mpsi)

Yield Strength 210 MPa

(30ksi)

280-1600 MPa

(40-232ksi)

Ultimate

Strength

340 Mpa

(49ksi)

340- 1900 MPa

(49-275ksi)

% Elongation 35 3- 40

Engr. Shahid Ullah

Please confirm the conversion from Pa to Psi.


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Types of Steel ShapesStandard Rolled Shapes


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Properties of Steels Used for Buildings and Bridges


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Uses of Various Steels


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Uses of Various Steels


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Primarily occur as:

Chord Members in trusses:

In diagonal bracing in bracing systems;

Cable elements in suspension roofs, main cables of

suspension bridges and suspenders.

Types of steel structuresTension Members


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Typical Tension Members

Types of Steel Shapes


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Primarily occur as: Columns in buildings; Chord Members in trusses and diagonal members

in end panels of trusses Stability is an important consideration in design and

behavior of compression members Area is generally spread out to maximize Radius of

Gyration

Types of steel structuresCompression Members


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Typical Compression Members Types of Steel Shapes


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Primarily loaded transverse to the longitudinal axis and resist loading by flexure

X-sectional area is located as far from the neutral axis as is practical

Commonly W shapes are used in most cases

For deeper beams I-shaped sections made by welding plates are commonly used

For smaller loads and spans open-web joists are commonly used

Instability due to lateral Torsional Buckling is an important consideration

Types of steel structuresBeam Members


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Typical Beam Members Types of Steel Shapes


Typical Beam Members (Contd.)

• Economical & popular for Roof & floor framing.• Bar joist are supported by bearing walls or steel girders.• Floor of thin concrete reinforced in both directions with

rebars or welded wire fabric.

Open-Web Steel Joist

Types of steel structures


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Difference between W and S shapes

W shape: Have wide flanges, efficient in resisting moments so used primarily as beamsS shape: Have wide webs, efficient in resisting shear (used in the past as railway tracks)


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Symbolic Representation of Various shapes

W30x90:

W represents shape of the section, I section in this case

30 is the depth of the section in inches

90 is the nominal weight in lb per ft

L3x2x1/2

L represents shape of the section, angle

3 is the length of one leg, inches

2 is the length of the other leg, inches

½ is the thickness of the angle, inches


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Classical Skeleton framing

Steel truss

Rigid frames

Arches

Domes

Cable supported Roofs



Classical system supported by beams, girders and columns.

Beams: W or S shapes, Channel shapes for roof purlins.

Columns: generally W shapes

Classical skeleton framing



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Classical skeleton framing



Triangular rigid structure

Most common double pitched roof trusses:

Fink & Pratt.

Most common flat trusses: Pratt &Warren

Steel trusses



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Steel trusses



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No span limit

Often prefabricated

Used with wood or steel purlins to support the roof.

Bracing: if resting on masonry walls:

Diagonal bracing in alternate bays

Continuous struts(angles, channels)

Steel trusses



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Truss Bridge

Built-up Members

Steel trusses: Example of steel truss with built up members



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Steel trusses



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• For large unobstructed floor areas and ceiling heights.

• Spans generally 40ft to 100ft• Members are connected by bolting and welding• Members: W shapes or web plates fillet welded to

flange plates• Connection to foundation with a base plate(bolted)

Rigid Frame



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Types of steel structures Rigid Frame


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Types of steel structuresBraced Frame


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Used in field houses, exhibition halls,… with span over 300ft

Most common type: three hinged arch

Types of steel structures Steel arch structures


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Lateral bracing;

Diagonal bracing in curved surfaces between arches

Lateral bracing of ribs with purlins or trussed purlins.



Used for large circular areas: Assembly halls, gymnasium, field houses….

Spans up to 400ft in diameter

Structural members:Perimeter (Tension ring)

Domes and rings are supported by columns braced laterally or by bearing walls

Purlins supports the roof deck and span between ribs

Types of steel structures Steel Domes




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Chicago O’Hare International Airport Restaurant

• Reinforced concrete compression ring 190’dia

• Ring is supported on 26 RCC columns 58ft above ground

• Tension ring W-shape 13’ dia

• Sag of cables 10.5’

• Ends of cables are anchored to the two rings

• Roof deck: precast RCC slabs 3.5” thick which fit between cables with projecting ends of reinforcement hooking over the cables.

Types of steel structuresCable supported roof


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Types of steel structuresCable supported roof


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Types of steel structures Cable supported roof


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Eiffel Tower, Paris

The World’s tallest structure at times(990ft).

Was originally built to last 20 years.

Examples of Famous steel structures


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James R. Thompson Center, Chicago

The building is enclosed by 17 story curtain walls.

The diameter of rotunda is 160ft

The rotunda projects as a cylinder and its top resembles a drum without a dome slanting towards plaza.

Office spaces are between rotunda’s walls and outer skin



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The Geodesic Dome at Walt Disney World

Spaceship earth is a huge golf ball, standing 180 ft.

The structure is designed to withstand wind speeds of 200 mph

Structure: steel framing clad with faceted aluminum panels, and stands on 3 pairs of steel legs.

Site was mostly swamp, filled with much, organic material with 95% water content.

As a solution, 2.5 million cubic yards of soil was removed and replaced by clean material



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Indoor Football Facility,University of Illinois, Urbana

The roof structure is semi-parabolic dome

A large single arched box truss spans the length of the field and supports 1/2 of roof load.



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• 110 Stories Tall• Total Height = 1725 ft • Based on revolutionary Bundled Tube Design• Rigid outer walls act as walls of hollow tube• There are 9 tubes in all• The number of tubes reduces with height• Designed by late Fazl-ur-Rehman from Bangladesh• Supported by 114 piles


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Lecture -2 Introduction to Steel Structures

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