Transcript N-W.F.P. University of Engineering and Technology Peshawar Design of Steel Structures CE-411 By: Prof Dr.

N-W.F.P. University of Engineering and
Technology Peshawar
Design of Steel Structures
CE-411
By: Prof Dr. Akhtar Naeem Khan
[email protected]
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N-W.F.P. University of Engineering and
Technology Peshawar
Lecture 02: Introduction to Steel
Structures
By: Prof Dr. Akhtar Naeem Khan
[email protected]
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Topics to be Covered
 What 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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Mention of Iron in Holy Quran
 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)
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Mention of Iron in Holy Quran
Al-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.
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Steel Making Process
http://www.tatasteel.com/products-and-processes/processes/steel-making-process.asp
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Steel Making Process
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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Steel Making Process
Chemical Reactions in Blast Furnace
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.
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Steel Making Process
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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Steel Making Process
Making Steel Rolled Shapes
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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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Treatments and Processes Affecting
Steel Properties
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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Treatments and Processes Affecting
Steel Properties
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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Treatments and Processes Affecting
Steel Properties
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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Treatments and Processes Affecting
Steel Properties
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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Treatments and Processes Affecting
Steel Properties
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%.
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Effect of Carbon percentage
on Steel Properties
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.
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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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Mechanical Properties of
Structural Steels
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Mechanical Properties of
Structural Steels
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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Mechanical Properties of Structural Steels
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.
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Mechanical properties
of structural steel
Stiffness: The resistance of structural
component to deformation.
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Lecture 01
Material
Length
X-Section
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Mechanical properties of
structural steel
Stiffness: Variation with Geometry and Material.
Lecture 01
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Mechanical properties of
structural steel
Stiffness: Variation with Geometry and Material.
Lecture 01
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Mechanical properties of
structural steel
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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Mechanical properties of
structural steel
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Mechanical properties of
structural steel
Material A is more Stiffer but less Tougher than Material C
Material A has more strength than Material C
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Mechanical Properties of Structural Steels
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.
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Mechanical Properties of Structural Steels
ASTM A36 Mild
(low-carbon)
steel
Composition & Property
Minimum
Properties
Chemistry
CE-411: Lecture 02
ASTM A572
Grade 50 steel
Ultimate Tensile Strength, psi 58,000 - 79,800
65,000 min
Yield Strength, psi
36,300
50,000 min.
Elongation
20.0%
18% min
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
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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
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Types of Steel Shapes
Standard Rolled Shapes
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Properties of Steels Used for
Buildings and Bridges
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Properties of Steels Used for
Buildings and Bridges
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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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Types of steel structures
Tension Members
 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.
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Types of Steel Shapes
Typical Tension Members
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Types of steel structures
Compression Members
 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

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Area is generally spread out to maximize Radius of
Gyration
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Types of Steel Shapes
Typical Compression Members
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Types of steel structures
Beam Members
 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
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Types of Steel Shapes
Typical Beam Members
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Types of steel structures
Typical Beam Members (Contd.)
Open-Web Steel Joist
• 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.
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Difference between W
and S shapes
W shape: Have wide flanges, efficient in resisting moments so used primarily as beams
S 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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Types of steel structures
 Classical Skeleton framing
 Steel truss
 Rigid frames
 Arches
 Domes
 Cable supported Roofs
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Types of steel structures
Classical skeleton framing
 Classical system supported by beams,
girders and columns.
 Beams: W or S shapes, Channel shapes
for roof purlins.
 Columns: generally W shapes
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Types of steel structures
Classical skeleton framing
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Types of steel structures
Steel trusses
 Triangular rigid
structure
 Most common
double pitched roof
trusses:
 Fink & Pratt.
 Most common flat
trusses: Pratt
&Warren
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Types of steel structures
Steel trusses
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Types of steel structures
Steel trusses
 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
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struts(angles, channels)
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Types of steel structures
Steel trusses: Example of steel truss with built up members
Truss Bridge
Built-up Members
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Types of steel structures
Steel trusses
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Types of steel structures
Rigid Frame
• 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)
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Types of steel structures
Rigid Frame
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Types of steel structures
Braced Frame
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Types of steel structures
Steel arch structures
 Used in field houses, exhibition halls,… with span
over 300ft
 Most common type: three hinged arch
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Types of steel structures
Steel arch structures
Lecture 01
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Types of steel structures
Steel arch structures
Lateral bracing;
Diagonal bracing in curved surfaces between arches
Lateral bracing of ribs with purlins or trussed purlins.
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Types of steel structures
Steel Domes
 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
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Types of steel structures
Steel Domes
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Types of steel structures
Steel Domes
Lecture 01
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Types of steel structures
Cable supported roof
Chicago O’Hare International Airport Restaurant
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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.
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Types of steel structures
Cable supported roof
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Types of steel structures
Cable supported roof
Lecture 01
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Examples of Famous steel
structures
Eiffel Tower, Paris
 The World’s tallest
structure at
times(990ft).
 Was originally built
to last 20 years.
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Examples of Famous steel
structures
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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Examples of Famous steel
structures
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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Examples of Famous steel
structures
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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Examples of Famous steel
structures
• 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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