Design of steel structures Introduction

DESIGN OF STEEL STRUCTURES
INTRODUCTION
From Civilrnd.com

1. What are steel structures
• In steel structures, structural steel is the main
load carrying material to transfer the load within
them and to transfer load to the ground
• Ex: - I-Beam, Tee section, [ - Channel section,
Steel plate etc..,
• Steel concrete composite structures are also used
in high-rise buildings but we are only going to
study about steel structures in this paper
Prakash Kumar Sekar from Civilrnd.com

2.Common Steel structures
1. Roof truss in factories, cinema halls, railways
etc.,
2. Crane girders, columns, beams
3. Plate girders, bridges
4. Transmission towers, water tank, chimney
etc.,
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Old Arch Bridge

Framed Building

Industrial Building
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Industrial Building

Truss Bridge

Suspension Bridge

Cable Stayed Bridge

Advantages
• High comp. & tensile strength per unit weight hence low
construction weight, saves space
• Good aesthetic view
• Good quality and durability
• Very high speed of construction
• Reusability and scrap value – env. Friendly
• Better solution to cover large span and tall structures
Disadvantages
• Highcost – Initial
• Corrosion
• Low fire resistance
2. Adv. & Disadv.
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3. Steel
Steel making
• First iron is extracted from iron ores like
haematite, limestone, magnetite in furnace
• Oxygen is passed through molten iron to
remove carbon and impurities to make steel.
• Magnese is added to strengthen the steel
• Adding chrome, nickel, phosphorous can
impart special properties in steel
From Civilrnd.com

Cont….
• Semi finished products from the machine is
hot rolled to different sections like bars,
plates, angles, sections etc..,
• Adding carbon increases the tensile strength
and hardness but lowers ductility and
toughness
• In building we use structural steel which has
low carbon of upto 0.1% to have ductility and
yield.

-Manganese
-Ferrite
--Carbon
-Ferrite
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Ingot slab bloom Billet
Basic shapes and their relative proportions
Primary rolls for plates
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4.Properties of steel
• Physical properties (IS800:2.2.4)
1. r = 7850 kg/m3 = 78.5kN/m3
2. E = 2x105 N/mm2
3. Poison ratio µ = 0.3
From Civilrnd.com

Type l Design-
ation
UTS
(MPa)
Yield strength (Mpa)
Thickness (mm)
<20 20-40 >40
Standard
structural
steel
Fe 410A 410 250 240 230
Fe 410B 410 250 240 230
Fe 410C 410 250 240 230
High tensile
structural
steel
St58HT 580 360 0.05 1.00
ST55-HTW 550 360 .05 1.00
Mechanical properties
FE 410 A W
IRON
ULTIMATE TENSILE
STRENGTH
GRADE
WELDABLE

Tensile test
specimen before
ruptureF
Area=S-
F
F
Standard tensile
test specimen
F
r
t
d
Lc
Area=S0-
L
From Civilrnd.com

Ductility
• Ability of material to change its shape without fracture
Mild steel – high ductility
High carbon steel – low ductility
Toughness & brittle fracture
• Ability of material to resist (absorb) impact load like earthquake load,
machine load etc..,
• Requires both strength and ductility
• At low temp. steel fails on impact loading due to reduction in ductility
and toughness called brittle fracture
Temp
At high temp strength reduces
Corrosion
Steel corrodes in moist air, sea water and acid. Adopt Painting,
metallic coating, plastic coating, using corrosion resistant steel to
resist corrosion

Hardness
• Resistance of the material to intentions and scratching
• Brinell harness, rockwell hardness number are used to
measure hardness
Fatigue
• Damage of material to cyclic loading
• Occurs due to moving loads, vibration in bridge
Residual stress
• Latent stress present in the steel sections due to uneven
heating and cooling during steel making
Stress concentration
• Under loading, stress is concentrated at places at abrubt
change in geomentry like holes bolts
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Steel sections
• Steel is rolled to a required shape during
fabrication.
• Commonly available
– I section – I
– Tee section – T
– Channel sections –
– Angle sections – l
- Steel bars , tubes, plates, sheets, strips
Refer structural engg handbook or steel table for
sectional details
From Civilrnd.com

Common Steel members

Rolled steel I - section
• ISJB – Indian standard junior beam
• ISLB – “ Light beam
• ISMB - “ Medium beam
• ISWB - “ Wide flange beam
• ISHB - “ Heavy beam
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Rolled steel I - section
• Example = ISMB 500 & 0.852 kN/m
Depth Weight per Unit length
500 mm
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Beam – Column construction
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Rolled steel Channel - section
• ISLC, ISMC, ISLC, ISSC(Indian standard special
section)
• Example ISMC 300 & 0.351 kN/m
300 mm

Rolled Steel Angle section
• ISA Equal angle – ISA 150 x 150 x 12
• ISA unequal angle – ISA 150 x 115 x 12 Thickness
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Rolled Tee- section
• ISNT – Indian standard Normal beam
• ISHT – “ Heavy beam
• ISLT - “ special legged
• ISLT - “ Light beam

Other Sections
• Steel bars
– ISRO – Round bars
– ISSQ – Square bars
• Steel Tubes
• Steel plates
From Civilrnd.com

Braced frame Moment resisting
frame
Core and suspended
floors

PylonLattice girderTapered portal
Knee brace Cable stayed bridge Suspension bridge

Choice of sections
• Governed by sectional properties and
availability
• Popular in India – ISMB, ISMC, equal angles
• Channels are used in purlins, Tee and angles in
truss, I section in beam and column
Other forms of sections
Built-up, stepped, wide flange, hybrid, cold
formed (formed from light gauge steel strips)
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6. Loads on Structures
• DL = Dead load – self weight and perm load (IS875 - part 1)
• LL = Live load - changes from time to time – person ,furniture
etc.., ( part 2)
• WL = Wind load – IS875 part 3
• AL = IS875 part 5
• EL = Earthquake load – IS1893
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Design philosophy
Working Stress method
– Stress at which the material starts to yield is taken as
permissible stress of the section. All sections are designed
not to exceed the permissible stress
Permissible stress = Yield stress / F.O.S
– Since steel can resist load after yield point, following this
principle results in bulky, uneconomical sections
Ultimate Load method (plastic design method)
– Permissible load is a load when all the fiber in the steel is
yielded
– This method does not ensure serviceability

• Limit State Method (IS800 : 2007)
– It takes both strength and serviceability to account
– Limit state of Serviceability
• Minimum deformation, deflection, crack, vibration,
corrosion to ensure aesthetic view, functionability and
safety to partitions etc.,
– Limit state of strength
• Structure should be stable and not collapse under load
γf = partial safety factor for load (table 4)
γm = partial safety factor for material (table 5)

Questions ???
Next Class: Connection

Design of steel structures Introduction

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