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DESIGN OF STEEL STRUCTURES
INTRODUCTION
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Prakash Kumar Sekar 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
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2.Common Steel structures
1. Roof truss in factories, cinema halls, railways etc.,
2. Crane girders, columns, beams3. Plate girders, bridges4. Transmission towers, water tank, chimney
etc.,
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Old Arch Bridge
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Framed Building
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Framed Building
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Industrial Building
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Industrial Building
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Truss Bridge
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Suspension Bridge
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Cable Stayed Bridge
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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
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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.
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-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/m32. E = 2x105 N/mm2 3. Poison ratio µ = 0.3
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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
GRADEWELDABLE
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Tensile test specimen before ruptureF
Area=S-
F
F
Standard tensile test specimen
F
rt
d
Lc
Area=S0-
L
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Ductility• Ability of material to change its shape without fracture
Mild steel – high ductilityHigh 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
TempAt high temp strength reduces
CorrosionSteel corrodes in moist air, sea water and acid. Adopt Painting, metallic coating, plastic coating, using corrosion resistant steel to resist corrosion
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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 bridgeResidual stress• Latent stress present in the steel sections due to uneven
heating and cooling during steel makingStress concentration• Under loading, stress is concentrated at places at abrubt
change in geomentry like holes bolts
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5. STEEL SECTIONS
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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
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Common Steel members
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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/mDepth 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
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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
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Other Sections
• Steel bars– ISRO – Round bars– ISSQ – Square bars
• Steel Tubes
• Steel plates
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Braced frame Moment resisting frame
Core and suspended floors
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PylonLattice girderTapered portal
Knee brace Cable stayed bridge Suspension bridge
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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 columnOther forms of sectionsBuilt-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 philosophyWorking 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
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• 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)
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Questions ???
Next Class: Connection