Design of steel structures Introduction

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


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


Framed Building


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

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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.



-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


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


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


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


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



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

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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 columnOther forms of sectionsBuilt-up, stepped, wide flange, hybrid, cold

formed (formed from light gauge steel strips)

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Prakash Kumar Sekar from Civilrnd.comFrom Civilrnd.com


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


• 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 ???

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Design of steel structures Introduction