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BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

INSTRUCTION DIVISION

FIRST SEMESTER 2012-2013

Course Handout Part II

Date: 03/08/2012

In addition to part -I (General Handout for all courses appended to the time table) this

portion gives further specific details regarding the course.

Course No. : CE C381

Course Title : Design of Steel Structures

Instructor-in-charge : MANOJ KUMAR

1. Scope and Objective of the Course

The course intends to impart design skills to common type of Civil Engineering Steel

Structures as found in practice as per revised code IS 800: 2007. An understanding of

basic design concepts, loads and stresses to be used as per Indian standards for

steel design work will be developed. The course deals with designing of steel

structural elements subjected to axial tension, axial Compression, bending, combined

twisting and bending. Moreover, emphasis will be also given to the special structures

such as beam-column, trusses, and plate girders. In addition, analysis and design of

various types of connections such as bolted and welded will be discussed for use in

fabrication of tension, compression, and flexural members in the framed structures.

All design approaches will be based on Limit State of strengths and serviceability.

Furthermore, a special chapter on Plastic design of steel will also be introduced.

Text Book

S. K. Duggal, “Limit State Design of Steel Structures”, Tata McGraw

Hill, New Delhi, 2010

Reference Books

(i) N. Subramanian, ‘Steel Structures: Design and Practice’, Oxford

University Press, New Delhi, 2011.

(ii) N. Subramanian, “Design of Steel Structures”, Oxford University

Press, New Delhi, 2010.

(iii) Teaching Material on Structural Steel Design, by Institute for Steel Development and growth (INSDAG), Calcutta, http://www.steel-insdag.org/new/contents.asp .

(iv) IS 800:2007 ‘Code of practice for General construction in steel’ Bureau of Indian Standards, New Delhi

(v) IS 875 : 1987 (parts I – IV) “Code of practice for design Loads”, Bureau of Indian Standards.

Lectur

e Nos.

Learning

Objectives

Topics to be covered Reference

Chap. No. of

TB

3 General

considerations

Introduction, Advantages &

Disadvantages of steel as structural

material, properties of structural steel,

rolled steel sections, Loads

considered for structural design, basis

for design, design philosophies

Chapter 1

3 Introduction to

Plastic Design

Bending of beams, Re-distribution of

moments and Reserve of Strength,

Shape factor, Load factor,

Mechanisms, Plastic Analysis and

Design of simple beams and frames

Chapter 2

3 Introduction to

Limit State

Design

Limit States (LS) design method for

Steel: LS of strength, LS of

Serviceability, probabilistic basis for

design, design criterion

Chapter 3

3 Design of

bolted and

Pined

connections

Types of connections, Introduction to

Riveted joints, Design of bolted

connections, Design of pin

connections

Chapter 4

Lecture

Nos.

Learning

Objectives

Topics to be covered Ref.Chap

No. of TB

3 Welded

Connections

Types of welds and their symbols, Design of

Groove welds, Design of Fillet welds: Fillet

weld specifications, Design strength, Design of

welds

Chapter 5

3 Design of

Tension

Members

Types of tension members, Net sectional area,

net effective area, design strength of tension

members, slenderness area, design of tension

members, lug angles, splices, gusset plate

Chapter 6

4 Design of

Compressio

n Members

Effective length, Slenderness ratio, types of

compression members, Design strength of

compression members, design of axially loaded

compression members, Design of built-up

columns, design of Lacings and Battens

Chapter 7

4 Design of

Beams

Types of beam sections, behavior of beams in

flexure, lateral stability of beams, bending

strength of (i) laterally supported and (ii)

laterally unsupported beams, shear strength of

beams, web buckling, web crippling, deflection,

design of rolled beams, design of built-up

beams, beam bearing plates

Chapter 8

Lecture

Nos.

Learning

Objectives

Topics to be covered Ref.Chap

No. of TB

3 Members

subjected

to axial

load and

moment

Design of crane members, behavior of beam

columns, design of beam columns Chapter 9

3 Column

bases and

caps

Types of column bases, design of slab bases,

design of gusset bases, design of bases of

columns subjected to axial load and moment

Chapter 10

4 Design of

plate

girders

Elements of plate girders, general design

considerations, proportioning of web,

proportioning of flange, flexural strength and

shear strength, design of plate girder, design of

stiffeners, flange curtailment, design steps

Chapter 11

2 Gantry

Girders

Loads, fatigue effects, design of Crane girders Chapter 12

4 Eccentric

connection

Beam column connections, Un-stiffened and

stiffened connections, Bolted bracket

connections, Welded bracket connections

Chapter 13

S. No Evaluation

Component

Duration Weight

age

Date & Time Remarks

1 Mid-Sem Test 90 mts 35 04/10 2:00 – 3:30 PM CB

2 Tutorials 50 mts 20 Every M 8:00 – 8:50 AM OB

3 Comprehensive

Examination

3 Hrs 45 01/12 2:00 – 5:00 PM CB

5. Make-up Policy

No Make-up will be given for Tutorials.

Make-up for Mid-Sem Test will be given only for genuine

cases if applied in advance.

6. Chamber Consultation Hour:

To be announced in the class. Students must adhere to the announced timing.

7. Notice:

Notice if any, concerning this course will be displayed on the Civil Engg. Dept. Notice Board.

5. Make-up Policy

No Make-up will be given for Tutorials.

Make-up for Mid-Sem Test will be given only for genuine

cases if applied in advance.

6. Chamber Consultation Hour:

To be announced in the class. Students must adhere to the announced timing.

7. Notice:

Notice if any, concerning this course will be displayed on the Civil Engg. Dept. Notice Board.

Instructor-in-charge

CE C381

• steel structure an assemblage of a group of members (elements)

• Members- sustain their share of applied forces and transfer them

• safely to the ground.

• Depending on the orientation of the member in the structure and its structural use, the member is subjected to forces either

• (i) axial, (ii) bending, or (iii) torsion, or a combination thereof.

• Axial load tensile—Tension Members (tie), Compressive – Compressive Members (Strut)

• Flexural Force Beams and girders

• Torsion Shafts (Not discussed here)

• Steel members are connected using the rivets, welds, bolts, pins

• The connection between steel members Joints

• Joints Rigid — can transfer moments)

Flexible —can transfer axial loads (shears);

Semi-rigid —that fall in between rigid and flexible

• Steel structures are used in:

• Roof trusses for factories, railway station platforms, cinema halls, auditoriums

• Bridges for railways

• Crane girders in industry

• Water tanks

• Telephone Towers

Steel Sections:

Steel sections are rolled in industry in the standard shapes

called rolled sections

The shapes of rolled sections are:

Steel I-Sections; Channel Sections; Angle Sections; Tee Sections;

Steel Bars; Steel Tubes; Steel Plates;

ADVANTAGES OF STEEL AS A STRUCTURAL MATERIAL

high strength per unit weight

small section little self-weight

members resists heavy loads smaller column sections

lesser columns in buildings

easy to transport prefabricated members can be used

Steel ductile material No sudden failure

Steel may be bent, hammered, sheared or even the bolt holes may be punched

without any visible damage.

Steel Properly maintained steel structures have a long life.

Steel properties mostly do not change with time

Additions and alterations can be made easily

Can be erected at a faster rate.

Highest scrap value amongst all building materials and can be reused and recycled

DISADVANTAGES OF STEEL AS A STRUCTURAL MATERIAL

When placed in exposed conditions corrosion

require frequent painting and maintenance

Strength reduces drastically in fire Needs fire-proof treatment

Needs additional Cost

Excellent heat conductor

may transmit enough heat from a burning location to adjoining room.

Fatigue one of the major drawbacks

At stress concentration locations steel may lose its ductility (tearing of steel)

Fatigue at very low temperatures aggravate the situation

Stress-Strain Curve for Steel

Stress-Strain Curve for Steel

Point A Limit of Proportionality; B elastic limit;

C’ upper yield point (strain increase with out increase in stress)

upto C’, stress is elastic

obtained by loading the specimen rapidly

not found in hot rolled steel due to residual stress

no practical significance

C Lower yield point,

stress at lower yield point called yield stress, fy.

obtained by loading the specimen slowly

CD plastic yielding;

strain at D = approx 10 times of strain at yield, provided ductility

DE strain hardening, Presently this strength is not used in design

E Ultimate stress, after this point, section area reduced locally necking

EF Strain softening,

stresses reduces in this zone and finally specimen breaks at point F

If fractured section makes cup-and-cone arrangement Ductile failure

Notes: • Same curve for tension as well as in compression.

• Actual behavior is different and indicates an apparently reduced yield stress in compression.

• Divergence from the ideal path is called the Bauchinger effect.

• The actual stress-strain curve may be idealized into bilinear or tri-linear form

• At High Temperature, curve will be more rounded with no clear yield point

Properties of Structural Steel:

Ultimate Strength or Minimum Guaranteed ultimate strength

or Engineering ultimate strength

tioncrossofareaOriginal

LoadTensileUltimateUTSstrengthTensileUltimate

sec)(

Actual Ultimate Strength

intsec)(

pobreakingattioncrossofarea

LoadTensileUltimateUTSstrengthTensileUltimateActual

Since area of cross-section varies with load, it becomes difficult to measure area at

different load stage

• Characteristic Ultimate Strength:

• The strength below which not more than 5% of samples falls.

64.1meank ff

1tan,

2

n

ffdeviationdardswhere mean

Design Strength: In order to incorporate the reduction in strength due to corrosion

and accidental damage, the partial safety factor of 1.1 is used. Thus

Design strength of steel = Characteristic strength/partial safety factor (1.1)

Ductility: capacity of steel to undergo large inelastic deformation without significant

loss of strength or stiffness

% Elongation

;100%lengthgauge

lengthgaugelengthelongatedelongation

tioncrossofareaAAlengthgauge sec;65.5 00

Toughness: Capacity to absorb energy, measure of fracture resistance under

impact. Area under the stress-strain curve is a measure of toughness

Properties of Structural Steel

Two types of steel in India: (i) Standard Structural steel ,

(ii) Micro-alloyed medium /High strength steel

Standard Structural Steel Designated as Fe 410 (IS 2062)

Characteristic yield strength

for thickness < 20 mm 250 MPa

for thickness 20-40 mm 240 MPa

for thickness > 40 mm 230 MPa

Available in three grades

Grade A used for structures subjected to normal conditions

Grade B used for situations where severe fluctuations are there but temp > 00 C

Grade C may be used upto – 400 C and have high impact properties

Modulus of Elasticity (E) = 2 105 N/mm2

Shear Modulus (G) = 0.769 105 N/mm2

Poission’s Ratio ( ) in elastic range = 0.3

in plastic range = 0.5

Coefficient of Thermal Expansion = 12 10-6 / 0C

Classification Based on manufacturing process, Two types of Section:

(i) Cold Formed Sections, (ii) Hot rolled Sections

Cold formed sections: produced by steel strips (thickness < 8mm )

Light in weight used for smaller loads where hot rolled becomes un-economical

Hot Rolled Sections Simply called as Rolled Sections

more commonly used as structural steel

Rolled Sections

• Sections produced by hot rolling process in rolling mill,

• Due to hot-rolling no loss of ductility

• In India available standard Sections as per IS 808:1989 are:

• Indian Standard Junior Beams (ISJB)

• Indian Standard Light-weight Beams (ISLB)

• Indian Standard Medium weight Beams (ISMB)

• Indian Standard Wide-Flange Beams (ISWB)

• Indian Standard Heavy weight Beams (ISHB)

• Indian Standard Column-Sections (ISSC)

• Indian Standard Junior Channels (ISJC)

• Indian Standard Junior Channels (ISJC)

• Indian Standard Light weight Channels (ISLC)

• Indian Standard Medium-weight Channels (ISMC)

• Indian Standard Angles (ISA)

• Indian Standard Normal Tee-Sections (ISNT)

• Indian Standard Deep-Legged Tee-Sections (ISDT)

• Indian Standard Light weight Tee-Sections (ISLT)

• Indian Standard Medium-weight Tee-Sections (ISMT)

LOADS

(1i Dead Load; (ii) Live loads; (iii) Environmental loads

DEAD LOADS (IS 875: Part I):

(i) Due to gravity: acts in the direction of gravity (due to self weight)

DL not known before design, so initially is assumed and later on is checked

(ii) Superimposed loads: permanent loads (such as partition walls)

LIVE LOADS (IS 875: Part IV):

Loads which may change in position and magnitude (Furniture, equipments, occupants)

Also some reduction in live loads are made for residential buildings.

No. of floors carried by member % reduction of Live load on all floors above

under consideration the member under consideration

1 0

2 10

3 20

4 30

5 to 10 40

Over 10 50

Impact Load:

• When a load is applied suddenly or load is in motion,

• Used for Lifts and Industrial buildings

Frames supporting lifts and hoists 100

Foundations, footings, piers supporting lifts and hoisting apparatus 40

Light machinery shaft motor units 20

Reciprocating machinery or power units 50

Installed machinery 20

Earth pressure: Used for Underground structures such as basement, retaining walls

Water current force: For piers and abutments

Thermal Loads: due to Temperature variations, may be up to 25% of LL in bridges

and Trusses

Environmental Loads: Wind Loads

Pressure, suction, uplift

More for tall structures

Design wind pressure at height ‘z’ above mean ground level, P = 0.6Vz2 (N/m2)

Vz = Vb k1 k2 k3

k1 probability or risk factor, k2 terrine (height) factor, k3 topography factor

Vb = basic design speed, increases with height (constant up 10 m from MGL)

Wind Force: F = Cf Ae pz

where Cf = force coefficient for buildings depends on shape of structure

BASIS FOR DESIGN

Steel Structure are designed for

• Integrity: Its constituent parts should constitute a stable and robust structure under normal loading and Columns must be anchored in two directions at right angles

• Stability: Remain fit with adequate reliability and are able to sustain all actions

• Durability: Not seriously damaged (collapse) under accidental events : Sway resistance is distributed throughout the building

METHODS OF ANALYSIS

Working Stress Method:

• An elastic method of design material behavior elastic

• Based on concept that maximum probable stresses due to applied loads ≤ permissible stresses

• Permissible stress = Material strength (yield strength) / Factor of safety

• Factor of safety is used (only to strength) to make structure safe

• Factor of Safety accounts: – Overloading under certain circumstances

– Secondary stresses due to fabrication, erection and thermal

– Stress concentrations

– Unpredictable natural calamities

• Disadvantage: (i) only limited material capacity is used, (ii) no check at overloading

Plastic Method

• Steel ductile material major portion of curve lies beyond the elastic limit (from the stress–strain curve)

• higher strength after elastic limit called reserve strength used in plastic design method

• Plastic method based on failure conditions rather than working load conditions

• Structure designed for collapse load rather than elastic loads excessive deformations at collapse

• Design Load in plastic design (Collapse Load) = working loads load factor

• At plastic stage Plastic Hinge formation infinite rotation More P Hinges Collapse of structure

• Since actual loads are less than design loads Structure do not collapses

• Disadvantage: Design at ultimate loads only , no check for serviceability

Limit State Method

• also known as load and resistance factor method

• overcomes the drawbacks of (i) Working stress and (ii) Plastic Design

• Limit State a state beyond which the structure is unable to function satisfactorily

• Two major categories of limit states:

• limit state of strength load carrying capacity (plastic strength, fracture, buckling, fatigue)

• limit state of serviceability perform. of str at service load (deflection., durability, vibrations, fire, etc)

Limit states of strength: • associated with failures (or imminent failure), under the action of probable and

most unfavorable combination of loads on the structure using the appropriate partial safety factors, which may endanger the safety of life and property.

It include:

• Loss of equilibrium of the structure as a whole or any of its parts or components

• Loss of stability of the structure (including the effect of sway where appropriate and overturning) or any of its parts including supports and foundations.

• Failure by excessive deformation, rupture of the structure or any of its parts or components,

• Fracture due to fatigue,

• Brittle fracture.

Limit state of serviceability:

• Deformation and deflections, which may adversely affect the appearance or effective use of the structure or may cause improper functioning of equipment or services or may cause damages to finishes and non-structural members.

It includes:

• Vibrations in the structure or any of its components causing discomfort to people, damages to the structure, its contents or which may limit its functional effectiveness. Special consideration shall be given to systems susceptible to vibration, such as large open floor areas free of partitions to ensure that such vibrations are acceptable for the intended use and occupancy (see Annex C).

• Repairable damage or crack due to fatigue.

• Corrosion and durability,

• Fire.

Objective of LSM design

• structure not to become unfit for use with an acceptable target reliability OR

• Very low probability to reach structure to LS during its lifetime

In Limit state design method

• Loads Characteristic loads (loads with small probability exceeding this value)

• Strength Characteristic strength (strength with small probability less than this value)

• Checks are made for serviceability

• However, in the LSDM, Structures are designed using the ‘Design Strength’ and ‘Design Load’.

• Design Load = Partial safety factors for load ( f) Charact. load

• Design Strength = Charact. strength / Partial safety factors for material ( m)

Partial Safety Factor for Loads:

• Partial safety factors are different for different load combinations:

• DL + LL / CL case for DL 1.5 for LL/CL 1.5

• DL + LL/CL + WL/EL case for DL 1.2 for LL/CL 1.2 for WL/EL 1.2

• DL + WL/EL case for DL 1.5 (0.9) ------ for WL/EQ 1.5

• DL + ER case for DL 1.2 (0.9) for ER 1.2

Design Strength

• Design Strength, Sd, is obtained as given below from ultimate strength, Su

and partial safety factors for materials, m given in Table 5 of code.

Sd=Su/ m where partial safety factor for materials, m account for:

• Possibility of unfavorable deviation of material strength from the

characteristic value,

• Possibility of unfavorable variation of member sizes,

• Possibility of unfavorable reduction in member strength due to fabrication

and tolerances, and

• Uncertainty in the calculation of strength of the members.

Introduction to riveted connections:

• Riveted connections are obsolete

• Rivet made up of round ductile material round bar called ‘shank’

• Classified based on head shape: ‘snap (common)’, ‘pan’, ‘flat countersunk’, ‘round countersunk’

• Diameter of shank nominal diameter

• Two types Hot Driven and Cold Driven also shop rivets and Field Rivets

Hot Driven Rivet

• Rivets are first heated increase in diameter heated diameter > shank diameter gross diameter

• On cooling length of rivet reduces joint becomes tighter,

Diameter of rivet reduces some space remains between rivet and hole

Cold Driven Rivet

• Needs high pressure

• Strength of cold driven rivets > hot driven rivets

Rivet Pattern:

• Chain Pattern; Staggered Pattern; Diamond Pattern; Staggered Diamond

Design of Riveted Sections are Same as Bolted Connections, with following differences

• In Case of Rivets, Diameter of Rivet = Diameter of Hole,

• in case of Bolt , Diameter of bolt = nominal diameter of bolt

• Design Stress of rivet > Design Stress of bolts

BOLTED CONNECTION

• Consists of bolt (shank with a head at one end and threaded at other end), nuts and washers.

• Washers are used to:

– Distribute the clamping pressure on bolted member,

– to prevent the threaded portion

• If the section is subjected to vibrations nuts are locked

Advantages of Connections over Riveted connections:

• Speedup erection of structure,

• Needs less skilled persons

• Overall cost of bolts less as compared to Rivet due to Reduction in labour and equipment cost

Objections on use of bolts:

• Cost of bolts > cost of rivet material

• Tensile strength of bolt < tensile strength of rivet (due reduction in area of cross-section at root of thread)

• May loose due to vibrations and shocks

Methods of making Holes for bolts:

(1) Drilling ,

(2) Punching simple:

– saves time and cost but reduction in ductility and toughness

– As per IS:800-2007permits punching, only when

• material yield stress < 360 MPA,

• thickness < 5600/fy mm

• If punching is to be used, holes are punched 2 mm less than required and 2 mm is drilled

TYPES OF BOLTED CONNECTIONS:

Classification On the basis of Resultant Force Transferred

• Concentric when load passes through CG of section (in case of axial loads)

• Eccentric load is away from CG of connection (such as in channels)

• Moment resisting joint subjected to moments (beam-column connection)

• Classification On the basis of Type of Force:

• Shear connection Load is transferred through shear (lap joint, butt joint)

• Tension connections load transfer by tension on bolts (hanger connections)

• Combined shear and tension connection (inclined member connected to a bracket) (bracing connections)

Types of Bolts: (1) Unfinished Bolts; (2)High Strength bolts

Unfinished bolts

• Also known as Ordinary, rough or black bolts

• Used for connecting light structures for static and secondary members (purlins, bracings, trusses etc)

• Not suitable for vibrations and fatigue

• Made from mild steel rods forged from low carbon steel

• Heads are made square or hexagonal (costly but better appearance)

• Available in 5 mm to 36 mm diameter designated as M5 to M36

• In structural steel generally bolts used are M16, M20, M24 and M30

• Ratio of net tensile area / nominal plane shank = 0.78 (as per IS 1367)

• In IS 800: above ratio is taken as 1.0

• As per IS 800 net tensile area is considered at root called stress area or proof area

• Bolts are designated by i.g. 4.6 Ultimate stress 400 MPa and yield stress 0.6x400 = 240 MPa

• Not significant clamping stress is developed

• Force is transferred through the interlocking and bearing bearing type joint

High Strength Bolts

• Made from medium carbon heat-treated steel and from alloy steel

• Tightened until they have very high tensile stress (twice ordinary bolts)

• Load resisted at high stress called proof load

• Connected parts are clamped together

• Loads are transferred primarily by friction not by shear called friction bolts

• to avoid slip surfaces to be connected must be free from rust, paint, grease

• Better vibration and impact resistance

• Available in 5 mm to 36 mm diameter designated as M5 to M36

• Commonly used grades of bolts are 8.8S, 10.9S (written at cap of bolt)

– 8.8S (diameter < 16 mm) ult. Stress 800 MPa yield stress 640 MPa

– 8.8S (diameter > 16 mm) ult. Stress 830 MPa yield stress 660 MPa

– 10.9S ult. Stress 1040 MPa yield stress 940 MPa

Advantages of High strength bolts:

• No slip between elements connected rigid joint high strength of connection

• No shearing and bearing failure

• No stress concentration in the hole more fatigue strength

• Uniform tensile stress in bolt

• No loosening in bolts

• Less man power (compared to rivets) cost saving

• Less Noise nuisance

• Less number of bolts are required as compared to rivets

Types of bolted joints

Two types: (i) Lap Joint, (ii) Butt Joint

Lap Joints

• Two members are overlapped and connected together

• May be single bolted or two bolted joint

• Loading axis of members do not match Load is eccentric uneven stress distribution

• Bending of joint couple is formed bolt may fail in tension at least two bolts must be used

Butt Joint

• Members are placed end to end

• Cover plates are provided in two ways (i) single cover plate butt jt. (ii) Double cover plate butt jt.

• Double cover plate joints better than lap joint since:

– SF in double cover plate = (1/2) of SF in lap joint

– Shear strength of double cover plate = 2 x Shear strength of Lap joint

– In double cover plate joint no bending

Failure of Bolted Joints:

Six-types of failures:

(i) Shear failure of bolt,

• Occurs when shear stress in bolt > Nominal shear stress

Shear failure : Two types

(i) Single shear Failure

shear failure at one section of bolt

occurs in case of Lap joint,

(ii) Double Shear Failure

shear failure at two sections of bolt

occurs in case of Butt joint

(ii)) Bearing Failure of bolt and (iii) Bearing Failure of Plate

• In general, transfer of force in connecting parts through bearing action

• half circumference of plate in contact with bolt get crushed (plate weaker than bolt)

• half circumference of bolt in contact with plate get crushed (bolt weaker than plate)

• or partially both plate and bolt are get crushed

(iv) Tension Failure of bolt

• If bolt in Tension and Tensile stress in bolt > Permissible stress Tension Failure at root of thread (weak)

(v) Tension or Tearing Rupture failure of plates

• In plates, holes in plate for connection Reduction in net effective area of plate Tension Failure of Plate

• Tension Failure of Plate may be prevented by (i) fewer holes, (ii) staggered holes

• Plate breaks along the bolt line

(vi) Shear Failure of Plate

• Due to insufficient end distance (distance from end of plate from center of nearest hole measured along force direction)

• portion of plate of width equal to diameter of hole is sheared

• to prevent shear failure provide enough end distance

(vi) Block Failure

• A combination of shear failure and Tension Failure

• A portion of plate (block) shears along the force direction

Pitch: C/C distance between individual fasteners (bolts) in a line/ rows

(measured parallel to load/stress)- p

• If bolts are in zig-zag pattern distance measured parallel to direction of load/stress staggered pitch- ps

Gauge: distance between adjacent gauge (bolt) lines (measured

perpendicular to force)

Limitations on Pitch:

• Minimum Pitch:

• Pitch 2.5 nominal diameter of bolt called Minimum pitch

• Pitch minimum pitch due to following reasons:

• To prevent bearing failure between two bolts

• for sufficient space to tight the bolts

• to avoid overlapping of washers

• to avoid tear-out of plate (between bolts in a rough

Line/row

p End distance

Edge distance

Maximum Pitch:

• In Tension Members: Pitch 16t or 200 mm, whichever is less (where t = thickness of thinner plate)

• In compression members: Pitch 12t or 200 mm, whichever is less, (where t = thickness of thinner plate)

• In compression members, where forces are transmitted through the butting facing:

• Pitch 4.5d for a distance of 1.5 b from the butting faces, where b= width of member

• Pitch for edge row of the outside plate (100 mm + 4t) or 200 mm, whichever is less

• If bolts are staggered at equal interval and gauge 75 mm, pitch for tension and compression members may be increased by 50% provided Pitch 32t or 300 mm

Pitch Maximum Pitch due to following reasons:

• To reduce the length of connection and gusset plateto have compact joint

• For Long joints (> 15 diameter of bolt) end bolts are stressed more progressive joint failure – called unbuttoning

• Unbuttoning is controlled by limiting the maximum pitch

• In case of built-up compression member joints buckling of cover plates

• In case of built-up Tension Member joint connected plates apt (tends) to gap apart (from cover plate in transverse direction)

• Edge Distance:

• Distance from center of any (extreme) bolt hole to edge of plate (measured perp. to load direction)

• End Distance:

• C/C distance of bolt holes to the edge of an element (measured along load direction)

• Minimum Edge Distance and Minimum end distance (book table is given for up to 32 mm dia) – 1.7 hole diameter in case of sheared or hand-flame cut edges (uneven)

– 1.5 hole diameter in case of rolled, machine-flame cut, sawn(saw) &

planed (plane) edges

If Edge distance < mini. Edge distance and End distance < mini. end distance:

• Plate may fail in tension

• Steel of plate opposite the hole may bulge out (in direction perp to load) crack

Hole Diameter:

• Hole diameter = Nominal diameter of bolt + clearance

• For nominal diameter from 12 mm to 14 mm clearance = 1. 0 mm

• For nominal diameter from 16 mm to 24 mm clearance = 2. 0 mm

• For nominal diameter > 24 mm clearance = 3. 0 mm

Maximum Edge Distance

• Edge distance to the nearest line of fasteners from an edge of any un-

stiffened part , where and t = thickness of the thinner

outer plate.

• Above is valid for fasteners interconnecting the components of back to

back tension members.

• Where members are exposed to corrosive influences,

edge distance (40 mm+4t), (t=thick. of thinner plate)

If Edge distance > maximum edge distance

• Edges may separate Moisture may reach between parts Corrosion

problem in joint

t12yf/250

Bearing Type Connections (in case of un-finished or ordinary bolts):

• Load transferred > friction resistance bearing action

• Bolts in Bearing type connection are checked for

(i) shear, and

(ii) bearing

Since single bolt may fail and result in collapse Minimum no. of = 2 or 3

Strength Bolt

Strength of bolt = Minimum of

(i) Strength of bolt in bearing, and

(ii) Strength of bolt in shearing

Strength of bolt connection = strength of one bolt no. of bolts

Strength of joint = Minimum of

(i) strength of bolt or bolt group, and

(ii) net tensile Strength of plate

Note: Bearing plane is considered in the threaded portion for safe design

(since bolts may be put in both ways)

boltoneofStrength

redbetransfertoLoadrequiredboltsofNo.

Determination of Shearing Strength of Bolt:

Shear strength of bolt depends on

(i) Ult. tensile strength of bolt, fub

(ii) No. of shear planes with threads, nn

(iii) No. of shear planes without threads (shanks), ns

(iv) Nominal area of shank, Asb,

(v) Net stress area of bolt Anb

sbA

sn

nbA

nnub

f

nsbV

3 bolt, ofcapacity Shear

Reduction factor in shear for Long Joints

If the length of joint > 15d Long Section

If the section is long Stress in outer bolts > inner bolts

Need to apply Reduction factor ( ij)

ij accounts for overloading of the end bolts

1.0β0.75butd200

l-1.075β lj

j

lj

Where, lj = length of joint

= distance between first and last row of bolts measured in direction of load

For uniform stress section (i.e. all bolts carry equal stress) ij = 1

Reduction factor in shear for Large grip length (i.e. more thickness of plates)

more thickness of plates more grip length of bolt More Bending Moment in Section

For the safe design Need to apply a reduction factor for large grip length ( lg)

If total thickness of the connected plates > 5 nominal diameter of the bolt

more grip length Use lg

lj

g

anddld

dlglg 8

3

8

Reduction factor for packing Plates

If packing plate thickness > 6 mm bending is developed in shank

Need to apply a reduction factor in shear capacity ( pkg)

pkgpkg t0125.01

where tpkg = thickness of packing plate

Nominal Strength of Joint

The nominal shear strength of the bolt taking in to account reduction factors

pkgljsbA

sn

nbA

nnub

f

nsbV lg

3

Factor of Safety of material ( mb)

For safety of joint in shear, a factor of safety for material is used ( mb)

pkglj

mbmb

nsb

sbA

sn

nbA

nnub

fV

sbV lg

3

where mb = partial safety factor of material of bolt = 1.25

For 4.6 grade bolt, fsb = 400 MPa

pkgljesb

pkglj

pkglj

AV

sbA

sn

nbA

nn

sbV

sbA

sn

nbA

nn

sbV

lg

lg

lg

75.184

75.184

25.13

400

shearDoubleinboltsforsb

As

nnb

An

n

sheargleinboltsforsb

As

nnb

An

nAWhere e

2

sin,

For normal bolts and members threads are excluded,

if considered very conservative design

Bearing Strength (Capacity) of bolt

Due to Bearing hole elongates

Due to Excessive bearing tearing of plate

To avoiding excessive elongation of hole

Bearing stress not greater than Nominal bearing strength of bolt

Nominal bearing strength of bolt = projected bearing area ultimate Tensile stress

uftdbnpb k2.5 V

0.1,25.03

,3

ofsmaller k Where,00

andf

f

d

p

d

e

u

ubb

d = nominal diameter of bolt;

d0 = diameter of hole

p = pitch of the bolt (along bearing direction);

e = end distance of the bolt (along bearing direction)

fub = ult tensile stress of bolt;

fu = ult. Tensile stress of plate

t = total thickness of the connected plates subjected to bearing stressing in the same direction

in case of countersunk bolts t = plate thickness – (1/2) depth of counter sunk

For safety of joint: need to use a Factor of safety, mb, mb = 1.25

mb

ubpb

mb

npb

pb

ftdkV

VV ..5.2

Tensile Strength of Plate

)(, boltingchainfortdnBAAreaNet hn

)(4

,1

2

boltingstaggeredfortg

pndBAAreaNet

m

i i

sihn

1

9.0 plate, ofstrength Tensilem

unnd

fAT

Where, fu = ultimate stress in MPa;

An = net effective area in mm2

m1 = partial safety factor = 1.25

Strength and Efficiency of Joint

Strength of bolted joint

= minimum of (strength of connection based on

(i) shear,

(ii) bearing ,

(iii) strength of main member) 100

int ,connection of Efficiency

lengthpitchperholesfordeductionswithoutplatesolidofStrength

lengthpitchperjoboltedofStrength

Tension Capacity of bolt

If the bolts are subjected to tension Tensile stress in bolt Tensile strength of bolt

Tensile of bolt is checked in (i) Shank zone as well as in (ii) threaded zone

zonethreadedin A f 0.9 T nbubb

For bolt to be safe in tension, Tension force in bolt, mb

db

mb

nbdb T

TT nbub A bf 0.9

Also the bearing strength is to be checked shank zone zoneshankin A fm0

mbsbybdbT

Where,

fub = ult tensile stress of bolt;

fyb = yield stress of bolt

Anb = net tensile stress area of bolt;

Asb = shank area of bolt

mb = partial safety factor for bolt material = 1.25

m0 = partial safety factor for bolt material

governed by yielding= 1.10

Bolt subjected to combined shear and tension

0.1

22

db

b

db

b

T

T

V

V

Vb = factored shear force on bolt;

Tb = factored Tensile force on bolt;

Vdb = design shear capacity

Tdb = design Tension capacity

Example: Two plates are connected by single bolted double cover butt joint using M20

bolts at 60 mm pitch; steel grade 410 MPa and bolts 4.6 grade; Calculate bolt efficiency

Strength of M20 bolt in shear

For M20 bolts Diameter of bolt = 20 mm Diameter of bolt hole = 22 mm

For Fe 410 Grade steel Net Tensile Stress area = 245 mm2 (from Table 4.3 of TB)

For Fe 410 Grade steel ult strength fu = 410 MPa

For 4.6 grade bolt ult strength of bolt, fub = 400 MPa

Partial safety factor for bolt material = 1.25

pkglj

mbsb

As

nnb

An

nubf

nsbV lg

3shearinboltofStrength

Since, there is only one line of bolts No question of overloading lj = 1

Since thickness of plates to be connected < 5 nominal dai of bolt i.e.5 20 mm lg = 1

Since, size of connecting plates is equal no packing plate pkg = 1

sbA

sn

nbA

nnub

f

nsbV

mb 3shearinboltofStrength

Assume bolt pitch = 60 mm

End distance = 30 mm

sbA

sn

nbA

nnub

f

sbV

mb 3shear in bolt ofStrength

Due to double cover butt joint The bolt will be in double shear,

Assuming, both the shear planes in net area section nn = 2, ns = 0, Anb = 157 mm2

32.02

3.02

33 mbmbmb

ubf

nbA

sbA

nbAub

f

sbA

nbAub

f

sbA

sn

nbA

nnub

f

sbV

kNsb

V 53.90325.1

4002452

Strength of M20 bolt in bearing mb

bnpb

k2.5 V uftd

0.1,25.03

,3

ofsmaller k Where,00

b andf

f

d

p

d

e

u

ub

d = nominal diameter of bolt = 20 mm ; d0 = diameter of hole = 22 mm

p = pitch of the bolt = 60 mm; e = end distance of the bolt = 30 mm

fub = ult tensile stress of bolt = 400 MPa; fu = ult. Tensile stress of plate = 410 MPa

t = minimum of (thickness of the connected plates, Sum of cover plates) 6 mm

454.00.1,975.0,659.0,454.0

0.1410

400,25.0

223

60,

223

30 ofsmaller k Where, b

ofSmaller

and

Strength of bolt = Minimum of strength bolt in shear and in bearing

= minimum of (90.53 kN and 43.58 kN) = 43.58 kN

kNftd u 58.43

25.1

400620454.05.2k2.5 V

mb

bnpb

Strength of plate per pitch length in Tension

kN

ftdp

fAT

m

u

m

unnd

3.6725.1

410622609.0

9.09.0 plate, ofstrength Tensile11

Determination of Strength joint per pitch length

Strength of joint per pitch length = Minimum of (strength bolt, strength of plate)

= minimum of (43.58kN, 67.3 kN) = 43.58 kN

Strength of solid plate per pitch length in Tension

kNf

ATm

upnd 27.106

25.1

4106609.09.0 plate, ofstrength Tensile

1

%4127.106

58.43

lengthpitchperplate solid ofStrength

length,pitchperjoint ofStrength joinof Efficiency

)( 1 factsafpartm

Shear strength of bolts = 2 90.53 kN = 181.06 kN

Bearing strength of bolts = 2 43.58 kN = 87.16 kN

Strength of plate = same as in one line case = 67.3 kN

Strength of joint = Minimum of above three = 67.3 kN

Strength of solid plate = 106.27 kN

%3.6327.106

3.67joinof Efficiency

Example: In previous example, find the efficiency of joint if joint have two lines

Slip Critical Connections

At service loads load < Friction resistance No significant slip

Large Member force or when connection length limited slip bearing action

To avoid bearing action High Strength Friction Grip (HSFG) bolts most suitable

Joint with HSFG bolts called slip resistant connection

At service loads No slip behave as Slip resistant connection

At ultimate loads Slip develops Bearing Bearing type connection

As the load go on increasing Slip occurs at particular load called critical load

Connection is called slip critical Connection

In Bridges Loading & unloading fatigue Slip critical connections not good

use slip resistant connection

Theoretically, at service loads In slip critical connections No Slip Load transfer

by Friction only (i.e. no shear, no bearing)

In practice, Slip occurs Slip Critical connections are designed for bearing also

Principal of HSFG Bolts

Bolts dia > Bolt dia bolt not fills complete hole

No bearing and shearing at loads < slip critical loads

Since, In the tightened to a very high load 90% of proof load

Bolts Initially load is transferred by ) Friction action (load<friction resis)

When Load > Friction resistance Load is transferred by

(i) shear and (ii) bearing actions Gap between shank and hole At No Slip stage shearing load transfer by Friction only

Friction force developed depends

(i) Tension in bolt and (ii) Friction Coeff. between plate and nut/washer

,ResistanceFriction Horizontal TF f

factorslipandBoltinTensionTWhere f,

Design of HSFG Bolts

Types of HSFG bolt connections:

HSFG Parallel Shank bolts no slip at service loads but may slip at ultimate load

Slip critical connections

HSFG Waisted Shank bolts No slip at service as well as at ultimate loads

Slip resistant connections

mfmf

nsfVohef

df

F K n µ V,Resistance SlipDesign

γmf = 1.10 (if slip resistance is designed at service load)

γmf = 1.25 (if slip resistance is designed at ultimate load)

Shear Connections with HSFG Bolts

ohefnsf F K n µ V,Resistance Slip

ne = number of effective interfaces offering frictional resistance to slip

Kh = 1.0 for fasteners in clearance holes (gap is there)

= 0.85 for fasteners in oversized and short slotted holes and for fasteners in the

long slotted holes loaded perpendicular to slot

= 0.7 for fasteners in long slotted holes loaded parallel to the slot.

Fo = minimum bolt tension (proof load) at installation ( = Anb fo)

Anb = net area of bolt at the threads;

fo = proof stress (= 0.70 fub), where fub = ultimate stress of bolt.

Correction For Long Joint

The above determined load is to be multiplied by long joint correction factor

1.0β0.75butd200

l-1.075β lj

j

lj

d = nominal shank diameter of bolt

Bearing Strength of HSFG (High-strength Shear-critical Friction Grip)

At ultimate load slip takes place Need to check the strength of connection

due to bearing at ult. Load In the same way as in case of black bolts.

Tensile Strength of HSFG Bolts

Same as for black bolts denoted as Tnf

Combined Shear and Tension For Slip Critical Connections

0.1

22

df

f

df

f

T

T

V

V

Vf = applied factored shear force at design load;

Tf = externally applied factored Tensile force at design load

Vdf = design shear strength

Tdf = design Tensile strength

Prying Action

When a channel section, with flexible flanges (in tension), is connected to roof

flange bends the CG of the compressive stress (prestress) in the plate shifts

towards the end additional force in bolt called Prying force

)(2272

as,given is forcePrying 2

4

0 braketintermSecondneglectingl

lT

ll

tbfT

l

lQ

e

ve

ve

ee

e

v

Where, lv = distance from bolt center-line of bolt to the

(i) toe of the fillet weld or

(ii) half the root radius for the rolled section (Fig)

Le = distance between prying force and bolt center-line

which is taken minimum of

yf

ft 01.1(ii) and distance end (i)

= 1 for pre-tensioned bolts, = 2 for non-pre-tensioned bolts,

= 1.5 for Limit State Design; be = effective width of flange;

f0 = proof stress; t = thickness of end plate,

To minimise the Prying Force:

• Use fully tensioned bolts (i.e. applied tensile force in the plate pre-stress in the plate)

• Use thick plate or stiffened the plate

• Limit the distance between bolt and plate edge

)(2272

as,given is forcePrying 2

4

0 braketintermSecondneglectingl

lT

ll

tbfT

l

lQ

e

ve

ve

ee

e

v

Design of Connection Subjected to Prying Force

Based on Trial and error Since the prying force depends on section thickness

and no. of bolts (controls the edge distance)

Based on plastic analysis, the thickness of the end plate may be determined as

ey

p

bf

Mt

40.4min

Provided where rotations are allowed

For satisfactory function minimum friction is required

Pinned connections make the structure determinate

Large force since only one pine is provided rather than more as in case of bolts

Pins are available in diameter 9 m to 330 mm

Pins application:

(i) Tie Rod (ii) Diagonal bracings in beams and columns (iii) Truss

Strength of Pined Connections

Shear Capacity:

(a) If no rotation is required and the pin is not intended to be removed;

Shear capacity = 0.6 fyp A

(b) If rotation is required or the pin is intended to be removed;

Shear capacity = 0.5 fyp A

Where, fyp = design strength of pin,

A = Cross-sectional area of pin

PIN CONNECTIONS

Bearing Capacity

(a) If no rotation is required and the pin is not intended to be removed;

Bearing capacity = 1.5 fy d t

(b) If rotation is required or the pin is intended to be removed;

Bearing capacity = 0.8 fy d t

Where, fy = lower of design strength of pin and connected part,

d = diameter of pin;

t = thickness of plate

Flexural Capacity of Pin

There is gap between the connected members due to following reasons:

(i) To prevent friction

(ii) To allow bolt heads if built-up connection

(iii) To facilitate painting

(a) If no rotation is required and the pin is not intended to be removed;

Moment capacity = 1.5 fyp Z

(b) If rotation is required or the pin is intended to be removed;

Moment capacity = 0.8 fyp Z

Where Z = Section modulus of pin

Note: Flexure is more critical Pin diameter is generally governed by Flexure 31

3

33.2132

5.15.1l,cylindrica ispin Ifyp

uypuypu

f

Md

dfMZfM

WELDED CONNECTIONS

two pieces of metal connected by heating them to a plastic or fluid state called fusion

Welding process two types Electric welding, (ii) Gas Welding

Assumptions in Welded Joints

Welded are homogeneous, isotropic and elastic

Welds are rigid and no deformation with-in welds

No residual stresses in welds (due welding process

Advantages of welded connections over bolted connections:

• No deductions for hole gross section is effective More efficient use of material

•small size of gusset plates more compact joint

• Economical due to saving in time in preparing drawings and fabrication

• Fast speed of fabrication and erection

• No connecting plates Less weight of structure economical

• Better for fatigue, impact and vibrations (earlier it was assumed not good in fatigue)

• Produces rigid connection produce one piece Construction less deformation

• requires less depth of beam reduced overall ht of building

• Less noise pollution

• Watertight / airtight connections good for liquid/gas storage tanks

• Avoids problem of hole alignment

• Needs power supply at site

Disadvantages of welded Joints:

• Needs skilled labour

• Needs costly equipments

• Difficult to inspect the joints needs NDT testing methods such as

Magnetic particle method, dye penetration method, ultrasonic method, radiography

• Welded joints if over rigid (than members) may fail in fatigue cracking in members

Types of Welded Connections

Welds classification based weld procedure

(i) Fillet weld, (ii) butt or groove weld (iii) plug weld (iv) slot weld

classification based on its location

(i) flat weld; (ii) horizontal weld (iii) Vertical weld (iv) overhead weld

Welds classification based on type of joint

(i) butt welded joint (ii) lap welded joint (iii) Tee Welded joint (iv) corner welded

Butt Joints

•Used when members to be joined are in a line or aligned in the same plane

•Needs edge preparation costly and time consuming

•The grooves have a slope of 30 -600

•If two plates are of different thicknesses thicker plate is made thin near joint

Reinforcement:

•makes the butt joint stronger under static load

•Smoothen the flow of forces

•concentration develops in case of fatigue loads leading to cracking and early.

•Not greater than 0.75 mm to 3 mm Extra reinforcement removed by machine

Butt welding of parts of unequal thickness and/or unequal width

Types of Butt (groove) Welds:

(i) Single and double square (ii) Single and double V (iii & iv) Single and

double Bevel (v & vi) Single and double U (vii & viii) single and double J

Incomplete penetration welds :- single V, single bevel,

Incomplete penetration stress concentration Complete penetration is better than

incomplete one

Square welds: Easy but are used for plate up to 8 mm thickness only

Fillet Joints:

•used to joint two members in different planes lap joints

•Easy to make

•Needs less material preparation

•High stress concentration

•For given amount of weld flat welds are poor than butt welds

•flat welds more common than butt (groove ) welds

Types of Fillet Welds

Triangular in shape: when two perpendicular members are connected and in lap joints

May be (i) Concave or (ii) Convex, (iii) Mitre

Lap Joints

Advantage of lap joints plates with different thicknesses can be joined

Drawback of a lap joint introduces some eccentricity of loads

(May be avoided if double lap joint is used)

Welded Lap Joints

T-Joints

Defects in Welds

DESIGN OF BUTT JOINTS

Forces in butt joints:-

(i) Axial: Tension or Compression), and (ii) shear if any

Design Specifications

Reinforcement:

Extra weld metal (above the plate level)

Reinforcement is required:

to avoid error in thickness of weld

to increase in static load capacity increase in efficiency of joint

Reinforcement thickness at least 10% of plate thickness but not more than 3 mm

Reinforcement is made at the time of welding later they are dressed flush

Reinforcement is ignored in calculation

Reinforcement is not provided in case of vibrations and impact

to avoid failure due to stress concentration

Size of Groove weld

Size of groove weld = size of throat called effective throat thickness (te)

In case of complete penetration of the groove weld

Throat dimensions = thickness of thinner member

Effective Throat thickness = 7/8 of thickness of thinner member

In calculation, Effective Throat thickness = 5/8 of thickness of thinner member

Effective area of weld:

Effective area of weld = Throat thickness (te) effective length of weld (Lw) (measured along width)

Effective length of weld = length over which required size of weld is done

Design Strength

Design Strength of groove weld in tension/compression

fy = smaller of ULTIMATE stress of weld (fyw) and parent material (fy)

Lw = effective length of weld in mm

te = effective throat thickness of throat in mm

mw = partial safety factor = 1.25 for shop weld and 1.5 for site weld

Design Strength of groove weld in shear

mw

ewy

dw

tLfT

fyw1 = smaller of yield stress of weld (fyw/ 3) and parent material (fy/ 3)

Design Procedure:

In Case of Complete penetration weld strength = member strength no

calculation required

In case of incomplete penetration determine throat thickness calculate length

required to develop strength of weld equal to member strength

mw

ewyw

dw

tLfV

1

Design of Fillet Weld

Used when two members overlap each other

Stresses developed

(i) Direct stress –Minor

(ii) Shear stress Major

Two more widely used fillet weld shapes concave and convex (mitre not common)

Concave weld

less penetration than convex

smaller throat (than convex)

on cooling outer face in tension cracks Not Good

Convex welds

More penetration

Large throat

convex weld stronger

on cooling Compression in outer face due to shrinkage Good

Note: If concave welds are desired in first pass they are made convex and in second concave

Size of Fillet welds

Weld size = minimum leg length of weld

Leg length = distance from the root to the toe of fillet weld

Leg lengths are measured along the largest right angle triangle inscribed within weld

Throat size perpendicular distance

Size of weld (i) equal and (ii) unequal

Equal is preferred

In some circumstances unequal is used to increase the throat size (hence

strength) (in butt angle connected to plate where thickness of weld on angle is

limited)

Weld Size Specifications

Thicker plates

Heat dissipation in horizontally as well as vertically

Lesser fusion depth due to high heat dissipation

Need to limit the maximum thickness of plate

Thin Plates:

Heat dissipation mostly along horizontally

Lesser depth may be sufficient

Maximum Size of weld

To avoid the melting of base material

If plate thickness < 6 mm Max Size of weld = Thickness o plate

If plate thickness > 6 mm Max size of weld (Thick. of thinner member – 1.5 mm)

(to avoid over stressing of weld at ends)

Max size of weld for round toes (3/4) thickness of toe thickness

Minimum Size of weld

In a thick member, Small size of weld no proper bond between weld and member

Thickness of Thicker member Minimum Weld size (as per IS 800)

0 – 10 mm 3mm

10 – 20 mm 5 mm

20 – 32 mm 6 mm

32 – 50 mm 8 in first run finally 10 mm

Also Minimum weld size thickness of thinner member

Generally minimum size of weld is preferred due to following reasons:

Only one run is required (more the size needs more than one runs such as in plates

of thick > 32 mm)

Less weld size – cheaper

Effective Throat Thickness

Shortest distance from root of fillet to the face line of weld i.e perpendicular distance

Effective throat thickness = K Size of weld = K S

In equal welds , K = 0.707 size of weld = 0.707 S

For other angles K is given as

Angle between fusion faces 600 – 900 910 – 1000 1010 – 1060 1070 – 1130 1140 – 1200

K 0.70 0.65 0.6 0.55 0.5

Effective thickness of throat 3 mm

0.7 thickness of thinner member

thickness of thinner member in spl circumstances

Effective Length of Weld

Effective length of weld = Overall length of weld – 2 S

Actual length of weld 4S

Slots in Welds

In the Tension/compression members, if l < d Stress concentration at ends

To avoid, this stress concentration, Code puts limit on length l as

Distance of longitudinal fillet welds 16 thickness of thinner member

If above condition is not satisfied, slots are made in plate

These slots are welded by welding of same strength as longitudinal weld

Some times slots are made to increase the length of weld

If slots are made, they need to be cheeked just behind the weld for failure

Correction for Long Welds

If length of weld > 150 throat size of weld reduction in weld strength

0.1150

2.02.1

1t

l j where, lj = length of joint and tt throat size of weld

Effective Area = Effective length of weld Effective throat size of weld

Overlap of plate in lap joints: 4 thickness of thinner plate or 40 mm whichever less

Transverse spacing of welds

Length of weld on either side Transverse spacing of welds

3 weld ofstrength nominal f Where, weld,fillet of stressDesign wn

u

mw

wnwd

fff

mw

uw

mw

utwwntwdw

fSKL

ftLftLP

33weld, ofstrength Design

Lw = effective length,

tt = throat thickness,

S = Size of weld ;

fu = smaller of ult strength of weld and parent material

mw = partial safety factor

= 1.25 for shop weld and 1.5 for site weld

Design Strength of Weld

Design Strength of Fillet Weld (in tension or shear):

Design Steps

Weld may be subjected to axial load shear Shear critical shear controls the design

Assume size of weld based on thickness of member

Determine length of weld = force transmitted / strength of weld

Length of weld is provided in sides and should not be less than ld

Also provide end returns of length equal to 2S

If length of weld > 150 tt apply length correction

Commonly weld is provided on all three sides (in this case no need to check transverse spacing)

Example on Butt or Groove Weld

Example on Fillet Weld : Design a connection to joint two plates of size 250 x 12 mm

of grade Fe 410, to mobilize full plate tensile strength using shop fillet welds, using

(i) a double cover butt joint with 8mm cover plate (ii) lap joint

(i) Connection using Double cover plate Butt joint

Assume width of cover plate = 250 - 2 x 15 = 220 mm

Area of cover plate = 220 x 8 = 1760 mm2

Required area of cover plate = 1.05 x 250 x 12/2 = 1575 mm2 < 1760 mm2

For the 8 mm thick plate well size between 3 and 8 mm Let us use a 5 mm fillet weld

Strength of the 5-mm weld = 410/( 3 1.25) x 0.7 x 5 = 661.5 N/mm

Required length of weld = 681.82 x 1000/661.5 = 1031 mm, say 1040 mm

Length of the connection = [(1040 - 2 x 220)/4] x 2 = 300 mm < 150 tt i.e. 150 x 0.7 x 8

Joint is not a long joint

Hence, provide two cover plates of size 300 x 220 mm

(ii) Connection using lap joint (large force)

Minimum = 5 mm ( from Table)

Maximum = 12 - 1.5 = 10.5 (clause 10.5.8.1 of IS 800)

Assuming weld size s = 8 mm (in order to reduce the connection length not taken minimum)

kNA

m

g82.681

1000

1

1.1

12250250f strength Plate

0

y

mmNks

m

/10581000

1

25.1

87.0

3

410.

3

f weld ofstrength

0

y

mmsaymm 6506441058

1000681.82 weld oflength Required

Weld length available at end = 150 mm

Length of weld on one side

= (650 - 150)/2 = 250 mm

< 150 x 0.7 x 8 = 840 mm

not a long joint

OK

Example on lap joint (limited force)

Determine the size and length of the site fillet weld for the lap joint to transmit a

factored load of 120 kN through a 8mm thick and 75 mm wide plate. Steel Fe 410

Solution

Minimum size of weld for a 8-mm thick section = 3 mm

Maximum size of weld = 8 - 1.5 = 6.5 mm

Choose the size of weld as 6 mm (in order to reduce the connection length)

Effective throat thickness = ks = 0.7 x 6 = 4.2 mm

Assuming that there are only two longitudinal (side) welds,

Length to be provided on each side

= 181/2 = 90.5 mm > 75 mm OK

Hence, provide 95 mm weld on each side

with an end return of 2s i.e. 2 6 = 12 mm.

Therefore, the overall length of the weld provided

= 2 x (90.5 + 2 x 6) = 205 mm

mmNks

m

/7.6621000

1

5.1

67.0

3

410.

3

f weld ofstrength

0

y

mm1817.662

10120 weld oflength Required

3

Alternative solutions:

Minimum size of weld for a 8 mm thick section = 3 mm

Maximum size of weld = 8 - 1.5 = 6.5 mm

Let us choose the size of weld as minimum specified i.e. 3 mm

Two possible solutions are shown in Figure

(i) If only longitudinal welds are provided,

Length of each side weld = 363/2 = 181.5 mm

Total length on each side including end return = 181.5 + 2 x 3 = 187.5 mm

(ii) If welds are provided on three sides

Length of each side weld

= (363 - 75)/2 = 144 mm

> 75 mm (width of the plate) OK

This solution is preferred since

connection is more compact

provides better stress distribution

mmNks

m

/4.3311000

1

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Fillet Weld for Truss Members:

The weld may be done on

(i) two sides (one side not recommended) or

(ii) three sides i.e. two sides as well as at end

For the angle section CG not at mid of width,

Weld is done such that CG of weld coincides with CG of angle section

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If welding is done on two sides only

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Design of intermittent weld

When design force for weld small use smaller size of weld and provide on full length

When loads are very small Length required to weld (even with smallest size)

< available length Intermittent weld is selected

Intermittent weld discontinuous welds two types

(i) chain,

(ii) staggered – better than chain weld

Shop weld 10 8

50 (100) 50 (100)

(140) 70 (140)

8 mm weld

50 50

100

140 70 140 70

10 mm weld

Design Specifications for Intermittent Weld:

Minimum effective length of intermittent weld = 4 weld size (except for plate girder)

Clear spacing between the intermittent weld in compression 12t also 200 mm

Clear spacing between the intermittent weld in Tension 16t also 200 mm

If end weld not done: Length of longitudinal weld at the end width of the member

If end weld done:

Total Length of weld (end long. + Transverse at the end) 2 width of member

Types of welds in tension

Two types:

(i) Single sided

(ii) Double sided

Single side weld

eccentricity between the line of action of the load and the throat centroid

creates a moment on the weld throat

should be avoided in practice

Specifications for Plug or Slot Welds

If t = thickness of plate in which slot is made then

Width or diameter of slot 3t and also 25 mm

Corners of slot made curved Corner radius of slotted hole 1.5t and also 12 mm

Clear distance between two holes (slots) 2t and also 25 mm

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Stresses due to individual forces

In case of beam column, joint are subjected to

(i) axial force (tensile or compressive due to bending) as well as

(ii) shear force.

If a joint is subjected to axial force P and shear load Q, then

Combination of normal and shear stresses

For Fillet Weld

When shear stress are in addition to tension or compression

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For Butt Welds

No Need to check for Equivalent stress Since:

Butt welds joints are generally axially loaded

In single or double bevel butt joints (Normal stress + Shear Stress) < fwd

Combination of bending normal, shear and bearing stresses

If a joint is subjected to (i) bending stress; (ii) Shear stress; and (iii) bearing stress,

then the equivalent stress are determined as

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222 3

TENSION MEMBERS

Members subjected to axial force only (i.e. not eccentric loads)

Examples of Tension Members truss and tower members, tie member in bridges

The fixidity at ends of truss develops moment at ends neglected in design or

permissible stresses are reduced

Self weight of members bending moment in member bending stresses small

Neglected

In case of bolted members Holes are made for bolts reduction in area called

Net area

Net area = gross area – deduction of area due to holes

Preferably put one bolt in a line (vertical line)

If force is large More number of bolts bolts may be arranged in (i) Chain and (ii)

staggered

To increase the net area Staggered better choice

Types of Tension Members :

(i) Single Sections Single flat bars, Single angle sections, Single channel sections,

Single I sections

(ii) Built-up Sections

Built-up section are selected when

Large cross-sectional area is required

For given area more moment of inertia is required

Load reversal Can resist tensile as well as compressive loads

In built-up sections:

•Tie plates are provided at regular interval to:

•Minimize the slenderness ration and

•To transfer any unequal load from one member to other

•These plate are not considered to increase area of section

Net Sectional Area in Plates: Same as discussed in connection chapter

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If pitch and gauge lengths are same for all bolts, then p1= p2 = …… = p

and g1= g2 = …… = g

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2

where m = no. of staggered pitches

Net Sectional Area in Angle Sections

Angles very common sections, angles have two legs

Angles may be connected by (i) both legs or by (ii) Single Leg

When angles are connected by both legs:

- for the analysis angle is unfolded (developed)

- now the section may be treated as flat (plate)

- if ga and gb are the distance of first bolts from root and g1 (in book gf ) is the

distance of second bolt from first bolt,

then after unfolding the angle, the first gauge length will be g2 = ga + gb - t

When angles are connected by Single leg only:

The leg connected to gusset plate called connecting leg

Other leg (not connected) called outstanding leg

Due to single leg connection Non-Uniform stress distribution in connecting leg Non-

uniform stress distribution due to following reasons:

(i) Load transfer from connecting leg to plate along the CG eccentricity

(ii) strain at junction of connecting leg and outstanding leg > strain at free end of

connecting leg shear-lag

Net Effective Area of Section

Net Effective Area of Section Depends on

(i) Shear Lag effect

(ii) Ductility of plate material

(iii) Method used for hole formation

(iv) Geometry factor

Net effective Area of Plate = k1 k2 k3 k4 An

Where, An = Net area of plate after making deduction for holes

k1 = factor to consider the Shear Lag effect

k2 = factor to consider influence of ductility of plate material

k3 = factor to consider influence of method used for hole formation

k4 = factor to consider the influence of geometry of connection

Ductility factor (k1):

Ductile material of plate better stress re-distribution at bolt holes at higher loads

No stress concentration stress are distributed better on whole width of plate

Rk 0032.082.01

For Ductile material k1 = 1.0 and for brittle material, R = 1 k1 = 0.8232

For Common structural steels assumed to have sufficient ductility k1 = 1.0

Factor for method of fabrication or hole forming factor (k2):

Methods of bolt hole formation

(i) Punching, and

(ii) Drilling

Punching Shear deformation in material around the hole

10-15% reduction material strength than drilling

In some Design Specifications:

For Drilled holes k2 = 1.0; For Punched holes k2 = 0.85

As per IS 800:2007 k2 = 1.0

Provided diameter of hole is increased by 2 mm in calculation of net area

Geometry factor (k3)

Small bolt diameter Less gauge can be provided

if (g/d) less less material between holes at critical section

force transferred to adjacent bolt

less deformation in material

more uniform stress in plate

more efficiency of joint

gne AAk 70.060.13

K3 varies between 0.9 to 1.14; As per IS 800: 2007 k3 = 1.0

Shear lag factor (k4):

In angle- and T- sections subjected to axial force

non-uniform stress distribution near joint shear lag

Stress near the junction of connecting leg and outstanding leg

high as compared to at toes

These non-uniform stresses becomes uniform after some distance

called Transition Length

Failure of member occurs earlier than predicted Due to shear lag

Due to shear lag more load is taken by connection leg and some by portion of

outstanding leg near to junction

unequal angles are generally preferred (small outstanding leg)

To incorporate shear lag factor, factor k4 is used

k4 is calculated as

L = Length of connection

Lxk 14

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The length of the connection for bolted and welded connections is calculated as

x of value Small and large value of ‘L’ large k4 large effective area

more strength of joint

In the IS 800:2007: k1= k2 = k3 = 1.0 and k4 = nne AA

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Design Strength of Tension Member:

In Tension members, at the joint holes in member, remaining length without hole

Tension Members without holes (beyond the joint):

As load is increased yielding of material due to strain hardening, strength more

than yield strength

But more elongation before fracture unserviceable

Tension Member with holes (within joint):

Due to excessive load --: there may be (i) net area failure, and (ii) Block shear failure

Thus, design strength of a tension member will be minimum of the following:

(i) Gross-section yielding: more yielding

more displacements before the fracture failure

(ii) Net Section Failure

failure at net cross-section

(iii) Block shear failure

a segment of block of material at end of member shears out in smaller joints

Thus there are two Limit States are to be considered:

Limit state of yielding in the gross-section

to avoid excessive deformation in member

to control excessive deformation stress in gross-section < fy

Limit State of fracture in the net section

to avoid fracture in net section, stress in net section < tensile strength

Limit State of block shear : combined shear and tension failure