Welding Connection 2

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Teaching Resources (c) IIT Madras,SERC Madras, ANNA Univ,

1

WELDED CONNECTIONS II




2

TRUSS CONNECTIONS

•

Type of connections to be fixed at conceptual stageplanar trusses

• Web members may be directly welded to chord

members

Eccentricities

• Element centroidal axes not intersecting at a point

• Connection centroid not coinciding with the

element centroid




3

DIRECT CONNECTION OF WEB MEMBERS

CONNECTION AT THE APEX OF A ROOF TRUSS

EAVES CONNECTION




4

(b)

e P

e

(a)

P

ECCENTRICITIES IN TRUSS CONNECTIONS

(a) PRATT TRUSS,

(b) CROSS BRACING BETWEEN PLATE GIRDERS




5

PORTAL FRAME CONNECTIONS

• Common frame spacing - 4.5m to 7.5m

• Eaves and apex locations

• Crit ical moment occurs at eaves

•Plastic analysis leads to redistribution and lightersections

• Haunched sections for increased rotation

capacity




6

MOMENT TRANSMISSION AT

CORNER USING A SHEAR PANEL

MOMENT TRANSMISSION AT CORNER

USING A DIAGONAL STIFFENER




7

VARIOUS EAVES CONNECTIONS OF PORTAL FRAMES

(a) (b)

(c) (d)




8

WELDED APEX CONNECTIONS

(a) GENERAL ARRANGEMENT

(b) – (d) DIFFERENT APPROACHES TO THE TENSION FLANGE

CONNECTION

(a) (b)

(d)(c)




9

• Structural sections are available only in specific

lengths• Splices are provided at non-critical locations

Column splice

• Partial penetration butt welds are economical.

Welded beam splice plates

Beam splice where bending moment is low spl icing

of webs due to

• Required length of plate is not available

• Girder may be cambered at the spl ice

• Thickness of the girder may be varied

COLUMN AND BEAM SPLICES






11

X

X

X

X

STAGGERED FORM OF ARRANGEMENT FOR

TEMPORARY SUPPORT




12

Only weld here if i tsabsence could lead to

corrosion

X-XX

X

BUTT – WELDED SPLICE PLATE CONNECTION

X

X

HYBRID CONNECTION




13

BEAM-BEAM AND BEAM-COLUMN CONNECTIONS

Types of beam connections

Rotational characteristics of connections

• Simple 0-20% moment resistance

• Semi rigid 20 -90% “

• Rigid >90% “




14

WELDED BEAM CONNECTIONS

•

Web angles• Beam seats Shear connections

• Stiffened beam seats

• Moment resistant connections

Two common ways of stress transfer at connection

• Bending forces occur in beam flanges and for

transfer, welds to be provided at the beam

flanges

• For transfer of shear forces welds to be provided

at webs




15

Erection bolt

FRAMED SIMPLE CONNECTION

End return

WELDED SEMIRIGID CONNECTION




16

(a) Simple Connections (0%)

(b) Rigid Connections (100%)

(c) Semi Rigid Connections (50%)

(d) Semi Rigid Connections (75%)

w kg /m

(c)

wl 2 / 24+ wl 2 / 24

wl 2 / 12

(b)

w kg /m

wl 2 / 24

+ wl 2 / 12wl 2 / 12

wl 2 / 8

(a)

w kg / m

+

w kg /m

(d)

wl 2 / 16

wl 2

/ 16 wl 2 / 16+




17

WELDED MOMENT - RESISTING CONNECTION

WELDED WEB ANGLES

End returns End returns

Erectio

n bolts




18

0.1

L

0.6L

0.3L

0.9LL

fhBeam web

R/2R/2

End returne

ECCENTRICITY OF REACTION FORCES




19

Optionallocation of top L

Erection Bolts

Top angle

Seat or shelf angle

Erection bolt

WELDED SEATED- BEAM CONNECTIONS




20

STIFFENED BEAM SEAT CONNECTION




21

MOMENT RESISTANT CONNECTIONS

• Continuous structures

• Connections are designed to resist ful l moments

•Efficient connections

• Moment resistance may reduce

– Bending of the column at the connection point

– Top connection plate tries to bend column f lange




22

End connection plate Fillet weld

Groove weld

Stiffened seat

T

C

(a)

(b)

MOMENT RESISTING CONNECTIONS -1




23

(a) OVERSTRESSING OF THE WELD,(b) COLUMN FLANGE STIFFENED WITH PLATES

Top connection plate

pulling away from connection

Weld is over stressed here

(a)

Reinforcing plate cut back

(b)




24

FATIGUE BEHAVIOUR

• Number of cycles or the time taken to attain a

pre- defined failure criteria eg. Bridges,Crane

girders

• High cycle low stress fatigue - 108 cycles

• Low-cycle high stress fatigue <105 cycles

• Corrosion fatigue

• Uncertain features due to

• Environmental effects

• Prediction of internal stresses

•

Time to failure• Methods of analysis

• S-N approach and fracture mechanics approach




25

CHARACTERISTICS OF FATIGUE FAILURE

• Poor design and fabrication are prime reasons

• Crack initiation and propagation

• Fatigue crack is transgranular

•Fracture surface may be either crystalline orfibrous

• Mechanism of fatigue

• Plastic flow due to stress concentration effects




26

FACTORS INFLUENCING FATIGUE BEHAVIOUR

•

Stress range• Stress concentration

• Rate of cyclic loading

• Residual stresses

• Size

• Geometry

• Environment

• Temperature

• Previous stress history




27

FRACTURED SURFACE OF A SPECIMEN



Teaching Resources (c) IIT Madras,SERC Madras, ANNA Univ, 28

10 -3 min

5*10 –6 mm

(a) Fine slip (b) Coarse bond produced by

alternating slip

FATIGUE MECHANISM




THREE LEVELS OF STRESS CONCENTRATION

• Structural action

Relative deformation between adjacent elements

Secondary members

• Macroscopic stress concentration

Geometric stress interruption to stress f low




• Local geometric stress concentration

• Crack tip effects occur in weld or HAZ

• For low stress range,frequency effect is

insignificant

• For high stress range ,increase in frequency

produces increase in apparent fatigue strength

• Effect of residual stresses varies considerably.

• Fatigue limit is 50% of ultimate stress

FI = 140 + 0.25FU




TYPICAL CONNECTION DETAILS

(a) Industrial roof structure (b) Bridge structure




BENDING STRESSES IN A DISCONTINUOUS BEAM

(a) Beam arrangement (b) Stress flow at change in

direction




STRESS CONCENTRATION AT THE TOE OF A

FILLET WELD

(a) Fillet weld arrangement (b) Tensile stress flow




Fatigue failure at a welded joint may occur due to

• Porosity,slag inclusion and defects

• Lack of fusion or microscopic cracks

• Crystalline change in base metal in HAZ

• Due to undercut at toe edge of the weld

Butt welded joints have better fatigue resistance




FATIGUE ANALYSIS

• Nominal stress

• Hot spot stress

• Number of stress reversals before failure




S-N curve approach

•

Cyclic stress range(S) to number of cycles to failures(N)• Log-log relationship is linear

NS

m =

• Goodman’s diagram

• Variable amplitude loading

• Stress spectrum

• Damage due to each band in stress spectrum

• Palmgren - Miner rule

0.1 N

n

N

n

N

n

n

n

2

2

1

1£+++ KKK






Number of cycles to failure (N) in thousands

25000 1000 1500 2000500

S – N CURVE (WOHLER CURVE)




S

Cycles – to – failure N (Log scale)

F1,00,000 = fatigue strength for 100,000 Cycles

F 2000,000 = fatigue strength for 2000,000

Cycles

NS r m =

Zero - to - tension cycle

F 2,000,000

F1,00,000

N

S – N CURVE PRESENTED ON A LOG-LOG SCALE




GOODMAN DIAGRAM

T e n s i l e o r c o m p r e s s i v e

s t r e s s ,

i n p e r c e n t o f

s t a t i c

u l t i m a t e s t r e n g t h

100

80

60

40

20

0

-20

-40 33% of Ultimate

33%

Ultimate tensile strength

Maximum stress in cycle

Minimum stress in cycle

Line of zero stress




s

t1

n4 n1 n2 n3 n5

s t2

s t3

s t4

s t5

True spectrum

STRESS RANGE SPECTRUM




Fracture mechanics

Presence of cracks is accounted

K = Y* *

Stress intensity factor,K

relationship is a sigmoidal curve

Paris equation

a´P

( )mk c

dNda D=

klogvsdN

dalog 1010 D




SCHEMATIC PRESENTATION OF CRACK GROWTH

Static failure mechanism

da / dN = C K m

-9

-6

-3

Log10 K

Threshold K th

Non –

continuum

mechanism

Continuum

mechanism

(striation

growth)

Kic final failure

B m

1



Teaching Resources (c) IIT Madras,

SERC Madras, ANNA Univ,

44

INDIAN STANDARD PRACTICE

• IS 1024 -1979 -Code of practice for use of

welding in bridges and structures

subjected to dynamic loading

• Working stress is reduced to allow effects of

fatigue





45

Seven clauses of weld details

Permissible stress in welds

Combined bending and shear

Combined shear,bearing and bending

( ) ( ) ( ) ( )22

bc

22

bte f 3f or f 3f f

SS ++=

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )2 b bc

2

b

2

bc

2

b bt

2

b

2

bce f 3f f f f or f 3f f f f f

SS +´-++´++=





46

CLASS E FULL PENETRATION

CRUCIFORM BUTT WELD

X

X

Class E stress refers to this member

TYPICAL CLASS F WELD DETAILS





47

TYPICAL CLASS F WELD DETAILS

X

X

In this case load is

resisted by bending of

the plate

Check principal stress in web at ends of stiffener to web welds (or equally at any other

attachment to a shear-loaded

m ember)

X

XX

These connections include

stiffener to flange welds

Ends of Flange

Plates WhetherWeld transversely

or not





48

Longitudinal load carrying f illet welds(a)

X

X

TYPICAL CLASS G WELD DETAILS







50

IMPROVEMENT OF FATIGUE STRENGTH

Improvement techniques Fatigue strength improvement by

• Weld geometry improvement

• Residual stress reduction

Crack initiation li fe can be extended by

• Removal of crack l ike defects

•

Reducing SC in weld• Reduction of tensile residual stresses





51

WELD IMPROVEMENT TECHNIQUES

• Weld toe grindingRemoval of small cracks (0.5 mm) by grinding

Concave surface

• Weld dressing

Weld profi le is smoothened by dressing

• Weld toe remelting

TIG or Plasma arc dressing

•

Hammer peening Application of repeated hammering





52

REPAIRS TO CRACKED WELDS

• Repaired weld may contain defects and may have

a lower fatigue life.

• Repaired weld should be to a revised detail.

• Addit ional stif fening to reduce stress range





53

FATIGUE RESISTANT DESIGN

General suggestionsButt weld

• Double-sided fil let weld

• Avoid stress concentration

• Avoid abrupt transition in sections

• Eliminate or reduce eccentricit ies

• Avoid attachments to parts subjected to fatigue

loading

• Use continuous welds





54

FATIGUE RESISTANT DESIGN-1

• Proper inspection during fabrication

• Deep penetration fi llet welds

• Proper inspection during service

• Provide structural redundancy

• Provide crack arrestors



Teaching Reso rces (c) IIT Madras 55

SUMMARY

• Types of beam-to-beam and beam-to-column

welded connections are described.

• Fatigue effects and factors affecting fatigue

behaviour of welded connections are explained.

• Methods of evaluating fatigue lives of welded

connections are presented.

• Techniques for improving fatigue performance

and fracture resistant design are explained.

• Indian Standard codal provisions are included.

Welding Connection 2

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