Welding Connection 2
Transcript of Welding Connection 2
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WELDED CONNECTIONS II
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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
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DIRECT CONNECTION OF WEB MEMBERS
CONNECTION AT THE APEX OF A ROOF TRUSS
EAVES CONNECTION
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(b)
e P
e
(a)
P
ECCENTRICITIES IN TRUSS CONNECTIONS
(a) PRATT TRUSS,
(b) CROSS BRACING BETWEEN PLATE GIRDERS
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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
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MOMENT TRANSMISSION AT
CORNER USING A SHEAR PANEL
MOMENT TRANSMISSION AT CORNER
USING A DIAGONAL STIFFENER
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VARIOUS EAVES CONNECTIONS OF PORTAL FRAMES
(a) (b)
(c) (d)
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WELDED APEX CONNECTIONS
(a) GENERAL ARRANGEMENT
(b) – (d) DIFFERENT APPROACHES TO THE TENSION FLANGE
CONNECTION
(a) (b)
(d)(c)
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• 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
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X
X
X
X
STAGGERED FORM OF ARRANGEMENT FOR
TEMPORARY SUPPORT
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Only weld here if i tsabsence could lead to
corrosion
X-XX
X
BUTT – WELDED SPLICE PLATE CONNECTION
X
X
HYBRID CONNECTION
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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% “
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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
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Erection bolt
FRAMED SIMPLE CONNECTION
End return
WELDED SEMIRIGID CONNECTION
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(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+
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WELDED MOMENT - RESISTING CONNECTION
WELDED WEB ANGLES
End returns End returns
Erectio
n bolts
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0.1
L
0.6L
0.3L
0.9LL
fhBeam web
R/2R/2
End returne
ECCENTRICITY OF REACTION FORCES
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Optionallocation of top L
Erection Bolts
Top angle
Seat or shelf angle
Erection bolt
WELDED SEATED- BEAM CONNECTIONS
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STIFFENED BEAM SEAT CONNECTION
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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
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End connection plate Fillet weld
Groove weld
Stiffened seat
T
C
(a)
(b)
MOMENT RESISTING CONNECTIONS -1
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(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)
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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
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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
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FACTORS INFLUENCING FATIGUE BEHAVIOUR
•
Stress range• Stress concentration
• Rate of cyclic loading
• Residual stresses
• Size
• Geometry
• Environment
• Temperature
• Previous stress history
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FRACTURED SURFACE OF A SPECIMEN
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10 -3 min
5*10 –6 mm
(a) Fine slip (b) Coarse bond produced by
alternating slip
FATIGUE MECHANISM
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THREE LEVELS OF STRESS CONCENTRATION
• Structural action
Relative deformation between adjacent elements
Secondary members
• Macroscopic stress concentration
Geometric stress interruption to stress f low
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• 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
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TYPICAL CONNECTION DETAILS
(a) Industrial roof structure (b) Bridge structure
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BENDING STRESSES IN A DISCONTINUOUS BEAM
(a) Beam arrangement (b) Stress flow at change in
direction
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STRESS CONCENTRATION AT THE TOE OF A
FILLET WELD
(a) Fillet weld arrangement (b) Tensile stress flow
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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
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FATIGUE ANALYSIS
• Nominal stress
• Hot spot stress
• Number of stress reversals before failure
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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
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Number of cycles to failure (N) in thousands
25000 1000 1500 2000500
S – N CURVE (WOHLER CURVE)
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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
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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
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s
t1
n4 n1 n2 n3 n5
s t2
s t3
s t4
s t5
True spectrum
STRESS RANGE SPECTRUM
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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
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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
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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
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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 +´-++´++=
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CLASS E FULL PENETRATION
CRUCIFORM BUTT WELD
X
X
Class E stress refers to this member
TYPICAL CLASS F WELD DETAILS
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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
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Longitudinal load carrying f illet welds(a)
X
X
TYPICAL CLASS G WELD DETAILS
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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
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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
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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
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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
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FATIGUE RESISTANT DESIGN-1
• Proper inspection during fabrication
• Deep penetration fi llet welds
• Proper inspection during service
• Provide structural redundancy
• Provide crack arrestors
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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.