welded connections final
TRANSCRIPT
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W e l d e d Connecti ons
Dr. S. RAVIRAJAssistant Professor of Civil Engineering
Sri Jayachamarajendra College of Engineering, Mysore 06
1.0 INTRODUCTION
Welding is the process to unite variouspieces of metal by creating a strong
metallurgicalbond.Bond is achieved by heat or pressure or both. Welding is
the most efficient and direct way ofconnecting the metalpieces. Over many
decades, differentwelding techniques have been developedtojoin metals.
2.0 TYPES OF WELDING
Welding is generally performed by eitherelectric or gas. Most of the welding is
done using electricsupply. Though gas welding is relatively cheaper, itis a slow
process. Hence this method is generally used for repair and maintenance
purposes.
2.1 Gas Welding
Fig. 1 Gas Welding2.2 Arc Welding
It is also called as oxy-acetylene welding.
Here the mixture of gases namely
acetyleneand oxygen is burned at the tip
of a torch, which produces a very hot
flame. This heats the metalpieces for
cutting and welding process. The features
ofa typical gas welding is shown in Fig. 1.
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Arc welding is used in most of the structuralwelding operations. Here, electric
energy which is usedas the heat source is produced by electric arc. The base
metal and welding rod (orelectrode)are heated to fusiontemperature by the
electric arc. The typical arc welding equipments and accessories are shown inFig. 2. Thewelding rod is connected to one terminal of the current source and
the object to be weldedis connected to the other terminal.Thetemperature in
the region ofwelding ranges from 3300o C to 5500o C.
Fig. 2 Arc welding equipment and accessories
2.2.1 Shielded Metal Arc Welding(SMAW)
This is the most popularmethod of arc welding.Heating is done by means of
electric arc between a coated electrode and the materialbeingjoined. If
uncoated or bare wireelectrodes are used,themoltenmetal gets exposed to
atmosphere and combines chemically with oxygen and nitrogen and forms
defective welds. The coating on the electrode forms a gaseous shield thathelpsto exclude oxygen and protects the moltenmetal from oxidation.The
flux of the electrodecoating,being lighterthan moltenmetal, hardens at the
surface of the weld.This can be removed by gentletapping or bybrushing.The
typical features ofshieldedmetal arc welding are shown in Fig. 3.
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Fig. 3 Shieldedmetal arc welding
The type ofwelding electrode used decides the weldproperties like strength,
ductility and corrosion resistance. The choice ofelectrode depends upon the
type of metalbeing welded, the amount of material to be added, and the
position of work.
There are two types ofelectrodes:
Lightly coated electrodes
Heavily coated electrodes
Heavily coated electrodes are used in structural welding. These electrodes
result in welds thatare stronger, more corrosion resistant, and more ductile
(compared to lightly coated electrodes). Usually the SMAW process is either
automatic or semi-automatic. The main advantage of SMAW is that high
quality welds can be made rapidly at a low cost. The grade and propertiesof
electrodes are listedin Table 1 ofIS 800 : 2007 which is as perIS 814 : 2004.
3.0 ADVANTAGES OF WELDING
There are many advantages of welding. Some of the important ones are as
follows.
1. Weldedjoints are aesthetical to look when compared toboltedjoints.
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2. Welded joints are more rigid than bolted joints. Hence the material at
varioussectionsareutilized more efficiently to resist stresses than that of
lessrigid connections.
3. Welding does not requiredriving ofholes.This reduces the cost incurred fordrilling. Hence, whilecomputing the tensile strength of members, the net
area remains the same as the gross area.
4. Weldedjoints are wellsuited forliquid and gas containingstructures.
5. Welding offers the possibilities of fabricating new sections like castellated
beams orcreatingcomplexjointsin tubulartruss.
4.0 DISADVANTAGES OF WELDING
Some of the disadvantages ofwelding are;
1. Welding requires greater skill than bolting and hence requireshighly skilled
human resources.
2. Improper welding will distort the members and its alignment, and hence
requiresmoreconcentration.
3. Theinspection of weldjointsis more difficult and cumbersome than bolted
joints.
4. The process ofwelding may leave a higherresidual stress in the material.
5. Welding equipment is more expensive and requires larger initial
investment.
6. Welding at siteis more difficult and alsorequires constant powersupply.
5.0TYPESOF WELDS
There are four types of welds.They are:
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1. Fillet welds
2. Groove welds
3. Slot welds,and
4. Plug welds
Of these welds, fil let is used to a large extent. Groove welds are used to a
lesserextent. However,slot andplug welds are rarelyused.
5.1FilletWelds
Fig. 4 Fillet weld and its cross section
Fillet welds(Fig. 4A) are widely used due to theireconomy, ease offabrication,
and adoptability at site.They are approximatelytriangularin cross section(Fig.
4). These welds require lessprecision in fittingup two sections, due to the
overlapping ofpieces.They are adopted in field as well as in shop welding. It
does not require any edge preparationand hence cheaper than groove welds.
Fillet welds are assumed to fail in shear. They can be present on one side
(single) or on both sides(double) of a member as shown in Table1 and Fig. 5.
Table 1 Single and double fillet welds
SINGLE DOUBLE
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FILLET
Fig. 5 Idealised and actual fillet weld
5.2 GROOVE WELDS
In this type of weld, grooves are generally made in the base metal before
welding and hence are called as groove weld. They are generally used to
connect structural members aligned in thesameplane, such as in butt joints.
Thedetails of a typical groove weld are shown in Fig. 6.
Fig. 6 Details of typical groove weld
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Some of the commonly used groove weldsinbutt joints are shown in Fig. 7.
Fig. 7 Types of groove welds in butt joint
The square groove weld is used to connect plates up to 8 mm thickness.They
are also used inT-connections. The grooves have a slope of 30O and 60O with
the vertical, which depend on the thickness of the plate and the welding
operation. Partialpenetration groove welds should not be used especially in
fatiguesituations.Root opening or gap (see Fig. 6) isprovided for the electrode
to access the base of the joint.Thebevelangle (see Fig. 6) for typical root
openingsis shown in Table2.
Table 2 Root openings and bevel angle for groove weld
Root openings Bevel angle
3 mm 60O
6 mm 45O
9 mm 30O
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Weld metalis more expensive than the base metal. Hence, the choice between
single or doublepenetration depends on the availability of access on both
sides, the thickness ofplate to be welded, the type of welding equipment
available and theposition of weld.
When the plate thickness is more than 12 mm, the groove can be either
double-bevelordouble-V type. When the platethicknessis more than 40 mm,
the groove can be eitherdouble - U ordouble - J type. Forplates between 12
to 40 mm, the groove can be withersingle-J and single-U type.
Groove welds are chosen in situations where the members need to transmit
the full load ofthe members they join. Hence, the strength of welds shouldbe
more than orequal to the strength of the members they join. To ensure this,
fullpenetration groove welds are used more frequently.
5.3 SLOT AND PLUGWELDS
Slot and Plug welds (Fig. 8) are limitedly used to connect the steel members.
They are generallyused to complement the fillet weldsin situations where itis
notpossible toprovidesufficientlengthoffillet welds due to some constraints.
These welds fail in shear. The extent ofpenetration of these welds into the
parent metal is difficult to determine since it is difficult to inspect it. They are
to be avoided when the members are subjected to tensile forces. The
calculation ofdesignstrength ofslot orplug welds are similarto that of fillet
welds.
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Fig. 8 Typical slot and plug welds
6.0WELDINGPOSITON
The weldingpositions can be of four types, which are:
Flat - On the floor
Overhead - Under the roof
Vertical - On the wall
Horizontal - On the wall
Figure 9 shows the different weld positions which exist during weldingoperation.
a) Flat - On the floor b) Overhead - Under the roof
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c) Vertical - On the wall d) Horizontal - On the wall
Fig. 9 Different weldpositions
7.0 TYPES OF JOINTS
There are fivebasic types of common joints.They are
Buttjoint
Lapjoint
Tjoint
Cornerjoint,and
Edgejoint
Each joint is suitable for a specific situation. The choice of the joint for a
particularjob depends on the size and shape of the members to be welded at
the joint, the type of loading, area available for welding at the joint, and
relative cost ofvarious types of welds.
7.1 ButtJoint
Fig. 10 Typicalbutt joint
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Buttjoints are used to join the ends of flatplates ofnearlyequalthickness.A
typicalbutt jointis shown in Fig. 10. Thisjointavoidseccentric transfer of force
at the connection.Itispreferable to havefullpenetration of welds at the butt
joints so that the jointis fully efficient.Thesize ofconnectionisquitesmall andhence is very economical.Itis aesthetical to look at. Face reinforcement(weld
beyond the surface) is normallyprovided in Butt joints. This increases the
efficiency of the jointand ensures that depth of weld is at leastequal to the
thickness of theplate.
7.2 LapJoint
Fig. 11 Typical lap joint
Lapjoints are easy to fit and join any two members. A typical lapjointis shown
in Fig. 11. Itis themost commonly used joint. It does not require any specialpreparation. Lap joints utilize fil let welds. They are well suited for shop and
field welding. Lap joints can accommodate minor errors in fabrication and
minoradjustment in length. The main advantage oflap joints is that it canjoin
plates with different thicknesses without any difficulty (Fig. 12). The main
disadvantage ofthisjointisthat itintroduceseccentric transfer ofloads at the
connection.
Fig. 12 Lap joint with plates of different thickness
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7.3 TJoints
Fig. 13 Typical T -joint
A T-joint is usually used to fabricatebuilt-up sections from simple members. Atypical T-shaped joint is shown in Fig. 13. Some of the commonly used built-up
shapes where T-joints are seen are I-sections,Plate girders, Hangers, Brackets,
and Stiffeners. The members in the built-up sections are joined by means of
fillet welds or groove welds.
7.4 CornerJoint
Cornerjoints are normally seen inbuilt - up rectangularbox sections.A typical
cornerjoint is shown in Fig. 14. They are generally seen at places which are
subjected to high torsionalmoments.
Fig. 14 Typical cornerjoint
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Corner joints are seen in the built - up rectangular box sections. A typical
cornerjoint is shown inFig. 14. They are commonly seen at placeswhich are
subjected to high torsionalmoments.
7.5 EdgeJoint
Fig. 15 Typical cornerjoint
Edge joints are generally not used in structuralapplications.They are used to
keep two or moreplates in position in a given plane. A typical edge joint is
shown in Fig. 15.
8.0 WELDSYMBOLAND WELDINGSYMBOL
A weldsymbol is a symbol which indicates the type of weld to be adopted to
joint the metalpieces. However, a welding symbol is a concise way of
describingallparticularinformationrelated to the weld on drawings.
8.1 Weld Symbol
Weld symbol is unique for each specific type of weld. Hence the weldsymbol
used is differentforfillet, groove,plug, and slot welds.
8.1.1 Basic Weld Symbols
Thebasic weldsymbols for the commonly used welds are shown in Table3.
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Table 3 Basic weld symbols
Weld symbol is only a part of the informationregarding the welding operation
to be performed at ajoint. As indicated in Table 3, whenever a weld symbol
consists of both vertical and inclined legs, theverticalis always drawn towards
the leftside of the inclinedline.
8.2 Welding Symbol
The welding symbol containsall the information necessary in connection with
a welding operation. Italso includes the type of weld, where welds are to be
located, the type ofjoint to be used, and thesize and amount of weldmetal tobe depositedin the joint.
The symbols used are standardized by the various codes ofpractice so that the
entireinformationcan be concisely represented in a drawing.
Thebasicwelding symbol comprises of three parts, namely
a reference line
an arrow, and
a tail
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Apart from this there are alsosupplementarywelding symbols to represent
Dimensions and otherdata
Supplementarysymbols
Finish symbols Specification,process or other reference
8.2.1 Basic Welding Symbols
Fig. 16 Basic welding symbols
Reference Line
The reference line is always drawn horizontally. It is mandatory and forms the
foundation of a welding symbol. All information with respect to the welding
process is to be indicated around thisline.
Arrow
The arrow line is present at one end of the reference line. Itsimply connects
one end ofthe reference line to the joint or area to be welded.Thedirection of
the arrow has nobearing on thesignificance of the reference line.Some of the
possible types of arrows used in the welding symbol are shown in Fig. 17.
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Fig. 17 Different types of arrows used in welding symbol
Tail
The tailis shown on the other end (away from arrow end) of the reference line.
It is not mandatory.The tail is used to specify a certainwelding process. It is
used only when necessary. Itis used tomention some specialcharacteristic ofthe weld like type of electrode, some type of reference or specification,
welding orcutting process, procedures or other supplementary information.If
additional information is not needed, then the tail will be omitted. The
representation oftai withadditionalinformationis shown in Fig. 18.
Fig. 18 Representation of tail with additional information
8.2.2 Interpretation of Symbols
Fillet weld on arrow side
Symbol Meaning
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Fig. 19 Fillet weld, Arrow side
Fillet weld on otherside
Symbol Meaning
Fig. 20 Fillet weld, Other side
Fillet weld onboth sides
Symbol Meaning
Fig. 21 Fillet weld, Bothsides
Bevel edge
Symbol Meaning
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The symbol indicates that one edge of a joint is to be beveled. The arrowshould points towards the member to be beveled. Hence, the arrow shouldshow a definite break so that the member to be beveled can be clearlyidentified.
Fig. 22 Bevel edge
8.2.3 StandardLocation ofElements of a Welding Symbol
The standard location ofvariouselements of a welding symbol to be indicated
in drawingissummarizedbelow.
Fig. 23 Standard location of welding symbols
8.3 Supplementary Symbols
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There are some supplementary weld symbols used in addition to the basic
weld symbols which are indicated in section 8.1. These include Finish and
contour symbols, All round weld, and Field or site weld which are shown in
Table4.
Table 4 Supplementary weld symbols
8.3.1 Finish and contoursymbols
Finish symbol shows the method of finish to be carried out to a weld.
Generally, the finish ofwelding is eitherby chipping (C) or by machining (M) or
by grinding (G). Contoursymbols are used with weldsymbols to show how the
face of the weldis to be formed. The face of the weldwillbe eitherflat, convex
or concave as shown in Table4.
8.3.2 All round weld
Fig. 24 Typical representation of all round weld
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The all round symbol (Table 4) indicates that the welds are continued all
around the joint.Atypical all round weld and itsrepresentationis shown in Fig.
24.
8.3.3 Field orsiteweld
Fig. 25 Representation of field or site weld
The symbol used for field or site weld is a flag (Table 4). Itpoints toward the
tail of welding symbol. If no symbol is present, it indicates the weld as shop
weld.A typical representation offieldorsite weldis shown in Fig. 25.
9.0 WELD D EFECTS
As mentionedearlier,welding requires greater skill so that the defects can be
avoided.Some ofthecommonly observed defects in weldsare;
1. Incompletefusion
2. Incompletepenetration
3. Porosity
4. Undercutting
5. Inclusion ofslag
6. Cracks
7. Lamellartearing
1. Incompletefusion
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This occurs when the surfaces have not been cleanedproperly, and are coated
with oxides, mill scales, and other foreign materials. Insufficient current
supplied by the welding equipment or high rate (speed) of welding can also
lead to incompletefusion.
2. Incompletepenetration
Incomplete penetration can be due to impropergrooves orunsuitable groove
design made forthewelding process. This can also be a result of the usage of
largesize electrodes,insufficient welding current, and excessivewelding rates.
3. Porosity
Improper welding techniques will result in air voids being entrapped in the
molten metal during the cooling process resulting in porosity. Some of the
common reasons are excessively high current, longerarc length, poorwelding
procedures, and careless use ofback-strips.
4. Undercutting
In case of groove welds, grooves are made at the edges of the base metal to
accommodate the welding process. If the grooves are not completely filled
with weld,itresultsin undercutting ofthe base metal(i.e., the thickness of the
base metal will be less in that region). This may lead toplaces of stress
concentrations during the process of force transfer and can be dangerous.
Hence to eliminateundercutting, it is mandatory to have face reinforcement
(welding over the surface ofbase metal)in all groovejoints.
5. Inclusion ofslag
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Slag is formed from the coating of the electrodes which are used to shield the
molten material fromoxides during the cooling process. The slag is generally
removed after the weld cools by either wirebrushing or by gentletapping. If
the process ofcooling is done rapidly, the slag gets trapped inside the weld.This weakens the weld strength and is not desirable. When the required weld
thicknessislarge,itis made by several passes. In such cases, the slag should be
removed after the completionofeach pass. If thisis not done properly,italso
resultsin the inclusion ofslag.
5. Cracks
Cracks are the most serious weld defects since it reduces the weld strength
directly. This resultsmostly due the relative differences in internal stresses in
the weld. The direction of the weld can be either along the longitudinal or
transverse direction of weld.They can be seen on the surface orpresent inside
the weld. They can be avoided by using good quality electrodes, adopting
uniform rate ofwelding and ensuringslowercoolingperiods.
6. Lamellartearing
This is the formation of cracksbeneath the weld. The high temperature during
welding causes largerelativestrainsin the base metal due to localised stresses
and results in tearing of the base metal. This can be prevented by choosing
properwelding techniques and adoptinguniform rate of welding.
10.0 WELDDISTORTIONS
During welding operation, proper care must be ensured to avoid weld
distortions. Improperwelding techniquesresults in welddistortions.Someof
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the commonly observed distortions during welding are Transverse shrinkage,
Longitudinal shrinkage, Angular change, Rotational distortion, Longitudinal
bendingdistortion and Bucking distortion.Thesedistortions are shown in Fig.
26.
a) Transverse shrinkage b) Longitudinalshrinkage
c) Angular change d) Rotationaldistortion
e) Longitudinal bending distortion f) Buckling distortion
Fig. 26 Typical weld distortions
11.0 IMPORTANT CODAL PROVISIO NS OF IS 800 : 2007 WITH RESPECT TO
WELDING
The welds and welding shall conform to the following codes:
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IS 816 : 1969 Code ofpractice for the use ofmetal arc welding forgeneral
constructionin mildsteel
IS 9595 : 1996 Metal arc welding of carbon and carbon manganese steels
Cl. 10.5.1.1 EndReturn s
>2a
a = weld
size
>2a
Fig. 27 Details of end returns ofweld
Fillet welds terminating at the ends or sides of parts should be returned
continuously aroundthe corners fordistance of not less than twice the size of
the weld, unless it is impractical. This isparticularly important on the tension
end of parts carryingbendingloads.
Cl. 10.5.1.2 Lap Joint
Fig. 28 Typical lapjoint
Inlapjoints the minimum lapshould be not less than fourtimes the thickness
of the thinnerpartjoined.Single end filletshould be used only when lapped
parts are restrained from openings.When end of an elementis connected only
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byparallel longitudinal fil let welds, the length of the weldalong eitheredge
should be not lessthan the transverse spacing between longitudinal welds.
Cl. 10.5.2 Size of weld
10.5.2.1 The size ofnormal fillets shallbe taken as the minimum weld leg size.
For deep penetration welds, where the depth ofpenetration beyond the root
run is 2.4 mm (minimum), the size of the fil letshould be taken as the minimum
leg sizeplus 2.4 mm.
Fig. 29 Size ofweld
10.5.2.2 For fillet welds made by semi-automatic or automatic processes,
where the depth ofpenetration is considerably in excess of 2.4 mm, the size
shall be taken considering actual depth ofpenetration subject to agreement
between the purchaser and the contractor.
10.5.2.3 Thesize offillet weldsshall not be less than 3 mm. Theminimumsize
of the first run or ofasingle run fillet weldshall be as given in Table 5, to avoid
the risk ofcracking in the absence ofpreheating.
Table 5 Minimum weld size
Thickness ofThickerPart Minimum Size
Over, mm Upto and including, mm mm
-- 10 3
10 20 5
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20 32 6
32 50 8 for first run
10 for min. size ofweld
10.5.2.4 The size of butt weld shall be specified by the effective throatthickness.
Fig. 30 Typicalbutt joint
Cl. 10.5.3 Effective Thro at Thickness, te
10.5.3.1
Fig. 31 Effective throat thickness of fillet weld
The effective throat thickness te (Fig. 31) of a fillet weldshall not be less than 3
mm and shallgenerally not exceed 0.7t, and 1.0t under specialcircumstances,
where t is the thickness of the thinnerplate ofelementsbeing welded.
10.5.3.2 For the purpose of stress calculation in fillet weldsjoining facesinclined to each other, the effective throat thicknessshall be taken as K times
the filletsize, where K is a constant, depending upon the angle between fusion
faces, as given in Table6.
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Anglebetweenfusionfaces, Constant K
600900 0.70
9101000 0.65
10101060 0.60
10701130 0.55
11401200 0.50
Table 6 Values of k for different angles between fusion faces
10.5.3.3 Theeffective throat thickness te (Fig. 32) of a completepenetration
butt weldshallbe taken as the thickness of the thinnerpart joined, and that of
an incompletepenetration butt weldte(Fig. 32) shall be taken as the minimum
thickness of the weld metal common to the parts joined, excluding
reinforcement.
Fig. 32 Effective throat thickness of butt weld
Cl. 10.5.4 Effective Length
10.5.4.1 Theeffective length of fillet weldshall be taken as only that length
which is of the specified size and required throat thickness. Inpractice the
actuallength of weldis made oftheeffectivelength shown in drawingplustwo
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times the weld size, but itshould not be less than four times the size of the
weld.
10.5.4.2 Theeffective length of butt weldshall be taken as the length of the
continuousfull size weld, but itshould not be less than fourtimes the size of
the weld.
Cl. 10.5.5 Intermittent Welds
10.5.5.1 Theintermittentfilletwelding shall have an effectivelength of not less
than fourtimesthe weldsize, with a minimum of 40 mm, except as otherwise
specified.
10.5.5.2 Theclearspacing between the effective lengths of intermittent fillet
weldshall not exceed 12 and 16 times the thickness ofthinnerplatejoined, for
compression and tensionjointrespectively, and in no case be more than 200mm.
10.5.5.3 The intermittent butt weldshall have an effective length of not less
than fourtimesthe weldsize and the longitudinal space between the effective
length of weldsshall not more than 16times the thickness of the thinnerpart
joined, except as otherwisespecified.Theintermittent weldsshall not be usedinpositions subject to dynamic,repetitive and alternatestresses.
Cl. 10.5.7 Design Stresses in Welds
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10.5.7.1 Shop welds
10.5.7.1.1 Fillet welds Thedesign strength of a fillet weld, fwd, shall be based
on its throat area.
fwd = fwn / mw in which fwn = fu / 3
where
fu = smallerof the ultimate stress of the weld and the parent metal
mw = 1.25 =partial safety factor
10.5.7.1.2 Butt welds Butt welds shall be treated as parent metal with athicknessequal to the throat thickness, and the stresses shall not exceed those
permittedin the parent metal.
10.5.7.2 SiteWelds Thedesign strength in shear and tension forsite welds
made during erection ofstructural members shall be calculated as per 10.5.7.1
but using apartial safety factor mw of1.5.
Cl. 10.5.8.1 Fillet Weld Applied to Edge of a Plate
Square edge
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10.5.8.1 Where a fillet weld is applied to the square edge of a part, the
specified size of the weld shouldgenerally be at least 1.5 mm less than the
edge thicknessin order to avoid washing down of the exposed arris(Fig. 33).
1.5 mm
a
Fig. 33 Fillet weld on square edge ofplate
10.5.8.2 Where the fillet weldis applied to the rounded toe of a rolledsection,the specifiedsize ofthe weldshouldgenerally not exceed 3/4 of the thickness
of the section at the toe (Fig. 34).
1/4 t
t
Fig. 34 Fillet weld on round toe of rolled section
10.5.8.3 Where the size specified for a fillet weldis such that the parent metal
will not project beyond the weld, no melting of the outer cover or covers shall
be allowed to occur to such an extent as to reduce the throat thickness(Fig.
35)
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most unfavorable conditions of loading, the equivalent stress fe is obtained
from the following formulae:
where
fe
=2
fb +2
fbr + fb fbr+ 3q2
fe = equivalentstress
fb = calculated stress due tobendinginN/mm2
fbr=calculated stress due tobearinginN/mm2, and
q = shearstress inN/mm2
Cl. 10.8 Intersections
Members or components meeting at a jointshall be arranged to transfer the
designactionsbetween the parts, whereverpracticable, withtheircentroidal
axes meeting at apoint. Where there iseccentricity at joints, the members and
components shall be designed for the designbending moments, which result
due to eccentricity.
The disposition of fillet welds to balance the design actions about the
centroidalaxis or axes for end connections of single angle,doubleangle and
similar type members is not required for statically loaded members but is
required for members, connection components subject to fatigueloading.
Eccentricity between the centroidal axes ofangle members and the gauge lines
for their bolted end connections may be neglected in statically loaded
members, but shall be considered in members and connection components
subject to fatigueloading.
Cl. 10.11 Analysis of a Bolt/WeldGroup
10.11.1 Bolt/Weld Group Subject to In-planeLoading
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10.11.1.1 General Method of Analysis The design force in a bolt/weld in a
bolt/weld group ordesign force perunitlengthin abolt/weld group subject to
in-planeloadingshall be determinedin accordance with the following:
a) Theconnectionplatesshall be considered to be rigid and to rotate relative
to each other about apoint known as the instantaneous centre ofrotation
of the group.
b) In the case of a group subject to a pure couple only (Fig. 37a), the
instantaneouscentreof rotationcoincides with the group centroid. In the
case of in-plane shear force applied at the group centroid (Fig. 37b), the
instantaneous centre of the rotation is at infinity and the design force isuniformly distributed throughout the group. In all other cases (Fig. 37c),
eithertheresults of independentanalyses for a pure couplealone and for
an in-plane shear force appliedat the group centroid shall be superposed,
or a recognized method ofanalysis shall be used.
c) Thedesign force in abolt ordesign force per unitlength at anypointin the
group shallbeassumed to act at rightangles to the radius from that pointto the instantaneous centre, and shall be taken as proportional to that
radius.
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a) Pure coupleatcentroid
b) In-plane shear forceatcentroid
c) In-plane shear forceaway from centroid
Fig. 37 Location of Pure couple and In-plane shear with respect to centroid
10.11.2 Bolt/Weld group Subject to Out-of-PlaneLoading
10.11.2.1 General Method ofAnalysis The design force of abolt inbolt group
ordesign force per unit length in the fillet weld group subject to out-of-plane
loading(Fig. 38) shall be determinedin accordance with the following:
a) Thedesign force in the bolts perunitlengthin the fillet weld group resulting
from any shear force oraxial force shall be considered to be equally shared
by allboltsin the group oruniformlydistributed over the length of the fillet
weld group.
b) The design force resulting from a design bending moment shall
be considered to vary linearly with the distance from the relevant
centroidal axes.
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i) Inbearing type ofbolt group plates in the compression side of the
neutralaxis and onlyboltsin the tensionside of the neutralaxis may be
considered forcalculating the neutralaxis and second moment ofarea.
ii) In the friction grip bolt group only the bolts shall be considered in thecalculation ofneutralaxis and second moment ofarea.
iii) Thefillet weld group shall be consideredin isolation from the connected
element; forthecalculation ofcentroid and second moment of the weld
length.
e V
Z Z
B r a c k e t
Fig. 38 Out-of-plane shear force with respect to weldplane
10.11.2.2 Alternative Analysis The design force per unit length in a fillet
weld/bolt group mayalternatively be determined by considering the fillet weld
group as an extension of the connected member and distributing the design
forces among the welds of the fillet weld group so as to satisfy equil ibrium
between the fillet weld group and the elements of the connected member.
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12.0 Numerical Problems
1. Twoplates of size 200 x10 mm and 200 x 8 mm are connectedby a weld groove having (i)Single
V groove weldjoint, and (ii) Double V groove weldjoint. Determinethe maximum
tension which thejoints can resist.Thesteelplates are of grade Fe 410 grade with yield
strength of250
MPa. Assume shop welding.
Solution
Case(i):SingleV groove weld (Fig. 39)
In this case, incompletepenetration results due to singleVgroove.
Fig. 39 Single V groove weld
Single V is an incomplete penetration welding. Hence the throat thickness is 5/8th ofthe
thickness of thinnerplate.
te = 5/8t = (5/8) x 8 = 5 mm
Effective length of weld Lw = width of plate = 200 mm.
Strength of weld, P = L x tt x fy / mw Cl. 6.2
= 200 x 5 x200/1.25 = 200,000N= 200kN
Case(ii) : DoubleV groove weld (Fig. 40)
In this case, complete penetration results due to DoubleVgroove.
Fig. 40 Double V groove weld
Effective throat thickness is 8mm whichis the thickness ofthe thinnerplate.
te = 8 mm
Strength of weld, P = L x tt x fy / mw Cl. 6.2
= 200 x 8 x200/1.25 = 320,000N = 320kN
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2. Find the size and length ofthefillet weld forthe lapjoint to transmit a factored load of
120 kN as shown inFig. 41. Assume site welds, Fe 410 grade steel and E41 electrode.
Assume width ofplate as 75 mm andthickness as 8 mm.
Fig. 41 Lap joint connection
S o lu tio n Minimum sizeofweld for8 mm thick section = 3 mm (Table5,
Cl. 10.5.2.3) Maximum sizeofweld = 81.5 = 6.5 mm (Cl. 10.5.8.1)
Choose the sizeofweld, a =6mm
Effective throat thickness = te = 0.70 a = 4.2 mm
Strength of6 mm weld / mm length = 4.2 x 410 /(3x 1.5) Cl.10.5.7.1.1
= 662.7
N/mm Assuming only two longitudinal welds along
the sides Required length ofweld =120x 103/
662.7= 181 mm Length tobe provided on each side
=181/2= 90.5 mm
> 75 mm (width ofplate) Hence,
provide 90.5 mm weld on each side with an end return of2x 6 = 12 mm
Overall length oftheweld provided = 2 x(90.5+ 2 x6)= 205 mm
3. Two plates are connected to form a fillet joint using 6mm weld. Welding is provided on
three sides with a lap of 300mm as shown in Fig.42. Find the strength of the joint. If
welding is provided on all foursides (Fig. 44), determine the strength ofthe joint. Also
find the percentage increase inthestrength. Use Fe 410 steelwith yield stress 250 MPa.
Assume shop welding.
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Solution
Fig. 42 Lap joint connection with weld on three sides
Case (i) : Welding on three sides (Fig. 42)
Lw= 300 + 200 + 300 =800mm
Design strength of fillet weld joint, P1 = 0.7a fwd Lw /mw Cl.10.5.7.1.1
= 0.7 x6x(410/3)x 800 /1.25
= 6,36,286
N Hence, allowable load = 6,36,286 / 1.5 = 4,24,191
N
Case (ii) : Welding on four sides (Fig.4 3)
Fig. 43 Lap joint connection with weld on foursides
Lw= 300 + 200 + 300 +200= 1000 mm
Design strength of fillet weld joint, P2 = 0.7a fwd Lw /mw Cl.10.5.7.1.1
= 0.7 x6x(410/3)x1000/1.25
= 7,95,358
N Hence, allowable load = 7,95,358 / 1.5 = 5,30,239
N
Percentage increase in strength = (P2P1)/ P1 x 100 = 25 %
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4. Determinethe size ofthe weld required forthe bracket connection shown in Fig. 44.
Assume shopwelding.
Solution
Eccentricity, e = 100 + 75 = 175 mm
Torsional moment ,M= 60 x 175 = 10,500 kN-
mm Considering unit thickness ofweld at root
Ip = Izz+Iyy = 3.08 x 106 mm4
Shear stress due to direct force, f1 = 60,000 / (400x1) = 150 MPa
Fig. 44 Bracket connection showing load and resultant stress in weld
Further, rmax = 125 mm
Shear stress due to torsional moment, f2 =Mx rmax /Ip
= 426 MPa
Resultant stress in the critical part of the weld / mm width
R= (f12
+ f22
+2f1f2 cos ) 0.5 = 530 MPa
Strength ofthe weld per unit length andthickness, P P = 0.7a fwd Lw /mw
= 0.7 a (410/3) 1 / 1.25 = 132.6 aN
Equating the resultant, Rwith the strength ofweld, P (i.e.,R= P)
a = 3.99 mm
Size of weld to be provided =4mm
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5. Determinethe load, V that can be applied on the bracket shown in Fig. 45. Use 6 mm
field fillet welding.
Solution
Shear stress due to direct force, q =V/(2x 200 x 1)
= 0.0025 V
N/mm2
Moment on weld, M= 200 VN-mm
Considering unit thickness ofweld at root,
Moment of inertia Iz= 2 (1x2003)/ 12 = 1.33 x 10
6mm
4
Fig. 45 Bracket connection showing load out ofplane
Normal stress due to bending in tension, fa = M y/Iz
Here, M= 200 VN-mm,y= 100 mm, Iz= 1.33 x 106 mm4
Hence, fa = 0.015 V
Equivalent stress, fe is computed as (cl. 10.5.10.1.1)
fe =( fa2
+ 3 q2 ) 0.5 =
0.0156 V For6 mm fillet weld (field
weld)
Strength ofweld/mm, fwd =0.7x6(410/3)/ 1.50 = 662.8N/mm
Equating the equivalent stress, fe with the strength of6 mm weld,
fwd fe =fwd
0.0156 V=662.8
Hence, V= 42487 N =42.5 kN Forthe current bracket
a load of42.5kN can be safelyapplied.
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