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Design of Steel Structures

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Santha Kumar

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It is distinguished from
other forms of mechanical connections, such as riveting or bolting, which are formed by
friction or mechanical interlocking
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Welding offers many advantages over bolting and riveting
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Hence, the weight of the joint is minimum
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It
involves less fabrication cost compared to other methods due to handling of fewer parts
and elimination of operations like drilling, punching etc
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Welding offers air tight and water tight joining and hence is ideal for
oil storage tanks, ships etc
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Welded structures are more rigid compared to
structures with riveted and bolted connections
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Generally welded joints are as strong or
stronger than the base metal, thereby placing no restriction on the joints
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Some of the disadvantages of welding are that it requires skilled manpower for
welding as well as inspection
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Welding in the field may be difficult due to the location or
environment
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Large
residual stresses and distortion are developed in welded connections
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The
tendency of atoms to bond is the fundamental basis of welding
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The diffusion may take place in the liquid, solid or mixed state
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In practice however, it is very difficult to achieve a perfect joint;
for, real surfaces are never smooth
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Therefore
the strength attained will be only a fraction of the full strength
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In the welding of such surfaces, the contaminants have to be removed for the
bonding of the surface atoms to take place
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In practical welding, both heat and pressure are applied to get a
good joint
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The relative amount of heat and pressure required to
join two materials may vary considerably between two extreme cases in which either
heat or pressure alone is applied
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Examples of such a
process are Gas Tungsten Arc Welding (GTAW), Shielded Metal Arc Welding (SMAW),
Submerged Arc Welding (SAW) etc
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There are other welding methods where both pressure and heat are
employed, such as resistance welding, friction welding etc
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Electric arc is by far the
most popular source of heat used in commercial welding practice
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Indian Institute of Technology Madras

Design of Steel Structures

Prof
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Santha Kumar

In gas welding a mixture of oxygen and some suitable gas is burned at the tip of
a torch held in the welder’s hand or by an automatic machine
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The flame
produced can be used both for cutting and welding of metals
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But, the process is slow compared to other means of welding
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The most common welding processes, especially for structural steel, use electric
energy as the heat source produced by the electric arc
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The
arc is a continuous spark formed when a large current at a low voltage is discharged
between the electrode and the base metal through a thermally ionised gaseous column,
called plasma
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A temperature of 33000 C to 55000
C is produced in the arc
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In arc welding, fusion takes place by
the flow of material from the welding rod across the arc without pressure being applied
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In Shielded Metal Arc Welding or SMAW (Fig
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12), heating is done by means of
electric arc between a coated electrode and the material being joined
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The
electrode coating on the welding rod forms a gaseous shield that helps to exclude
oxygen and stabilise the arc
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After cooling, the slag can be easily
removed by hammering and wire brushing
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Fig
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12 Shielded metal arc welding (SMAW) process

Fig
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12 Shielded metal arc welding (SMAW) process
The type of welding electrode used would decide the weld properties such as
strength, ductility and corrosion resistance
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The two general classes of electrodes are lightly coated and
heavily coated
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The resulting welds are stronger, more corrosion resistant and more ductile compared
to welds produced by lightly coated electrodes
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The term weldability is defined as the ability to obtain economic welds, which are
good, crack-free and would meet all the requirements
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The effects of heating
and cooling associated with fusion welding are experienced by the weld metal and the
Heat Affected Zone (HAZ) of the base metal
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For most steels, weld
cracks become a problem as the thickness of the plates increases
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A variety of joints is used in structural steel work
and they can be classified into four basic configurations namely, Lap joint, Tee joint,
Butt joint and Corner joint
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The T- joints form a Tee and in Corner joints, the
ends are joined like the letter L
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Plug and slot welds are not generally used in structural
steel work
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Butt welds can be of complete penetration or
incomplete penetration depending upon whether the penetration is complete through
the thickness or partial
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Though fillet welds are weaker than butt welds, about 80% of the connections are
made with fillet welds
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For butt welds, the members to be connected have to fit perfectly when they
are lined up for welding
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To ensure full penetration and a sound weld, a backup
plate is temporarily provided as shown in Fig
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15

Butt welds:
Full penetration butt welds are formed when the parts are connected together
within the thickness of the parent metal
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For thicker parts, edge preparation may have to be done to
achieve the welding
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They
are shown in Fig
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13 In order to qualify for a full penetration weld; there are certain
conditions to be satisfied while making the welds
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Flat welds are the most economical to make while overhead welds are
the most difficult and expensive
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A few of the many different butt welds are shown in Fig
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Each type of butt weld requires a specific edge preparation and is named accordingly
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Butt welds have high strength, high resistance to impact and cyclic stress
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But their major
disadvantages are: high residual stresses, necessity of edge preparation and proper
aligning of the members in the field
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Indian Institute of Technology Madras

Design of Steel Structures

Prof
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Santha Kumar

Fig
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14 Common types of welds

Fig
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15 Shaping of surface and backup plate

Fig
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16 Typical connections with butt weld
To minimise weld distortions and residual stresses, the heat input is minimised
and hence the welding volume is minimised
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Hence for thicker plates, double Butt welds and U welds are generally
used
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The smaller the root gap the
greater the angle of the bevel
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Roughly, the penetration is about 1 mm per 100A
and in manual welding the current is usually 150 – 200 A
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This
groove is filled with the molten metal from the electrode
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For good penetration,
the root faces must be melted
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Fig
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17 Butt weld details

Fillet welds:
Owing to their economy, ease of fabrication and adaptability, fillet welds are
widely used
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Another advantage of fillet welds is that
special preparation of edges, as required by Butt welds, is not required
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A
typical fillet weld is shown in Fig
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Indian Institute of Technology Madras

Design of Steel Structures

Prof
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A
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Santha Kumar

Fig
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18 Typical fillet weld
The root of the weld is the point where the faces of the metallic members meet
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The throat area equals the theoretical throat distance times the length of
the weld
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But
it is more vulnerable to shrinkage and cracking than the convex surface and has a much
reduced throat area to transfer stresses
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For statically loaded structures, a
slightly convex shape is preferable, while for fatigue – prone structures, concave
surface is desirable
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For reasons
of economy, it is desirable to choose weld sizes that can be made in a single pass
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Indian Institute of Technology Madras

Design of Steel Structures

Prof
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Santha Kumar

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4 Weld symbols
The information concerning type, size, position, welding process etc
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The symbolic
representation includes elementary symbols along with a) supplementary symbol, b) a
means of showing dimensions, or c) some complementary indications
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Elementary symbols represent the various categories of the weld and look similar
to the shape of the weld to be made
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Elementary symbols are shown in Table 3
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Table 3
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Supplementary symbols are shown in Table 3
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The
weld locations are defined by specifying, a) position of the arrow line, b) position of the
reference line, and c) the position of the symbol
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Indian Institute of Technology Madras

Design of Steel Structures

Prof
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Santha Kumar

Table 3
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Supplementary symbols
Flat (flush) single – V butt
weld

Convex double – V butt weld

Concave fillet weld

Flat (flush) single – V butt
with flat (flush) backing run

Position of symbols in drawings:
Apart from the symbols as covered earlier, the methods of representation (Fig
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19)
also include the following:
· An arrow line for each joint

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The arrow line joins one end of the continuous reference line such that it forms an angle
with it and shall be completed by an arrowhead or a dot
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The symbol is placed either above or beneath the reference line
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It has been observed from tests conducted on tensile coupons
containing a full penetration butt weld normal to the applied load that the welded joint
had higher strength than the parent metal itself
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The butt weld is normally designed for direct tension or compression
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Design strength value is often taken the same
as the parent metal strength
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Effective
length of the butt weld is taken as the length of the continuous full size weld
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For a full penetration butt weld, the
throat dimension is usually assumed as the thickness of the thinner part of the
connection
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Such reinforcements often have a negative effect, producing stress concentration,
especially under cyclic loads
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Intermittent butt welds are used to resist shear only and the effective
length should not be less than four times the longitudinal space between the effective
length of welds nor more than 16 times the thinner part
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Some modern codes do not
allow intermittent welds in bridge structures
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This is applicable in cases where the difference in thickness
exceeds 25 % of the thickness of the thinner part or 3
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The
slope provided at the joint for the thicker part should not be steeper than one in five
[Figs
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20 (a) & (b)]
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3
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Where reduction of the wider
part is not possible, the ends of the weld shall be returned to ensure full throat
thickness
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7
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For field welds, the permissible
stresses in shear and tension calculated using a partial factor γmw of 1
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(Cl
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5
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2)

Design of fillet welds:
Fillet welds are broadly classified into side fillets and end fillets (Fig
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21)
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But the ductility is
minimal
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The weld is subjected to shear and the weld shear strength is
limited to just about half the weld metal tensile strength
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For intermediate weld positions, the value of strength and ductility show
intermediate values
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3
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In order to apply this method, it is important to establish
equilibrium with the applied load
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For equal leg lengths, making the
direction of applied tension nearly parallel to the throat leads to a large reduction in
strength
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A small variation in the side fillet connections has negligible effect on
strength
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Fig
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21 Fillet (a) side welds and (b) end welds

Indian Institute of Technology Madras

Design of Steel Structures

Prof
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R
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A
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Santha Kumar

Fig
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22 Average stress in the weld throat
A simple approach to design is to assume uniform fillet weld strength in all
directions and to specify a certain throat stress value
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This method is limited in usage to cases of pure shear, tension or compression
(Fig
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23)
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For the simple method, the stress is taken as the vector sum of the force
components acting in the weld divided by the throat area
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25 for shop welds and = 1
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The size of a normal fillet should be taken as the minimum leg size (Fig
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25)

Indian Institute of Technology Madras

Design of Steel Structures

Prof
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R
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A
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Santha Kumar

Fig
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25 Sizes of fillet welds
For a deep penetration weld, the depth of penetration should be a minimum of
2
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Then the size of the weld is minimum leg length plus 2
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The size of a
fillet weld should not be less than 3 mm or more than the thickness of the thinner part
joined
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4
(Cl
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5
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3)
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7 t and 1
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Table 3
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Values of K for different angles between tension fusion
faces are given in Table 3
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10
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3
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Fillet welds are normally used for connecting
parts whose fusion faces form angles between 60° and 120°
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Minimum effective
length should not be less than four times the weld size
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5 mm less than the edge

Indian Institute of Technology Madras

Design of Steel Structures

Prof
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Santha Kumar

thickness [Fig
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26 (a)]
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3
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10
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8
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Fig
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26 (a) Fillet welds on square edge of plate, (b) Fillet
Welds on round toe of rolled section
Table 3
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Value of K for different angles between fusion faces
Angle between fusion faces
Constant K

60° - 90°
0
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65

101°-106°
0
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55

114°-120°
0
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The length of intermediate welds should not be less than 4 times the weld
size with a minimum of 40 mm
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Chain intermittent welding is better than staggered intermittent welding
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For lap
joints, the overlap should not be less than five times the thickness of the thinner part
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5 times the thickness or 12 mm whichever is greater, and

Indian Institute of Technology Madras

Design of Steel Structures

c)

Prof
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Santha Kumar

The distance between the edge of the part and the edge of the slot or hole, or

between adjacent slots or holes, should be not less than twice the thickness and not
less than 25 mm for the holes
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Plug welds are not designed to carry stresses
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The high stress concentration at ends of welds is minimised by providing welds
around the ends as shown in Fig
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Most designers
neglect end returns in the effective length calculation of the weld
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The end returns are provided for a distance not less than twice the size of the
weld
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In such a situation, the required strength may
be built up by welding along the back of the channel at the edge of the plate if sufficient
space is available
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3
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Another way of developing the
required strength is by providing slot or plug welds
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3
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On certain occasions, plug welds
are used to fill the holes that are temporarily made for erection bolts for beam and

Indian Institute of Technology Madras

Design of Steel Structures

Prof
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A
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Santha Kumar

column connections
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The limitations given in specifications for the maximum sizes of plug and slot
welds are necessary to avoid large shrinkage, which might be caused around these
welds when they exceed the specified sizes
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This area is usually referred to as the faying surface and is equal to the area of
contact at the base of the slot or plug
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3
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15)



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