Chapter

Welding and Welded

3 Connections

3.1 INTRODUCTION
Welding is the process of joining two pieces of metal by creating a strong
them by heating or pressure or both. It is distinguished from other forms metallurgical
bond between
of mechanical
such as riveting or bolting, which are formed connections
by friction or mechanical interlocking. It is one of the
oldest and reliable method of
joining.
Welding offers many advantages over bolting and riveting. Welding enables direct transfer of stress
between members eliminating gusset and
splice plates necessary for bolted structures. Hence. the weight
of the joint is minimum. In the case of tension members, the absence of
holes improves the efficiency of
the section. It involves less fabrication cost
compared to other methods due to handling of fewer
parts
and elimination of operations like drilling,
punching etc., and consequently less labour leading to
economy. Welding offers air tight and water tight joining and hence is ideal for oil storage tanks.
etc. Welded structures also have a neat
ships
appearance and enable the connection of complicated shapes.
Welded structures are more rigid compared to structures with riveted and bolted connections. A
truly
continuous structure is formed by the process effusing the members
together. Generally welded joints
are as strong as or
stronger than the base metal, thereby placing no restriction on the joints. Stress
concentration effect is also considerably less in a welded connection.
Some of the disadvantages of welding are that it requires skilled manpower for welding as well as
inspection. Also, non-destructive evaluation may have to be carried out to detect defects in welds.
Welding in the field may be difficult due to the location or environment. Welded joints are highly prone
to cracking under fatigue loading. Large residual stresses and distortion are developed in welded
connections.

3.2 FUNDAMENTALS OF WELDING

A welded joint is obtained when two clean surfaces are brought into contact with each other and either
pressure or heat, or both are applied to obtain a bond. The tendency of atoms to bond is he fundamental
basis of welding. The inter-diffusion between the materials that are joined is the underlying principie in
all welding processes. The diffusion may take place in the liquid, solid or mixed state. In welding the
metallic materials are joined by the formation of metallic bonds and a perfect connection is formed. In

73
 74 Design of Steel Structures

practice however, it is very difficult to achieve a

welding, contact is established only perfect joint; for, real surfaces are never smooth. When
at a few points
in the
bonding occurs. Therefore the strength attained will be surface, joins irregular surfaces where atomic
irregular surface may not be very clean, being contaminated only a fraction of the full strength. Also, the
layer etc. In the welding of such surfaces, the with adsorbed moisture, oxide film,
contaminants have to be removed for the grease
surface atoms to take place. This can be bonding of the
welding. both heat and pressure are applied accomplished by applying either heat or pressure. In practical
to get a
As pointed out
good joint.
earlier, any welding process needs some
two materials. The form of energy, often heat, to connect the
relative amount of heat and
considerably between two extreme cases in which pressure required to join two materials may vary
either heat or pressure alone is
alone is applied to make
the joint. pressure is used applied. When heat
Examples of such a process are Gas Tungsten Arc merely keep to the joining members together.
(SMAW) as shown in Fig. 3.1, and Welding (GTAW), Shielded Metal Arc Welding
alone is used to make the Submerged Arc Welding (SAW) etc. On the
other hand pressure
bonding by plastic deformation, examples being cold
ultrasonic weldingetc. There other welding, roll welding.
Such as resistance
are
welding methods where both pressure and heat are employed,
welding,
friction welding etc. A flame, an arc or resistance to an electric current,
produces the required heat. Electric arc is by far the most
welding practice. popular source of heat used in commercial

Electrode

Coating on electrode
Arc stream

Slag Gaseous shield

Weld Molten pool

Penetration depth
Base metal
Fig. 3.1 Shielded metal arc welding (SMAW) processs

3.3 TYPE OF JOINTS AND WELDS

By means of welding, it is
possible to make continuous, load bearing
structure. A variety of joints is used in structural steel work and joints between the members of a
configurations namely. Lap joint, Tee joint, Butt joint and Cornerthey can be classified into four basic
members are overlapped and for butt joint. For lap joints, the ends of two
a Tee and in
Corner joints. the ends are
joints, the two members are
placed end to end. The T-joints form
fillet weld or the butt (also joined like the letter L. Most
common joints are made
steel work. Fig. 3.2 shows
calling groove) weld. Plug and slot welds are not
up of
fillet welds are suitable for generally used in structural
and corner joints. Butt welds lap joints and Tee joints and groove welds for
be of
complete penetration or incomplete
can butt
whether the penetration complete
is penetration depending upon
joints requires an indication of the through
the thickness or
partial. Generally a
description of welded
type of both the joint and the weld.
Though fillet welds are weaker than butt welds, about 80%
fillet welds. The reason for the of the connections are
wider use of fillet welds is that made with
members are lapped over each in the case of fillet
other, large tolerances are allowed in welds, when
members to be connected have to erection. For butt
fit perfectly when welds, the
they are lined up for welding. Further butt
 Welding and Welded Connections 75

(a) Butt welds

(b) Fillet weld

Ends shall be semi

circular
A A A

Section A-A Section A-A

(c) Slot weld

(d) Plug weld
Fig. 3.2 Types of welds
welding requires the shaping of the surfaces to be
To ensure full penetration joined.
and a sound weld, a backup
plate is temporarily provided as shown in Fig. 3.3.

3.3.1 Butt Welds Backup plate

Fig. 3.3 Shaping of surface and backup plate
Full penetration butt welds are formed when the
parts are connected together within the thickness of the
parent metal. For thin parts, it is possible to achieve full
preparation may have to be done to achieve the welding.penetration of the weld. For thicker
There are nine different
parts, edge
square, single V, double V, single U, double U, types of butt
are shown in
single J, double J, single bevel and double bevel.joints:
Fig. 3.4. In order to qualify for a full
penetration They
weld; there are certain conditions to be
satisfied while making the welds.

(a) Square (b) Single V (c) Double V

(d) Single bevel

(e) Double bevel ( Single U (o) Double U

(h) Single J

i) Double J G) Double V-finished flush

on both sides

Fig. 3.4 Types of butt welds

 76 Design of Steel Structures

The main use of butt welds is to connect

structural members, which are in the same
strength, high resistance to impact and cyclic stress. They are most directplane. and
welds have high Butt
introduce least eccentricity in the joint. But their joints
of edge preparation and
major disadvantages are: high residual stresses,
necessity
proper aligning of the members in the field. Therefore, field butt
rarely used. joints are

3.3.2 Fillet Welds

Owing to their economy, ease of fabrication and adaptability, fillet welds are widely used.
less precision in the fitting up because the They require
plates being joined
can be moved about than the butt welds. Another
more Weld and leg size
advantage fillet weld is that special preparation of edges,
of
as required by butt welds, is not Face of weld
required. In a fillet weld the
stress condition in the weld is
quite different from that of the Theoretical throat
connected parts. A typical fillet weld is shown in Fig. 3.5. (t 0.707 s)
The root of the weld is the point where the faces of the
metallic members meet. The theoretical throat of a weld is Root of weld
the shortest distance from the the
root to hypotenuse of the
triangle. The throat area equals the theoretical throat distance
times the length of the weld.
The concave shape of free surface provides a smoother Fig. 3.5 Typical fllet weld

transition between the connected parts and hence, causes less stress concentration than a convexsurface.
But it is more vulnerable to shrinkage and cracking than the convex surface and has a much reduced
throat area to transfer stresses. On the other hand, convex shapes provide extra weld metal or reinforcement
for the throat. For statically loaded structures, a slightly convex shape is preferable, while for fatigue
prone structures, concave surface is desirable. Large welds are invariably made up ofa number of layers
or passes. For reasons of economy, it is desirable to choose weld sizes that can be made in a single pass.
Large welds can be made in a single pass by an automatic machine, though manually, 8 mm fillet is the
largest single-pass layer.
&
3
3.3.3 Weld Symbols
The information concerning type, size, position, welding process etc., of the welds in welded joints is
conveyed by standard symbols in drawings. The symbolic representation includes elementary symbols
along with: (a) supplementary symbol, (b) a means of showing dimensions, or (c) some complementary
indications. IS:813 "Scheme of Symbols for Welding" gives all the details of weld representation in

drawings.
Elementary symbols represent the various categories of the weld and look similar to the shape of the
weld to be made. Combination of elementary symbols may also be used, when required. Elementary

symbols are shown in Table. 3.1.

 Welding and Welded Connections

Table. 3.1 Elementary symbols

Illustration (Fig) Symbol Symbol description

Butt weld between plates with raised edges (the
J raised edges being melted down completely)

Square butt weld

V Single-V butt weld

V Single-bevel butt weld

Y Single-V butt weld with broad root face

Fillet weld

Plug weld; plug or slot weld

(Contd...
 78 Design of Steel Structures

llustration(Fig) Symbol Symbol description e

O
O Spot weld

Seam weld

3.4 DESIGN OF WELDS

3.4.1 Design of Butt Welds
For butt welds the most critical form of
loading is tension applied in the transverse direction. 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. The yield stress of the
weld metal and the parent metal in the heat-affected zone (HAZ) region was found to be much higher
than the parent metal.
The butt weld is normally designed for direct tension or
compression. However, a provision is
made to protect it from shear. Design strength value is often taken the same as the
parent metal strength.
For design purposes, the effective area of the but-welded connection is taken as
the effective length of
the weld times the throat size. Effective
length of the butt weld is taken as the length of the continuous
full size weld. The throat size is specified by the effective throat thickness. For a full
weld, the throat dimension is usually assumed as the thickness of the thinner part of the connection.
penetration
butt

Even though a butt weld may be reinforced on both sides to ensure full
cross-sectional areas, its effect
is neglected while estimating the throat dimensions. Such reinforcements often have a negative effect,
producing stress concentration, especially under cyclic loads.
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. They are not to be used in locations
subjected to dynamic or alternating stresses. Some
modern codes do not allow intermittent welds in bridge structures. For butt
welding parts with unequal
cross sections, say unequal width, or thickness, the dimensions of the
wider or thicker part should be
reduced at the butt joint to those of the smaller This is
part. applicable in cases where the difference in
thickness exceeds 25% of the thickness of the thinner
part 3.0 mm, whichever is greater. The slope
or
provided at the joint for the thicker part should not be steeper than one in five. In instances, where this
 Welding and Welded Connections 79

ot practicable, the weld metal is built up at the junction equal to a thickness which is at least 25 %
eater than the thinner part or equal to the dimension of the thicker part. Where reduction of the wider
at is not possible, the ends of the weld shall be returned to ensure full throat thickness. Stresses for
utt welds are assumed same as for the parent metal with a thickness equal to the throat thickness
S:800, CI.10.5.7.1). For field welds, the permissible stresses in shear and tension calculated using a
gartial factor Ym of 1.5. IS:800, CI. 10.5.7.2).

34.2 Design of Fillet Welds

Filet welds are broadly classified into side fillets and end fillets Fig. 3.6. When a connection with end
let is loaded in tension, the weld develops high strength and the stress developed in the weld is equal
tothe value of the weld metal. But the ductility is minimal. On the other hand, when a specimen with
ide weld is loaded, the load axis is parallel to the weld axis. The weld is subjected to shear and the weld
shear strength is limited to just about half the weld metal tensile strength. But ductility is considerably
mproved. For intermediate weld positions, the value of strength and ductility show intermediate values.

a) (6)
Fig. 3.6 Fillet (a) side welds and (b) end welds

The design strength of a fillet weld, fwshall be based on its throat area (IS:800, CI.No. 10.5.7).

Twd Jwn 3.1)

where, = nominal strength offillet weld (3.2)

where f, smaller of the ultimate stress of the weld and the parent metal and Y partial safety factor
1.25 for shop welds and = 1.5 for field welds). The design strength shall be reduced appropriately for
ong joints as prescribed in the code. The size of a normal fillet should be taken as the minimum leg size
as shown in Fig. 3.7.
The design stress of fillet weld and groove weld is given by:

Paw Ju-
3Y. ...(3.3)

where, L = effective length of the weld in mm

, throat thickness in mm = KxS

S size of weld in mm

Pdw design strength of weld in N

K=aconstant, shown in Table 3.3.
 Structures
80 Design of Steel
Size
Size (Min. leg size)
Size
L

Fillets of equal leg length

Fillets of unequal leg length

- Leg length
Penetration

+ 2.4 mm
Size =
leg length
3.7 Sizes of fillet welds
Fig.

Note welds madeduring erection of structural members

and tension for site
The design strength in shear factor yw 1.5. of
should be calculated by Eq.
(3.5) but with partial safety considered as cqual to the
weld the thickness of weld (1) is
weld or groove
In case of butt
thickness of thinner plate.
Then the
minimum of 2.4 mm.
should be a
the depth of penetration less than 3
For apenetration weld,
deep size of a fillet weld should not be
2.4 mm. The of fillet welds is
nminimum leg length plus
size of the weld is Minimum size requirement
the thinner part joined. thickness should not
be less
mm or more
than the thickness of Effective throat
Table. 21, CI. 10.5.2.3). is the thickness of
below in Table 3.2 (IS:800, circumstances, where 't
given 0.7t and 1.0t under special
should not exceed
than 3 mm and
thinner part. run fillet weld
run or of a single
Minimum size of first
Table 3.2

Thickness of thicker part (mm), Minimum size (mm

S.No. Up to and including
Over
3
10
.
5
20
2.
10 6
32 10
20 8 mm of first run mm
3. 50
32 for minimum size
of weld
4.

taken as K times fillet size, where K

thickness should be
the effective throat
fusion faces are given in Table 3.3
For stress calculations, between tension
different angles whose fusion faces
form
Values of K for
is constant.
used for connecting parts
the effective length plus
a
Fillet welds are normally
(IS:800, CI.10.5.3.2). is taken as the length having
120°. The actual length the weld size. When
a
between 60° and not be less than four times
angles Minimum effective length
should
mm less than
the edge
twice the weld size. weld size should be at least 1.5
to square edge part, the of a exceed 3/4 thickness
of the
fillet weld is provided the weld size should not
rounded toe of a
rolled section,
thickness. For the
CI.10.5.8.1).
section at the toe (IS:800,
 Welding and Welded Connections 81
Table 3.3 Value of K for different angles between fusion faces

107-113° I140-120
ngle between fusion faces 60-90 91-100° 101-106°

onstant K 0.70 0.65 0.60 0.55 0.50

Intermittent fillet welds may be provided where the strength required is less than that can be
by a continuous fillet weld of the smallest allowable size for the parts joined. The of
eveloped welds should not be less than 4 times the weld size with a minimum of 40 mm. The clear
length
termediate
cing between the effective lengths of the intermittent welds should be less than or equal to 12 times
thickness of the thinner member in compression and 16 times in tension; in no case the length should
Kceed 20 cm. Chain intermittent welding is better than staggered intermittent welding. Intermittent
let welds are not used in main members exposed to weather. For lap joints, the overlap should not be
s than five times the thickness of the thinner part. For fillet welds to be used in slots and holes, the
mension of the slot or hole should comply with the following limits:
(a) The width or diameter should not be less than three times the thickness or 25 mm whichever is
greater.
(b) Corners at the enclosed ends or slots should be rounded with a radius not less than 1.5 times
the thickness or 12 mm whichever is greater, and
c) 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.
The high stress concentration at ends of welds is minimized by providing welds around the ends as
in Fig. 3.8. These are called end
shown
eturns. Most designers neglect end re-
uns in the effective length calculation End weld
of the weld. End returns are invariably
provided for welded joints that are sub- P
ect to eccentricity, impact or stress re-
versals. The end returns are provided for
distance not less than twice the size of
the weld.
Fig. 3.8 End returns for side welds

35 DESIGN OF PLUG AND SLOT WELDS

n.certain instances, the lengths available for the normal longitudinal fillet welds
to resist the loads. In such a may not be sufficient
situation, the required strength may be built up by welding
of the channel at the edge of the plate if sufficient along the back
space is available. Another way of
required strength is by providing slot or developing the
plug welds. Slot and plug welds as shown in Fig. 3.9 are
generally used along with fillet welds in lap joints. On certain occasions,
holes that are temporarily made for erection bolts for beam and column plug welds are used to fill the
connections. However, their
strength may not be considered in the overall strength of the
The limitations given in
joint
specifications for the maximum sizes of
shrinkage, which might be caused around these weldsplug
to avoid large and slot welds are
necessary
sizes. The strength of a when they exceed the
plug or slot weld is calculated by specified
nominal area in the shearing plane. This area is considering the allowable stress and its
usually referred to as the faying surface and is
equal to
 82 Design of Steel Structures

the area of contact at the base of the slot or plug. The length of the slot weld can be obtained from the
following relationship:
L Load/(Width) x allowable stress ...(3.4)
Ends shall be
semi circular
or have corners
rounded to a

L radius not less

than thickness
L/hS
Section A-A Section A-A

(a) (b)
Fig. 3.9 Slot and plug welds

3.6 STRESSES DUE TO INDIVIDUAL FORCES

When subjected to either compressive or tensile or shear force alone, the stress in the weld is given by:
for q = Pl, ..(3.5)
where, S= calculated normal stress due to axial force in N/mm.
q shear stress in N/mm.
P force transmitted (axial force T or the shear force Q).
t, = effective throat thickness of weld in mm.

effective length of weld in mm.

3.7 COMBINATION OF STRESSES

3.7.1 Fillet Welds (1S:800-2007, CL. 10.5.10, page 80)
When subjected to a combination of normal and shear stresses, the
equivalent stress f, shall satisfy the
following:

I= +34 s (3.6)
3
where, f. =
normal stresses, compression or tension, due to axial force or bending moment determined
from Eq. (3.5)
q shear stress due to shear force or tension determined from Eq. (3.5)
For the fillet welds, check for combination of stresses need not be done for:
(a) Side fillet welds joining cover plates and flange
plates, and
(b) Fillet welds where sum of normal and shear stresses does not
exceed
Eq. (3.1). fu determined from
 Connections
Welding and Welded

37.2 Butt Welds

of stresses in butt welds need not be carried

out provided that
or butt welds, check for the combination
(a) butt welds are axially loaded, and
of normal and shear stresses does not exceed the
(6) in single and double bevel welds the sum

normal stress and the shear stress does not exceed 50% of the design shear stress.
design

3.7.3 Combined Bearing, Bending and Shear

or compressive), f, and
shear stresses, q
here bearing stress, fhr is combined with bendingin(tensile
butt welds, the equivalent stress. f, as
obtained
conditions of loading
nder the most unfavourable
from the following formula, shall not the values allowed for the parent metal

..(3.7)

where, f. equivalent stress, in N/mm-

=

Scalculated stress due to bending, in N/mm

ar calculated stress due to bearing, in N/mm and

shear stress, in N/mm

3.8 WELDED JOINT VS BOLTED JOINTS

Welded joints are economical because of big size gusset plates, bolt materials and splices are completely
elemented. For welded connection, size of gusset plate required is smaller because of reduced length
of connection.
Welded structures are permanent. Once a weld is made the part must be damaged or destroyed to
disassemble it. Whereas bolted connection are not permanent and parts can be disassembled if get
damaged.
Depending on the welding method used, it can create a continuous joint with lower stress concentrations,
while bolting will always have stress concentrations at each bolt.
Opportunities for corrosion are minimized as compared to bolted joint.
Welding can be performed faster than many other method due to reduced preparation time (drilling
holes, gathering fasteners and tools, etc.)
In most cases welded joints maintain the same strength as the material being joined compared to
bolted connection.
The
efficiency of welded joint is more than that of a bolted joint.
Welding usually requires more skill person as compared to bolted joint.
Welding is difficult to perform in wet, widely. or submerged situations as compared to bolted joint.
Special instruments are required for inspection of welded joints, whereas bolted joints can be inspected
by tapping the joint with a hammer.