Welded Connections

1.0 INTRODUCTION

Welding is the process to unite various pieces of metal by creating a strong

metallurgical bond. Bond is achieved by heat or pressure or both. Welding is

the most efficient and direct way of connecting the metal pieces. Over many

decades, different welding techniques have been developed to join metals.

2.0 TYPES OF WELDING

Welding is generally performed by either electric or gas. Most of the

welding is done using electric supply. Though gas welding is relatively

cheaper, it is a slow process. Hence this method is generally used for

repair and maintenance purposes.

2.1 Gas Welding

It is also called as oxy-acetylene welding.

Here the mixture of gases namely

acetylene and oxygen is burned at the tip

of a torch, which produces a very hot

flame. This heats the metal pieces for

cutting and welding process. The features

of a typical gas welding is shown in Fig. 1.

Fig. 1 Gas Welding2.2 Arc Welding

Arc welding is used in most of the structural welding operations. Here, electric

energy which is used as the heat source is produced by electric arc. The base

metal and welding rod (or electrode) are heated to fusion temperature by the

electric arc. The typical arc welding equipments and accessories are shown in

Fig. 2. The welding rod is connected to one terminal of the current source and

the object to be welded is connected to the other terminal. The temperature in

the region of welding 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 popular method of arc welding. Heating is done by means of

electric arc between a coated electrode and the material being joined. If

uncoated or bare wire electrodes are used, the molten metal gets exposed to

atmosphere and combines chemically with oxygen and nitrogen and forms

defective welds. The coating on the electrode forms a gaseous shield that helps

to exclude oxygen and protects the molten metal from oxidation. The flux of the

electrode coating, being lighter than molten metal, hardens at the surface of the

weld. This can be removed by gentle tapping or by brushing. The typical

features of shielded metal arc welding are shown in Fig. 3.

Fig. 3 Shielded metal arc welding

The type of welding electrode used decides the weld properties like

strength, ductility and corrosion resistance. The choice of electrode

depends upon the type of metal being welded, the amount of material to

be added, and the position of work.

There are two types of electrodes:

• Lightly coated electrodes

• Heavily coated electrodes

Heavily coated electrodes are used in structural welding. These electrodes

result in welds that are 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 properties of

electrodes are listed in Table 1 of IS 800 : 2007 which is as per IS 814 : 2004.

3.0 ADVANTAGES OF WELDING

There are many advantages of welding. Some of the important ones are

as follows.

1. Welded joints are aesthetical to look when compared to bolted joints.

2. Welded joints are more rigid than bolted joints. Hence the material at

various sections are utilized more efficiently to resist stresses than that

of less rigid connections.

3. Welding does not require driving of holes. This reduces the cost

incurred for drilling. Hence, while computing the tensile strength of

members, the net area remains the same as the gross area.

4. Welded joints are well suited for liquid and gas containing structures.

5. Welding offers the possibilities of fabricating new sections like

castellated beams or creating complex joints in tubular truss.

4.0 DISADVANTAGES OF WELDING

Some of the disadvantages of welding are;

1. Welding requires greater skill than bolting and hence requires highly

skilled human resources.

2. Improper welding will distort the members and its alignment, and hence requires more concentration.

3. The inspection of weld joints is more difficult and cumbersome than

bolted joints.

4. The process of welding may leave a higher residual stress in the material.

5. Welding equipment is more expensive and requires larger initial

investment.

6. Welding at site is more difficult and also requires constant power supply.

5.0 TYPES OF WELDS

There are four types of welds. They are:

1. Fillet welds

2. Groove welds

3. Slot welds, and

4. Plug welds

Of these welds, fillet is used to a large extent. Groove welds are used to

a lesser extent. However, slot and plug welds are rarely used.

5.1 Fillet Welds

Fig. 4 Fillet weld and its cross section

Fillet welds (Fig. 4A) are widely used due to their economy, ease of fabrication,

and adoptability at site. They are approximately triangular in cross section (Fig.

4). These welds require less precision in ‘fitting up’ two sections, due to the

overlapping of pieces. They are adopted in field as well as in shop welding. It

does not require any edge preparation and 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

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 the same plane, such as in butt

joints. The details of a typical groove weld are shown in Fig. 6.

Fig. 6 Details of typical groove weld

Some of the commonly used groove welds in butt 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 in T-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. Partial penetration groove welds should not be

used especially in fatigue situations. Root opening or gap (see Fig. 6) is

provided for the electrode to access the base of the joint. The bevel angle

(see Fig. 6) for typical root openings is shown in Table 2.

Table 2 Root openings and bevel angle for groove weld

Root openings Bevel angle

3 mm 60O

6 mm 45O

9 mm 30O

Weld metal is more expensive than the base metal. Hence, the choice

between single or double penetration depends on the availability of

access on both sides, the thickness of plate to be welded, the type of

welding equipment available and the position of weld.

When the plate thickness is more than 12 mm, the groove can be either

double-bevel or double-V type. When the plate thickness is more than 40

mm, the groove can be either double - U or double - J type. For plates

between 12 to 40 mm, the groove can be wither single-J and single-U type.

Groove welds are chosen in situations where the members need to transmit

the full load of the members they join. Hence, the strength of welds should

be more than or equal to the strength of the members they join. To ensure

this, full penetration groove welds are used more frequently.

5.3 SLOT AND PLUG WELDS

Slot and Plug welds (Fig. 8) are limitedly used to connect the steel

members. They are generally used to complement the fillet welds in

situations where it is not possible to provide sufficient length of fillet welds

due to some constraints. These welds fail in shear. The extent of

penetration 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 of design strength of slot

or plug welds are similar to that of fillet welds.

Fig. 8 Typical slot and plug welds

6.0 WELDING POSITON

The welding positions 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 welding

operation.

a) Flat - On the floor b) Overhead - Under the roof

c) Vertical - On the wall d) Horizontal - On the wall

Fig. 9 Different weld positions

7.0 TYPES OF JOINTS

There are five basic types of common joints. They are

• Butt joint

• Lap joint

• T – joint

• Corner joint, and

• Edge joint

Each joint is suitable for a specific situation. The choice of the joint for a

particular job 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 of various types of welds.

7.1 Butt Joint

Fig. 10 Typical butt joint

Butt joints are used to join the ends of flat plates of nearly equal thickness. A

typical butt joint is shown in Fig. 10. This joint avoids eccentric transfer of

force at the connection. It is preferable to have full penetration of welds at

the butt joints so that the joint is fully efficient. The size of connection is quite

small and hence is very economical. It is aesthetical to look at. Face

reinforcement (weld beyond the surface) is normally provided in Butt joints.

This increases the efficiency of the joint and ensures that depth of weld is at

least equal to the thickness of the plate.

7.2 Lap Joint

Fig. 11 Typical lap joint

Lap joints are easy to fit and join any two members. A typical lap joint is

shown in Fig. 11. It is the most commonly used joint. It does not require

any special preparation. Lap joints utilize fillet welds. They are well suited

for shop and field welding. Lap joints can accommodate minor errors in

fabrication and minor adjustment in length. The main advantage of lap

joints is that it can join plates with different thicknesses without any

difficulty (Fig. 12). The main disadvantage of this joint is that it introduces

eccentric transfer of loads at the connection.

Fig. 12 Lap joint with plates of different thickness

7.3 T – Joints

Fig. 13 Typical T - joint

A T-joint is usually used to fabricate built-up sections from simple

members. A typical 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 Corner Joint

Corner joints are normally seen in built - up rectangular box sections. A

typical corner joint is shown in Fig. 14. They are generally seen at places

which are subjected to high torsional moments.

Fig. 14 Typical corner joint

Corner joints are seen in the built - up rectangular box sections. A typical

corner joint is shown in Fig. 14. They are commonly seen at places which

are subjected to high torsional moments.

7.5 Edge Joint

Fig. 15 Typical corner joint

Edge joints are generally not used in structural applications. They are

used to keep two or more plates in position in a given plane. A typical

edge joint is shown in Fig. 15.

8.0 WELD SYMBOL AND WELDING SYMBOL

A weld symbol is a symbol which indicates the type of weld to be adopted

to joint the metal pieces. However, a welding symbol is a concise way of

describing all particular information related to the weld on drawings.

8.1 Weld Symbol

Weld symbol is unique for each specific type of weld. Hence the weld

symbol used is different for fillet, groove, plug, and slot welds.

8.1.1 Basic Weld Symbols

The basic weld symbols for the commonly used welds are shown in Table 3.

Table 3 Basic weld symbols

Weld symbol is only a part of the information regarding the welding

operation to be performed at a joint. As indicated in Table 3, whenever a

weld symbol consists of both vertical and inclined legs, the vertical is

always drawn towards the left side of the inclined line.

8.2 Welding Symbol

The welding symbol contains all the information necessary in connection

with a welding operation. It also includes the type of weld, where welds

are to be located, the type of joint to be used, and the size and amount of

weld metal to be deposited in the joint.

The symbols used are standardized by the various codes of practice so

that the entire information can be concisely represented in a drawing.

The basic welding symbol comprises of three parts, namely

• a reference line• an arrow, and• a tail

Apart from this there are also supplementary welding symbols to represent

• Dimensions and other data• Supplementary symbols• 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 this line.

• Arrow

The arrow line is present at one end of the reference line. It simply connects

one end of the reference line to the joint or area to be welded. The direction of

the arrow has no bearing on the significance of the reference line. Some of the

possible types of arrows used in the welding symbol are shown in Fig. 17.

Fig. 17 Different types of arrows used in welding symbol

• Tail

The tail is shown on the other end (away from arrow end) of the reference line.

It is not mandatory. The tail is used to specify a certain welding process. It is

used only when necessary. It is used to mention some special characteristic of

the weld like type of electrode, some type of reference or specification, welding

or cutting process, procedures or other supplementary information. If additional

information is not needed, then the tail will be omitted. The representation of tai

with additional information is 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

Fig. 19 Fillet weld, Arrow side

• Fillet weld – on other side

Symbol Meaning

Fig. 20 Fillet weld, Other side

• Fillet weld – on both sides

Symbol Meaning

Fig. 21 Fillet weld, Both sides

• Bevel edge

Symbol Meaning

The symbol indicates that one edge of a joint is to be beveled. The arrow should points towards the member to be beveled. Hence, the arrow should show a definite break so that the member to be beveled can be clearly identified.

Fig. 22 Bevel edge

8.2.3 Standard Location of Elements of a Welding Symbol

The standard location of various elements of a welding symbol to be

indicated in drawing is summarized below.

Fig. 23 Standard location of welding symbols

8.3 Supplementary Symbols

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 Table 4.

Table 4 Supplementary weld symbols

8.3.1 Finish and contour symbols

Finish symbol shows the method of finish to be carried out to a weld.

Generally, the finish of welding is either by chipping (C) or by machining

(M) or by grinding (G). Contour symbols are used with weld symbols to

show how the face of the weld is to be formed. The face of the weld will

be either flat, convex or concave as shown in Table 4.

8.3.2 All round weld

Fig. 24 Typical representation of all round weld

The all round symbol (Table 4) indicates that the welds are continued all

around the joint. A typical all round weld and its representation is shown

in Fig. 24.

8.3.3 Field or site weld

Fig. 25 Representation of field or site weld

The symbol used for field or site weld is a flag (Table 4). It points toward the

tail of welding symbol. If no symbol is present, it indicates the weld as shop

weld. A typical representation of field or site weld is shown in Fig. 25.

9.0 WELD DEFECTS

As mentioned earlier, welding requires greater skill so that the defects

can be avoided. Some of the commonly observed defects in welds are;

1. Incomplete fusion

2. Incomplete penetration

3. Porosity

4. Undercutting

5. Inclusion of slag

6. Cracks

7. Lamellar tearing

1. Incomplete fusion

This occurs when the surfaces have not been cleaned properly, 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 incomplete fusion.

2. Incomplete penetration

Incomplete penetration can be due to improper grooves or unsuitable groove

design made for the welding process. This can also be a result of the usage of

large size electrodes, insufficient welding current, and excessive welding 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, longer arc length, poor

welding procedures, and careless use of back-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, it results in undercutting of the base metal (i.e., the thickness of

the base metal will be less in that region). This may lead to places of stress

concentrations during the process of force transfer and can be dangerous.

Hence to eliminate undercutting, it is mandatory to have face reinforcement

(welding over the surface of base metal) in all groove joints.

5. Inclusion of slag

Slag is formed from the coating of the electrodes which are used to shield

the molten material from oxides during the cooling process. The slag is

generally removed after the weld cools by either wire brushing or by gentle

tapping. If the process of cooling is done rapidly, the slag gets trapped inside

the weld. This weakens the weld strength and is not desirable. When the

required weld thickness is large, it is made by several passes. In such

cases, the slag should be removed after the completion of each pass. If this

is not done properly, it also results in the inclusion of slag.

5. Cracks

Cracks are the most serious weld defects since it reduces the weld strength

directly. This results mostly 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 or present

inside the weld. They can be avoided by using good quality electrodes,

adopting uniform rate of welding and ensuring slower cooling periods.

6. Lamellar tearing

This is the formation of cracks beneath the weld. The high temperature during

welding causes large relative strains in the base metal due to localised stresses

and results in tearing of the base metal. This can be prevented by choosing

proper welding techniques and adopting uniform rate of welding.

10.0 WELD DISTORTIONS

During welding operation, proper care must be ensured to avoid weld

distortions. Improper welding techniques results in weld distortions. Some of

the commonly observed distortions during welding are Transverse

shrinkage, Longitudinal shrinkage, Angular change, Rotational distortion,

Longitudinal bending distortion and Bucking distortion. These distortions

are shown in Fig. 26.

a) Transverse shrinkage b) Longitudinal shrinkage

c) Angular change d) Rotational distortion

e) Longitudinal bending distortion f) Buckling distortion

Fig. 26 Typical weld distortions

11.0 IMPORTANT CODAL PROVISIONS OF IS 800 : 2007 WITH

RESPECT TO WELDING

The welds and welding shall conform to the following codes:

• IS 816 : 1969 – Code of practice for the use of metal arc welding for

general construction in mild steel

• IS 9595 : 1996 – Metal arc welding of carbon and carbon manganese steels

Cl. 10.5.1.1 End Returns

>2a a = weldsize

>2a

Fig. 27 Details of end returns of weld

Fillet welds terminating at the ends or sides of parts should be returned

continuously around the corners for distance of not less than twice the

size of the weld, unless it is impractical. This is particularly important on

the tension end of parts carrying bending loads.

Cl. 10.5.1.2 Lap Joint

Fig. 28 Typical lap joint

In lap joints the minimum lap should be not less than four times the thickness of

the thinner part joined. Single end fillet should be used only when lapped parts

are restrained from openings. When end of an element is connected only

by parallel longitudinal fillet welds, the length of the weld along either edge

should be not less than the transverse spacing between longitudinal welds.

Cl. 10.5.2 Size of weld

10.5.2.1 The size of normal fillets shall be taken as the minimum weld leg

size. For deep penetration welds, where the depth of penetration beyond

the root run is 2.4 mm (minimum), the size of the fillet should be taken as

the minimum leg size plus 2.4 mm.

Fig. 29 Size of weld

10.5.2.2 For fillet welds made by semi-automatic or automatic processes,

where the depth of penetration is considerably in excess of 2.4 mm, the

size shall be taken considering actual depth of penetration subject to

agreement between the purchaser and the contractor.

10.5.2.3 The size of fillet welds shall not be less than 3 mm. The

minimum size of the first run or of a single run fillet weld shall be as given

in Table 5, to avoid the risk of cracking in the absence of preheating.

Table 5 Minimum weld size

Thickness of Thicker Part Minimum SizemmOver, mm Upto and including, mm

-- 10 3

10 20 5

20 32 6

32 50 8 for first run10 for min. size of weld

10.5.2.4 The size of butt weld shall be specified by the effective throat

thickness.

Fig. 30 Typical butt joint

Cl. 10.5.3 Effective Throat Thickness, te

10.5.3.1

Fig. 31 Effective throat thickness of fillet weld

The effective throat thickness te (Fig. 31) of a fillet weld shall not be less than 3

mm and shall generally not exceed 0.7t, and 1.0t under special circumstances,

where t is the thickness of the thinner plate of elements being welded.

10.5.3.2 For the purpose of stress calculation in fillet welds joining faces

inclined to each other, the effective throat thickness shall be taken as K

times the fillet size, where K is a constant, depending upon the angle

between fusion faces, as given in Table 6.

Table 6 Values of k for different angles between fusion faces

Angle between fusion faces, Constant K

600– 900 0.70

910 – 1000 0.65

1010– 1060 0.601070– 1130 0.55

1140– 1200 0.50

10.5.3.3 The effective throat thickness te (Fig. 32) of a complete

penetration butt weld shall be taken as the thickness of the thinner part

joined, and that of an incomplete penetration butt weld te (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 The effective length of fillet weld shall be taken as only that length

which is of the specified size and required throat thickness. In practice the

actual length of weld is made of the effective length shown in drawing plus two

times the weld size, but it should not be less than four times the size of

the weld.

10.5.4.2 The effective length of butt weld shall be taken as the length of

the continuous full size weld, but it should not be less than four times the

size of the weld.

Cl. 10.5.5 Intermittent Welds

10.5.5.1 The intermittent fillet welding shall have an effective length of not

less than four times the weld size, with a minimum of 40 mm, except as

otherwise specified.

10.5.5.2 The clear spacing between the effective lengths of intermittent

fillet weld shall not exceed 12 and 16 times the thickness of thinner plate

joined, for compression and tension joint respectively, and in no case be

more than 200 mm.

10.5.5.3 The intermittent butt weld shall have an effective length of not less

than four times the weld size and the longitudinal space between the effective

length of welds shall not more than 16 times the thickness of the thinner part

joined, except as otherwise specified. The intermittent welds shall not be used

in positions subject to dynamic, repetitive and alternate stresses.

Cl. 10.5.7 Design Stresses in Welds

10.5.7.1 Shop welds

10.5.7.1.1 Fillet welds − The design strength of a fillet weld, fwd, shall be

based on its throat area.

fwd = fwn / γmw in which fwn = fu / √ 3

where

fu = smaller of the ultimate stress of the weld and the parent metal

γmw = 1.25 = partial safety factor

10.5.7.1.2 Butt welds − Butt welds shall be treated as parent metal with a

thickness equal to the throat thickness, and the stresses shall not exceed

those permitted in the parent metal.

10.5.7.2 Site Welds − The design strength in shear and tension for site welds made during erection of structural members shall be calculated as

per 10.5.7.1 but using a partial safety factor γmw of 1.5.

Cl. 10.5.8.1 Fillet Weld Applied to Edge of a Plate

• Square edge

10.5.8.1 Where a fillet weld is applied to the square edge of a part, the specified

size of the weld should generally be at least 1.5 mm less than the edge

thickness in order to avoid washing down of the exposed arris (Fig. 33).

1.5 mm

a

Fig. 33 Fillet weld on square edge of plate

10.5.8.2 Where the fillet weld is applied to the rounded toe of a rolled

section, the specified size of the weld should generally not exceed 3/4 of

the thickness of the section at the toe (Fig. 34).

1/4 tt

Fig. 34 Fillet weld on round toe of rolled section

10.5.8.3 Where the size specified for a fillet weld is 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)

Fig. 35 Full size fillet weld applied to the edge of a plate or section

10.5.8.4 When fillet welds are applied to the edges of a plate or section in

members subject to dynamic loading, the fillet weld shall be of full size,

that is, with its leg length equal to the thickness of the plate or section,

with the limitations enumerated in 10.5.8.3.

Fig. 36 End fillet weld normal to direction of force

10.5.8.5 End fillet weld normal to the direction of force shall be of unequal

size with a throat thickness not less than 0.5t where t is the thickness of

the part as shown in Fig. 36. The difference in thickness of the welds

shall be negotiated at a uniform slope.

Cl. 10.5.9 Stresses Due to Individual forces

When subjected to either compressive or tensile or shear force alone, the

stress in the weld is given by:

fa or q =

where

Ptt lw

fa = calculated normal stress due to axial force in N/mm2 q = shear stress in N/mm2

P = force transmitted (axial force N or the shear force Q)

tt = effective throat thickness of weld in mm

lw= effective length of weld in mm

Cl. 10.5.10 Combination of stresses

10.5.10.1 Fillet Welds

10.5.10.1.1 When subjected to a combination of normal and shear stress,

the equivalent stress fe shall satisfy the following

f u2 2f e f a 3 q <

3γ mw

where

fa = normal stresses, compression or tension, due to axial force or bending moment (10.5.9), and

q = shear stress due to shear force or tension (10.5.9)

10.5.10.2.2 Combined bearing, bending and shear - Where bearing stress, fbr is

combined with bending (tensile or compressive) and shear stresses under the

most unfavorable conditions of loading, the equivalent stress fe is

obtained from the following formulae:

2 2

f br 3q 2f e f b f br f bwhere

fe = equivalent stress

fb = calculated stress due to bending in N/mm2

fbr =calculated stress due to bearing in N/mm2, and q = shear stress in N/mm2

Cl. 10.8 Intersections

Members or components meeting at a joint shall be arranged to transfer

the design actions between the parts, wherever practicable, with their

centroidal axes meeting at a point. Where there is eccentricity at joints,

the members and components shall be designed for the design bending

moments, which result due to eccentricity.

The disposition of fillet welds to balance the design actions about the

centroidal axis or axes for end connections of single angle, double angle and

similar type members is not required for statically loaded members but is

required for members, connection components subject to fatigue loading.

Eccentricity between the centroidal axes of angle 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 fatigue loading.

Cl. 10.11 Analysis of a Bolt/Weld Group

10.11.1 Bolt/Weld Group Subject to In-plane Loading

10.11.1.1 General Method of Analysis − The design force in a bolt/weld in a

bolt/weld group or design force per unit length in a bolt/weld group subject to

in-plane loading shall be determined in accordance with the following:

a) The connection plates shall be considered to be rigid and to rotate

relative to each other about a point known as the instantaneous centre

of rotation of the group.

b) In the case of a group subject to a pure couple only (Fig. 37a), the

instantaneous centre of rotation coincides 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 is

uniformly distributed throughout the group. In all other cases (Fig. 37c),

either the results of independent analyses for a pure couple alone and for

an in-plane shear force applied at the group centroid shall be

superposed, or a recognized method of analysis shall be used.

c) The design force in a bolt or design force per unit length at any point in

the group shall be assumed to act at right angles to the radius from

that point to the instantaneous centre, and shall be taken as

proportional to that radius.

a) Pure couple at b) In-plane shear force c) In-plane shear forcecentroid at centroid away 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-Plane Loading

10.11.2.1 General Method of Analysis −The design force of a bolt in bolt

group or design force per unit length in the fillet weld group subject to out-of-

plane loading (Fig. 38) shall be determined in accordance with the following:

a) The design force in the bolts per unit length in the fillet weld group

resulting from any shear force or axial force shall be considered to be

equally shared by all bolts in the group or uniformly distributed 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.

i) In bearing type of bolt group plates in the compression side of the neutral

axis and only bolts in the tension side of the neutral axis may be

considered for calculating the neutral axis and second moment of area.

ii) In the friction grip bolt group only the bolts shall be considered in the calculation of neutral axis and second moment of area.

iii) The fillet weld group shall be considered in isolation from the

connected element; for the calculation of centroid 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 weld plane

10.11.2.2 Alternative Analysis − The design force per unit length in a fillet

weld/bolt group may alternatively 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 equilibrium

between the fillet weld group and the elements of the connected member.

12.0 Numerical Problems

1. Two plates of size 200 x 10 mm and 200 x 8 mm are connected by a weld groove having (i) Single

– V groove weld joint, and (ii) Double – V groove weld joint. Determine the

maximum tension which the joints can resist. The steel plates are of grade Fe

410 grade with yield strength of 250 MPa. Assume shop welding.

Solution

Case (i) : Single – V groove weld (Fig. 39)

In this case, incomplete penetration results due to single – V groove.

Fig. 39 Single V groove weld

Single V is an incomplete penetration welding. Hence the throat thickness is

5/8th of the thickness of thinner plate.

te = 5/8 t = (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 / γ mw Cl. 6.2

= 200 x 5 x 200/1.25 = 200,000 N = 200 kN

Case (ii) : Double – V groove weld (Fig. 40)

In this case, complete penetration results due to Double – V groove.

Fig. 40 Double V groove weld

Effective throat thickness is 8mm which is the thickness of the thinner plate.

te = 8 mm

Strength of weld, P = L x tt x fy / γ mw Cl. 6.2

= 200 x 8 x 200/1.25 = 320,000 N = 320 kN

2. Find the size and length of the fillet weld for the lap joint to transmit a factored

load of 120 kN as shown in Fig. 41. Assume site welds, Fe 410 grade steel and

E41 electrode. Assume width of plate as 75 mm and thickness as 8 mm.

Fig. 41 Lap joint connection

Solution

Minimum size of weld for 8 mm thick section = 3 mm (Table 5, Cl. 10.5.2.3)

Maximum size of weld = 8 – 1.5 = 6.5 mm (Cl. 10.5.8.1)

Choose the size of weld, a = 6 mm

Effective throat thickness = te = 0.70 a = 4.2 mm

Strength of 6 mm weld / mm length = 4.2 x 410 / (√3 x 1.5) Cl. 10.5.7.1.1

= 662.7 N/mm

Assuming only two longitudinal welds along the sides

Required length of weld = 120 x 103 /662.7 = 181 mm

Length to be provided on each side = 181/2 = 90.5 mm

>75 mm (width of plate)

Hence, provide 90.5 mm weld on each side with an end return of 2x 6 = 12

mm Overall length of the weld provided = 2 x (90.5 + 2 x 6) = 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 four sides (Fig. 44), determine the strength of the joint. Also find the percentage

increase in the strength. Use Fe 410 steel with yield stress 250 MPa. Assume shop welding.

Fig. 42 Lap joint connection with weld on three sides

Solution

Case (i) : Welding on three sides (Fig. 42)

Lw = 300 + 200 + 300 = 800 mm

Design strength of fillet weld joint, P1 = 0.7a fwd Lw / γ mw Cl. 10.5.7.1.1

= 0.7 x 6 x (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. 43)

Fig. 43 Lap joint connection with weld on four sides

Lw = 300 + 200 + 300 + 200 = 1000 mm

Design strength of fillet weld joint, P2 = 0.7a fwd Lw / γ mw Cl. 10.5.7.1.1

= 0.7 x 6 x (410 /√3) x 1000 / 1.25

= 7,95,358 N

Hence, allowable load = 7,95,358 / 1.5 = 5,30,239 N

Percentage increase in strength = (P2 – P1) / P1 x 100 = 25 %

4. Determine the size of the weld required for the bracket connection shown in Fig. 44. Assume shop welding.

Solution

Eccentricity, e = 100 + 75 = 175 mm

Torsional moment , M = 60 x 175 = 10,500 kN-mm

Considering unit thickness of weld 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 = M x rmax / Ip

= 426 MPa

Resultant stress in the critical part of the weld / mm width

R = (f12 + f22 + 2 f1 f2 cos ) 0.5 = 530 MPa

Strength of the weld per unit length and thickness, P

P = 0.7a fwd Lw / γmw

= 0.7 a (410/√3) 1 / 1.25 = 132.6 a N

Equating the resultant, R with the strength of weld, P (i.e., R = P)

a = 3.99 mm

Size of weld to be provided = 4mm

5. Determine the 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 / (2 x 200 x 1)

= 0.0025 V N/mm2

Moment on weld, M = 200 V N-mm

Considering unit thickness of weld at root,

Moment of inertia Iz = 2 (1x 2003) / 12 = 1.33 x 106 mm4

Fig. 45 Bracket connection showing load out of plane

Normal stress due to bending in tension, fa = M y / Iz

Here, M = 200 V N-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)2 2

For 6 mm fillet weld (field weld)

Strength of weld/mm , fwd =0.7 x 6 (410 / √3) / 1.50 = 662.8 N/mm

Equating the equivalent stress, fe with the strength of 6 mm weld, fwd

fe = fwd

0.0156 V = 662.8

Hence, V = 42487 N = 42.5 kN

For the current bracket a load of 42.5 kN can be safely applied.