Design of Machine Members-I Unit-3 Lecture Notes – … · Contents: Unsymmetrical welded joints ... Eccentrically Loaded Welded Joints An eccentric load may be imposed on welded

Design of Machine Members-I

Contents: Unsymmetrical welded joints

Axially Loaded Unsymmetrical WeldedSections

Sometimes unsymmetrical sections such as angles,

flange edges are loaded axially as shown in Fig.

proportioned in such a way that the sum of resisting

axis is zero. Consider an angle section as shown in Fig.

Let la = Length of weld at the top,

lb = Length of weld at the bottom,

l = Total length of weld = la + lb

P = Axial load,

a = Distance of top weld from gravity axis,

b = Distance of bottom weld from gravity axis, and

f = Resistance offered by the weld per unit length.

Fig. Axially loaded unsymmetrical welded section

Moment of the top weld about gravity axis

And moment of the bottom weld about gravity axis

Since the sum of the moments of the weld about the gravity axis must be zero, therefore,

We know that l =

From equations (i) and (ii), we have

Lecture Notes – 34

Contents: Unsymmetrical welded joints

Axially Loaded Unsymmetrical WeldedSections

Sometimes unsymmetrical sections such as angles, channels, T-sections etc., welded on the

axially as shown in Fig. In such cases, the lengths of weld

proportioned in such a way that the sum of resisting moments of the welds about the gravity

section as shown in Fig.

= Length of weld at the top,

= Length of weld at the bottom,

= Distance of top weld from gravity axis,

= Distance of bottom weld from gravity axis, and

= Resistance offered by the weld per unit length.

Axially loaded unsymmetrical welded section

gravity axis

= la × f × a

nd moment of the bottom weld about gravity axis

= lb × f × b


la × f × a – lb × f × b = 0

or la × a = lb × b ...(i)

= la + lb ...(ii)

we have

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sections etc., welded on the

In such cases, the lengths of weld should be

moments of the welds about the gravity


Design of Machine Members-I Unit-3Lecture Notes – 34

References:

1. Machine Design - V.Bandari .

2. Machine Design – R.S. Khurmi

3. Design Data hand Book - S MD Jalaludin.


Eccentrically Loaded Welded Joints

An eccentric load may be imposed on welded joints in many ways. The stresses induced on

the joint may be of different nature or of the same nature. The induced stresses are combined

depending upon the nature of stresses.

simultaneously present in a joint

Maximum normal stress,

And Maximum shear stress,

Where σb = Bending stress, and

τ = Shear stress.

When the stresses are of the same nature, these may be combined

We shall now discuss the two cases of eccentric loading as follows:

Case 1

Consider a T-joint fixed at one end and subjected to an eccentric load

shown in Fig. 1

Let s = Size of weld,

l = Length of weld, and

t = Throat thickness.

The joint will be subjected to the following two types of stresses:

1. Direct shear stress due to the shear force

2. Bending stress due to the bending moment

We know that area at the throat,

A = Throat thickness × Length of weld

= t × l × 2 = 2

= 2 × 0.707 s × l = 1.414

Shear stress in the weld (assuming uniformly distributed),


Eccentrically Loaded Welded Joints


joint may be of different nature or of the same nature. The induced stresses are combined

upon the nature of stresses. When the shear and bending stresses are

(see case 1), then maximum stresses are as follows:

Fig.1. Eccentrically loaded welded joint

When the stresses are of the same nature, these may be combined vectorially (see case 2).

We shall now discuss the two cases of eccentric loading as follows:

joint fixed at one end and subjected to an eccentric load P at a distance

The joint will be subjected to the following two types of stresses:

Direct shear stress due to the shear force P acting at the welds, and

stress due to the bending moment P × e.

= Throat thickness × Length of weld

× 2 = 2 t × l ... (For double fillet weld)

= 1.414 s × l ... (since, t = s cos 45° = 0.707 s)

(assuming uniformly distributed),

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joint may be of different nature or of the same nature. The induced stresses are combined

When the shear and bending stresses are

(see case 1), then maximum stresses are as follows:

oaded welded joint

vectorially (see case 2).

at a distance e as


Section modulus of the weld metal through the throat,

Bending moment, M = P × e

We know that the maximum normal stress,

And maximum shear stress,

Case 2

When a welded joint is loaded eccentrically

stresses are induced:

1. Direct or primary shear stress, and

2. Shear stress due to turning moment.

Fig.2


Section modulus of the weld metal through the throat,

We know that the maximum normal stress,

When a welded joint is loaded eccentrically as shown in Fig.2, the following two types of the

Direct or primary shear stress, and

Shear stress due to turning moment.

Fig.2 eccentrically loaded welded joint.

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the following two types of the


Let P = Eccentric load,

e = Eccentricity i.e. perpendicular distance

centre of gravity (G) of the throat section

l = Length of single weld,

s = Size or leg of weld, and

t = Throat thickness.

Let two loads P1 and P2 (each equal to

weld system. The effect of load

be uniform over the entire weld length. The effect of load

moment of magnitude P × e which tends of rotate the joint about

the weld system. Due to the turning

We know that the direct or primary shear stress,

Since the shear stress produced due to the turning moment (

proportional to its radial distance from

proportional to AG (r2) and is in a direction at right angles to

Where τ2 is the shear stress at the maximum distance (

distance r. Consider a small section of the weld having area

Shear force on this small section

And turning moment of this shear force about

Total turning moment over the whole weld


. perpendicular distance between the line of action of

) of the throat section or fillets,

= Length of single weld,

= Size or leg of weld, and

(each equal to P) are introduced at the centre of gravity ‘

system. The effect of load P1 = P is to produce direct shear stress which is assumed to

the entire weld length. The effect of load P2 = P is to produce a turning

which tends of rotate the joint about the centre of gravity ‘

the weld system. Due to the turning moment, secondary shear stress is induced.

We know that the direct or primary shear stress,

Since the shear stress produced due to the turning moment (T = P × e) at any section is

to its radial distance from G, therefore stress due to P × e at the point

is in a direction at right angles to AG. In other words,

is the shear stress at the maximum distance (r2) and τ is the shear stress at any

Consider a small section of the weld having area dA at a distance r from

= τ × dA

And turning moment of this shear force about G,

Total turning moment over the whole weld area,

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between the line of action of load and

) are introduced at the centre of gravity ‘G' of the

is to produce direct shear stress which is assumed to

is to produce a turning

the centre of gravity ‘G' of

) at any section is

at the point A is

. In other words,

) and τ is the shear stress at any

from G.

Design of Machine Members-I Unit-3Lecture Notes – 35

Where J = Polar moment of inertia of the throat area about G.

�Shear stress due to the turning moment i.e. secondary shear stress,

In order to find the resultant stress, the primary and secondary shear stresses are combined

vectorially.

Resultant shear stress at A,

References:

1. Machine Design - V.Bandari .


3. Design Data hand Book - S MD Jalaludin


Problem:

A welded joint as shown in Fig. 10.24, is subjected to an eccentric load of 2 kN.

of weld, if the maximum shear stress in the weld

Problem:

A bracket carrying a load of 15 kN is to required if the allowable shear stress is not to exceed 80 MPa.


A welded joint as shown in Fig. 10.24, is subjected to an eccentric load of 2 kN.

of weld, if the maximum shear stress in the weld is 25 MPa.

A bracket carrying a load of 15 kN is to be welded as shown in Fig. Find the size of weld required if the allowable shear stress is not to exceed 80 MPa.

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A welded joint as shown in Fig. 10.24, is subjected to an eccentric load of 2 kN. Find the size

the size of weld


References:

1. Machine Design - V.Bandari


3. Design Data hand Book - S MD Jalaludin.


R.S. Khurmi

S MD Jalaludin.

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Design of Machine Members-I Unit-3 Lecture Notes – … · Contents: Unsymmetrical welded joints ... Eccentrically Loaded Welded Joints An eccentric load may be imposed on welded