Design of welded joints problems

1. Design of Riveted and
Welded joints
Syllabus:
Riveted & Welded joints: Boiler Joints &
Lozenge Joint, Design of riveted joints under
eccentric loading and eccentrically loaded
welded joints
Unit-III

2. The joints used in mechanical assemblies are
classified into two groups-permanent and separable.
Permanent joints are those joints which cannot be
disassembled without damaging the assembled
parts. Riveted and welded joints are permanent joints.
Separable joints are those joints which permit
disassembly and reassembly without damaging the
assembled parts. Bolted joints, cotter joints and
splined connections are the examples of separable
joints.

3. Welding can be defined as a process of
joining metallic parts by heating to a
suitable temperature with or without the
application of pressure. Welding is an
economical and efficient method for
obtaining a permanent joint of metallic
parts.

4. Advantages of Welded joints over
Riveted Joints
Welded joints offer the following advantages compared with
riveted joints:
(i)Riveted joints require additional cover plates, gusset
plates, straps, clip angles and a large number of rivets,
which increase the weight. Since there are no such
additional parts, welded assembly results in lightweight
construction. Welded steel structures are lighter than the
corresponding iron castings by 50% and steel castings by
30%.
(ii) Due to the elimination of these components, the cost of
welded assembly is lower than that of riveted joints.

5. (iii) The design of welded assemblies can be easily and
economically modified to meet the changing product
requirements. Alterations and additions can be easily made
in the existing structure by welding.
(iv) Welded assemblies are tight and leakproof as
compared with riveted assemblies.
(v) The production time is less for welded assemblies.
(vi) When two parts are joined by the riveting method, holes
are drilled in the parts to accommodate the rivets. The
holes reduce the cross-sectional area of the members and
result in stress concentration. There is no such problem in
welded connections.

6. (vii) A welded structure has smooth and pleasant
appearance. The projection of rivet head adversely affects
the appearance of the riveted structure.
(viii) The strength of welded joint is high. Very often, the
strength of the weld is more than the strength of the plates
that are joined together.
(ix) Machine components of certain shape, such as circular
steel pipes, find difficulty in riveting. However, they can be
easily welded.

7. Disadvantages of Welded
joints over Riveted Joints
Welded joints have the following disadvantages:
(i)As compared with cast iron structures, the capacity of
welded structure to damp vibrations is poor.
(ii) Welding results in a thermal distortion of the parts,
thereby inducing residual stresses. In many cases, stress-
relieving heat treatment is required to relieve residual
stresses. Riveted or cast structures do not require such
stressrelieving treatment.

8. (iii) The quality and the strength of the welded joint depend
upon the skill of the welder. It is difficult to control the
quality when a number of welders are involved.
(iv) The inspection of the welded joint is more specialised
and costly compared with the inspection of riveted or cast
structures.

9. BUTT JOINTS
Welded joints are divided into two groups-butt
joints and fillet joints.
A butt joint can be defined as a joint between two
components lying approximately in the same
plane. A butt joint connects the ends of the two
plates. The types of butt joints are illustrated in Fig.
The selection of the types of butt joints depends
upon the plate thickness and the reliability. Some
guidelines are as follows:

11. FILLET/LAP JOINTS
A fillet joint, also called a lap joint, is a joint
between two overlapping plates or components. A
fillet weld consists of an approximately triangular
cross-section joining two surfaces at right angles
to each other. There are two types of fillet joints -
transverse and parallel

12. Types of Welded Joints

13. A fillet weld is called transverse, if the direction of the weld
is perpendicular to the direction of the force acting on the
joint. It is shown in Fig. (a) and (b).

14. A fillet weld is called parallel or longitudinal, if the
direction of weld is parallel to the direction of the force
acting on the joint. It is shown in Fig. (c).

15. There are two types of cross-sections for fillet weld -
normal and convex, as shown in Fig. The normal weld
consists of an isosceles triangle - a triangle having two
equal sides. It is shown in Fig. (a).
A convex weld is shown in Fig. (b). A convex weld requires
more filler material and more labour. There is more stress
concentration in a convex weld compared to a triangular
weld.
Therefore, normal weld is preferred over convex weld.

17. Strength of Transverse Fillet Welded
Joints
The transverse fillet welds are designed for tensile
strength. Let us consider a single and double transverse
fillet welds as shown in Fig. (a) and (b) respectively.

20. If σt is the allowable tensile stress for the weld metal, then
the tensile strength of the joint for single fillet weld,
P =Throat area × Allowable tensile stress = 0.707 s × l × σt
and tensile strength of the joint for double fillet weld,
P = 2 × 0.707 s × l × σt = 1.414 s × l × σt
Note: Since the weld is weaker than the plate due to slag
and blow holes, therefore the weld is given a reinforcement
which may be taken as 10% of the plate thickness.

21. Strength of Parallel Fillet Welded Joints

22. The parallel fillet welded joints are designed for shear
strength. Consider a double parallel fillet welded joint as
shown in Fig. (a).
We know that the minimum area of weld or the throat area,
A = 0.707 s × l
If is the allowable shear stress for the weld metal, then
the shear strength of the joint for single parallel fillet weld,
P=Throat area × Allowable shear stress
= 0.707 s × l ×
and shear strength of the joint for double parallel fillet weld,
P = 2 × 0.707 × s × l × τ = 1.414 s × l × τ

23. If there is a combination of single transverse and double
parallel fillet welds as shown in Fig. (b), then the strength of
the joint is given by the sum of strengths of single
transverse and double parallel fillet welds.
Mathematically,
P = 0.707s × l1 × σt + 1.414 s × l2 × τ
where l1 is normally the width of the plate.
In order to allow for starting and stopping of the bead, 12.5
mm should be added to the length of each weld obtained
by the above expression.
For reinforced fillet welds, the throat dimension may be
taken as 0.85 t.

24. Two plates are joined togther by means of fillet
welds as shown in fig. The leg dimensions of the
welds is 10 mm and permissible shear stress at
the throat cross-section is 75 Mpa. Determine
the length of each weld, if 15mm weld length is
required for starting and stopping of the weld
run.

27. A plate 75mm wide and 12.5mm thick is joined
with another plate by a single transverse fillet
weld and a double parallel fillet weld. Tensile
stress=70N/mm2 and shear stress=
56N/mm2.Find the length of each parallel fillet
weld.

28. A plate 100 mm wide and 10 mm thick is to be welded to
another plate by means of double parallel fillets. The plates
are subjected to a static load of 80 kN. Find the length of
weld if the permissible shear stress in the weld does not
exceed 55 MPa.

29. A steel plate, 80 mm wide and 10 mm thick is joined to another
steel plate by means of a single transverse and double parallel
fillet weds, as shown in fig. The strength of the welded joint
should be equal to the strength of the plates to be joined. The
permissible tensile stress and shear stress for the weld material
and the plates are 100 and 70 Mpa. Find the length of each
parallel fillet weld. Assume that the tensile force passes through
centre of gravity of these welds.