Welded Connection Introduction | Design of Steel Structures

WELDED CONNECTIONS

Advantages

1. Welded structures are lighter due to absence of gusset plates, connecting angle, etc.

2. Absence of making holes for bolts, etc.

3. More adoptable and 100% efficiency can be achieved

4. Good aesthetic appearance

5. Less noise

6. Alterations in connections can be easily made

7. No problem of mismatching of holes.

Disadvantages

1. Needs skilled workers

2. Not good to resist fatigue stresses

3. Likely to get distorted due to uneven heating and cooling

4. Inspection of joints is difficult and expensive

5. Proper welding in the field is difficult

Types of Welded Joints

* Butt welds Square, V, U, J welds, may be single or double.

* Fillet welds Approximately triangular connections when the cross section of fillet weld is isoceles

triangle with face at 45°, it is known as a standard fillet weld. In this case t = = 0.7s where s is

size and t is throat thickness.

* Slot weld and plug weld To make slot weld a circular hole is made in one plate, kept on another

plate and fillet welding is made along the periphery of the hole.

Plug weld is made by making small holes in one of the plates to be connected, and then weld

material is filled in the holes after keeping the plate on the other plate to be connected.

Important Specifications for Welding

Butt weld It is specified by throat thickness

1. In case of double U, double V, double J and double level butt welds, the thickness of weld, is

taken as complete penetration of butt weld and hence it is equal to the thickness of thinner plate

2. In case of single U, single V, single J and single level, it is taken as × thickness of thinner plate

connected.

3. Minimum weld length shall be 4 × the size of weld.

4. If intermittent butt welding is used, it shall have an effective length of not less than 4 × the weld

size and space between the consecutive weld shall not be more than 16 times the thickness of

thinner plate joined.

Fillet Weld

1. The size of normal fillet weld shall be taken as the minimum leg size.

2. Minimum size of s, specified are:

For less than 3 mm plates 3 mm

For 10 to 20 mm plates 5 mm

For 20 to 32 mm plates 6 mm

For 32 to 50 mm plates 8 mm

3. Effective throat thickness shall be ks where s is size and k is constant, depending upon angle

between fusion faces as given below

Angle of fusion face 60° – 90° 91° – 100° 101° – 106° 107° – 10° 114° to 120°

k 0.7 0.65 0.60 0.55 0.5

4. In the drawing only effective lengths are shown. It is to be noted that actual length shall be equal to

the effective length + twice the size of weld.

5. Lap joint shall be a minimum of 4 times the thickness of thinner member or 40 mm which-ever is

more.

6. If intermittent weld is used it shall not be less than 4 times the weld size or 40 mm whichever is

more.

Plug weld The effective area of a plug weld shall be considered the nominal area of the hole.

* Design stresses

1. In butt weld, same as that in parent metal.

2. In fillet, slot or plug welds fwd =

where fu = smaller of ultimate stress in weld or of the parent metal

gmw = 1.25 for shop welds

= 1.5 for field welds

The following provisions are made in the code :

(i) If fillet weld is to the square edge,

max size = t – 1.5 mm

(ii) If fillet weld is to the rounded toe of rolled section

s = × thickness of section at toe

(iii) Reduction in design stresses for long joints:

If the length of welded joint l

j

, is greater than 150 t, where t is throat thickness, the design capacity

of weld fwd shall be reduced by the factor

(iv) Combined axial and shear stress

The equivalent stress fe shall satisfy the following

where fa = axial stress

q = shear stress

\ If eccentric connection gives rise to moment at right angle to the plane of weld.

Direct shear stress

Bending stress

Combined axial and shear stresses on welds

Take trial depth take h = 1.1 h¢ where