Saturday, March 1, 2014

Welding (Design of Steel Structure)

Welding is a method in which metals are joined together by melting them. In this post going to cover Fillet welds and Slot weld, just in brief with you.

Fillet Welds

  • Size of fillet weld. It should not be less than the minimum allowable value given in the table  in the image above. Size of the fillet weld used along tee of an angle or rounded edge of a flange should not exceed three-forth the nominal thickness of an angle of flange leg.
  • Throat of fillet weld. It is length of perpendicular from right angle corner to the hypotenuse.
Effective throat thickness = k*fillet size
Value of k depends upon angle between fusion faces. Value of k decreases with increase in angle between fusion face.
In most cases, a right angled fillet weld is used, for which k = 0.7.
  • Effective length of fillet weld. It is equal to its overall length minus twice the weld size. Effective length of a fillet weld designed to transmit the loading should not be less than four times weld size. Equal return should be made equal to twice the size of the weld.
  • Overlap. Overlap in lap joint should not be less than five times the thickness of the thinner plate as shown in the figure to right.
  • Side Fillet. In a lap joint made by a side or longitudinal fillet weld, length of each fillet weld should not be less than perpendicular distance between them; the perpendicular distance between side fillets should not exceed sixteen times thickness of the thinner part connected. 
  • Intermittent fillet weld. Any section of an intermittent fillet weld should have an effective length of not less than four time the weld size or 40 mm, whichever is greater. Clear spacing between ends or effective lengths of intermittent weld carrying stresses should not exceed 12.t for compression and 16.t for tension and in no case should be more than 20cm, 't; is the thickness of thinner part joined.
  • Permissible stress and strength of fillet weld. Permissible stress in fillet weld is 108 MPa or 1100 kgf/cm2.
Permissible stresses in shear and tension are reduced to 80% for the fillet welds made during erection. Permissible stresses are increased by 25% if wind or earthquake load are taken into account. However, size of the weld should not be less than the size required when the wind or earthquake load is considered or neglected.

Slot or Plug Weld

Following specifications are used for the design of slot or Plug welds:
  1. Width or diameter of slot should not be less than three times the thickness of the part in which slot is formed or 25 mm, whichever is greater.
  2. Corners at the enclosed ends should be rounded to a radius not less than 1.5 times the thickness of upper plate or 12 mm, whichever is greater.
  3. Distance between edges of the plates and slot or between edges of adjacent slots should not be less than twice the thickness of the upper plate.
  4. Permissible stress is taken as 108 MPa or 1100 kgf/cm2.
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