Plug and slot weld design concept

Plug and slot welds

A plug weld is made by filling a circular hole in one lapped plate with weld metal so that it fuses with the plate beneath. A slot weld is the same in an elongated (slot) hole. They transfer shear between overlapping plates when the edge fillet length is not enough, and they also stop lapped plates from buckling or separating.

   top plate  =====[ O ]=====     plug (circular hole filled)
   bottom plate ===============
   top plate  ===[ ====== ]===    slot (elongated hole)

Design concepts

• The weld is assumed to resist shear on the area of the hole/slot in the plane of the faying (contact) surface: $\text{strength}=A_w\,f_{wd}$ with $f_{wd}=\dfrac{f_u}{\sqrt3\gamma_{mw}}$, where $A_w$ is the nominal area of the hole or slot.
• The weld is not designed for tension or bending across the plate.

Requirements (IS 800:2007, slot and plug welds)

1. Hole/slot size: width of the slot or diameter of the hole not less than $3t$ ($t$ = thickness of the plate containing it) or 25 mm, whichever is greater.
2. Slot ends: corners at the enclosed ends of slots rounded with a radius not less than $1.5t$ or 12 mm, whichever is greater.
3. Spacing and edge distance: distance between the plate edge and a slot or hole, and between adjacent slots or holes, not less than $2t$ and not less than 25 mm.
4. Effective area: the nominal area of the hole or slot in the plane of the faying surface.
5. Filling: in thin plates (up to about 16 mm) the hole is filled completely; thicker plates may be filled to at least $t/2$ (not less than 16 mm).
6. Fusion: the weld must penetrate and fuse with the underlying plate; good welding preparation is required.
7. Plug/slot welds are used mainly for lapped joints, built-up members and to stiffen thin plates, and are not used alone in primary tension members.