Permanent backing
Applied to joint
Supplementary symbol applied to fillet weld
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Description
A supplementary symbol indicating that a permanent backing strip or bar is to be used on the root side of the joint. The backing material remains in place after welding and becomes part of the completed joint. Symbol contains 'M' for material.
In plain English
A steel strip or bar is tacked to the back of the joint before you weld. It supports the root pass and stops the weld pool from falling through. After welding, you leave it in place -- it stays there permanently as part of the joint. The symbol is a rectangle with 'M' inside it. Common on structural steel moment connections, beam splices, and anywhere you cannot backgouge or access the root side after welding. Permanent backings can create a crevice that may be a corrosion concern in some environments.
Symbol position
Rectangle with 'M' on the root side of the weld symbol (opposite side from the main weld).
Size notation
Backing material type may be noted in the tail. Thickness and width shown on the drawing.
Common uses
- Structural steel beam-to-column moment connections
- Beam splice joints
- Single-sided groove welds where root access is impossible
- Field welding of structural steel connections
