What is a backing bar?

• A. A bar used to align the joint on the weldment.
• B. A filler bar placed on the underside of the joint in groove welds to support root fusion (often removed after welding).
• C. A protective shield used during welding.
• D. A bar that adjusts heat input.

Answer: (B)

The backing bar is a strip placed along the underside of a groove weld to support the root fusion. It gives a solid surface for the root pass to fuse against and helps control penetration, preventing burn-through at the bottom of the weld. After welding, this bar is often removed, since it is used only to back up the weld root rather than become part of the finished joint. It’s not used for aligning the joint, not a protective shield, and not a device for changing heat input.