In a groove weld, what is root opening and what is throat?

• A. Groove weld fills a groove with no root opening; throat is distance along axis
• B. Groove weld fills a groove in a joint; root opening is the gap at the root; throat is the minimum distance between weld face and root, controlling strength
• C. Groove weld is the same as a fillet weld; root opening is thickness; throat is width
• D. Groove weld does not involve a root opening; throat is weld length

Answer: (B)

In groove welding, two dimensions define how the weld fuses and how strong it will be: the root opening and the throat. The root opening is the gap at the root of the joint before welding—the space between the pieces where the weld root will form. This opening matters because it affects penetration and the ability to fuse the root properly; if it’s too small you can get lack of fusion at the root, and if it’s too large you’ll need more heat and filler and may risk distortion.

The throat is the minimum distance from the weld face to the root across the weld cross-section. It’s the part of the weld that actually carries the load, so it largely determines the joint’s strength. A larger throat means a thicker effective cross-section and typically a stronger weld, within practical limits.

Other descriptions misstate the concepts: groove welds do involve a root opening, and the throat is not simply a length or width along the axis—it’s the smallest cross-sectional distance from root to face that governs strength. Groove welds are a different geometry from fillet welds, so the terms don’t map to fillet dimensions.