Weld symbol
Partial joint penetration (PJP) symbol
A groove weld deliberately not carried through the full thickness. Numbers to the left of the symbol give the depth of preparation and, in parentheses, the effective throat the design relies on.
Reads as: Arrow side: V-groove weld, 1/2 size, 3/8 effective throat, 60° groove angle.
How to read it
Partial penetration is a design decision, not a shortfall. Many joints do not need the full section fused: the load path only requires a certain depth of weld, and filling more would waste metal, add heat, and increase distortion. The symbol carries a size precisely because the depth is being controlled rather than left at full thickness.
Two numbers commonly appear to the left. The first is the depth of preparation, meaning how far the groove is cut into the member. The second, in parentheses, is the effective throat, meaning how much of that depth the design actually credits as load-carrying. Where the two are equal only one number is shown, and where the weld is not expected to fuse to the very bottom of the preparation the throat is the smaller of the two.
That gap between preparation depth and effective throat is where the notation earns its keep. A groove might be cut half an inch deep, but if the welding process cannot be relied on to fuse the last eighth of an inch at the root, the designer credits only three eighths. Reading only the first number and stopping there gives you the shop instruction but misses what the joint is designed around.
Partial penetration joints are usually examined at the surface rather than volumetrically, because there is no expectation of a fully fused cross section to compare against. The unfused land at the root is intentional. Calling it a defect is a classic error, and it is why the letters in the tail are worth reading before writing up a joint.
Where each dimension goes
- Depth of preparation (first number left of the symbol)
- How far the groove is cut into the member before welding.
- Effective throat (in parentheses)
- The portion of the depth the design credits as load-carrying, which can be less than the preparation depth.
- Groove angle
- Read exactly as on any groove weld — included angle across two prepared faces, or single-face angle on a bevel or J.
- Tail note
- Names the partial-penetration intent so nobody reads the unfused root as a defect.
Worked examples
PJP with preparation depth and throat both stated
“Cut the bevel 5/8 deep and credit the weld with a 1/2 effective throat.”
The first number is how deep the groove is machined and the parenthesised number is what the design counts on, the difference allowing for incomplete fusion at the root.
PJP from both faces
“Weld partial penetration grooves 3/8 deep from each face of the joint.”
Symbols above and below the line each carry their own depth, so the joint has weld metal at both faces and an intentional unfused land through the middle of the thickness.
Common mistakes
- Reporting the intentional unfused land at the root of a partial penetration joint as incomplete penetration.
- Reading only the preparation depth and ignoring the effective throat in parentheses.
- Welding a partial penetration joint deeper than called for on the assumption that more is better.
- Expecting volumetric inspection results to look like a full-penetration joint's.
Typical joints
Related symbols
Practise reading welds, not just symbols
The drawing tells you what the weld should be. The gauge trainer is where you find out whether you can tell what it actually is.