Weld symbol
Concave contour symbol
An arc dishing back toward the reference line calls for a weld face that curves inward between its toes, easing the transition into the base metal at the cost of throat thickness.
Reads asArrow side: fillet weld, 5/16 size, concave contour.
How to read it
The concave contour symbol is an arc drawn beyond the weld symbol's face, curving back toward the reference line. It asks for a dished weld face: instead of standing proud, the surface hollows inward between the two toes, so the weld is thinnest in the middle of its face and blends smoothly into the base metal at each edge.
The reason to want that shape is fatigue. A smooth, gradual transition where the weld meets the base metal spreads stress out instead of concentrating it at a sharp toe angle. On joints that see repeated loading, a concave face can extend life substantially compared with the same weld left with a heavy crown, which is why the symbol shows up on cyclically loaded details.
The cost is throat. A fillet's effective throat is measured from the root to its face, and dishing that face inward pulls the throat down. A concave fillet with the same leg size as a flat one carries less, so when a drawing calls for both a concave face and a specific throat, the legs have to be run larger to compensate. Reading the contour without checking the size requirement is how joints end up undersized.
Concavity also has a defect twin. A weld face that dishes because the welder ran too hot or too fast, rather than because the drawing asked for it, is a different thing entirely and often comes with undercut at the toes. The symbol asks for a controlled, smooth hollow blending cleanly into the plate, not a starved bead with sharp edges.
Where each dimension goes
- Arc dishing toward the reference line
- Calls for a weld face that curves inward between its toes rather than standing proud.
- Weld size
- Read to the left of the weld symbol as always, and often increased to compensate for the throat the hollow removes.
- Effective throat
- Reduced by concavity, because the throat is measured from the root to the dished face.
- Finish letter
- Names the method where the concave face is produced by dressing rather than by welding technique alone.
Worked examples
Each one is a drawing, the sentence it comes out as, and then the mark-by-mark account of why. Read the sheet first and say the sentence yourself before you look down.
Example 1: Concave fillets on a fatigue-loaded tee
TitleConcave fillets on a fatigue-loaded teeRefConcave contourEx1 / 2Reads as
Run 1/2 fillets both sides with the faces dished smoothly into the plate.
Which mark says so
- fillet symbolBelow the reference lineA fillet weld on the arrow side of the joint
- 1/2Left of the arrow-side symbolLeg size of the fillet
- concave contourBeyond the outer face of the arrow-side symbolThe completed face is to be concave
- fillet symbolAbove the reference lineA fillet weld on the far side of the joint
- 1/2Left of the other-side symbolLeg size of the fillet
- concave contourBeyond the outer face of the other-side symbolThe completed face is to be concave
The arc curving toward the reference line on each side calls for concave faces, and the larger leg size compensates for the throat that dishing the face removes.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: Concave face produced by grinding
TitleConcave face produced by grindingRefConcave contourEx2 / 2Reads as
Weld the fillet, then grind the face into a smooth hollow at the toes.
Which mark says so
- fillet symbolBelow the reference lineA fillet weld on the arrow side of the joint
- 3/8Left of the symbolLeg size of the fillet
- concave contourBeyond the outer face of the symbolThe completed face is to be concave
- GBeyond the contour on the symbolThe method used to reach that contour, not the finish itself
The concave arc with a G says the shape comes from grinding rather than from welding technique, which gives tighter control of the toe blend on a critical detail.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- Nothing sits above the reference line, so the far face of the joint stays bare.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Keeping the same leg size on a concave fillet and ending up with less throat than the design needs.
- Confusing a specified concave face with an accidentally starved bead, which usually brings undercut with it.
- Grinding a hollow that leaves sharp edges at the toes, which defeats the reason for the shape.
- Applying concavity to a groove weld face where reinforcement is actually required.