GaugePass

Weld symbol

Double fillet weld symbol

A fillet triangle above and below the reference line means both faces of the same joint get welded. Each triangle carries its own size and length, so the two welds do not have to match.

Interactive drawing

3/83/8
TitleDouble fillet symbol
ViewAs drawn
Sheet1 / 2
The symbol as it appears on the drawing
arrowarrow sideother side
TitleDouble fillet joint
ViewSection
Sheet2 / 2
The joint it produces, in cross-section

Reads asArrow side: fillet weld, 3/8 size. Other side: fillet weld, 3/8 size.

How to read it

A double fillet is not a special symbol. It is the ordinary fillet triangle used twice on one reference line, once below for the arrow side and once above for the other side. Both refer to the same joint that the single arrow points at, so you never need a second arrow or a second symbol to call for welding both faces of a tee or a corner.

Read each triangle completely on its own before comparing them. Take the size from the left of that triangle, the length and pitch from the right of it, and the contour from beyond its outer face. Only then look at whether the two match. Symmetric double fillets are the common case, but a heavier weld on one face is perfectly normal where loading or access differs side to side.

The doubled symbol is the workhorse of structural fabrication because a fillet on each face roughly doubles the joint's shear capacity while keeping preparation at zero - no bevelling, no root opening, just two surfaces brought together and welded. It also balances shrinkage across the joint, which keeps the upright member from pulling toward the welded side.

Sequence is not part of the symbol. If the order matters - for instance alternating passes side to side to control distortion on a long member - that has to come from a note, the tail, or a written procedure. Reading a double fillet symbol tells you what to deposit, never in what order to deposit it.

Where each dimension goes

Size on each side
Leg length for that face. The two faces are dimensioned separately and can differ.
Length on each side
Run length for that face, or full length of the joint when omitted.
Symbol above the line
The face of the joint opposite the one the arrow touches.
Symbol below the line
The face of the joint the arrow tip lands on.

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.

  1. Unequal double fillet

    5/161/2
    TitleUnequal double fillet
    RefDouble fillet
    Ex1 / 2

    Reads as

    Weld a 1/2 fillet on the arrow side and a 5/16 fillet on the far side.

    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
    • fillet symbolAbove the reference lineA fillet weld on the far side of the joint
    • 5/16Left of the other-side symbolLeg size of the fillet

    Each triangle carries its own size. Nothing about the doubled symbol forces the two faces to match, so a heavier weld goes where the load is and a lighter one seals the back.

    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.

  2. Double fillet welded all around

    1/41/4
    TitleDouble fillet welded all around
    RefDouble fillet
    Ex2 / 2

    Reads as

    Weld 1/4 fillets on both faces and carry them all the way around the joint.

    Which mark says so

    • fillet symbolBelow the reference lineA fillet weld on the arrow side of the joint
    • 1/4Left of the arrow-side symbolLeg size of the fillet
    • fillet symbolAbove the reference lineA fillet weld on the far side of the joint
    • 1/4Left of the other-side symbolLeg size of the fillet
    • Circle at the elbowWhere the leader meets the reference lineWeld all the way around the joint

    The circle at the elbow applies to the whole symbol, so both the arrow-side and other-side fillets continue around every side of the attachment instead of stopping at the corners.

    And what it does not say

    • No length is given, so each weld runs the full length of the joint.

Common mistakes

  • Assuming the two triangles must carry the same size and welding both faces to whichever number you read first.
  • Reading the doubled symbol as two separate joints instead of two faces of one joint.
  • Overlooking a length or pitch that appears on only one of the two triangles.
  • Expecting the symbol to tell you a welding sequence. It never does.

Typical joints

Tee joints in structural framesColumn base plate to shaftGirder web to flangeInside corner joints on heavy sections

Related symbols

Fillet weldThe right triangle on the reference line calls for a fillet weld. The number to its left is the leg size, and the side of the line it sits on tells you which face of the joint gets welded.Intermittent filletTwo numbers to the right of the fillet triangle turn a continuous weld into a series of segments: the first is how long each segment runs, the second is the centre-to-centre spacing between them.Unequal leg filletTwo numbers separated by a multiplication sign to the left of the fillet triangle call for legs of different lengths, and the drawing has to show which leg goes on which member.Double bevelOne member bevelled from both faces while the other stays square, giving a K-shaped cross section. Two half-vee symbols sit on the reference line, one above and one below.