Double U-groove weld symbol

Two U symbols on one reference line mean the joint is prepared from both faces. In cross section it resembles two shallow bowls meeting back to back with a land between them. Each U carries its own included angle beyond the open end and its own depth to the left, and the root opening is written once for the joint as a whole.

This is the preparation reached for when the plate is thick enough that even a double-V would take too much weld metal. Because a U's cross-sectional area grows slowly with depth, splitting it across two faces keeps the total volume remarkably low on joints several inches through. Less deposited metal means less shrinkage, less residual stress, and far less arc time.

The trade is preparation cost and access. Both faces have to be machined or gouged to a radiused profile, and the joint has to be turned partway through so the second side can be reached. Where the part cannot be rolled, a designer will usually choose a single-sided U with a backing arrangement instead, so seeing a double U tells you the assembly is meant to be turned.

The two preparations need not be identical. Read the depth and angle on each symbol before setting up the cut, and expect the deeper preparation on whichever face has better access or better welding position. As with any two-sided joint, the root is normally cleaned back to sound metal before the second side is filled.