For flare bevel groove welds made with a backing bar, how should the effective throat thickness be determined?

• A. It should equal the lesser of the thicknesses of the material being welded
• B. It should be fixed at the thinner material thickness always
• C. It should equal the greater of the two thicknesses
• D. It should be twice the fillet size

Answer: (A)

In a flare bevel groove weld with a backing bar, the effective throat thickness is controlled by the thinner base metal. The throat is the portion of the weld that carries the shear load, and when two pieces of different thicknesses are joined, the narrowest member limits how deep the weld root and throat can effectively be. The backing bar supports the underside, but it doesn’t allow the weld throat to exceed the thickness of the thinner plate. So the correct effective throat thickness is the lesser of the two base metal thicknesses. The other options either overstate the throat (using the greater thickness or twice a fillet size) or imply a fixed condition that doesn’t reflect the geometry of the thinner piece.