The image above shows the preparation for a groove weld with permanent backing bar.
The image below from a finite element model illustrates how such a configuration can create a stress concentration. The model consists of a pair of 4 inch long x 1 inch thick steel plates with a 1 inch long x 1/4 inch thick permanent steel backing bar attached to the bottom of the plate interface, simulating the final condition for a CJP weld. The left end of the model has been restrained, and a tensile pressure of 1 ksi has been applied to the right end. The centroid of the plate's cross section aligns with the pressure's force resultant everywhere except at the backing, whose presence shifts the centroid of the material downward, creating eccentricity and causing the plate to bend locally at the backing. As a result of the bending, the bottom of the backing bar takes compression while the top of the plate takes tensile stresses in addition to the applied pressure, for a tensile stress total (in this case) of 1.1 ksi, a 10% increase - enough to negate a factor of safety. It can be counterintuitive to think that the addition of material could make a design less conservative, but such is the case here. For this reason, it is advisable to encourage the use of removable backing bars, and ensure that they do get removed. Alternatively, the problem could be avoided by instead designing a weld that doesn't require backing at all, such as a PJP.
