The magnitude of the residual stress is on par with the yield strength of the base metal and filler metal. The commentary is not part of the code and while it is intended to be "helpful" it does not add or subtract from or to the requirements of the code.

The fact that the backing was tack welded to the structural member is not going to increase the magnitude of the residual stress. If the cross section of the backing bar was relatively large in comparison to the members being welded, the resulting residual stress could cause small tack welds to fail if they were located outside the groove. If the tack welds were inside the groove and if the welder used small stringer beads for the root beads, one along the root face of each member, and if the welder were to allow the interpass temperature to drop to ambient such that there was sufficient contraction due to the delta T, it would be possible to cause the root beads to fail. It would appear that was not the case since the UT detected no cracks or unacceptable reflectors in the completed weld. The weld is sound.

It is best if there is no restraint when correcting the excessive root opening to mitigate the chance of crack initiation. However, there are situations where there are no alternatives but to accommodate the residual stresses that are a fact of life when welding. That is why the selection of base metals has to include due consideration of factors such as chemistry, state of heat treatment, and ductility rather than strength alone. The material selected has to have sufficient ductility to allow for some plastic deformation during welding. If there is insufficient ductility, the material will be too brittle and cracks will result.

The prequalified base metals listed in D1.1 and D1.5 typically have "relatively" low carbon equivalencies to mitigate the probability of forming hard brittle martensite in the HAZ. Filler metals are formulated with low carbon equivalencies for the same reason. Alternatives to carbon are utilized as alloying constituents to ensure adequate strength and ductility without increasing the carbon equivalency beyond a certain threshold to reduce the probability of forming hard brittle microstructures and to mitigate cracking problems while welding. 

If the material involved in this situation was high strength alloy steel, the outcome could have been different, but it would appear the welds are sound, free of cracks, and the stars were in proper alignment. It would appear the connection provided sufficient flexibility, i.e., it was unrestrained, to accommodate the contraction that occurred as the connection cooled from its maximum interpass temperature to ambient temperature. The residual stress causes plastic flow to accommodate the stress, which in turn results in strain that can strain hardening which increases the weld/BM strength (in the area that is subject to stain).  If the residual stresses were not accommodated by plastic deformation, i.e., the base metal lack sufficient ductility, cracks will result when the residual stresses equal the tensile strength of the base metal (at the temperature at which the cracks initiate). That is one reason why the Bridge Code has limitations on the yield strength of the weld when qualifying the WPS. As the value of the ratio of tensile strength to yield strength approaches unity (1), the likelihood of cracking increases. This is exactly the situation when welding high strength quenched and tempered steels where the TS/YS ratio is nearly “1”. Q&T steels with poor ductility are more prone to cracking than low strength steels that have better ductility and where TS/YS ratios are greater than 1.

All is good; fortune was on the side of the welder. Accept the welds and move on to the next problem.

Best regards - Al