It depends on how the welder was qualified. Did he qualify with backing or without backing and backgouge the root, or did he weld from one side with complete burnthru or penetration. Is this tube to plate weld full pen? If he was qualified with backing or he backgouged the root then he is not qualified to weld that joint detail unless you use backing. Look at table 4.9 under groove weld type-production box tube welding qualified, cjp note 4.
Brian J. Maas