Section 179.400-11 - Welding(a) Except for closure of openings and a maximum of two circumferential closing joints in the cylindrical portion of the outer jacket, each joint of an inner tank and the outer jacket must be a fusion double welded butt joint.(b) The closure for openings and the circumferential closing joints in the cylindrical portion of the outer jacket, including head to shell joints, may be a single welded butt joint using a backing strip on the inside of the joint.(c) Each joint must be welded in accordance with the requirements of AAR Specifications for Tank Cars, appendix W (IBR, see § 171.7 of this subchapter).(d) Each welding procedure, welder, and fabricator must be approved.Amdt. 179-32, 48 FR 27708, June 16, 1983, as amended at 68 FR 75763 , Dec. 31, 2003