I have heard that lengths of shelf angles should not be welded together to make them continuous. Why?
Welding sections of shelf angles prevents them from properly expanding or contracting due to temperature changes. The Brick Institute of America (BIA) in Technical Notes 18A "Differential Movement," states "A small space should be left between lengths of angles to allow for thermal movements." BIA Technical Notes 21, "Brick Masonry Cavity Walls," and 28A, "Brick Veneer, Existing Construction," give similar recommendations.