Structural steel hollow sections can be well suited to structural applications because they have a higher strength to weight ratio than open section profiles such as “I” beams and columns, channels or angles, and a lower area to be covered by a protective coating.
Although they are more expensive than open section profiles on a per tonne basis, the overall weight saving of steel and reduced cost of a coating will often result in a cost effective fabrication.
The welding of hollow sections for welded structural steelwork is covered by AS/NZS 1554. As a structural standard AS/NZ 1554 generally requires prequalified joints to be welded from both sides, or against backing. Table E4 in the Standard provides the prequalified RHS joint options for full penetration butt welds where welding from both sides is not possible.
Setting up backing for butt welds in RHS entails some attention to detail; the backing has to cater for the radiused corners of the section, and usually there is a weld flash from the RHS manufacturing process protruding on the inside of the section that may interfere with the fitting of the backing.
Fabricators commonly use one of two methods for RHS joints with backing; either four individual pieces are fitted in, or a one piece “plug” forms the backing. For both these options the bar or plug must extend into the ends of both RHS so there is no risk of lack of root fusion or incomplete penetration.
Regardless of the backing arrangement chosen the joint set-up must comply with the including Standard Section 5.2.4 of that specifies the minimum thickness of a bar and the maximum separation between the backing and the wall of the RHS (1.5mm max.).
While set-up time can be reduced by accepting generous gaps in joints there may be no saving overall when the extra weld metal required to fill the gaps is taken into account. For example: for a RHS with a 6mm wall a standard prequalified single V butt with a 1.5mm gap between the backing and will require 50% more weld metal than if the gap was only 0.5mm.
The old adage that preparation is half the weld is as relevant to welds in RHS as to any other joint.