Sheet Metal & Air Conditioning Contractors’ National Association


Basic design factors to consider for welded joints

Jun 19, 2012

Basic design factors to consider for welded joints

Many factors must be considered for welded joint design in the sheet metal industry. The basic factors of size, type of weld, edge preparation, metal thickness, reinforcement members, and distortion are the most prevalent.

Take for example, a welded joint for 16 gage hot-rolled steel (low carbon steel) rectangular duct fittings. The proper reinforcement of the duct fitting could be of primary importance and without proper reinforcement the side of the duct fitting may “breathe” or vibrate excessively due to turbulent or pulsating air flow. This excess vibration would expose the welded joint to potential metal fatigue. This situation becomes more critical when working with materials such as 300 series stainless steels that are more subject to metal fatigue than low carbon steels.

Distortion is another common problem encountered when welding light gage sheet metal. Reinforcement can reduce distortion, however, it is important to consider weld location, clamping, or fixturing of the assembly, intermittent welding if suitable, and the selection of less heat-constrained welding processes.

The welding process and the skill set of the welder must also be compatible. For example, a vertical corner weld on 16 gage 304 stainless steel using GTAW may be easy for a particular welder but that same weld using SMAW may be very difficult for that person.

Weld position within the sheet metal shop is less of an issue. However, in the field the problem of accessibility is usually of greater importance. Therefore, it is important where possible that all field welds be such that they are clear of all obstructions such as adjacent ductwork, piping, or building steel, so the welder has clear access to the entire joint.

Edge preparation is another important consideration since the configuration and condition of the edges to be welded will affect the quality of the weld, including depth penetration, porosity, and impurities in the weld.

These are highlights of some of the important factors to consider in the basic design of a welded joint. Four SMACNA manuals available in the SMACNA Bookstore that cover welding are: “Sheet Metal Welding Guide,” “Accepted Industry Practice for Industrial Duct Construction,” “Round Industrial Duct Construction Standards,” and “Rectangular Industrial Duct Construction Standards.” For further information on welding, you may want to visit the SMACNA Publications Store.