From time to time, questions come up on projects about welding two Nelson studs together to achieve a longer effective length. This practice is often referred to as “piggybacking” or “stacking” of studs, and is commonly shown in online videos or informal write-ups, usually from a contractor or fabrication perspective.
To be clear, welding one stud on top of another is physically possible. With the right equipment and technique, two pieces of steel can be fused together, and in a controlled shop environment, short tests may appear to show acceptable strength. However, physical feasibility does not automatically translate into code compliance or engineering acceptability.
Standard Nelson stud welding is defined for stud-to-base-plate connections, with specific assumptions regarding geometry, material, and acceptance criteria. These provisions do not extend to stud-to-stud splices. In particular, a headed stud is designed for anchorage into concrete, not to serve as a welding substrate. Welding to the head of a stud introduces a joint condition that was never intended or addressed in standard stud welding provisions.
Once a stud is welded to another stud, a new joint type is created with additional failure modes. Any lack of perfect axial alignment introduces eccentricity, which in turn creates bending moments at the splice location. These bending demands are not captured in typical shop tests and are not part of the original stud design assumptions.
Shop testing alone is therefore not sufficient to eliminate uncertainty. Shop tests are performed under controlled conditions with ideal alignment, restraint, and load application. In real structures, especially temporary or high-risk systems, actual load paths, construction tolerances, installation variability, and boundary conditions can differ significantly from test setups. Performance observed in a shop environment does not necessarily represent in-situ behavior.
From a code standpoint, stud welding is tightly defined. Welding practices outside standard stud-to-plate configurations fall outside prequalified provisions and require project-specific engineering, qualified welding procedures, and supporting test data. This adds complexity, cost, and risk that must be carefully justified.
For low-risk or non-critical applications, an Engineer of Record may choose to review and approve a properly engineered alternate detail. However, such approval still requires a qualified Welding Procedure Specification supported by procedure qualification testing representative of the proposed joint. EOR approval alone does not eliminate the need for documented welding qualification and supporting test data.
For high-risk applications such as shoring systems, excavation support, or other temporary works where failure consequences are severe, this approach carries additional risk that is difficult to justify. In these cases, reliance on non-standard details, even if tested in isolation, is generally not appropriate.
If a specified stud length is not available in the market, the appropriate path forward is to raise the issue with the Engineer of Record. The EOR may revise the design, specify a different connection approach, or formally review and approve an engineered alternate. What should be avoided is treating stud stacking as an assumed full-strength substitute without proper engineering justification.
The takeaway is simple. When a detail falls outside standard code-recognized practice, the burden of proof increases, and engineering judgment matters more, not less.