Welding a steel tab onto a box section beam is a common fabrication task — whether you're adding a bracket for a gate hinge, a mounting point for equipment, or a connection plate for a structure. With the right preparation, technique, and safety gear, a competent DIY welder can achieve a strong, clean result. However, if the beam is part of a structural or load-bearing assembly, you should always involve a qualified professional.
Understanding the Job Before You Start
A box section beam (also called RHS — Rectangular Hollow Section) is a hollow steel profile with four welded walls. Welding a tab (a flat steel plate or bracket) onto its surface requires a fillet weld around the tab's perimeter, or at least along the sides that will carry the load.
Before picking up a welding torch, ask yourself:
- Is this beam structural (load-bearing, part of a building frame, or supporting people or vehicles)? If so, stop and read the section on professional help below.
- What are the material thicknesses? Matching your filler wire or electrode to the parent metal is critical.
- Is the steel mild steel (the most common and easiest to weld) or something else like high-strength or galvanised steel?
Tools and Materials You'll Need
- MIG welder (most practical for this job) or a stick (MMA) welder
- Appropriate welding wire or electrodes — for mild steel, ER70S-6 MIG wire or E6013/E7018 stick electrodes are standard choices
- Angle grinder with flap disc and cutting disc
- Wire brush and rag
- Clamps or magnets to hold the tab in position
- Steel square to check alignment
- Personal protective equipment (PPE): auto-darkening welding helmet, leather gloves, welding jacket or leather sleeves, steel-capped boots, and hearing protection
You can find welding consumables and PPE at Bunnings or Mitre 10.
Step-by-Step: Welding the Tab
1. Prepare the Surfaces
Clean metal is the foundation of a good weld. Use an angle grinder with a flap disc to remove any mill scale, rust, paint, or galvanising from both the beam surface and the tab where they will join. Never weld over galvanised steel without proper preparation — the zinc coating releases toxic zinc oxide fumes. If the steel is galvanised, grind back at least 25–50 mm around the weld zone and ensure excellent ventilation. See WorkSafe NZ for guidance on welding fume hazards.
2. Mark and Position the Tab
Mark the exact position on the beam with a marker or scribe. Use a steel square and clamps or magnetic welding squares to hold the tab firmly in place at the correct angle. Double-check alignment before you strike an arc — it's much easier to adjust now than to grind out a weld later.
3. Tack Weld First
Place small tack welds (short, temporary welds) at two or more corners to lock the tab in position. Check alignment again after tacking, as heat can cause slight movement. Adjust if needed before committing to the full weld.
4. Run the Fillet Welds
A fillet weld sits in the 90° corner between the tab and the beam face. For most mild steel tabs up to 6 mm thick, a single-pass fillet weld is sufficient. Key tips:
- Travel angle: Hold the torch or electrode at roughly 45° into the joint, with a 10–15° drag or push angle depending on your process.
- MIG settings: As a rough guide, 6 mm plate typically suits around 130–160 A and a wire speed to match — but always refer to your welder's chart.
- Weld all load-bearing sides of the tab, not just one edge.
- Allow each pass to cool slightly and chip slag (stick welding) or brush the bead before the next pass.
- Aim for a consistent, slightly convex bead with good fusion at both toes of the weld.
5. Inspect and Finish
Once cool, inspect the weld visually for undercut, porosity, cracks, or incomplete fusion. Grind any rough areas smooth with a flap disc if needed. For a tidy finish, a light grind along the weld face can help, but don't grind so much that you reduce the weld throat size.
Working Safely
Welding carries serious risks — UV radiation, fumes, fire, and electric shock. Always:
- Weld in a well-ventilated area or use local exhaust ventilation
- Keep a fire extinguisher nearby and clear flammables from the area
- Never weld near LPG, petrol, or other flammables
- Ensure bystanders are protected from arc flash with welding screens
- Check WorkSafe NZ for full guidance on safe welding practices
Structural Steel and the NZ Building Code
If the box section beam is part of a building structure — for example, a portal frame, a beam supporting a floor or roof, or any element designed under NZS 3404 (Steel Structures Standard) — then welding work may require a building consent and must be carried out or supervised by a qualified structural engineer and certified welder. The MBIE Building Performance website explains consent requirements and Schedule 1 exemptions. Always confirm with your local council before proceeding.
For structural connections, weld quality is typically verified by inspection and may require non-destructive testing (NDT). This is not DIY territory.
When to Call a Professional
Call a qualified welder or structural engineer if:
- The beam is load-bearing or structural
- You are unsure of the steel grade or condition
- The tab will carry significant loads (e.g., lifting points, vehicle attachments, staircase connections)
- The steel is thick (over 10–12 mm) and requires pre-heat or multi-pass welds
- The job requires a weld procedure specification (WPS) or inspection certificate
- You simply don't have the equipment or experience to do it safely
A qualified local welder or fabricator can be found easily on tradedirectory.co.nz.
Site Safe
SiteWise
ISO 9001
