The exclusion of intumescent paint on the top flange of the beam in your installation is justified by the need to ensure proper installation of shear studs for any Trapezoidal or re-entrant profile.
Why the Top Flange Must Be Left Uncoated Before Thru-Deck Stud Welding
Leaving the upper flange of a steel beam free from intumescent paint (or any thick, non-weldable coating) is standard practice in composite floor construction because it protects weld quality, structural performance and code compliance.
1 Stud-weld integrity demands bare steel
- Composite action relies on shear studs achieving a 360° fusion weld with the beam flange. Any substantial coating even an intumescent layer only 250 µm thick acts as an insulator, introduces gas porosity and prevents full fusion, leading to failed bend tests under the BCSA’s 30° pre-production regime.
- BS EN 1994 and SCI P300 both assume studs are welded onto clean S350 or S450 flanges; weld-through capability is permitted only for very thin, specialist primers (≤0.8 mil / ≈20 µm) that vaporise completely during the arc.
2 Concrete slab already furnishes the fire protection
- In a composite slab the top flange sits within or directly beneath the concrete; the slab insulates the steel and provides the required fire resistance period to Eurocode 4 without added paint on the embedded flange.
- Any fire protection that is genuinely needed applies to the exposed lower flange and web, not the portion buried in concrete.
3 Industry standards list acceptable and unacceptable coatings
| Coating on top flange | Weld-through status | Typical dry-film limit | Action required |
|---|---|---|---|
| Inorganic “pre-construction” zinc primer | Acceptable if specifically certified as weldable | ≤ 0.8 mil (≈20 µm) | Qualification bend test and WPQR before production |
| Conventional inorganic zinc at 2–3.5 mil/Lead based zinc | Unacceptable | > 1 mil | Remove locally before welding |
| All organic primers (epoxy, urethane, alkyd) | Unacceptable | Any | Remove before welding |
| Intumescent coatings (all types) | Unacceptable | Any | Prohibit until after studs are welded |
Even when a weldable inorganic primer is specified, the deck/steelwork contractor must verify performance with ten test studs bent to 30° and retain records as required by the BCSA Code §8.71.
4 Practical sequence keeps risks and costs down
- Supply beams with the top flange stripped or masked.
- Fix decking and perform thru-deck stud welding on bare steel.
- Conduct visual, ring-tone and bend tests; record results.
- Apply intumescent or other protection only to the exposed steel after weld inspection.
- Where a fully coated member is essential (e.g. car-park environments), switch to shot-fired connectors-still on bare steel strips or use board encasement.
5 Conclusion
Excluding intumescent paint from the top flange is not a shortcut; it is a code-supported necessity that:
- Guarantees full-strength shear connections and satisfactory bend tests.
- Avoids the cost and uncertainty of site grinding or re-welding.
- Relies on the concrete slab itself to deliver the fire rating prescribed by Eurocode.
If a client later requests additional fire performance, use post-weld intumescent coatings, board encasement or cementitious spray never a pre-weld coating over the stud-weld zone. The concrete and steel deck is acceptable fire protection to the top flange of the beam nfor fire protection and written into the national annexe.
Conclusion:
Given the necessity for shear stud installation and the inherent fire protection offered by the composite slab, excluding intumescent paint on the top flange is a standard and justified approach. If additional fire resistance is required, alternative methods such as fire-resistant board encasement or post-weld intumescent application could be considered.
If in doubt please speak to our technical department at main office on 01787 275055
