Fire protection for thin-walled steel

Why “steel does not burn” is not enough

Steel is non-combustible, but thin-walled structures can heat up very quickly because their cross-section is small relative to the exposed surface. As they heat, sections can twist and develop enough stress to buckle out of the wall.

This is not merely a theoretical risk. If fire protection is not resolved correctly, a fire can cause serious damage even in parts of the structure the flames never reached. We call this a sensitive point of the technology not because the structure is poor, but because this aspect must be taken seriously.

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Why intumescent paint is not the answer

The obvious idea is to coat the structure with intumescent fire protection as is common for hot-rolled steel. In practice, this is not a viable route for two reasons.

The physics works against it

Required coating thickness depends on the section factor: the thinner the steel relative to its fire-exposed surface, the thicker the coating must be. Thin-walled sections have an extremely unfavourable ratio.

There is no supporting test evidence

Manufacturers’ assessment tables apply to specified section ranges. Extrapolating beyond those ranges is prohibited by international certification bodies. In our experience, below roughly 4 mm wall thickness, it is almost impossible to find a manufacturer that provides suitable evidence for its product.

And the cost

Even if someone attempted it, the enormous surface area would make the coating cost unrealistic for a residential building.

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Fire-rated board alone is not enough either

Another obvious proposal is to add fire-rated plasterboard carrying an REI classification. This involves a common misunderstanding.

An REI classification applies to the assembly, not the board

Fire resistance describes the performance of the complete assembly, including its ability to retain load-bearing capacity. A board alone cannot support the wall, so a standalone “REI value” for the board is not meaningful. Boards that can be considered independently without a tested system do exist, but they are extremely expensive.

Fire-rated plasterboard is otherwise a good material for this task. It is meaningful only as part of a system, tested with the specified frame, build-up and fixing arrangement.

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What verifies a load-bearing wall?

This is the key point. A load-bearing light-gauge wall does not automatically have a system certificate; one must be obtained. There are two routes.

NMÉ or ETA

A technical assessment issued in the name of the manufacturer or system owner, defining the tested build-up and configuration. Read more in NMÉ, ETA and CE — when they are required and when they are not.

Project-specific fire simulation

A system document can be replaced by simulation, but only when the responsible designer provides and signs that simulation for the specific project. One generic simulation cannot be circulated between projects.

This second route works, and we use it regularly. It is neither simple nor widely practised: relatively few professionals in Hungary carry out this type of simulation properly. We have the necessary technical background and a specialist who performs it.

The point to remember

Fire protection is the one part of system documentation with distinct technical substance that cannot be avoided. Build-ups and structural calculations must be designed in every case by responsible professionals. Fire performance, however, must be demonstrated either through an NMÉ or a signed, project-specific simulation. If somebody says it does not matter because “steel does not burn”, that is the point at which to stop and ask questions.