Fire resistance by calculation

The logic

Fire resistance design is simply a second design for different conditions. Cold design asks whether the structure carries the load in normal conditions. Fire design asks whether it carries the — typically smaller — load present during a fire at the moment when the steel has heated up and become weaker.

It has three steps:

1. The fire action

Which temperature curve has to be considered. The standard fire curve is the basis for comparison; in more complex cases a natural fire model may be used if the input data can be justified.

2. Heating of the structure

How much the steel heats up in the given time. This is where the section factor and the effect of the protective lining come in: the lining does not stop the heating, it slows it.

3. Capacity in the heated state

The strength properties of steel fall with temperature. The question is whether the remaining capacity covers the action present during the fire until the end of the required period.

Why this matters particularly for this technology

Because the other two routes are often closed. Intumescent coating cannot be verified on thin sections, and there is no off-the-shelf system certificate for a load-bearing wall. What remains is a test-based technical assessment — issued in the system owner’s name — or calculation produced for the project.

What makes it verification

Not the existence of the calculation, but the fact that someone takes responsibility for it: the engineer carrying out the fire design, for that project, with their signature. A general calculation made for an earlier project cannot be passed around — it does not apply to the structure being built.

It also means the construction has to follow the design: if the build-up or the fixing differs, the basis of the verification is gone.

Whose task it is

The meeting point of structural and fire safety competence. In practice the structural model comes from the structural engineer, and the fire assessment and signature from the fire safety engineer. In Hungary few people do this seriously, so the service is not available everywhere.

The point

Fire design is not a loophole around system documentation but the proper engineering route: it verifies the same level of safety, by calculation, for that particular building.