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CROSS Safety Report

Fire protection of steelwork supporting a timber floor

Report ID: 1234 Published: 20 February 2024 Region: CROSS-UK


A reporter shares concerns that developers and Building Control officials are assuming that, if a timber joist floor construction is suitable to provide 30 minutes fire resistance loadbearing capacity, then the structural steel supporting members within it will be similarly protected despite the absence of test data to support this approach.

Key Learning Outcomes

For fire protection product manufacturers:

  • Supply literature and test data that accurately reflects the project application

For designers and engineers:

  • Beware the term ‘fire-rated’, as it is often used without consideration of the individual aspects of loadbearing capacity (R), integrity (E) and insulation (I), which place very different requirements on products/constructions

  • Even when products have been tested and given an appropriate REI rating, if the tested construction does not represent the project application reasonably well, the actual performance could be significantly worse than expected

Full Report

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In their submission, the reporter poses a scenario where a structural steel beam supports a timber joist floor in the same plane as the joists - a common detail in individual dwellings which prevents beam encasement. This scenario specifically considers intermediate floors within a dwelling, so the fire resistance (loadbearing) requirement is 30 minutes, but these principles will apply to other uses.

The reporter explains that, in their experience, developers and Building Control officials appear to be working on the assumption that, if the timber joist floor construction is suitable to provide 30 minutes fire resistance and maintain its loadbearing capacity, the supporting structural steelwork within it will be similarly protected. On a particular project the reporter is familiar with, they state the developer is seeking to avoid the cost associated with the application of intumescent coatings to the structural steelwork on this basis.

The reporter shares that they have struggled to find any test data that would support this developer's case. In the reporter's view, either ceilings or encasements are tested as an imperforate membrane, not accounting for penetrations for any recessed light fittings, extract fans, or whole floor build-ups are tested but not considering primary steelwork within them.

One test the reporter found is an example of the latter, prepared for a fire protection company to demonstrate that their products can maintain the 30 minute REI rating of a metal web joist floor. For the duration of the test (40 minutes), the floor did not collapse (criterion R), no flames came through the floor (criterion E), and the temperature rise on the upper surface was limited to below 140oC (criterion I).

However, this test was conducted on a complete floor assembly and demonstrates that after 30 minutes the temperature rise on the top surface of the floor was limited to between 43 and 48oC. Nevertheless, temperatures within the floor void, which are critical when considering the ceiling as a standalone element and the steels within the void, are in some cases as high as 430oC - highlighting the significant effect of the cavity depth and floorboards. Depending on their level of stress, steels supporting the joists could fail at temperatures as low as 350oC.

The reporter suggests this scenario demonstrates:

  • a common misunderstanding of the concepts of integrity, insulation and loadbearing capacity
  • an oversimplification of terms such as 'fire rated'
  • a generalisation about fire protection methodology, and
  • a failure to consider project specifics

The reporter concludes that, in this scenario, the steel beams may have been better protected by the application of intumescent paint. 

Expert Panel Comments

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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.

It is the view of the Panel that designers cannot rely entirely on a fire resistance test on a timber-framed floor, tested in isolation and without any internal structural steelwork, to demonstrate adequate fire resistance for the internal steelwork which is then inserted into the floor in a real building. 

The fire resistance of the structural steelwork needs to be demonstrated, or suitably justified using calculation or expert judgement, for the R30 steel beam in its own right.

If designers can provide suitable justifications (with reliance on test data, calculations, or expert judgement) that the temperature of the steel beam will remain below its critical temperature (which will also depend on various factors, including its utilisation), then in principle this could be sufficient - but such justifications need to be explicitly provided.


Note this report was updated on the 26/03/24

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