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Comments On Some Issues with the Fire Resistance Testing

 

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By: Ulf Wickström, FSFPE , Luleå University of Technology and TASEF Ltd, Sweden

In the Q3 2019 issue of SFPE Europe Magazine Węgrzyński, Turkowski and Roszkowski criticized the fire resistance furnace test standard and concludes that the ‘disregard of the energy balance in fire testing seems to be a design flaw of the method, originating from the limited knowledge related to heat transfer in the time of the first tests’. To be frank, I do not agree. The standard furnace test is one of the most ‘reliable’ we have in FSE. It is one of the very few methods where we can predict the test results by calculations based on physical material properties.

When analysing a test, we may consider accuracy, verification and validation.

By accuracy I mean how well can we generate test conditions. In this case I limit myself to thermal exposure conditions. The standard prescribes a furnace time-temperature curve as measured with plate thermometers. The standard requires that this curve must be followed within a few percent, except for the first few minutes. The fire laboratories have usually no problems doing that.

By verification I mean how relevant is the measured PT temperature for the temperature development in fire exposed structures. That issue was addressed when PT was introduced about 30 years ago by testing a very well-defined test specimen and measure its temperature when exposed to furnace conditions controlled by PT measurements. Thus, a so-called calibration element was designed and tested in several European furnaces. The conclusion that exercise was that if furnaces are run according to the standard and controlled by PT measurements, the calibration elements obtained the same temperatures. Shortly thereafter PTs were introduced in both the ISO and the corresponding CEN furnace test standards.

By validation I mean how relevant is the standard curve for real fires. Here we must rely on basic compartment fire theory. For details see e.g. ref. [1]. If we are considering structural fire resistance, we think about fully developed fires. Such fires are ventilation controlled, i.e. the heat released in the compartment is limited by the amount of air/oxygen entering the compartment. Additional fuel will not burn inside the compartment in will therefore not contribute to any temperature increase. The fire temperature will increase with time as the surrounding structures heat up and will eventually, if the fuel load is large enough, reach temperature in the order of 1300°C to 1400°C depending on burning efficiency. The rate of fire temperature increase depends on the ratio between the opening factor and the thermal inertia of the surrounding structures, cf. parametric fire curves in Eurocode EN 1991-1-2, Annex 1. For a certain value of this ratio (according to Eurocode a gamma factor equal unity) the theoretical fire temperature curve is almost identical to the standard fire curve. That implies that the standard curve has a good bases in theory but only for gamma equal unity. For other values it is possible to obtain other design fires according to Eurocode. Thus, according to the theory, the amount of fuel, i.e. the fuel load, does not influence the rate of temperature increase but only the fire duration. You may argue that a fully developed fire in a compartment lined with e.g. wood panels will be much hotter than a compartment lined with non-combustible concrete. Yes, that is true, but not because the wood burns but because the thermal inertia of wood is much lower than that of concrete. If we replace the wood with e.g. mineral wool, we might get an even considerably hotter fire although the mineral wool does not contribute to the combustion. In summary, for a given compartment additional fuel will not increase the fire temperature for fully developed fires. However, it will increase the fire duration and the risks of fire spread due to unburnt fuel being released by flames out through openings. In addition, the likelihood of a fully developed compartment fire to occur will increase if the surrounding structures are made of combustibles like wood panels rather than of non-combustibles like gypsum boards.

Of course, a single fire curve could not represent all fires. But in a similar way, no fire test method can represent all fire scenarios. The question is then can we improve the test and design conditions.

Yes, we can by considering the openings and properties of the compartment in question as suggested by the parametric fire curves of Eurocode or by applying methods as presented by Byström and Wickström and in ref. [1], or other more advanced methods like CFD calculations. But can we afford doing that more often, and do we have enough competence in the FSE community to make the analyses needed without depending of classifications based on the standard curve?

References

  1. [1] Wickström, U., Temperature Calculation in Fire Safety Engineering, Springer International Publishing, 2016

 

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