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Fire Engineering for Healthcare Premises – Structural Fire Protection – Part 20

February 1, 2021 1:24 pm

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 19 of Fire Engineering for Healthcare Premises, LWF considered time-temperature curves and parametric time-temperature curves and their role in structural fire protection. In part 20, we will discuss time-equivalent exposure and structural modelling.

Structural Fire Design: The Role of Time Equivalence G.C. THOMAS, A.H. BUCHANAN and C.M. FLEISCHMANN, stated the following:

The time equivalence concept is used to relate the expected real fire exposure to the standard test fire, to allow the use of published fire resistance ratings […].  The equal area concept has little theoretical significance because the units of area are not meaningful. This concept can give a very poor comparison of heat transfer for fires with different shaped time temperature curves, because heat transfer from fully developed fires is mostly by radiation, the balance by convection. Radiative heat transfer is proportional to the fourth power of the absolute temperature, so heat transfer in a short hot fire may be much greater than in a long cool fire, even if both have equal areas under the time temperature curves.”

The limitations of time-equivalent formulae mean that it is not possible to adequately reflect the heat transfer mechanism, on the basis of determining equivalence of equal areas under time-temperature curves for a real fire and a standard curve.

Time-equivalent formulae may not be applicable to fire scenarios other than assumed in the original empirical deviation, such as non-cellulosic fuels, larger rooms, different types of fire protection, different levels of glazing/ventilation or different types of structural member.  Equally, time-equivalent formulae are not intended for unprotected steel or timber structures.

In the case of structural modelling, most models are theoretically similar. Differences can arise due to the ways in which material properties are modelled and the data is used. Where thermal models are used by those with relevant experience, they tend to be adjusted to improve the fit with experimental measurements.

Thermal analysis can be inappropriate to use on concrete structures. Moisture within the concrete can affect the analysis without treatment and if ignored, significant errors can result. Equally, structural analysis can only be undertaken where the temperature of the structure is accurately predicted using thermal model predictions.

In Part 21 of LWF’s blog series, LWF will look at the use of calculations for prediction of flames from windows and radiation from windows. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact LWF on freephone 0800 410 1130.

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