# The LWF Blog

## Fire Engineering for Healthcare Premises – Structural Fire Protection – Part 43

July 12, 2021 11:14 am

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 42 of Fire Engineering for Healthcare Premises, discussed the use of structural fire protection in healthcare buildings. In part 43, we discuss fire resistance and fire severity, the calculations and variables.

While fire resistance ratings are useful in comparing products for use, but do not indicate the survival time in a real fire situation, it can be helpful to understand the calculation methods used. All calculation methods should begin with a definition or calculation of the heating regime. This means that the calculation can work out the heat transfer from the atmosphere of the compartment (surrounding enclosure) to the structure (or part under consideration).

The temperature (distribution) within the structure can be evaluated in this way and then used as an input to structural stability calculations, or to directly estimate the insulation-resistance criteria.

While it would be most useful to calculate the point of loss of integrity, e.g. the point at which small fissures may appear through which hot gases can pass and ignite materials on the other side of the fire-resistant barrier, this is not currently possible.

Fire severity can be defined in various ways as a measure of the destructive potential of a fire. The most common and convenient way is in terms of time. The time would be established through exposure to an equivalent heating regime in a furnace test, causing a similar level of damage.

The use of time as a measure allows comparisons between fire resistance and fire severity. The fire safety design objective is that the resistance time exceeds the severity time by an appropriate safety margin.

Both fire severity and fire resistance are random variables and even when working within the appropriate safety margin, there is the potential for unforeseen probability of failure, e.g. an untimely collapse of structure. The design objective, while acknowledging random variables, aims to keep this risk acceptably small.

The severity of a fire depends on various factors – the temperature the fire reaches within the compartment, the duration of time since fire ignition and in turn, both of these factors rely on other variables – the amount of fuel available to the fire, the physical disposition (whether in thin wall linings or solid blocks of material) and how much ventilation is available.  It should be noted that the area of openings providing ventilation may vary during a fire.