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Passive Fire Protection | Calculating Section FactorsFires resistance is expressed in units of time (Generally 30, 60, 90, 180 and 240 minutes) so one of the contributory factors to fire resistance is the heating rate of the member, which governs the time taken to reach it’s specific failure temperature. The heating rate of each section varies according to its dimensions. A short heavy section will heat up more slowly than a tall light section.
This effect is stated as the ‘Section Factor’ and is denoted at the Hp/A value of the steel section. The Hp/A value is calculated from the dimensions of the steel section.
The larger the surface area relative to the cross sectional area the higher Hp/A value. The higher Hp/A value the more material required. Example 1: Section, 4-sided UB 457x191, 98.3kg/m.
Approx calculation:
Because the short heavy sections heat up more slowly, they require less protection in the form of film thickness than the tall light sections. Also beams with less than 4-sides exposed to the fire, such as a beam supporting a concrete floor will require less protection than the same section with all 4 sides exposed. Example 1: Section, 3-sided UB 457x191, 98.3kg/m.
Also for list of British Standard steel sections sizes, and the associated section factors please download the following PDF file:
Nullifire can provide specific loading tables, which detail individual section factors, and the required amount of material for a given performance at various failure temperatures. For further information please contact Nullifire Technical Services Department:
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