Effect of section dimensions

Fire resistance is expressed in units of time so one of the contributory factors to fire resistance is the heating rate of the member. This governs the time taken to reach it’s failure (or limiting) temperature and varies according to the dimensions of the section. Clearly, a heavy, massive section will heat up more slowly (and thus have a higher fire resistance) than will a light, slender section. This massivity effect is quantified in the 'Section Factor' (Hp/A)* Concept, Figure 14 shown below.

 

	Heat Perimeter (Hp)
Section Factor =
	Cross Sectional Area (A)

Hp/A concept

The heating rate of a steel section in a fire depends upon:

a) The perimeter of the steel exposed to flames - Hp(m)

b) The cross sectional area of the section - A(m² )

 

High Hp/Low A = Fast heating

Low Hp/High A = Slow heating

 

An example of this concept is given in Figure 15 shown below which shows the heating rate for three unprotected beams when subjected to the standard fire test.

Unprotected beams - Design temperature

 

Because heavy sections (lower Hp/A) heat up more slowly than light sections (higher Hp/A), a heavy section will require less insulation than a light section to achieve the same fire resistance.

Beams supporting non-composite concrete floor slabs with section factors less than 90m-1 heat so slowly that, where the load ratio is less than 0.6, they do not reach their limiting temperature for over 30 minutes, thus achieving 1/2 hour fire resistance without any fire protection. Columns in simple construction achieve 30 minutes fire resistance under the same circumstances when the section factor is less than 50m-1.

* sometimes written as A/V (Area/Volume)