What is steel section factor?

The dry film thickness required for a particular fire resistance time depends upon various factors.

One of those factors is steel section factor and, to ensure the correct intumescent paint coating specification, it is important to understand steel section factor and how it is calculated.

Steel section factor, also known as Hp/A or ksm, is the ratio of perimeter to cross sectional area of structural steel members. Different section sizes heat up at different rates - the section factor is determined by dividing a section's perimeter by its cross-sectional area.

Hp/A gives an indication of the heating rate and dictates selection of the steel sections that can achieve the required FRL.

Generally, a larger, thicker steel member will have a lower Hp/A and lower heating rate than a smaller, thinner steel member.

This is important to consider because structural steel members with high Hp/A will require more passive fire protection than those with low a Hp/A. We need to apply thicker coatings (i.e. more insulation) to thinner sections than we do to thicker sections to achieve the same level of protection.

In a nutshell:

  • Hp/A, A/V and Ksm are all measurements of the heating rate of a structural steel section in a fire. They are all different terms describing essentially the same thing - steel section factor
  • Hp/A depends on two factors:
  • Hp - The heated perimeter of the steel that is exposed to the flames
  • A - the cross sectional area of the structural section
  • Ksm depends on one factor:
  • The ratio of the exposed surface area to the mass of the section in square metres per tonne
  • Whichever terminology is used, the sectional shape, size and mass of the steel determines the exposed surface area to mass ratio or perimeter to cross sectional surface area

The bottom line of all this is that section size information is critical to ensure adequate protection and accurate budgeting.

Low Hp + high A = slow heating
High Hp + low A = fast heating

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