Why the Size of Steel is Important in Fire Proofing: Not All Steel is Created Equal

During a building fire, different steel sections can heat at different rates. The measure of mass (weight of the structural steel in comparison to the amount of steel exposed to fire) dictates how quickly the steel heats up, where steel that is less massive heats more quickly than steel with greater mass. This is important to consider when fireproofing steel and trying to establish the necessary amount of intumescent applied to a specific steel section.

Several calculations are used when considering the massivity of steel; Hp/A, W/D, and A/P ratio being the most common. In the United States, W/D ratio is utilized for Wide Flange and miscellaneous steel shapes, whereas A/P ratio is employed for Hollow Steel Sections (HSS).

Hp/A, which is known as “Section Factor”, can be applied as a calculation for all steel profiles, and is determined by dividing the perimeter of a section by its cross-sectional area. These calculations are available for all standard steel profiles, typically from the manufacturer, or through engineering resources.

As a general rule thicker steel members will heat at a slower rate than thinner steel members, and therefore will require a lesser dry film thickness. As such, thinner steel members will generally require greater fire protection than thicker ones. This is demonstrated below, where pieces of steel with the same weight per linear foot, but different cross sectional dimensions, are shown to have different W/D ratios.

Another important factor to consider is the orientation and exposure of the steel. The same piece of steel will have a different W/D ratio, or Hp/A, when used as a 4 sided column vs as a 3 sided beam. This is because the heated perimeter (or area of the steel exposed to a fire) changes depending upon the orientation.

Generally, as the heated perimeter decreases, the rateability of the steel increases, because less of the steel is exposed to fire. This relationship is demonstrated below, where the same W10x100 steel member is shown to have two different section factors for 4 sided column and 3 sided beam exposures.

Understanding these calculations can go a long way in understanding how intumescent coating thicknesses are specified. As the W/D ratio and A/P ratio increase, the amount of intumescent coating required decreases. Hp/A is inversely related: as Hp/A increases, so does the required coating thickness.