Section Factor Information

PRINCIPLE
The thickness of coating required for any particular section and period of fire resistance is based on its surface area exposed to the fire (Heated Perimeter - Hp) to its mass (Cross Sectional Area - A).

EXPLANATION
A steel section with a large exposed perimeter (Hp) will receive more heat than one with a smaller perimeter. Also, the greater the cross sectional area (A) of the section, the greater the heat sink (capacity to absorb heat). Therefore, a small, thick section will be slower to increase in temperature than a large thin one. Hp/A is thus a measure of the rate at which a section will heat up in a fire and the higher its value the greater will be the fire protection thickness required.

CALCULATION OF HP/A

FAILURE TEMPERATURES
Steel fully exposed and stressed in accordance with BS449 or BS5950 part 1 will lose its design strength at around 550ºC.

Where a steel beam is in direct contact with a dense concrete slab this acts as an additional heat sink and a failure temperature of a fully stressed section is taken as 620ºC.

In accordance with BS5950 part 8, a member that has reduced stress levels will fail at temperatures in excess of 550ºC (4 sides exposed) and 620ºC (3 sides exposed).

Bollom Fire Protection carry out all tests to a temperature of >800ºC, thus data is used in our Fire Engineering assessment work. Reductions in the thickness of the coating protection are therefore possible and you should contact our Technical Department to discuss your project.

Material thicknesses are based on independent assessments and fire tests carried out in accordance with BS476 part 20/21: 1987.

To calculate the dry film thickness for the required level of fire protection, firstly determine the Hp/A value of the section from the steel section tables in this brochure. Next consult the film thickness data tables detailing the wet and dry film thicknesses required to achieve the specified level of fire protection.

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