Steelwork Section Factor, definition.
The section factor for a given steel profile is a measure of the heat sink capacity it provides under fire attack conditions and is used to select the appropriate thickness of passive fire protection system being applied to the steel.

A steelwork section factor has more than one standard notation that's commonly used and is usually expressed as either an A/V value or a H_p/A value, and is given in SI units of m^-1. Although current calculations are based on the H_p/A principle, it's still common for manufacturers and specifiers to refer to an A/V value or a section factor, these three descriptors are effectively interchangeable.

Section factors are dependent on the type of passive fire protection system being applied, where more than one figure can be arrived at for the same steel profile based on whether there is a boarded or boxed encasement system being used - this is the box section factor - or a spray or paint treatment - this is the profile section factor.

As the calculation is a geometrical relationship between the heated perimeter (H_p) of the steel profile and the cross-sectional area (A), when a boarded encasement system is used to protect the steel, the smallest rectangular box that can be drawn around the steel profile can be used for the H_p value, whereas for a profiled protection system, all other elements of the steel profile geometry must be taken into account, making the profile section factor calculation process more complex than that for a box section factor.

Where one or more sides of the steel profile requiring protection are covered by a solid, load-bearing element of the building construction that's of equal or greater fire performance to that required of the encasement, usually either brickwork, blockwork, concrete or masonry walls or soffits, only the other exposed sides of the profile or box need to be used in the H_p calculation. The cross-sectional area of the steel profile, by contrast, remains the same regardless of how many sides or faces are exposed to fire attack.

The Section Factors for most standard steel profiles and sizes are already calculated, and are included in tables published in Chapter 3 of the Promat Fire Protection Handbook as well as the ASFP Yellow book.

Steelwork profiles requiring special consideration.

When a cellular or castellated beam (beams with holes in the web sections), or a lattice beam requires protection, the same rules cannot be applied as for standard solid steel profiles when calculating the section factors. Similarly, selecting the appropriate thickness of passive fire protection that needs to be applied is subject to additional testing and certification, in addition to the standard testing and certification used for solid steel profiles.

For any composite steelwork sections, where more than one steel profile is used in combination with another in order to provide the required structural support, the section factor calculations can also become more complex.

For further advice on any such calculations and guidance on how these can then be used to select an appropriate Promat protection system, please contact the Technical Services Department who will be happy to offer our additional assistance.