When steel is exposed to fire, the thermal energy will cause the steel temperature to increase. The rate of temperature increase depends on the geometry of the steel:
The greater the exposed external surface of the steel element, the faster the thermal energy will flow into the steel element
The smaller the volume of the steel, the less thermal energy is needed to heat it up.
In European and British standards, both aspects are combined in the section factor, which is calculated by dividing the fire exposed external surface (in m2) by the internal steel volume (in m3). An alternative simple approach which gives the same outcome is to divide the exposed external perimeter (in m) by the cross-section area of the element (in m2). In ASTM standards, a similar approach is used where the calculation is based on the ratio between weight per linear foot (lb) and heated perimeter surface (inch).
Typically, section factors are in a range of 50m-1 to 300m-1. A low section factor means relatively slow heating, a high section factor means faster heating. Therefore, elements with a high section factor usually need thicker fire protection.
Even if a steel element is the same, the section factor can vary depending on some conditions:
Exposed surface: for beams under a concrete floor, only the sides and bottom of the beam are exposed to the fire. This is called “three sided exposure”. However a free standing column is exposed on four sides. Three or four sided exposure is reflected in the exposed surface which is used to calculate the section factor.
Box or contour protection: for sections with an H shape, the external exposed surface follows the contour of the H shape. However, if the fire protection consists of a rectangular box placed around the H shape, the external exposed surface is reduced to the size of the box. For intumescent paints and for sprays, the contour shape is relevant. For board protection, normally the box shape is used.
For more information, please contact your local Promat office.