Hot rolled H and I sections

When proprietary passive fire protection is necessary to achieve fire resistance, the required thickness can be determined from manufacturer’s published data. Much of this information has been consolidated into a reference text commonly known as 'The Yellow Book', Figure 16 shown below published by the Association of Specialist Fire Protection (ASFP) and The Steel Construction Institute. This publication is easy to use and gives valuable guidance on approved proprietary fire protection systems.

 

Manufacturer’s recommendations generally relate the thickness of protection to the section factor (Hp/A) and the fire resistance time required. In general, protection thickness recommendations are derived from the BS476 Standard Fire Test  and are designed to restrict steelwork in fire to a limiting temperature of 550°C (or 620°C for intumescent coated, 3 side exposed beams). However, where manufacturer’s data for other limiting temperatures is available, it may be used and could yield economies.

For typical building construction using universal I and H sections, the value of Hp/A is usually in the range 20-325m^-1, the value of 20m^-1 being associated with the heavy 356 x 406 x 634 kg/m column for three sided box protection (eg boards), whilst the light 127 x 76 x 13 beam has a Hp/A value of 325 for four sided profile protection (e.g. intumescent coatings). In published tables, values of Hp/A are normally rounded to the nearest 5 units.

Figure 17, shown below, shows four protection configurations for a 533 x 210 x 82 kg/m beam are shown below. To determine the thickness of a spray protection for a three sided profile to give 1 hour fire resistance, first define the section factor – 160m^-1 – then refer to manufacturer’s data or 'The Yellow Book', which shows the required thickness to be 16 mm, shown below in Figure 18.

 

This procedure provides a relatively simple method for establishing the protection requirements for most sizes of steel section and fire resistance periods.