Effect of section dimensions
Fire resistance is expressed in units of time so one of the contributory factors to fire resistance is the heating rate of the member, which governs the time taken to reach its failure (or limiting) temperature. This varies according to the dimensions of the section. Clearly, a heavy, massive section will heat up more slowly (and thus have a higher fire resistance) than will a light, slender section. This massivity effect is quantified in the “Section Factor” (Hp/A) Concept. (Figure 13).
An example of this concept is given in Figure 14 which shows the heating rate for three unprotected beams when subjected to the standard fire test. Because heavy sections (lower Hp/A) heat up more slowly than light sections (higher Hp/A), a heavy section will require less insulation than a light section.
Beams supporting concrete floor slabs with section factors less than 90m-1 heat so slowly that, where the load ratio is less than 0.6, they do not reach their limiting temperature for over 30 minutes, thus achieving 1/2 hour fire resistance without any fire protection. Columns in simple construction achieve 30 minutes fire resistance under the same circumstances when the section factor is less than 50m-1
Hot Rolled H and I sections
When proprietary passive fire protection is necessary, 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” 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 40 -300m-1, the value of 40m-1 being associated with the heavy 305 x 305 x 198 kg/m column for three sided box protection (eg. boards), whilst the light 152 x 152 x 23 kg/m column has a Hp/A value of 300 for four sided profile protection (eg. sprayed coatings). In published tables, values of Hp/A are normally rounded to the nearest 5 units.
Figure 16 shows four protection configurations for a 533 x 210 x 82 kg/m beam. 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 supplied in the Yellow Book, (Figure 17) for a typical product of this type, which shows the required thickness to be 16mm.
This procedure provides a relatively simple method for establishing the protection requirements for most sizes of steel section and fire resistance periods.
Castellated and cellular beams
For castellated or cellular beams the thickness of the fire protection material should be 1.2 times the thickness determined from the section factor of the original, uncut section. Therefore an 800 x 210 x 82 kg/m castellated beam formed from the 533 x 210 x 82 kg/m section used in the previous example would require 1.2 x 16 = 19.2 mm, (rounded up to 20 mm), protection thickness.
Hot rolled unfilled hollow sections
For unfilled hollow sections, the required thickness of fire protection is also determined from values of section factor. For board and spray fire protection materials the thickness required for an unfilled hollow section may be obtained by reference to the thickness required for an I or H section with the same section factor. Where the thickness of a board or spray fire protection material was originally assessed from tests using boxed systems which enclose the section, the same protection thickness can be used. Where the thickness of a board or spray fire protection material was originally assessed from tests using sprayed systems, a modified thickness must be used. The modification factor is calculated as:-
For a section factor, Hp/A <250m-1 Thickness = t (1 + (Hp/A)/1000).
For a section factor, Hp/A >250m-1 Thickness = 1.25t
where t is the thickness of fire protection material calculated for the equivalent I or H section. This method is not applicable to intumescent coating systems. In this situation confirmation must be sought from the manufacturers regarding required thicknesses. Some suppliers do clearly differentiate between open (H & I) and closed (hollow) sections in their specifications, others do not.