Fire resistance of steel framed buildings – section factor and protection thickness assessment

Topics Covered

Effect of section dimensions
Hot Rolled H and I sections
Castellated and cellular beams
Hot rolled unfilled hollow sections

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. 

Source: Corus             

 

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