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. Most suppliers clearly differentiate between open (H & I) and closed (hollow) sections in their specifications.

Concrete filled hollow sections are discussed Filled Hollow sections in fire.