Castellated and cellular beams

For castellated or cellular beams, or fabricated beams with holes, the thickness of the fire protection material should be 20% more than the thickness determined from the section factor of the original, uncut section for boards and sprays. 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.

The 20% rule is not suitable for use with intumescents and recent testing has indicated that the amount of added protection is product specific. The advice of the intumescent manufacturer should be sought