Beams with web openings, including traditional castellated beams, cellular' beams with circular openings, and beams with multiple openings of varying size and shape have traditionally been fire protected using the guidance given in Fire protection for structural steel in buildings, the "Yellow Book". This gives an empirical rule for calculating the passive fire protection thickness to be applied to castellated and cellular beams, in which the thickness required for the solid parent beam is increased by 20 percent in recognition of the fact that castellated beams have been found to heat up at a slightly faster rate than solid beams. The limited tests from which this conclusion was drawn were carried out on loaded castellated beams protected with a thick insulating, spray applied, fire protection material. The Yellow Book states that the rule applies to passive materials and that no general guidance is available for active (intumescent) materials. However, because no other guidance was available, the rule has been widely applied to perforated or cellular beams protected using intumescent coatings. In the absence of other guidance, the rule was given without restriction in the SCI publication, Structural fire design, Off site applied thin film intumescent coatings, and was adopted in the current version of BS5950-8. Although the BS5950-8 rule relates to castellated beams, it is widely interpreted as applying to cellular beams with circular openings.
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