Analytical calculation, for structures in steel and in a mix of steel and concrete, can only be carried out according to the criteria laid down in Eurocodes EN1991-1-2, EN1993-1-2 and EN1994-1-2. For passive protection systems using soft plaster or plasterboard, it is possible to do the calculations with either advanced or simplified methods, provided that the values used for the thermal properties (λ) of the materials (density, specific heat, conductibility coefficient) have been obtained using the prescribed experimental procedure. It should be noted that this option is not available for reactive protection systems (intumescent paints) as the calculation can only be done using experimental assessment procedures.
Assessment using comparison tables makes it possible to work out the thickness of the protective coating on the basis of the type of material used, the fire-resistance rating and the section factor of the steel element. If the comparison table method is to be used, it is important to check that the elements to be protected correspond precisely to those referred to in the tables in question.
The tables can only be used for the purpose of fire-resistance rating if the manufacturer states, on the basis of their own testing experience, that the product guarantees the performance claimed for it, accepting liability for this statement. The manufacturer must also state that the product demonstrates the necessary adhesion and cohesion for the time period corresponding to the required fire-resistance rating. This statement must be accompanied by instructions for applying the coatings or for installation. This method does not apply to reactive paints given the enormous variations in performance associated with the chemical and physical reaction that occurs during exposure to fire.
Assessment of protective systems for steel structures through experimental testing involves laboratory testing carried out in compliance with the provisions of the document EN 13501-2. For fire-proofing coatings this document refers to the standards EN 13381-4 or EN 13381-8: “Test methods for determining the contribution to the fire resistance of structural members”. Part 4 or 8: “Applied passive protection products to steel members”. In some European countries Part 4 is required, in others Part 8, but whichever part, it is the essential basis for CE marking and for European Technical Assessments (ETAs). Laboratory testing of reactive coatings requires a long series of tests in which the various steel elements are subject to controlled fire to check the effectiveness of the protective system applied, in terms of duration, adhesion and a number of other parameters. By interpolating the results of all these tests it is possible to arrive at an assessment of the thickness of the protective coating to produce the required fire-resistance, taking into account the various mass factors and different critical collapse temperatures.