The building regulations require certain elements of structure to have fire resistance for a specified minimum period of time. The amount of fire protection required to achieve this depends on the following:
-Duration of fire resistance specified
-Type of protection used
-Perimeter of steel section exposed to fire
-Shape and size of steel section
Several factors contribute to determining the thickness of fireproof coatings which is as follows: a) Section Factor
The rate of increase in temperature of a steel cross-section is determined by the ratio of the heated surface area to the volume A/V or Hp/A. In earlier publications the section factor was written as Hp/A while in new European testing and design standards, section factor is presented as A/V which has the same numerical value as Hp/A. So the degree of fire protection depends on the section factor of steel section. The higher the Hp/A, the faster the steel section heats up and so the greater thickness of fire protection material is required. According to the above principle, the Hp/A ratio should be known to determine the thickness of fireproof coating. b) Critical temperature
Steel structures lose their load-bearing capacity in a fire so that strength of steel at 550°C is about 60% of its room temperature strength.
All materials weaken with increasing temperature and steel is no exception. Strength loss for steel is generally accepted to begin at about 300°C and increases rapidly after 400°C. By 550°C the most common grades of structural steel retain about 60% of its room temperature yield strength. Structural steels that are heated above 600°C will lose some of their properties on cooling. The extent of this loss is a function of the grade of steel. c) Duration of fire resistance
This parameter is determined based on the distance from fire station, area and number of floors of building, amount of flammable materials in the building and the type of fire.
Thickness calculation method of fireproof coatings