General

Regulations require certain elements of structure to have fire resistance. Whether or not an element requires fire resistance depends upon such things as size, use of building and the function of the element.

When exposed to fire all commonly used structural materials lose some of their strength. Heavily loaded steel will lose its design margin of safety at temperatures around 550°C regardless of the grade of steel.

Where structural steel members are required to have fire resistance, they can be protected by applying insulating materials. In considering any fire protection system it is important to distinguish between profile, box and solid methods of application.

Calculation

Methods of assessing the performance of fire protection materials have been developed which enable the thickness of protection for a wide range of situations to be predicted. The procedure is in two parts. Firstly, a carefully designed program of fire tests is carried out on both loaded and unloaded specimens and, secondly, a mathematical procedure is applied to the results of the tests which enable predictions of required thickness to be made. These programs of tests are designed to determine both the insulation characteristics of a fire protection material and its physical performance under fire conditions for a range of steel sizes. They generate the maximum amount of data from a minimum number of tests. (ENV13381-4)

Using fire design codes such as the Structural Eurocodes, EC3–1.2 and EC4-1.2, designated EN 1993-1.2 and 1994-1.2, the load on the structure at the time of the fire can be calculated by treating it as an accidental limit state. If used, this will allow designers to specify to the protection contractor a limiting or failure temperature for a given structural section.

The protection contractor will then be able to use the required thickness of material to ensure that the steel section does not exceed this temperature, within the fire resistance period. This process could be simplified by the designer specifying a maximum steel temperature, based on the worst case, for all beams or columns on one floor level.

Thicknesses are given for a range of steel temperatures. It is the responsibility of the design engineer, using design codes such as ENV1993-1-2, to specify the appropriate limiting steel temperatures.

Steel columns and beams

The rate of increase in temperature of a steel cross-section is determined by the ratio of the heated surface area (A) to the volume (V). This ratio, A/V, has units of m-1 and is known as the "Section Factor". Members with low Section Factors will heat up more slowly.

A steel section with a large surface area (A) (m²/m) will receive more heat than one with a smaller surface area. Also, the greater the volume (V) (m³/m) of the section, the greater is the heat sink. It follows therefore, that a small thick section will be slower to increase in temperature than a large thin one.

The Section Factor (A/V) is thus a measure of the rate at which a section will heat up in a fire and the higher its value, the greater will be the protection thickness required. In calculating the Section Factor values the full volume, V, is used whether the section is exposed on three or four sides as the whole of the steel section will be receiving heat. A, however, is the exposed surface area and that depends on the configuration of the fire protection.