FIRE RESISTANCE

The fire resistance limit is the time interval (in hours or minutes) from the beginning of exposure to fire till the occurrence of one of the limit conditions of elements and structures.

Limit conditions of building structures

Marking	Name	Criterion
R	loss of load-bearing capacity	Loss of ability of the structure or element to withstand the set impacts during a fire, according to the defined criterion
Е	loss of integrity	Loss of the ability of the enclosing element of the building structure exposed to fire from the one side to prevent the penetration of flames and hot gases, and to prevent the occurrence of flames on the side that is not exposed to fire
I	loss of heat-insulating ability	Loss of the ability of the enclosing element of the building structure exposed to fire from the one side to limit the temperature increase to a certain level on the surface not exposed to heat

Building structures are divided into fire resistance classes depending on the standardized fire resistance limit condition and fire resistance limits. Marking of fire resistance classes of building structures consists of conditional letter designations of limit conditions (R, E, I) and the number corresponding to the standardized fire resistance limit, in minutes selected from the following: 15; 30; 45; 60; 90; 120; 150; 180; 240; 360.

To standardize fire resistance classes of building structures, which directly perform only the load-bearing function in the building, only the letter designation of the limit condition R is used — for columns, beams, trusses, arches, frames. For structures performing only the enclosing functions (not load-bearing, self-load-bearing), the letter designation of the limit condition E, I is used — for external non-load-bearing walls, internal partitions.

If different fire resistance limits are standardized by different limit conditions for a structure, the designation of the fire resistance class consists of two or three parts separated by a slash.

For example: Fire resistance class R 120 means that the value of the fire resistance limit must be not less than 120 minutes and not more than 150 minutes by the criterion of loss of load-bearing capacity of the structure;

The fire resistance class REI 150 means that the value of the fire resistance limit must be at least 150 minutes and not more than 180 minutes by the criteria of loss of load-bearing capacity, loss of integrity and insulating capacity, regardless of which of these three limit conditions occurs earlier.

The main fire resistance indicator for standardizing fire resistance classes of load-bearing steel structures is the loss of load-bearing capacity of structures and assemblies R.

The fire resistance limit of most unprotected steel structures is extremely low and is 10-15 minutes depending on the metal thickness. The exception is steel columns with thick solid cross-section, the fire resistance class of which can reach R30-R45 without systems of fire protective coatings, but the use of such structures in practice is extremely limited.

As a result of exposure to heat, the limit condition of a steel structure by the criterion of loss of bearing capacity (R) occurs. With other conditions being equal, the value of R depends on the section factor and the critical temperature of the steel structure.

There are several options for calculating section factors of steel structures, but they are all in direct or inverse relationship with each other — profile, box section factors, adjusted thickness of the metal, etc. The thicker the steel structure is, the slower it responds to heat and fire loads and has a higher fire resistance limit. Thinner steel structures have less inertia to the heating of the entire structure, and as a result, they have a lower fire resistance limit.

Critical temperature of the steel structure is the temperature at which failure of an element of the steel structure is expected subject to uniform temperature distribution over the cross-sectional area for a given level of load (i.e. loss of load-bearing capacity of the element).

The critical temperature of a metal structure primarily depends on the type of steel used.

Grade of steel in the structure	Critical temperature, °C
Low-alloy steel 30HG2S	500
Low-alloy steel 25G2S	550
Steel of grade 10X17H13M2T	700

A temperature of about 500 °C was used in Ukraine for a long time as the main critical (design) temperature of steel structures with fire protective coatings and cladding according to DSTU B V.1.1-4-98*.

However, the adoption of the standards DSTU-N B EN 1993-1-2: 2010 and DSTU-N B V.2.6-211: 2016 in Ukraine allows to apply a differential approach to determining the critical temperature of steel elements and calculating the fire resistance of steel structures in accordance with Eurocodes 3.

The use of design values of critical temperatures in designing fire protective treatment can significantly reduce the cost of the fire protective material, and thus the costs for fire protection works.