What is fire resistance and fire resistance limit?

Fire resistance of a structure - the ability of a building structure to resist fire and limit the spread of fire, as well as maintain the required performance at high temperatures in a fire. It is characterized by the limits of fire resistance and fire propagation. Fire protection of building structures is the main task in the design and construction of buildings and structures. Increasing the fire resistance of building structures directly proportionally increases the fire safety of people at the facility and people extinguishing a fire in the event of a fire.

Fire resistance limit - the time in minutes (hours) from the start of the fire to the failure of the structure (until the loss of bearing capacity, collapse, achievement of irreversible deformations or until the formation of through cracks), or warming up until the temperature rises on the surface opposite to the fire of the order of 220 ° C, above which self-ignition of organic materials is possible. (Source: Dictionary of architectural and construction terms).

In other words, the fire resistance limit is the time in minutes (hours) during which the building structure retains its fire resistance.

The limiting state of a structure in terms of fire resistance is a state of a structure in which it loses the ability to maintain one of its fire-fighting functions.

Fire resistance limit of building structures

The limits of fire resistance of building structures are determined by their fire tests according to the standard method and are expressed by the time (hours or minutes) of the action on the structure of the so-called standard fire (see below) until it reaches one of the following limit states:

1. loss of bearing capacity (collapse or deflection) under the design support scheme and the action of the standard load - constant from own. the weight of the structure and temporary, long-term, from the weight, for example, of stationary equipment (machine tools, apparatus and machines, electric motors, etc.);
2. an increase in the temperature of an unheated surface by an average of more than 160 ° C or at any point of it more than 190 ° C in comparison with the initial temperature or more than 220 ° C, regardless of the temperature of the structure before testing;
3. the formation of through cracks or holes in the structure through which combustion products or flame penetrate;
4. the achievement of the critical temperature (i.e., the temperature at which irreversible changes in physical and mechanical properties occur) during testing of an unloaded structure of its load-bearing elements or parts protected by fire-retardant coatings and facings; characterizes the loss of bearing capacity.

The limits of fire propagation are determined by the dimensions (cm) of their damage due to burning or charring outside the zone of influence of a standard fire.

The following limiting states are used to standardize the fire resistance limits of load-bearing and enclosing structures: For columns, beams, trusses, arches and frames - only the loss of the bearing capacity of the structure and nodes - R; For external load-bearing walls and coatings - loss of bearing capacity and integrity - R, E, for external curtain walls - E. For non-load-bearing internal walls and partitions - loss of thermal insulation capacity and integrity - E, I; For load-bearing internal walls and fire barriers - loss of bearing capacity, integrity and thermal insulation capacity - R, E, I. - limit designations.

The designation of the limit of fire resistance of building structures consists of the symbols normalized for a given structure of limit states, a figure corresponding to the time to reach one of the limit states (the first in time) in minutes.

For example :
R 120 - fire resistance 120 minutes - loss of bearing capacity;
RE 60 - fire resistance limit of 60 minutes - for loss of bearing capacity and loss of integrity, regardless of which of the two limit states occurs earlier;
REI 30 - fire resistance limit of 30 minutes - for the loss of bearing capacity, integrity and thermal insulation capacity, no matter which one occurs earlier.

If different fire resistance limits for different limit states are normalized for a structure, the designation of the fire resistance limit consists of two or three parts, separated by an oblique line.

For example :
R 120 / EI 60 - fire resistance limit of 120 minutes for loss of bearing capacity / fire resistance limit of 60 minutes for loss of integrity or thermal insulation, regardless of which of the latter two occurs earlier.
R 120 / EI 60 - fire resistance limit of 120 minutes for loss of bearing capacity / fire resistance limit of 60 minutes for loss of integrity or thermal insulation, regardless of which of the latter two occurs earlier.

Basic requirements for fire resistance of building structures laid down in SNiP 21-01-97 *, FZ "Technical regulations on fire safety requirements" dated 22.07.08 No. 123.

What is the actual fire resistance of steel structures?

Metals are highly sensitive to high temperatures and fire. They heat up quickly, which significantly reduces their strength properties. The actual limit of fire resistance of steel structures, depending on the thickness of the section elements and the effective stresses, is from 0.1 to 0.4 hours. While the minimum values ​​of the required limits of fire resistance of the main building structures, including metal, are from 0.25 to 2.5 hours, depending on the degree of fire resistance of buildings and the type of structures. To meet these requirements, it is necessary to take measures for the fire protection of metal surfaces.