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Classification of buildings and structures by fire resistance.

In assessing the fire safety qualities of buildings and structures great value has their fire resistance.

Fire resistance is the ability of building structural elements buildings to perform load-bearing and enclosing functions in fire conditions for a certain time. It is characterized by fire resistance.

The fire resistance limits of facility structures must be such that the structures retain their load-bearing and enclosing functions throughout the entire duration of the evacuation of people or their stay in places of collective protection. In this case, fire resistance limits must be assigned without taking into account the impact of extinguishing agents on the development of a fire.

The fire resistance limit of building structures is determined by the time (hours) from the start of the fire until one of the signs occurs: a) the formation of through cracks in the structure; b) an increase in temperature on the unheated surface of the structure by an average of more than 140 ° C or at any point on this surface by more than 180 ° C compared to the temperature of the structure before testing, or by more than 220 ° C regardless of the temperature of the structure before testing; d) loss of load-bearing capacity of the structure.

The fire resistance limit of individual building structures depends on their dimensions (thickness or cross-section) and the physical properties of the materials. For example, the stone walls of a building are 120 mm thick. have a fire resistance limit of 2.5 hours, and with a thickness of 250 mm the fire resistance limit increases to 5.5 hours.

The degree of fire resistance of a building depends on the degree of flammability and the fire resistance limit of its main building structures. All buildings and structures are divided into five degrees according to fire resistance (Table 32).

Table 32 Classification of buildings and structures by fire resistance.

Fire resistance degree	Basic building structures
Fire resistance degree	load-bearing walls, walls stairwells, columns	external curtain panel walls and external half-timbered walls	slabs, decking and others load-bearing structures interfloor and attic floors	slabs, decking and other load-bearing structures of coverings	internal load-bearing walls (partitions)	fire walls
I	Fireproof (2.5)	Fireproof (0.5)	Fireproof (1.0)	Fireproof (0.5)	Fireproof (0.5)	Fireproof (2.5)
II	Fireproof (2.0)	Fireproof (0.25); fire-resistant (0.5)	Fireproof (0.75)	Fireproof (0.25)	Fire-resistant (0.25)	Fireproof (2.5)
III	Fireproof (2.0)	Fireproof (0.25); fire-resistant (0.15)	Fire-resistant (0.75)	Combustible	Fire-resistant (0.25)	Fireproof (2.5)
IV	Fire-resistant (0.5)	Fire-resistant (0.25)	Fire-resistant (0.25)	»	Fire-resistant (0.25)	Fireproof (2.5)
V	Combustible	Combustible	Combustible	»	Combustible	Fireproof (2.5)

Note. The fire resistance limits (h) are indicated in parentheses.

This division into degrees was introduced by SNiP II-A. 5-70, which gives nine notes to keep in mind when using the table.

IIIa from SNiP 2.01.02-85* APPENDIX 2 Reference
SAMPLE CONSTRUCTION CHARACTERISTICS OF BUILDINGS
DEPENDING ON THEIR DEGREE OF FIRE RESISTANCE
1. Fire resistance level
2. Design characteristics

I
Buildings with load-bearing and enclosing structures made of natural or artificial stone materials, concrete or reinforced concrete using sheet and slab non-combustible materials

II
Same. It is allowed to use unprotected steel structures in building coverings

III
Buildings with load-bearing and enclosing structures made of natural or artificial stone materials, concrete or reinforced concrete. For floors it is allowed to use wooden structures, protected by plaster or low-flammability sheets, as well as slab materials. There are no requirements for fire resistance limits and fire spread limits for coating elements, while attic wood roofing elements are subject to fire retardant treatment

IIIa
Buildings predominantly with frame design diagram. The frame elements are made of unprotected steel structures. Enclosing structures - made of profiled steel sheets or other non-combustible sheet materials with low-flammability insulation

IIIb
The buildings are predominantly one-story with a frame structural design. Frame elements are made of solid or laminated wood, subjected to fire retardant treatment, ensuring the required limit of fire spread. Enclosing structures - made of panels or element-by-element assembly, made using wood or wood-based materials. Wood and other combustible materials of enclosing structures must be subjected to fire retardant treatment or protected from fire and high temperatures in such a way as to ensure the required fire spread limit.

IV
Buildings with load-bearing and enclosing structures made of solid or laminated wood and other combustible or low-combustible materials, protected from fire and high temperatures by plaster or other sheet or slab materials. There are no requirements for fire resistance limits and fire spread limits for coating elements, while attic wood roofing elements are subject to fire retardant treatment

IVа
The buildings are predominantly one-story with a frame structural design. The frame elements are made of unprotected steel structures. Enclosing structures - made of profiled steel sheets or other non-combustible materials with combustible insulation

V
Buildings, the load-bearing and enclosing structures of which are not subject to requirements for fire resistance limits and fire spread limits

Note. The building structures given in this appendix must meet the requirements of Table. 1 and other standards of this SNiP.

The highest degree of fire resistance is I (mausoleum).

When assessing the fire characteristics (properties) of various buildings or structures special attention the degree of fire resistance is taken into account. Fire resistance means the functional ability of structural components of structures to suppress the spread of fire without losing their properties. performance characteristics. These properties include load-bearing and enclosing abilities. Let's look at these concepts in more detail.

Fire resistance limit of a building: definition, factors influencing its values

With the loss of load-bearing capacity, the integrity of the building is violated, and the loss of enclosing capacity entails the appearance of cracks and through holes, up to the penetration of fire into the buildings, followed by burning.

The fire resistance limit of a building is the time from the start of combustion in a fire until the time signs of loss appear, namely such as:

the appearance of through-type cracks;
an increase in temperature indicators on the unheated part above 140°C or in any place above 180°C in comparison with the temperature of the entire structure before testing;
loss of structural functional characteristics.

The value of the fire resistance limit is influenced by the dimensions and physical properties materials. The thicker the walls, the longer (in time) the fire resistance limit will be. The degree of fire resistance of a building is influenced by:

number of storeys of the building;
square;
type of building (administrative, residential, etc.);
quality and degree of fire resistance of materials.

The degree of fire resistance of a building depends on the fire resistance of building structures. They are divided into three main groups:

fireproof (stone, brick, metal structures);
fire-resistant (combustible materials, the surface of which is protected by a non-combustible mixture);
easily combustible (wood).

Classification of buildings by degree of fire resistance

The fire resistance of a building is determined in strict accordance with building codes and rules (SNiP). Thus, according to the degree of fire resistance, all buildings are divided into five main groups. First group. Buildings most protected from negative consequences arising as a result of fire. The main materials used for these structures are concrete and stone, which are resistant to elevated temperatures and fire.

Second group also covers buildings with fire-resistant structures, as in the first case, with a slight allowance for the use of unprotected elements in steel structures. By third grade include buildings in the structural structure of which there are fireproof and fire-resistant materials. If the structure includes combustible materials, then they must be treated with a special fire-retardant mixture.

Buildings that are assigned fourth degree of fire resistance, must have fire-resistant walls in their design, and non-combustible materials must be used for load-bearing walls. For buildings included to the fifth group, the use of combustible materials is typical, however for load-bearing walls, as for buildings of the fourth degree of fire resistance, materials of a fireproof nature are used. The degree of fire resistance of a building (structure) must coincide with the explosion and fire safety of the premises.

Buildings made of brick have a high degree of fire protection - the first degree of fire resistance. Brick is a material that is resistant to combustion processes - it does not burn or smolder, which is why most construction companies prefer to build houses from this material.

Factors that influence the degree of fire resistance of a residential building

The degree of fire resistance of any residential building is affected by its number of floors and area - the higher the residential building and the larger the area, the higher the degree of fire resistance. Basically, brick, stone or concrete are used for residential buildings, so they are given the first degree of fire resistance. If brick and concrete block elements are used to build such a structure, then this is the second class of fire resistance. For houses built on metal frame, with sheathing made of fire-resistant materials, are assigned a third degree of fire resistance.

Houses with a base of wooden frame assign the fourth degree of fire resistance, and the fifth class includes houses that are most susceptible to fire.

In connection with fires that occur in administrative and residential premises, much attention during the construction of buildings is paid to such a criterion as the fire resistance of buildings. The fire resistance of any building is calculated taking into account the above features and building codes and regulations (SNiP).

Fires caused by humans have become quite common and widespread. Thousands of fires occur every year, causing a range of unpleasant consequences. Therefore, when constructing structures, the degree of fire resistance of the building is of great importance. Each constructed object is assigned a specific fire resistance number, according to the existing classification. Next, we will consider the classification in more detail and describe the parameters of each class.

What is the degree of fire resistance?

Fire resistance level of the structure	Fire safety class of the structure	Maximum permissible height of the structure, cm	Allowable floor S, cm2
I	Co Co Cl	7500 5000 2800	250000 250000 220000
II	Co Co Cl	2800 2800 1500	180000 180000 180000
III	Co Cl C2	500 500 200	10000 80000 120000
IV	Without rationing	500	50000
V	Without rationing

SNiP 31-01-03

This definition refers to the ability of structures to restrain the expansion of the flammable area without the building losing its ability further exploitation. The list of these properties consists of enclosing and load-bearing abilities.

If a structure loses its load-bearing capacity, it will certainly collapse. It is by destruction that this definition is meant. As for the barrier ability, its loss is considered to be the level of heating of materials before the formation of cracks or holes through which combustion products can spread into adjoining rooms or heating to the temperature at which the combustion process of the material begins.

The indicator of the maximum degree of fire resistance of structures is the time interval from the moment of fire formation to the appearance of signs of such losses (measured in hours). To test the performance of materials under fire conditions, a prototype is taken and placed in equipment for such experiments - a special oven. In a kiln environment, the test item is subjected to high-temperature fire, which places stress on the material that is specific to the specific project.

The degree of fire resistance, when determining its limit, also depends on the ability to increase the temperature at individual points or the average value of the increase in temperature indicators over the surface, which is compared with the original one. The structural elements of the structure made of metal have the minimum resistance to fire, and the maximum resistance is reinforced concrete, in the manufacture of which cement with high fire resistance characteristics was used. The maximum fire resistance level can reach 2.5 hours.

Also, when determining the ability of a structure to withstand fire, the limit of fire spread is taken into account. It is equivalent to the extent of damage in areas that were outside the combustion zone. This figure can be 0-40 cm.

We can safely say that the degree of fire resistance of structures directly depends on the ability of the materials used in its construction to withstand high temperatures affecting the surface in a fire environment.

According to the degree of combustion, materials are divided into 3 groups:

Fireproof (reinforced concrete structures, brick, stone elements).
Refractory (materials from the combustible group, the fire resistance of which is increased by treatment with special means).
Combustible (quickly ignites and burns well).

To classify materials, a special set of documents is used - SNIP.

How is it determined?

The degree of fire resistance is a representative of the most significant parameters of a structure, not inferior in importance to the design features in terms of fire safety and functional characteristics. But what should you pay attention to in order to determine it with utmost accuracy? To do this, you need to consider the following construction parameters:

Number of storeys.
Actual area of the structure.
Nature of the building's purpose: industrial, residential, commercial, etc.

To determine the degree of fire resistance (I, II, etc.) it is necessary to determine exclusively the regulatory documents and those given in SNIP. Also for such purposes and design high-rise buildings DBN 1.1-7-2002 is used, to determine the fire safety of multi-storey structures, 4 DBN V.2.2-15-2005 are used, and to become familiar with the fire safety requirements for structures with a large number of floors, 9 DBN V.2.2-24:2009 are used. Only the use of special documentation will allow us to obtain the most complete information about the degrees of fire resistance of buildings with different design features.

Similar questions have arisen repeatedly. I keep an extract from the Soviet norms
IIIa from SNiP 2.01.02-85* APPENDIX 2 Reference
SAMPLE CONSTRUCTION CHARACTERISTICS OF BUILDINGS
DEPENDING ON THEIR DEGREE OF FIRE RESISTANCE
1. Fire resistance level
2. Design characteristics

I
Buildings with load-bearing and enclosing structures made of natural or artificial stone materials, concrete or reinforced concrete using sheet and slab non-combustible materials

II
Same. It is allowed to use unprotected steel structures in building coverings

III
Buildings with load-bearing and enclosing structures made of natural or artificial stone materials, concrete or reinforced concrete. For floors, it is allowed to use wooden structures protected by plaster or low-flammability sheet and slab materials. There are no requirements for fire resistance limits and fire spread limits for coating elements, while attic wood roofing elements are subject to fire retardant treatment

IIIa
Buildings predominantly have a frame structural design. The frame elements are made of unprotected steel structures. Enclosing structures - made of profiled steel sheets or other non-combustible sheet materials with low-flammable insulation

IIIb
The buildings are predominantly one-story with a frame structural design. Frame elements are made of solid or laminated wood, subjected to fire retardant treatment, ensuring the required limit of fire spread. Enclosing structures - made of panels or element-by-element assembly, made using wood or wood-based materials. Wood and other combustible materials of enclosing structures must be subjected to fire retardant treatment or protected from exposure to fire and high temperatures in such a way as to ensure the required limit of fire spread.

V
Buildings, the load-bearing and enclosing structures of which are not subject to requirements for fire resistance limits and fire spread limits

Note. The building structures given in this appendix must meet the requirements of Table. 1 and other standards of this SNiP.

The highest degree of fire resistance is I (mausoleum).