. .

Limitfire resistance of the structure- the period of time from the beginning of fire exposure under standard test conditions until the onset of one of the limit states normalized for a given design.

For load-bearing steel structures, the limit state is the load-bearing capacity, that is, the indicator R.

Although metal (steel) structures are made of fireproof materials, the actual fire resistance limit is on average 15 minutes. This is explained by a fairly rapid decrease in the strength and deformation characteristics of the metal at elevated temperatures during a fire. The intensity of heating of MC depends on a number of factors, which include the nature of heating of structures and methods of protecting them.

There are several fire temperature regimes:

Standard fire;

Fire mode in the tunnel;

Hydrocarbon fire mode;

External fire modes, etc.

When determining fire resistance limits, a standard temperature regime is created, characterized by the following dependence

Where T- temperature in the furnace corresponding to time t, degrees C;

That- temperature in the furnace before the start of thermal exposure (taken equal to the temperature environment), deg. WITH;

t- time calculated from the beginning of the test, min.

The temperature regime of a hydrocarbon fire is expressed by the following relationship

Reaching the fire resistance limit metal structures occurs as a result of loss of strength or due to loss of stability of the structures themselves or their elements. Both cases correspond to a certain heating temperature of the metal, called critical, i.e. at which the formation of a plastic hinge occurs.

Calculation of the fire resistance limit comes down to solving two problems:static and thermal engineering.

The static problem aims to determine bearing capacity structures taking into account changes in metal properties during high temperatures ah, that is determining the critical temperature at the moment the limiting state occurs in a fire.

As a result of solving the thermal engineering problem, the heating time of the metal is determined from the onset of the fire until the critical temperature is reached in the design section, i.e. solving this problem allows us to determine the actual fire resistance limit of the structure.

The basics of modern calculation of the fire resistance limit of steel structures are presented in the book "Fire Resistance building structures" *I.L. Mosalkov, G.F. Plyusnina, A.Yu. Frolov Moscow, 2001 Special equipment), where section 3 on pp. 105-179 is devoted to the calculation of the fire resistance limit of steel structures.

The method for calculating the fire resistance limits of steel structures with fire retardant coatings is set out in the VNIIPO Methodological Recommendations "Fire protection means for steel structures. Calculation and experimental method for determining the fire resistance limit of load-bearing metal structures with thin-layer fire retardant coatings."

The result of the calculation is a conclusion about the actual fire resistance limit of the structure, including taking into account decisions on its fire protection.

To solve a thermotechnical problem, i.e. tasks in which it is necessary to determine the time for heating a structure to a critical temperature, it is necessary to know the design loading scheme, the reduced thickness of the metal structure, the number of heated sides, steel grade, sections (moment resistance), as well as heat-shielding properties fire retardant coatings.

The effectiveness of fire protection means for steel structures is determined according to GOST R 53295-2009 “Fire protection means for steel structures. General requirements. Method for determining fire retardant efficiency." Unfortunately, this standard cannot be used to determine fire resistance limits, this is directly written in paragraph 1 "Scope":" Real the standard does not apply to the determination limitsfire resistance of building structures with fire protection".

The fact is that according to GOST, as a result of tests, the time for heating the structure to a conditionally critical temperature of 500C is established, while the calculated critical temperature depends on the “safety margin” of the structure and its value can be either less than 500C or more.

Abroad, fire protection products are tested for fire retardant effectiveness upon reaching critical temperatures of 250C, 300C, 350C, 400C, 450C, 500C, 550C, 600C, 650C, 700C, 750C.

The required fire resistance limits are established by Art. 87 and table No. 21 Technical regulations on fire safety requirements.

The degree of fire resistance is determined in accordance with the requirements of SP 2.13130.2012 "Systems fire protection. Ensuring the fire resistance of protected objects."

In accordance with the requirements of clause 5.4.3 SP 2.13130.2012 .... allowed use unprotected steel structures regardless of their actual fire resistance limit, except in cases where the fire resistance limit of at least one of the elements load-bearing structures(structural elements of trusses, beams, columns, etc.) according to test results is less than R 8. Here the actual fire resistance limit is determined by calculation.

In addition, the same paragraph limits the use of thin-layer fire-retardant coatings (fire-retardant paints) for load-bearing structures with a reduced metal thickness of 5.8 mm or less in buildings of fire resistance degrees I and II.

Load-bearing steel structures are in most cases elements of the frame-braced frame of a building, the stability of which depends both on the fire resistance limit of the load-bearing columns and on the covering elements, beams and ties.

In accordance with the requirements of clause 5.4.2 SP 2.13130.2012 "TO load-bearing elements buildings include load-bearing walls, columns, braces, stiffening diaphragms, trusses, elements of floors and roofless coverings (beams, crossbars, slabs, decking), if they are involved in ensuring the overall sustainability and geometric immutability of the building in case of fire. Information about supporting structures that are not involved in providing general sustainabilityand geometric immutability of the building are given by the design organization in technical documentation on the building".

Thus, all elements of the frame-braced frame of the building must have a fire resistance limit according to the highest of them.

Determination of fire resistance limits of structures, limits of fire spread through structures and flammability groups of materials

(Benefit)

The manual contains data on standardized fire resistance indicators and fire danger building structures and materials.

In cases where the information given in the manual is insufficient to establish the appropriate indicators of structures and materials, you should contact the TsNIISK im. Kucherenko or NIIZhB of the USSR State Construction Committee. The basis for establishing these indicators can also be the results of tests performed in accordance with standards and methods approved or agreed upon by the USSR State Construction Committee.

2. BUILDING STRUCTURES. FIRE RESISTANCE LIMITS AND FIRE SPREAD LIMITS

2.1. The fire resistance limits of building structures are determined according to the CMEA standard 1000-78 " Fire regulations construction design. Method of testing building structures for fire resistance."

The limit of fire spread through building structures is determined according to the methodology.

Fire resistance limit

2.2. The fire resistance limit of building structures is taken to be the time (in hours or minutes) from the start of their standard fire test until the occurrence of one of the fire resistance limit states.

2.3. The SEV 1000-78 standard distinguishes the following four types of limit states for fire resistance: for loss of load-bearing capacity of structures and components (collapse or deflection depending on the type of structure;) for thermal insulation capacity - an increase in temperature on an unheated surface by an average of more than 160 ° C or at any point on this surface by more than 190°C compared to the temperature of the structure before testing, or more than 220°C regardless of the temperature of the structure before testing; by density - the formation in structures of through cracks or through holes through which combustion products or flames penetrate; for structures protected by fire-retardant coatings and tested without loads, the limiting state will be the achievement of a critical temperature of the material of the structure.

For external walls, coverings, beams, trusses, columns and pillars, the limiting state is only the loss of the load-bearing capacity of structures and components.

2.4. The limit states of structures for fire resistance specified in clause 2.3 will be further referred to, for brevity, as I, II, III and IV limit states of structures for fire resistance, respectively.

In cases of determining the fire resistance limit at loads determined on the basis detailed analysis conditions that arise during a fire and differ from the standard ones, the limiting state of the structure will be designated 1A.

2.5. The fire resistance limits of structures can also be determined by calculation. In these cases, tests may not be carried out.

Determination of fire resistance limits by calculation should be carried out according to methods approved by the Glavtekhnormirovanie of the USSR State Construction Committee.

2.6. For an approximate assessment of the fire resistance limit of structures during their development and design, one can be guided by the following provisions:

a) the fire resistance limit of layered enclosing structures in terms of thermal insulation capacity is equal to, and, as a rule, higher than the sum of the fire resistance limits of individual layers. It follows that increasing the number of layers of the enclosing structure (plastering, cladding) does not reduce its fire resistance limit in terms of heat-insulating ability. In some cases, the introduction of an additional layer may not have an effect, for example, when facing with sheet metal on the unheated side;

b) the fire resistance limits of enclosing structures with an air gap are on average 10% higher than the fire resistance limits of the same structures, but without an air gap; the efficiency of the air gap is higher, the further it is removed from the heated plane; with closed air gaps their thickness does not affect the fire resistance limit;

c) the fire resistance limits of enclosing structures with an asymmetrical arrangement of layers depend on the direction of the heat flow. On the side where the likelihood of a fire is higher, it is recommended to place fireproof materials with low thermal conductivity;

d) an increase in the humidity of structures helps to reduce the rate of heating and increase fire resistance, except in cases where an increase in humidity increases the likelihood of sudden brittle destruction of the material or the appearance of local spalls; this phenomenon is especially dangerous for concrete and asbestos-cement structures;

e) the fire resistance limit of loaded structures decreases with increasing load. The most stressed section of structures exposed to fire and high temperatures, as a rule, determines the value of the fire resistance limit;

f) the fire resistance limit of a structure is higher, the smaller the ratio of the heated perimeter of the cross-section of its elements to their Area;

g) the fire resistance limit of statically indeterminate structures, as a rule, is higher than the fire resistance limit of similar statically indeterminate structures due to the redistribution of forces to less stressed elements that are heated at a lower rate; in this case, it is necessary to take into account the influence of additional forces arising due to temperature deformations;

h) the flammability of the materials from which the structure is made does not determine its fire resistance limit. For example, structures made of thin-walled metal profiles have a minimum limit of fire resistance, and structures made of wood have a higher limit of fire resistance than structures made of steel with the same ratio of the heated perimeter of the section to its area and the magnitude of the operating stresses to the temporary resistance or yield strength. At the same time, it should be taken into account that the use of combustible materials instead of difficult-to-burn or non-combustible materials can reduce the fire resistance limit of the structure if the rate of its burnout is higher than the rate of heating.

To assess the fire resistance limit of structures based on the above provisions, it is necessary to have sufficient information about the fire resistance limits of structures similar to those considered in shape, materials used and design, as well as information about the main patterns of their behavior in case of fire or fire tests.

2.7. In cases where in the table. 2-15 fire resistance limits are indicated for similar structures various sizes, the fire resistance limit of a structure having an intermediate size can be determined by linear interpolation. For reinforced concrete structures, interpolation should also be carried out based on the distance to the reinforcement axis.

Fire spread limit

2.8. Testing building structures for fire spread consists of determining the extent of damage to the structure due to its combustion outside the heating zone - in the control zone.

2.9. Damage is considered to be charring or burning of materials that can be detected visually, as well as melting of thermoplastic materials.

The limit of fire spread is taken to be maximum size damage (cm), determined according to the test method.

2.10. Structures made using combustible and non-combustible materials, usually without finishing or cladding, are tested for the spread of fire.

Structures made only from non-combustible materials should be considered not to spread fire (the limit of fire spread through them should be taken equal to zero).

If, when testing for fire spread, damage to structures in the control zone is no more than 5 cm, it should also be considered not to spread fire.

2.11. For a preliminary assessment of the fire spread limit, the following provisions can be used:

a) structures made of combustible materials have a horizontal fire spread limit (for horizontal structures- floors, coverings, beams, etc.) more than 25 cm, and vertically (for vertical structures - walls, partitions, columns, etc.) - more than 40 cm;

b) structures made of combustible or hardly combustible materials, protected from fire and high temperatures by non-combustible materials, may have a horizontal fire spread limit of less than 25 cm, and a vertical limit of less than 40 cm, provided that the protective layer is in place during the entire test period (until the structure has completely cooled) will not warm up in the control zone to the ignition temperature or the beginning of intense thermal decomposition of the protected material. The structure may not spread fire provided that the outer layer, made of non-combustible materials, does not warm up in the heating zone to the ignition temperature or the beginning of intense thermal decomposition of the protected material during the entire test period (until the structure has completely cooled down);

c) in cases where a structure may have a different limit for the spread of fire when heated from different sides (for example, with an asymmetrical arrangement of layers in the enclosing structure), this limit is set according to its maximum value.

Concrete and reinforced concrete structures

2.12. The main parameters that influence the fire resistance limit of concrete and reinforced concrete structures are: the type of concrete, binder and filler; reinforcement class;

type of construction; cross-sectional shape; element sizes;

conditions for their heating; load magnitude and concrete moisture content.

2.13. The increase in temperature in the concrete cross-section of an element during a fire depends on the type of concrete, binder and fillers and on the ratio of the surface affected by the flame to the cross-sectional area. Heavy concrete with silicate filler warms up faster than with carbonate filler. Lightweight concrete warms up more slowly, the lower its density. The polymer binder, like the carbonate filler, reduces the rate of heating of concrete due to the decomposition reactions occurring in them, which consume heat. Massive structural elements better resist the effects of fire; the fire resistance limit of columns heated on four sides is less than the fire resistance limit of columns with one-sided heating; The fire resistance limit of beams when exposed to fire on three sides is less than the fire resistance limit of beams heated on one side.

2.14. The minimum dimensions of elements and distances from the axis of the reinforcement to the surfaces of the element are taken according to the tables of this section, but not less than those required by chapter SNiP 11-21-75 “Concrete and reinforced concrete structures”.

2.15. Distance to the reinforcement axis and minimum dimensions elements to ensure the required fire resistance limit of structures depend on the type of concrete. Lightweight concrete has a thermal conductivity of 10-20%, and concrete with coarse carbonate aggregate is 5-10% less than heavy concrete with silicate aggregate. In this regard, the distance to the reinforcement axis for a structure made of lightweight concrete or heavy concrete with carbonate filler can be taken less than for structures made of heavy concrete with silicate filler with the same fire resistance limit for structures made from these concretes.

Rice. 1. Distance to the reinforcement axis.

The values of fire resistance limits given in table. 2-6, 8, refer to concrete with coarse silicate rock aggregate, as well as dense silicate concrete.

Rice. 2. Average distance

to the axis of the reinforcement.

When using carbonate rock filler, the minimum dimensions of both the cross-section and the distance from the axes of the reinforcement to the surface of the bending element can be reduced by 10%. For lightweight concrete, the reduction can be 20% with a concrete density of 1.2 t/m3 and by 30% for bending elements (see Tables 3, 5, 6, 8) with a concrete density of 0.8 t/m3 and expanded clay. perlite concrete with a density of 1.2 t/m3.

2.16. During a fire, a protective layer of concrete protects the reinforcement from rapid heating and reaching its critical temperature, at which the fire resistance of the structure reaches its limit.

If the distance adopted in the project to the axis of the reinforcement is less than required to ensure the required fire resistance limit of structures, it should be increased or additional heat-insulating coatings should be applied on the surfaces of the element exposed to fire (Additional heat-insulating coatings can be carried out in accordance with the “Recommendations for the use of fire-retardant coatings for metal structures” - M., Stroyizdat, 1984). Thermal insulation coating made of lime-cement plaster (15 mm thick), gypsum plaster(10 mm) and vermiculite plaster or mineral fiber insulation (5 mm) is equivalent to an increase of 10 mm in the thickness of the heavy concrete layer. If the thickness of the protective layer of concrete is more than 40 mm for heavy concrete and 60 mm for lightweight concrete, the protective layer of concrete must have additional reinforcement on the fire side in the form of a reinforcement mesh with a diameter of 2.5-3 mm (cells 150x150 mm). Protective thermal insulation coatings with a thickness of more than 40 mm must also have additional reinforcement.

In table 2, 4-8 show the distances from the heated surface to the axis of the reinforcement (Fig. 1 and 2).

In cases where the fittings are located in different levels the average distance to the reinforcement axis (A1, A2, ..., An) and the corresponding distances to the axes (a1, a2, ..., an), measured from the nearest heated (bottom or side) surface of the element, according to the formula:

2.17. All steels reduce their tensile or compressive strength when heated. The degree of resistance reduction is greater for hardened high-strength steel reinforcing wires than for mild steel reinforcement bars.

TsNIISK them. Kucherenko Gosstroy USSR

to determine the fire resistance limits of structures, the limits of fire spread across structures and groups

flammability of materials

(KSNiP II-2-80)

Moscow 1985

ORDER OF THE RED BANNER OF LABOR CENTRAL RESEARCH INSTITUTE OF BUILDING STRUCTURES named after. V. A. KUCHERENKO SHNIISK nm. Kucherenko) GOSSTROYA USSR

TO DETERMINE THE LIMITS OF FIRE RESISTANCE OF A STRUCTURE,

LIMITS OF FIRE SPREAD BY STRUCTURES AND GROUPS

FLAMMABILITY OF MATERIALS (to SNiP I-2-80)

Approved

A manual for determining the fire resistance limits of structures, the limits of fire propagation through structures and flammability groups of materials (to SNiP II-2-80) / TsNIISK nm. Kucherenko.- M.: Stroyizdat, 1985.-56 p.

Developed for SNiP 11-2-80 “Fire safety standards for the design of buildings and structures.” Reference data is provided on the limits of fire resistance and fire spread for building structures made of reinforced concrete, metal, wood, asbestos cement, plastics and other building materials, as well as data on the flammability groups of building materials.

For engineering and technical workers of design, construction organizations and state fire supervision authorities.

Table 15, fig. 3.

3206000000-615 047(01)-85

Instruction-norm. (I issue - 62-84

PREFACE

This Manual has been developed for SNiP 11-2-80 “Fire safety standards for the design of buildings and structures.” It contains data on the standardized fire resistance and fire hazard indicators of building structures and materials.

Sec. I manual was developed by TsNIISK them. Kucherenko (Doctor of Technical Sciences, Prof. I. G. Romanenkov, Candidate of Technical Sciences, V. N. Zigern-Korn). Sec. 2 developed by TsNIISK named after. Kucherenko ( Dr. Tech.. Sciences I. G. Romanenkov, Candidates of Engineering. Sciences V. N. Zigern-Korn, L. N. Bruskova, G. M. Kirpichenkov, V. A. Orlov, V. V. Sorokin, engineers A. V. Pestritsky, |V. Y. Yashin |); NIIZHB (Doctor of Technical Sciences V.V. Zhukov; Doctor of Technical Sciences, Prof. A.F. Milovanov; Candidate of Physical and Mathematical Sciences A.E. Segalov, Candidates of Technical Sciences A. A. Gusev, V. V. Solomonov, V. M. Samoilenko; engineers V. F. Gulyaeva, T. N. Malkina); TsNIIEP im. Mezentseva (candidate of technical sciences L. M. Schmidt, engineer P. E. Zhavoronkov); TsNIIPromzdanny (Candidate of Technical Sciences V.V. Fedorov, engineers E.S. Giller, V.V. Sipin) and VNIIPO (Doctor of Technical Sciences, Prof. A.I. Yakovlev; Candidates of Technical Sciences V. P. Bushev, S. V. Davydov, V. G. Olimpiev, N. F. Gavrikov; engineers V. Z. Volokhatykh, Yu. A. Grinchnk, N. P. Savkin, A. N. Sorokin, V. S. Kharitonov, L. V. Sheinina, V. I. Shchelkunov). Sec. 3 developed by TsNIISK named after. Kucherenko (Doctor of Technical Sciences, Prof. I.G. Romanenkov, Candidate of Technical Sciences N.V. Kovyrshina, Engineer V.G. Gonchar) and the Institute of Mining Mechanics of the Georgian Academy of Sciences. SSR (candidate of technical sciences G. S. Abashidze, engineers L. I. Mirashvili, L. V. Gurchumelia).

When developing the Manual, materials from the TsNIIEP of housing and the TsNIIEP of educational buildings of the State Civil Engineering Committee, MIIT Ministry of Railways of the USSR, VNIISTROM and NIPIsilicate concrete of the Ministry of Industrial Construction Materials of the USSR were used.

The text of SNiP II-2-80 used in the Guide is typed in bold. Its points are double numbered; the numbering according to SNiP is given in brackets.

In cases where the information given in the Manual is insufficient to establish the appropriate indicators of structures and materials, you should contact the TsNIISK im. Kucherenko or NIIZhB of the USSR State Construction Committee. The basis for establishing these indicators can also be the results of tests performed in accordance with standards and methods approved or agreed upon by the USSR State Construction Committee.

Please send comments and suggestions regarding the Manual to the following address: Moscow, 109389, 2nd Institutskaya St., 6, TsNIISK im. V. A. Kucherenko.

1. GENERAL PROVISIONS

1.1. The manual has been compiled to help design, construction*# organizations and bodies fire department in order to reduce the cost of time, labor and materials to establish the fire resistance limits of building structures, the limits of fire spread through them and the flammability groups of materials standardized by SNiP II-2-80.

1.2. (2.1). Buildings and structures are divided into five levels according to fire resistance. The degree of fire resistance of buildings and structures is determined by the fire resistance limits of the main building structures and the limits of fire spread through these structures.

1.3. (2.4). Based on flammability, building materials are divided into three groups: non-combustible, non-combustible and combustible.

1.4. The fire resistance limits of structures, the limits of fire spread through them, as well as the flammability groups of materials given in this Manual should be included in the design of structures, provided that their execution fully complies with the description given in the Manual. Materials from the Manual should also be used when developing new designs.

2. BUILDING STRUCTURES.

FIRE RESISTANCE LIMITS AND FIRE SPREAD LIMITS

2.1 (2.3). The fire resistance limits of building structures are determined according to the CMEA standard 1000-78 “Fire safety standards for building design. Method of testing building structures for fire resistance."

The limit of fire spread through building structures is determined according to the methodology given in the appendix. 2.

FIRE RESISTANCE LIMIT

2.3. The SEV 1000-78 standard distinguishes the following four types of limit states for fire resistance: loss of bearing capacity of structures and components (collapse or deflection depending on the type

structures); in terms of thermal insulation ability - an increase in temperature on an unheated surface by an average of more than 160°C or at any point on this surface by more than 190°C compared to the temperature of the structure before testing, or more than 220°C regardless of the temperature of the structure before testing; by density - the formation in structures of through cracks or through holes through which combustion products or flames penetrate; for structures protected by fire-retardant coatings and tested without loads, the limiting state will be the achievement of a critical temperature of the material of the structure.

For external walls, coverings, beams, trusses, columns and pillars, the limiting state is only the loss of the load-bearing capacity of structures and components.

2.4. The limit states of structures for fire resistance specified in clause 2.3 will be further referred to as I, 11, 111 and IV limit states of structures for fire resistance, respectively, for brevity.

In cases of determining the fire resistance limit under loads determined on the basis of a detailed analysis of the conditions that arise during a fire and differ from the standard ones, the limiting state of the structure will be designated 1A.

2.5. The fire resistance limits of structures can also be determined by calculation. In these cases, tests may not be carried out.

Determination of fire resistance limits by calculation should be carried out according to methods approved by the Glavtekhnormirovanie of the USSR State Construction Committee.

2.6. For an approximate assessment of the fire resistance limit of structures during their development and design, one can be guided by the following provisions:

b) the fire resistance limits of enclosing structures with an air gap are on average 10% higher than the fire resistance limits of the same structures, but without an air gap; the efficiency of the air gap is higher, the further it is removed from the heated plane; with closed air gaps, their thickness does not affect the fire resistance limit;

c) fire resistance limits of enclosing structures with asymmetrical

The exact arrangement of the layers depends on the direction of the heat flow. On the side where the likelihood of a fire is higher, it is recommended to place fireproof materials with low thermal conductivity;

d) an increase in the humidity of structures helps to reduce the rate of heating and increase fire resistance, except in cases where an increase in humidity increases the likelihood of sudden brittle destruction of the material or the appearance of local punctures; this phenomenon is especially dangerous for concrete and asbestos-cement structures;

f) the fire resistance limit of a structure is higher, the smaller the ratio of the heated perimeter of the cross-section of its elements to their area;

h) the flammability of the materials from which the structure is made does not determine its fire resistance limit. For example, structures made of thin-walled metal profiles have a minimum fire resistance limit, and structures made of wood have a higher fire resistance limit than steel structures with the same ratio of the heated perimeter of the section to its area and the magnitude of the operating stresses to the temporary resistance or yield strength. At the same time, it should be taken into account that the use of combustible materials instead of difficult-to-burn or non-combustible materials can reduce the fire resistance limit of the structure if the rate of its burnout is higher than the rate of heating.

To assess the fire resistance limit of structures based on the above provisions, it is necessary to have sufficient information about the fire resistance limits of structures similar to those considered in shape, materials used and design, as well as information about the basic patterns of their behavior in case of fire or fire tests.-

2.7. In cases where in the table. 2-15 fire resistance limits are indicated for similar structures of various sizes; the fire resistance limit of a structure having an intermediate size can be determined by linear interpolation. For reinforced concrete structures, interpolation should also be carried out based on the distance to the reinforcement axis.

FIRE SPREAD LIMIT

2.8. (Appendix 2, paragraph 1). Testing building structures for fire spread consists of determining the extent of damage to the structure due to its combustion outside the heating zone - in the control zone.

2.9. Damage is considered to be charring or burning of materials that can be detected visually, as well as melting of thermoplastic materials.

The limit of fire spread is taken to be the maximum size of damage (cm), determined according to the test procedure set out in appendix. 2 to SNiP II-2-80.

2.10. Structures made using combustible and non-combustible materials, usually without finishing or cladding, are tested for the spread of fire.

Structures made only from fireproof materials should be considered not to spread fire (the limit of fire spread through them should be taken equal to zero).

If, when testing for the spread of fire, the damage to structures in the control zone is no more than 5 cm, it should also be considered not to spread fire.

2.11: For a preliminary assessment of the fire spread limit, the following provisions can be used:

a) structures made of combustible materials have a fire spread limit horizontally (for horizontal structures - floors, coverings, beams, etc.) of more than 25 cm, and vertically (for vertical structures - walls, partitions, columns, etc.) .i.) - more than 40 cm;

CONCRETE AND REINFORCED CONCRETE STRUCTURES

2.12. The main parameters that influence the fire resistance limit of concrete and reinforced concrete structures are: the type of concrete, binder and filler; reinforcement class; type of construction; cross-sectional shape; element sizes; conditions for their heating; load magnitude and concrete moisture content.

2.13. The increase in temperature in the concrete cross-section of an element during a fire depends on the type of concrete, binder and fillers, and on the ratio of the surface affected by the flame to the cross-sectional area. Heavy concrete with silicate filler warms up faster than with carbonate filler. Lightweight and lightweight concretes warm up more slowly, the lower their density. The polymer binder, like the carbonate filler, reduces the rate of heating of concrete due to the decomposition reactions occurring in them, which consume heat.

Massive structural elements are better resistant to fire; the fire resistance limit of columns heated on four sides is less than the fire resistance limit of columns with one-sided heating; The fire resistance limit of beams when exposed to fire on three sides is less than the fire resistance limit of beams heated on one side.

2.14. The minimum dimensions of elements and distances from the axis of the reinforcement to the surfaces of the element are taken according to the tables of this section, but not less than those required by the chapter of SNiP I-21-75 “Concrete and reinforced concrete structures”.

2.15. The distance to the reinforcement axis and the minimum dimensions of elements to ensure the required fire resistance limit of structures depend on the type of concrete. Lightweight concrete has a thermal conductivity of 10-20%, and concrete with coarse carbonate filler is 5-10% less than heavy concrete with silicate filler. In this regard, the distance to the reinforcement axis for a structure made of lightweight concrete or heavy concrete with carbonate filler can be taken less than for structures made of heavy concrete with silicate filler with the same fire resistance limit for structures made from these concretes.

The values of fire resistance limits given in table. 2-b, 8, refer to concrete with coarse silicate rock aggregate, as well as dense silicate concrete. When using carbonate rock filler, the minimum dimensions of both the cross-section and the distance from the axes of the reinforcement to the surface of the bending element can be reduced by 10%. For lightweight concrete, the reduction can be 20% at a concrete density of 1.2 t/m 3 and 30% for bending elements (see Tables 3, 5, 6, 8) at a concrete density of 0.8 t/m 3 and expanded clay perlite concrete with a density of 1.2 t/m 3.

If the distance adopted in the project to the axis of the reinforcement is less than that required to ensure the required fire resistance limit of structures, it should be increased or additional heat-insulating coatings should be applied to the surfaces of the element 1 exposed to fire. Thermal insulation coating of lime cement plaster (15mm thick), gypsum plaster (10mm) and vermiculite plaster or mineral fiber insulation (5mm) is equivalent to a 10mm increase in the thickness of the heavy concrete layer. If the thickness of the protective layer of concrete is more than 40 mm for heavy concrete and 60 mm for lightweight concrete, the protective layer of concrete must have additional reinforcement on the fire side in the form of a reinforcement mesh with a diameter of 2.5-3 mm (cells 150X150 mm). Protective thermal insulation coatings with a thickness of more than 40 mm must also have additional reinforcement.

In table 2, 4-8 show the distances from the heated surface to the axis of the reinforcement (Fig. 1 and 2).

Rice. 1. Distances to the reinforcement axis Fig. 2. Average distance to wasps*

fittings

In cases where reinforcement is located at different levels, the average distance to the reinforcement axis a must be determined taking into account the areas of the reinforcement (L Lg, ..., L p) and the corresponding distances to the axes (оь а-1.....Qn), measured from the nearest heating

wash (bottom or side) surfaces of the element, according to the formula

. . . , . „ 2 Ai a (

L|0| -j~ LdOg ~f~ ■ . . +A p a p __ j°i_

L1+L2+L3 , . +L I 2 Ai

2.17. All steels reduce tensile or compressive strength

1 Additional heat-insulating coatings can be carried out in accordance with the “Recommendations for the use of fire-retardant coatings for metal structures” - M.; Stroyizdat, 1984.

when heated. The degree of resistance reduction is greater for hardened high-strength steel reinforcing wires than for low-carbon steel reinforcement bars.

The fire resistance limit of bent and eccentrically compressed elements with a large eccentricity for loss of bearing capacity depends on the critical heating temperature of the reinforcement. The critical heating temperature of the reinforcement is the temperature at which the tensile or compression resistance decreases to the value of the stress arising in the reinforcement from the standard load.

2.18. Table 5-8 are compiled for reinforced concrete elements with non-prestressed and prestressed reinforcement under the assumption that the critical heating temperature of the reinforcement is 500°C. This corresponds to reinforcing steels classes A-I, A-N, A-1v, A-Shv, A-IV, At-IV, A-V, At-V. The difference in critical temperatures for other classes of reinforcement should be taken into account by multiplying those given in table. 5-8 fire resistance limits per factor<р, или деля приведенные в табл. 5-8 расстояния до осей арматуры на этот коэффициент. Значения <р следует принимать:

1. For floors and coverings made of prefabricated reinforced concrete flat slabs, solid and hollow-core, reinforced:

a) steel class A-III, equal to 1.2;

b) steels of classes A-VI, At-VI, At-VII, B-1, BP-I, equal to 0.9;

c) high-strength reinforcing wire classes V-P, VR-P or reinforcing ropes of class K-7, equal to 0.8.

2. For. prefabricated floors and coverings reinforced concrete slabs with longitudinal load-bearing ribs “down” and box-shaped, as well as beams, crossbars and girders in accordance with the specified classes of reinforcement: a) (p = 1.1; b) q> => 0.95; c) av = 0.9.

2.19. For structures made of any type of concrete, the following must be observed: minimum requirements requirements for structures made of heavy concrete with a fire resistance limit of 0.25 or 0.5 hours.

2.20. Fire resistance limits of load-bearing structures in table. 2, 4-8 and in the text are given for full standard loads with the ratio of the long-term part of the load G $or to the full load Veer equal to 1. If this ratio is 0.3, then the fire resistance limit increases by 2 times. For intermediate values of G 8e r/V B er, the fire resistance limit is adopted by linear interpolation.

2.21. The fire resistance limit of reinforced concrete structures depends on their static operating pattern. The fire resistance limit of statically indeterminate structures is greater than the fire resistance limit of statically determinable structures, if the necessary reinforcement is available in the areas of negative moments. The increase in the fire resistance limit of statically indeterminate bendable reinforced concrete elements depends on the ratio of the cross-sectional areas of the reinforcement above the support and in the span according to Table. 1.

The ratio of the area of reinforcement above the support to the area of reinforcement in the span

Increase in the fire resistance limit of a bendable statically indeterminate element, %. compared to the fire resistance limit of a statically determined element

Note. For intermediate area ratios, the increase in fire resistance limit is taken by interpolation.

The influence of static indetermination of structures on the fire resistance limit is taken into account if the following requirements are met:

a) at least 20% of the upper reinforcement required on the support must pass above the middle of the span;

b) the upper reinforcement above the outer supports of a continuous system must be inserted at a distance of at least 0.4/ in the direction of the span from the support and then gradually break off (/ - span length);

c) all upper reinforcement above the intermediate supports must continue to the span for at least 0.15/ and then gradually break off.

Flexible elements embedded on supports can be considered as continuous systems.

2.22. In table 2 shows the requirements for reinforced concrete columns made of heavy and light concrete. They include requirements for the size of columns exposed to fire on all sides, as well as those located in walls and heated on one side. In this case, dimension b applies only to columns whose heated surface is at the same level with the wall, or for part of the column protruding from the wall and bearing the load. It is assumed that there are no holes in the wall near the column in the direction of the minimum size b.

For solid columns round section their diameter should be taken as dimension b.

Columns with the parameters given in table. 2, have an eccentrically applied load or a load with a random eccentricity when reinforcing columns of no more than 3% of the cross-section of concrete, with the exception of joints.

Fire resistance limit reinforced concrete columns with additional reinforcement in the form of welded transverse mesh installed in increments of no more than 250 mm should be taken according to table. 2, multiplying them by a factor of 1.5.

table 2

Type of concrete	Width b of the column and distance to reinforcement a	Minimum dimensions, mm, of reinforced concrete columns with fire resistance limits, h
Type of concrete	Width b of the column and distance to reinforcement a





(Y® “ 1.2 t/m 3)

2.23. Fire resistance limit of non-structural concrete and reinforced concrete partitions and their minimum thickness / n are given in table. 3. The minimum thickness of the partitions ensures that the temperature on the unheated surface of the concrete element will increase on average by no more than 160°C and will not exceed 220°C during a standard fire resistance test. When determining t n, additional protective coatings and plasters according to the instructions in paragraphs. 2.16 and 2.16.

Table 3

2.24. For load-bearing solid walls, the fire resistance limit, wall thickness t c and the distance to the reinforcement axis a are given in table. 4. These data apply to reinforced concrete centrally and eccentrically

compressed walls, provided that the total force is located in the middle third of the width of the cross section of the wall. In this case, the ratio of the height of the wall to its thickness should not exceed 20. For wall panels with platform support and thicknesses of at least 14 cm, fire resistance limits should be taken according to table. 4, multiplying them by a factor of 1.5.

Table 4

The fire resistance of ribbed wall slabs should be determined by the thickness of the slabs. The ribs must be connected to the slab with clamps. The minimum dimensions of the ribs and the distance to the axes of the reinforcement in the ribs must meet the requirements for beams and given in table. 6 and 7.

External walls made of two-layer panels, consisting of an enclosing layer with a thickness of at least 24 cm made of large-porous expanded clay concrete class B2-B2.5 (HC = 0.6-0.9 t/m 3) and a load-bearing layer with a thickness of at least 10 cm, with compressive stresses in it no more than 5 MPa, have a fire resistance limit of 3.6 hours.

When using combustible insulation in wall panels or ceilings, it is necessary to provide for the perimeter protection of this insulation with non-combustible material during manufacture, installation or installation.

Walls made of three-layer panels, consisting of two ribbed reinforced concrete slabs and insulation, made of fireproof or fire-resistant mineral wool or fiberboard slabs with a total cross-sectional thickness of 25 cm, they have a fire resistance rating of at least 3 hours.

External non-structural and self-supporting walls from three-layer solid panels (GOST 17078-71 as amended), consisting of outer (at least 50 mm thick) and internal reinforced concrete layers and a middle layer of combustible insulation (PSB foam plastic according to GOST 15588-70 as amended, etc.) , have a fire resistance limit with a total cross-sectional thickness of 15-22 cm for at least 1 hour. For similar load-bearing walls with layers connected by metal bonds with a total thickness of 25 cm,

with an internal load-bearing layer of reinforced concrete M 200 with compressive stresses in it no more than 2.5 MPa and a thickness of 10 cm or M 300 with compressive stresses in it no more than 10 MPa and a thickness of 14 cm, the fire resistance limit is 2.5 hours.

The fire spread limit for these structures is zero.

2.25. For tensile elements, fire resistance limits, cross-sectional width b and distance to the reinforcement axis a are given in Table. 5. These data apply to tensile elements of trusses and arches with non-prestressed and prestressed reinforcement, heated from all sides. The total cross-sectional area of the concrete element must be at least 2b 2 Mi R, where b min is the corresponding size for b, given in table. 5.

Table 5

Type of concrete

]Minimum cross-sectional width b and distance to the reinforcement axis a

Minimum dimensions of reinforced concrete tensile elements, mm, with fire resistance limits, h

(y" = 1.2 t/m 3)

2.26. For statically determined simply supported beams heated on three sides, fire resistance limits, beam width b and distances to the reinforcement axis a, flu. (Fig. 3) are given for heavy concrete in table. 6 and for light (y in = 1.2 t/m 3) in Table 7.

When heated on one side, the fire resistance limit of beams is taken according to table. 8 as for slabs.

For beams with inclined sides, the width b should be measured at the center of gravity of the tensile reinforcement (see Fig. 3).

When determining the fire resistance limit, holes in the beam flanges may not be taken into account if the remaining cross-sectional area in the tension zone is not less than 2v2,

To prevent concrete spalling in the ribs of the beams, the distance between the clamp and the surface should not be more than 0.2 of the rib width.

Minimum distance from

Rice. Reinforcement of beams and

distance to the axis of the element surface reinforcement to the axis

of any reinforcement bar must be no less than required (Table 6) for a fire resistance limit of 0.5 hours and no less than half a.

Table b

Fire resistance limits. h	Maximum dimensions of reinforced concrete beams, mm	Minimum rib width b w. mm

With a fire resistance limit of 2 hours or more, simply supported I-beams with a distance between the centers of gravity of the flanges of more than 120 cm must have end thickenings equal to the width of the beam.

For I-beams in which the ratio of the flange width to the wall width (see Fig. 3) b/b w is greater than 2, it is necessary to install transverse reinforcement in the rib. If the ratio b/b w is greater than 1.4, the distance to the axis of the reinforcement should be increased to 0.85аУл/bxa. For bjb v > 3, use the table. 6 and 7 are not allowed.

In beams with large shearing forces, which are perceived by clamps installed near the outer surface of the element, distance a (Tables 6 and 7) also applies to clamps provided they are located in zones where the calculated value of tensile stresses is greater than 0.1 of the compressive strength of concrete . When determining the fire resistance limit of statically indeterminate beams, the instructions of clause 2.21 are taken into account.

Table 7

Fire resistance limits, h	Beam width b and distance to the reinforcement axis a	Minimum dimensions of reinforced concrete beams, mm	Minimum rib width “V mm

The fire resistance limit of beams made of reinforced polymer concrete based on furfural acetone monomer with &=|160 mm and a = 45 mm, a>= 25 mm, reinforced with steel of class A-III, is 1 hour.

2.27. For simply supported slabs, the fire resistance limit, slab thickness /, distance to the reinforcement axis a are given in Table. 8.

The minimum thickness of the slab t ensures the heating requirement: the temperature on the unheated surface adjacent to the floor will, on average, increase by no more than 160°C and will not exceed 220°C. Backfill and flooring made of non-combustible materials are combined into the overall thickness of the slab and increase its fire resistance limit. Combustible insulation elephant laid on cement preparation, do not reduce the fire resistance limit of the slabs and can be used. Additional layers of plaster can be attributed to the thickness of the slabs.

Effective thickness hollow core slab to assess the fire resistance limit, it is determined by dividing the cross-sectional area of the slab, minus the void areas, by its width.

When determining the fire resistance limit of statically indeterminate slabs, clause 2.21 is taken into account. In this case, the thickness of the slabs and the distances to the axis of the reinforcement must correspond to those given in table. 8.

Fire resistance limits of multi-hollow structures, including those with voids.

located across the span, and ribbed panels and decking with ribs up should be taken according to table. 8, multiplying them by a factor of 0.9.

The fire resistance limits for heating two-layer slabs of light and heavy concrete and the required layer thickness are given in Table. 9.

Table 8

Type of concrete and slab characteristics		Minimum slab thickness t and distance to the reinforcement axis a. mm	Fire resistance limits, c
Type of concrete and slab characteristics
	Slab thickness
	Support on two sides or along a contour at 1у/1х ^ 1.5
	Support along the contour /„//*< 1,5
	Slab thickness
	Support on both sides or along the contour at /„//* ^ 1.5
	Support along contour 1 at Tskh< 1,5

Table 9

If all the reinforcement is located at the same level, the distance to the axis of the reinforcement from the side surface of the slabs must be no less than the thickness of the layer given in tables b and 7.

2.28. During a fire and fire tests of structures, spalling of concrete may be observed in case of high humidity, which, as a rule, can be present in structures immediately after their manufacture or during operation in rooms with high relative humidity. In this case, a calculation should be made according to the “Recommendations for the protection of concrete and reinforced concrete structures from brittle destruction in a fire” (M, Stroyizdat, 1979). If necessary, use those specified in these Recommendations protective measures or perform control tests.

2.29. During control tests, the fire resistance of reinforced concrete structures should be determined at a concrete moisture content corresponding to its humidity under operating conditions. If the moisture content of concrete under operating conditions is unknown, then it is recommended to test the reinforced concrete structure after storing it in a room with a relative air humidity of 60 ± 15% and a temperature of 20 ± 10 ° C for 1 year. To ensure the operational humidity of concrete, before testing structures, it is allowed to dry them at an air temperature not exceeding 60°C.

STONE STRUCTURES

2.30. The fire resistance limits of stone structures are given in table. 10.

2.31. If in column b of table. 10 indicates that the fire resistance limit of masonry structures is determined by the II limit state; it should be assumed that the I limit state of these structures does not occur earlier than II.

1 Walls and partitions made of solid and hollow ceramic and sand-lime bricks and stones according to GOST 379-79. 7484-78, 530-80

Walls made of natural, lightweight concrete and gypsum stones, lightweight brickwork with filling lightweight concrete, fireproof or difficult to burn thermal insulation materials

Table 10

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TsNIISK them. Kucherenko Gosstroy USSR

Benefit

Moscow 1985

ORDER OF THE RED BANNER OF LABOR CENTRAL RESEARCH INSTITUTE OF BUILDING STRUCTURES named after. V. A. KUCHERENKO SHNIISK them. Kucherenko) GOSSTROYA USSR

Benefit

TO DETERMINE THE LIMITS OF FIRE RESISTANCE OF STRUCTURES,

LIMITS

DISTRIBUTIONS

fire on structures

FLAMMABILITY OF MATERIALS (to SNiP P-2-80)

Approved

1®Ш

MOSCOW STROYIZDAT 1985

when heated. The degree of resistance reduction is greater for hardened high-strength steel reinforcing wires than for low-carbon steel reinforcement bars.

2.18. Table 5-8 are compiled for reinforced concrete elements with non-prestressed and prestressed reinforcement under the assumption that the critical heating temperature of the reinforcement is 500°C. This corresponds to reinforcing steels of classes A-I, A-II, A-1v, A-Shv, A-IV, At-IV, A-V, At-V. The difference in critical temperatures for other classes of reinforcement should be taken into account by multiplying those given in table. 5-8 fire resistance limits by coefficient f, or dividing those given in table. 5-8 distances to the reinforcement axes by this factor. The values of f should be taken:

1. For floors and coverings made of prefabricated reinforced concrete flat slabs, solid and hollow-core, reinforced:

a) steel class A-III, equal to 1.2;

b) steels of classes A-VI, At-VI, At-VII, B-1, BP-I, equal to 0.9;

c) high-strength reinforcing wire of classes V-P, Vr-N or reinforcing ropes of class K-7, equal to 0.8.

2. For. floors and coverings made of prefabricated reinforced concrete slabs with longitudinal load-bearing ribs “down” and box-section, as well as beams, crossbars and girders in accordance with the specified classes of reinforcement: a) f = 1.1; b) f = 0.95; c) f = 0.9.

2.19. For structures made of any type of concrete, the minimum requirements for structures made of heavy concrete with a fire resistance limit of 0.25 or 0.5 hours must be met.

2.20. Fire resistance limits of load-bearing structures in table. 2, 4-8 and in the text are given for full standard loads with a ratio of the long-term part of the load G eor to the full load Veer equal to 1. If this ratio is 0.3, then the fire resistance limit increases by 2 times. For intermediate values of G S er/Vser, the fire resistance limit is adopted by linear interpolation.

Note. For intermediate area ratios, the increase in fire resistance limit is taken by interpolation.

The influence of static indetermination of structures on the fire resistance limit is taken into account if the following requirements are met:

a) at least 20% of the upper reinforcement required on the support must pass above the middle of the span;

b) the upper reinforcement above the outer supports of a continuous system must be inserted at a distance of at least 0.4/ towards the span from the support and then gradually break off (/ - span length);

c) all upper reinforcement above the intermediate supports must continue to the span for at least 0.15/ and then gradually break off.

Flexible elements embedded on supports can be considered as continuous systems.

For columns of solid circular cross-section, their diameter should be taken as dimension b.

The fire resistance limit of reinforced concrete columns with additional reinforcement in the form of welded transverse mesh installed in increments of no more than 250 mm should be taken according to table. 2, multiplying them by a factor of 1.5.

table 2

Type of concrete	Width I b of column and distance to OCF reinforcement a	Minimum dimensions, mm, of reinforced concrete columns with fire resistance limits, h
Type of concrete	Width I b of column and distance to OCF reinforcement a





(Yb = 1.2 t/m3)

2.23. The fire resistance limit of non-load-bearing concrete and reinforced concrete partitions and their minimum thickness t u are given in table. 3. The minimum thickness of the partitions ensures that the temperature on the unheated surface of the concrete element will increase on average by no more than 160°C and will not exceed 220°C during a standard fire resistance test. When determining t n, additional protective coatings and plasters should be taken into account in accordance with the instructions in paragraphs. 2.16 and 2.16.

Table 3

	Minimum fire resistance partition thickness, h	with limits
Type of concrete

[y and = 1.2 t/m 3)
Cellular KYb = 0.8 t/m 3)

compressed walls, provided that the total force is located in the middle third of the width of the cross section of the wall. In this case, the ratio of the height of the wall to its thickness should not exceed 20. For wall panels with platform support and thicknesses of at least 14 cm, the fire resistance limits should be taken according to table. 4, multiplying them by a factor of 1.5.

Table 4

Type of concrete	Thickness t c and distance to the reinforcement axis a	Minimum dimensions of reinforced concrete walls, mm, with fire resistance limits, h
Type of concrete	Thickness t c and distance to the reinforcement axis a



<Ув = 1,2 т/м 3)

The fire resistance of ribbed wall slabs should be determined by

thickness of the slabs. The ribs must be connected to the slab with clamps. The minimum dimensions of the ribs and the distance to the axes of the reinforcement in the ribs must meet the requirements for beams and given in table. 6 and 7.

External walls made of two-layer panels, consisting of an enclosing layer with a thickness of at least 24 cm made of large-porous expanded clay concrete class B2-B2.5 (in - 0.6-0.9 t/m 3) and a load-bearing layer with a thickness of at least 10 cm , with compressive stresses not exceeding 5 MPa, have a fire resistance limit of 3.6 hours.

External non-load-bearing and self-supporting walls made of three-layer solid panels (GOST 17078-71 as amended), consisting of outer (at least 50 mm thick) and internal reinforced concrete layers and a middle layer of combustible insulation (PSB foam plastic according to GOST 15588 - 70 as amended) ., etc.), have a fire resistance limit with a total cross-sectional thickness of 15-22 cm for at least 1 hour. For similar load-bearing walls with layers connected by metal connections with a total thickness of 25 cm,

The fire spread limit for these structures is zero.

2.25. For tensile elements, fire resistance limits, cross-sectional width b and distance to the reinforcement axis a are given in Table. 5. These data apply to tensile elements of trusses and arches with non-tensioned and pre-stressed reinforcement, heated from all sides. The total cross-sectional area of the concrete element must be at least 25 2 Min, where b min is the corresponding size for 6, given in table. 5.

Table 5

Type of concrete	Minimum cross-section width b and distance to the reinforcement axis a	Minimum dimensions of reinforced concrete tensile elements, mm, with fire resistance limits, h
Type of concrete



(Yb =* 1.2 t/m 3)

2.26. For statically determined simply supported beams heated on three sides, fire resistance limits, beam width b and

the distances to the reinforcement axis a, a yu (Fig. 3) are given for heavy concrete in table. 6 and for light (sh = (1.2 t/m3) in Table 7.

When heated on one side, the fire resistance limit of beams is taken according to table. 8 as for slabs.

For beams with inclined sides, the width b should be measured at the center of gravity of the tensile reinforcement (see Fig. 3).

When determining the fire resistance limit, holes in the beam flanges may not be taken into account if the remaining cross-sectional area in the tension zone is not less than 2v2,

To prevent concrete spalling in the ribs of the beams, the distance between the clamp and the surface should not be more than 0.2 of the rib width.

Minimum distance a! from the surface of the element to the axis

/ £36")

Rice. 3. Ball reinforcement and distance to the reinforcement axis

of any reinforcement bar must be no less than required (Table 6) for a fire resistance limit of 0.5 hours and no less than half a.

Table b

Fire resistance limits, h	Beam width b and distance to the reinforcement axis a	Dimensions of reinforced concrete beams, mm	Minimum rib width b w . mm

With a fire resistance limit of 2 hours or more, freely supported I-beams with a distance between the centers of gravity of the flanges of more than 120 cm must have end thickenings equal to the width of the beam.

For I-beams in which the ratio of the flange width to the wall width (see Fig. 3) bjb w is greater than 2, it is necessary to install transverse reinforcement in the rib. If the ratio b/b w is greater than 1.4, the distance to the axis of the reinforcement should be increased to

0.S5ayb/b w . For bjb w > 3, use the table. 6 and 7 are not allowed.

Table 7

Fire resistance limits, h	Beam width b and distance to the reinforcement axis a	Minimum dimensions of reinforced concrete beams, mm	Minimum rib width b w , mm

The fire resistance limit of beams made of reinforced polymer concrete based on furfuralacetone monomer with 5 = Ts60 mm and a-45 mm, a w = 25 mm, reinforced with steel of class A-III, is 1 hour.

2.27. For simply supported slabs, the fire resistance limit, slab thickness t, distance to the reinforcement axis a are given in Table. 8.

The minimum thickness of the slab t ensures the heating requirement: the temperature on the unheated surface adjacent to the floor will, on average, increase by no more than 160°C and will not exceed 220°C. Backfill and flooring made of non-combustible materials are combined into the overall thickness of the slab and increase its fire resistance limit. Combustible insulating layers laid on cement preparation do not reduce the fire resistance limit of the slabs and can be used. Additional layers of plaster can be attributed to the thickness of the slabs.

The effective thickness of a hollow-core slab for assessing the fire resistance limit is determined by dividing the cross-sectional area or< ты, за вычетом площадей пустот, на ее ширину.

Fire resistance limits of multi-hollow structures, including those with voids*

located across the span, and ribbed panels and decking with ribs up should be taken according to table. 8, multiplying them by a factor of 0.9.

Location of concrete on the fire side	Minimum thickness of layers 11 of light concrete and 1 2 of heavy concrete, mm	Fire resistance limits, h
Location of concrete on the fire side



(Yb = 1.2 t/m3)

The fire resistance limits for heating two-layer slabs of light and heavy concrete and the required layer thickness are given in Table. 9.

Table 8

Type of concrete and characteristics		Minimum slab thickness t and dis-	Fire resistance limits, c
stickn plates		distance to the reinforcement axis a, mm
	Slab thickness

	Support along the contour lyjlx< 1,5
	Slab thickness
(Yb = 1.2 t/m3)	Support on both sides or along the contour when
	Support along the contour 1у/1х< 1,5
				Table 9

If all the reinforcement is located at one level, the distance to the axis of the reinforcement from the side surface of the slabs must be no less than the thickness of the layer given in table. 6 and 7.

2.28. During a fire and fire tests of structures, spalling of concrete may be observed in case of high humidity, which, as a rule, can be present in structures immediately after their manufacture or during operation in rooms with high relative humidity. In this case, a calculation should be made according to the “Recommendations for the protection of concrete and reinforced concrete structures from brittle destruction in a fire” (M, Stroyizdat, 1979). If necessary, use the protective measures specified in these Recommendations or perform control tests.

STONE STRUCTURES

2.30. The fire resistance limits of stone structures are given in table. 10.

2.31. If in column 6 of table. 10 indicates that the fire resistance limit of masonry structures is determined by the II limit state; it should be assumed that the I limit state of these structures does not occur earlier than II.

Table 10

Scheme (section) of the structure

Dimensions a, cm	Fire resistance limit, h	Limit state for fire resistance (see clause 2.4)

Scientific Council of the TsNIISK named after. Kucherenko State Construction Committee of the USSR.

A manual for determining the fire resistance limits of structures, the limits of fire propagation through structures and flammability groups of materials (to SNiP P-2-80) / TsNIISK im. Kucherenko.- M.: Stroyizdat, 1985.-56 p.

Developed for SNiP P-2-80 “Fire safety standards for the design of buildings and structures.” Reference data is provided on the limits of fire resistance and fire spread for building structures made of reinforced concrete, metal, wood, asbestos cement, plastics and other building materials, as well as data on the flammability groups of building materials.

For engineering and technical workers of design, construction organizations and state fire supervision authorities.

Table 15, fig. 3.

and-Instruction-norm. II issue - 62-84

Continuation of the table. 10

3.7 2.5 (based on test results)

PREFACE

This Manual has been developed for SNiP II-2-80 “Fire safety standards for the design of buildings and structures.” It contains data on the standardized fire resistance and fire hazard indicators of building structures and materials.

Sec. 1 manual was developed by TsNIISK named after. Kucherenko (Doctor of Technical Sciences, Prof. I. G. Romanenkov, Candidate of Technical Sciences, V. N. Zigern-Korn). Sec. 2 developed by TsNIISK named after. Kucherenko (Doctor of Technical Sciences)

I. G. Romanenkov, candidates of technical sciences. Sciences V. N. Zigern-Korn,

L. N. Bruskova, G. M. Kirpichenkov, V. A. Orlov, V. V. Sorokin, engineers A. V. Pestritsky, |V. I. Yashin)); NIIZhB (Doctor of Technical Sciences)

V. V. Zhukov; Dr. Tech. sciences, prof. A. F. Milovanov; Ph.D. physics and mathematics Sciences A.E. Segalov, Candidates of Engineering. Sci. A. A. Gusev, V. V. Solomonov, V. M. Samoilenko; engineers V.F. Gulyaeva, T.N. Malkina); TsNIIEP im. Mezentseva (candidate of technical sciences L. M. Schmidt, engineer P. E. Zhavoronkov); TsNIIPromzdanny (Candidate of Technical Sciences V.V. Fedorov, engineers E.S. Giller, V.V. Sipin) and VNIIPO (Doctor of Technical Sciences, Prof. A.I. Yakovlev; Candidates of Technical Sciences V. P. Bushev, S. V. Davydov, V. G. Olimpiev, N. F. Gavrikov, engineers V. Z. Volokhatykh, Yu. A. Grinchik, N. P. Savkin, A. N. Sorokin, V. S. Kharitonov, L. V. Sheinina, V. I. Shchelkunov). Sec. 3 developed by TsNIISK named after. Kucherenko (Dr. Tech. Science, Prof. I. G. Romanenkov, Candidate of Chemical Sciences N. V. Kovyrshina, engineer V. G. Gonchar) and the Institute of Mining Mechanics of the Academy of Sciences of Georgia. SSR (candidate of technical sciences G. S. Abashidze, engineers L. I. Mirashvili, L. V. Gurchumelia).

When developing the Manual, materials from the TsNIIEP of housing and the TsNIIEP of educational buildings of the State Civil Engineering Committee, MNIT Ministry of Railways of the USSR, VNIISTROM and NIPIsilicate concrete of the Ministry of Industrial Construction Materials of the USSR were used.

The text of SNiP II-2-80 used in the Guide is typed in bold. Its points are double numbered; the numbering according to SNiP is given in brackets.

In cases where the information provided in the Manual is insufficient to establish the appropriate indicators of structures and materials, you should contact TsNIISK nm for consultations and applications for fire tests. Kucherenko or NIIZhB of the USSR State Construction Committee. The basis for establishing these indicators can also be the results of tests performed in accordance with standards and methods approved or agreed upon by the USSR State Construction Committee.

Please send comments and suggestions regarding the Manual to the following address: Moscow, 109389, 2nd Institutskaya St., 6, TsNIISK im. V. A. Kucherenko.

1. GENERAL PROVISIONS

1.1. Is the manual compiled to help design and construction projects? organizations and fire protection authorities in order to reduce the cost of time, labor and materials to establish the fire resistance limits of building structures, the limits of fire spread through them and the flammability groups of materials standardized by SNiP 11-2-80.

1.3. (2.4). Based on flammability, building materials are divided into three groups: non-combustible, non-combustible and combustible.

2. BUILDING STRUCTURES.

FIRE RESISTANCE LIMITS AND FIRE SPREAD LIMITS

The limit of fire spread through building structures is determined according to the methodology given in the appendix. 2.

FIRE RESISTANCE LIMIT

structures); in terms of thermal insulation ability - an increase in temperature on an unheated surface by an average of more than 160°C or at any point on this surface by more than 190°C in comparison with the temperature of the structure before testing, or by more than 220°C regardless of the temperature of the structure before testing; by density - the formation in structures of through cracks or through holes through which combustion products or flames penetrate; for structures protected by fire-retardant coatings and tested without loads, the limiting state will be the achievement of a critical temperature of the material of the structure.

For external walls, coverings, beams, trusses, columns and pillars, the limiting state is only the loss of the load-bearing capacity of structures and components.

2.4. The limiting states of structures for fire resistance, specified in clause 2.3, in the future, for brevity, we will call l t II, III and IV, respectively, the limiting states of structures for fire resistance.

2.5. The fire resistance limits of structures can also be determined by calculation. In these cases, tests may not be carried out.

Determination of fire resistance limits by calculation should be carried out according to methods approved by the Glavtekhnormirovanie of the USSR State Construction Committee.

2.6. For an approximate assessment of the fire resistance limit of structures during their development and design, one can be guided by the following provisions:

b) the fire resistance limits of enclosing structures with an air gap are on average 10% higher than the fire resistance limits of the same structures, but without an air gap; the efficiency of the air gap is higher, the further it is removed from the heated plane; with closed air gaps, their thickness does not affect the fire resistance limit;

c) fire resistance limits of enclosing structures with asymmetrical

d) an increase in the humidity of structures helps to reduce the rate of heating and increase fire resistance, except in cases where an increase in humidity increases the likelihood of sudden brittle destruction of the material or the appearance of local cracks, this phenomenon is especially dangerous for concrete and asbestos-cement structures;

f) the fire resistance limit of a structure is higher, the smaller the ratio of the heated perimeter of the cross-section of its elements to their area;

h) the flammability of the materials from which the structure is made does not determine its fire resistance limit. For example, structures made of thin-walled metal profiles have a minimum fire resistance limit, and structures made of wood have a higher fire resistance limit than steel structures with the same ratio of the heated perimeter of the section to its area and the magnitude of the operating stresses to the temporary resistance or yield strength. At the same time, it should be taken into account that the use of combustible materials instead of difficult-to-burn or non-combustible materials can reduce the fire resistance limit of the structure if the rate of its burnout is higher than the rate of heating.

FIRE SPREAD LIMIT

2.9. Damage is considered to be charring or burning of materials that can be detected visually, as well as melting of thermoplastic materials.

The limit of fire spread is taken to be the maximum size of damage (cm), determined according to the test procedure set out in appendix. 2 to SNiP II-2-8G.

2.10. Structures made using combustible and non-combustible materials, usually without finishing or cladding, are tested for the spread of fire.

Structures made only from fireproof materials should be considered not to spread fire (the limit of fire spread through them should be taken equal to zero).

If, when testing for the spread of fire, the damage to structures in the control zone is no more than 5 cm, it should also be considered not to spread fire.

2Л For a preliminary assessment of the fire spread limit, the following provisions can be used:

CONCRETE AND REINFORCED CONCRETE STRUCTURES

2.14. The minimum dimensions of elements and distances from the axis of the reinforcement to the surfaces of the element are taken according to the tables of this section, but not less than those required by the chapter of SNiP I-21-75 “Concrete and reinforced concrete structures”.

In table 2, 4-8 show the distances from the heated surface to the axis of the reinforcement (Fig. 1 and 2).

Rice. 1. Distances to the reinforcement axis Fig. 2. Average distance to axle

fittings

In cases where reinforcement is located at different levels, the average

the distance to the axis of the reinforcement a must be determined taking into account the areas of the reinforcement (L l L 2, ..., L p) and the corresponding distances to the axes (a b a-2, > Yap), measured from the nearest heated

wash (bottom or side) surfaces of the element, according to the formula

A\I\\A^

Ljfli -f- A^cl^ ~b. . N~L p Dp __ 1_

L1+L2+L3. . +Lp 2 Lg

2.17. All steels reduce tensile or compressive strength

1 Additional heat-insulating coatings can be carried out in accordance with the “Recommendations for the use of fire-retardant coatings for metal structures” - M.; Stroyizdat, 1984.

The essence of the calculation method

The purpose of the calculation is to determine the time after which a building structure at standard temperature conditions will lose (will run out) its load-bearing or heat-insulating capacity (1 and 3 limit states of structures for fire resistance), i.e. until the time of onset of P f.

The onset time (Pf) for the second limit state of the structure for fire resistance cannot yet be calculated.

Based on the 3rd limit state of a structure for fire resistance, it is calculated interior walls, partitions, ceilings.

Considering that individual structures are both load-bearing and enclosing, they are calculated according to both 1 and 3 limit states for fire resistance, for example: structures of internal load-bearing walls and ceilings.

The same applies to determining the fire resistance limit of structures and according to the reference manual, technical information (“to help the GPN inspector”) and, naturally, by the method of full-scale fire tests.

IN general case The methodology for calculating the fire resistance limit of a load-bearing building structure consists of from thermotechnical and static parts (enclosing - only from thermal engineering).

Thermal engineering part calculation methods involve determining temperature changes (during exposure to standard temperature conditions) both at any point along the thickness of the structure and its surfaces.

Based on the results of this calculation, it is possible to determine not only the indicated temperature values, but also the time for warming up the enclosing structure to extreme temperatures (140°C+tn), i.e., the time of occurrence of its fire resistance limit according to the 3rd limit state of the structure for fire resistance.

Static part The methodology involves calculating changes in bearing capacity (by strength, amount of deformation) heated structure during a standard fire test.

Calculation schemes

When calculating the fire resistance limit of a structure, the following calculation schemes are usually used:

The 1st design scheme (Fig. 3.1) is used when the fire resistance limit of a structure occurs as a result of the loss of its heat-insulating ability (3rd limit state for fire resistance). Calculation based on it comes down to solving only the thermotechnical part of the fire resistance problem.

Rice. 3.1. The first calculation scheme. a – vertical fence; b – horizontal fence.

The 2nd calculation scheme (Fig. 3.2) is used when the fire resistance limit of a structure occurs as a result of the loss of its load-bearing capacity (when heated above the critical temperature - t cr of metal structures or working reinforcement of a reinforced concrete structure).

Rice. 3.2. Second calculation scheme. a – metal lined column; b – frame metal wall; c – reinforced concrete wall; d – reinforced concrete beam.

Critical – temperature - t cr load-bearing metal structure or working reinforcement of a bending reinforced concrete structure - the temperature of its heating at which the yield strength of the metal, decreasing, reaches the value of the standard (working) stress from the standard (working) load on the structure, respectively.

Its numerical value depends on the composition (brands) metal, product processing technology and standard value (worker - the one who operates in the constructed building) load on the structure. The slower the yield strength of the metal decreases when heated and the smaller the external load on the structure, the higher the value of t cr, i.e., the higher the Pf of the structure.

There are structures, in particular wooden ones, the destruction of which in a fire occurs as a result of a reduction in their cross-sectional area to a critical value - F cr during charring of the wood.

As a result, the voltage value - s from the external load in the remaining (working) part of the cross-section of the structure increases, and when this value reaches the value of the standard resistance - R nt of wood (adjusted for temperature) the structure collapses because it reaches its limiting state for fire resistance (loss of bearing capacity), i.e. P f. For this case, design scheme 3 is used.

Calculation of the actual fire resistance limit of the structure according to 3rd design scheme comes down to determining the point in time of a standard fire resistance test of a structure, upon reaching which (with a known wood charring rate - n l) cross-sectional area - S designs (its load-bearing part) will decrease to a critical value.

Rice. 3.3. Third calculation scheme. A - wooden beam; b – reinforced concrete column.

Using this calculation scheme, it is also possible to calculate the actual fire resistance limit of the load-bearing reinforced concrete column structure with sufficient accuracy of the result for practical purposes, taking the assumption that the standard resistance (tensile strength) of concrete heated above the critical temperature is equal to zero, and within the critical area of the “cross section” it is equal to the original value - Rn.

With the use of computers appeared 4 design diagram, which provides, simultaneously with the solution of the thermotechnical part of the fire resistance problem, the calculation and changes in the load-bearing capacity of the structure before its loss (i.e. before the onset of the P f of the structure for the first limit state for fire resistance - Fig. 3.5), when:

N t N n ; or M t =M n. (3.1)

where N t ; M t - load-bearing capacity of the heated structure, N; N×m;

Nn; M n - standard load (moment from the standard load on the structure) N, N×m.

Using this calculation scheme, the temperature is calculated using a PC at each point of the calculation grid (Fig. 3.5), superimposed on the cross section of the structure, at calculated time intervals (good convergence of calculation results with the results of full-scale fire tests - with a counting step D t £ 0.1 min).

Simultaneously with calculating the temperature at each point of the calculation grid, the PC also calculates the strength of the material at these points - at the same times - at the corresponding temperatures (i.e. solves the static part of the fire resistance problem). At the same time, the PC summarizes the strength indicators of the construction materials at the points of the calculation grid and thus determines the total load-bearing capacity, i.e., the load-bearing capacity of the structure as a whole at a given point in time during a standard fire resistance test of the structure.

Based on the results of such calculations, a graph of changes in the load-bearing capacity of the structure versus the time of the fire test is constructed manually (or using a PC) (Fig. 3.4), from which the actual fire resistance limit of the structure is determined.

Rice. 3.4. Change (decrease) in the load-bearing capacity of a structure (for example, a column) to the standard load when it is heated under full-scale fire test conditions.

Thus, design schemes 2 and 3 are special cases of the 4th.

As already mentioned, building structures that perform both load-bearing and enclosing functions are calculated according to both the 1st and 3rd limit states of the structure for fire resistance. In this case, the 1st design scheme, as well as the 2nd one, is used, respectively. An example of such a design is the ribbed reinforced concrete floor slab, for which, according to the first design scheme, the time of occurrence of the 3rd limit state of the structure for fire resistance is calculated - when the shelf is heated. Then the time of occurrence of the 1st limit state of the structure for fire resistance is calculated - as a result of heating the working reinforcement of the slab to - t cr - according to the 2nd calculation scheme - until the destruction of the slab due to a decrease in its load-bearing capacity (working reinforcement in the ribs) to normative (working) loads.

Due to the insufficient results of experimental and theoretical studies, the following basic assumptions are usually introduced into the methodology for calculating the fire resistance limits of structures:

1) subject to calculation separate design- without taking into account its connections (joints) with other structures;

2) the vertical rod structure during a fire (full-scale fire test) is heated evenly over its entire height;

3) there is no heat leakage at the ends of the structure;

4) temperature stresses in the structure resulting from its uneven heating (due to changes in the deformative properties of materials and different quantities thermal expansion layers of material), are missing.

Art. Lecturer at the Department of Physical Safety and Emergency Medicine

Art. internal service lieutenant G.L. Shidlovsky

”_____” _______________ 201_

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