Calculation of the fire resistance limit of a reinforced concrete floor slab. LLC is an architectural manufacturing company. Determination of fire resistance limits of reinforced concrete columns

As mentioned above, the fire resistance limit of bent reinforced concrete structures may occur due to heating to a critical temperature of the working reinforcement located in the stretched zone.

In this regard, the calculation of the fire resistance of a multi-hollow floor slab will be determined by the time of heating up to the critical temperature of the stretched working reinforcement.

The section of the slab is shown in Figure 3.8.

b p b p b p b p b p

h h 0

A s

Fig.3.8. Estimated section of a hollow-core floor slab

To calculate the slab, its cross section is reduced to a tee (Fig. 3.9).

b´ f

x topic ≤h´ f

h´ f

h h 0

x topic >h' f

A s

a∑b R

Fig.3.9. Tee section of a hollow-core slab for calculating its fire resistance

Subsequence

calculation of the fire resistance limit of flat flexible multi-hollow reinforced concrete elements

3. If, then  s , topic is determined by the formula

Where instead b used ;

If
, then it must be recalculated according to the formula:

According to 3.1.5 is determined t s , cr(critical temperature).

The Gaussian error function is calculated by the formula:

According to 3.2.7, the argument of the Gaussian function is found.

The fire resistance limit P f is calculated by the formula:

Example number 5.

Given. Hollow-core floor slab freely supported on both sides. Section dimensions: b=1200 mm, working span length l= 6 m, section height h= 220 mm, protective layer thickness A l = 20 mm, class A-III tension reinforcement, 4 rods Ø14 mm; heavy concrete class B20 on crushed limestone, weight moisture content of concrete w= 2%, average dry concrete density ρ 0s\u003d 2300 kg / m 3, void diameter d n = 5.5 kN/m.

Define the actual fire resistance limit of the slab.

Solution:

For concrete class B20 R bn= 15 MPa (clause 3.2.1.)

R bu\u003d R bn / 0.83 \u003d 15 / 0.83 \u003d 18.07 MPa

For reinforcement class A-III R sn = 390 MPa (clause 3.1.2.)

R su= R sn /0.9 = 390/0.9 = 433.3 MPa

A s= 615 mm 2 = 61510 -6 m 2

Thermophysical characteristics of concrete:

λ tem \u003d 1.14 - 0.00055450 \u003d 0.89 W / (m ˚С)

with tem = 710 + 0.84450 = 1090 J/(kg ˚C)

k= 37.2 p.3.2.8.

k 1 = 0.5 p.3.2.9. .

The actual fire resistance limit is determined:

Taking into account the hollowness of the slab, its actual fire resistance must be multiplied by a factor of 0.9 (clause 2.27.).

Literature

Shelegov V.G., Kuznetsov N.A. "Buildings, structures and their stability in case of fire". Textbook for the study of discipline. - Irkutsk.: VSI MIA of Russia, 2002. - 191 p.

Shelegov V.G., Kuznetsov N.A. Building construction. Help Guide in the discipline "Buildings, structures and their stability in case of fire." - Irkutsk.: VSI Ministry of Internal Affairs of Russia, 2001. - 73 p.

Mosalkov I.L. and others. Fire resistance of building structures: M .: CJSC "Spetstechnika", 2001. - 496 p., illustration

Yakovlev A.I. Fire resistance calculation building structures. - M .: Stroyizdat, 1988.- 143s., Ill.

Shelegov V.G., Chernov Yu.L. "Buildings, structures and their stability in case of fire". A guide to completing a course project. - Irkutsk.: VSI Ministry of Internal Affairs of Russia, 2002. - 36 p.

Manual for determining the fire resistance limits of structures, the limits of fire propagation along structures and the flammability groups of materials (to SNiP II-2-80), TsNIISK im. Kucherenko. – M.: Stroyizdat, 1985. – 56 p.

GOST 27772-88: Rolled products for building steel structures. Are common specifications/ Gosstroy of the USSR. - M., 1989

SNiP 2.01.07-85*. Loads and impacts / Gosstroy of the USSR. - M.: CITP Gosstroy USSR, 1987. - 36 p.

GOST 30247.0 - 94. Building structures. Test methods for fire resistance. General requirements.

SNiP 2.03.01-84*. Concrete and reinforced concrete structures / Ministry of Construction of Russia. - M.: GP TsPP, 1995. - 80 p.

1ELLING - a structure on the shore with a specially arranged sloping foundation ( slipway), where the ship's hull is laid down and built.

2 Viaduct - a bridge over land routes (or over land routes) at their intersection. Provides movement on them at different levels.

3FLASHBACK - a construction in the form of a bridge for passing one path over another at the point of their intersection, for mooring ships, and also in general for creating a road at a certain height.

4 STORAGE TANK - container for liquids and gases.

5 GAS CONTAINER– facility for acceptance, storage and release of gas to the gas network.

6blast furnace- shaft furnace for smelting pig iron from iron ore.

7Critical temperature- the temperature at which the standard resistance of the metal R un decreases to the value of the standard voltage  n from external load on the design, i.e. at which there is a loss of bearing capacity.

8 Nagel - a wooden or metal rod used to fasten parts of wooden structures.

Reinforced concrete structures, due to their incombustibility and relatively low thermal conductivity, quite well resist the effects of aggressive fire factors. However, they cannot indefinitely resist fire. Modern reinforced concrete structures, as a rule, are thin-walled, without a monolithic connection with other elements of the building, which limits their ability to perform their working functions in a fire to 1 hour, and sometimes less. Wet reinforced concrete structures have an even lower fire resistance limit. If an increase in the moisture content of a structure to 3.5% increases the fire resistance limit, then a further increase in the moisture content of concrete with a density of more than 1200 kg / m 3 during a short-term fire can cause an explosion of concrete and a rapid destruction of the structure.

The fire resistance limit of a reinforced concrete structure depends on the size of its section, the thickness of the protective layer, the type, quantity and diameter of the reinforcement, the class of concrete and the type of aggregate, the load on the structure and its support scheme.

The fire resistance limit of enclosing structures for heating - the surface opposite to fire by 140 ° C (ceilings, walls, partitions) depends on their thickness, type of concrete and its moisture content. With an increase in thickness and a decrease in the density of concrete, the fire resistance increases.

The fire resistance limit on the basis of the loss of bearing capacity depends on the type and static support scheme of the structure. Single-span freely supported bending elements (beam slabs, panels and floorings, beams, girders) are destroyed by fire as a result of heating of the longitudinal lower working reinforcement to the limiting critical temperature. The fire resistance limit of these structures depends on the thickness of the protective layer of the lower working reinforcement, the reinforcement class, the working load and the thermal conductivity of concrete. For beams and purlins, the fire resistance limit also depends on the width of the section.

With the same design parameters, the fire resistance limit of beams is less than that of slabs, since in case of fire the beams are heated from three sides (from the bottom and two side faces), and the slabs are heated only from the bottom surface.

The best reinforcing steel in terms of fire resistance is class A-III grade 25G2S. The critical temperature of this steel at the moment of the onset of the fire resistance limit of a structure loaded with a standard load is 570°C.

Large-hollow prestressed floorings made of heavy concrete with a protective layer of 20 mm and bar reinforcement made of class A-IV steel, produced by factories, have a fire resistance limit of 1 hour, which makes it possible to use these floorings in residential buildings.

Slabs and panels of solid section made of ordinary reinforced concrete with a protective layer of 10 mm have fire resistance limits: steel reinforcement classes A-I and A-II - 0.75 h; A-III (grades 25G2S) - 1 hour

In some cases, thin-walled bending structures (hollow and ribbed panels and floorings, crossbars and beams with a section width of 160 mm or less, without vertical frames at the supports) under the action of a fire can be destroyed prematurely along the oblique section at the supports. This type of destruction is prevented by installing vertical frames at least 1/4 of the span on the supporting sections of these structures.

Plates supported along the contour have a fire resistance limit much higher than simple bending elements. These slabs are reinforced with working reinforcement in two directions, so their fire resistance additionally depends on the ratio of reinforcement in short and long spans. For square slabs having this ratio, equal to one, the critical temperature of the reinforcement at the onset of the fire resistance limit is 800°C.

With an increase in the ratio of the sides of the plate, the critical temperature decreases, therefore, the fire resistance limit also decreases. With aspect ratios of more than four, the fire resistance limit is practically equal to the fire resistance limit of plates supported on two sides.

Statically indeterminate beams and beam slabs, when heated, lose their bearing capacity as a result of the destruction of the supporting and span sections. The sections in the span are destroyed as a result of a decrease in the strength of the lower longitudinal reinforcement, and the supporting sections are destroyed due to the loss of concrete strength in the lower compressed zone, which heats up to high temperatures. The heating rate of this zone depends on the size of the cross section, so the fire resistance of statically indeterminate beam plates depends on their thickness, and beams - on the width and height of the section. At large sizes cross-section, the fire resistance limit of the structures under consideration is much higher than that of statically determinable structures (single-span freely supported beams and slabs), and in some cases (for thick beam slabs, for beams with strong upper supporting reinforcement) practically does not depend on the thickness of the protective layer at the longitudinal bottom reinforcement.

Columns. The fire resistance limit of columns depends on the load application scheme (central, eccentric), cross-sectional dimensions, percentage of reinforcement, type of large concrete aggregate and thickness of the protective layer at the longitudinal reinforcement.

The destruction of columns during heating occurs as a result of a decrease in the strength of reinforcement and concrete. Eccentric load application reduces the fire resistance of the columns. If the load is applied with a large eccentricity, then the fire resistance of the column will depend on the thickness of the protective layer at the tension reinforcement, i.e. the nature of the operation of such columns when heated is the same as that of simple beams. The fire resistance of a column with a small eccentricity approaches the fire resistance of centrally compressed columns. Concrete columns on crushed granite have less fire resistance (by 20%) than columns on crushed limestone. This is explained by the fact that granite begins to collapse at a temperature of 573 ° C, and limestone begins to collapse at a temperature of the beginning of their firing of 800 ° C.

Walls. During fires, as a rule, the walls are heated on one side and therefore bend either towards the fire or in the opposite direction. The wall from a centrally compressed structure turns into an eccentrically compressed one with an eccentricity increasing in time. Under these conditions, fire resistance bearing walls largely depends on the load and on their thickness. As the load increases and the wall thickness decreases, its fire resistance decreases, and vice versa.

With an increase in the number of storeys of buildings, the load on the walls increases, therefore, to ensure the necessary fire resistance, the thickness of the load-bearing transverse walls in residential buildings is assumed to be (mm): in 5 ... 9-storey buildings - 120, 12-storey buildings - 140, 16-storey buildings - 160 , in houses with a height of more than 16 floors - 180 or more.

Single-layer, double-layer and three-layer self-supporting panels of exterior walls are exposed to light loads, so the fire resistance of these walls usually meets the fire protection requirements.

The bearing capacity of walls under the action of high temperature is determined not only by a change in the strength characteristics of concrete and steel, but mainly by the deformability of the element as a whole. The fire resistance of walls is determined, as a rule, by the loss of bearing capacity (destruction) in a heated state; the sign of heating the "cold" surface of the wall by 140 ° C is not characteristic. The fire resistance limit is dependent on the working load (factor of safety of the structure). The destruction of walls from unilateral impact occurs according to one of three schemes:

• 1) with the irreversible development of deflection towards the heated surface of the wall and its destruction in the middle of the height according to the first or second case of eccentric compression (along heated reinforcement or "cold" concrete);
• 2) with the deflection of the element at the beginning in the direction of heating, and at the final stage in the opposite direction; destruction - in the middle of the height along heated concrete or along "cold" (stretched) reinforcement;
• 3) with a variable deflection direction, as in scheme 1, but the destruction of the wall occurs in the support zones along the concrete of the "cold" surface or along oblique sections.

The first failure scheme is typical for flexible walls, the second and third - for walls with less flexibility and platform supported. If the freedom of rotation of the supporting sections of the wall is limited, as is the case with platform support, its deformability decreases and therefore the fire resistance increases. Thus, the platform support of the walls (on non-displaceable planes) increased the fire resistance limit on average by a factor of two compared to the hinged support, regardless of the element destruction scheme.

Reducing the percentage of wall reinforcement with hinged support reduces the fire resistance limit; with platform support, a change within the usual limits of wall reinforcement has practically no effect on their fire resistance. When the wall is heated simultaneously from both sides ( interior walls) it does not have a thermal deflection, the structure continues to work on central compression and therefore the fire resistance limit is not lower than in the case of one-sided heating.

Basic principles for calculating the fire resistance of reinforced concrete structures

The fire resistance of reinforced concrete structures is lost, as a rule, as a result of a loss of bearing capacity (collapse) due to a decrease in strength, thermal expansion and thermal creep of reinforcement and concrete when heated, as well as due to heating of the surface not facing fire by 140 ° C. According to these indicators - the fire resistance limit of reinforced concrete structures can be found by calculation.

IN general case The calculation consists of two parts: thermal engineering and static.

In the heat engineering part, the temperature is determined over the cross section of the structure in the process of heating it according to the standard temperature regime. In the static part, the bearing capacity (strength) of the heated structure is calculated. Then they build a graph (Fig. 3.7) of reducing its bearing capacity over time. According to this schedule, the fire resistance limit is found, i.e. heating time, after which the bearing capacity of the structure will decrease to the working load, i.e. when the equality will take place: M pt (N pt) = M n (M n), where M pt (N pt) is the bearing capacity of a bending (compressed or eccentrically compressed) structure;

M n (M n), - bending moment (longitudinal force) from the normative or other working load.

The most common material in
construction is reinforced concrete. It combines concrete and steel reinforcement,
rationally laid in the design for the perception of tensile and compressive
efforts.

Concrete has good compressive strength and
worse - stretching. This feature of concrete is unfavorable for bending and
stretched elements. The most common flexible building elements
are slabs and beams.

To compensate for adverse
concrete processes, it is customary to reinforce structures with steel reinforcement. Reinforce
slabs with welded meshes, consisting of rods located in two mutually
perpendicular directions. Grids are laid in slabs in such a way that
the rods of their working reinforcement were located along the span and perceived
tensile forces arising in structures during bending under load, in
according to the diagram of bending loads.

IN
under fire conditions, the slabs are exposed to high temperatures from below,
a decrease in their bearing capacity occurs mainly due to a decrease in
strength of heated tensile reinforcement. Typically, these elements
are destroyed as a result of the formation of a plastic hinge in the cross section with
maximum bending moment by reducing the tensile strength
heated stretched reinforcement to the value of operating stresses in its cross section.

Providing fire
building security requires increased fire resistance and fire safety
reinforced concrete structures. For this, the following technologies are used:

• reinforcing slabs to produce
  only knitted or welded frames, and not loose individual rods;
• to avoid bulging of the longitudinal reinforcement when it is heated during
  during a fire, it is necessary to provide structural reinforcement with clamps or
  transverse rods;
• the thickness of the lower protective layer of concrete of the ceiling should be
  sufficient so that it warms up no higher than 500 ° C and after a fire does not
  influenced the further safe operation of the structure.
  Studies have established that with a standardized fire resistance R = 120, the thickness
  the protective layer of concrete should be at least 45 mm, at R = 180 - at least 55 mm,
  at R=240 - not less than 70 mm;
• in the protective layer of concrete at a depth of 15–20 mm from the bottom
  the floor surface should be provided with an anti-splinter reinforcement mesh
  from a wire with a diameter of 3 mm with a mesh size of 50–70 mm, which reduces the intensity
  explosive destruction of concrete;
• reinforcement of the supporting sections of thin-walled ceilings of the transverse
  fittings not provided for by the usual calculation;
• increase in fire resistance due to the location of the plates,
  supported along the contour;
• application of special plasters (using asbestos and
  perlite, vermiculite). Even with small sizes of such plasters (1.5 - 2 cm)
  the fire resistance of reinforced concrete slabs increases several times (2 - 5);
• increase in fire resistance due to false ceiling;
• protection of nodes and joints of structures with a layer of concrete with the required
  fire resistance limit.

These measures will ensure proper fire safety building.
The reinforced concrete structure will acquire the necessary fire resistance and
fire safety.

Used Books:
1. Buildings and structures, and their sustainability
in case of fire. Academy of State Fire Service EMERCOM of Russia, 2003
2. MDS 21-2.2000.
Guidelines for calculating the fire resistance of reinforced concrete structures.
- M. : State Unitary Enterprise "NIIZhB", 2000. - 92 p.

Determination of fire resistance limits of building structures

Determination of the fire resistance limit of reinforced concrete structures

Initial data for reinforced concrete slab overlappings are given in table 1.2.1.1

Type of concrete - lightweight concrete with a density of c = 1600 kg/m3 with coarse expanded clay aggregate; slabs are multi-hollow, with round voids, the number of voids is 6 pcs, the slabs are supported on two sides.

1) The effective thickness of a hollow-core slab teff for assessing the fire resistance limit in terms of heat-insulating ability in accordance with paragraph 2.27 of the Manual to SNiP II-2-80 (Fire resistance):

2) We determine according to the table. 8 Allowances for the fire resistance of the slab on the loss of thermal insulation capacity for a slab of lightweight concrete with an effective thickness of 140 mm:

The fire resistance limit of the plate is 180 min.

3) Determine the distance from the heated surface of the plate to the axis of the rod reinforcement:

4) According to Table 1.2.1.2 (Table 8 of the Handbook), we determine the fire resistance limit of the slab according to the loss of bearing capacity at a = 40 mm, for lightweight concrete when supported on two sides.

Table 1.2.1.2

Fire resistance limits of reinforced concrete slabs

The desired fire resistance limit is 2 hours or 120 minutes.

5) According to clause 2.27 of the Handbook, a reduction factor of 0.9 is applied to determine the fire resistance limit of hollow core slabs:

6) We determine the total load on the plates as the sum of permanent and temporary loads:

7) Determine the ratio of the long-acting part of the load to the full load:

8) Correction factor for load according to paragraph 2.20 of the Handbook:

9) According to clause 2.18 (part 1 b) of the Benefit, we accept the coefficient for reinforcement

10) We determine the fire resistance limit of the slab, taking into account the coefficients for the load and for the reinforcement:

The fire resistance limit of the plate in terms of bearing capacity is

Based on the results obtained in the course of calculations, we obtained that the fire resistance limit of a reinforced concrete slab in terms of bearing capacity is 139 minutes, and in terms of heat-insulating capacity is 180 minutes. It is necessary to take the smallest fire resistance limit.

Conclusion: fire resistance limit of reinforced concrete slab REI 139.

Determination of fire resistance limits of reinforced concrete columns

Type of concrete - heavy concrete with a density of c = 2350 kg/m3 with a large aggregate of carbonate rocks (limestone);

Table 1.2.2.1 (Table 2 of the Handbook) shows the values ​​of the actual fire resistance limits (POf) of reinforced concrete columns with different characteristics. In this case, POf is determined not by the thickness of the concrete protective layer, but by the distance from the surface of the structure to the axis of the working reinforcing bar (), which includes, in addition to the thickness of the protective layer, also half the diameter of the working reinforcing bar.

1) Determine the distance from the heated surface of the column to the axis of the bar reinforcement by the formula:

2) According to clause 2.15 of the Handbook for structures made of concrete with carbonate aggregate, the cross-sectional size can be reduced by 10% with the same fire resistance limit. Then the width of the column is determined by the formula:

3) According to Table 1.2.2.2 (Table 2 of the Handbook), we determine the fire resistance limit for a lightweight concrete column with the parameters: b = 444 mm, a = 37 mm when the column is heated from all sides.

Table 1.2.2.2

Fire resistance limits of reinforced concrete columns

The desired fire resistance limit is between 1.5 hours and 3 hours. To determine the fire resistance limit, we use the linear interpolation method. Data are given in table 1.2.2.3

TO THE QUESTION OF THE CALCULATION OF BEAM-FREE SLABS FOR FIRE RESISTANCE

TO THE QUESTION OF THE CALCULATION OF BEAM-FREE SLABS FOR FIRE RESISTANCE

V.V. Zhukov, V.N. Lavrov

The article was published in the publication “Concrete and reinforced concrete - ways of development. Scientific works of the 2nd All-Russian (International) conference on concrete and reinforced concrete. September 5-9, 2005 Moscow; In 5 volumes. NIIZhB 2005, Volume 2. Section reports. Section “Reinforced concrete structures of buildings and structures”, 2005.”

Consider the calculation of the fire resistance limit of a beamless ceiling using an example that is quite common in construction practice. Beamless reinforced concrete floor has a thickness of 200 mm from concrete of class B25 in compression, reinforced with a mesh with cells of 200x200 mm from reinforcement of class A400 with a diameter of 16 mm with a protective layer of 33 mm (to the center of gravity of the reinforcement) at the lower surface of the floor and A400 with a diameter of 12 mm with a protective layer 28 mm (up to c.t.) at the top surface. The distance between the columns is 7m. In the building under consideration, the ceiling is a fire barrier of the first type according to and must have a fire resistance limit for the loss of heat-insulating ability (I), integrity (E) and bearing capacity (R) REI 150. The assessment of the fire resistance limit of the ceiling according to existing documents can be determined by calculation only by thickness protective layer (R) for a statically determinate structure, the thickness of the ceiling (I) and, if possible, brittle fracture in a fire (E). At the same time, the calculations of I and E give a fairly correct assessment, and the bearing capacity of the ceiling in case of fire as a statically indeterminate structure can only be determined by calculating the thermally stressed state, using the theory of elastic-plasticity of reinforced concrete during heating or the theory of the method of limit equilibrium of the structure under the action of static and thermal loads in case of fire . The latter theory is the simplest, since it does not require the determination of stresses from a static load and temperature, but only the forces (moments) from the action of a static load, taking into account changes in the properties of concrete and reinforcement during heating until plastic hinges appear in a statically indeterminate structure when it turns into mechanism. In this regard, the assessment of the bearing capacity of a beamless floor in case of fire was made according to the limit equilibrium method, and in relative units to the bearing capacity of the floor under normal operating conditions. The working drawings of the building were reviewed and analyzed, calculations were made for the fire resistance limits of a reinforced concrete beamless ceiling upon the onset of signs of limit states normalized for these structures. The calculation of the fire resistance limits for the bearing capacity is made taking into account the change in the temperature of concrete and reinforcement for 2.5 hours of standard tests. All thermodynamic and physical-mechanical characteristics of construction materials given in this report are taken on the basis of data from VNIIPO, NIIZHB, TsNIISK.

FIRE RESISTANCE LIMIT OF THE LOSS OF THERMAL INSULATING CAPABILITY (I)

In practice, the heating of structures is determined by a finite-difference or finite-element calculation using a computer. When solving the problem of thermal conductivity, changes in the thermophysical properties of concrete and reinforcement during heating are taken into account. Calculation of temperatures in the structure at standard temperature regime produced under the initial condition: the temperature of the structures and the environment is 20C. The temperature of the environment tc during a fire varies depending on time according to . When calculating temperatures in structures, convective Qc and radiant Qr heat transfers between the heated medium and the surface are taken into account. The calculation of temperatures can be performed using the conditional thickness of the considered concrete layer Xi* from the heated surface . To determine the temperature in concrete, calculate

Let us determine by formula (5) the temperature distribution over the thickness of the floor after 2.5 hours of fire. Let us determine by formula (6) the thickness of the floors, which is necessary to achieve a critical temperature of 220C on its unheated surface in 2.5 hours. This thickness is 97 mm. Therefore, a 200 mm thick overlap will have a fire resistance limit for the loss of heat-insulating ability of at least 2.5 hours.

FLOOR SLAB LOSS FIRE RESISTANCE LIMIT (E)

In case of fire in buildings and structures in which concrete and reinforced concrete structures are used, brittle fracture of concrete is possible, which leads to loss of structural integrity. Destruction occurs suddenly, quickly and therefore is the most dangerous. Brittle fracture of concrete begins, as a rule, after 5-20 minutes from the beginning of the fire impact and manifests itself as a spall from the heated surface of the structure of pieces of concrete, as a result, the structure may appear through hole, i.e. the structure can achieve premature fire resistance by loss of integrity (E). The brittle destruction of concrete may be accompanied by a sound effect in the form of a light pop, crackling of varying intensity, or an "explosion". In the case of brittle fracture of concrete, pieces weighing up to several kilograms can be scattered over a distance of up to 10–20 m. steam filtration through the concrete structure. The brittle fracture of concrete during a fire depends on the structure of concrete, its composition, humidity, temperature, boundary conditions and external load, i.e. it depends both on the material (concrete) and on the type of concrete or reinforced concrete structure. The assessment of the fire resistance limit of a reinforced concrete floor in terms of loss of integrity can be performed by the value of the brittle fracture criterion (F), which is determined by the formula given in:

LOSS LOSS FIRE RESISTANCE LIMIT (R)

According to the bearing capacity, the fire resistance of the ceiling is also determined by calculation, which is allowed. Thermal engineering and static problems are solved. In the thermotechnical part of the calculation, the temperature distribution over the thickness of the slab is determined under standard thermal exposure. In the static part of the calculation, the bearing capacity of the slab is determined in case of a fire with a duration of 2.5 hours. The load and support conditions are taken in accordance with the design of the building. Combinations of loads for calculating the fire resistance limit are considered as special. In this case, it is allowed not to take into account short-term loads and include only permanent and temporary long-term standard loads. The loads on the slab in case of fire are determined according to the NIIZhB method. If the calculated bearing capacity of the slab is R under normal operating conditions, then the calculated load value is P = 0.95 R. The standard load in case of fire is 0.5R. The design resistances of materials for calculating the fire resistance limits are accepted with a reliability factor of 0.83 for concrete and 0.9 for reinforcement. The fire resistance limit of reinforced concrete floor slabs reinforced with bar reinforcement may occur for reasons that must be taken into account: reinforcement slippage on a support when the contact layer of concrete and reinforcement is heated to a critical temperature; reinforcement creep and fracture when the reinforcement is heated to a critical temperature. In the building under consideration, monolithic reinforced concrete floors are used and their load-bearing capacity in case of fire is determined by the limit equilibrium method, taking into account changes in the physical and mechanical properties of concrete and reinforcement during heating. It is necessary to make a small digression about the possibility of using the limit equilibrium method to calculate the fire resistance limit of reinforced concrete structures under thermal exposure during a fire. According to the data, “as long as the limit equilibrium method remains in force, the limits of the bearing capacity are completely independent of the actual self-stresses that arise, and, consequently, of factors such as temperature deformations, displacements of supports, etc.” But at the same time, it is necessary to take into account the fulfillment of the following prerequisites: structural elements should not be brittle before reaching the limiting stage, self-stresses should not affect the limiting conditions of the elements. In reinforced concrete structures, these prerequisites for the applicability of the limit equilibrium method are preserved, but for this it is necessary that there is no slippage of the reinforcement in the places where plastic hinges form and brittle fracture of structural elements until the limit state is reached. In case of fire, the greatest heating of the floor slab is observed from below in the zone of maximum moment, where, as a rule, the first plastic hinge is formed with sufficient anchoring of the tensile reinforcement with its significant deformability from heating to rotate in the hinge and redistribute forces to the support zone. In the latter, the increase in the deformability of the plastic hinge is facilitated by heated concrete. “If the limit equilibrium method can be applied, then self-stresses (available in the form of stresses from temperature - authors' note) do not affect the internal and external limit of the bearing capacity of structures.” When calculating by the limit equilibrium method, it is assumed, for this there is corresponding experimental data, that in a fire under the action of a load the slab breaks into flat links connected to each other along the fracture lines by linear plastic hinges. The use of a part of the design bearing capacity of the structure under normal operating conditions as a load in case of fire and the same scheme of destruction of the slab under normal conditions and in case of fire make it possible to calculate the fire resistance limit of the slab in relative units, independent of the geometric characteristics of the slab in plan. Calculate the fire resistance of a heavy concrete slab of compressive strength class B25 with a standard compressive strength of 18.5 MPa at 20 C. A400 class rebar with standard tensile strength (20C) 391.3 MPa (4000 kg/cm2). Changes in the strength of concrete and reinforcement during heating are taken according to. Fracture analysis of a separate strip of panels is made on the assumption that in the considered strip of panels linear plastic hinges are formed parallel to the axis of this strip: one linear plastic hinge in the span with crack opening from below and one linear plastic hinge at the columns with crack opening from above. The most dangerous in case of fire are cracks from below, where the heating of tensile reinforcement is much higher than in cracks from above. The calculation of the bearing capacity R of the floor as a whole in case of fire is carried out according to the formula:

The temperature of this reinforcement after 2.5 hours of fire is 503.5 C. The height of the compressed zone in the concrete of the slab in the middle plastic hinge (in stock without taking into account the reinforcement in the compressed zone of concrete).

Let us determine the corresponding calculated bearing capacity of the floor R3 under normal operating conditions for a floor with a thickness of 200 mm, with the height of the compressed zone for the middle hinge at xc = ; the shoulder of the inner pair Zc=15.8 cm and the height of the compressed zone of the left and right hinges Хс = Хn=1.34 cm, the shoulder of the inner pair Zx=Zn=16.53 cm. The calculated bearing capacity of the floor R3 20 cm thick at 20 C.

In this case, of course, the following requirements must be met: a) at least 20% of the top reinforcement required on the support should pass over the middle of the span; b) the upper reinforcement above the extreme supports of the continuous system is started at a distance of at least 0.4l in the direction of the span from the support and then gradually breaks off (l is the span length); c) all upper reinforcement above the intermediate supports should extend to the span by at least 0.15 l.

CONCLUSIONS

1. To assess the fire resistance limit of a beamless reinforced concrete floor, calculations of its fire resistance limit must be performed according to three signs of limit states: loss of bearing capacity R; loss of integrity E; loss of heat-insulating ability I. In this case, the following methods can be used: limit equilibrium, heating and crack mechanics.
2. Calculations have shown that for the object under consideration, for all three limit states, the fire resistance limit of a slab 200 mm thick made of concrete of compressive strength class B25, reinforced reinforcing mesh with cells of 200x200 mm steel A400 with a thickness of the protective layer of reinforcement with a diameter of 16 mm at the bottom surface of 33 mm and an upper diameter of 12 mm - 28 mm is at least REI 150.
3. This beamless reinforced concrete floor can serve as a fire barrier, the first type according to.
4. The assessment of the minimum fire resistance limit of a beamless reinforced concrete floor can be performed using the limit equilibrium method under conditions of sufficient embedment of tension reinforcement in places where plastic hinges are formed.

Literature

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