RU2674570C1 - Method for evaluating fire resistance of reinforced concrete slab with pinched contour - Google Patents

Abstract

FIELD: fire safety.SUBSTANCE: invention relates to the field of fire safety of buildings and structures and can be used to classify reinforced concrete slabs with pinched contour. Essence of the invention lies in the fact that the test of reinforced concrete slabs is carried out without destruction, using a complex of individual quality indicators, estimating their value using statistical control. For this purpose, the geometrical dimensions of the reinforced concrete slab, the heating pattern of the design cross section under fire conditions, the placement of the reinforcement in the cross section, the depth of embedding and the degree of its fire protection, the indicator of thermal diffusion of concrete, the value of the test load on a concrete slab with pinching along the contour and the intensity of the stress in the rods of the working reinforcement are determined. Fire resistance limit of reinforced concrete slab is determined by the sign of loss of bearing capacity F, using analytical equation (1). When describing the process of resistance of a reinforced concrete slab to the fire impact of a standard fire, the following parameters are taken into account: the degree of fire protection of the reinforcement C, cm, the intensity of its stress J(within 0.1–1.0) and the index of thermal diffusion of concrete D, mm/min as well as features of the reinforcement of reinforced concrete slabs and static scheme of its operation.EFFECT: providing the possibility of determining the design fire resistance of a reinforced concrete slab without full-scale fire exposure and increasing the reliability of statistical quality control and non-destructive testing.11 cl, 4 dwg

Description

The invention relates to the field of fire safety of buildings and structures (hereinafter - buildings). In particular, it can be used to classify reinforced concrete slabs with contamination along the contour according to their resistance to high temperatures of a standard fire. This makes it possible to justify the use of existing structures with a design fire resistance limit in buildings of various purposes.

The need to assess the fire resistance of reinforced concrete floor slabs arises during the reconstruction of a building, strengthening its structures, bringing the actual fire resistance of building structures in accordance with the requirements of modern standards, during the examination and restoration of reinforced concrete slabs after a fire.

A known method for evaluating the fire resistance of a reinforced concrete structure according to the results of a study of the consequences of a natural fire. This method includes determining the position of the beam slab in the building, assessing the condition of the slab by inspecting and measuring, manufacturing control samples of concrete and reinforcement, determining the time of the onset of the ultimate state of the reinforced concrete slab by the loss of the structural bearing capacity, that is, collapse under conditions of power load and high temperature full-scale fire / Ilyin N.A. The consequences of fire on reinforced concrete structures. - M., Stroyizdat, 1979, (see p. 34-35; 90) / [1].

The reasons that impede the achievement of the technical result indicated below when using the known method include the fact that in the known method, the fire resistance limits of a reinforced concrete slab are determined approximately from the results of a study of a past fire. A detailed investigation of the fire involves the expert’s continued work. At the same time, it is impossible to assess the fire resistance of full-scale reinforced concrete floor slabs having different sizes and different power loads. It is impossible to compare the results with standard fire tests of similar reinforced concrete slabs. Therefore, the known method of roads is laborious and dangerous for testers.

There is a method of evaluating the fire resistance of a reinforced concrete slab of a building by testing, including conducting a technical inspection, establishing the type of concrete and reinforcement of the slab, identifying the conditions for its support and fastening, determining the time of the ultimate state of the reinforced concrete structure under standard load under standard fire exposure / GOST 30247.1-94 . Building constructions. Fire test methods. Bearing and enclosing structures [2].

The reasons that impede the achievement of the following technical result when using the known method include the fact that in the known method, the tests are carried out on a design sample, which is affected by the design regulatory loads. Plate tests are carried out on special bench equipment in fire furnaces until it is destroyed. The dimensions of the slab sample are limited depending on the dimensions of the opening of the stationary furnace. Therefore, standard fire tests are time-consuming, not effective, not safe, have little technological capabilities for testing various sizes and variously loaded reinforced concrete floor slabs with experience. The results of the fire test are single and do not take into account the diversity in fixing the ends of the structures, their actual dimensions, the actual reinforcement and the heating circuit of the dangerous section of the tested reinforced concrete slab in a fire. The economic costs of testing increase due to the costs of dismantling the structure, transportation to the installation site of the heating furnace and the creation of a standard thermal regime in them.

There is a method of evaluating the fire resistance of a reinforced concrete slab based on the contour, including conducting a technical inspection, establishing the type of concrete and reinforcement of the slab, identifying the conditions for its support, identifying the limiting (critical) temperature of heating of reinforcing steel; determination of temperatures in stretched plate reinforcement; The calculation of the fire resistance of a reinforced concrete slab contains two parts: heat engineering and static; the solution of the heat engineering problem determines the temperature field in the cross section of the plate using power algebraic formulas and auxiliary tables: the degree of heating of the tensile reinforcement is determined at specified time intervals of a standard fire test with an interval of 30-60 minutes; then build a graph of changes in temperature of the reinforcement at predetermined intervals; the time through which the temperature of the tensile reinforcement (by interpolation) will be equal to the limit (critical) is taken as the fire resistance of a reinforced concrete slab with a contour bearing / see Bushev V.P., Pchelintsev V.A., Yakovlev A.I. and other Fire resistance of buildings. - M .; Stroyizdat, 1970 .-- 261 p. (see chapter IV Fire resistance of reinforced concrete structures, p. 2 Calculation of temperatures in flat structures; p. 13 Slabs supported along the contour, pp. 108-109) [3].

For reasons that impede the achievement of the technical result indicated below when using the known method, the known method does not take into account the features of the static operation of the reinforced concrete structure: monolithic interface nodes between the elements of the floors; schemes of external loads, the integral degree of voltage of the working reinforcement; fracture schemes for a monolithic reinforced concrete slab, the formation of plastic hinges; features for determining bending moments along a short and long span of a slab; the analytical equation for calculating the critical (limiting) heating temperature does not take into account the class of reinforcement and its main strength characteristics; the use of a graph of the change in the heating temperature of tensile reinforcement during a standard fire test increases the errors in calculating the fire resistance of the plate; the calculation algorithm (especially the heat engineering parts) requires a significant amount of computer RAM.

The closest method of the same purpose to the claimed invention by the totality of features is a method for assessing the fire resistance of a reinforced concrete structure by testing, including technical inspection, establishing the type of concrete and reinforcement of the beam structure, identifying the conditions for its support and fastening, determining the time of the onset of the ultimate state of fire resistance, this test of reinforced concrete slab is carried out without destruction according to a set of individual quality indicators, complement technical inspection instrumental measurements of the geometric dimensions of the slab and its dangerous sections, determine the number and nominal diameter of the bars of the working reinforcement, their relative position and the thickness of the protective layer of concrete, identify the shape of the reinforced concrete slab, the heating circuit of the design cross-section in case of fire and the heating conditions of the working reinforcement, establish the depth of the working rods reinforcement and the degree of its fire protection, determine the thermal diffusion of concrete, evaluate the characteristics of concrete to compressive strength and longitudinal working arm resistance to tensile strength, set the standard load on the beam structure and find the intensity of the working reinforcement from it, and using the obtained integral parameters of the beam structure, calculate its actual fire resistance based on the loss of bearing capacity / Pat. 2615048 RF, IPC G01N 25/50. A method of evaluating the fire resistance of a reinforced concrete beam structure of a building. / Ilyin N.A., Panfilov D.A., application. SASAS: 11/05/2015; [4] - taken as a prototype.

The reasons that impede the achievement of the technical result indicated below when using the known method include insufficient accuracy in determining the design fire resistance on the basis of loss of bearing capacity of continuous reinforced concrete slab. The characteristic properties of the static work of the reinforced concrete structure are not taken into account; the features of estimating the values of bending moments along the short and long spans of the plate are not taken into account. The analytical equation for identifying the limiting temperature of heating of the working reinforcement does not take into account the type of reinforcement and its main characteristics of its thermal fluidity.

The essence of the invention is to establish indicators of fire safety of the floors of the building in terms of the guaranteed duration of the resistance of the reinforced concrete slab with a rigid bearing on the contour in a fire; in determining the actual fire resistance limits of a reinforced concrete slab of a building based on the loss of bearing capacity F _{U (R)} , min.

The technical result is a decrease in the complexity of evaluating the fire resistance of a reinforced concrete slab; expanding the technological capabilities of determining the actual fire resistance of variously loaded plates of any sizes by signs of loss of bearing capacity; the ability to test floor slabs for fire resistance without disturbing the functional process in the building; reduction in economic costs of testing; maintaining the serviceability of the building during the inspection and testing of the slab; simplification of conditions and reduction of the test time of the slab for fire resistance; increased accuracy and expressiveness of the test; determination of the real resource of reinforced concrete structures for fire resistance; increasing the reliability of determining the degree of fire protection of the longitudinal working reinforcement of the plate, the depth and conditions of its heating; simplification of taking into account the influence on the fire resistance limit of the slab of the features of the static scheme of its operation; determination of the design limit of fire resistance of a reinforced concrete slab with pinching along the contour depending on the design parameters on the basis of loss of bearing capacity.

The specified technical result in the implementation of the invention is achieved by the fact that in the known method of evaluating the fire resistance of a reinforced concrete structure, including conducting a technical inspection, establishing the type of concrete and reinforcement of the reinforced concrete structure, identifying the conditions for its support and fastening, determining the time of occurrence of the ultimate state of fire resistance, testing the reinforced concrete structure without destruction of a set of individual quality indicators, instrumental measurements of the geometric dimensions of the iron ton structure and its design section, determining the number and diameter of the bars of the working reinforcement, their relative position and the thickness of the concrete protective layer, revealing the forms of the reinforced concrete structure, heating schemes of the calculated section in case of fire and heating conditions of the working reinforcement, establishing the depth of the working reinforcement rods and their degree fire protection, setting the value of the thermal diffusion index of concrete of the protective layer, determining the characteristics of concrete to compressive strength and reinforcing resistance to traction, establishing the value of the test load on the reinforced concrete structure and the values of the maximum span and supporting moments, the peculiarity is that the reinforced concrete structure is adopted reinforced concrete slab with contraction along the contour, while the design limit of fire resistance of the reinforced concrete slab on the basis of loss of bearing capacity (F _{U ( R)} , min) is determined using the analytical equation (1):

where e is a natural number (e = 2.718), J _σs is the stress intensity of the working reinforcement of the reinforced concrete slab; n is the indicator of thermal fluidity of the working reinforcement; t _{s, u} - limit temperature (° C) of heating of the working reinforcement, C - degree of fire protection with concrete of the working reinforcement; k ₀ - an indicator of continuity of a reinforced concrete slab.

The degree of fire protection of concrete working reinforcement (C) is calculated according to equation (2):

- показатель термодиффузии бетона, мм²/мин.where a _min - the depth of the working reinforcement, mm;

- indicator of thermal diffusion of concrete, mm ² / min.

The thermal fluidity of the working reinforcement (n and t _{s, cr} , ° С) of the reinforced concrete slab is taken depending on the class of reinforcement with the following values:

, мм²/мин, определяют при температуре

по аналитическому выражению (3):Thermal diffusion index of concrete protective layer

mm ² / min, determined at temperature

according to the analytical expression (3):

- эмпирические числа для определения показателя теплопроводности бетона, Вт/(м⋅°С); С₀ и d - эмпирические числа для определения удельной теплоемости, кДж/(кг⋅°С); ρ_с - плотность сухого бетона, кг/м³; ω - влажность бетона массовая, %.where λ ₀ and

- empirical numbers for determining the thermal conductivity of concrete, W / (m⋅ ° С); С ₀ and d are empirical numbers for determining specific heat, kJ / (kg⋅ ° С); ρ _s is the density of dry concrete, kg / m ³ ; ω - concrete moisture mass,%.

The intensity of the stresses in the cross section of the working reinforcement (J _σs ≤1) of the reinforced concrete slab is calculated by the analytical equation (4):

where t _{s, cr} and t _{s, u} are the critical and limiting heating temperatures of reinforcing steel, ° C, respectively; n is an indicator of thermal fluidity of reinforcing steel.

The maximum heating temperature (t _{s, u} , ° С) of the stretched reinforcement of a reinforced concrete slab is calculated by the analytical equation (5):

where t _{s, cr is the} critical heating temperature of the tensile reinforcement, ° C; ƒ _a is the ratio of the areas of the working reinforcement in the span of the plate A _to / A _∂ ; And _to and And _∂ - respectively, the design area of the working reinforcement located along the short and long span of the plate, mm ² .

; кН⋅м) моментов для расчетной полосы железобетонной плиты определяют, используя уравнения (6) и (7):The value of the maximum span (M, kN⋅m) and reference (

; kN⋅m) of moments for the calculated strip of the reinforced concrete slab is determined using equations (6) and (7):

; α_i и -β_i - расчетные коэффициенты для определения соответственно пролетных и опорных изгибаемых моментов;

- общая нагрузка на расчетную полосу плиты, кН/м²; L_k и L_∂ - соответственно длина короткого и длинного пролета плиты, м.Where

; α _i and -β _i are the calculated coefficients for determining the span and reference bending moments, respectively;

- total load on the design strip of the plate, kN / m ² ; L _k and L _∂ - respectively, the length of the short and long span of the plate, m

The values of the coefficients (α _k ) for determining the maximum span bending moments of a reinforced concrete slab acting in the direction of the short sides of the slab are identified using the analytical expression (8):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ).

The values of the coefficients (α _∂ ) for determining the maximum span bending moments of a reinforced concrete slab acting in the direction of the long sides of the slab are found using the analytical expression (9):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ).

The values of the coefficients (β _k ) for determining the maximum reference bending moments of a reinforced concrete slab acting in the direction of the short sides of the slab are found using the analytical expression (10):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ).

The values of the coefficients (β _∂ ) for determining the maximum reference bending moments of a reinforced concrete slab acting in the direction of the long sides of the slab are found using the analytical expression (11):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ).

The causal relationship between the totality of features and the technical result is as follows.

The exclusion of fire tests of a reinforced concrete slab and their replacement with non-destructive tests reduces the complexity of evaluating their fire resistance, expands the technological capabilities for detecting design fire resistance of variously loaded slabs of any size, makes it possible to test a reinforced concrete slab for fire resistance without disturbing the functional process of the building under examination, as well as comparing the results obtained with standard tests of similar plates and maintaining the serviceability of the subject buildings without disturbing the bearing capacity of its reinforced concrete floor slabs during the test. Therefore, the conditions for testing the reinforced concrete slab for fire resistance are greatly simplified.

Reducing the economic costs of testing include reducing the cost of dismantling, transportation and fire tests of a sample of reinforced concrete slabs.

The use of a mathematical description of the process of resistance of a reinforced concrete slab with a rigid support to high temperatures and the use of the constructed analytical expressions (1) - (5) increase the accuracy and expressiveness of the estimation of its fire resistance based on the loss of bearing capacity.

The use of integral structural parameters, such as: the degree of fire protection of the working reinforcement, the intensity of the power voltage and the thermal diffusion index of concrete, simplifies the mathematical description of the resistance process of a loaded reinforced concrete slab with pinching to high temperature effects.

Assessment of the fire resistance of a reinforced concrete slab by only one quality indicator, for example, by the thickness of the concrete protective layer, leads, as a rule, to underestimation of its design fire resistance limit, since variations in individual quality indicators have different signs on it, and a decrease in the fire resistance due to one indicator can be compensated by others. As a result, in the proposed method, the evaluation of the fire resistance of a reinforced concrete slab with jamming by loss of bearing capacity is provided for not by one indicator, but by a set of individual indicators of its quality. This allows you to more accurately take into account the real life of the design fire resistance of a reinforced concrete floor slab.

Simplified features of the static scheme of the reinforced concrete slab, reinforcement of the design section, the maximum temperature of heating of the working reinforcement and the continuity of the reinforced concrete slab by the value of the fire resistance.

и

- тоже, максимальные опорные изгибаемые моменты, кН/м².In FIG. 1 shows a diagram for calculating the fire resistance of reinforced concrete slabs with pinching along the contour; the dashed line shows the fracture lines of the slab; L _k and L _∂ - design spans of the plate (distance in the light between the bearing beams), m; ϕ is the angle of rotation of the links of the plate, deg; M _k and M _∂ are the maximum flying bending moments acting in the direction of the short and long sides of the plate, kN⋅m;

and

- also, the maximum reference bending moments, kN / m ² .

и

- тоже, максимальные опорные изгибаемые моменты, кН/м²; L_k и L_∂ - расчетные пролеты плиты (расстояние в свету между несущими балками).In FIG. 2 shows bending moments M _k and M _∂ - maximum span bending moments acting in the direction of the short and long sides of the plate, kN⋅m;

and

- also, the maximum reference bending moments, kN / m ² ; L _k and L _∂ - design spans of the plate (the distance in the light between the bearing beams).

класса А240, шаг стержней 200 мм; 2 - поперечная арматура

класса А240, шаг стержней 200 мм;

и

- длина и ширина арматурной сетки, мм.In FIG. 3 shows the design of the reinforcing mesh with working reinforcement in both directions; (for example, calculation): 1 - longitudinal reinforcement

A240 class, pitch of rods 200 mm; 2 - transverse reinforcement

A240 class, pitch of rods 200 mm;

and

- the length and width of the reinforcing mesh, mm.

и

- тоже, максимальные опорные изгибаемые моменты, кН/м².In FIG. 4 shows diagrams of bending moments of a reinforced concrete slab: M _k and M _∂ — maximum span bending moments acting in the directed short and long sides of the slab, kN⋅m;

and

- also, the maximum reference bending moments, kN / m ² .

Information confirming the possibility of carrying out the invention with obtaining the above technical result.

The method of evaluating the fire resistance of a reinforced concrete slab is carried out in the following sequence. First, a visual inspection of the building is carried out. Assign a set of individual quality indicators for reinforced concrete slabs affecting its actual fire resistance. The conditions of abutment, fixing the ends of the reinforced concrete slab and its design section are revealed. Then, individual quality indicators of the reinforced concrete slab and its integral parameters are evaluated, and the fire resistance limit of the tested reinforced concrete slab with pinching is estimated from them.

Visual inspection means checking the condition of a reinforced concrete slab, including identifying the conditions of support of the slab, determining the type and strength class of concrete, the thickness of its protective layer, the presence of cracks and spalls, breaking the adhesion of working reinforcement to concrete, and the presence of corrosion of reinforcing steel.

The number and location of the control plots in which the quality indicators of the reinforced concrete slab are determined are determined as follows. In a reinforced concrete slab having one design section, control sections are placed only in this section. In a reinforced concrete slab having several design sections, control sections are assigned with the obligatory arrangement of part of the control sections in the design section.

The main single quality indicators of reinforced concrete slabs with pinching, providing design fire resistance, include: the geometric dimensions of the reinforced concrete slab and the height of the design section; occurrence depth, strength class, nominal diameter, intensity of power voltage and yield strength of working reinforcement; concrete compressive strength, moisture and its density in natural conditions; the thickness of the protective layer and the rate of thermal diffusion of concrete.

For statically indefinable reinforced concrete slabs with jamming, design sections are assigned in spans and on supports. The calculated cross section is then found by the largest ordinate of the envelope of the moment diagram.

The dimensions of the reinforced concrete slab with pinching are checked with an accuracy of 1 mm; crack width - with an accuracy of 0.05 mm.

By the depth of the working reinforcement is understood the normal distance between the concrete surface of the reinforced concrete slab with pinching and the longitudinal axis of the working reinforcement.

For a continuous plate reinforced with nets or individual rods, with their unilateral heating (m ₀ = 1), the depth of the longitudinal working reinforcement ( a _min , mm) in the cross section is determined by the algebraic expression (13):

- толщина защитного слоя бетона, мм; d - номинальный диаметр стержня, мм.Where

- thickness of the protective layer of concrete, mm; d is the nominal diameter of the rod, mm

The intensity of the stresses (J _σs ) in the _tensile reinforcement in the design section and slab is determined by equation (4):

.

.

The maximum heating temperature (t _{s, u} , ° С) of the stretched reinforcement of a reinforced concrete slab is calculated by the analytical equation (5):

where t _{s, cr is the} critical heating temperature of the tensile reinforcement, ° C; ƒ _a is the ratio of the areas of the working reinforcement in the span of the plate A _to / A _∂ ; here A _to and A _∂ are, respectively, the design areas of the working reinforcement located along the short and long span of the plate, mm ² .

Example. Given: reinforced concrete slab with pinched contour; plate thickness h = 140 mm; plate dimensions in terms of L _∂ = 7.6 m; L _k = 5.8 m.

Calculation data: column pitch 6 × 8 m; temporary standard load on the ceiling - 9 kN / m ² ; constant normative load from the mass of the floor - 1.0 kN / m ² ; concrete class B25 (density ρ = 25 kN / m ³ , rated resistance R _b = 14.5 MPa); the level of responsibility of the building is normal (γ _n = 1,0).

(γ=1,1); временная

(γ=1,2); общая нагрузка

.Estimated load: constant

(γ = 1,1); temporary

(γ = 1,2); total load

.

,

), показатель термодиффузии бетона

.Heavy concrete class B15; (

,

), an indicator of thermal diffusion of concrete

.

Armature class A240 (R _sn , = 240 MPa, R _s = 210 MPa); critical temperature t _{s, cr} = 510 ° C; (n = 2.8). A diagram for calculating the fire resistance of a reinforced concrete slab with pinching is shown in FIG. one.

It is required to set the design limit of fire resistance of a reinforced concrete slab with pinching along the contour.

Solution: 1) The ratio of spans of reinforced concrete slabs:

n ₀ = L _∂ / L _k = 7.6 / 5.8 = 1.31

where L _k and L _∂ are the calculated spans of the slab (the distance in the light between the supporting beams.

2) The calculated coefficients for determining the maximum passing bending moment of a reinforced concrete slab, acting in the direction of the short sides of the slabs (α _k ), are calculated according to equation (8):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ).

also, the coefficient (α _∂ ) for the maximum moment acting in the direction of the long sides of the plate is calculated by equation (9):

3) The calculated coefficient for determining the maximum reference bending moment of a reinforced concrete slab, acting in the direction of the short sides of the slab (β _k ), is calculated according to equation (10):

β _k = (5 + n ₀ ) / 100⋅n ₀ = (5 + 1.31) / 100⋅1.31 = 6.31 / 131 = 0.048;

also, the coefficients (β _∂ ) for the moment acting in the direction of the long sides of the plate are calculated by equation (11):

;

;

) вычисляют по уравнениям (6) и (7):4) the values of max passing and supporting moments for a strip of a slab 1 m wide (at

) are calculated according to equations (6) and (7):

M _k = α _to ⋅Q = 0.022-535.57 = 11.78 kN⋅m;

M _∂ = α _∂ ⋅Q = 0.0125⋅535.57 = 6.695 kN⋅m

;

;

.

.

Bending moment diagrams are shown in FIG. 2.

5) Strength calculation of the normal section of a reinforced concrete slab in the span

In the short direction

The relative statistical moment of the compressed zone of concrete is determined:

where b = 1000 mm is the width of the calculated strip; h ₀ = h- a _min = 140-20 = 120 mm - working section height; h is the thickness of the floor slab; a _min = 20 mm is the distance from the resultant force in the stretched reinforcement to the nearest section edge, taking into account the location of the adopted reinforcement in the short direction; M _k - calculated bending moment in the short direction of the plate; R _b is the calculated concrete compressive strength.

The reinforcement area for a strip 1 m wide is

.

.

where R _s is the calculated tensile strength of the reinforcement.

А240 (A-I), фактическая площадь арматуры A_s=565 мм², шаг стержней 200 мм.Accept

A240 (AI), the actual area of reinforcement A _s = 565 mm ² , the pitch of the rods 200 mm.

In the long direction

The relative statistical moment of the compressed zone of concrete is determined:

where b = 1000 mm is the width of the calculated strip; h ₀ = h- a _min = 140-32 = 108 mm - working section height; a _min = 32 mm is the distance from the resultant force in the stretched reinforcement to the nearest section edge, taking into account the location of the adopted reinforcement in the short direction.

The reinforcement area for a strip 1 m wide is

.

.

А240 (A-I), фактическая площадь арматуры A_s=393 мм², шаг стержней 200 мм.Accept

A240 (AI), the actual area of the reinforcement is A _s = 393 mm ² , the pitch of the rods is 200 mm.

From the design rods of the short and long directions, a welded mesh is constructed (Fig. 3) and installed in the lower part of the reinforced concrete structure.

6) The ratio of the design area of reinforcement (ƒ _a ) in the span of a reinforced concrete slab:

ƒ _a = A _k / A _∂ = 565/393 = 1,438;

А240, установленная по короткому пролету; А_∂ - тоже

А240, установленная по длинному пролету плиты.here A _k is the design area of the working reinforcement

A240, installed on a short span; And _∂ is also

A240 installed over a long span of the slab.

7) The maximum heating temperature (t _{s, u} ° C) of the tensile reinforcement (at k ₀ = 1.2) is determined using equation (5):

t _s , _u = t _s.cr + [(350 / ƒ _a ) +150] / ƒ _a ⋅k ₀ == 510 + [(350 / 1,438) +150] / 1,438⋅1,2 = 653 <800 ° FROM,

where t _{s, cr is the} critical heating temperature of the tensile reinforcement, ° C; ƒ _a is the ratio of the areas of the working reinforcement in the span of the slab A _k / А _∂ ; here A _k and A _∂ are the design areas of the working reinforcement located along the short and long span of the plate, mm ² , respectively.

8) The degree of fire protection of concrete reinforcement short span slab is calculated according to equation (2):

;

;

also a long span of the slab:

,

,

- показатель термодиффузии бетона, мм²/мин.where a _mjn is the depth of the working reinforcement, mm;

- indicator of thermal diffusion of concrete, mm ² / min.

9) The stress intensity in the working reinforcement of a reinforced concrete slab is calculated by equation (4):

.

.

10) The fire resistance of a reinforced concrete slab on the basis of loss of bearing capacity (F _{U (R)} , min) of a short span of the slab (at k ₀ = 1.2) is determined by equation (1)

.

.

also for a long span of the slab:

,

,

where e is a natural number (e = 2.718), J _σs is the voltage intensity of the working reinforcement of a reinforced concrete slab; n is the index of thermal fluidity of the class of working reinforcement; t _{s, u} is the limiting temperature (° С) of heating of the working reinforcement, C is the degree of fire protection with concrete of the working reinforcement.

Therefore, the weakest in terms of heat is the section of the reinforced concrete slab in the direction of a short span; and design, on the basis of loss of bearing capacity, the fire resistance of a reinforced concrete structure is F _{U (J)} = 365 min.

The fire resistance of a reinforced concrete slab based on the loss of heat-insulating ability (F _{U (J)} , min) is calculated by the power function:

,

,

- показатель термодиффузии бетонаwhere h is the thickness of the slab, mm;

- indicator of thermal diffusion of concrete

), арматура класса В500, показали пределы огнестойкости, F_U(R)=360 мин, F_U(J)=235 мин. Следовательно, заявленное изобретение соответствует условию "промышленная применимость".The proposed method was applied during field inspection of reinforced concrete slabs covering the building in Samara. The results of non-destructive tests of reinforced concrete slabs with contamination along the contour of 5.8 × 7.6 × 0.14 m in size, heavy concrete, class B 35 (

), valves of class B500, showed fire resistance limits, F _{U (R)} = 360 min, F _{U (J)} = 235 min. Therefore, the claimed invention meets the condition of "industrial applicability".

Information sources

1. Ilyin N. A. Consequences of fire exposure on reinforced concrete structures. - M., Stroyizdat, 1979, - 128 p. (see p. 16; 34-35).

2. GOST 30247.1-94. Building constructions. Fire test methods. Bearing and enclosing structures.

3. Bushev V.P., Pchelintsev V.A., Yakovlev A.I. and other Fire resistance of buildings. - M., Stroyizdat, 1970. - 261 p.) (See chap. IV. Fire resistance of reinforced concrete structures; p. 2. Calculation of temperatures in flat structures; p. 13. Plates (panels) supported along the contour, p. 108 -109).

4. Pat. 2615048 RF, IPC G01N 25/50. A method for assessing the fire resistance of a reinforced concrete beam structure of a building / Ilyin N.A., Panfilov D.A., Sgasu: 11/05/2015.

Claims (33)

1. A method for assessing the fire resistance of a reinforced concrete structure, including conducting a technical inspection, establishing the type of concrete and reinforcement of the reinforced concrete structure, identifying the conditions for its support and fastening, determining the time of the onset of the ultimate state of fire resistance, testing the reinforced concrete structure without destruction by a set of individual quality indicators, instrumental measurements of geometric the dimensions of the reinforced concrete structure and its design section, determining the number and diameter of the rods of the working arm Ura, their relative position and thickness of the protective layer of concrete, revealing the shape of the reinforced concrete structure, heating scheme of the design cross-section in case of fire and the conditions of heating of the working reinforcement, establishing the depth of the rods of the working reinforcement and the degree of their fire protection, establishing the value of the thermal diffusion coefficient of concrete of the protective layer, determining the characteristics of concrete compressive strength and working reinforcement tensile strength, establishing the magnitude of the test load on the reinforced concrete structure and the values maximum span and support moments, characterized in that the concrete structure receiving concrete slab with a pinching along the contour, the design limit of the fire resistance of concrete slab on the ground bearing capacity loss _{(F U (R),} min) is determined using an analytical equation (1 ):

where e is a natural number (e = 2.718), J _os is the voltage intensity of the working reinforcement of a reinforced concrete slab; n is the indicator of thermal fluidity of the working reinforcement; t _{s, u} is the limiting temperature (° С) of heating of the working reinforcement, C is the degree of fire protection with concrete of the working reinforcement; k ₀ - an indicator of continuity of a reinforced concrete slab. 2. The method according to p. 1, characterized in that the degree of fire protection with concrete of the working reinforcement (C) is calculated according to equation (2):

where a _min - the depth of the working reinforcement, mm;

- indicator of thermal diffusion of concrete, mm ² / min. 3. The method according to p. 1, characterized in that the thermal fluidity of the working reinforcement (n and t _{s, cr} ° C) of the reinforced concrete slab is taken depending on the class of reinforcement with the following values:

4. The method according to p. 1, characterized in that the thermal diffusion index of the concrete protective layer

mm / min, determined at temperature

according to the analytical expression (3):

where λ ₀ and in are empirical numbers for determining the thermal conductivity of concrete, W / (m⋅ ° С); С ₀ and d are empirical numbers for determining specific heat, kJ / (kg⋅ ° С); ρ _c is the density of dry concrete, kg / m ³ ; ω - concrete moisture mass,%. 5. The method according to p. 1, characterized in that the intensity of the power stresses in the cross section of the working reinforcement (J _σs ≤1) of the reinforced concrete slab is calculated by the analytical equation (4):

where t _{s, cr} and t _{s, u} are the critical and limiting heating temperatures of reinforcing steel, respectively, ° C; n is an indicator of thermal fluidity of reinforcing steel. 6. The method according to p. 1, characterized in that the limiting heating temperature (t _{s, u} , ° C) of the tensile reinforcement of the reinforced concrete slab is calculated by the analytical equation (5): t _{s, u} = t _{s, cr} + [(350 / ƒ _a ) +150] / ƒ _a , where t _{s, cr is the} critical heating temperature of the tensile reinforcement, ° C; ƒ _a is the ratio of the areas of the working reinforcement in the span of the plate A _to / A _∂ ; And _to and And _∂ - respectively, the design area of the working reinforcement located along the short and long span of the plate, mm ² . 7. The method according to p. 1, characterized in that the maximum span (M, kN⋅m) and reference (

; kN⋅m) of moments for the calculated strip of the reinforced concrete slab is determined using equations (6) and (7): M = α _i ⋅ Q;

where Q = g⋅L _k ⋅L _∂ ; α _i and -β _i are the calculated coefficients for determining the span and reference bending moments, respectively; g is the total load on the design strip of the plate, kN / m ² ; L _k and L _∂ - respectively, the length of the short and long spans of the plate, m 8. The method according to p. 1 or 7, characterized in that the values of the coefficients (α _k ) for determining the maximum span bending moments of a reinforced concrete slab acting in the direction of the short sides of the slab are identified using the analytical expression (8):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ). 9. The method according to p. 1 or 7, characterized in that the values of the coefficients (α _∂ ) for determining the maximum span bending moments of a reinforced concrete slab acting in the direction of the long sides of the slab are found using the analytical expression (9):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ). 10. The method according to p. 1 or 7, characterized in that the values of the coefficients (β _k ) for determining the maximum reference bending moments of the reinforced concrete slab, acting in the direction of the short sides of the slab, are found using the analytical expression (10):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ). 11. The method according to p. 1 or 7, characterized in that the values of the coefficients (β _∂ ) for determining the maximum reference bending moments of the reinforced concrete slab, acting in the direction of the long sides of the slab, are found using the analytical expression (11):

where n ₀ is the ratio of the spans of the plate (L _∂ / L _k ).

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