RU2161793C2 - Method for determination of fire resistance of flexible reinforced-concrete building constructions - Google Patents

Abstract

FIELD: fire safety. SUBSTANCE: the flexible reinforced-concrete building constructions are subjected to a non-destructive test according to a complex of single quality indices estimating their value with the aid of statistical quality control. To this end, determined are the geometric dimensions of the reinforced- concrete construction, heating system of dangerous sections in the conditions of fire; arrangement of reinforcement in sections, depth of foundation and the degree of its fire protection; density, moisture content and thermal conductivity of concrete; value of proof loads on the reinforced-concrete construction and the degree of stress of the principal reinforcement. The fire-resistance limit of the reinforced concrete constructions is determined by a nomogram. The description of fire resistance of the reinforced-concrete constructions to the thermal effect of a standard fire is presented by a mathematical dependence, which takes into account the degree of fire protection of the reinforcement, degree of its stress and thermal conductivity of concrete, as well as the specific features of reinforcing of the reinforced - concrete construction and the physical wear of it. EFFECT: possibility to determine the fire resistance of reinforced-concrete constructions without actual thermal effect, enhanced truth of statistical quality control and non-destructive tests, reduced economic expenditures. 23 cl, 19 dwg

Description

 The invention relates to the field of fire safety of buildings and structures, further - buildings. In particular, it can be used to classify reinforced concrete structures (reinforced concrete structures) of buildings according to their resistance to fire. This makes it possible to justify the use of existing structures with an actual fire resistance limit in buildings of various categories for their fire and explosive hazard.

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

 During the reconstruction of a building, it is possible to reconstruct and redevelop the premises, change their functional purpose, replace building structures and equipment. This affects the change in the required fire resistance of the building and its structures.

 A known method of determining the fire resistance of flexible concrete structures of a building according to the results of a study of the consequences of a natural fire. This method includes determining the position of the structure in the building, assessing the state of the structure by inspection and measurement, making control samples of concrete and reinforcement, determining the time of the onset of the limiting state by the loss of the bearing capacity of the structure, that is, collapse under the conditions of an external load and the thermal effect of a natural fire; see Ilyin N.A. Consequences of fire on reinforced concrete structures. - M., Stroyizdat, 1979, see p. 34-35; 90 [1].

 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 fire resistance limits are determined approximately by the results of a study of the consequences of a past fire. A detailed study determines the long-term work of an expert. At the same time, it is impossible to determine the fire resistance of full-scale structures having other sizes and other external loads. It is difficult to compare the results obtained with standard fire tests of similar designs. Therefore, this method is expensive, has little technological ability to re-test, time-consuming and dangerous for testers.

 There is a method of evaluating the fire resistance of flexible concrete structures of buildings according to the results of full-scale fire tests of a building fragment in which the structures are inspected, the moisture content of concrete is determined, the static load is assigned to the structure according to the actual operating conditions of the building, the factors affecting the fire resistance of the tested structure are determined, and the fire resistance limit value ; see airbag 233-97. Buildings and fragments of buildings. The method of full-scale fire tests. General requirements. - M., VNIIPO, 1997, p. 6-12 [2].

 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 there are high economic costs for conducting fire tests, monitoring the state of structures in an experimental fire is difficult and unsafe, due to differences in the thermal regime of the experimental and standard fires determination of the true values of the fire resistance limits of structures, the reasons for the destruction of the bending structures of the fragment may not be valid ovleny due manifold fire simultaneously operating factors. The ultimate state of fire resistance of flexible structures may not be reached due to earlier destruction of the compressed walls of the fragment; see Fire resistance of buildings. // V.P. Bushev, V.A. Pchelintsev, B.C. Fedorenko, A.I. Yakovlev. - M., Stroyizdat, 1970, p. 252-256. [3].

 The closest method of the same purpose to the claimed invention according to the totality of features is a method for determining the fire resistance of bent reinforced concrete structures of a building by testing, including technical inspection, establishing the type of concrete and reinforcement of the structure, identifying the conditions for its support and fastening, determining the time of occurrence of the ultimate state by the sign of loss load-bearing capacity of the structure under standard load under standard thermal exposure; see GOST 30247.1-94.

 Building constructions. Fire test methods. Bearing and enclosing structures. - M., Publishing house of standards, 1995. - 7 c. [4]; - adopted as a prototype.

 The reasons that impede the achievement of the following technical results when using the known method adopted for the prototype include the fact that in the known method, the tests are carried out on a design sample that is exposed to only constant and continuous loads in their calculated values with a reliability coefficient equal to one, i.e. design regulatory loads. Tests are carried out on special bench equipment in fire furnaces until the destruction of structural samples. The size of the samples is limited depending on the openings of stationary furnaces. Therefore, standard fire tests are time-consuming, not effective, not safe, have little technological capabilities for testing various sizes and variously loaded structures, do not provide the necessary information about the effect of individual design quality indicators on its fire resistance. By a small number of tested samples (2-3 pieces) it is impossible to judge the actual state of the structures, the ceiling of the building. 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, physical wear, actual reinforcement and the heating circuit of the dangerous section of the tested structure in a fire. Determining the fire resistance of reinforced concrete structures by a single quality indicator, for example, by the thickness of the concrete protective layer, as a rule, underestimates the suitability of the structure in a building with a given degree of fire resistance. The economic costs of testing increase due to the costs of dismantling the structure, transportation to the installation site of the heating furnaces and the creation of a standard thermal regime in them.

 The invention consists in the following. The problem to which the claimed invention is directed is to establish fire safety indicators of a building in terms of the guaranteed duration of resistance of reinforced concrete structures in a fire; in determining the actual fire resistance limits of flexible concrete structures during the design, construction and / or operation of a building; in reducing economic costs when testing structures for fire resistance.

 EFFECT: elimination of fire tests of a structure in a building or its fragment; reducing the complexity of determining the fire resistance of structures; expanding the technological capabilities for determining the actual fire resistance of variously loaded structures of any size and the possibility of comparing the results with tests of similar building structures; the ability to test structures for fire resistance without disturbing the functional process in the building; lower economic costs of testing; maintaining the serviceability of the building during inspection and non-destructive testing of structures; simplification of conditions and reduction of the test time of structures for fire resistance; use of a polyparametric nomogram to determine the fire resistance of structures; increased accuracy and expressiveness of the test; getting the opportunity to solve the inverse problems of fire resistance of structures and using the method of selecting variable values of its structural parameters; the use of integral structural parameters to determine the fire resistance of structures and simplification of the mathematical description of the process of thermal resistance of loaded structures; increasing the reliability of the test results of a group of similar designs; accounting for the real resource of the design for fire resistance using a set of individual indicators of their qualities; increasing the reliability of determining the fire protection measure of the working reinforcement of the structure, the depth and conditions of its heating in a fire; simplification of taking into account the influence on the fire resistance of structures of the features of the static scheme of work, manufacturing and physical wear clarification of individual quality indicators of structures that affect their fire resistance and determination of the minimum number of tests; reducing the sample of tested structures to a minimum, ensuring sufficient reliability of the test results; the ability to determine the guaranteed fire resistance of a flexible concrete element by its structural parameters.

 The specified technical result in the implementation of the invention is achieved by the fact that in the known method for determining the fire resistance of flexible concrete structures of a building by testing, including technical inspection, establishing the type of concrete and reinforcement of the structures, identifying the conditions for their support and fastening, determining the time of occurrence of the ultimate state based on the loss of bearing the ability of structures under standard load in conditions of standard heat exposure, a feature is the fact that the test of structures is carried out without destruction according to a set of individual quality indicators, the technical inspection is supplemented by instrumental measurements of the geometric dimensions of structures and their dangerous sections, the number and diameter of the working bars of the reinforcement, their relative position and thickness of the concrete protective layer are determined, the shape of the structure, their schemes are revealed heating in case of fire and the conditions for heating the working reinforcement in dangerous sections, establish the depth of the rods of the working reinforcement and the measure of its fire protection, experiment The density of concrete and its moisture content in the natural state and / or the value of the averaged coefficient of thermal diffusivity of concrete are determined, the characteristics of concrete are evaluated for compressive strength and working reinforcement, tensile strength, the value of the applied standard load on the structures is determined, and the degree of tension of the working reinforcement is determined from it and, using the integral design parameters, according to the nomogram, calculate the actual limit of its fire resistance.

(1)
где J_σc - степень напряжения продольной рабочей арматуры в опасном сечении конструкции;
n и D_c - параметры вида и класса арматуры;
C_a - мера огнезащиты рабочей арматуры, см;
L_br - осредненный коэффициент температуропроводности защитного слоя бетона, см²/ч;
К_i - поправочные коэффициенты, которыми учитывают статическую схему работы конструкций; особенности армирования их опасных сечений, размеры диаметров стержней рабочей арматуры; физический износ конструкции, особенности изготовления конструкций; например, для сплошной плиты K₁ = 1, для многопустотной панели K₁= 0,8.In addition, the peculiarity of the method lies in the fact that the duration of the resistance is F _(R) , min, from the beginning of the standard heat exposure to the loss of bearing capacity is determined by the formula (1):

(1)
where J _σc is the degree of tension of the longitudinal working reinforcement in a dangerous section of the structure;
n and D _c are the parameters of the type and class of reinforcement;
C _a - measure of fire protection of the working reinforcement, cm;
L _br - the average coefficient of thermal diffusivity of the protective layer of concrete, cm ² / h;
To _i - correction factors that take into account the static scheme of the structures; features of reinforcing their dangerous sections, diameters of the rods of the working reinforcement; physical wear of the structure, features of the manufacture of structures for example, for a continuous slab K ₁ = 1, for a multi-hollow panel K ₁ = 0.8.

The next feature of the proposed method is that the degree of stress - J _{σc of the} longitudinal working reinforcement of the structure from the current standard load is determined from condition (2):
J _σc = σ _n / R _sn ≅ 1, (2)
where σ _n - stresses in the working reinforcement from the bending moment, MPa;
R _sn - standard tensile strength of the reinforcement, MPa.

A feature of the method is that the values of the fire resistance parameters n and D _{c of} flexible structures reinforced with various types of steel are taken depending on the class of reinforcement (see table).

The fire protection measure of the longitudinal working reinforcement - C _a is determined by the formula (3):
C _a = m a _min , (3)
where m is an indicator of the heating conditions of the working reinforcement in the cross section of the structure during thermal exposure;
a _min - the minimum depth of the working reinforcement along one of the coordinate axes of the cross section, see

где a_x1, a_x2 и a_y - соответственно глубина залегания арматуры от обогреваемой грани конструкции по осям координат поперечного сечения, см, при a_y < a_x1 - под корнем принимают обратное соотношение, то есть a_x1/a_y.The value of the indicator of the heating condition - m of longitudinal reinforcement with 2- or 3-sided heating it, for a _x1 ≅ a _x2 and a _y ≥ a _x1 , is determined by the formula (4):

where a _x1 , a _x2 and a _y are, respectively, the depth of the reinforcement from the heated face of the structure along the coordinate axes of the cross section, cm, for a _y <a _x1 - the inverse ratio is taken under the root, that is, a _x1 / a _y .

The value of the averaged coefficient of thermal diffusivity of concrete - L _br , cm ² / h, is determined experimentally or found from expression (5):
L _br = 36 · 10 ³ · λ _br · (1 + 0.01 · β) / (c _br + 0.05 · β) · ρ, (5)
where λ _br and с _br are the thermal conductivity, W / m · ^o C) and specific heat of concrete, respectively, kJ / (kg · ^o C), at a temperature of 450 ^o C;
β and ρ are the density of concrete in its natural state, kg / m ³ , and its moisture content,% by weight.

 A feature of the proposed method for determining the fire resistance of reinforced concrete structures of a building is that non-destructive tests are carried out for a group of structures of the same type, the differences between the strength of concrete and the fluidity of reinforcement which are caused mainly by random factors.

 Schemes for heating sections of the test beam structures under fire conditions are determined depending on the actual location of the building parts.

 A further feature of the method is that for beam structures with a symmetric cross-section and with symmetrical heating under heat exposure, the depth of the group of rods of the working reinforcement from the heated surface is determined averaged by finding the geometric center of the areas of the corresponding rods for each reinforced section zone divided by the axis of symmetry .

 If the group of rods of the working reinforcement is located only along the symmetry axis of the structural section, the depth of its occurrence is determined averaged by finding the geometric center of the areas of all the rods of the reinforced section zone.

The depths of the working reinforcement a _x , and a _y , cm, from the faces of the test structure heated under the influence of heat exposure are determined by the formulas (6 and 7):
_{a x = ΣA i · x i} / ΣA i, ( 6)
a _y = ΣA _i · y _i / ΣA _i (7)
where x _i and y _i - the axial distance of the i-th rod along the coordinate axes, cm;
n and A _i - the number of rebar rods and the area of the i-th rod, cm ² .

The depth of the reinforcement - a _x , cm, of the test structure normal to the heated face located at an angle α, deg, to the ordinate axis, is determined by the formula (8):
a _x = b ₁ cos α + a _y sin sin, (8)
where b ₁ - part of the width of the cross section along the bottom of the structure, measured from the heated face to the axis of the reinforcement, cm;
a _y - depth of reinforcement along the ordinate axis, see

A feature of the method is that the correction to the value of the fire resistance limit of structures having working reinforcement of various diameters is taken into account by the coefficient K _⌀ equal to:
K _⌀ = d $\genfrac{}{}{0ex}{}{\text{0}}{\text{c}}$ ^{, 05} , (9)
where d _c is the nominal diameter of the working reinforcement, see

где d₁ и d₂ - номинальный диаметр первого и второго спаренных стержней, см.When grouping the rods of the working reinforcement in the cross section of the structure, the paired rods are replaced by one conventional rod with a reduced diameter d _c , cm, equal to:

where d ₁ and d ₂ - the nominal diameter of the first and second paired rods, see

The amendment to the value of the fire resistance limit of floor structures having a certain physical wear, take into account the coefficient K _f equal to:
K _f = 0.1 · (100-F _and ), (11)
where f _and - the amount of physical deterioration of structures,%.

An increase in the fire resistance of statically indefinable bending structures is taken into account by a coefficient K _m equal to:
K _m = 0.9 (1 + A _on / A), (12)
where A _on and A are, respectively, the cross-sectional area of the longitudinal working reinforcement above the support and in the span of the structure, cm ² .

 A feature of the proposed method is that for the individual indicators of the quality of the structure, affecting the limit of fire resistance, take the geometric dimensions of the structures and the height of the dangerous section; depth, class, diameter, stress degree and yield strength of reinforcement; the compressive strength of concrete, its moisture and density, the thickness of the protective layer and the coefficient of thermal diffusivity of concrete.

The number of tests n _{and a} single indicator of the quality of structures, with a probability of a result of 0.95 and an accuracy of 5%, is determined by the formula (13):
n _and = 0.15 · ν ² ≥ 6, (13)
where ν is the sample coefficient of variation,%.

где H - число однотипных конструкций в здании, шт.In the case when all the individual indicators of the quality of the structure with H more than 9 are within the control limits, the minimum integer number of structures in the sample according to the plan of shortened tests H _c , pcs, is assigned from condition (14):

where H is the number of similar structures in the building, pcs.

В случае, когда хота бы один из единичных показателей качества конструкций выходит за границы допустимых пределов или H ≅ 5 шт, испытанию подвергают все однотипные конструкции здания поштучно.In the case when at least one of the single indicators of the quality of the structures goes beyond the control limits, the minimum number of structures in the sample according to the test norm H _n , pcs, is assigned from condition (15):

In the case when at least one of the single indicators of the quality of structures goes beyond the limits of permissible limits or H ≅ 5 pcs, all the same type building structures are tested individually.

 A feature of the proposed method is that the guaranteed limit of fire resistance of structures is calculated by the above nomogram by solving the inverse problem of fire resistance.

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

 The elimination of fire tests of the structures of an existing building and their replacement with non-destructive tests reduces the complexity of determining their fire resistance, extends the technological possibilities of identifying the actual fire resistance of variously loaded structures of any size, makes it possible to test structures 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 designs and maintenance the suitability of the building being inspected without violating the bearing capacity of its structures during the test. Therefore, the conditions for testing structures for fire resistance are greatly simplified.

 Reducing the economic costs of testing include reducing the cost of dismantling, transportation and fire testing of structural samples.

 The use of a mathematical description of the process of resistance of flexible concrete structures to standard thermal testing and the use of the constructed parametric nomograms increase the accuracy and expressiveness of their fire resistance assessment.

 The use of the nomogram is convenient due to its simplicity, visibility, the possibility of solving inverse problems of fire resistance of structures and the use of the method of selecting variable values of its design parameters.

 The use of integral structural parameters, such as: measures of fire protection of reinforcement, its degree of stress and coefficient of thermal diffusivity of concrete, simplifies the mathematical description of the process of resistance of a loaded structure to thermal effects.

 The proposed technical solution provides for testing not one, but groups of similar designs. This allows you to 5-15 times increase the number of tested structures and increase the reliability of the test results and technical inspection of the building.

 Determination of the fire resistance of structures by only one quality indicator, for example, by the thickness of the concrete protective layer, leads, as a rule, to underestimating their fire resistance limit, since the influence of variations in individual quality indicators on it has different signs, and a decrease in fire resistance due to one indicator can be compensated by others. As a result, in the proposed method, the assessment of the fire resistance of structures is provided not for one indicator, but for a complex of individual indicators of their quality. This allows you to more accurately take into account the real resource of fire resistance of structures.

 In the proposed technical solution, the error in determining the fire protection measure of the working reinforcement is reduced by evaluating its value depending on the depth and conditions of its heating in case of fire.

 The indicator of the heating condition of the working reinforcement is determined by the mathematical dependence, taking into account the number of directions of heat supply to it and the location of its rods with respect to the bisector of the angle of the heated section. This allows you to more accurately determine the heating conditions of the working reinforcement in the cross section of the structure with symmetric heating.

 For structures with a symmetric cross-section and symmetric heating in a fire, determining the depth of reinforcement from the heated surface by means of the geometric centers of the reinforcement rods identified for each reinforced zone of the cross section separated by the axis of symmetry allows a more accurate description of the heating conditions of the reinforcement and simplifies the design scheme sections of the structure.

 The features are simplified: a static scheme of the operation of structures, reinforcement of hazardous sections, diameter sizes of reinforcing bars, fabrication of structures and physical wear by the value of their fire resistance.

 The complex of single quality indicators of flexible reinforced concrete structures that affect their fire resistance, determined by non-destructive tests, has been clarified.

 The minimum number of non-destructive tests of a single indicator of the quality of the structure has been clarified.

 The accepted size of the sample from the total number of similar structures of the building provides reliability, reduces the time and complexity of testing.

 The analysis of the prior art by the applicant, including a search by patent and scientific and technical sources of information, and the identification of sources containing information about analogues of the claimed invention, allowed to establish that the applicant did not find a source characterized by signs that are identical to all the essential features of the claimed invention. The definition from the list of identified analogues of the prototype, as the closest in the totality of the features of the analogue, allowed us to establish a set of significant distinctive features in relation to the applicant’s perceived technical result in the claimed method set forth in the claims. Therefore, the claimed invention meets the condition of novelty.

 To verify the compliance of the claimed invention with the condition of the inventive step, the applicant conducted an additional search for known solutions in order to identify signs that match the distinctive features of the claimed method from the prototype. The search results showed that the claimed invention does not follow explicitly from the prior art for the specialist, since the influence of the transformations provided for by the essential features of the claimed invention is not revealed from the prior art determined by the applicant to achieve a technical result.

 The described invention is not based on a change in quantitative features, the presentation of such features in relationship or a change in its appearance. Therefore, the claimed invention meets the condition of inventive step.

 In FIG. 1 shows a polyparametric nomogram for assessing the fire resistance of flexible structures, heating from below. The given nomogram is mesh type. Inclined lines are drawn inside the grids. An arrow is drawn on each beam of lines or rays, which shows the direction of increase of the variable parameter. The name of the parameter in alphabetic terms and in units of measurement is placed on the arrow. To the left and right of the arrow on the rays are the values of the variable parameter. At the top left of the nomogram is the sampling order or “key”, on the right is the longitudinal section of the structure and its main parameters.

 In FIG. 2 to assess the depth of the working reinforcement and the conditions of its heating in case of fire, a section of the cross section of a continuous beam plate reinforced with nets or individual rods is shown. Here a is the depth of the reinforcement; d is the diameter of the rod; C is the thickness of the protective layer of concrete; t is the temperature of the fire medium; x, y - coordinate axes; 1 - working rods of reinforcement.

In FIG. 3 to assess the depth of the reinforcement and the conditions of its heating, the reinforcement zone is shown with one rod of the cross-sectional section of the beam structure, heated from both sides. Here a _x and a _y are the depth of reinforcement, c _x and c _y are the thickness of the concrete protective layer along the coordinate axes.

In FIG. 4 and 5, to determine the depth of the reinforcement and the conditions for its heating, an elongated cross-sectional area of the beam structure reinforced asymmetrically with two rods is shown, heating on both sides (Fig. 4) and on three sides (Fig. 5). Here 1 and 2 are rods with diameters d ₁ , d ₂ ; x _i and y _i ; - the depth of reinforcement along the coordinate axes.

In FIG. 6 and 7, to assess the depth of the reinforcement in the form of a group rod and the conditions for its heating, a section of the extended zone of the cross section of the beam structure is shown. The group rod is formed by the convergence of two rods 1 and 2 with diameters d ₁ and d ₂ vertically (Fig. 6) and horizontally (Fig. 7). The diameter of the conditional rod when d ₁ = d _{2 is} taken equal to d _c = 1,41 · d ₁ ; indicated by a dotted line. Heated conditional rod on both sides (Fig. 6) and on three sides (Fig. 7).

In FIG. Figure 8 shows the cross section of the ribbed slab of the building in the coordinate axes (abbreviated as r.p.), its main geometric dimensions, the location of the rods 1 and 2 of the working reinforcement in the longitudinal edges of the slab, and the direction of heat supply to the reinforcement in a fire. Here b is the width of the main rib along the bottom of the slab section; b ' _p is the same, at the top of the section, b' _f and h ' _f are the width and height of the section shelf in the compressed zone; h, h _o - height and working height of the section; a _xi and a _yi - depth of reinforcement along the coordinate axes; b ₁ - part of the width of the cross section along the bottom of the structure, measured from the heated face to the axis of the reinforcement; α - the angle of the heated face of the section from the coordinate axis, deg.

 In FIG. 9 is a cross-sectional view of a ribbed plate for evaluating its fire resistance. A section with a single reinforcement is represented with a symmetrical heating of the trapezoidal rib with the deviation of its faces from the vertical by an angle α, deg.

 In FIG. 10 and 11 to determine the depth of the working reinforcement and the conditions for its heating, zones of dense reinforcement of the longitudinal edges of the plate reinforced with rods (Fig. 10) and high-strength wire (Fig. 11) are shown. Here, T. A is the geometric center of the areas of all the rods of the corresponding zone of dense section reinforcement, divided by the axis of symmetry.

 In FIG. 12-15, to determine the depth of the working reinforcement and the conditions of its heating, the reinforcement of the cross section of the beam structures is shown: rectangular (Fig. 12 and 13), T-shaped (Fig. 14) and I-beams (Fig. 15), having an axis of symmetry (abbreviated - about .s.) and symmetric heating in a fire. The section is divided into two parts by the axis of symmetry.

 The depth of reinforcement in the form of their geometric centers (conditionally - T. A) is shown for each zone of dense section reinforcement, in FIG. 13 and 15 are dotted circles.

In FIG. 16 and 17 to assess the depth of the working reinforcement and the conditions of its heating, the reinforcement of the cross section of the beam structure with the rod (Fig. 16) and wire reinforcement (Fig. 17) is shown. Each section is divided into two zones by an axis of symmetry. Here T. A is the geometric center of the areas of all the rods of the zone of dense section reinforcement; b _f is the width of the shelf of the T-section in the stretched zone; e and e ₁ - the distance between the centers of the wire reinforcement; 1-8 - reinforcement rods.

 In FIG. 18 and 19 to identify the depth of the working reinforcement along the abscissa axis and the conditions of its heating, the reinforcement of the beam structures of the T-shaped (Fig. 18) and trapezoidal (Fig. 19) cross-sections is shown at a known angle of inclination of the side face - α, deg. The case of the location of the working reinforcement only along the symmetry axis of the cross section of the structure is shown in FIG. 18. A variant of the entry of the rods into both zones of dense reinforcement of a symmetrical section is shown in FIG. 19; 1, 2 and 3 - rods of working fittings.

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

 The sequence of steps of the method for determining the fire resistance of flexible concrete structures of buildings is as follows.

 First, a visual inspection of the building is carried out. Then determine the group of similar structures and their total number in it. The sample size of the structures of the same type is calculated. Assign a set of individual indicators of the quality of the structure, affecting fire resistance. Identify the conditions of support, fastening of the ends and dangerous sections of structures. The number of tests of a single indicator of the quality of a design is calculated depending on its statistical variability. Then, single structural quality indicators and their integral parameters are evaluated, and finally, the fire resistance of the tested structures is found from them.

 Visual inspection means checking the condition of structures, including identifying the conditions of support of individual structures, determining the type of concrete and the thickness of its protective layer, the presence of cracks and spalls, the disengagement of reinforcement with concrete, the presence of corrosion of reinforcing steel.

 In the process of inspection, groups of similar structures are determined.

 A group of structures in a building is understood to mean structures of the same type manufactured and built under similar technological conditions and under similar operating conditions.

 Schemes for heating cross sections of beam structures in fire conditions are determined depending on the actual location of the elements of the building parts, the installation of suspended ceilings, and the laying of adjacent structures that reduce the number of sides of the heating.

 The minimum integer number of structures in the sample according to the plan of normal or reduced tests is assigned from the conditions (14 or 15).

по сокращенному плану

При числе конструкций в группе H ≅ 5, их проверяют поштучно.Example 1. With the number of structures of the same type in the group H = 120 pcs, the number of test plates is taken at a rate

on a shortened plan

With the number of constructions in the group H ≅ 5, they are checked individually.

 The number and location of sites in which quality indicators of structures are determined are determined as follows. In bendable structures having one dangerous section, sections are placed only in this section. In structures having several dangerous sections, the sections are evenly distributed over the surface with the obligatory arrangement of part of the sections in dangerous sections.

 The main single quality indicators of flexible reinforced concrete structures providing fire resistance include: geometric dimensions of the structure and the height of the dangerous section; occurrence depth, class, diameter, degree of stress and yield strength of reinforcement; concrete compressive strength, moisture and its density in natural conditions; the thickness of the protective layer and the coefficient of thermal diffusivity of concrete.

средняя ошибка

здесь m_i - результат i-го испытания; Σ(x_i)² - сумма квадратов всех отклонений от среднего.The number of tests of a single indicator of the quality of structures, with a probability of a result equal to 0.5, an accuracy rate of 5%, is determined by the formula (13); the coefficient of variation of the sample is ν = ± 100 · σ / M, arithmetic mean M = (1 / n) · Σm _i , standard deviation from mean

average error

here m _i is the result of the i-th test; Σ (x _i ) ² is the sum of the squares of all deviations from the mean.

 Checked geometric dimensions are: for constructions of slabs of continuous section - its height; for multi-hollow slabs - section height, dimensions and position of voids, shelf thickness; for ribbed plates, the total height of the cross section of the ribs and the thickness of the shelf; for beams - the width and height of the section.

 Dangerous sections for beam structures are assigned in places of greatest moments from the action of the normative load or at points of maximum convergence of the envelope of the moment diagram and the diagram of structural materials. So, in gable beams, the most dangerous section is assigned not in the middle of the span, but at a distance from the support equal to 0.4 spans.

 For statically indefinable structures, dangerous sections are assigned in spans and on supports. In this case, dangerous sections are found by the largest ordinates of the envelope of the moment diagram.

 The dimensions of the structure are checked with an accuracy of ± 1 mm; crack width - with an accuracy of 0.05 mm.

 Strength testing of concrete structures included in the sample or checked individually, is carried out by non-destructive tests using mechanical and ultrasonic devices [1, p. 31-38].

 The minimum depth of the rod of the working reinforcement is taken along one of the coordinate axes of the cross section of the structures.

 By the depth of reinforcement is understood the distance according to the norms between the surface of the concrete structure and the longitudinal axis of the reinforcement.

For a solid slab reinforced with nets or finished with rods, with their unilateral heating (see Fig. 2), the depth of reinforcement a, cm, in the cross section is determined by the formula (19):
a = c + (d / 2), (19)
where c is the thickness of the protective layer of concrete, cm;
d is the nominal diameter of the rod, cm;
indicator of the heating condition of the reinforcement m ₁ = 1.

где x_i и y_i - осевые расстояния для 1 и 2 стержня, см;
при асимметричном расположении арматурного стержня в сечении, но симметричном двухстороннем обогреве его, условия нагрева арматуры, при x₁ ≅ x₂, определяют по формуле (23) показателем
m₂ = 1/[1 + (x₁/x₂)²] ≅1, (23)
для балочной конструкции, армированной стержнями, распложенными раздельно по высоте, при трехстороннем обогреве (см. фиг. 5), глубину залегания арматуры по осям координат поперечного сечения a_x и a_y, см, определяют по формулам (24 и 25):
a_x = (x₁ + x₂)/2, (24)
a_y = (A₁ · y₁ + A₂ · y₂)/(A₁ + A₂), (25)
условия нагрева арматуры, при a_x ≅ a_y и x₁ ≅ x₂, оценивают по формуле (26) показателем

где A₁ и A₂ - площади сечения, см², стержней арматуры диаметром d₁, d₂;
x_i и y_i - осевые расстояния для 1 и 2 стержня, см;
при a_y < a_x1 - под корнем в формуле (26) принимают обратное соотношение, то есть: a_x1/a_y.For a beam structure with working reinforcement from two asymmetrically located rods horizontally in cross section and bilateral heating of each rod (see Fig. 4), the depth of the reinforcement a _x and a _y , cm, is determined by the formulas (20 and 21):
a _x = (x ₁ + x ₂ ) / 2, (20)
a _y = (y ₁ + y ₂ ) / 2; (21)
the heating conditions of the reinforcement, at a _x ≅ a _y , are determined by formula (22) by the indicator

where x _i and y _i are the axial distances for 1 and 2 rods, cm;
with an asymmetric arrangement of the reinforcing bar in the cross section, but symmetric two-sided heating of it, the conditions for heating the reinforcement, at x ₁ ≅ x ₂ , are determined by formula (23) by the indicator
m ₂ = 1 / [1 + (x ₁ / x ₂ ) ² ] ≅1, (23)
for a beam structure reinforced with rods arranged separately in height, with three-sided heating (see Fig. 5), the depth of reinforcement along the coordinate axes of the cross section a _x and a _y , cm, is determined by the formulas (24 and 25):
a _x = (x ₁ + x ₂ ) / 2, (24)
a _y = (A ₁ · y ₁ + A ₂ · y ₂ ) / (A ₁ + A ₂ ), (25)
the heating conditions of the reinforcement, at a _x ≅ a _y and x ₁ ≅ x ₂ , are estimated by formula (26) by the indicator

where A ₁ and A ₂ - cross-sectional area, cm ² , reinforcing bars with a diameter of d ₁ , d ₂ ;
x _i and y _i - axial distances for 1 and 2 rods, cm;
when a _y <a _x1 - under the root in the formula (26) take the inverse relationship, that is: a _x1 / a _y .

 For beam structures with a symmetric cross-section and symmetric heating under thermal conditions, the depth of the group of rods of the working reinforcement from the heated surface is determined averaged in the form of the geometric center of the areas of all the rods for each reinforced section area divided by the axis of symmetry (see Fig. 12-13, 15-17 and 19).

 If the group of rods of the working reinforcement is located only along the axis of symmetry of the section of the structure, its depth is determined averaged in the form of the geometric center of the areas of all the rods of the reinforced section zone (see Figs. 14 and 18).

The depths of the longitudinal working reinforcement a _x and a _y , cm, from the face of the structure heated under the influence of heat exposure are determined by the formulas (6 and 7).

 The depth of the reinforcement of the test structure normal to the heated face located at an angle to the ordinate axis, for example, in the case of a trapezoidal cross section, is determined by the formula (8).

According to the measurement results, the minimum depth of the working reinforcement is determined along one of the coordinate axes of the structural cross section - a _min , cm, and the value of the heating condition index m of the working reinforcement during thermal exposure. Then, using the values of m and a _min , the integral parameter is established - the measure of fire protection of the working reinforcement according to the formula (3).

следовательно мера огнезащиты арматуры бетоном C_a = m₃ · a_min = 0,52 · 3,5 = 1,82 см.Example 2. Indicators of checking the condition of reinforced concrete ribbed slabs are given: reinforcement on a slab 2⌀20 A _t- IV; d _c = 2 cm; c = 2.5 cm;
a _y = 2.5 + 2/2 = 3.5 cm = a _min ; b ₁ = 3.75 cm; α = 8.5 degrees;
a _x1 = b ₁ · cos α + a _y · sin α = 3.75 · 0.989 + 3.5 · 0.148 = 4.23 cm;
e = 2 · (bb ₁ ) + 1.5 = 2 · (7.5-3.75) + 1.5 = 8 cm;
e = 8 <2 · a _x1 = 2 · 4.23 = 8.46 cm, therefore, the heat supply to the rod from the third side of the cross section is taken into account; a _x2 = e + a _x1 = 8 + 4.23 = 12.23 cm, for a _y <a _x1 , the indicator of the heating conditions of the rods is determined by the formula (26):

therefore, the measure of fire protection of reinforcement with concrete C _a = m ₃ · a _min = 0.52 · 3.5 = 1.82 cm.

 The integral parameter of the degree of tension of the longitudinal working reinforcement of a flexible concrete structure is determined from condition (2).

Для прямоугольного сечения конструкций с одиночной арматурой, в зависимости от высоты сжатой зоны
x = (A_s · R_sn)/(b · R_bn); (29)
степень напряжения арматуры определяют по формуле (30):
J_σc= M_н/[A_s·R_sn·(h₀-0,5·x)] ≅ 1; (30)
где M_н - изгибающий момент от действия нормативных нагрузок, кН · м;
A_s и A_s1 - площади сечения арматуры в растянутой и сжатой зоне сечения, см²;
R_sn и R_scn - нормативные сопротивления арматуры на растяжение и на сжатие, МПа;
h₀ и b - полезная высота и ширина сечения элемента, см;
a' - глубина заложения сжатой арматуры, см;
R_sn - нормативные сопротивления бетона сжатию, МПа.For a rectangular section with reinforcement concentrated near the stretched and compressed faces of the structure, depending on the height of the compressed zone
x = (A _s R _sn - A _s - R _scn ) / (b R _bn ),
the degree of tension of the reinforcement is determined by the formula (28):

For a rectangular section of structures with single reinforcement, depending on the height of the compressed zone
x = (A _s · R _sn ) / (b · R _bn ); (29)
the degree of tension of the reinforcement is determined by the formula (30):
J _σc = M _n / [A _s · R _sn · (h ₀ -0.5 · x)] ≅ 1; (thirty)
where M _n - bending moment from the action of regulatory loads, kN · m;
A _s and A _s1 are the cross-sectional areas of the reinforcement in the stretched and compressed section zone, cm ² ;
R _sn and R _scn are the standard tensile and compression resistance of the reinforcement, MPa;
h ₀ and b - the useful height and width of the section of the element, cm;
a 'is the depth of the compressed reinforcement, cm;
R _sn - standard concrete compressive strength, MPa.

The coefficient of thermal diffusivity of concrete of a fire-retardant layer under thermal exposure is determined averaged at 450 ^o C. To calculate its integral parameter by formula (5), determine the density of concrete in its natural state, its moisture content, as well as the thermal conductivity and heat capacity of concrete at 450 ^o C.

Using the obtained integral parameters C _a , cm; Jσ _c ; L _br , cm ² / h, the above nomogram (see Fig. 1) find the fire resistance of the beam structures of the building.

 With the same values of the integral parameters, the fire resistance of structures with different static working patterns, physical wear, cross-sectional shapes and features of its reinforcement is calculated by formula (I).

The guaranteed fire resistance of flexible concrete structures, F _(R) , min, is calculated by the nomogram (see Fig. 1) with a corresponding change in design parameters: fire protection measures of working reinforcement C _a , cm, averaged coefficient of thermal diffusivity of concrete L _br , cm ² / h, and the degree of reinforcement stress Jσ _c .
Example 3. Prefabricated flat plate made of heavy concrete L _br = 13 cm ² / h, working reinforcement class A-III, the depth of the reinforcement - the measure of fire protection of the working reinforcement - C _a = 2.5 cm, the degree of tension of the reinforcement - Jσ _c = 0.66.

Answer: fire resistance limit of reinforced concrete slab F _(R) = 115 min (see Fig. 1 - continuous working line of the nomogram).

Example 4. Inverse problem: the guaranteed fire resistance limit F _(R) = 60 min for a flat reinforced concrete slab of Example 1 is provided by the depth of working reinforcement _Ca = 2 cm (see Fig. 1 - dashed line of the nomogram).

Thus, the above information indicates that when using the claimed method the following set of conditions:
a) a tool that embodies the claimed method in its implementation, is intended for use in the construction industry, namely in the classification of reinforced concrete structures of buildings according to their resistance to fire;
b) for the claimed method in the form described in the independent clause of the claims, the possibility of its implementation using the means and methods described in the application is confirmed;
c) the proposed method was applied during field inspection of reinforced concrete structures of the coating of the warehouse block with an area of 2160 m ^{2 of an} industrial building in Samara. The results of non-destructive testing of ribbed slabs measuring 6х3х0.3 m, heavy concrete, class B 35, reinforcement 2⌀16 AV, showed a fire resistance limit of 90 min (1.5 h); for gable grating beams with a span of 18 m, At-V class reinforcement, fire resistance limit - 70 min (1.15 h).

 Therefore, the claimed invention meets the condition of "industrial applicability".

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

 2. Buildings and fragments of buildings. The method of full-scale fire tests. General requirements. Fire safety standards. NPB 233-97. - M., VNIIPO, 1997-14 p.

 3. Fire resistance of buildings // V.P. Bushev, V.A. Pchelintsev, V.S. Fedorenko, A.I. Yakovlev. - M., Stroyizdat, 1970, - 261 p. (see p. 252-256).

 4. GOST 30247.1-94. Building constructions. Test methods for fire resistance. Bearing and enclosing structures. - M., IPK Publishing House of Standards, 1995. - 7 p.

Claims (22)

 1. A method for determining the fire resistance of bent reinforced concrete structures of a building by testing, including technical inspection, establishing the type of concrete and reinforcement of structures, identifying the conditions for their support and fastening, determining the time of the onset of the ultimate state based on the loss of the bearing capacity of the structure under standard load under standard thermal exposure , characterized in that the test of structures is carried out without destruction according to a set of individual quality indicators, technical This inspection is supplemented by instrumental measurements of the geometric dimensions of structures and their dangerous sections, the number and diameter of working reinforcement rods, their relative position and thickness of the concrete protective layer are determined, the shape of structures, their heating schemes for dangerous sections during a fire and the heating conditions of working reinforcement are determined, and the depth is established rods of working reinforcement and the measure of its fire protection, experimentally determine the density of concrete and its moisture content in a natural state and / or the value averaged the thermal coefficient of concrete, concrete estimate characteristics of resistance and compression of the working armature resistance tensile set value applied regulatory burden on the design and on it are working degree armature voltage, and using the obtained integral design parameters calculated at the present nomogram actual limit its fire resistance. 2. The method according to p. 1, characterized in that the duration of resistance, F _(R) , min of bending reinforced concrete structures from the beginning of the standard heat exposure to the loss of bearing capacity is determined by the formula

where J _σc is the degree of tension of the longitudinal working reinforcement in a dangerous section of structures;
n and D _with - parameters of the type and class of reinforcement;
With _a - a measure of fire protection of reinforcement, cm;
L _br - the average coefficient of thermal diffusivity of concrete protective layer, cm ² / h;
K _i - correction factors that take into account the static scheme of the structures, the features of reinforcing their dangerous sections, the diameters of the rods of the working reinforcement, the physical wear of structures, for example, for a continuous plate K ₁ = 1, for a multi-hollow panel K ₁ = 0.8. 3. The method according to any one of claims 1 and 2, characterized in that the degree of stress - J _{σc of the} longitudinal working reinforcement of the structure from the current standard load is determined from the condition
J _σc = σ _n / R _sn ≅ 1,
where σ _n - stresses in the working reinforcement from the bending moment, MPa;
R _sn - standard tensile strength of the reinforcement, MPa. 4. The method according to claim 2, characterized in that the values of the fire resistance parameters n and D _{c of} flexible structures reinforced with various types of steel are adopted, depending on the class of reinforcement, as follows: (see graphic part). 5. The method according to claim 2, characterized in that the fire protection measure of the longitudinal working reinforcement - Ca, cm, is determined by the dependence
C _a = m · a _min,
where m is an indicator of the heating conditions of the working reinforcement in the cross section of the structure during thermal exposure;
a _min - the minimum depth of the working reinforcement along one of the coordinate axes of the cross section, see 6. The method according to any one of claims 2 and 5, characterized in that the value of the heating condition indicator is m of longitudinal working reinforcement with 2- or 3-sided heating of it, with a _x1 ≅ a _x2 and a _y ≥ a _x1 , determined by the formula

where a _x1 , a _x2 and a _y are, respectively, the depth of the reinforcement from the heated face of the structure along the coordinate axes of the cross section, cm, for a _y <a _x1 - the inverse ratio is taken under the root, that is, a _x1 / a _y . 7. The method according to claim 2, characterized in that the value of the averaged coefficient of thermal diffusivity of concrete L _br , cm ² / h, is determined experimentally or found from the expression
L _br = 36 · 10 ³ · λ _br · (1 + 0.01 · β) / (c _br + 0.05 · β) · ρ,
where λ _br and C _br - respectively, thermal conductivity, W / (m · ^o C), and specific heat of concrete, kJ / (kg · ^o C), at a temperature of 450 ^o C;
β and ρ are the density of concrete in its natural state, kg / m ³ , and its moisture content,% by weight.  8. The method according to claim 1, characterized in that non-destructive tests are carried out for a group of structures of the same type, the differences between the strength of concrete and the fluidity of the reinforcement which are caused mainly by a random factor.  9. The method according to claim 1, characterized in that the heating patterns of the cross-sections of the test beam structures under fire conditions are determined depending on the actual location of the parts of the building.  10. The method according to claim 1, characterized in that for beam structures with a symmetrical cross-section and with symmetrical heating under heat exposure, the depth of the group of rods of the working reinforcement from the heated surface along the normal to it is determined averaged by finding the geometric center of the areas of the corresponding rods for each reinforced section zone, divided by an axis of symmetry.  11. The method according to any one of claims 1 and 10, characterized in that if the group of rods of the working reinforcement is located only along the symmetry axis of the structural cross section, its depth is determined averaged by finding the geometric center of the areas of all the rods of the reinforced section zone. 12. The method according to any one of claims 1, 10 and 11, characterized in that the depths of the working reinforcement a _x and a _y , cm, from the faces of the test structure heated under the influence of heat exposure are determined by the formulas
a _x = ΣA _i · x _i / ΣA _i ; a _y = ΣA _i · y _i / ΣA _i ,
where x ₁ and y _i - axial distance of the i-th rod along the coordinate axes, cm;
n and A _i - the number of rebar rods and the area of the i-th rod, cm ² . 13. The method according to any one of claims 1 and 12, characterized in that the depth of the reinforcement - a _x , cm, of the test structure normal to the heated face located at an angle α, deg, to the ordinate axis, is determined by the formula
a _x = b ₁ cos α + a _y sin sin,
where b ₁ - part of the width of the cross section at the bottom of the structure measured from the heated face to the axis of the reinforcement, cm;
and _y is the depth of reinforcement along the ordinate axis, see 14. The method according to claim 2, characterized in that the correction to the fire resistance limit of structures having working reinforcement of various diameters is taken into account by the coefficient K _⌀ = d _c ^0.05 , where d _c is the nominal diameter of the working reinforcement, see 15. The method according to any one of paragraphs. 2 and 14, characterized in that in the group arrangement of the rods of the working reinforcement in the cross section of the structure, the paired rods are replaced by one conditional rod with a reduced diameter

where d ₁ and d ₂ - the nominal diameter of the first and second paired rods, see 16. The method according to claim 2, characterized in that the amendment to the value of the fire resistance limit of the floor structures having a certain physical wear, take into account the coefficient K _f = 0.1 (100 - F _and ), where F _and - the amount of physical wear of structures,% . 17. The method according to claim 2, characterized in that the increase in the fire resistance of statically indefinable bending structures is taken into account by the coefficient
K _m = 0.9 · (1 + A _on / A);
where A _on and A are, respectively, the cross-sectional area of the longitudinal working reinforcement above the support and in the span of the structure, cm ² .  18. The method according to claim 1, characterized in that the unit dimensions of the quality of structures that affect the fire resistance, take the geometric dimensions of the structures and the height of the dangerous section, the depth, class, diameter, stress and yield strength of the reinforcement, concrete compressive strength , humidity and density, the thickness of the protective layer and the coefficient of thermal diffusivity of concrete. 19. The method according to any one of claims 1 and 18, characterized in that the number of tests n _{and a} single indicator of the quality of the structures, with a probability of a result of 0.95 and an accuracy of 5%, take n _and = 0.15ν ² ≥ 6. 20. The method according to claim 1, characterized in that in the case when all the individual indicators of the quality of the structures, when H is more than 9, are within the control limits, the minimum integer number of structures in the sample according to the plan for shortened tests N _s , pcs, is assigned from the condition

where H is the number of structures of the same type in the building, pcs. 21. The method according to each of claims 1 and 20, characterized in that in the case when at least one of the individual indicators of the quality of the structures goes beyond the control limits, the minimum number of structures in the sample is normal

22. The method according to each of paragraphs 1, 20 and 21, characterized in that in the case when at least one of the individual indicators of the quality of the structures goes beyond the permissible limits or H ≅ 5 pieces, all the same building structures are tested individually.  23. The method according to claim 1, characterized in that the guaranteed limit of fire resistance of structures is calculated according to the nomogram by solving the inverse problem of fire resistance.

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