RU2320982C1 - Method to determine steel building beam fire resistance - Google Patents

Abstract

FIELD: fire-fighting, particularly building structure fire safety.

SUBSTANCE: method involves performing nondestructive building beam testing to determine simple quality indexes by determination of geometrical steel beam dimensions and beam securing conditions; measuring normative working load applied to beam and intensity of normative stresses in weak metal beam sections; determining heat diffusion indexes of fire-protective material of the beam. Fire-protective beam resistance to heat applied from ordinary fire is given as function, which takes into consideration of normative force stresses intensity in beam cross-section caused by working normative load application and heat diffusion indexes of fire-protective material. Fire-resistance rating of fire-protected steel beams is determined from nomograph.

EFFECT: increased test reliability and decreased test time.

16 cl, 3 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 metal structures of buildings according to their resistance to fire. This makes it possible to justify the use of beams with an actual fire resistance limit in buildings of various classes of constructive and functional fire hazard.

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

A known method for determining the fire resistance of steel beams 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 reference samples of structural steel, determining the time of the onset of the limit state by the loss of the bearing capacity of the structure, that is, collapse under the conditions of external load and the thermal effect of a natural fire. / Ilyin N.A. Technical expertise of buildings damaged by fire. - M .: Stroyizdat, 1983. - S.90-91 / [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 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 steel beams according to the results of full-scale fire tests of a building fragment in which the structures are inspected, the steel grade is determined, the static load on the structure is assigned 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 / airbag limit value 233-97. Buildings and fragments of buildings. The method of full-scale fire tests. General requirements. - M .: VNIIPO, 1997. - S.6-12 / [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 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 causes of the destruction of the steel beams of the fragment may not be established in a consequence of the diversity of simultaneously acting fire factors. The ultimate fire resistance of steel beams may not be reached due to earlier destruction of the coating elements or fragment walls / Fire resistance of buildings. / V.P. Bushev, V.A. Pchelintsev, B.C. Fedorenko, A.I. Yakovlev. - M.: Stroyizdat, 1970. - S.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 steel fire-protected beams of a building by testing, including conducting a technical inspection, establishing the grade of steel and the type of material, identifying the conditions for supporting and attaching the ends of the beams, determining the time of the onset of the ultimate state according to loss of the bearing capacity of the structure under standard load under standard thermal exposure / GOST 30247.1-94. Building constructions. Fire test methods. Bearing and enclosing structures. - M .: Publishing house of standards, 1995. - 7 p. / [4] - adopted as a prototype.

The reasons that impede the achievement of the technical result indicated below when using the known method adopted as a prototype include the fact that in the known method the tests are carried out on a design sample that is subjected to constant and continuous loads in their calculated values with a safety factor equal to one, then There are 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, inefficient, unsafe, have little technological capabilities for testing various sizes and variously loaded structures, and do not provide the necessary information about the influence of individual design quality indicators on its fire resistance. Determining the fire resistance of steel fire-protected beams by a single quality indicator, for example, by the thickness of the fire-retardant coating, 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 steel beams, transportation to the installation site of heating furnaces and the creation of a standard thermal regime in them. By the small number of test samples (2-3 pcs.) It is impossible to judge the actual state of the building beams. The results of the fire test are sporadic and do not take into account the diversity in fixing the ends of the beams, their actual dimensions, the actual scheme of heating the dangerous section of the test structure in a fire.

The invention consists in the following. The task 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 metal structures in a fire; in determining the actual fire resistance limits of steel fireproof beams 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 of 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; reduction in 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; the use of nomograms 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 the application of the method of selection of 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 thickness of the fire retardant coating and the heating conditions of the steel beam in a fire; 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 steel beam according to its design 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 steel fire-protected beams of a building by testing, including technical inspection, instrumental measurement of the geometric characteristics of the beams when bending in their dangerous sections; identification of the conditions of support and fastening of beams, heating schemes of their cross sections; establishing the type of fire-retardant material and the steel grade of the beams, the characteristics of the metal resistance to bending and tension; determination of the magnitude of the working normative load on the beams, schemes for its application, the intensity of power stresses in the metal in dangerous sections of the beams; determining the time of the onset of the limiting state on the basis of loss of the bearing capacity of the beams under the working normative load under standard heat exposure, the peculiarity is that the fire resistance of steel fireproof beams of a building is determined without fire exposure by non-destructive testing methods using a set of unit quality indicators of steel structures, determine the number and location of sites in which single quality indicators are determined, while visual inspection is supplemented by the definition of a group of steel beams of the same type, instrumental determination of the geometric dimensions of the thickness of the fire retardant coating and the bearing rods of steel beams, experimentally determine the density of fire protection materials, their moisture in the natural state and / or the coefficient of thermal diffusion of the fire protection material, take geometric beam characteristics, critical temperature of steel heating in a fire; degree of stress and yield strength of metal; calculate the integral design parameters D _ar , δ ₀ , m ₀ , J _σs , τ _us ;

where D _ar is the coefficient of thermal diffusion of the fire protection material, mm ² / min;

δ ₀ - the thickness of the fire-retardant coating of steel beams, mm;

m ₀ - an indicator of the heating conditions of the bearing rod of steel beams with a fire retardant coating in case of fire;

J _σs is the intensity of normal power stresses in the cross section of steel beams from the working standard load (0≤J _σs ≤1);

τ _us - the duration of resistance to thermal effects of the bearing rod of steel beams without taking into account the fire retardant coating, min;

and using the received parameters flame retardancy steel beams nomogram calculated actual fire limit F _{u (R),} min.

The next feature of the proposed method is that the intensity of normal power stresses in the cross section of the supporting rod of steel fire-protected beams from the working standard load, operating in the conditions of standard fire tests, is calculated by the formula (1):

where M _ρ is the bending moment in a dangerous section from the working standard load acting on the beams under test conditions, kN · m;

W _n - moment of resistance of the cross section relative to the horizontal axis of the beam, cm ³ ;

R _yn is the standard tensile, compression and bending resistance to yield strength of steel according to TU or GOST, MPa; (see Appendix 1 SNiP II-23-81 *. "Steel structures" - M., 1990, - p.63-66 [5]).

A further feature of the proposed method is that for various values of the limiting temperature of heating the steel of the supporting rod of metal beams t _u , ° С, different from the critical temperature of steel t _cr , ° С, the actual intensity of normal power stresses in the cross section of steel beams is calculated by formula (2):

where J _σs is the intensity of normal power stresses in the cross section of steel fireproof beams (0 ÷ 1);

t _u is the limiting temperature of steel heating (° C) at which variously loaded steel fireproof beams lose their bearing capacity;

t _cr is the critical temperature of steel heating (° C) of metal beams (at a standard intensity of power stresses J _n = 0.625 for construction mild steel of grade St.3 t _cr = 510 ° C);

n - empirical indicators depending on the grades of steel (n = 2.8 for construction steel grade St.3).

The next feature of the proposed method is that at t _u = t _cr ± 50 ° C, the intensity of normal power stresses in the cross section of steel beams is calculated by the formula (3)

where J _n = 0.625 is the normative intensity of power stresses in the cross section of steel beams under conditions of fire tests;

t _cr and t _u - respectively, the critical and ultimate temperature of steel heating, ° C.

The next feature of the proposed method is that the indicator of the heating conditions of the bearing rod of steel beams with a structural fire retardant coating under the standard fire test (at δ _x ≤α _y1 ) is calculated by the formula (4)

where δ _x is the thickness of the fire-retardant coating of the shelf (wall) of the bearing rod of steel beams along the x axis, mm;

α _y1 , α _y2 - the ordinates of the calculated point of the section, directionally moved along the y axis, mm

Under the condition α _x > α _y , in the formula (4) these factors are interchanged;

when changing the coordinate axes, the width of the section b, mm, of the bearing rod of metal columns is replaced by a height h, mm, here b is the width of the section of the bearing rod of beams, mm.

The next feature of the proposed method is that the ordinates of the calculated points of the cross section M, directionally displaced along the y axis, determining the location of the average temperature along the width of the shelf or the wall height of the unevenly heated supporting beam of the steel beam in a fire, are calculated by formulas (5a) and (5 B):

where α _y1 and α _y2 are the ordinates of the design points of the unevenly heated cross section of steel beams, mm;

δ _y is the thickness of the fire-retardant coating of the shelf or wall of the supporting rod of the steel beam along the y axis, mm;

b and H - respectively, the width of the cross section of the supporting rod and the height of the steel fireproof beam, mm

The next feature of the proposed method is that the coefficient of thermal diffusion of the material of the fire-retardant coating of steel beams, D _ar , mm ² / min, is determined experimentally or calculated at an averaged temperature t _m = 450 ° C according to the formula (6)

where λ ₀ and b are empirical numbers for calculating the coefficient of thermal conductivity of the fire protection material;

t _m is the average heating temperature (450 ° C) of the fire protection material;

C ₀ and d are empirical numbers for calculating the specific heat of the fire protection material;

w is the moisture content of the fire protection material mass,%;

ρ _{with the} density of the dry material of fire protection, kg / m ³ ;

k _p = 0.75 at ρ _with more than 1000 kg / m ³ ; k _p = 1 at ρ _s less than 1000 kg / m ³ .

The averaged values of the heat diffusion coefficients for fire protection materials at their natural moisture content (1 cm ² / h = 1.67 mm ² / min) are given in the table.

The next feature of the proposed method is that the duration of resistance to thermal effects of the bearing rod of steel beams without taking into account the fire retardant coating τ _us , min, is determined by the formula (7):

where J _σs is the intensity of normal stresses in the cross section of the bearing rod of steel beams from the action of the working standard load in case of fire (0≤J _σs ≤1);

T _sr - reduced thickness of the metal of the supporting rod of steel beams, cm, calculated by the formula (8)

where A _s is the metal area of the cross section of the supporting rod, cm ² ;

P ₀ - heating perimeter of the cross section of steel beams, see

Table Thermophysical characteristics of fire-resistant materials of steel beams Flame retardant material Density ρ _s , kg / m ³ Humidity ω,% The parameters of thermal conductivity, W / m · ° C, and heat capacity of the material, J / kg · ° C Heat diffusion coefficient D _ar , mm ² / min λ ₀ b C ₀ d 1. Asbestos cement slabs 1800 9 0.31 0.08 0.84 0.63 9.0 2. Asbestos perlite cement slabs 960 6 0,087 0.23 0.84 0.63 6.0 3. Plaster concrete slabs 1300 twenty 0.4 0.08 0.84 0.6 12.0 4. Gypsum plaster 900 twenty 0.2 0.35 one 0.6 9.0 5. Plasterboard 1000 - - - - - 12.0 6. Gypsum boards reinforced with wood shavings 700 - - - - - 18.0 7. Gypsum slag plates 1400 - - - - - 18.0 8. Gypsum boards 800 - - - - - 19.0 9. Expanded clay 750 6 0.18 0.08 0.32 0.48 12.0 10. Expanded clay 950 6 0.23 0.13 0.85 0.58 13.0 11. Expanded clay 1400 6 - - - - 14.0 12. Perlite concrete 1250 6 0.4 0.05 0.85 0.38 19.0 13. Perlite cement-based plaster 500 8 17.0 14. Cemento - lime-sand plaster 1700 - - - - - 20,0 15. Heavy concrete on crushed stone 2250 3 1.15 -0.55 0.71 0.84 24.0 16. Heavy concrete with granite aggregate 2400 3 1,2 -0.3 0.72 0.2 28.0 17. Fireproof coating OFP-PM 250 16.5 0,046 0.35 0.92 0.63 24.2 18. Cement-sand plaster 1930 2 - - - - 24.0 19. Sand concrete 2000 3 1.16 0.62 0.75 0.77 35.6 20. Glass wool mats one hundred 5 - - - - 80.0

A feature of the proposed method is that for the individual quality indicators of steel beams that affect their fire resistance limit, they take: geometric characteristics when bending the cross section, conditions for fixing the ends of the beams, the length of the perimeter of the heating of the cross section, the standard resistance of steel to bending, compression and tension by yield strength, the size of the working standard load and the scheme of its application; the magnitude of the bending moment and the transverse force, the intensity of the normative power stresses in the metal in dangerous sections of steel beams, the critical temperature of heating of building steel under conditions of fire exposure, the moisture and density of the flame retardant material in its natural state, the thickness of the flame retardant coating, the thermal diffusion of the flame retardant material.

A feature of the proposed method is that non-destructive tests are carried out for a group of the same type of steel beams, the differences between the geometric dimensions of the cross sections and the fluidity of steel which are caused mainly by random factors.

The next feature of the proposed method is that the number of tests n _{uc of a} single quality indicator of steel fireproof beams, with a probability of a result of 0.95 and an accuracy of 5%, is taken according to the formula (9)

where υ is the sample coefficient of variation of the test results,%.

A feature of the proposed method lies in the fact that the heating scheme of the cross sections in the fire conditions of the tested steel beams is determined depending on the actual location of the building parts.

A feature of the proposed method is that in the case when all individual quality indicators of steel beams (with M more than 9 pcs.) Are within the control limits, the minimum integer number of metal structures in the sample according to the plan of shortened tests M _min , pcs., Is prescribed from condition (10)

where M is the number of the same type of steel beams in the building, pcs.

In the case when at least one of the single quality indicators of steel beams goes beyond the control limits, the minimum number of structures in the sample is determined by the norm according to the formula (11)

In the case when at least one of the individual quality indicators of steel beams goes beyond the limits of permissible limits or M≤5 pcs, all the same type building structures are subjected to a non-destructive test piece by piece.

A feature of the proposed method is that it additionally calculates the guaranteed fire resistance limit of steel fire-protected beams according to the nomogram by solving the inverse fire resistance problem.

A feature of the proposed method for determining the fire resistance of steel beams of a building is that non-destructive tests are carried out for a group of the same type of beams, the differences between the thickness of the fire-retardant coating and the fluidity of steel which are caused mainly by random factors.

Schemes for heating sections of test steel fire-protected beams under fire conditions are determined depending on the actual location of the building parts.

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

The elimination of fire tests of steel beams of an existing building and their replacement with non-destructive tests reduces the complexity of determining their fire resistance, expands the technological capabilities of detecting the actual fire resistance of variously loaded steel beams 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 results with standard tests of similar steel beams and maintenance translational fitness inspected the building without disturbing the bearing capacity of its beams in the testing process. Therefore, the conditions for testing steel beams for fire resistance are greatly simplified.

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

The use of a mathematical description of the process of resistance of steel fireproof beams to standard thermal testing and the use of the constructed parametric nomograms increase the accuracy and expressiveness of the assessment of their fire resistance limits.

The use of integral structural parameters, such as: an indicator of the heating conditions of the supporting rod of steel fire-protected beams in case of fire, the intensity of steel stress and the coefficient of thermal diffusion of fire protection materials, simplifies the mathematical description of the process of resistance of loaded steel fire-protected beams to thermal exposure.

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

The proposed technical solution provides for testing not one but a group of the same steel beams. This allows you to 5-10 times increase the number of tested steel beams 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 fire retardant coating, 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 steel fireproof beams is provided for not by one indicator, but by a set of individual indicators of their quality. This allows you to more accurately take into account the real fire resistance of steel beams.

The complex of individual quality indicators of steel beams that affect their fire resistance, determined by non-destructive tests, has been clarified. The minimum number of non-destructive tests of a single quality indicator of steel fire-protected beams has been clarified.

The accepted sample size from the total number of the same steel beams of the building provides reliability, reduces the time and complexity of testing.

Figure 1 shows the nomogram for assessing the fire resistance of steel fireproof beams, heating from 4 sides. 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.

The procedure for the production of samples or "key":

Figure 2 shows a diagram of a steel beam with a fire-retardant coating, four-sided sectional heating, the thermal regime of a standard fire; the cross-sectional dimensions of the beam B × H = 150 × 250 mm;

1 - bearing rod - steel tee No. 20, b × h × d = 10 × 2 × 0.52 cm; reduced metal thickness T _sr = 0.34 cm; sectional area A _s = 26.8 cm ² ; steel grade St.3, temperature limit t _u = 505 ° С; critical temperature t _cr = 510 ° C;

2 - filling the internal cavity - heavy concrete on limestone crushed stone, density 2200 kg / m ³ ; (k ₀ = 1.33);

3 - plaster - composition 1: 1: 0.5 (cement: lime: sand), humidity 1.3%; thickness 25 mm; coefficient of thermal diffusion D _ar = 20.0 mm ² / min.

Figure 3 shows the calculated cross-sectional diagram of a steel fire-protected beam with a three-way supply of heat to the control points M directed along the axis of the obscissus:

1 - the supporting rod of the steel beam - I-beam No. 20;

2 - filling with heavy concrete;

3 - cement-lime-sand plaster, δ _x = δ _y = 25 mm;

4 - points M, determining the location of the average temperature of the unevenly heated shelves of the I-beam.

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 steel fire-protected beams of a building is as follows.

First, a visual inspection of the building is carried out. Then determine the group of the same type of steel beams and their total number in it. The sample size of the same steel beams is calculated. Assign a set of individual indicators of the quality of the structure, affecting fire resistance. The conditions for fixing the ends and dangerous sections of steel beams are revealed. The number of tests of a single quality indicator of steel fire-protected beams is calculated depending on its statistical variability. Then, single quality indicators of steel fire-protected beams 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 steel fireproof beams, including identifying the conditions for fixing and heating steel beams, determining the material of the structural protection of steel beams from heating in case of fire (ordinary concrete, expanded clay concrete, hollow blocks, gypsum boards, vermiculite plates, plaster, perlite-asbestos cladding etc.), type of hire (I-beam, channel, corner), cross-sectional shape of the bearing rod of a steel beam, its geometric dimensions, steel grade, working standard load.

During the inspection, groups of the same type of steel beams are determined. By a group of structures in a building is meant the same type of steel beams made and erected in similar technological conditions and under similar operating conditions.

Schemes for heating cross sections of steel fire-protected beams in a fire are determined depending on the actual location of the building parts, fire protection devices, laying adjacent structures, reducing the number of sides of the heating.

The minimum integer number of steel beams in the sample according to the plan of normal or reduced tests is assigned from the conditions (10 and 11).

Example 1. With the number of structures of the same type in the group M = 100 pcs., The number of tested steel beams is taken at a rate

M _n = 5 + M ^0.5 = 5 + 100 ^0.5 = 15 pcs., According to an abridged plan

M _min = 0.3 · (15 + M ^0.5 ) = 0.3 · (15 + 100 ^0.5 ) = 7.5≈8 pcs.

When the number of designs in the group M≤5, they are checked individually.

The number and location of sites in which the quality indicators of steel beams are determined are determined as follows. In a steel beam having one dangerous section, sections are placed only in this section. In a steel beam having several dangerous sections, the test areas are placed evenly on the surface with the obligatory location of part of the sections in dangerous sections.

The main single quality indicators of steel fire-resistant beams that provide fire resistance include: coefficient of thermal diffusion and density of the material of the fire-retardant coating, an indicator of the heating conditions of the bearing rod of the steel fire-protected beam, the thickness of the fire-retardant coating, the grade of steel, its yield strength, critical temperature, the reduced thickness of the carrier metal rod, the intensity of normal stresses in the cross section of the bearing rod, the resistance time to the thermo-force action of the bearing rod became Nnoy beam without fire protection.

The number of tests of a single indicator of the quality of structures, with a probability of a result equal to 0.95, and an accuracy rate of 5%, is determined by the formula (9); the coefficient of variation of the sample υ = ± 100 · σ / A;

arithmetic mean A = (1 / n) · Σm _i ,

average error ΔА = ± σ / (2 · n) ^0.5 ;

standard deviation σ = ± [(1 / (n-1)) · Σ (x _i ) ² ] ^0.5 ;

here n is the number of tests; 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: the thickness of the fire retardant coating, the width and height of the cross section of the bearing rod of the steel beam. Dangerous sections of steel beams are assigned in places of greatest moments and transverse forces from the action of the working standard load. The dimensions of the structure are checked with an accuracy of ± 1 mm.

The coefficient of thermal diffusion of the fire protection material under thermal exposure is determined at 450 ° C. To calculate its integral parameter by the formula (6), the density of the fire protection material in its natural state, humidity, as well as its thermal conductivity and heat capacity at 450 ° C are determined.

Using the obtained integral parameters

D _ar (mm ² / min), δ ₀ (mm), m ₀ , J _σs , τ _us (min),

according to the nomogram (see figure 1), the fire resistance limits of the steel fire-protected beams of the building are found.

Example 2. Given: The depth of the flange of the I-beam (see figure 2)

δ _x = δ _y = 25 mm; δ ₀ = k ₀ · δ _x = 1.33 · 25 = 33.3 mm; an indicator of heating of a shelf of an I-beam m ₀ = 0.87; the intensity of the stresses in the metal power J _σs = 0,63; the degree of fire protection of the point M of the flange of the I-beam C = 33.3 mm; the duration of resistance to the thermal force of an unprotected supporting rod is τ _us = 14 min. The fire resistance of the steel fireproof beam according to the nomogram (see figure 1) at D _ar = 20 mm ² / min; δ ₀ = 33.3 mm; m ₀ = 0.87; J _σs = 0.63; τ _us = 14 min is equal to F _ur = 110 min (1.83 h; experiment - 1.9 h).

Checking the result of determining the fire resistance of steel beams with a fireproof coating according to the nomogram is carried out by calculation according to the formula (12):

where J _σs is the intensity of normal stresses in the cross section of the bearing rod of steel beams from the working standard load, operating in the conditions of a standard fire test (0≤J _σs ≤1);

τ _us - the duration of resistance to thermal effects of the bearing rod of steel beams without taking into account fire protection, min;

C is the degree (measure) of fire protection of the supporting rod of steel beams with a fireproof coating, mm, calculated by the formula (13)

where m ₀ is the indicator of the heating conditions in the fire of the bearing rod of steel beams with fireproof coating (0.25≤m ₀ ≤1);

D _ar — coefficient of thermal diffusion of the fire protection material, mm ² / min;

k ₀ - coefficient taking into account the voidness of the cross section on the fire resistance of the beams;

δ ₀ - the thickness of the fire protection material of the steel beam, mm

The guaranteed fire resistance of steel fireproof beams, F _ur , min, is calculated by the nomogram (see figure 1) with a corresponding change in design parameters:

J _σs is the intensity of normal stresses in the cross section of the bearing rod of the steel beam from the standard load (0 ÷ 1);

δ ₀ - the minimum thickness of fire protection of a steel beam, mm;

m ₀ - an indicator of the heating conditions of the bearing rod of a steel fire-protected beam in fire conditions (0.25 ÷ 1.0);

D _ar (mm ² / min) - indicator of the thermal diffusion flame retardant coating material (thermal diffusivity) at 450 ° C;

τ _us (min) - the duration of the resistance to the thermal force of the supporting rod of the steel beam without fire protection;

T _sr (cm) is the reduced thickness of the metal of the cross section of the beams.

Example 3. The initial data of the experimental steel fireproof beam are shown in figure 2 (the experimental limit of fire resistance is 1.9 hours), the design scheme is shown in figure 3.

The algorithm for solving the problem.

1) The depths of the calculated points M along the y-axis of the I-beam are calculated by formulas (5a) and (5b):

2) The indicator of the heating conditions of the I-beam shelf is calculated by the formula (4)

3) The reduced metal thickness (cm) is calculated by the formula (8)

4) The intensity of the stresses in the metal is calculated by the formula (3)

J _σs = J _n · t _cr / t _u = 0.625 · 510/505 = 0.63.

5) The duration of resistance to fire of the supporting rod of a steel beam without taking into account the fire retardant coating is calculated by the formula (7)

τ _us = 110 · [(1-J _σs ) ^0.5 ] + 6 · T _sr = 110 · [(1-0.63) ^0.5 -0.5] + 6 · 0.34 = 14 min.

6) The degree of fire protection of the supporting rod of the steel beam is calculated by the formula (13)

0.1 · C = 3.7.

7) The fire resistance of steel fireproof beams is calculated by the formula (12)

Thus, the above information indicates that when using the claimed method the following set of conditions:

a) a tool embodying the claimed method in its implementation, is intended for use in the construction industry, namely in the classification of metal columns 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 is applied in assessing the fire resistance of lined steel beams tested in the VNIIPO fire furnace.

Therefore, the claimed invention meets the condition of "industrial applicability". The error of the calculation values in comparison with the experiment:

ξ = 100 · (F _{u, on} - F _{u, pac} ) / F _{u, on} = 100 · (1.9-1.87) / 1.9 = 1.6% <5%.

Information sources

1. Ilyin N.A. Technical expertise of buildings damaged by fire. - M.: Stroyizdat, 1983 .-- 128 p. (see p. 90-91, 131, 134).

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, B. C. 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.

5. SNiP II-23-81 *. Steel structures. Design Standards. - M .: GUP TsPP, 1990. - 95 p. (see p. 63-66).

Claims (16)

1. A method for determining the fire resistance of steel fire-protected beams of a building by testing, including conducting a technical inspection, instrumental measurement of the geometric characteristics of beams during bending in their dangerous sections; identification of the conditions of support and fastening of beams, heating schemes of their cross sections; establishing the type of fire-retardant material and the steel grade of the beams, the characteristics of the metal resistance to bending and tension; determination of the magnitude of the working normative load on the beams, schemes for its application, the intensity of power stresses in the metal in dangerous sections of the beams; determining the time of the onset of the limit state on the basis of loss of the bearing capacity of the beams under the working standard load under standard thermal exposure, characterized in that the determination of the fire resistance of steel fire-protected beams of the building is carried out without fire by non-destructive testing methods, using a set of unit quality indicators of steel structures, determine the number and the location of the sites in which individual quality indicators are determined, with technical inspection p is supplemented by the definition of a group of steel beams of the same type, the instrumental determination of the geometric dimensions of the thickness of the fire retardant coating and the bearing rods of steel beams, experimentally determine the density of fire protection materials, their moisture content in the natural state and / or the value of the coefficient of thermal diffusion of the fire protection material, the geometric characteristics are taken as unit quality indicators beams, the critical temperature of steel heating in a fire; degree of stress and yield strength of metal; calculate integral structural parameters D _ar , δ ₀ , m ₀ ; J _σs , τ _us, where D _ar is the coefficient of thermal diffusion of the fire protection material, mm ² / min; δ ₀ - the thickness of the fire-retardant coating of steel beams, mm; m ₀ - an indicator of the heating conditions of the bearing rod of steel beams with a fire retardant coating in case of fire; J _σs is the intensity of normal power stresses in the cross section of steel beams from the working standard load; τ _us - the duration of resistance to thermal effects of the bearing rod of steel beams without taking into account the fire retardant coating, min; and, using the obtained parameters of steel fireproof beams, the actual fire resistance limit F _{u (R)} , min. 2. The method according to claim 1, characterized in that the intensity of normal power stresses in the cross section of the supporting rod of steel fire-protected beams from the working standard load, operating under standard fire tests, is calculated by the formula (1) J _σs = M _ρ / (W _n · R _yn ), where M _ρ is the bending moment in a dangerous section from the working standard load acting on the beams under test conditions, kN · m; W _n - moment of resistance of the cross section relative to the horizontal axis of the beam, cm ³ ; R _yn - regulatory resistance to tensile, compression and bending of the yield strength of steel according to TU or GOST, MPa. 3. The method according to claim 1, characterized in that at different values of the limiting temperature of heating the steel of the supporting rod of the metal beams t _u , ° C, different from the critical temperature of the steel t _cr , ° C, the actual intensity of normal power stresses in the cross section of steel beams are calculated by the formula (2)

where J _σs is the intensity of normal power stresses in the cross section of steel fireproof beams; t _u is the limiting temperature of steel heating (° C) at which variously loaded steel fireproof beams lose their bearing capacity; t _cr is the critical temperature of steel heating (° C) of metal beams (at a standard intensity of power stresses J _n = 0.625 for construction mild steel of grade St.3 t _cr = 510 ° C); n - empirical indicators depending on the grades of steel (n = 2.8 for construction steel grade St.3). 4. The method according to claim 1, characterized in that at t _u = t _cr ± 50 ° C, the intensity of normal power stresses in the cross section of steel beams is calculated by the formula (3) J _σs = J _n · t _cr / t _u = 0.625 · t _cr / t _u where J _n = 0.625 is the normative intensity of power stresses in the cross section of steel beams under conditions of fire tests; t _cr and t _u - respectively, the critical and ultimate temperature of steel heating, ° C. 5. The method according to claim 1, characterized in that the indicator of the heating conditions of the bearing rod of steel beams with a structural fire retardant coating under the standard fire test (at δ _x ≤α _y1 ) is calculated by the formula (4) m ₀ = (α _y1 / δ _x ) ^0.5 / (1.5+ (α _y1 / α _y2 ) ⁴ ), where δ _x is the thickness of the fire-retardant coating of the shelf (wall) of the bearing rod of steel beams along the x axis, mm; α _y1 , α _y2 - the ordinates of the calculated point of the cross-section, directionally moved along the y axis, mm, under the condition δ _x > α _y1 , in the formula (4) these factors are interchanged; when changing the coordinate axes, the width of the section b, mm, of the bearing rod of metal columns is replaced by a height h, mm, where b is the width of the section of the bearing rod of beams, mm. 6. The method according to claim 1, characterized in that the ordinates of the calculated points of the cross section M directed along the y axis, determining the location of the average temperature along the width of the shelf or the wall height of the unevenly heated supporting beam of the steel beam in a fire, are calculated by the formulas (5a ) and (5b):

α _y2 = H-α _y1, where α _y1 and α _y2 are the ordinates of the design points of the unevenly heated cross section of steel beams, mm; δ _y - the thickness of the fire-retardant coating of the shelf or wall of the supporting rod of the steel beam along the y axis, mm; b and H - respectively, the width of the cross-section of the supporting rod and the height of the steel fireproof beam, mm 7. The method according to claim 1, characterized in that the coefficient of thermal diffusion of the material of the fire retardant coating of steel beams, D _ar , mm ² / min, is determined experimentally or calculated at an averaged temperature t _m = 450 ° C according to the formula (6)

where λ ₀ and b are empirical numbers for calculating the coefficient of thermal conductivity of the fire protection material; t _m is the average heating temperature (450 ° C) of the flame retardant material; C ₀ and d are empirical numbers for calculating the specific heat of the fire protection material; ω is the moisture content of the fire protection material mass,%; ρ _{with the} density of the dry material of fire protection, kg / m ³ ; k _ρ = 0.75 at ρ _with more than 1000 kg / m ³ ; k _ρ = 1 at ρ _s less than 1000 kg / m ³ . 8. The method according to claim 1, characterized in that the duration of resistance to thermal effects of the bearing rod of steel beams without taking into account the fire retardant coating τ _us , min, is determined by the formula (7):

where J _σs is the intensity of normal stresses in the cross section of the bearing rod of steel beams from the action of the working standard load in case of fire; T _sr - reduced thickness of the metal of the supporting rod of steel beams, cm, calculated by the formula (8) T _sr = A _s / P ₀ , where A _s is the metal area of the cross section of the supporting rod, cm ² ; P ₀ - perimeter of the heating of the cross section of steel beams, see 9. The method according to claim 1, characterized in that for the individual quality indicators of steel beams that affect their fire resistance, take the geometric characteristics when bending the cross section, the conditions for fixing the ends of the beams, the length of the perimeter of the heating of the cross section, the standard resistance of steel to bending, compressive and tensile yield strength, the size of the working standard load and the scheme of its application; the magnitude of the bending moment and the transverse force, the intensity of the normative power stresses in the metal in dangerous sections of steel beams, the critical temperature of heating of building steel under conditions of fire exposure, the moisture and density of the flame retardant material in its natural state, the thickness of the flame retardant coating, the thermal diffusion of the flame retardant material. 10. The method according to claim 1, characterized in that non-destructive tests are carried out for a group of the same steel beams, the differences between the geometric dimensions of the cross sections and the fluidity of the steel which are caused mainly by random factors. 11. The method according to claim 1, characterized in that the number of tests n _{uc of a} single quality indicator of steel fireproof beams, with a probability of a result of 0.95 and an accuracy of 5%, is determined by the formula (9) n _uc = 0.15 · υ ² ≥6, where υ is the sample coefficient of variation of the test results,%. 12. The method according to claim 1, characterized in that the heating scheme of the cross sections in a fire condition of the test steel beams is determined depending on the actual location of the building parts. 13. The method according to claim 1, characterized in that in the case when all the individual quality indicators of steel beams (with M more than 9 pcs.) Are within the control limits, the minimum integer number of metal structures in the sample according to the plan of shortened tests M _min , pcs., appoint from condition (10) M _min = 0.3 · (15 + M ^0.5 ) ≥5, where M is the number of the same type of metal beams in the building, pcs. 14. The method according to claim 1, characterized in that in the case when at least one of the individual quality indicators of steel beams goes beyond the control limits, the minimum number of structures in the sample is determined by the norm according to the formula (11) M _n = 5 + M ^0.5 ≥8. 15. The method according to claim 1, characterized in that in the case when at least one of the individual quality indicators of the steel beams goes beyond the permissible limits or M≤5 pcs., All the same building structures are subjected to a non-destructive test piece by piece. 16. The method according to claim 1, characterized in that it further computes the guaranteed fire resistance limit of steel fireproof beams from the nomogram by solving the inverse fire resistance problem.

Images