RU2357245C2 - Method of determining flame resistance of building brick piers - Google Patents

Abstract

FIELD: fire safety.

SUBSTANCE: invention relates to fire safety of the buildings and can be used to classify brick piers and separating walls with respect to their flame resistance characteristics. The proposed method comprises performing tech inspection, defining the bricklaying type, types of bricks and mortar, the bricklaying elastic stability, the pier support and attachment conditions, determining the probable time of occurrence of limiting flame resistance conditions of plain brick piers at rated load in the conditions of standards fire. Testing the said plain brick piers involves no destruction and incorporates a set of separate quality properties of the plain brick piers. The date and place are selected for quality characteristics to be defined. The aforesaid tech inspection is supplemented by instrument measurements of brick pier geometrical sizes at critical sections. The pattern of heating in aforesaid critical sections of piers in fire is determined, as well as temporary compression resistance of bricklaying. The rated load onto brick piers is selected for flame resistance test conditions and strains in the pier critical sections are defined.

EFFECT: determination of plain brick pier and separation wall flame resistance involving no natural heating; higher validity of quality control checks and non-destructive tests.

12 cl, 5 dwg, 2 ex

Description

The invention relates to the field of fire safety of buildings and structures (hereinafter - “buildings”). In particular, it can be used to classify unreinforced brick pillars of buildings according to their resistance to fire. This makes it possible to justify the use of existing brick pillars with an actual fire resistance limit in buildings of various classes for their constructive fire hazard.

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

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

A known method of determining the fire resistance of brick pillars 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 poles in the building, assessing the condition of the poles by inspection and measurement, making control samples of brick and stone, determining the time of the onset of the limiting state by the loss of the bearing capacity of the structure, that is, collapse under conditions of external load and heat exposure / Ilyin N. BUT. The consequences of fire on reinforced concrete structures. - M .: Stroyizdat, 1979. - 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 brick pillars having other sizes and other external loads. It is difficult to compare the results with standard fire tests of similar brick pillars. Therefore, this method is expensive, has little technological ability to re-test, time-consuming and dangerous for testers.

A known method for evaluating the fire resistance of brick pillars of a building according to the results of full-scale fire tests of a building fragment in which the structures are inspected, determine the moisture of the masonry materials, assign a static load to the structure according to the actual operating conditions of the building, determine the factors that affect the fire resistance of the tested structures, and the value of the fire resistance / NPB 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 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 brick pillars in an experimental fire is difficult and unsafe, due to differences in the thermal regime of the experimental and standard fires it is difficult to determine the true values of the fire resistance limits of brick pillars, the reasons for the destruction of brick pillars of a fragment can be e established due to the diversity of simultaneously acting fire factors. The ultimate fire resistance state of brick pillars may not be reached due to earlier destruction of the bending elements of the fragment coating / 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 brick pillars of a building by testing, including technical inspection, establishing the type of brick, stone and masonry mortar, identifying the conditions for their support and fastening, determining the time of occurrence of the limit state for loss bearing capacity of columns under standard load in a standard fire / GOST 30247.1-94. Building constructions. Test methods for fire resistance. 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, tests are carried out on samples of brick poles, which are affected only by constant and continuous loads in their calculated values with a reliability factor equal to unity , i.e. design regulatory loads.

Tests are carried out on special bench equipment in fire furnaces until the destruction of samples of brick pillars. 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 sized and differently loaded brick pillars, do not provide the necessary information about the effect of individual quality indicators of brick pillars on its fire resistance. Determining the fire resistance of brick pillars by a single quality indicator, for example, by the size of the smaller side of the rectangular cross-section of a column, as a rule, underestimates the suitability of brick pillars in a building of a given degree of fire resistance. The economic costs of testing increase due to the cost of erecting a sample of brick poles at the installation site of the heating furnaces and to create a standard thermal regime in them. By the small number of tested samples (2-3 pcs.) It is impossible to judge the actual state of the brick pillars of the building. The results of the fire test are single and do not take into account the diversity in the brick pillars of stone walls, their actual dimensions, the heating scheme of the dangerous section of the tested pillars.

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 unreinforced brick pillars in a fire; in determining the actual (design) fire resistance limits of brick pillars 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 brick pillars in a building or its fragment; reducing the complexity of determining the fire resistance of brick pillars; expanding the technological capabilities of determining the actual fire resistance of variously loaded brick pillars of any size and the possibility of comparing the results with tests of similar building structures; the possibility of testing brick pillars 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 brick pillars; simplification of conditions and shortening of the test time of brick pillars for fire resistance; the use of polyparametric dependence to determine the fire resistance of brick pillars; increased accuracy and expressiveness of the test; getting the opportunity to solve the inverse problems of fire resistance of pillars and the application of the method of selection of variable values of its design parameters; the use of integral structural parameters to determine the fire resistance of brick pillars and the simplification of the mathematical description of the process of thermal resistance of loaded brick pillars; increasing the reliability of the test results of a group of the same type of brick pillars; taking into account the real resource of brick pillars to the fire resistance using a complex of individual indicators of their qualities; clarification of individual quality indicators of unreinforced brick pillars affecting their fire resistance; the ability to determine the guaranteed fire resistance of brick pillars by 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 brick pillars of a building by testing, including technical inspection, establishing the type of masonry, grades of brick and mortar, the elastic characteristics of the masonry, identifying the conditions of support and fastening of the pillars, determining the time of the onset fire resistance conditions of unreinforced brick pillars under standard load in a standard fire, a feature is the fact that the test of unreinforced brick pillars is carried out without destruction, using a set of individual quality indicators of unreinforced brick pillars, the number and location of sites in which the quality indicators are determined, technical inspection is supplemented by instrumental measurements of the geometric dimensions of the brick pillars in their dangerous sections, the scheme is revealed heating dangerous sections of brick pillars in case of fire, find temporary resistance to compression of the brickwork, set the standard load Ku on brick columns under fire test, determine the magnitude of force of the stress intensity in hazardous sections of the pillars and, - using the received indicators of the quality parameters of single unreinforced brick pillars;

m _o - coefficient of the conditions for the heating of the cross section;

φ is the coefficient of longitudinal bending of brick pillars (0.05 ÷ 1);

J _σ0 is the intensity of power stresses in a dangerous section (0 ÷ 0.95);

h is the size of the smaller side of the rectangular cross-section of the columns, mm;

D _kk - an indicator of thermal diffusion of masonry, mm ² / min;

R _u - temporary compressive strength of the masonry, MPa,

- calculate the fire resistance of unreinforced brick pillars by the loss of bearing capacity F _u (R), min, according to the polyparametric dependence (1):

The next feature of the proposed method is that the coefficient of the conditions for heating the cross section (m _o ) is determined by the formula (2):

where m _o is the coefficient of the conditions for the heating of the cross section;

P and P _o - respectively, the perimeter of the cross section and part of the perimeter, heated in a fire, mm

A feature of the proposed method is that the coefficient of longitudinal bending of unreinforced brick pillars (φ) is determined by the formula (3):

where ξ _k is the masonry deformability index, which for a rectangular section of an unreinforced brick pillar is calculated by the formula (4):

α is the elastic characteristic of unreinforced masonry;

h is the smaller side of the rectangular sections of the pillars, mm;

l _about - the estimated height of the brick pillars, mm;

for a stone structure of any cross-sectional shape, the deformation index of masonry is calculated by the formula (5):

where r is the radius of inertia of the section, mm

A feature of the proposed method lies in the fact that the intensity of power stresses in a dangerous section of unreinforced brick pillars from the standard load when tested for fire resistance is determined from the condition (6):

where J _σo is the intensity of power stresses in a dangerous section (0 ÷ 0.95);

γ _n is the coefficient of the level of responsibility of building structures;

N _cc ; N _ρ - respectively, the bearing capacity and standard load when testing brick pillars for fire resistance, kN;

R _u ; R is the temporary and calculated compressive strength of the masonry, MPa.

A feature of the proposed method is that the heat diffusion index of the masonry of unreinforced brick pillars D _kk , mm ² / min, is determined experimentally at an average temperature t _m = 450 ° C or is found from the analytical expression (7):

where λ ₀ and С _о are respectively the thermal conductivity coefficient, W / (m · ° С), and specific heat, kJ / (kg · ° С), masonry at t _н = 20 ° С;

t _m - masonry heating temperature along the cross-section of the pillars (450 ° C);

b and d - thermal indicators of the coefficient of thermal conductivity, W / (m · ° C), and specific heat, kJ / (kg · ° C), masonry;

ω and γ _{with the} humidity of the masonry,%, by weight and average density of the dry masonry material, kg / m ³ .

In the formula (7), the plus sign is used for ceramic bricks; minus sign - for silicate brick.

A feature of the proposed method is that for individual quality indicators of unreinforced brick pillars that affect the fire resistance of the loss of bearing capacity F _{u (R)} , min, take: geometric dimensions, the conditions of support and fastening of the pillars, the size of the smaller side of the cross sections of the pillars, the coefficient of longitudinal bending, the strength of the brick and mortar, the elastic characteristic of the masonry, the temporary compressive strength of the masonry, the bearing capacity of the pillars and the standard load on them when tested for fire capacity, safety factor for bearing capacity of columns, intensity of power stresses in a dangerous section; indicators of humidity, density and diffusion of masonry heat.

A feature of the proposed method is that non-destructive tests are carried out for a group of the same type of unreinforced brick pillars, the differences between the strength of the brick and mortar which are caused mainly by a random factor.

A feature of the proposed method is that the number of tests n _{uc of a} single quality indicator of unreinforced brick pillars, with a probability of a result of 0.95 and an accuracy of 5%, is taken according to the formula (8):

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

A feature of the proposed method is that in the case when all the individual quality indicators of unreinforced brick pillars, when M is more than 9 pieces, are within the control limits, the minimum integer number of columns in the sample according to the plan of shortened tests M _min , pieces, is prescribed (9):

where M is the number of the same type of brick pillars in the building, pcs.

A feature of the proposed method is that in the case when at least one of the individual quality indicators of unreinforced brick pillars goes beyond the control limits, the minimum number of tested structures in the sample is calculated according to the norm according to the formula (10):

A feature of the proposed method lies in the fact that in the case when at least one of the individual quality indicators of unreinforced brick pillars goes beyond the permissible limits or M≤5 pcs, all the same type building poles are subjected to non-destructive testing individually.

A feature of the proposed method lies in the fact that in the case when at least one of the individual quality indicators of the stone walls goes beyond the boundaries of permissible limits or M≤5 pcs, all the same type of building walls are subjected to non-destructive testing individually.

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

The elimination of fire tests of brick pillars of an existing building and their replacement with non-destructive tests reduces the complexity of determining their fire resistance, expands the technological capabilities for detecting the actual fire resistance of variously loaded brick pillars of any size, makes it possible to test brick pillars for fire resistance without disturbing the functional process of the building under examination, as well as comparing results with standard tests of similar unreinforced bricks pillars and the continued serviceability of the surveyed building without disturbing the bearing capacity of its structures during the test. Therefore, the conditions for testing brick pillars for fire resistance are greatly simplified.

Reducing the economic costs of testing include reducing the cost of the construction and fire tests of structural samples.

The use of a mathematical description of the process of resistance of unreinforced brick pillars to standard thermal testing and the use of polyparametric dependencies increase the accuracy and expressiveness of their fire resistance assessment.

The use of integral parameters, such as: the degree of stress of the calculated section of unreinforced brick pillars and the thermal diffusion index of masonry, simplifies the mathematical description of the process of resistance of load-bearing structures to thermal effects.

The proposed technical solution provides for testing not one, but groups of the same type of unreinforced brick pillars. 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. Determining the fire resistance of unreinforced brick pillars by only one quality indicator, for example, by the thickness of brick pillars, leads, as a rule, to underestimating their fire resistance limit, since the influence on it of variations of individual quality indicators of unreinforced brick pillars has different signs, and the fire resistance is reduced due to one indicator can be offset by others. As a result, in the proposed method, the assessment of the fire resistance of unreinforced brick pillars is provided not for one indicator, but for a set of individual indicators of their quality. This allows you to more accurately take into account the real fire resistance of unreinforced brick pillars. The complex of individual quality indicators of unreinforced brick pillars that affect their fire resistance limits, determined by non-destructive tests, has been clarified.

The minimum number of non-destructive tests of a single quality indicator of unreinforced brick pillars has been clarified. The accepted sample size from the total number of the same type of brick pillars of the building provides reliability, reduces the time and complexity of testing.

Figure 1 shows a longitudinal section of an unreinforced brick pillar;

Figure 2 is a cross section 1-1 unreinforced brick pillar with 4-sided heating in the conditions of a standard fire test;

Figure 3 - shows a 1-sided heating section of an unreinforced brick pillar;

Figures 4 and 5 show options for 3-sided heating of a cross section of an unreinforced brick pillar under standard fire test conditions:

1 - unreinforced brick pillar; longitudinal section;

2 - heat flow standard fire test, t _cm , ° C;

3 is a cross section of an unreinforced brick pillar with dimensions b × h, mm;

4 - masonry joints filled with mortar, δ _joint = 10 ÷ 12 mm;

5 - standard load on an unreinforced brick pillar under fire test conditions, N _ρ , kN;

6 - heated part of the perimeter of the cross section;

7 - unheated part of the perimeter of the cross section.

Thermophysical characteristics of building materials for masonry (including values of D _km , mm ² / min) are given in table 1.

Table 1
Thermophysical characteristics of building materials Material Density ρ _s , kg / m ³ Humidity ω,% The parameters of thermal conductivity, W / (m · ° С), and heat capacity of the material, kJ / (kg · ° С) Heat diffusion coefficient D _km , mm ² / min λ ₀ b C ₀ d one 2 3 four 5 6 7 8 1 Cement, lime and gypsum mortars 1.1 Cement-sand 1800 2 0.58 0.35 0.84 0.63 20.1 1.2 Complex (sand, lime, cement) 1700 2 0.52 0.35 0.84 0.63 19.54 1.3 Lime and sand 1600 2 0.47 0.35 0.84 0.63 19.23 1.4 Cement slag 1400 2 0.41 0.35 0.84 0.63 19.90 1.5 The same 1200 2 0.35 0.34 0.84 0.62 20.63 1.6 Cement-perlite 1000 7 0.21 0.33 0.84 0.62 14.64 1.7 The same 800 7 0.16 0.32 0.84 0.62 15,52 1.8 Gypsum-perlite 600 10 0.14 0.31 0.84 0.60 17.26 1.9 The same, porous 500 6 0.12 0.30 0.84 0.60 21.56 1.10 The same 400 6 0.09 0.30 0.84 0.58 24.10 1.11 Plaster plates (GOST 6428) 1200 four 0.35 0.8 0.84 0.62 26.91 1.12 The same 1000 four 0.23 0.78 0.84 0.62 26.43 1.13 Facing gypsum sheets (dry plaster) (GOST 6266) 800 four 015 0.74 0.84 0.62 27.46 1.14 The same, γ = 900 kg / m ³ 900 four 0.16 0.76 0.84 0.62 25.37 2 Brickwork made of ceramic stone and brick on a heat-insulating cement-perlite mortar (γ _о = 800 kg / m ³ ) 2.1 Stone large-format ceramic hollow from porous ceramics, γ _about = 600 kg / m ³ 630 one 0.12 0.21 0.88 0.38 18.55 2.2 The same, γ _about = 800 kg / m ³ 800 one 0.17 0.22 0.88 0.40 18.2 2.3 Ceramic hollow stone (250 × 120 × 138 mm) γ _о = 800 kg / m ³ 800 2 0.19 0.22 0.88 0.40 18.7 2.4 The same, γ _about = 1000 kg / m ³ 970 2 0.23 0.23 0.88 0.42 17.65 2.5 The same, γ _about = 1200 kg / m ³ 1140 2 0.25 0.23 0.88 0.42 15.92 2.6 The same, γ _o = 1400 kg / m ³ 1300 2 0.28 0.23 0.88 0.42 15.14

Continuation of table 1 one 2 3 four 5 6 7 8 2.7 Brick trepilny corpulent single thickened, γ _about = 900 kg / m ³ 960 2 0.28 0.22 0.88 0.42 20.3 2.8 The same, γ _o = 1000 kg / m ³ 1110 2 0.30 0.22 0.88 0.42 18.45 2.9 Brick ceramic hollow single thickened, γ _about = 1000 kg / m ³ 960 2 0.23 0.22 0.88 0.42 17.6 2.10 The same, γ _o = 1200 kg / m ³ 1110 2 0.27 0.22 0.88 0.42 06/17 2.11 The same, γ _o = 1400 kg / m ³ 1270 2 0.3 0.22 0.88 0.42 16.12 2.12 Bricks of a ceramic full-body single thickened, γ _о = 1600 kg / m ³ 1430 2 0.45 0.22 0.88 0.42 19.7 2.13 The same, γ _about = 2000 kg / m ³ 1750 2 0.53 0.22 0.88 0.42 18.45 3. Brickwork of silicate brick and stone on a cement-sand mortar (γ _o = 1800 kg / m ³ ) (GOST 379) 3.1 Full single brick, γ _о = 2000 kg / m 1960 2 1.06 -0.36 0.88 0.61 21.91 3.2 The same, γ _about = 1800 kg / m ³ 1800 2 0.78 -0.35 0.88 0.60 24.1 3.3 Hollow single and thickened bricks, γ _о = 1600 kg / m ³ 1640 2 0.68 -0.35 0.88 0.60 26.41 3.4 Hollow 11-hollow thickened brick and stone, γ _о = 1400 kg / m ³ 1500 2 0.64 -0.35 0.88 0.60 15.44 3.5 Hollow 14-hollow thickened brick and stone, γ _о = 1300 kg / m ³ 1400 2 0.52 -0.35 0.88 0.60 12,42 3.6 The same, γ _about = 1200 kg / m ³ 1300 2 0.43 -0.35 0.88 0.60 10.1 3.7 Masonry from slag brick and stone (γ _о = 1400 kg / m) on a cement-sand mortar (γ _о = 1800 kg / m ³ ) 1500 1,5 0.52 -0.35 0.88 0.60 11.84

Here λ ₀ and С ₀ are the values of thermal conductivity, W / (m · ° С), and heat capacity, kJ / (kg · ° С), of building materials at t _н = 20 ⁰ С, respectively;

b and d are respectively the thermal coefficients of thermal conductivity and heat capacity of materials multiplied by 1000.

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 unreinforced brick pillars of buildings is as follows.

First, a visual inspection of the building is carried out. Then determine the group of the same type of unreinforced brick pillars and their total number in it. The sample size of the structures of the same type is calculated. Assign a set of individual quality indicators of unreinforced brick pillars that affect fire resistance. The conditions for fixing the ends and dangerous sections of unreinforced brick pillars are revealed. The number of tests of a single quality indicator of unreinforced brick pillars is calculated depending on its statistical variability. Then, single quality indicators of unreinforced brick pillars and their integral parameters are evaluated, and finally, the fire resistance of the tested pillars is found from them.

Visual inspection means checking the condition of unreinforced brick pillars, including identifying the conditions for fixing individual pillars, determining the brand of brick and mortar, the presence of cracks and spalls, the minimum size of the smaller side of the rectangular cross-section of the pillars.

During the inspection, groups of similar structural elements are determined. By a group of structural elements in a building is meant the same type of unreinforced brick pillars made and erected in similar technological conditions and under similar operating conditions.

For verification calculations of the bearing capacity of unreinforced brick pillars determine: the height and width of the cross section of the pillars, the distance between the floors of the building;

type of supports of brick pillars: rigid (l ₀ = 0.7 · N), elastic;

the thickness of the solution under the pillars;

type of brick pillars: from brick or ceramic stones, from concrete or natural stones; from cellular concrete stones;

type of masonry, depending on the brands of brick and stones; masonry group depending on the grades of the solution; type of masonry: reinforced, unreinforced;

The minimum integer number of structures in the sample according to the plan of normal or reduced tests is assigned from the conditions (9 and 10).

Example 1. With the number of the same type of unreinforced brick pillars in the group M = 120 pcs., The number of subjects is taken according to the norm M _n = 5 + M ^0,5 = 5 + 120 ^0,5 = 16 pcs.

according to the shortened plan M _min = 0.3 · (15 + M ^0.5 ) = 0.3 · (15 + 120 ^0.5 ) · 8 pcs.

When the number of unreinforced brick pillars in the group M≤5, they are checked individually.

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

The main single quality indicators of unreinforced brick pillars that determine fire resistance include: the geometric dimensions of the pillars and the smaller side of the dangerous section; support conditions for the pillars, the value of the coefficient of longitudinal bending; masonry compressive strength, humidity, density, thermal conductivity and heat capacity in natural conditions; masonry thermal diffusion index (thermal diffusivity) in a fire.

The number of tests of a single quality indicator of brick pillars, with a probability of result equal to 0.95, and an accuracy rate of 5%, is determined by the formula (8);

the coefficient of variation of the sample υ = ± 100 · σ / A;

arithmetic mean A = (1 / n) · Σm _i ,

(here m _i is the result of the i-th test);

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

(here Σ (x _i ) ² is the sum of the squares of all deviations from the mean);

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

Checked geometric dimensions are: the width and height of the cross section of the column, the perimeter of the dangerous section and part of the perimeter, heated in case of fire; floor height.

Dangerous sections of brick pillars are assigned in places of the greatest moments from the action of the normative load during fire tests.

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

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

The masonry thermal diffusion in a fire is determined at 450 ° C. To calculate its integral parameter by the formula (7), the density of the masonry in its natural state, humidity, as well as the coefficient of thermal conductivity and specific heat of the masonry at 450 ° C are determined.

Using the obtained integral parameters m _o ; φ; h mm J _σo , D _km , mm ² / min; Ru, MPa, by the formula (1) find the fire resistance limit F _u (R), min, unreinforced brick pillars of the building.

The temporary compressive strength of the masonry Ru, MPa, is determined by the formula (3) SNiP II-22-81 [5]:

R _u = k · R;

where k = 2.0 - for masonry of bricks and stones of all kinds; k = 2.5 - for masonry from cellular concrete blocks (see table 14 SNiP II-22-81);

R is the estimated compressive resistance of the masonry, MPa, taken according to table 2 ÷ 9 SNiP II-22-81 [5].

Example 2. The source data:

Unreinforced brick pillar of hollow silicate brick;

design height l _about = N = 4800 mm;

cross-sectional dimensions h × b = 510 × 640 mm;

sectional area A = h × b = 51 × 640 = 3264 cm ² ;

cross-sectional heating 4-sided; the perimeter of the cross section and the heated part of the perimeter P = P _o = 230 cm;

rated continuous load N _dl = 350 kN (35 tf);

brick grade 150, cement-sand mortar brand 75,

the elastic characteristic of the masonry is a = 750 (according to Table 15 of SNiP II-22-81 [5]);

the coefficient of thermal diffusion of masonry from hollow silicate brick on a cement-sand mortar γ _о = 1800 kg / m ³ , - (see table 1), is equal to D _кк = 26.41 mm ² / min;

level of responsibility of building structures - II (second): γ _n = 0.95;

temporary and estimated compressive strength of the masonry: R _u = 4 MPa; R = 2 MPa.

It is required to calculate the fire resistance by the loss of the bearing capacity of a column of hollow silicate brick F _{u (R)} , min.

Solution: 1) The coefficient of the conditions for heating the column cross section is calculated by the formula (2):

2) The deformability index of masonry unreinforced brick pillar is calculated by the formula (4):

3) The coefficient of longitudinal bending of an unreinforced brick pillar is calculated by the formula (3):

4) The bearing capacity of an unreinforced brick pillar under central compression (with m _g = 1) is calculated by the formula (10), SNiP II-22-81:

5) The standard load during the fire test (with k _n = 1,2):

6) The intensity of power stresses in a dangerous section of a brick pillar is calculated by the formula (6):

7) The fire resistance of an unreinforced brick pillar of hollow silicate brick by the loss of bearing capacity F _{u (R)} , min, with 4-sided heating of the section is determined by the dependence (1):

The proposed method was applied during field inspection of brick pillars of a public building in Samara. The results of non-destructive testing of brick pillars h × b = 510 × 640 mm; D _bm = 26.41 mm ² / min; φ = 0.864; J _σo = 0.246; R _u = 4 MPa, is the fire resistance of brick pillars, by the loss of bearing capacity F _{u (R)} = 410 min (7 h).

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

Information sources

1. Ilyin N.A. The consequences of fire 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. NPB 233-97. - M .: VNIIPO, 1997 .-- 14 p.

3. Fire resistance of buildings / V. P. Bushev, V. A. Pchelintsev, 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 .; 1995 .-- 7 p.

5. SNiP II-22-81. Stone and arched structures. Design Standards. - M .: Stroyizdat, 1983 .-- 40 p.

Claims (12)

1. A method for determining the fire resistance of brick pillars of a building by testing, including technical inspection, establishing the type of masonry, grades of brick and mortar, the elastic characteristics of the masonry, identifying the conditions of support and fastening of the pillars, determining the time of the onset of the ultimate state of fire resistance of unreinforced brick pillars under standard load in standard fire conditions, characterized in that the test of unreinforced brick pillars is carried out without destruction, using a set of individual quality indicators of unreinforced brick pillars, designate the number and location of sites in which quality indicators are determined, technical inspection is supplemented by instrumental measurements of the geometric dimensions of brick poles in their dangerous sections, the heating pattern of dangerous sections of brick poles in a fire is revealed, temporary resistance to compression of the brickwork is found, establish the standard load on the brick poles during the fire test, determine the magnitude of the intensity of the power voltage in a dangerous section of the column, and using the obtained parameters of individual indicators of the quality of unreinforced brick pillars:
m ₀ is the coefficient of the conditions for the heating of the cross section;
φ is the coefficient of longitudinal bending of brick pillars (0.05 ÷ 1);
J _σ0 is the intensity of power stresses in a dangerous section (0 ÷ 0.95);
h is the size of the smaller side of the rectangular cross-section of the columns, mm;
D _kk - an indicator of thermal diffusion of masonry, mm ² / min;
R _u - temporary compressive strength of the masonry, MPa,
calculate the fire resistance of unreinforced brick pillars by the loss of bearing capacity F _{u (R)} , min, according to the polyparametric dependence (1):

2. The method according to claim 1, characterized in that the coefficient of the conditions for the heating of the cross section (m ₀ ) is determined by the formula (2)

where m ₀ is the coefficient of the conditions for heating the cross section;
P and P ₀ - respectively, the perimeter of the cross section and part of the perimeter, heated in a fire, mm 3. The method according to claim 1, characterized in that the coefficient of longitudinal bending of unreinforced brick pillars (φ) is determined by the formula (3)

where ξ _k is the masonry deformability index, which for a rectangular section of an unreinforced brick column is calculated by the formula (4)

where α is the elastic characteristic of unreinforced masonry;
h is the smaller side of the rectangular sections of the pillars, mm;
l ₀ - the estimated height of the brick pillars, mm,
for a stone structure of any cross-sectional shape, the masonry deformability index is calculated by the formula (5)

where r is the radius of inertia of the section, mm 4. The method according to claim 1, characterized in that the intensity of the power voltage in a dangerous section of unreinforced brick pillars from the standard load when tested for fire resistance is determined from the condition (6)

where J _σ0 is the intensity of power stresses in a dangerous section (0 ÷ 0.95);
γ _n is the coefficient of the level of responsibility of building structures;
N _cc ; N _ρ - respectively, the bearing capacity and standard load when testing brick pillars for fire resistance, kN;
R _u ; R is the temporary and calculated compressive strength of the masonry, MPa. 5. The method according to claim 1, characterized in that the heat diffusion index of the masonry of unreinforced brick pillars D _kk , mm ² / min, is determined experimentally at an average temperature t _m = 450 ° C or is found from the analytical expression (7):

where λ ₀ and C ₀ - respectively, the coefficient of thermal conductivity, W / (m · ° C), and specific heat, kJ / (kg · ° C), masonry at t _n = 20 ° C;
t _m - masonry heating temperature along the cross-section of the pillars (450 ° C);
b and d - thermal indicators of the coefficient of thermal conductivity, W / (m · ° C), and specific heat, kJ / (kg · ° C), masonry;
ω and γ _s are the humidity of the masonry,%, by weight, and the average density of the dry masonry material, kg / m ³ , moreover, in the formula (7) the plus sign is used for ceramic bricks; minus sign - for silicate brick. 6. The method according to claim 1, characterized in that for the individual quality indicators of unreinforced brick pillars affecting the fire resistance of the loss of bearing capacity F _{u (R)} , min, take the geometric dimensions, the conditions of support and fastening of the pillars, the size of the smaller side of the transverse cross-sections of columns, the coefficient of longitudinal bending, the strength of the brick and mortar, the elastic characteristic of the masonry, the temporary compressive strength of the masonry, the bearing capacity of the columns and the standard load on them during the fire test, coefficient pass on the bearing capacity of the pillars, the intensity of the power voltage in the dangerous section, humidity indicators, density and thermal conductivity of masonry. 7. The method according to claim 1, characterized in that the non-destructive tests are carried out for a group of the same type of unreinforced brick pillars, the differences between the strength of the brick and mortar which are caused mainly by a random factor. 8. The method according to claim 1, characterized in that the number of tests n _{uc of a} single quality indicator of unreinforced brick pillars with a probability of a result of 0.95 and an accuracy of 5% is taken according to the formula (8)

where υ is the sample coefficient of variation of the test results,%. 9. The method according to claim 1, characterized in that in the case when all the individual quality indicators of unreinforced brick pillars, when M is more than 9 pieces, are within the control limits, the minimum integer number of columns in the sample according to the plan of shortened tests M _min , pieces ., is assigned from condition (9)

where M is the number of the same type of brick pillars in the building, pcs. 10. The method according to claim 1, characterized in that in the case when at least one of the individual quality indicators of unreinforced brick pillars goes beyond the control limits, the minimum number of tested structures in the sample is calculated according to the formula (10)

11. The method according to claim 1, characterized in that in the case when at least one of the individual quality indicators of unreinforced brick pillars goes beyond the permissible limits or M≤5 pieces, all the same building columns are subjected to non-destructive testing individually. 12. The method according to claim 1, characterized in that in the case when at least one of the individual indicators of the quality of the stone walls goes beyond the acceptable limits or M≤5 pcs, all the same type of building walls are subjected to non-destructive testing individually.

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