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

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

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

The cross section of the slab is shown in Fig. 3.8.

b p b p b p b p b p

h h 0

A s

Fig.3.8. Design cross-section of a hollow-core floor slab

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

b´ f

x tem ≤h´ f

h´ f

h h 0

x tem >h´ f

A s

a∑b R

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

Subsequence

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

3. If, then  s , tem determined by the formula

Where instead b used ;

If
, then it must be recalculated using the formula:

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

The Gaussian error function is calculated using the formula:

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

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

Example No. 5.

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

Define actual fire resistance limit of the slab.

Solution:

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

R bu= R bn /0.83 = 15/0.83 = 18.07 MPa

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

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

A s= 615 mm 2 = 61510 -6 m 2

Thermophysical characteristics of concrete:

λ tem = 1.14 – 0.00055450 = 0.89 W/(m·˚С)

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

k= 37.2 p.3.2.8.

k 1 = 0.5 p.3.2.9. .

The actual fire resistance limit is determined:

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

Literature

Shelegov V.G., Kuznetsov N.A. “Buildings, structures and their stability in case of fire.” Textbook for studying the discipline. – Irkutsk: VSI Ministry of Internal Affairs of Russia, 2002. – 191 p.

Shelegov V.G., Kuznetsov N.A. Building construction. Reference Guide in the discipline “Buildings, structures and their stability in case of fire.” – Irkutsk: All-Russian Research Institute of the Ministry of Internal Affairs of Russia, 2001. – 73 p.

Mosalkov I.L. and others. Fire resistance of building structures: M.: ZAO "Spetstekhnika", 2001. - 496 pp., illus.

Yakovlev A.I. Fire resistance calculation building structures. – M.: Stroyizdat, 1988.- 143 p., ill.

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

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

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

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

GOST 30247.0 – 94. Building structures. Fire resistance test methods. General requirements.

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

1BOARDSHIP – a structure on the shore with a specially constructed inclined foundation ( slipway), where the ship's hull is laid and built.

2 Overpass – a bridge across land routes (or over a land route) where they intersect. Movement along them is provided at different levels.

3OVERSTAND – a structure in the form of a bridge for carrying one path over another at the point of their intersection, for berthing ships, and also generally for creating a road at a certain height.

4 STORAGE TANK - container for liquids and gases.

5 GAS HOLDER– a facility for receiving, storing and distributing gas into the gas pipeline network.

6blast furnace- a shaft furnace for smelting cast iron from iron ore.

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

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

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

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

The fire resistance limit of enclosing structures by heating the surface opposite to fire by 140°C (floors, walls, partitions) depends on their thickness, type of concrete and its humidity. With increasing thickness and decreasing density of concrete, the fire resistance limit increases.

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

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

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

Factory-produced large-hollow prestressed decks made of heavy concrete with a protective layer of 20 mm and rod reinforcement made of class A-IV steel have a fire resistance limit of 1 hour, which allows the use of these decks in residential buildings.

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

In some cases, thin-walled flexible structures (hollow and ribbed panels and decking, crossbars and beams with a section width of 160 mm or less, without vertical frames at the supports) may collapse prematurely in the event of a fire along the oblique section at the supports. This type of destruction is prevented by installing vertical frames with a length of at least 1/4 of the span on the supporting areas of these structures.

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

As the aspect ratio of the slab increases, the critical temperature decreases, and therefore the fire resistance limit also decreases. With aspect ratios of more than four, the fire resistance limit is almost equal to the fire resistance limit of slabs supported on two sides.

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

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

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

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

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

Single-layer, double-layer and three-layer self-supporting external wall panels are subject to light loads, so the fire resistance of these walls usually satisfies fire safety requirements.

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

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

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

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

Basic principles for calculating the fire resistance of reinforced concrete structures

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

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

In the thermal engineering part, the temperature is determined across the cross section of the structure during its heating according to the standard temperature regime. In the static part, the load-bearing capacity (strength) of the heated structure is calculated. Then a graph is built (Fig. 3.7) of the decrease in its load-bearing capacity over time. Using this graph, the fire resistance limit is found, i.e. heating time, after which the load-bearing capacity of the structure will decrease to the working load, i.e. when the equality takes place: M rt (N rt) = M n (M n), where M rt (N rt) is the load-bearing capacity of the bending (compressed or eccentrically compressed) structure;

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

The most common material in
construction is reinforced concrete. It combines concrete and steel reinforcement,
rationally laid out in a structure to absorb tensile and compressive forces
effort.

Concrete resists compression well and
worse - sprain. This feature of concrete is unfavorable for bending and
stretched elements. The most common flexible building elements
are slabs and beams.

To compensate for unfavorable
concrete processes, structures are usually reinforced with steel reinforcement. Reinforce
slabs with welded meshes consisting of rods located in two mutually
perpendicular directions. The grids are laid in slabs in such a way that
the rods of their working reinforcement were located along the span and perceived
tensile forces arising in structures when bending under load, in
in accordance with the diagram of bending loads.

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

Providing fire protection
building safety requires increased fire resistance and fire safety
reinforced concrete structures. The following technologies are used for this:

• reinforcement of slabs
  only knitted or welded frames, and not loose individual rods;
• to avoid buckling of the longitudinal reinforcement when it is heated in
  during a fire, it is necessary to provide structural reinforcement with clamps or
  cross bars;
• the thickness of the lower protective layer of the floor concrete should be
  sufficient so that it warms up no higher than 500°C and after a fire does not
  influenced the further safe operation of the structure.
  Research has established that with the normalized fire resistance limit R=120, the thickness
  the protective layer of concrete must be at least 45 mm, at R=180 - at least 55 mm,
  at R=240 - no less than 70 mm;
• in a protective layer of concrete at a depth of 15–20 mm from the bottom
  the floor surface should be provided with anti-splinter reinforcement mesh
  made of wire with a diameter of 3 mm with a mesh size of 50–70 mm, reducing intensity
  explosive destruction of concrete;
• strengthening the supporting sections of thin-walled transverse floors
  reinforcement not provided for in the usual calculations;
• increasing the fire resistance limit due to the arrangement of the slabs,
  supported along the contour;
• the use of special plasters (using asbestos and
  perlite, vermiculite). Even with small sizes of such plasters (1.5 - 2 cm)
  the fire resistance of reinforced concrete slabs increases several times (2 - 5);
• increasing the fire resistance limit due to a suspended ceiling;
• protection of components and joints of structures with a layer of concrete with the required
  fire resistance limit.

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

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

Determination of fire resistance limits of building structures

Determination of the fire resistance limit of reinforced concrete structures

Initial data for reinforced concrete slab ceilings are shown in table 1.2.1.1

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

1) Effective thickness of a hollow-core slab teff for assessing the fire resistance limit based on thermal insulation ability according to clause 2.27 of the Manual to SNiP II-2-80 (Fire resistance):

2) Determine according to the table. 8 Manuals fire resistance limit of a slab based on loss of thermal insulation capacity for a slab made of lightweight concrete with an effective thickness of 140 mm:

Fire resistance limit of the slab is 180 min.

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

4) Using table 1.2.1.2 (Table 8 of the Manual), we determine the fire resistance limit of the slab based on the loss of load-bearing capacity at a = 40 mm, for lightweight concrete when supported on two sides.

Table 1.2.1.2

Fire resistance limits of reinforced concrete slabs

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

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

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

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

8) Correction factor for load according to clause 2.20 of the Manual:

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

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

The fire resistance limit of the slab in terms of load-bearing capacity is

Based on the results obtained during the calculations, we found that the fire resistance limit of a reinforced concrete slab in terms of load-bearing capacity is 139 minutes, and in terms of thermal insulation capacity is 180 minutes. It is necessary to take the lowest fire resistance limit.

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

Determination of fire resistance limits of reinforced concrete columns

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

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

1) Determine the distance from the heated surface of the column to the axis of the rod reinforcement using the formula:

2) According to clause 2.15 of the Manual for structures made of concrete with carbonate filler, the cross-sectional size can be reduced by 10% with the same fire resistance limit. Then we determine the width of the column using the formula:

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

Table 1.2.2.2

Fire resistance limits of reinforced concrete columns

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

ON THE QUESTION OF CALCULATING BEAMLESS SLOBS FOR FIRE RESISTANCE

ON THE QUESTION OF CALCULATING BEAMLESS SLOBS FOR FIRE RESISTANCE

V.V. Zhukov, V.N. Lavrov

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

Let's consider the calculation of the fire resistance limit of a beamless floor using an example that is quite common in construction practice. The beamless reinforced concrete floor has a thickness of 200 mm from concrete of compression class B25, reinforced with a mesh with cells 200x200 mm from class A400 reinforcement with a diameter of 16 mm with a protective layer of 33 mm (to the center of gravity of the reinforcement) at the bottom surface of the floor and A400 with a diameter of 12 mm with a protective layer 28 mm (to center point) at the top surface. The distance between columns is 7m. In the building under consideration, the floor is a fire barrier of the first type and must have a fire resistance limit for loss of thermal insulation capacity (I), integrity (E) and load-bearing capacity (R) REI 150. An assessment of the fire resistance limit of the floor according to existing documents can be determined by calculation only by thickness protective layer (R) for a statically definable structure, according to the thickness of the floor (I) and the possibility of brittle destruction in a fire (E). In this case, a fairly correct estimate is given by calculations of I and E, and the load-bearing capacity of the floor in a fire as a statically indeterminate structure can only be determined by calculating the thermally stressed state, using the theory of elastic-plasticity of reinforced concrete when heated or the theory of the limit equilibrium method of a structure under the action of static and thermal loads in a fire . The last theory is the simplest, since it does not require determining the stresses from the static load and temperature, but only the forces (moments) from the action of the static load, taking into account the change in the properties of concrete and reinforcement when heated until plastic hinges appear in the statically indeterminate structure when it turns into mechanism. In this regard, the assessment of the load-bearing capacity of a beamless floor during a fire was made using the limit equilibrium method, and in relative units to the load-bearing capacity of the floor under normal operating conditions. Working drawings of the building were reviewed and analyzed, calculations were made of the fire resistance limits of a reinforced concrete beamless floor based on the occurrence of limit state signs normalized for these structures. The calculation of fire resistance limits based on load-bearing capacity was carried out taking into account changes in the temperature of concrete and reinforcement during 2.5 hours of standard tests. All thermodynamic and physical-mechanical characteristics of construction materials given in this report are based on data from VNIIPO, NIIZHB, TsNIISK.

FIRE RESISTANCE LIMIT OF COVERING BY LOSS OF THERMAL INSULATING ABILITY (I)

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

Using formula (5), we determine the temperature distribution over the thickness of the floor after 2.5 hours of fire. Using formula (6), we determine the thickness of the floors, which is necessary to achieve a critical temperature of 220C on its unheated surface in 2.5 hours. This thickness is 97 mm. Consequently, a 200 mm thick floor will have a fire resistance limit for loss of thermal insulation capacity of at least 2.5 hours.

FIRE RESISTANCE LIMIT OF FLOOR PLATE BY LOSS OF INTEGRITY (E)

In case of fire in buildings and structures that use concrete and reinforced concrete structures, brittle destruction of concrete is possible, which leads to loss of structural integrity. Destruction occurs suddenly, quickly and is therefore the most dangerous. Brittle destruction of concrete begins, as a rule, 5-20 minutes after the start of fire exposure and manifests itself as the breaking off of pieces of concrete from the heated surface of the structure; as a result, the structure may appear through hole, i.e. the structure can achieve premature fire resistance due to loss of integrity (E). Brittle destruction of concrete may be accompanied by a sound effect in the form of a light pop, a crack of varying intensity, or an “explosion.” In the case of brittle fracture of concrete, pieces weighing up to several kilograms can scatter over a distance of up to 10-20 m. In a fire, the greatest influence on the brittle fracture of concrete is exerted by: intrinsic temperature stresses from the temperature gradient across the cross section of the element, stresses from the static indetermination of structures, from external loads and from steam filtration through the concrete structure. The brittle destruction of concrete in a fire depends on the structure of the concrete, its composition, humidity, temperature, boundary conditions and external load, i.e. it depends both on the material (concrete) and on the type of concrete or reinforced concrete structure. The fire resistance limit of a reinforced concrete floor based on loss of integrity can be assessed by the value of the brittle fracture criterion (F), which is determined by the formula given in:

FIRE RESISTANCE LIMIT OF THE SLOVER BY LOSS OF LOAD-LOADING CAPACITY (R)

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

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

Let us determine the corresponding design load-bearing capacity of the floor R3 under normal operating conditions for a floor with a thickness of 200 mm, at the height of the compressed zone for the middle hinge at xc = ; shoulder of the internal pair Zc = 15.8 cm and the height of the compressed zone of the left and right hinges Xc = Xn = 1.34 cm, shoulder of the internal pair Zx = Zn = 16.53 cm. Design load-bearing capacity of the floor R3 with a thickness of 20 cm at 20 C.

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

CONCLUSIONS

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

Literature

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