How to increase the fire resistance of reinforced concrete coatings and floors? Determination of fire resistance limits of reinforced concrete columns Fire resistance limit of ribbed floor slabs.

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Table 2.18

Lightweight concrete density? = 1600 kg/m3 with coarse expanded clay aggregate, slabs with round voids in the amount of 6 pieces, the slabs are supported freely on both sides.

1. Let’s determine the effective thickness of the hollow-core slab teff to assess the fire resistance limit based on thermal insulation ability according to clause 2.27 of the Manual:

where is the thickness of the slab, mm;

- slab width, mm;
- number of voids, pcs.;
- diameter of voids, mm.
2. Determine according to the table. 8 Guidelines for the fire resistance limit of a slab based on the loss of thermal insulation capacity for a slab made of heavy concrete part with an effective thickness of 140 mm:

Fire resistance limit of the slab based on loss of thermal insulation ability

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

where is the thickness of the protective layer of concrete, mm;

- diameter of working fittings, mm.
4. According to table. 8 Manuals We determine the fire resistance limit of a slab based on the loss of load-bearing capacity at a = 24 mm, for heavy concrete and when supported on two sides.

The required fire resistance limit is in the range between 1 hour and 1.5 hours, we determine it by linear interpolation:

The fire resistance limit of the slab without taking into account correction factors is 1.25 hours.

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 slab 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 a) Benefits, do we accept the coefficient? for A-VI fittings:
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 R 98.

The fire resistance limit of the slab is taken to be the lesser of two values - the loss of thermal insulation capacity (180 min) and the loss of load-bearing capacity (98 min).

Conclusion: fire resistance limit reinforced concrete slab is REI 98

To solve the static part of the problem, we reduce the cross-sectional shape of a reinforced concrete floor slab with round voids (Appendix 2, Fig. 6) to the calculated T-shaped one.

Let us determine the bending moment in the middle of the span due to the action of the standard load and the slab’s own weight:

Where q / n– standard load per 1 linear meter of slab, equal to:

The distance from the bottom (heated) surface of the panel to the axis of the working fittings will be:

mm,

Where d– diameter of reinforcing bars, mm.

The average distance will be:

mm,

Where A– cross-sectional area of the reinforcing bar (clause 3.1.1.), mm 2.

Let us determine the main dimensions of the calculated T-section of the panel:

Width: b f = b= 1.49 m;

Height: h f = 0,5 (h-П) = 0.5 (220 – 159) = 30.5 mm;

Distance from the unheated surface of the structure to the axis of the reinforcing bar h o = h – a= 220 – 21 = 199 mm.

We determine the strength and thermophysical characteristics of concrete:

Standard tensile strength R bn= 18.5 MPa (Table 12 or clause 3.2.1 for concrete class B25);

Reliability factor  b = 0,83 ;

Design strength of concrete by ultimate strength R bu = R bn /  b= 18.5 / 0.83 = 22.29 MPa;

Coefficient of thermal conductivity  t = 1,3 – 0,00035T Wed= 1.3 – 0.00035 723 = 1.05 W m -1 K -1 (clause 3.2.3.),

Where T Wed– average temperature during a fire equal to 723 K;

Specific heat WITH t = 481 + 0,84T Wed= 481 + 0.84 · 723 = 1088.32 J kg -1 K -1 (section 3.2.3.);

Given thermal diffusivity coefficient:

Coefficients depending on the average density of concrete TO= 39 s 0.5 and TO 1 = 0.5 (clause 3.2.8, clause 3.2.9.).

Determine the height of the compressed zone of the slab:

We determine the stress in tensile reinforcement from an external load in accordance with the adj. 4:

because X t= 8.27 mm h f= 30.5 mm, then

Where As– the total cross-sectional area of reinforcing bars in the tensile zone of the cross-section of the structure, equal for 5 bars12 mm 563 mm 2 (clause 3.1.1.).

Let us determine the critical value of the coefficient of change in the strength of reinforcing steel:

Where R su– design resistance of the reinforcement in terms of ultimate strength, equal to:

R su = R sn /  s= 390 / 0.9 = 433.33 MPa (here  s– reliability factor for reinforcement, taken equal to 0.9);

R sn– standard tensile strength of reinforcement equal to 390 MPa (Table 19 or clause 3.1.2).

Got that  stcr1. This means that the stresses from the external load in the tensile reinforcement exceed the standard resistance of the reinforcement. Therefore, it is necessary to reduce the stress from the external load in the reinforcement. To do this, we will increase the number of reinforcing bars of the panel12mm to 6.Then A s= 679 10 -6 (section 3.1.1.).

MPa,

Let us determine the critical heating temperature of the load-bearing reinforcement in the tension zone.

According to the table in clause 3.1.5. Using linear interpolation, we determine that for class A-III reinforcement, steel grade 35 GS and  stcr = 0,93.

t stcr= 475C.

The time it takes for the reinforcement to warm up to the critical temperature for a slab of solid cross-section will be the actual fire resistance limit.

s = 0.96 h,

Where X– argument of the Gaussian (Crump) error function equal to 0.64 (clause 3.2.7.) depending on the value of the Gaussian (Crump) error function equal to:

(Here t n– the temperature of the structure before the fire is taken equal to 20С).

The actual fire resistance limit of a floor slab with round voids will be:

P f =  0.9 = 0.960.9 = 0.86 hours,

where 0.9 is a coefficient that takes into account the presence of voids in the slab.

Since concrete is a non-combustible material, then, obviously, the actual fire hazard class of the structure is K0.

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 further safe operation designs.
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.

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 made of concrete with compression class B25, mesh reinforced with cells 200x200 mm from A400 class reinforcement with a diameter of 16 mm with a protective layer of 33 mm (to the center of gravity of the reinforcement) at the lower surface of the ceiling and A400 with a diameter of 12 mm with a protective layer of 28 mm (to the center of gravity) at the upper 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 be determined only 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 in normal conditions operation. 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 and mechanical characteristics 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 initial conditions: temperature of structures and external environment 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. 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. Fire resistance limit assessment reinforced concrete floor loss of integrity can be achieved 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 physical and mechanical properties 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

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.
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.
This beamless reinforced concrete floor can serve as a fire barrier, the first type according to.
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

Instructions for calculating the actual fire resistance limits of reinforced concrete building structures based on the use of computers. – M.: VNIIPO, 1975.
GOST 30247.0-94. Building structures. Test methods for fire resistance. M., 1994. – 10 p.
SP 52-101-2003. Concrete and reinforced concrete structures without prestressing reinforcement. – M.: FSUE TsPP, 2004. –54 p.
SNiP-2.03.04-84. Concrete and reinforced concrete structures designed to operate in conditions of high and high temperatures. – M.: CITP Gosstroy USSR, 1985.
Recommendations for calculating fire resistance limits of concrete and reinforced concrete structures. – M.: Stroyizdat, 1979. – 38 p.
SNiP-21-01-97* Fire safety buildings and structures. State Unitary Enterprise TsPP, 1997. – 14 p.
Recommendations for the protection of concrete and reinforced concrete structures from brittle destruction in fire. – M.: Stroyizdat, 1979. – 21 p.
Recommendations for the design of hollow-core floor slabs with the required fire resistance. – M.: NIIZhB, 1987. – 28 p.
Guide to the calculation of statically indeterminate reinforced concrete structures. – M.: Stroyizdat, 1975. P.98-121.
Methodological recommendations for calculating fire resistance and fire safety of reinforced concrete structures (MDS 21-2.000). – M.: NIIZhB, 2000. – 92 p.
Gvozdev A.A. Calculation of the bearing capacity of structures using the limit equilibrium method. State publishing house of construction literature. – M., 1949.

Determination of fire resistance limits of building structures

Determination of the fire resistance limit of reinforced concrete structures

The initial data for the reinforced concrete floor slab is given in Table 1.2.1.1

Type of concrete - lightweight concrete 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 from 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

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