Fire resistance limit of hollow plates. Determination of fire resistance limits of reinforced concrete columns. To the question of calculating the booming overlaps on fire resistance

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

In this regard, the calculation of fire resistance multi-back plate Overlapping will be determined by the warm-up time until the critical temperature of the stretched working fittings.

The slab cross section is represented in Fig.3.8.

b. p. b. p. b. p. b. p. b. p.

h. h. 0

A. s.

Fig.3.8. Settlement section of a crowded floor slab

To calculate the plate, its cross section is given to the taving (Fig. 3.9).

b' f.

x. tem. ≤h' f.

h' f.

h H. 0

x. tem. \u003e h' f.

A. s.

a ΣB. r

Fig.3.9. Taving section of a crowded plate for calculating it on fire resistance

Sequence

calculation of the limit of fire resistance of flat bends of multi-console reinforced concrete elements

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

Where instead b. used ;

If a
, it must be counted according to the formula:

3.1.5 is determined t. s. , cR (Critical temperature).

The Gauss error function is calculated by the formula:

3.2.7 is the argument of the Gauss function.

The limit of fire resistance n f by the formula is calculated:

Example number 5.

Dano. Multiplass slab overlap, freely based on two sides. Size dimensions: b.\u003d 1200 mm, work span length l. \u003d 6 m, section height h. \u003d 220 mm, protective layer thickness but l. \u003d 20 mm, stretched fittings of class A-III, 4 rod Ø14 mm; Heavy Class B20 concrete on limestone crushed it, webiness of concrete w. \u003d 2%, average concrete density in dry condition ρ 0s \u003d 2300 kg / m 3, void diameter d. n. \u003d 5.5 kN / m.

Determine The actual fire resistance plate.

Decision:

For concrete class B20 R. bN. \u003d 15 MPa (p. 3.2.1.)

R. bu. \u003d R bn / 0.83 \u003d 15 / 0.83 \u003d 18.07 mp

For fittings class A-III R. sN. \u003d 390 MPa (clause 3.1.2.)

R. sU. \u003d R Sn / 0.9 \u003d 390 / 0.9 \u003d 433.3 MPa

A. s. \u003d 615 mm 2 \u003d 61510 -6 m 2

Thermophysical characteristics of concrete:

λ TEM \u003d 1.14 - 0.00055450 \u003d 0.89 W / (M · ˚С)

with TEM \u003d 710 + 0.84450 \u003d 1090 J / (kg · ˚С)

k. \u003d 37.2 p.3.2.8.

k. 1 \u003d 0.5 p.3.2.9. .

The actual limit of fire resistance is determined:

Taking into account the hollowness of the plate, its actual limit of fire resistance must be multiplied by the coefficient of 0.9 (paragraph 2.27.).

Literature

Shelegov V.G., Kuznetsov N.A. "Buildings, facilities and their fire sustainability." Textbook on the study of discipline. - Irkutsk.: WSI of the Ministry of Internal Affairs of Russia, 2002. - 191 p.

Shelegov V.G., Kuznetsov N.A. Building construction. Reference manual for the discipline "Buildings, structures and their stability in the fire". - Irkutsk.: WSI of the Ministry of Internal Affairs of Russia, 2001. - 73 p.

Mosalkov I.L. and others. Fire resistance of building structures: M.: SpecTeechnic CJSC, 2001. - 496 p., Ill

Yakovlev A.I. Calculation of fire resistance building structures. - M.: Stroyzdat, 1988.- 143С., IL.

Shelegov V.G., Chernov Y.L. "Buildings, facilities and their fire sustainability." Manual for the implementation of the course project. - Irkutsk.: WSI of the Ministry of Internal Affairs of Russia, 2002. - 36 p.

Manual for determining the limits of fire resistance of structures, the limits of the spread of fire in designs and groups of material marks (K SNIP II-2-80), CNIIs. Kucherenko. - M.: Stroyzdat, 1985. - 56 p.

GOST 27772-88: Rental for construction steel structures. General technical conditions/ Gosstroy USSR. - M., 1989

Snip 2.01.07-85 *. Loads and exposure / Gosstroy USSR. - M.: CITP Gosstroy USSR, 1987. - 36 p.

GOST 30247.0 - 94. Construction construction. Fire test methods. General requirements.

SNIP 2.03.01-84 *. Concrete and reinforced concrete structures / MinStroy Russia. - M.: GP CPP, 1995. - 80 s.

1Elling -construction on the shore with a specially arranged inclined foundation ( stipel), where the hull of the vessel is laid and built.

2 Overpass -bridge through the land paths (or above the land way) on the place of their intersection. Movement on them in different levels is ensured.

3Estakada -the construction in the form of a bridge for holding one path over the other at the place of their intersection, for the pier of the courts, and in general to create a road at some height.

4 STORAGE TANK - Capacity for liquids and gases.

5 Gazgolder - Construction for Acceptance, Storage and Gas Vacation in the gas pipeline.

6blast furnace - Shaft furnace for smelting cast iron from iron ore.

7Critical temperature- The temperature at which the normative impedance of metalR Unit decreases to the value of the normative voltage Н from the external load on the structure, i.e. at which the loss of bearing ability comes.

8 Tower-keys or metallic rod used to fasten parts of wooden structures.

Reinforced concrete structures due to their non-combustible and relatively low thermal conductivity quite well resist the effects of aggressive fire factors. However, they can not be able to resist fire. Modern reinforced concrete structures are usually performed by thin-walled, without monolithic communication with other elements of the building, which limits their ability to carry out their operating functions in a fire conditions up to 1 h, and sometimes less. Moistheld reinforced concrete structures have a smaller limit of fire resistance. If the increase in the humidity of the structure up to 3.5% increases the limit of fire resistance, then further increase in the humidity of the concrete with a density of more than 1200 kg / m 3 with short-term fire action can cause a burst of concrete and rapid destruction of the design.

The limit of the fire resistance of the reinforced concrete structure depends on the size of its cross section, the thickness of the protective layer, the form, the amount and diameter of the reinforcement, the concrete class and the type of aggregate, the load on the design and the design of its support.

The limit of fire resistance of the enclosing structures on heating - the opposite surface of the surface at 140 ° C (overlapping, walls, partitions) depends on their thickness, type of concrete and its humidity. With increasing thickness and decrease in concrete density, the limit of fire resistance increases.

Fire resistance limit carrier ability Depends on the type and static design of the design. Unpaired freely opened bending elements (beam plates, panels and flooring, beams, beams) in the action of fire are destroyed as a result of heating the longitudinal bottom working reinforcement to the limiting critical temperature. The limit of fire resistance of these structures depends on the thickness of the protective layer of the lower working reinforcement, the class of fittings, workload and thermal conductivity of concrete. At beams and runs, the limit of fire resistance depends on the width of the section.

With the same design parameters, the limit of fire resistance beams is less than the plates, since the beams are heated with three sides (from the side of the lower and two side faces), and the plates are only on the side of the lower surface.

Best reinforcement steel from the point of view of fire resistance is steel class A-III brand 25G2C. The critical temperature of this steel at the time of the occurrence of the fire resistance limit of the structure loaded by the regulatory load is 570 ° C.

Large-standing pre-stressed flooring of heavy concrete with a protective layer of 20 mm and a rod fit of steel class A-IV have a limit of fire resistance of 1 hour, which allows the use of filling data in residential buildings.

The plates and panels of solid cross-section from the usual reinforced concrete with a protective layer of 10 mm have fire resistance: reinforcement of steel classes a-i and A-II - 0.75 h; A-III (25G2C) - 1 h.

In some cases, thin-walled bends (hollow and ribbed panels and flooring, riglels and beams with a width of a section of 160 mm and less that do not have vertical frames at the supports) under the action of a fire can be collapsed prematurely by oblique cross section of the supports. This nature of the destruction is prevented by installing on the pre-portions of these designs of vertical frameworks with a length of at least 1/4 of the span.

Plates, opened along the contour, have a limit of fire resistance significantly higher than simple bending elements. These plates are reinforced by the working reinforcement in two directions, so their fire resistance depends on the ratio of reinforcement in a short and long span. In square plates with this ratio, equal to one, the critical temperature of the reinforcement at the occurrence of the fire resistance limit is 800 ° C.

With an increase in the aspect ratio of the slab, the critical temperature decreases, therefore, the limit of fire resistance is reduced. With the ratios of the parties, more than four fire resistance limits are almost equal to the limit of fire resistance of plates, opened on two sides.

Statically indefinable beams and beam plates during heating lose the carrying capacity as a result of the destruction of reference and span sections. The cross sections in the span are destroyed as a result of a decrease in the strength of the lower longitudinal reinforcement, and the support sections - due to the loss of concrete strength in the lower compressed zone heating to high temperatures. The heating rate of this zone depends on the size cross sectiontherefore, the fire resistance of statically indefinable beams depends on their thickness, and the beams are from the width and height of the section. With large cross-sectional size, the limit of fire resistance of the designs under consideration is significantly higher than the statically determined structures (single-span freely opened beams and plates), and in some cases (in thick beams, at the beams with strong upper support fittings) practically does not depend on the thickness of the protective Layer at longitudinal lower reinforcement.

Columns. The fire resistance limit of the columns depends on the scheme of the application of the load (central, outcidentary), the size of the cross section, the percentage of reinforcement, the type of concrete aggregate and the thickness of the protective layer at the longitudinal reinforcement.

The destruction of the columns during heating occurs as a result of reducing the strength of the reinforcement and concrete. Especially the load application reduces the fire resistance of the columns. If the load is applied with a large eccentricity, the fire resistance of the column will depend on the thickness of the protective layer in the stretched fittings, i.e. The nature of such columns when heated is the same as simple beams. Fire resistance of the column with a small eccentricity approaches the fire resistance of the centrally compressed columns. Concrete columns on granite rubble Possess less fire resistance (by 20%) than columns on lime crushed. This is explained by the fact that granite begins to collapse at a temperature of 573 ° C, and limestones begin to collapse at the temperature of the beginning of their firing 800 ° C.

Walls. In case of fires, as a rule, the walls are heated on the one hand and therefore bend or towards the fire, or in the opposite direction. The wall from the central-compressed design turns into an eccentrally compressed with an eccentricity-increasing time. Under these conditions, fire resistance carriers largely depends on the load and from their thickness. With an increase in the load and reducing the thickness of the wall, its limit of fire resistance decreases, and vice versa.

With increasing floors of buildings, the load on the wall increases, therefore, to provide the necessary fire resistance, the thickness of the carrier transverse walls in residential buildings take equal (mm): in 5 ... 9-storey buildings - 120, 12-storey - 140, 16-storey - 160 , in houses with a height of more than 16 floors - 180 or more.

Single-layer, two-layer and three-layer self-supporting panels of exterior walls are exposed to small loads, so the fire resistance of these walls usually meets fireproof requirements.

The bearing ability of walls under the action of high temperature is determined not only by change strength characteristics Concrete and steel, but mainly the deformativity of the element as a whole. The fire resistance of the walls is determined, as a rule, loss of bearing capacity (destruction) in the heated state; The sign of heating the "cold" wall surface is 140 ° C is not characteristic. Fire resistance limit is depending on the workload (stock strength). The destruction of walls from one-sided exposure occurs in one of three schemes:

1) with the irreversible development of the deflection towards the heated surface of the wall and its destruction in the middle of the height on the first or second case of the extracentrate compression (on the heated fittings or the "cold" concrete);
2) with the deflection of the element at the beginning towards heating, and at the final stage in the opposite direction; Destruction - in the middle of height along the heated concrete or on "cold" (stretched) fittings;
3) from a variable direction of deflection, as in Scheme 1, but the destruction of the wall occurs in the prevail zones along the cold "surface or on oblique sections.

The first destruction scheme is characteristic of flexible walls, the second and third - for the walls with less flexibility and platform are supported. If you limit the freedom of rotation of the wall supporting sections, as is the case with platform support, its deformability decreases and therefore the fire resistance limit increases. Thus, the platform support of the walls (for non-displaced plane) increased the limit of fire resistance on average twice as compared to hinged content regardless of the destruction scheme of the element.

Reducing the percentage of wall reinforcement with hinged content reduces the limit of fire resistance; With the platform support, the change in the usual limits of the reinforcement of the walls on their fire resistance practically does not affect. When the wall is heated simultaneously on both sides (interroom walls), it does not occur with temperature deflection, the design continues to work on the central compression and therefore the limit of fire resistance is not lower than in the case of one-sided heating.

Basic principles for calculating fire resistance of reinforced concrete structures

Fire resistance of reinforced concrete structures is lost, as a rule, as a result of the loss of the bearing capacity (collapse) by reducing the strength, thermal expansion and temperature creep of the reinforcement and concrete when heated, as well as the warm-up of the surface of the surface by 140 ° C. According to these indicators - The limit of fire resistance of reinforced concrete structures can be found by calculation.

IN general The calculation consists of two parts: heat engineering and static.

In the heat engineering part determine the temperature in cross section of the structure in the process of its heating according to the standard temperature regime. In the static part, the carrying ability (strength) of heated design is calculated. Then build a graph (Fig. 3.7) to reduce its carrier ability over time. For this graphics find the limit of fire resistance, i.e. The heating time, after which the carrying capacity of the structure decreases to the workload, i.e. When there is an equality: M q (n pt) \u003d M n (M n), where M q (N РT) is the bending ability (compressed or echocently compressed) design;

M n (M n), - bending moment (longitudinal force) from the regulatory or other workload.

The most common material in
Construction is reinforced concrete. It combines concrete and steel reinforcement,
rationally laid in the design for the perception of tensile and compressive
effort.

Concrete is well resisting compression and
Worse - stretching. This feature of concrete is unfavorable for bends and
stretched elements. The most common bent elements of the building
are plates and beams.

For adverse compensation
Concrete processes, construction is made to reinforce steel reinforcement. Reinforced
Plates welded gridsconsisting of rods located in two mutually
perpendicular directions. Grids are stacked in stoves in such a way that
The rods of their working reinforcement were located along the span and perceived
Stretching efforts arising in bending structures under load in
accordance with the plot of bending loads.

IN
Fire conditions plates are exposed to high temperatures below,
Reducing their bearing capacity takes place mainly due to reduction
The strength of the heating stretched fittings. As a rule, such elements
collapsed as a result of the formation of a plastic hinge in a cross section with
Maximum bending torque due to reduction of strength
Heating stretched fittings up to the magnitude of operating stresses in its section.

Providing fireman
Building safety requires increased fire resistance and fire-circuit
reinforced concrete structures. For this, the following technologies are used:

reinforcement plates produce
only with knitted or welded frames, and not separate stems of the scatter;
to avoid releasing the longitudinal reinforcement when it is heated in
The fire time must be provided for constructive reinforcement of clamps or
transverse rods;
the thickness of the lower protective layer of the cement concrete should be
sufficient so that it warms up not above 500 ° C and after the fire is not
influenced further safe operation Designs.
Studies found that with the normalized fire resistance limit R \u003d 120, thickness
The protective layer of concrete should be at least 45 mm, at r \u003d 180 - at least 55 mm,
at r \u003d 240 - not less than 70 mm;
in a protective layer of concrete at a depth of 15-20 mm from the side of the lower
The surface of the overlap should be provided for the anti-hard reinforcement grid
From the wire with a diameter of 3 mm with a cell size of 50-70 mm, reduced intensity
explosive destruction of concrete;
strengthening the pre-prigid sections of thin-walled crossing
reinforcement not provided for by the usual calculation;
increasing fire resistance due to plates location,
Opened contour;
the use of special plasters (using asbestos and
Perlite, vermiculitis). Even with low values \u200b\u200bof such plasters (1.5 - 2 cm)
Fire resistance of reinforced concrete slabs increases several times (2 - 5);
an increase in fire resistance due to suspended ceiling;
protection of nodes and joint designs with concrete layer with the required
The limit of fire resistance.

These measures will provide proper fire safety of the building.
Reinforced concrete design will acquire the necessary fire resistance and
Fire cohesion.

Used Books:
1. Women and structures, and their stability
With fire. Academy of GPS Emergencies Ministry of Russia, 2003
2. MDS 21-2.2000.
Methodical recommendations for calculating fire resistance of reinforced concrete structures.
- M.: GUP "NIIZB", 2000. - 92 p.

Determination of fire resistance of building structures

Determination of the limit of fire resistance of reinforced concrete structures

Source data for reinforced concrete plate The overlaps are shown in Table 1.2.1.1

The type of concrete is a lightweight concrete with a density C \u003d 1600 kg / m3 with a large aggregate of clay; Multiplass plates, with round voids, the amount of emptiness - 6 pcs, supporting plates - on two sides.

1) Effective thickness of the TEF multi-consistency plate to estimate the limit of fire resistance by heat insulating ability in accordance with clause 2.27 SNIP II-2-80 (fire resistance):

2) Determine the table. 8 benefits limit of fire resistance plate for the loss of thermal insulating capacity for the slab lung concrete With an effective thickness of 140 mm:

Fire resistance limit 180 min.

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

4) Table 1.2.1.2 (Table 8 of the manual) determine the limit of fire resistance plate for loss of carrier capacity at a \u003d 40 mm for lightweight concrete when working on two sides.

Table 1.2.1.2

Fire resistance limits of reinforced concrete slabs

The desired limit of fire resistance 2 h or 120 minutes.

5) According to paragraph 2.27, the benefits for determining the limit of fire resistance of hollow slabs are applied by a lower coefficient of 0.9:

6) Determine the full load on the plates as the amount of constant and temporary loads:

7) Determine the ratio of a long existing part of the load to the full load:

8) correction coefficient for load according to paragraph 2.20 of the manual:

9) by paragraph 2.18 (part 1 b) benefits accept the coefficient for reinforcement

10) Determine the limit of fire resistance of the slab taking into account the load coefficients and on the valve:

The limit of fire resistance plate on the bearing capacity is

Based on the results of the calculations obtained during the calculations, we obtained that the limit of fire resistance of the reinforced concrete slab on the supporting capacity of 139 min., And the heat insulating capacity of 180 minutes. It is necessary to take the smallest limit of fire resistance.

Conclusion: The limit of fire resistance reinforced concrete plate 139.

Determination of the limits of fire resistance of reinforced concrete columns

The type of concrete is a heavy concrete with a density of C \u003d 2350 kg / m3 with a large aggregate of carbonate rocks (limestone);

Table 1.2.2.1 (Table 2 allowances) are given values \u200b\u200bof the actual limits of fire resistance (Pof) reinforced concrete columns from various characteristics. In this case, the POP is determined not by the thickness of the protective layer of concrete, but at the distance from the surface of the structure to the axis of the work reinforcement rod (), which includes in addition to the thickness of the protective layer, also half the diameter of the working reinforcement rod.

1) Determine the distance from the heated surface of the column to the axis of the rod fittings by the formula:

2) According to paragraph 2.15, the manual for concrete structures with carbonate aggregate The size of the cross section is allowed to be reduced by 10% with the same fire resistance limit. Then the column width define the formula:

3) Table 1.2.2.2 (Table 2 of the manuals) determine the limit of fire resistance for a lung concrete column with parameters: b \u003d 444 mm, a \u003d 37 mm when heating the columns from all sides.

Table 1.2.2.2

Fire resistance limits of reinforced concrete columns

The required limit of fire resistance is in the range between 1.5 hours and 3 hours. To determine the limit of fire resistance, we use the linear interpolation method. Data is shown in Table 1.2.2.3

To solve the static part of the problem, the shape of the cross section of the reinforced concrete slab overlap with round voids (Appendix 2. Fig. 6.) Lead to the calculated brake.

We determine the bending moment in the middle of the span from the action of the regulatory load and the own weight of the slab:

where q. / n. – regulatory load For 1 mongo meter plate, equal:

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

mm,

where d.- diameter of reinforcement rods, mm.

The average distance will be:

mm,

where BUT- cross-sectional area of \u200b\u200bthe reinforcement rod (clause 3.1.1.), Mm 2.

We define the main dimensions of the calculated brake cross section of the panel:

Width: b. f. = b.\u003d 1.49 m;

Height: h. f. = 0,5 (h.-P) \u003d 0.5 (220 - 159) \u003d 30.5 mm;

Distance from non-heated design surface to the axis of the reinforcement rod h. o. = h. – a.\u003d 220 - 21 \u003d 199 mm.

Determine the strength and thermophysical characteristics of concrete:

Regulatory resistance to the strength limit R. bN. \u003d 18.5 MPa (Table 12 or clause 3.2.1 for concrete class B25);

Reliability coefficient  b. = 0,83 ;

Calculated concrete resistance on the limit of strength R. bu. = R. bN. /  b. \u003d 18.5 / 0.83 \u003d 22.29 MPa;

Coefficient of thermal conductivity  t. = 1,3 – 0,00035T. cf. \u003d 1.3 - 0.00035 723 \u003d 1.05 W M -1 to -1 (clause 3.2.3.),

where T. cf. - average temperature in a fire, equal to 723 K;

Specific heat FROM t. = 481 + 0,84T. cf. \u003d 481 + 0.84 · 723 \u003d 1088.32 Jg kg -1 to -1 (p. 3.2.3.);

The reduced coefficient of temperature ward:

Coefficients depending on the average density of concrete TO\u003d 39 with 0.5 and TO 1 \u003d 0.5 (clause 3.2.8, paragraph 3.2.9.).

We determine the height of the compressed zone of the slab:

We determine the voltage in the stretched fittings from the external load in accordance with the ad. four:

as h. t. \u003d 8.27 mm h. f. \u003d 30.5 mm, then

where As- the total cross-sectional area of \u200b\u200bthe reinforcement rods in the stretched cross-sectional area of \u200b\u200bthe structure, equal to 5 rods12 mm 563 mm 2 (clause 3.1.1.).

We define the critical value of the coefficient of change of the strength of reinforcement steel:

where R. sU. - Estimated immaturity resistance on the limit of strength, equal:

R. sU. = R. sN. /  s. \u003d 390 / 0.9 \u003d 433.33 MPa (here  s. - reliability coefficient for reinforcement, taken equal to 0.9);

R. sN. - regulatory immaturity resistance on the limit of strength equal to 390 MPa (Table 19 or clause 3.1.2).

Received that  sTCR. 1. So, the voltages from the external load in the stretched fittings exceed the regulatory resistance of the reinforcement. Consequently, it is necessary to reduce the voltage from the external load in the valve. To do this, we increase the number of reinforcement rods panel 112mm to 6.Then A. s. \u003d 679 10 -6 (clause 3.1.1.).

MPa

We define the critical temperature of heating the carrier reinforcement in the stretched zone.

Table clause 3.1.5. Using linear interpolation, we define that for the class A-III reinforcement, 35 GS steel brands and  sTCR. = 0,93.

t. sTCR. \u003d 475c.

The time of warming up the armature to the critical temperature for the slab of the solid cross section will be the actual limit of fire resistance.

c \u003d 0.96 hours,

where H.- The argument of the Gauss error function (Crampa), equal to 0.64 (p.3.2.7.) Depending on the value of the Gauss error function (KRAMP), equal to:

(here t. n. - Design temperature to fire, accept equal to 20С).

The actual limit of fire resistance Plate of overlap with round voids will be:

P f. =  0.9 \u003d 0.960.9 \u003d 0.86 h,

where 0.9 is a coefficient that takes into account the presence in the stove of emptiness.

As concrete - non-combustible material, Obviously, the actual fire hazard class K0.