Determination of fire resistance limits of reinforced concrete columns. Fire resistance of reinforced concrete structures Fire resistance limit of hollow-core floor slabs
Table 2.18
Lightweight concrete density? = 1600 kg/m3 with coarse expanded clay aggregate, slabs with round voids, 6 pcs., slab support - free, on both sides.
1. Let's determine the effective thickness of the hollow-core slab teff to assess the fire resistance limit in terms of heat-insulating ability in accordance with clause 2.27 of the Handbook:
where is the plate thickness, mm;
- - plate width, mm;
- - number of voids, pcs.;
- - void diameter, mm.
- 2. We determine according to the table. 8 Allowances for the fire resistance of the slab on the loss of thermal insulation capacity for a slab of heavy concrete part with an effective thickness of 140 mm:
The fire resistance limit of the plate for the loss of heat-insulating ability
3. Determine the distance from the heated surface of the plate to the axis of the rod reinforcement:
where is the thickness of the concrete protective layer, mm;
- - diameter of the working reinforcement, mm.
- 4. According to the table. 8 Allowances determine the fire resistance limit of the slab by the loss of bearing capacity at a = 24 mm, for heavy concrete and when supported on two sides.
The desired fire resistance limit is in the range between 1 hour and 1.5 hours, we determine it by the method of linear interpolation:
The fire resistance limit of the plate without correction factors is 1.25 hours.
- 5. According to paragraph 2.27 of the Manual for determining the fire resistance limit 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 paragraph 2.20 of the Handbook:
- 9. According to clause 2.18 (part 1 a) of the Benefit, we accept the coefficient? for fittings A-VI:
- 10. We determine the fire resistance limit of the slab, taking into account the coefficients for the load and for the reinforcement:
The fire resistance limit of the plate in terms of bearing capacity is R 98.
For the fire resistance limit of the slab, we take the smaller of the two values \u200b\u200b- for the loss of heat-insulating ability (180 min) and for the loss of bearing capacity (98 min).
Conclusion: the fire resistance limit of a reinforced concrete slab is REI 98
As mentioned above, the fire resistance limit of bent reinforced concrete structures can occur due to heating to a critical temperature of the working reinforcement located in the tension zone.
In this regard, the calculation of the fire resistance of a multi-hollow floor slab will be determined by the time of heating up to the critical temperature of the stretched working reinforcement.
The section of the slab is shown in Figure 3.8.
b p b p b p b p b p
h h 0
A s
Fig.3.8. Estimated section of a hollow-core floor slab
To calculate the slab, its cross section is reduced to a tee (Fig. 3.9).
b´ f
x theme ≤h´ f
h´ f
h h 0
x theme >h' f
A s
a∑b R
Fig.3.9. Tee section of a multi-hollow slab for calculating its fire resistance
Subsequence
calculation of the fire resistance limit of flat flexible multi-hollow reinforced concrete elements
3. If, then s , theme is determined by the formula
Where instead b
used 
;
If a 
, then it must be recalculated according to the formula:
According to 3.1.5 is determined t s , cr(critical temperature).
The Gaussian error function is calculated by the formula:
According to 3.2.7, the argument of the Gaussian function is found.
The fire resistance limit P f is calculated by the formula:
Example number 5.
Given. Hollow-core floor slab freely supported on both sides. Section dimensions: b=1200 mm, working span length l= 6 m, section height h= 220 mm, protective layer thickness a l = 20 mm, class A-III tension reinforcement, 4 rods Ø14 mm; heavy concrete class B20 on crushed limestone, weight moisture content of concrete w = 2%, average density dry concrete ρ 0s\u003d 2300 kg / m 3, void diameter d n = 5.5 kN/m.
Define the actual fire resistance limit of the slab.
Solution:
For concrete class B20 R bn= 15 MPa (clause 3.2.1.)
R bu\u003d R bn / 0.83 \u003d 15 / 0.83 \u003d 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 = 61510 -6 m 2
Thermophysical characteristics of concrete:
λ tem \u003d 1.14 - 0.00055450 \u003d 0.89 W / (m ˚С)
with tem = 710 + 0.84450 = 1090 J/(kg ˚C)
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 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 the study of discipline. - Irkutsk.: VSI MIA of Russia, 2002. - 191 p.
Shelegov V.G., Kuznetsov N.A. Building construction. Reference manual for the discipline "Buildings, structures and their stability in case of fire". - Irkutsk.: VSI Ministry of Internal Affairs of Russia, 2001. - 73 p.
Mosalkov I.L. and others. Fire resistance of building structures: M .: CJSC "Spetstechnika", 2001. - 496 p., illustration
Yakovlev A.I. Fire resistance calculation building structures. - M .: Stroyizdat, 1988.- 143s., 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.
Manual for determining the fire resistance limits of structures, the limits of fire propagation along structures and the 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. General specifications/ Gosstroy of the USSR. - M., 1989
SNiP 2.01.07-85*. Loads and impacts / Gosstroy of the USSR. - M.: CITP Gosstroy USSR, 1987. - 36 p.
GOST 30247.0 - 94. Building structures. Test methods for fire resistance. General requirements.
SNiP 2.03.01-84*. Concrete and reinforced concrete structures / Ministry of Construction of Russia. - M.: GP TsPP, 1995. - 80 p.
1ELLING - a structure on the shore with a specially arranged sloping foundation ( slipway), where the ship's hull is laid down and built.
2 Viaduct - a bridge across land routes (or over a land route) at their intersection. Provides movement on them at different levels.
3FLASHBACK - a construction in the form of a bridge for passing one path over another at the point of their intersection, for mooring ships, and also in general for creating a road at a certain height.
4 STORAGE TANK - container for liquids and gases.
5 GAS CONTAINER– facility for acceptance, storage and release of gas to the gas network.
6blast furnace- shaft furnace for smelting pig iron from iron ore.
7Critical temperature- the temperature at which the standard resistance of the metal R un decreases to the value of the standard voltage n from external load on the design, i.e. at which there is a loss of bearing capacity.
8 Nagel - a wooden or metal rod used to fasten parts of wooden structures.
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
efforts.
Concrete has good compressive strength and
worse - stretching. This feature of concrete is unfavorable for bending and
stretched elements. The most common flexible building elements
are slabs and beams.
To compensate for adverse
concrete processes, it is customary to reinforce structures with steel reinforcement. Reinforce
plates welded meshes, consisting of rods located in two mutually
perpendicular directions. 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 during bending under load, in
according to the diagram of bending loads.
AT
under fire conditions, the slabs are exposed to high temperature bottom,
a decrease in their bearing capacity occurs mainly due to a decrease in
strength of heated tensile reinforcement. Typically, these elements
are destroyed as a result of the formation of a plastic hinge in the cross section with
maximum bending moment by reducing the tensile strength
heated stretched reinforcement to the value of operating stresses in its cross section.
Providing fire
building security requires increased fire resistance and fire safety
reinforced concrete structures. For this, the following technologies are used:
- reinforcing slabs to produce
only knitted or welded frames, and not loose individual rods; - to avoid bulging of the longitudinal reinforcement when it is heated during
during a fire, it is necessary to provide structural reinforcement with clamps or
transverse rods; - the thickness of the lower protective layer of concrete of the ceiling should be
sufficient so that it warms up no higher than 500 ° C and after a fire does not
influenced the future safe operation designs.
Studies have established that with a standardized fire resistance R = 120, the thickness
the protective layer of concrete should be at least 45 mm, at R = 180 - at least 55 mm,
at R=240 - not less than 70 mm; - in protective layer concrete at a depth of 15–20 mm from the bottom
the floor surface should be provided with an anti-splinter reinforcement mesh
from a wire with a diameter of 3 mm with a mesh size of 50–70 mm, which reduces the intensity
explosive destruction of concrete; - reinforcement of the supporting sections of thin-walled ceilings of the transverse
fittings not provided for by the usual calculation; - increase in fire resistance due to the location of the plates,
supported along the contour; - application 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); - increase in fire resistance due to false ceiling;
- protection of nodes and joints of structures with a layer of concrete with the required
fire resistance limit.
These measures will ensure proper fire safety of the 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. Academy of State Fire Service EMERCOM of Russia, 2003
2. MDS 21-2.2000.
Guidelines for calculating the fire resistance of reinforced concrete structures.
- M. : State Unitary Enterprise "NIIZhB", 2000. - 92 p.
Reinforced concrete structures, due to their incombustibility and relatively low thermal conductivity, quite well resist the effects of aggressive fire factors. However, they cannot indefinitely resist fire. Modern reinforced concrete structures, as a rule, are thin-walled, without a monolithic connection with other elements of the building, which limits their ability to perform their working functions in a fire to 1 hour, and sometimes less. Wet 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 a rapid destruction of the structure.
The fire resistance limit of a reinforced concrete structure depends on the size of its section, the thickness of the protective layer, the type, quantity and diameter of the 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 for heating - the surface opposite to fire by 140 ° C (ceilings, walls, partitions) depends on their thickness, type of concrete and its moisture content. With an increase in thickness and a decrease in the density of concrete, the fire resistance increases.
The fire resistance limit on the basis of the loss of bearing capacity depends on the type and static support scheme of the structure. Single-span freely supported bending elements (beam slabs, panels and floorings, beams, girders) are destroyed by fire as a result of heating of the longitudinal lower working reinforcement to the limiting critical temperature. The fire resistance limit of these structures depends on the thickness of the protective layer of the lower working reinforcement, the reinforcement class, the working load and the thermal conductivity of 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 case of fire the beams are heated from three sides (from the bottom and two side faces), and the slabs are heated only from the bottom surface.
The best reinforcing steel in terms of fire resistance is class A-III grade 25G2S. The critical temperature of this steel at the moment of the onset of the fire resistance limit of a structure loaded with a standard load is 570°C.
Large-hollow prestressed floorings made of heavy concrete with a protective layer of 20 mm and rod reinforcement made of class A-IV steel, produced by factories, have a fire resistance limit of 1 hour, which makes it possible to use these floorings 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 h; A-III (grades 25G2S) - 1 hour
In some cases, thin-walled bending structures (hollow and ribbed panels and floorings, crossbars and beams with a section width of 160 mm or less, without vertical frames at the supports) under the action of a fire can be destroyed prematurely 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 sections of these structures.
Plates supported along the contour have a fire resistance limit significantly higher than simple bending elements. These slabs are reinforced with working reinforcement in two directions, so their fire resistance additionally depends on the ratio of reinforcement in short and long spans. At square slabs having this ratio, equal to one, the critical temperature of the reinforcement at the onset of the fire resistance limit is 800°C.
With an increase in the ratio of the sides of the plate, the critical temperature decreases, therefore, the fire resistance limit also decreases. With aspect ratios of more than four, the fire resistance limit is practically equal to the fire resistance limit of plates supported on two sides.
Statically indeterminate beams and beam slabs, when heated, lose their bearing capacity as a result of the 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 due to the loss of concrete strength in the lower compressed zone, which heats up to high temperatures. The heating rate of this zone depends on the size cross section, therefore, the fire resistance of statically indeterminate beam plates depends on their thickness, and beams - on the width and height of the section. At large sizes cross-section, the fire resistance limit of the structures under consideration is much higher than that of statically determinable structures (single-span freely supported beams and slabs), and in some cases (for thick beam slabs, for beams with strong upper supporting reinforcement) practically does not depend on the thickness of the protective layer at the longitudinal bottom reinforcement.
Columns. The fire resistance limit of columns depends on the load application scheme (central, eccentric), cross-sectional dimensions, percentage of reinforcement, type of large concrete aggregate and thickness of the protective layer at the longitudinal reinforcement.
The destruction of columns during heating occurs as a result of a decrease in the strength of reinforcement and concrete. Eccentric load application reduces the fire resistance of the 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 at the tension 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. Concrete columns on crushed granite have less fire resistance (by 20%) than columns on crushed limestone. 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 the beginning of their firing 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 from a centrally compressed structure turns into an eccentrically compressed one with an eccentricity increasing in time. Under these conditions, fire resistance bearing walls largely depends on the load and on their thickness. As the load increases and the wall thickness decreases, its fire resistance decreases, and vice versa.
With an 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 assumed to be (mm): in 5 ... 9-storey buildings - 120, 12-storey buildings - 140, 16-storey buildings - 160 , in houses with a height of more than 16 floors - 180 or more.
Single-layer, double-layer and three-layer self-supporting panels of exterior walls are exposed to light loads, so the fire resistance of these walls usually meets the fire protection requirements.
The bearing capacity of walls under the action of high temperature is determined not only by a change 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 bearing capacity (destruction) in a heated state; the sign of heating the "cold" surface of the wall by 140 ° C is not characteristic. The fire resistance limit is dependent on the working load (factor of safety 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 according to the first or second case of eccentric compression (along heated reinforcement or "cold" concrete);
- 2) with the deflection of the element at the beginning in the direction of heating, and at the final stage in the opposite direction; destruction - in the middle of the height along heated concrete or along "cold" (stretched) reinforcement;
- 3) with a variable deflection direction, 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 scheme is typical for flexible walls, the second and third - for walls with less flexibility and platform supported. If the freedom of rotation of the supporting sections of the wall is limited, as is the case with platform support, its deformability decreases and therefore the fire resistance increases. Thus, the platform support of the walls (on non-displaceable planes) increased the fire resistance limit on average by a factor of two compared to the hinged support, regardless of the element destruction scheme.
Reducing the percentage of wall reinforcement with hinged support reduces the fire resistance limit; with platform support, a change within the usual limits of wall reinforcement has practically no effect on their fire resistance. When the wall is heated simultaneously from both sides ( interior walls) it does not have a thermal 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 a loss of 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 fire by 140 ° C. Based on these indicators, the fire resistance limit of reinforced concrete structures can be found by calculation.
In the general case, the calculation consists of two parts: thermal and static.
In the heat engineering part, the temperature is determined over the cross section of the structure in the process of heating it according to the standard temperature regime. In the static part, the bearing capacity (strength) of the heated structure is calculated. Then they build a graph (Fig. 3.7) of reducing its bearing capacity over time. According to this schedule, the fire resistance limit is found, i.e. heating time after which load bearing capacity structure will be reduced to the working load, i.e. when the equality will take place: M pt (N pt) = M n (M n), where M pt (N pt) is the bearing capacity of a bending (compressed or eccentrically compressed) structure;
M n (M n), - bending moment (longitudinal force) from the normative or other working load.
