Fire resistance of 90 minutes reinforced concrete floor slabs. Fire resistance of reinforced concrete structures. Determination of fire resistance limits of building structures
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 tee.
Let us determine the bending moment in the middle of the span from the action of the standard load and the own weight of the slab:
Where q / n- standard load per 1 linear meter of the slab, equal to:
The distance from the lower (heated) surface of the panel to the axis of the working reinforcement will be:
mm,
Where d– diameter of reinforcing bars, mm.
The average distance will be:
mm,
Where A- cross-sectional area of \u200b\u200bthe reinforcing bar (clause 3.1.1.), mm 2.
Let us determine the main dimensions of the calculated tee cross-section of the panel:
Width: b f = b= 1.49 m;
Height: h f = 0,5 (h-P) = 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 thermal characteristics of concrete:
Normative resistance to tensile strength R bn= 18.5 MPa (Table 12 or clause 3.2.1 for class B25 concrete);
Reliability factor b = 0,83 ;
Design resistance of concrete according to tensile strength R bu = R bn / b= 18.5 / 0.83 = 22.29 MPa;
Coefficient of thermal conductivity t = 1,3 – 0,00035T Wed\u003d 1.3 - 0.00035 723 \u003d 1.05 W m -1 K -1 (clause 3.2.3. ),
Where T Wed- the average temperature during a fire, equal to 723 K;
Specific heat WITH t = 481 + 0,84T Wed\u003d 481 + 0.84 723 \u003d 1088.32 J kg -1 K -1 (clause 3.2.3.);
Reduced coefficient of thermal diffusivity:
Coefficients depending on the average density of concrete TO= 39 with 0.5 and TO 1 = 0.5 (clause 3.2.8, clause 3.2.9.).
Determine the height of the compressed zone of the plate:
We determine the stress in the tensile reinforcement from the external load in accordance with 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 stretched zone of the cross-section of the structure, equal to 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 reinforcement in terms of tensile strength, equal to:
R su = R sn / s= 390 / 0.9 = 433.33 MPa (here s- reliability coefficient for reinforcement, taken equal to 0.9);
R sn- standard resistance of reinforcement in terms of tensile strength, equal to 390 MPa (Table 19 or clause 3.1.2).
Got that stcr1. This means that the stresses from the external load in the tensile reinforcement exceed the normative resistance of the reinforcement. Therefore, it is necessary to reduce the stress from the external load in the armature. To do this, we will increase the number of reinforcing bars of the panel12mm to 6. Then A s= 679 10 -6 (clause 3.1.1.).
MPa
.
Let us determine the critical heating temperature of the supporting 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= 475C.
The heating time of the reinforcement to the critical temperature for a slab of a solid cross section will be the actual fire resistance limit.
c = 0.96 h,
Where X– argument of the Gaussian (Krump) error function equal to 0.64 (section 3.2.7. ) depending on the value of the Gaussian (Krump) error function equal to:
(Here t n- the temperature of the structure before the fire, we take equal to 20С).
The actual fire resistance limit of a floor slab with round voids will be:
P f = 0.9 = 0.960.9 = 0.86 h,
where 0.9 is a coefficient that takes into account the presence of voids in the slab.
Since concrete is a non-combustible material, it is obvious that the actual fire hazard class of the structure is K0.
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 according to the load according to paragraph 2.20 of the Manual:
- 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: fire resistance limit reinforced concrete slab is REI 98
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 bar 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 of the cross section, so 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.
Bearing capacity of walls in action high temperature determined not only by the change strength characteristics 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 thermal creep of reinforcement and concrete when heated, as well as due to heating of the surface not facing fire by 140 ° C. According to 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 the bearing capacity of the structure will decrease 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.
Determination of fire resistance limits of building structures
Determination of the fire resistance limit of reinforced concrete structures
The initial data for a reinforced concrete floor slab are given in Table 1.2.1.1
Type of concrete - lightweight concrete with a density of c = 1600 kg/m3 with coarse expanded clay aggregate; slabs are multi-hollow, with round voids, the number of voids is 6 pcs, the slabs are supported on two sides.
1) The effective thickness of a hollow-core slab teff for assessing the fire resistance limit in terms of heat-insulating ability in accordance with paragraph 2.27 of the Manual to SNiP II-2-80 (Fire resistance):
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 lightweight concrete with an effective thickness of 140 mm:
The fire resistance limit of the plate is 180 min.
3) Determine the distance from the heated surface of the plate to the axis of the rod reinforcement:
4) According to Table 1.2.1.2 (Table 8 of the Handbook), we determine the fire resistance limit of the slab according to the loss of 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 desired fire resistance limit is 2 hours or 120 minutes.
5) According to clause 2.27 of the Handbook, a reduction factor of 0.9 is applied to determine the fire resistance limit of hollow core slabs:
6) We determine the total load on the plates 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 b) of the Benefit, we accept the coefficient for reinforcement
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
Based on the results obtained in the course of calculations, we obtained that the fire resistance limit of a reinforced concrete slab in terms of bearing capacity is 139 minutes, and in terms of heat-insulating capacity is 180 minutes. It is necessary to take the smallest 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 density c = 2350 kg/m3 with coarse filler of carbonate rocks (limestone);
Table 1.2.2.1 (Table 2 of the Handbook) shows the values of the actual fire resistance limits (POf) of reinforced concrete columns with different 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 includes, in addition to the thickness of the protective layer, also half the diameter of the working reinforcing bar.
1) Determine the distance from the heated surface of the column to the axis of the bar reinforcement by the formula:
2) According to clause 2.15 of the Handbook for structures made of concrete with carbonate aggregate, the cross-sectional size can be reduced by 10% with the same fire resistance limit. Then the width of the column is determined by the formula:
3) According to Table 1.2.2.2 (Table 2 of the Handbook), we determine the fire resistance limit for a lightweight concrete column 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 desired fire resistance limit is between 1.5 hours and 3 hours. To determine the fire resistance limit, we use the linear interpolation method. Data are given in table 1.2.2.3
