Calculation of foundations of shallow laying is necessary to refine its geometric dimensions and to choose a variety of foundation. It boils down to the calculation of three factors: the magnitude of the building's pressure on the ground, the pressure of the soil as a result of frost heaving, and the strength of the foundation frame.
Calculation of small foundations
Building load is a set of transmitted loads, in simplified calculation - the mass of the entire structure, distributed on 1 m2 of the lower plane of the foundation. The strength of the deformation of the drill is determined from the reference data for a particular type of soil. The strength of the frame depends on the geometry of the foundation and the reinforcement used.
Building load calculation
In order to calculate the mass of a building, it is necessary to have a design that takes into account the dimensions of the structure, materials, construction and other features. Calculation of the load on the foundation takes into account the winter snow load. The calculation method is simple: calculate the masses of individual structural parts of the structure, sum them and divide by the area of the basement base. Determine for this type of soil the specific design resistance of the soil R0 and compare it with the obtained value of the specific load N. If R0 & lt;N, review the size or shape of the foundation: perform it with a wide sole, with an extension downwards or increase its width.
An example of calculation: a brick one-storey house 10х8 m, with walls of full-bodied red brick with a width of 0.4 meters, with reinforced concrete floor covering and wooden ceilings of the ceiling. The roof is gable, covered with corrugated board. The construction is planned on heavy loamy soil, the region is Moscow.
According to the calculation procedure given by , the load is N = 23 t / m2.
Frosty punching and selection of the type of shallow foundation
The concept of "puchinistye primers" of many confounders. Let's try to figure out what it is. Different soils have different ability to accumulate moisture. Coarse-grained sand, rock rocks do not retain water, and clays, on the contrary, bind it, become plastic and remain wet for a long time. The water expands when frozen, and the soil containing a lot of moisture increases in size. This phenomenon was called "frost whipping".
Depending on the composition and size of the particles, the soils can be divided into 5 groups - they are given in Table 1.
Soil types( Soil types)
Depending on the soil group, the design of the shallow foundation is selected according to the drawing and its overall dimensions, depth and heightpouring. Calculate the area of the lower base of the foundation Af.
According to the table and the figure, we select the group of foundations corresponding to the type of soil III "heavy loams".This is a foundation, not buried in the ground, on a sandy-gravel podsypke. The width of the basement is 0,4 m;height - 0,7 m;the thickness of the underfill is 0.5 m.
The calculation procedure for deformations
The calculation is carried out under two conditions:
- , the calculated value of the deformation of the punching does not exceed the allowable ultimate deformation;
- the relative deformation of the soil taking into account the load does not exceed the limiting relative for a particular type of structure.
Limit deformations for a particular type of structure are determined from Table 2.
Table 2 - Permissible deformations
To determine the specified strain values for a particular structure, a number of complex calculations must be performed.
The drill deformation is calculated by the formula
The formula for calculating the drill deformation
In this formula, N is the specific pressure of the whole structure on the ground, it is calculated by the in a separate procedure and is expressed in tonnes per 1 m2.
The coefficient b depends on the ratio of the thickness of the substrate to the width of the substrate, it is determined from Table 3.
Table 3 - Determination of the coefficient
The value of Pr is on the bottom of the soil, for the strip foundation it is calculated by the formula:
The value of Pr is on the sole of the heavingthe
Indicator b - the width of the foundation tape, and ss - the resistance of the frozen ground, it can be found in SNiP 2.02.01-83.
The thickness of a layer of soil subjected to swelling under the foundation dz is defined as dz = df - d - hp, where df is the average depth of freezing determined from Table 4, and the values of d and hp are the height of the foundation and the thickness of the underfill, in meters.
Table 4 - Average depth of soil freezing by regions
Average depth of soil freezing by regions
After calculating the thickness of the soil layer, dz is determined from the graphs the coefficient of working conditions of the freezing soil ka determined from the graphs as a function of the value of dz and the value of the basement area of the base Af per unitlength.
Graph - coefficient of working conditions of the freezing soil k
The deformation of the punching of the unloaded base hfi is found by the formula from Table 5 corresponding to the type of the chosen foundation and its geometric dimensions: the depth of foundation d and the thickness of the pillow hn.
Table 5 - Calculation formulas for different types of soils
Calculation formulas for different soil types
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- Determine the thickness of the freezing layer of soils dz = df - d - hp. The calculated freezing depth df for Moscow according to Table 4 is 1.4 m. Dz = 1.4-0.7-0.5 = 0.2 m.
- Determine the specific area of the foundation per meter of its length, with a basement width of 0.4m area is 0.4 m2.
- By the figure, we determine the coefficient ka, it is equal to 0.56.
- We find in SNiP 2.02.01-83 the value of σs-64.
- Define by the formula t / m2.
- We find the formula m
- We find the coefficient b in Table 3 for the base of the tape type: for the selected ratio of the thickness of the substrate to the width of the base 0,5 / 0,4 = 1,25 it is equal to the base 0,88.
- The building load is calculated to be 23 t / m2.
- Define m = 0,1 cm.
The permissible deformation of the swelling in Table 2 is 2.5 cm. The condition is met.
The relative deformation of the punching, taking into account the rigidity of the frame structure, is found by the formula
Formula - for deformation of the punching, taking into account the rigidity of the frame
The index w, depending on the flexibility coefficient of the structures of structure l in BCH 29-85, is determined according to the graph.
The index W is determined from the graph of
Dhfp-the difference in the deformation of the whipping at the maximum and minimum of the pre-winter soil moisture.
L is the length of the building wall, m.
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- We define the value of the structural flexibility index l - 0.55 according to the BCH 29-85 procedure.
- We determine the value of the indicator w by the graph - 0,03.
- We determine the difference in the deformation of the bundle according to the procedure of BCH 29-85.Dhfp = 0.0022 m.
- The length of the walls of the building is 10 and 8 m.
- Relative deformation considering the rigidity of the frame for a long wall m.
- For a short wall m
- The admissible value in Table 2 is 0.0005 m. The condition is met.
If, as a result of calculation, it turns out that the conditions are not met, it is necessary to increase the calculated cushion thickness or foundation area by changing the width of the base.
