The calculation of the foundation is a crucial stage in the preparation for construction. It is necessary to perform it in order to understand what section dimensions are needed, how much reinforcement is needed and what diameter. Before you correctly calculate the supporting part of the building, you will need to collect the initial data. The accuracy of the calculations will depend on their accuracy.
What should be done
Most often in private construction use a strip foundation. This type allows you to make a basement in the house, but in some cases it can be economically disadvantageous. To make an estimate for the work (or roughly estimate how much investment will be required), you need to calculate the reinforcement for the strip foundation, also calculate the volume of concrete and its geometric dimensions.
The calculation method involves the calculation of three quantities. The calculation of the strip foundation as a result should give the following information about the design:
- the depth of the sole;
- base width;
- width over the entire height.
The calculation of the foundation for a house of brick or other materials must begin with the determination of the depth of the foundation. It depends on the heaving of the soil, the level of groundwater and climate. If this characteristic is incorrectly calculated, the building may collapse under the influence of frost heaving forces. The tape will be exposed to moisture and cold at the same time, which will lead to uneven deformations and cracks.
The width of the base must be sufficient to evenly transfer the mass of the building to the ground. The lower the strength of the soil, the wider the sole required. Due to the large area, it is possible to distribute the load from the strip foundation for the house to the foundation so that each of its sections accounts for no more than the permissible value.
The width of the tape over the entire height is usually taken constructively. It should be slightly larger than the outer walls. This takes into account the method of manufacturing the tape. For a monolithic foundation, a section width of 200-300 mm may be sufficient, while it is recommended that a precast be made at least 400-600 mm. Also, this indicator depends on the depth of the laying. The larger it is, the stronger the overturning effects will be (more powerful basement walls will be required).
Preparatory work
Before calculating the foundation for the house, the designer needs to find out the geological data of the site. For large buildings, special geological surveys are performed. In private construction, it is permissible to conduct research on your own. In this case, all characteristics are assigned by visual inspection.
To correctly calculate the foundation, the soil is examined in two ways:
- a fragment of pits, which are deep holes with dimensions of 1x2 m in plan (on average);
- drilling by hand drill.
In the first case, the type of soil is looked at along the pit walls. In the second - check the soil on the blades of the drill.
The studies are carried out to a depth that is 50 cm higher than the expected laying of the tape (which was prescribed only at the freezing mark). During the work, it is necessary to find out the following characteristics:
- type of soil at the level of the sole;
- groundwater level (GWT) location;
- presence of weak soil in the lens area.
To accurately understand the GWA, you will need to conduct a study at several points. At least one of these points should be in the lowland of the site. Working in a drought does not give an exact result, since moisture can go deep into the ground.
It is best to find out the GWP in the spring. In this case, the strip foundation will not be afraid even of floods.
Lenses of weak soil can be difficult to find. To do this, you need to make pits or wells very often. In most situations this is not necessary.If during construction such a nuisance is found, it is covered with gravel, gravel or sand-gravel mixture.
If the groundwater level is deep in the area, then you can use a deep laying tape (more than 1.5 m). In this case, water should be located 50 cm below the bottom of the building. With the location of the groundwater surface at a distance of less than 1.5 m from the surface, it is reasonable to choose a shallow structure. But this type has limitations. If the moisture is higher, it is worth considering another option for the foundation: a slab or piles.
To perform the calculation of the foundation foundation, you need to know the strength of the soil. The characteristic features of each type of soil can be found in GOST 25100-2011. Particular attention should be paid to the annexes to this document. The bearing capacity of each type is taken from the table below.
Base type | Maximum bearing capacity in kg / cm2 |
---|---|
Clay mixed pebbles | 4,50 |
Gravel | 4,00 |
Coarse sand | 6,00 |
Medium sand | 5,00 |
Fine sand | 4,00 |
Dusty sand | 2,00 |
Loam or sandy loam | 3,50 |
Clayey | 6,00 |
Subsidence | 1,50 |
Bulk with seal | 1,50 |
Bulk without compaction | 1,50 |
Types that have a strength of 2 or less kg / cm2 are not recommended for use as a base. Before construction, it will be necessary to replace them with medium or large sand.
Depth determination
To correctly calculate the foundation, you need to consider three parameters at the same time:
- UGV (the sole should be at least 50 cm higher);
- basement floor mark (the sole is at least 20-30 cm lower);
- freezing mark (the sole should be at least 30 cm lower).
The freezing depth is calculated by the formulas from regulatory documents. To simplify the task, you may need ready-made tables. They provide values for large settlements.
Load calculation
Before you calculate the foundation for the house, you need to calculate the load. It is more convenient to collect the loads on the foundation in a tabular form. All loads are divided into two types: permanent and temporary. The latter are provisional, as they include furniture, equipment, etc. Constants consist of a mass of building structures.
The calculation of the load on the foundation can be performed completely independently, taking into account the exact characteristics of the materials used. But it will be enough to use the table below. It contains average values, but the load on the foundation from this will change uncritically.
Design | Load value, kg / m2 | Reliability factor |
---|---|---|
510 mm brick wall | 920 | 1,3 |
640 mm brick wall | 1150 | |
Beam wall 150 mm | 120 | 1,1 |
Beam wall 200 mm | 160 | |
Wall on a wooden frame with insulation of 150 mm | 30-50 | |
Plasterboard partitions 80 mm | 30 | |
Overlap of PC slabs with cement screed | 625 | 1,2 |
Overlapping wooden with insulation | 150 | 1,1 |
Reinforced concrete foundation in kg / m3 (!) | 2500 | 1.2 - for the national team 1.3 - for monolithic |
Roof according to type of coating | ||
Metal | 60 | 1,05 |
Ceramics | 120 | 1,2 |
Bitumen materials | 70 | 1,1 |
Temporary load | ||
From people and furniture | 150 | 1,2 |
Snow cover | According to the joint venture "Loads and impacts" tab. 10.1 taking into account the location of the construction site | 1,4 |
The load on the foundation of each type, in order to correctly calculate the cross section, is multiplied by the coefficient of reliability.
Calculation of the sole by bearing capacity
The strip foundation, the calculation of which must be performed, requires the use of only one formula. To choose the dimensions of the strip foundation, consider this:
B = P / (L * R),
Here, the letter B indicates the width of the foundations to be found. P is the mass of the entire building, taking into account the underground part, which will help to find the calculated collection of loads. R is the strength of the base from the first table of the article. L is the overall perimeter of the tape structure. In order to correctly calculate the foundation, both the external and internal walls of the basement must be included in the perimeter.
Calculation Example
The calculations include the following steps:
- selection of geometric parameters;
- calculation of concrete on the foundation;
- and the calculation of the reinforcement of the strip foundation.
Geometry calculation example
To calculate the foundation, we take a two-story brick house with an external wall of 510 mm, the total height of the external wall is 4.5 m. There are no internal walls. It is located in Moscow, the soil on the site is medium-grained sand (R = 5 kg / cm2). Overlappings (2 pcs., Above the basement and above the first floor) made of PC boards, plasterboard partitions 2.7 m high and a total length of 20 m. Floor height - 3 m, plan dimensions - 6x6 m. Water on the plot lies low, therefore a decision was made to build a buried foundation with a height of 2 m. The four-sloped roof with a coating of metal. The slope of the ramp is 30 °.
The calculation example begins with the collection of loads in the form of a table.
Type of loading | Calculations |
---|---|
The foundation is monolithic (previously, 0.6 m wide along the perimeter of the building, equal to 36 m) | 36m * 0.6m * 2m * 2500kg / m3 * 1.3 = 140400 kg |
Brick wall | 6m * 4.5m * 4pcs. * 920 kg / m2 * 1.3 = 129,168 kg |
Plasterboard partitions | 20m * 2.7m * 30kg / m2 * 1.1 = 1782 kg |
Overlap | 2pcs * 6m * 6m * 625 kg / m2 * 1.2 = 54000 kg |
Roof | 6m * 6m * 60kg / m2 * 1.05 = 2268 kg 2268 kg / cos30 ° = 2607 kg |
Useful | 2 floors * 36m2 * 150kg / m2 * 1.2 = 12960 kg |
Snow | 36m2 * 180kg / m2 * 1.4 = 9072 kg |
Amount | 349 989 kg |
B = P / (L * R) = 349989kg / (36000cm * 5kg / cm2) = 1.94m. The design is calculated.
We round off the calculated size of the width to 2 m. For a width over the entire height this is a lot, it will be enough 50 cm under the walls 51 cm. An overhang of 1 cm is allowed (maximum 4 cm in one direction). The width of the sole is greater than that used in the calculation, but over the entire height the size is smaller than the original. For this reason, there is no need to redo the calculations with the new mass of the underground structure.
Concrete counting
Before buying a mixture, its required cubic capacity must be calculated. To do this, you just need to find the volume of the tape. It is recommended to add a margin of 5-7% to the amount of concrete for the strip foundation.
Reinforcement
Reinforcement for the strip foundation is needed to compensate for bending effects. Which reinforcement should be used correctly for reinforcement? It all depends on the height of the underground part and its length. To understand what kind of reinforcement is needed as a working one, simple calculations are done. The calculation of the amount of reinforcement is performed so that its total section is 0.1% of the section of the concrete structure. At the same time, there are minimal design requirements:
- What reinforcement is needed for a structure with a side length of less than 3 m? The answer will be a cross section of 10 mm.
- With a side length of more than 3 m, 12 mm reinforcement for the foundation will be required.
The calculation is performed approximately. Only a professional can calculate reinforcement more accurately. The pitch of the working rods is selected so that they are evenly distributed. It is advisable to use the same step, placing the elements in the lower part of the tape, at the top and in the middle.
Next, you need to calculate the number for clamps. They connect the working parts of the frame to each other. The layout of the reinforcement in the strip foundation assumes the presence of vertical and horizontal clamps. They are made of rods with a diameter of 8 mm. The pitch is prescribed within 20-30 cm. In the corners, the pitch is halved.
Calculating the amount of reinforcement for a strip foundation helps save time and money. Knowing the exact amount of reinforcement for each diameter and its step, you can easily reinforce the tape and purchase materials.