The foundation is the foundation of the building, and its correct calculation is the foundation of the longevity of the entire structure. In order to calculate the required number of screw piles, their width and other parameters necessary for the construction of the foundation, it is necessary to adhere to a verified standardized methodology. It includes a set of formulas in which it is necessary to substitute geodetic data on the specifics of a particular area and tabular values that correspond to the desired foundation parameters. In order to calculate the number of screw piles for the foundation in a private house, you need to delve into all the features and subtleties of the calculations.
The foundation on screw piles is an excellent solution for terrains with complex terrain, which also has a moderate price. The specificity of this technology allows the installation of supports within 3 days and at the same time guarantees the reliability of the foundation for at least 100 years. To obtain a qualitative result, it is necessary to take into account all the factors inherent in the manufacturing process: uniform load distribution, soil features, soil freezing depth, the presence and specificity of groundwater, etc.
As a result of all the calculations, data appears that provide answers to questions such as:
- required height of screw piles;
- diameter of screw piles;
- depth of their installation;
- the required number of screw piles;
- total cost of materials.
Always the first step in any job is design.
For calculations, you can use the standardized methodology for screw piles described in SNiP 2.02.03–85. It is based on data on geodetic surveys of a particular plot of land.
They include the following information:
- land plot description;
- soil composition and density;
- groundwater level;
- soil freezing depth;
- precipitation in the built-up region.
Using this data, the number of screw piles for the foundation (K) is calculated.
For calculations, you need such indicators:
- the total load on the foundation (P), which is the sum of the masses of all materials used;
- reliability coefficient (k), which is a corrective indicator for the value of the total load on the piles;
- soil bearing capacity - table value;
- the area of the heel of the pile, which is directly dependent on its diameter, is the tabular value;
- maximum permissible load (S), indicator for one pile - table value.
The following is a substitution of values in a formula of the following form: K = P * k / S.
Reliability coefficient (k) correlates with the total number of piles and has the corresponding values:
- k = 1.4 if the piles are from 11 to 22 pieces;
- k = 1.65 - from 5 to 10 pieces;
- k = 1.75 - from 1 to 5 pieces.
Each pile carries a load equal to the total load divided by the number of supports. The fewer there are, the stronger the load on one pile and the faster it becomes worthless, and with it the whole foundation and house.
The correct calculation is to select the number of piles that is enough for the entire period of operation of the building, but without excessive surplus, which is a waste of money.
Using the above formula, the coefficient for screw piles, the calculation of loads and further construction is not fraught with special difficulties.
In the final calculations, it is necessary to distribute the loads under the supporting structures and critical points with excessive pressure on the foundation, taking into account:
- type of piles (hanging or racks);
- values of roll effort.
When calculating the screw foundation and the loads exerted on it, the following indicators must be taken into account:
- the total mass of the structure (constant), measured in kilograms, is the sum of the masses of such elements:
- walls and partitions;
- additional loads (temporary, variables):
- mass of snow on the roof;
- the mass of all items in the house: furniture, equipment, decoration materials and residents (average value 350 kg / sq. m);
- short-term dynamic loads arise from the effects of:
- gusts of wind;
- sedimentary processes;
- temperature fluctuations.
Depending on the structure (form) of a screw pile, the specifics of its application vary.
Such common types are distinguished:
- wide-plate with a cast tip - used for small buildings with simple soil;
- multi-layer with several blades at different levels - used with increased load on complex soil;
- with a variable perimeter - a narrow-profile product for specific conditions;
- narrow-layer with a cast notched tip - used in permafrost and rocky soil.
There are several basic technical characteristics of screw piles.
- barrel length and material of manufacture;
- pile diameter;
- variety of blades and their method of fixing on the trunk.
Piles are made with standardized dimensions for the implementation of the relevant tasks:
- 89 mm (blade diameter 250 mm) - with a design load of one support no more than 5 tons, these are mainly frame one-story houses;
- 108 mm (blade diameter 300 mm) - with a design load on one support of not more than 7 tons: frame one- and two-story houses, timber structures and foam concrete structures;
- 133 mm (blade diameter 350 mm) - with a design load of one support of not more than 10 tons: brick and aerated concrete houses using metal elements.
The choice of pile lengths is based on an indicator of soil density: the pile should rest only on hard soils.
Also, their length depends on the existing height differences in the area:
- loam depth less than 1 meter - pile length 2.5 meters;
- with loose soils or quicksand, the length of the pile is determined by the depth of immersion of the drill to hard formations;
- with roughnesses in the plot, the difference in the length of the piles can vary from 0.5 meters or more, depending on the particular case.
The number of supports and the interval of their location
The tabular values of the location of the supports relative to each other include the following values:
- from 2 to 2.5 meters - for houses with a wooden frame and block buildings;
- 3 meters - for buildings made of timber or logs.
When arranging the foundation piles for the even distribution of loads, the following rules for their arrangement should be taken into account:
- at every corner of the house;
- at the intersection of the bearing wall and the internal partition;
- near the entrance portal;
- inside the perimeter of the building with an interval of 2 meters;
- at least 2 piles under the fireplace;
- under the load-bearing wall, at the location of the balcony, mezzanine or similar structure.
The grillage is the foundation element necessary for uniform distribution of the load exerted by the structure on the foundation. To ensure reliability, the grillage needs to calculate a number of parameters, while the type of grillage does not matter.
The calculations include:
- force forcing the foundation;
- punching force acting on each corner separately;
- bending force.
If a high grillage is used, the entire load acts on the piles. The vertical load acts from below, the deforming load - from the side. Such calculations are very complex and require professional knowledge. For calculations, you must use the standards of individual construction.
They define the following standards:
- supports can be connected to the grillage in two ways: rigid and free;
- the depth of entry of the pile head into the grillage is at least 10 cm;
- the distance between the ground and the grillage is at least 20 cm;
- the thickness of the grillage cannot be less than the thickness of the walls and is at least 40 cm;
- grillage should have a height of more than 30 cm;
- grillage is strengthened by longitudinal and transverse reinforcement with a cross section of a rod from 10 to 12 mm.
This example serves the purpose of showing in detail the application of formulas in the calculations of a pile-screw foundation.
The initial data for a house with a perimeter of 10x10 are:
- a house built using frame technology, the roof is covered with slate, there is a porch;
- foundation dimensions - 10x10, construction height - 3 meters;
- two partitions are installed inside, which, intersecting, divide the room into 3 rooms;
- roof slope - 60 degrees;
- the frame is made of timber with a section of 150x150;
- grillage is made of timber with a section of 200x200;
- the walls are made of SIP panels.
Next, the calculations of the following structural elements are carried out:
- wall area:
- bearing: 10 * 3 * 4 = 120 sq. m;
- partitions: 10 * 3 + 5 * 3 = 45 sq. m;
- mass of walls (mass of 1 sq. m of a wall from a bar and a partition taken from the table of average values):
- bearing: 50 kg * 120 = 6000 kg;
- partitions: 30 kg * 45 = 1350 kg;
- total: 6000 + 1350 = 7350 kg;
- weight of ceilings per 100 square meters. m .:
- basement: 150 kg * 100 = 15000 kg;
- attic: 100 kg * 100 = 10000 kg;
- roof: 50 kg * 100 = 5000 kg;
- total: 15000 * 10000 + 5000 = 30000 kg;
- weight of additional elements (internal content of the house, type of household appliances, decoration, number of residents, etc.), a tabular average value for 1 square meter is taken. m in 350 kg:
- total mass of the building:
- 35,000 + 30,000 + 7350 = 72,350 kg;
- for example, a reliability factor of 1.4 is taken;
- the maximum load on the heel of piles with a diameter of 300 mm is 2600 kg, provided that the soil resistance is 3 kg / cu. cm (soil with medium density, deep water and freezing not more than 1 meter);
- calculate the number of piles according to the formula K = P * k / S: K = 72350 * 1.4 / 2600 = 39 piles.
In the process of calculating the number of piles and their distribution over the entire area of the foundation, there are many small features, each of which in one way or another affects the improvement of the final result:
- when installing the foundation from screw piles on complex unstable soil, to reinforce the supporting structure, a harness using a metal corner or channel at the base level is used;
- in the absence of geodetic data for calculations, it is better to use the parameters corresponding to the minimum design load, that is, create a maximum margin of safety;
- to improve the quality of calculations, in addition to formulas and tabular data, it is worth using a design program: it will recalculate all parameters and refute or confirm manual calculation;
- the least durable piles have trunks of suture pipes with welded blades;
- according to the standards, the base should not rise more than 60 cm above the ground, while the supply of piles along the length should be from 20 to 30 cm.
The estimated number of piles is not always optimal: there may be additional circumstances requiring the use of more of them. In addition, a small margin of safety favorably affects the durability of the foundation.
When installing piles in an uneven area, it is advisable to leave a margin of 20–50 cm in length. In the future, the surplus can be cut off or pulled out. But with a lack - you have to hammer a new pile.
See how to calculate the number of screw piles in the video below.