Rules and methods for calculating the foundation

 Rules and methods for calculating the foundation

No matter what the walls in the house, furniture and design decoration. All this can be depreciated in an instant, if mistakes were made during the construction of the foundation. And blunders relate not only to its qualitative features, but also to the main quantitative parameters.

Special features

When calculating the foundation, SNiP can be an invaluable assistant. But it is important to correctly understand the essence of the recommendations set out there. The fundamental requirement is the complete elimination of wetting and freezing of the substrate under the house.

These requirements are particularly relevant if the soil has an increased tendency to heave. Having explored accurate soil information on the site,You can already safely refer to the construction norms and rules - there are scrupulous recommendations for construction in any climatic zone and on any mineral materials existing on Earth.

It should be understood that only professionals can make a fairly correct and deep presentation. When designing the foundation is conducted by amateurs trying to save on the services of architects, it turns out a lot of problems - skewing houses, always damp and cracked walls, musty smells from below, weakening of the carrying capacity and so on.

The professional project takes into account the properties of specific materials and financial constraints. Because of this, it allows you to balance the loss of funds and the results achieved.

Type of

The stability of the base under the house is directly dependent on the type of it. There are clear minimum requirements for the characteristics of various types of foundations. So, under the house with dimensions of 6x9 m you can lay tapes 40 cm wide, this will allow you to have a double margin of safety compared with the recommended value. If, however, mount bored piles, extending down to 50 cm, the area of ​​a single support will reach 0.2 square meters. m, and you need 36 piles.More detailed data can be obtained only with a direct acquaintance with a specific situation.

What does it depend on?

Designing foundations even within the same type can be quite different. The main boundary lies between the bases of shallow and deep foundations.

The minimum bookmark level is determined by:

  • soil properties;
  • the level of water in them;
  • arrangement of basements and basements;
  • distance to the basements of neighboring buildings;
  • other factors that professionals should already consider.

When using slabs, one cannot lift their upper edge more than 0.5 m to the surface of the building. If a single-storey industrial object is built that will not be subjected to dynamic loads, or a residential (public) building of 1-2 floors, there is a certain subtlety - such buildings on soils freezing to a depth of 0.7 m are erected, replacing the lower part of the foundation with pillow

For the formation of this pillow is used:

  • gravel;
  • gravel;
  • sand of large or medium fraction.

Then the stone block must have a height of at least 500 mm; for the case of medium-sized sand, the base is prepared in such a way that it rises above groundwater.The foundation for internal columns and walls in heated structures may not adapt to the water level and the amount of freezing. But for it the minimum value will be 0.5 m. It is necessary to start a tape construction under the freezing line by 0.2 m. At the same time, it is prohibited to lower it more than 0.5-0.7 m from the lower planning point of the structure.

Methods

General recommendations on dimensions and depth may be useful, but it would be much more correct to focus on the results of professional-level calculations. Of great importance in their execution is the method of layer-by-layer summation. It allows you to confidently assess the draft of the base resting on a natural substrate of sand or soil. Important: there are separate restrictions for the applicability of such a method, but only specialists can understand this deeply.

The required formula includes:

  • dimensionless coefficient;
  • the average stress of the elementary soil layer under the action of external loads;
  • module of damage to the soil mass during primary loading;
  • it is also on the secondary boot;
  • weighted average stress of the elementary soil layer under its own weight, extracted during the preparation of the soil pit.

The lower line of the compressible array is now determined by the total voltage, and not by the additional effect, as recommended by building codes. During laboratory tests of soil properties, loading with a pause (temporary release) is now considered. First, the base under the foundation is conventionally divided into layers of identical thickness. Then measure the voltage at the joints of these layers (strictly under the middle of the sole).

After that, you can set the voltage generated by its own mass of soil at the outer boundaries of the layers. The next step is the definition of the lower line of the sequence that is subjected to compression. And only after all this can finally be calculated how should the draft of the foundation as a whole.

To calculate the eccentrically loaded base of the house, a different formula is practiced. It proceeds from the fact that it is required to strengthen the external border of the carrier block. After all, the bulk of the load will be applied there.

It is possible to compensate for the change in the vector of application of force through reinforcement, but it must be carried out in strict accordance with the design conditions. Sometimes reinforced sole or put the column.The beginning of the calculation involves the establishment of forces that act along the perimeter of the foundation. Simplify the calculations helps to reduce all forces to a limited set of resulting indicators by which to judge the nature and intensity of the applied loads. It is very important to correctly calculate the points at which the resultant forces will be applied to the plane of the sole.

Then they do the actual calculation of the characteristics of the foundation. Begin by determining the area that he should have. The algorithm is approximately the same as that used for the center-loaded block. Of course, accurate and final figures can only be obtained by shifting to the required values. Professionals operate with such an indicator as a plot of ground pressure.

It is recommended to make its value equal to an integer from 1 to 9. Such a requirement is related to ensuring the reliability and stability of the structure. Be sure to calculate the proportion of the smallest and largest loads on the project. The features of the building itself and the use of heavy equipment during construction should be taken into account.When a crane is designed to affect the foundation structure loaded outside the center, it is not allowed that the minimum voltage be less than 25% of the maximum value. In cases where construction will be carried out without the use of heavy machines, the acceptable level is any positive number.

The highest permissible resistance of the soil mass should be 20% higher than the most significant level of impact arising from the bottom of the sole. It is recommended to calculate the reinforcement of not only the most loaded sections, but also the structures adjacent to them. The fact is that the applied force can shift along the vector due to wear, reconstruction, major repairs or other adverse factors. It is very important to take into account all those phenomena and processes that can have a harmful effect on the foundation and degrade its characteristics. Consultation by professional builders will therefore not be superfluous.

How to calculate?

Even the most carefully calculated loads do not exhaust the numerical preparation of the project. It is also necessary to calculate the cubic capacity and width of the future foundation in order to know what excavation for the pit to make and how much materials to prepare for the work.It may seem that the calculation is very simple; for example, for a slab with a length of 10, a width of 8 and a thickness of 0.5 m, the total volume will be 40 cubic meters. m. But if you pour just this amount of concrete, you may experience significant problems.

The fact is that the school formula does not take into account the expenditure of space on the reinforcing mesh. And let its volume be limited to 1 cu. m., rarely it turns out more than this figure - you still need to prepare just as much material as is required. Then you will not have to overpay for the unnecessary, nor search feverishly where to buy the missing reinforcement. Calculations are somewhat different when using strip foundations, which are empty inside and therefore require less solution.

The necessary variables are:

  • the width of the employee for laying the pit (adjusted for the thickness of the walls and formwork to be mounted);
  • length of the bearing wall blocks and the partitions located between them;
  • the depth at which the base is embedded;
  • subspecies of the base itself - with monolithic concrete, of prefabricated blocks, of rubble stones.

The simplest case is calculated according to the parallelepiped volume formula minus the size of internal voids.It is even easier to determine the necessary parameters at the foundation of pillar execution. It will only be necessary to calculate the values ​​of two parallelepipeds, one of which will be the lowest point of the pillar, and the other the base of the structure itself. The result is required to be multiplied by the number of pillars, which are placed under the grillage with an interval of 200 cm.

The single principle is valid for screw and pile-grillage foundations, they summarize the volumes of used pillars and plate parts.

When using bored or screwed-in piles produced at the factory, only the tape segments will have to be counted. Values ​​of pillars are ignored, with the exception of the forecast for the value of earthworks. In addition to the volume of the foundation, the calculation of its precipitation is also very important.

A graphical representation of the layer-by-layer summation method indicates that attention should be paid to:

  • mark the surface of natural terrain;
  • entry of the base of the base deep into;
  • depth of groundwater;
  • the lowest line of compressed rock;
  • the magnitude of the vertical stress created by the mass of the soil itself (measured in kPa);
  • additional stresses due to external influences (also measured in kPa).

The specific gravity of the soil between the groundwater level and the line of the underlying aquifer is calculated with the correction for the presence of liquid. The voltage that occurs in the water bed itself under the ground’s own weight is determined by ignoring the weighting effect of water. A great danger in the operation of foundations is created by loads that can cause tilting. Calculate their value will not work without determining the total bearing capacity of the base.

When collecting data can be used:

  • dynamic test reports;
  • static test reports;
  • tabulated data theoretically calculated for a particular locale.

It is recommended that you read all this information at once. In case of any inconsistency, discrepancies, it is better to immediately find and understand its cause, rather than engage in risky construction. For amateur builders and customers, the easiest way to calculate the parameters affecting tipping is in accordance with the provisions of SP 22.13330.2011. The previous edition of the rules was published back in 1983, and, of course, their compilers simply could not reflect all the modern technological innovations and approaches.

It is advisable to take into account all the work that will be carried out to reduce the deformations of the very future of the foundation and foundations under nearby buildings.

There is a set of situations of instability that have been developed by generations of builders and architects that should be modeled. First of all, they count how the base soils can move, dragging the foundation with them.

Additionally, conduct calculations:

  • flat shear when the sole is in contact with the surface;
  • horizontal displacement of the foundation itself;
  • vertical displacement of the basement itself.

For 63 years, a uniform approach has been used - the so-called limit state technique. Construction rules require two such states to be calculated: according to the carrying capacity and the occurrence of cracks. The first group includes not only complete destruction, but also, for example, a downward slope.

To the second - all sorts of bends and partial cracks, limited sediment and other violations that complicate the operation, but do not exclude it at all. For the first category, the calculation of retaining walls and work aimed at deepening the existing basement.

It also applies if there is another trench next to it, a steep slope on the surface, or underground structures (including mines, mines). There are stable or temporary loads.

Prolonged or permanently influencing factors are:

  • the weights of all the components of the buildings and the soils additionally filled in; substrates;
  • hydrostatic pressure from deep and surface waters;
  • prestressing in reinforced concrete.

All other impacts that can only touch the foundation are accounted for as part of the temporary group. A very important point - to correctly calculate the possible roll; Tens and hundreds of houses collapsed prematurely just because of lack of attention to it. It is recommended to count as a roll under instant action, and under the load applied in the center of the base.

It is possible to assess the admissibility of the result obtained by comparing it with the instructions from the SNiP or with the technical design task. In most cases, the limitation is a factor of 0.004, only for the most critical structures the level of tolerance is lower.

When it turns out that by default the level of bank does not exceed the regulatory framework, this problem is solved in one of four ways:

  • full change of soil (most often use bulk pillows of sand and ground mass);
  • compaction of the existing array;
  • increasing the strength characteristics by fixing (helps to cope with loose and watery substrates);
  • the formation of piles of sand.

Important: whatever approach is chosen, you will have to re-calculate all the parameters. Otherwise, you can make another mistake and only waste money, time and materials.

Choosing a specific option for small bookmarks, first calculate the technological and economic parameters of the reinforced concrete base. Then carry out a similar calculation for the pile support. Comparing the results obtained and once again re-checking them, you can make a final conclusion about the optimal appearance of the foundation.

When determining the number of cubes of materials on the base plate, carefully assess the consumption of boards for formwork, as well as the length and width of reinforcing cells, their diameter. In some cases, the number of rows of rebar may be different.Next, analyze the optimal proportions of concrete in dry form and in solution. The final cost of any bulk materials, including auxiliary aggregates for concrete, is determined by their mass, and not on the basis of volume.

The average pressure under the base of the foundation structure is determined taking into account the eccentricity of the resultant of various forces relative to the center of gravity of the structure. In addition to finding out the calculated soil resistance, a weak underlying layer should be checked throughout its area and thickness for extrusion. Almost always, the maximum thickness of the elementary layers in the calculations is taken to be no more than 1 m. When a strip foundation is constructed, reinforcement is used no thicker than 1-1.2 cm.

Tips

It is very important not only to carry out all the calculations qualitatively, but also to clearly understand what the finished foundation should be. In the case of the construction of a very small outbuilding, it is worthwhile to carry out calculations for the construction of an asbestos-cement pipe. Belt and pile supports are chosen mainly for houses that create a very serious load.

Accordingly, it is determined by:

  • base cross section in diameter;
  • reinforcing reinforcement diameter;
  • stacking step of the strengthening lattice.

On sands, the layer of which is below 100 cm below the building, it is best to form light foundations with a depth of 40-100 cm. The same size should be followed if there is a pebble or a mixture of sand and stone below.

Important: these figures are only indicative and relate exclusively to light bases of small section, obtained in the form of a ribbon with weak reinforcement or pillars, saturated with broken stones. Approximate parameters do not exempt from the need in more detail and carefully calculate the actual requirements.

At loams, houses are most often built along a massive ribbon monolith, penetrated by reinforcing contours from below and above. The sides should be covered with hand-compacted sand, the layer of which ranges from 0.3 m along the entire height of the ribbon. Then the extrusion effect of the stresses is minimized or completely suppressed. When construction goes on the ground represented by sandy loam, it is required to analyze the ratio of sand and clay, and then make the final decision.When calculating the construction on the peat space, the organic mass is usually taken out to a strong substrate below it.

When it is very difficult and the work on the construction of the tape or pillars are disproportionately heavy and expensive, you need to count the piles. They are also necessarily brought to a dense point where a stable support is created. Absolutely any kind of foundation is supposed to wind up below the freezing line. If you do not do this, the power of frost repression and destruction will crush as many strong and solid structures as you like. It is desirable to include such a type of earthworks in projects as digging along the perimeter of trenches 0.3 m wide.

Correct information about the properties of the soil for the calculations can not be obtained by simply digging up the garden or focusing on the words of the neighbors, even if they are conscientious people. Experts advise to drill exploration wells with a depth of 200 cm. In some cases they can be even deeper if this is necessary for technical reasons.

It is useful to order a chemical and physical analysis of the extracted mass, otherwise it may give unexpected surprises. Ideally, you should generally abandon self-design and only check the calculations provided by the construction company.

In the next video you will find the calculation of the foundation of the house according to the bearing capacity.

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Information provided for reference purposes. For construction issues, always consult a specialist.

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