Consequences of incorrect calculation of the bearing capacity of the foundation Immediately after the commissioning of any structure, a process of slow lowering of the foundation occurs due to the applied loads. The foundation is always lowered to the calculated depth; this value is always taken into account and laid down when carrying out calculations.
Large, uneven settlements of the foundations entail deformation of structures with further destruction of the building. As a rule, the reason lies in incorrect calculation of the bearing capacity of foundations, as well as errors in calculating permissible loads on soils.
Let's look at examples
Example of calculating the weight of a building
Rice. 1. Calculation of soil resistance and its types.
In Fig. 2 shows the approximate weight parameters of the elements of a residential building, which will help you make the correct calculation. More accurate figures are given in reference books on building codes and regulations.
As an example, let’s take a two-story residential building without a basement, the dimensions of the base being 12x12 m.
First, the total weight of the building is determined - the weight of the roof, building frame, furniture, plinth and foundation is added up.
The weight of the roof is calculated. It depends on the weight of the rafters, floors and roof frame, the weight of the roof and the wind and snow load. The latest parameters can be found out in regional construction organizations or established according to SNiP “Loads and Impacts”.
Let’s assume that the roof is wooden, covered with metal tiles, and the snow and wind loads are insignificant: 3000+800+2000=5800 kg.
The weight of the building box will be determined. It is calculated from the weight of the house box itself, main walls, main floors and partitions.
Rice. 2. Approximate weight of residential building structures.
The construction of a two-story house of a given area will require approximately 15,000 units. facing brick (each block weighs 4 kg) and shell rock (each block weighs 15 kg): 15000*4+2500*15 = 60000+80700=140700 kg.
For the construction of main walls, partitions and non-foldings, red brick is used, 1 block of which weighs 3.8 kg: 12000 * 3.8 kg = 45000 kg.
The floors are erected from round-hollow reinforced concrete slabs. The weight of one 6x1.2 m slab is 200 kg: 34*2200=74800 kg.
It is necessary to take into account the weight of the mortar for laying brick and shell rock, screed, and rough finishing with plaster. The whole thing will weigh approximately 63,000 kg.
The total weight of equipment supporting the house and furniture is determined to be 5000 kg.
When all these parameters are summed up, the total weight of the building box is 329,100 kg.
The weight of the base and foundation is determined. For the construction of the base, brick is used, each block weighing 3.8 kg, and for the foundation, concrete blocks are taken, each weighing 1600 kg: 6500 * 3.8 + 40 * 1600 = 24700 + 64000 = 88700 kg.
It is also necessary to take into account the weight of pouring concrete screed, mortar for brickwork and installation of blocks and iron reinforcement. The total is 106,080 kg.
When summing up the weight of all elements of the house, the total weight of the building is 440,980 kg. That is, there will be 441 tons of pressure on the ground.
Example of safety factor calculation
First you need to calculate the area of support of the house on the ground. To do this, take the width of the blocks from which the foundation will be mounted (for example, 50 cm) and multiply it by the perimeter of the foundation. In this example, this figure will be equal to 4800 cm: 4800*50= 240000 cm2.
Let us assume that the type of soil on which the building will rest is lamellar clay, capable of a load of 2 kg per 1 cm2.
Let's calculate the weight that the soil under the building can support. To do this, we multiply the supporting area of the house by the calculated load on the ground: 240000*2=480000 kg/cm2.
Now you can calculate the safety factor. To do this, subtract the total weight of the building from the calculated load on the soil:
480000-440989=39011 kg.
That is, with a foundation width of 50 cm, the safety margin is 39 tons. This is quite enough to build a fairly large permanent residential building.
For light buildings such safety margins are not needed. Therefore, their foundations are not built from blocks, but use a strip structure, the width of which is usually equal to the width of a concrete block - 40-50 cm. But if the soil on the site is loamy and the groundwater level is very high, then you will have to build a monolithic slab or a floating foundation. It will help solve problems with uneven soil heaving. But in this case, calculation of the soil strength factor is still necessary.
If the calculations for the house are carried out correctly and all the nuances are taken into account, the building will last a long time and no problems will arise with it.
Determining the required width of the sole (“pillow”) of the strip foundation
The required width of the sole is determined by the ratio of the calculated resistance of the base to the linearly distributed load.
Previously, we determined the linear load acting at the level of the foundation base - 7925 kg/m. Our accepted soil resistance was 2.15 kg/cm2. Let's bring the load into the same units of measurement (meters into centimeters): 7925 kg/m = 79.25 kg/cm.
The width of the base of the strip foundation will be: (79.25 kg/cm) / (2.15 kg/cm2) = 36.86 cm.
The width of the foundation is usually taken as a multiple of 10cm, that is, rounded up to 40cm. The resulting width of the foundation is typical for light houses built on fairly dense loamy soils. However, for structural reasons, in some cases the foundation is made wider. For example, a wall will be faced with façade bricks with 50mm thick insulation. The required thickness of the base part of the wall will be 40 cm of aerated concrete + 12 cm of cladding + 5 cm of insulation = 57 cm. Aerated concrete masonry can be “hung” by 3-5 cm along the inner edge of the wall, which will reduce the thickness of the base part of the wall. The width of the sole must be at least this thickness.
When do you need to calculate foundations for bearing capacity?
An example of calculating a strip foundation for a house made of aerated concrete based on the bearing capacity of the soil
Drawing for calculating the foundation based on bearing capacity
- If the existing or new foundation is subject to significant horizontal loads, especially from houses being built nearby or regular vibrations from highways and industrial enterprises.
- The structure was built on a slope or the slope formed over time, exposing the outer part of the foundation.
- If the base of the foundation is installed on moisture-saturated soils.
- When the base can be affected by buoyant forces of various origins.
- If you need to check the stability of natural and artificial slopes.
If visible structural deformations have already appeared at a construction site or in the foundation of an existing building, always first pay attention to the condition of the soil under the sole and determine their condition. Therefore, according to the standards, there are several different types of soil deformations, which depend on internal and external factors
The need for geological research
To determine the type of foundations, as well as to calculate the approximate subsidence of soil in the construction zone, geological studies are mandatory. With their help, soil type, freezing depth, groundwater level, soil structure and other parameters are determined. Therefore, the load-bearing area of the foundation must be such that its mass, together with the future building, does not exceed the calculated resistance of the soil at the construction site.
Only then will you get a high-quality, reliable foundation capable of withstanding horizontal and vertical loads. At the same time, it is prohibited to build additional floors without strengthening the existing foundation, since in this case the mass of the object as a whole increases sharply.
Stages of soil deformation in the classical form
Design and calculation of the foundation for a house, how to calculate and make a foundation with your own hands
Scheme of development of deformations and possible soil movements in the event of incorrect calculation of bearing capacity
In modern literature, it is customary to distinguish three main phases of soil deformation:
- Initial. This is the stage of soil compaction under the influence of external factors, which occurs due to the reduction of pores between soil particles under the sole. The phase is different in that now there is no shift of the foundation, because all tangential loads are equivalent and are compensated by the load. But the load always arises spontaneously, it is distributed unevenly. As a result, the deformation may be slight at one point and severe at another. As a result, foundation shifts occur.
- The second stage is the phase of shear of the base. As the load increases, the soil compresses more and more, covers new areas, and a significant shift of the sole occurs towards a higher load. The standard balance is disrupted, a dense ball of soil is formed under the sole, and empty space is formed on the sides. The foundation material tends to take up the vacated space due to natural gravitational forces, which is why cracks and breaks appear in the foundation, and then in the load-bearing walls of the house.
- The third phase is the destruction of the sole. Here the material of the sole sticks out from the dense ball of soil and immediately becomes deformed.
This situation occurs with those foundations that are laid above the limiting depth of soil freezing or above the groundwater horizons. A slightly different picture occurs with deeply laid foundations. In such cases, a dense layer of soil also forms under the sole, but it does not stick out to the surface due to the large overlap area of the sole. Therefore, such a foundation has better load-bearing capabilities than a shallow foundation.
Once the process of soil deformation begins, it is sometimes impossible to stop it. The only way out is to install special protective structures that can level the loads or minimize their impact.
How to determine the type of soil on the site yourself
Soil classification is a comparison of the mechanical and physical parameters of the desired soil with the characteristics used in the standards. Self-assessment is indicative and approximate, therefore, when calculating, the bearing capacity is taken with some margin.
Visual determination method:
- Clay soil, when rubbed dry, gives the feeling of powder; clods are difficult to crush. Moistened clay remains soft and plastic, smeared on your fingers, and rolled into a sausage. When squeezed, the cake turns out without edge cracks.
- Dry loams give the feeling of sandy grains; the clods easily crumble when struck. The wet mass rolls into a sausage, but when bent, it cracks, and the cake ends up with breaks along the edges.
- Sandy loam soil when dry resembles flour or dust. The wet mass forms lumps of low strength that crumble. The wet mass lacks plasticity; it does not roll into a ring or flatten into a cake.
Sand is a loose mass without cohesion between small particles. When dry it spills between the fingers, but when wet it lacks plasticity, stickiness and cohesion.
Calculation of building materials
Do-it-yourself columnar foundation for a frame house
At the next stage, it is necessary to estimate the volume of building materials that will be required to build the foundation of the house: the amount of concrete mixture, reinforcement, formwork - in some cases, it is even necessary to calculate the bricks for the foundation. A competent approach will allow you to avoid unnecessary transportation costs and significantly save time on building the foundation.
Armature
We described the specifics of calculating reinforcement for the foundation in the corresponding article. There you will also find a detailed description of calculations for different types of reinforced concrete foundations. For a strip foundation, a frame is usually used of two belts of longitudinal reinforcement, 2 rods each, with a step of transverse (horizontal and vertical) reinforcement of 0.3-0.5 m. As an example of calculating the foundation, consider the same foundation of a house 6 × 9 m with one internal wall, we take the height of the tape equal to 1.5 m, width – 0.4 m.
The cross section of the tape has an area of: 0.4×1.5=0.6 m2=6000 cm2. Of this, 0.001% should be occupied by reinforcement, which is 6 cm2. Using the table below, we determine the required diameter of the rods - 14 mm. The number of meters of such reinforcement is approximately equal to: (6×3+9×2)×4=144 m. Smooth reinforcement, which, in fact, only plays the role of a connecting link for longitudinal rods, with a step of 0.5 m will require: (36/ 0.5)×(0.4×2+1.5×2)=273.6 m, where (36/0.5) is the number of smooth reinforcement connections, (0.4×2+1.5×2 ) – the perimeter of a rectangular element formed by smooth reinforcement.
Concrete
It doesn’t matter whether you plan to order a concrete mixture from a manufacturer, or are thinking about preparing it yourself - it’s simply necessary to estimate the volume of concrete! This is very easy to do using simple mathematical formulas and taking into account the geometry of the foundation. We talked about how to calculate the volume of concrete mixture in one of the articles, but just in case, we will give an example of calculation for our case: a 6x9 house with one internal wall, tape width - 0.4 m, height - 1.5 m The volume of our foundation, also known as the volume of concrete, will be: (9 × 0.4 × 2 + (6-0.8) × 0.4 × 3) × 1.5 = 20.16 m3 or 21 cubic meters of solution.
We talked about how to calculate the volume of concrete mixture in one of the articles, but just in case, we will give an example of calculation for our case: a 6x9 house with one internal wall, tape width - 0.4 m, height - 1.5 m The volume of our foundation, also known as the volume of concrete, will be: (9 × 0.4 × 2 + (6-0.8) × 0.4 × 3) × 1.5 = 20.16 m3 or 21 cubic meters of solution.
The same applies to situations in which you decide to prepare concrete yourself. In this case, information on the characteristics of the concrete mixture for the foundation will help you, as well as an article on how to calculate the amount of cement for concrete. They describe the work procedure in a simple and accessible way and present all the necessary calculations.
Calculation of formwork for the foundation
Of course, if you are going to pour concrete into pipes - use a bored pile foundation, then the issue with formwork will be resolved by itself. But when constructing a strip or slab reinforced concrete foundation, it is problematic to do without formwork. You can rent construction formwork kits, but it is expensive, especially if the construction time frame is unclear. Therefore, in some cases you have to make the formwork yourself - from lumber. Moreover, it must be done in such a way that the boards after stripping can be used, for example, for a subfloor or scaffolding. The cheapest option is to buy ordinary one-inch boards, which can be knocked down into fairly reliable panels. In an article devoted to calculations of formwork for a foundation, we described several examples of how formwork can be selected: based on the thickness of the boards and the distance between the braces, so that it is resistant to loads from the concrete mixture.
We hope that the information presented will help you solve difficult construction problems!
What is taken into account
When calculating the load on the foundation, the total weight of the entire structure, as well as the expected impacts (wind, precipitation), are summed up.
Basic
- The weight of the foundation itself (concrete, reinforcement) and the base.
- Interfloor ceilings, floors (rough + finishing), logs.
- External and internal walls (partitions).
- Rafter system and roofing.
- Flights of stairs, observation platforms and the like.
- All finishing and insulating materials (hydro-, steam- and heat protection).
- Door and window frames, frames.
- Fastening elements. If you take into account how many different nails, staples, self-tapping screws and other elements for fixing the position are needed, then in total there will be a lot of “hardware”.
- Engineering equipment. This includes not only units and special devices (boiler, plumbing, etc.), but also pipe lines and various shut-off valves.
- Furniture, household equipment, household utensils, personal belongings of family members.
The weight of all structural elements is determined by their dimensions (walls, ceilings, etc.). The materials from which they are made must also be taken into account. To simplify the calculations of foundation parameters, all parts of the house can be divided into appropriate groups (wood, brick). Their specific gravity (kg/m3) can be found in the corresponding tables (for example, SNiP II-3 from 1979).
In addition, it is necessary to take into account the intensity of precipitation in a given area in winter.
Here the roof configuration is taken into account. We are talking about whether the snow will linger on it and what layer it will fall on (thickness)?
Experts recommend, after making calculations for a small private house, to add about 2 – 2.5 tons (“reserve”) to the final value of the total weight of the building.
Determining the dimensions of the foundation
As a rule, all strip structures are made according to the “parallel and perpendicular” principle. This makes calculations easier. The developer is interested not only in the pure “geometry” of the house, but also in the expected consumption of building materials for the foundation - concrete, reinforcing bars, wood for formwork.
It is necessary to determine the length and width of the entire structure, as well as the total length of all internal walls (partitions). The technique is simple. The total length of the foundation strip is calculated, and its width and height are already known. As a result of multiplying the values, the volume of the “monolith” is obtained.
Next, the total volume of all internal partitions is calculated. Their width will be different, and the depth of installation can be reduced in order to save money.
This value is constant for each location. Therefore, it is not necessary to make calculations using complex formulas; it is enough to consult with a specialist.
Calculation of loads
Before calculating the foundation for the house, you will need to calculate the load. It is more convenient to collect foundation loads in tabular form. All loads are divided into two types: permanent and temporary. The latter are temporary, conditionally, since they include furniture, equipment, etc. The permanent ones consist of the mass of the building structures.
The calculation of the load on the foundation can be done completely independently, taking into account the exact characteristics of the materials used. But it will be enough to use the table below. It shows average values, but the load on the foundation will change uncritically.
| Design | Load value, kg/m2 | Reliability factor |
| Brick wall 510 mm | 920 | 1,3 |
| Brick wall 640 mm | 1150 | |
| Timber wall 150 mm | 120 | 1,1 |
| Timber wall 200 mm | 160 | |
| Wall on a wooden frame with 150 mm insulation | 30-50 | |
| Partitions made of plasterboard 80 mm | 30 | |
| Flooring made of PC slabs with cement screed | 625 | 1,2 |
| Wooden ceiling with insulation | 150 | 1,1 |
| Reinforced concrete foundation in kg/m3 (!) | 2500 | 1.2 - for prefabricated 1.3 - for monolithic |
| Roof depending on the type of covering | ||
| Metal | 60 | 1,05 |
| Ceramics | 120 | 1,2 |
| Bituminous materials | 70 | 1,1 |
| Live loads | ||
| From people and furniture | 150 | 1,2 |
| Snow cover | According to SP “Loads and impacts” table. 10.1 taking into account the location of the construction site | 1,4 |
In order to correctly calculate the cross-section, the load on the foundation of each type is multiplied by the reliability coefficient.
Foundation calculator
The foundation is the basis of any structure. Therefore, it is very important to choose the right and suitable foundation, as well as competent calculation of concrete for the foundation, which will allow you to draw up an approximate construction estimate.
It should be noted that an estimate must be drawn up so that you do not have to stop work and waste time purchasing missing materials or because the budget is exhausted. In order to understand how much money will need to be spent on the base of the structure, you can use a special concrete calculator for the foundation.
A strip foundation, which can be monolithic or prefabricated, is a closed strip of reinforced concrete that divides the load of the structure onto the soil and passes under the load-bearing walls of the structure. Such a foundation prevents the building from settling, changing its shape or deforming the walls. Strip foundation is the most commonly used type of foundation in the construction of private houses, basements and basements.
This foundation can also be shallow or deeply buried, which depends on the characteristics of the soil and the expected load on it. When constructing any foundation, correct calculation is important, which will avoid annoying mistakes and unnecessary waste of money. The cube calculator will help you determine the volume of concrete required for construction and stock up on all the components in the required quantity in advance. The foundation calculation calculator is useful for determining the weight of the concrete mixture and the load on the soil. You can also calculate the consumption of cement mortar.
Typically, the concrete calculator is calculated using such characteristics as the length, height and width of the designed foundation. For more accurate calculations, you can use additional parameters, for example, indicate the brand of concrete used and the composition of the mixture. A specialized concrete foundation calculator will also allow you to then analyze how much reinforcement may be needed or calculate the formwork.
It should also be taken into account that when preparing the mixture for construction yourself, concrete calculations are carried out depending on the fraction of sand and crushed stone, their density and the proportions used.
The calculator for a strip foundation consists not only of characteristics such as strip width, strip height and strip length, but also depends on the brand of concrete chosen and the thickness of the strip.
Foundation calculator
The foundation calculator will help you independently calculate the required volume of concrete for pouring the foundation, and will also calculate the amount of formwork and reinforcement. It is worth noting that the “Foundation height” parameter includes both the depth of the underground part and the height of the above-ground part.
If your interior partitions are not represented by a load-bearing type structure, then a lighter foundation layer is used under them, which has its own geometric parameters, and you need to calculate the foundation for the partitions separately in a calculator, and then summarize the data obtained.
How to calculate concrete for the foundation yourself
You can determine the cubic capacity of the mixture yourself using simple formulas. To do this, you need to know the width of the strip foundation, its height and total length. The length of the tape is determined as the sum of the perimeter and load-bearing walls. The height is the sum of the above-ground part and the depth, the width is taken from a preliminary calculation of the bearing capacity of the foundation.
Conventionally, we take the width to be 0.3 m, the height to be 1.6 m, and the length to be 40 m. Concrete is calculated as the volume of a parallelepiped:
V = 0.3 x 1.6 x 40 = 19.2 m3.
To simplify the calculation of the amount of concrete and avoid errors, you can use a calculator program. To do this, prepare standard initial data:
- strip base diagram;
- length and width of the house;
- width and height of the tape.
The program specifies in which units the linear parameters should be expressed. Typically, the calculator allows you to calculate not only concrete: in parallel, the calculation of the profile, length and total weight of reinforcement, formwork dimensions, volume of thermal insulation materials is performed
As an example, it is proposed to determine the amount of consumables needed to lay the foundation for a one-room country house. The parameters from Table 1 are entered into the calculator.
Table 1
| Scheme | Square |
| Concrete, brand | M200 |
| Foundation width | 6 m |
| Length | 6 m |
| Tape height | 70 cm |
| Tape width | 40 cm |
After entering the data, options are selected - for example, calculation of reinforcement or formwork. There are programs in which reinforcement is calculated by default, based on dimensional parameters and in accordance with building codes SNiP 52-01-2003.
As a result of calculations, the calculator produces results formed in the form of a table.
table 2
| Parameter name | Parameter value | Unit | Note |
| Total length of strip base | 22,4 | m | The calculation was performed along the center line of the tape. |
| Sole area | 8,96 | m2 | The surface area on which the foundation rests. It is used to determine the dimensions of the waterproofing. |
| The area of the outer side surface of the tape | 16,8 | m2 | It is equal to the area of the insulation with which the foundation is covered from the outside. |
| Net volume of concrete mixture | 6,3 | m3 | Due to shrinkage, concrete should be ordered with a 10-15% margin. |
| Weight of solution | 14,74 | T | This is the approximate mass taking into account the average density of the M200 solution |
| The pressure exerted by the foundation on the soil | 0,165 | kgf/cm2 | Distributed load per unit area of support |
If the additional option for calculating reinforcement is selected, the calculator displays the following information: the minimum diameter of longitudinal reinforcing bars, the number of rows of reinforcement in each chord, the smallest diameter of transverse stirrups, the pitch of the reinforcement, its total length and weight.
Calculation of the volume of solution for a strip foundation for a fence
This type of base is used to install a fence made of almost any material. It is used as concrete, brick, metal, wood. To obtain a high-quality foundation, take into account the density of the soil, the depth of its freezing, and the level of groundwater. Since the fence is considered a lightweight structure, “lean” concrete of grade 150 is usually poured into its base. If the soil is light or rocky, a concrete mixture of grade 100 with a low cement content is suitable. In areas with difficult terrain and loose soil, it is advisable to use 200-gauge concrete.
To calculate the volume of the mixture, you first need to find out the dimensions of the tape. Its length corresponds to the length of the fence, the width is most often 0.4 m. The average depth of a strip foundation for a fence is 0.5 m. It is optimal for wooden and metal profile fencing. For more massive structures, a deep foundation is made, passing below the freezing level of the soil (usually the difference is 30 cm).
Calculation example
It is required to make a base for the fence from reinforced concrete blocks with a total length of 25 m. The fence is installed on an area with dusty sandy soil, freezing to a depth of 1.5 m. Concrete for pouring the tape is calculated as follows:
H = 25 x 0.4 x (1.6 + 0.3) = 19 m3.
If you make the mixture yourself, you should remember: the foundation will be strong only if the concrete recipe is drawn up in accordance with building codes.
Determination of resistance to displacement of frozen soil relative to the foundation
1. The resistance of displaced frozen soil relative to the foundation is determined from the table of this appendix depending on the heaving rate u t
and the estimated temperature of the freezing soil
Td
under the foundation.
2. Soil heaving rate Ut
, m/day, determined from the expression
,(1)
where hfi
- heaving deformation of an unloaded base, determined in accordance with;
td
— duration of the period, in months, of soil freezing under the foundation
,(2)
Here t
— duration of the period with negative air temperatures, in months, determined in accordance with chapter SNiP 2.01.01-82.
d ,
h p , df
- the same notations as in.
3. The estimated temperature of the soil under the foundation is determined by the formula
,(3)
at
,(4)
where Tmin
— average air temperature of the coldest month of the winter period, °C, determined in accordance with chapter SNiP 2.01.01-82.
Table
Values s s
| Estimated temperature of the soil under the foundationTd, °C | Average speed of soil heaving uf ´102 m/day freezing under the base of the foundation | |||||||||||||||||||
| 0,02 | 0,04 | 0,06 | 0,08 | 0,1 | 0,12 | 0,14 | 0,16 | 0,18 | 0,2 | 0,25 | 0,3 | 0,35 | 0,4 | 0,45 | 0,5 | 0,55 | 0,6 | 0,65 | 0,7 | |
| -0,6 | 0,5 | 1,1 | 1,6 | 2,2 | 2,7 | 3,3 | 3,8 | 4,4 | 4,9 | 5,5 | 6,8 | 8,2 | 9,6 | 11,0 | 12,3 | 13,7 | 15,1 | 16,4 | 17,8 | 19,2 |
| -0,8 | 0,6 | 1,2 | 1,8 | 2,4 | 3,0 | 3,6 | 4,2 | 4,8 | 5,4 | 6,0 | 6,6 | 9,1 | 10,6 | 12,1 | 13,6 | 15,2 | 16,7 | 18,2 | 19,7 | 21,2 |
| -1 | 0,7 | 1,3 | 2,0 | 2,7 | 3,0 | 4,0 | 4,7 | 5,4 | 6,1 | 6,7 | 8,4 | 10,1 | 11,8 | 13,5 | 15,2 | 16,9 | 18,6 | 20,2 | 21,0 | 23,6 |
| -1,2 | 0,75 | 1,5 | 2,2 | 3,0 | 3,8 | 4,5 | 5,2 | 6,0 | 6,7 | 7,5 | 9,4 | 11,2 | 13,1 | 15,0 | 16,9 | 18,8 | 20,6 | 22,5 | 24,4 | 26,2 |
| -1,4 | 0,8 | 1,6 | 2,5 | 3,3 | 4,1 | 5,0 | 5,8 | 6,7 | 7,5 | 8,3 | 10,4 | 12,5 | 14,6 | 16,7 | 18,8 | 20,8 | 22,9 | 25,0 | 27,1 | 29,2 |
| -1,6 | 0,9 | 1,8 | 2,8 | 3,7 | 4,6 | 5,6 | 6,5 | 7,4 | 8,3 | 9,3 | 11,6 | 13,9 | 16,2 | 18,5 | 20,8 | 23,2 | 25,4 | 27,8 | 30,1 | 32,4 |
| -1,8 | 1,0 | 2,0 | 3,1 | 4,1 | 5,1 | 6,2 | 7,2 | 8,2 | 9,3 | 10,3 | 12,8 | 15,4 | 18,0 | 20,6 | 23,1 | 25,7 | 28,3 | 30,8 | 33,4 | 36,0 |
| -2 | 1,1 | 2,3 | 3,4 | 4,6 | 5,7 | 6,9 | 8,0 | 9,1 | 10,3 | 11,4 | 14,3 | 17,1 | 20,0 | 22,8 | 25,7 | 28,6 | 31,4 | 34,2 | 37,1 | 40,0 |
| -2,2 | 1,3 | 2,5 | 3,8 | 5,1 | 6,3 | 7,6 | 8,9 | 10,1 | 11,4 | 12,7 | 15,8 | 19,0 | 22,2 | 25,4 | 28,6 | 31,7 | 34,9 | 38,0 | 41,2 | 44,4 |
| -2,4 | 1,4 | 2,8 | 4,2 | 5,6 | 7,0 | 8,5 | 9,8 | 11,3 | 12,7 | 14,1 | 17,6 | 21,1 | 24,7 | 28,2 | 31,7 | 35,2 | 38,8 | 42,3 | 45,8 | 49,3 |
| -2,6 | 1,5 | 3,1 | 4,7 | 6,2 | 7,8 | 9,4 | 10,9 | 12,5 | 14,1 | 15,6 | 19,5 | 23,5 | 27,4 | 31,3 | 35,2 | 39,1 | 43,0 | 47,0 | 50,9 | 54,8 |
| -2,8 | 1,7 | 3,5 | 5,2 | 6,9 | 8,7 | 10,4 | 12,1 | 13,9 | 15,6 | 17,4 | 21,7 | 26,0 | 30,4 | 34,8 | 39,1 | 43,5 | 47,8 | 52,1 | 56,5 | 60,8 |
| -3 | 1,9 | 3,8 | 5,8 | 7,7 | 9,6 | 11,6 | 13,5 | 15,4 | 17,4 | 19,3 | 24,1 | 28,9 | 33,8 | 38,6 | 43,4 | 48,3 | 53,1 | 57,9 | 62,8 | 67,6 |
| -3,2 | 2,1 | 4,2 | 6,4 | 8,6 | 10,7 | 12,9 | 15,0 | 17,2 | 19,3 | 21,5 | 26,8 | 32,2 | 37,6 | 42,9 | 48,3 | 53,7 | 59,0 | 64,4 | 69,8 | 75,1 |
| -3,4 | 2,4 | 4,7 | 7,2 | 9,5 | 11,9 | 14,3 | 16,7 | 19,1 | 21,5 | 23,8 | 29,8 | 35,8 | 41,7 | 47,7 | 53,6 | 59,6 | 65,6 | 71,5 | 77,5 | 83,4 |
| -3,6 | 2,6 | 5,3 | 7,9 | 10,6 | 13,2 | 15,9 | 18,5 | 21,2 | 23,8 | 26,5 | 33,1 | 39,7 | 46,3 | 53,0 | 59,6 | 66,2 | 72,8 | 79,4 | 86,1 | 92,7 |
| -4 | 3,3 | 6,5 | 9,8 | 13,1 | 16,3 | 19,6 | 22,9 | 26,1 | 29,4 | 32,7 | 40,8 | 49,0 | 57,2 | 65,3 | 73,5 | 81,7 | 89,8 | 98,0 | 106,2 | 114,3 |
| -4,2 | 3,6 | 7,2 | 10,9 | 14,5 | 18,I | 21,8 | 25,4 | 29,0 | 32,7 | 36,3 | 45,4 | 54,4 | 63,5 | 72,6 | 81,6 | 90,7 | 99,8 | 108,8 | 117,9 | 127,0 |
| -4,4 | 4,0 | 8,1 | 12,1 | 16,6 | 20,1 | 24,2 | 28,2 | 32,2 | 36,3 | 40,3 | 50,4 | 60,4 | 70,5 | 80,6 | 90,7 | 110,8 | 120,9 | 131,0 | 131,0 | 141,0 |
| -4,6 | 4,5 | 9,0 | 13,4 | 17,9 | 22,4 | 26,9 | 31,3 | 35,8 | 40,3 | 44,8 | 55,9 | 67,1 | 78,3 | 89,5 | 100,7 | 111,9 | 123,1 | 134,3 | 145,5 | 156,7 |
| -4,8 | 5,0 | 9,9 | 14,9 | 20,0 | 24,9 | 29,8 | 34,8 | 39,8 | 44,7 | 49,7 | 62,1 | 74,6 | 87,0 | 99,4 | 111,9 | 124,3 | 136,7 | 149,1 | 161,6 | 174,0 |
| -5 | 5,5 | 11,0 | 16,6 | 22,1 | 27,6 | 33,1 | 38,7 | 44,2 | 49,7 | 55,2 | 69,0 | 82,8 | 96,6 | 100,4 | 121,2 | 138,0 | 151,9 | 165,7 | 170,5 | 193,3 |
| -5,2 | 6,1 | 12,3 | 18,4 | 24,5 | 30,7 | 36,8 | 42,9 | 49,1 | 55,2 | 61,3 | 76,7 | 92,0 | 107,3 | 122,7 | 138,0 | 153,3 | 168,7 | 184,0 | 199,3 | 214,7 |
| -5,4 | 6,8 | 13,6 | 20,4 | 27,2 | 34,1 | 40,9 | 47,7 | 54,5 | 61,3 | 68,1 | 85,2 | 102,2 | 119,7 | 136,2 | 153,3 | 170,3 | 187,3 | 204,4 | 221,4 | 238,4 |
| -5,6 | 7,6 | 15,1 | 22,7 | 30,3 | 37,8 | 45,4 | 53,0 | 60,5 | 68,1 | 75,7 | 94,6 | 113,5 | 132,4 | 151,3 | 170,2 | 189,2 | 208,1 | 227,0 | 246,0 | 264,8 |
| -5,8 | 8,4 | 16,8 | 25,2 | 33,6 | 42,0 | 50,4 | 58,8 | 67,2 | 75,6 | 84,0 | 106,1 | 126,1 | 147,1 | 168,1 | 189,1 | 210,1 | 231,1 | 252,1 | 273,1 | 294,1 |
| -6 | 9,3 | 18,7 | 28,0 | 37,3 | 46,7 | 56,0 | 65,3 | 74,7 | 84,0 | 93,3 | 116,7 | 140,0 | 163,4 | 186,7 | 210,0 | 233,4 | 256,7 | 280,0 | 303,4 | 326,7 |
Note. For intermediate values of Td
and
Uf
value
s s
is taken by interpolation.
What to consider when calculating the foundation
Features of the soil and what type of foundation to make
There are several types of foundation: slab, strip, pile, on pillars. Sometimes slightly modified designs are used, which are “modifications” of the main types. However, measures are often taken to modify the soil. For example, if the area is swampy, then partial excavation is done and more durable material is filled in. Granular slag is often used, which gradually turns into concrete;
Foundation laying depth
This parameter depends on two factors. It is necessary to determine how deep the groundwater (underground) water lies in this place. It also takes into account the depth to which the soil freezes in winter;
What load should the foundation withstand?
You need to understand that the entire structure, which has a certain weight, rests on the foundation (base). Walls, roof, ceiling, floor have weight. Plus, doors, window frames, etc. also have weight. How to calculate this weight? The material is known, the areas are known, the specific gravity of each type of material is known (there are many reference tables on this topic on the Internet). Therefore, the total weight of the entire structure can be calculated. We must not forget that there will be something in the building: furniture, household appliances, etc. All this will also add to the total weight, even including people in the premises of the house.
Trench for the foundation
Naturally, before making the foundation, you should prepare a trench. You can roughly indicate its maximum depth for some soils:
- sandy or gravelly 1 m
- sandy loam 1.25 m
- clay, loam 1.5 m
But there is also a certain minimum bookmark:
- dry soil 0.7 m
- wet soil 1.2 m
If the building has a basement, then the minimum depth is 0.4 m (from the basement floor level). In individual construction, a strip foundation is most often used. It has the advantage of a rigid connection of all elements in any direction (longitudinal, transverse).
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Limit loads
It is clear that the “total” weight of the structure should not exceed the load that the soil can withstand. Here are some data on the maximum load depending on the type of soil (in kg/cm²).
- coarse sand, gravelly 3.5 – 4.5
- fine wet sand 2 - 3
- hard clay 3 - 6
- gravel, crushed stone 5 - 6
In this case, the weight of the foundation itself should also be taken into account. In any case, you need to understand that such calculations are made individually for each building.
https://youtube.com/watch?v=S33UTty4_Z0
Preparatory work
Before calculating the foundation for a house, the designer needs to find out the geological data of the site. For large buildings, special geological surveys are carried out. In private construction, it is permissible to conduct research yourself. In this case, all characteristics are assigned based on visual inspection.
To correctly calculate the foundation, the soil is examined in two ways:
- excerpts of pits, which are deep holes with plan dimensions of 1x2 m (on average);
- drilling wells with a hand drill.
In the first case, the type of soil is looked at by the walls of the pit. In the second, they check the soil on the drill blades.
To study the soil, the walls of the pit are inspected
Research is carried out to a depth that is 50 cm greater than the intended laying of the tape (which was determined only by the freezing mark). When carrying out work, you need to find out the following characteristics:
- soil type at the base level;
- location of the groundwater level (GWL);
- the presence of weak soil in the lens area.
To accurately understand the GWL, it will be necessary to conduct research at several locations. At least one of these points must be located at the bottom of the site. Working in a drought does not give an accurate result, since moisture can go deep into the ground.
It is best to find out the ground level in the spring. In this case, the strip foundation will not be afraid even of floods.
Soft ground lenses 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 such a nuisance is discovered during construction, it is covered with crushed stone, gravel or a sand-gravel mixture.
If the groundwater level on the site is deep, then you can use a deep tape (more than 1.5 m). In this case, the water should be located 50 cm below the base of the building. When the groundwater level is located 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 foundation option: slab or piles.
The choice of foundation depth depends on the groundwater level
To calculate the foundation base, you will 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 appendices to this document. The load-bearing capacity of each type is taken from the table below.
| Base type | Maximum load-bearing capacity in kg/cm2 |
| Pebbles mixed with clay | 4,50 |
| Gravel | 4,00 |
| Coarse sand | 6,00 |
| Medium sand | 5,00 |
| Fine sand | 4,00 |
| Sand of silt fraction | 2,00 |
| Loam or sandy loam | 3,50 |
| Clayey | 6,00 |
| subsidence | 1,50 |
| Bulk with compaction | 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, you will need to replace them with medium or coarse sand.
Calculation of the foundation area for a house
At this stage, it is determined what size the base of the building should be so that it can withstand the calculated load and at the same time not push through the soil. If you plan to put the house on a monolithic foundation in the form of a slab, then it does not require calculations. Its area is equal to the dimensions of the house - this helps to distribute the load evenly, the slab provides sufficient resistance to the ground.
The minimum area of strip, column and pile foundations must be calculated using the formula:
S > γn* F/ γc*R,
where γn is the safety factor, assumed to be 1.2;
F is the total load on the sole, calculated earlier;
γc is a coefficient that depends on the combination of soil type and future structure and is in the range of 1.0 – 1.4 (for example, if a stone house is placed on plastic clay, γc = 1.0; when building any structure on fine sand, γc = 1 ,3);
R – calculated soil resistance when laying the foundation to a depth of 1.5 to 2 m.
To calculate soil resistance, you need to know the resistivity value - it depends not only on the composition of the soil, but also on its porosity and moisture content. The calculation of any foundation is based on a preliminary soil analysis of the site. According to its results, the reference value Ro is taken. It is adjusted according to the formula:
R=0.005Ro* (100+h/3), in which h is the depth of the foundation.
Calculation of the amount of concrete for the foundation and reinforcement to strengthen it
The method for determining the volume of concrete mixture has some nuances depending on the type of foundation.
- Tape. Its volume is the product of the total length (including under the walls) by the depth and width. For a house measuring 6 x 10 m, with a total length of the walls of 12 m, the total length of the foundation is determined: (6 + 10) * 2 + 12 = 44 m. With a foundation depth of 1.6 m and a strip width of 0.4 m, its volume (and amount of concrete) will be 44 * 1.6 * 0.4 = 28.2 m3.
- Columnar. Let’s say that concrete supports with a diameter of 0.2 m and a length of 1.5 m are used to make the base. The cross-sectional area of the pillar is 3.14 * 0.22/4 = 0.03 m2. By dividing the total square footage of the foundation by this number, the number of support pillars is determined. It is multiplied by the volume of one column (0.03 * 1.5 = 0.045 m3) and the cubic capacity of concrete for the foundation is obtained.
- Slab. The thickness of the monolithic slab varies from 15 to 40 cm - it is selected depending on the weight of the building. With a parameter of 40 cm and a house area of 60 m2, the required amount of concrete mixture will be 0.4 * 60 = 24 m3.
- Plate. For a light frame or panel house standing on hard clay or rocky ground, reinforcement with a diameter of 10 mm is suitable. If you plan to put a brick house on a slab and the soil is weak, then purchase thicker rods - from 14 to 16 mm. The frame is usually made in increments of 20 cm, making two reinforcement belts. A bunch of belts is made at each intersection point of longitudinal and transverse reinforcement. If the thickness of the slab is 40 cm, the length of the connecting rods will be 30 cm (both belts are 5 cm away from the base planes).
- Ribbon. It is less susceptible to bending, so reinforcement 10 - 12 cm thick is sufficient. It is also laid in two layers, each of which is buried 5 cm in the concrete. With a foundation width of 0.4 m, two longitudinal rods in each tier are enough. If the base is wider, 3 to 4 lines of reinforcement will be required. Transverse elements and vertical connections can be installed every half meter. They, as in a slab foundation, should be 5 cm from the surfaces of the tape.
- Pillars. They are reinforced with rods 10–12 mm thick. Vertical rods (2 - 6 pieces) are evenly distributed throughout the volume of the column - their length corresponds to the length of the support. Cross braces made of smooth reinforcement with a diameter of 6 mm are placed at intervals of 0.4 - 0.5 m in height.
Knowing the consumption rates of reinforcement and its location, it is not difficult to determine how much is needed to create the foundation of a particular house
Influence of soil on foundation depth
When calculating the foundation, it is necessary to know the characteristics of the soil and future loads.
Depth into the soil depends on the type of building and its design. For the calculation, loads acting on the foundation are collected. Geological conditions, degree of soil heaving, sedimentation and freezing are important indicators for foundation calculations. The installation depth is assumed to be no less than 0.5 m (excluding rocks).
In each case, the deepening is calculated according to individual rules so as to take the minimum value to reduce the volume of excavation and reduce the work on restructuring the soil below the base of the pit. Water drainage from the site is simplified when choosing the optimal degree of penetration of the base into the ground.
Rules for determining the depth of placement:
- the sole sinks into the thickness of the load-bearing layer by 10 - 15 cm;
- the presence of soil of small thickness under the base of the base is not allowed if its technical characteristics are inferior to the properties of the underlying layer;
- the backfill is made above the elevation level of the soil fluid to avoid work on water reduction during construction.
The amount of deepening of the foundations is assigned without taking into account the degree of freezing under the internal walls in buildings with heating, if the soils are protected from moisture from the beginning of construction until commissioning.
Calculation of reference area
When choosing a foundation, it is important to correctly determine the minimum permissible area of its support on the ground. It can be calculated using the formula S= γn · F / (γc · Ro), where:
- γc – operating conditions coefficient;
- γn – safety factor taken equal to 1.2;
- F – full (total) load on the ground.
The coefficient of operating conditions (working conditions) depends on the nature of the soil and structure. So, on clay soils for brick structures it is taken equal to 1.0, and for wooden ones – 1.1.
In the case of sandy soil: γc is equal to 1.2 for large and long buildings, rigid small houses; 1.3 – for any small buildings; 1.4 – for large, non-rigid houses.
Weight of the structure
The calculation is based on the load arising from the weight of all elements of the structure, including the foundation itself. Of course, it is quite difficult to accurately calculate the mass of all structural parts, and therefore average values of the specific gravity per unit surface area are taken.
Wall structures:
- frame houses with insulation with a wall thickness of 15 cm - 32-55 kg/m²;
- log and block house - 72-95 kg/m²;
- brickwork 15 cm thick – 210-260 kg/m²;
- walls made of reinforced concrete panels 15 cm thick - 305-360 kg/m².
Floors:
- attic, wooden floor, porous insulation - 75-100 kg/m²;
- the same, but with dense insulation - 140-190 kg/sq.m;
- floor covering (basement), wooden beams – 110-280 kg/m²;
- covering with concrete slabs – 500 kg/m².
Roof:
- metal roofing made of sheets – 22-30 kg/sq.m;
- roofing felt, roofing felt – 30-52 kg/sq.m;
- slate – 40-54 kg/sq.m;
- ceramic tiles – 60-75 kg/sq.m.
Calculating the weight of a structure taking into account the given specific gravities comes down to determining the area of the corresponding element and multiplying it by this indicator. In particular, to obtain the area of the walls, you need to know the perimeter of the house and the height of the walls. When calculating the roof, it is necessary to take into account the angle of the slope.
Foundation weight and snow load
The support area of the structure is determined at the level of the base, which means that the weight of the foundation must also be taken into account in the total load on the ground. The calculation method depends on its type:
- Strip foundation. First of all, the depth (Df) is determined, which should be below the freezing level. For example, at a level of 1.3 m, the normal depth is 1.7 m. Then, the perimeter of the tape (P) is determined as 2(a + b), where a and b are the length and width of the house, respectively. The width of the tape (bl) is selected taking into account the thickness of the wall. On average, it is 0.5 m. Accordingly, the volume of the strip foundation is V=P x bl x Nf. Multiplying it by the density of reinforced concrete (on average 2400 kg/m³), we obtain the estimated weight of the strip foundation.
- Columnar foundation. The calculation is carried out for each support. The weight of one column will be determined as the product of concrete density and pouring volume (V=SxНф, where S is the area of the column). In addition, the weight of the grillage must be taken into account, which is calculated similarly to a strip foundation.
- To determine the weight of a monolithic concrete slab, its volume is calculated (V=SxНф, where S is the area of the slab). The depth is usually about 40-50 cm.
In winter, the load on the ground can increase significantly due to the accumulation of snow on the roof. It is generally accepted that when the roof slopes at an angle of more than 60 degrees, snow does not accumulate and the snow load can be ignored.
If the roof angle is smaller, it must be taken into account. Long-term observations give the following parameters for this load:
- northern regions - 180-195 kg/m²;
- middle zone of the Russian Federation - 95-105 kg/m²;
- southern regions – up to 55 kg/m².
After determining all of the specified weight parameters, you can begin to calculate the minimum area of the sole using the above formula. The total ground load (F) is determined as the sum of the weight of the walls, floors, roof, foundation and snow load.
When calculating a column and pile foundation, the total load is divided by the number of supports, because the grillage distributes it evenly onto the supports.
Calculation of reinforcement requirements
Before starting work, it is important to correctly assess the need for materials to provide foundation reinforcement. The calculation is carried out as follows
Strip foundation
For it, 2 horizontal rows of steel reinforcement of a periodic profile with a diameter of 10-14 mm are usually used.
For vertical and transverse linking, you can use smooth rods with a diameter of 8-10 mm.
The connection between the rods is ensured by steel binding wire.
An example of calculation for a house 6x8 m. The total length of the foundation is 28 m. For longitudinal reinforcement, reinforcement with a diameter of 12 mm is used, and it is laid in 2 pieces in each row (in cross-section - 4 pieces). The standard length of the rods is 6 m.
When connecting, an overlap of 0.2 m is used, and at least 5 joints are required per 28 m. For horizontal reinforcement you need 28x4 = 112 m. Additionally, for overlaps - 5x4x0.2 = 4 m. The total is 116 m.
For vertical linking, rods with a diameter of 8 mm are needed. With a foundation height of 1.4 m, the length of each rod will be 1.2 m. They are installed in increments of 0.6 m, i.e. the number of rods for the entire length is 2x28/0.6 = 94 pieces.
The total length will be 94x1.2=113 m. In the transverse direction, the ligament is provided at the same points. With a tape width of 0.4 m, the length of each rod is 0.3 m. The need is determined as 94x0.3 = 29 m. The total need for reinforcement with a diameter of 8 mm will be 142 m.
The need for binding wire is determined by the number of knots. There are 4 of them in one section, and the total number is 4x28/0.6 = 188. One bundle will require about 0.3 m of wire. The total requirement is 0.3x188=57 m.
Online calculation of dimensions, reinforcement and concrete needs
Columnar
The reinforcement is installed in a vertical position (rods with a diameter of 10-12 mm), tied in cross section with rods with a diameter of 6-8 mm. 4 main rods are required per post, and the tying is done in 3 places.
In the example under consideration (depth 1.4 m), one column requires 4x1.4 = 5.6 m of periodic profile reinforcement with a diameter of 10 mm. For transverse tying, rods 0.3 m long are used.
Their total need is 3x4x0.4 = 4.8 m. Knitting wire needs 3x4x0.3 m = 3.6 m.
Online calculation of dimensions, reinforcement and concrete needs
Slab
Typically, reinforcement is made from steel rods with a diameter of 6-8 mm, laid in a grid in one row. The laying step is 0.3 m. For a house 6x8 m, you will need 6/0.3 = 20 rods in width, and 8/0.3 = 27 in length.
The total length will be (27x6)+(20x8) =382 m. The number of intersections of the rods is 27x20=540, i.e. knitting wire you need 540x0.3=162 m.
