Minimum tape width. How wide should a strip foundation be? Requirement for materials for the construction of a monolithic tape

Strip foundation is perhaps the most popular type of foundation used in low-rise construction. This is primarily due to its versatility, since it can be used to build a house from almost any material. Another question is that this is not always economically feasible, but more on that later. What this type of foundation is is clear from its name. This is a single structure in the form of a tape made of a certain solid building material located under all load-bearing walls of the building.

Based on their design, a distinction is made between monolithic and prefabricated strip foundations. Monolithic - made of monolithic reinforced concrete, prefabricated - a foundation made of FBS blocks or small-piece materials (brick, rubble stone).

According to their depth, strip foundations are divided into buried and shallow, which were discussed separately

This article will consider a recessed monolithic strip foundation.

Main advantages:

high strength and ability to withstand significant weight of the house;
greater reliability and durability;
the ability to build on your own;
opportunity to build ground floor(basement).

Flaws:

significant labor costs due to the large volume of excavation and concrete work;
significant material costs for concrete and reinforcement;
Let’s face it, it’s a dubious prospect to make a high-quality foundation without using construction equipment (we’ll talk about this later).

You cannot choose a recessed strip foundation when building on organic, loess soils, on peat bogs, on fatty water-saturated (even seasonally) clays, on fine and silty sands, which are especially susceptible to moisture.

Important: Level groundwater ideally should not rise closer than 2 meters to the base of the foundation. Otherwise, the possibility of choosing this type of foundation (especially when building a massive brick house) is better to establish when conducting geological and geodetic surveys, because it will be determined precisely by the composition of the soil and its uniformity on the site. Perhaps this type of foundation will have to be abandoned or it will be necessary to do drainage system. Remember that for some soils, when moistened, the bearing capacity changes greatly. This can lead to dire consequences.

The main mistakes during construction.

a thoughtless and unsubstantiated choice of the basic geometric parameters of the foundation strip, such as its height and width.
pouring concrete directly into the dug trench, without taking measures to waterproof and insulate it;
errors when performing foundation reinforcement and when laying household communications into the tape;
other errors related to the technology of work execution.

Now let's look at how to avoid these negative factors.

Calculation of strip foundation.

When calculating, it is necessary to compare the total weight of the entire house and the foundation itself with the bearing capacity of the soil. The first should be less than the second, and with a certain margin. This can be done in the following sequence:

I) We will inspect the building site. All information on this issue is presented

Based on the data obtained, we accept the foundation depth to be 30-50 cm greater than the calculated freezing depth. At the same time, you must understand that based on the calculated depth, you will have to comply with the selected thermal regime in the house in the first winter. Simply put, the house must be heated. Otherwise, if the house remains cold in the winter, the standard freezing depth is taken into account.

The width of the foundation strip is initially assumed to be 20 cm. This is the minimum value, which will be increased in further calculations if necessary.

II) Determine the weight of the house, which will act on the load-bearing soil layer.

The approximate specific gravity of individual structural elements of the house is given in the following table:

Also note that the snow load when the slopes are inclined more than 60º is assumed to be zero.

III) We calculate the weight of the foundation itself. From the house design we know the total length of the foundation strip. Its height and width were determined above, in paragraph I. We multiply these values to get the volume of the tape. We multiply it by the specific gravity of reinforced concrete, equal to 2500 kg/m³, and thereby obtain the weight of the foundation.

We add this figure to the weight of the house (point II) and we obtain the total load on the load-bearing soil (P, kg).

IV) Now we calculate the minimum acceptable value of the required width of the foundation base B (cm) according to the formula:

B = 1.3×P/(L×Ro) ,

where 1.3 is the load-bearing capacity safety factor;

P - total weight of the house with the foundation (point III), kg;

L is the length of the tape (converted to centimeters), cm;

Ro—resistance of load-bearing soil, kg/cm². Its value is approximately taken from the table below:

Let us note once again that the load-bearing capacity values in the table are given for soils of normal moisture. When the groundwater level rises to the bearing layer, the Ro values will change greatly (for example, for oily clay it can decrease by almost 6 times, and for fine sand - almost 4 times).

V) If the resulting value for the width of the tape turns out to be less than 20 cm chosen at the beginning, then we accept the final width as exactly 20 cm. You cannot do less, because the compressive strength of the foundation will not be ensured.

If we get a width that exceeds the initially selected 20 cm by more than 5 cm, then we need to repeat the calculation, starting with III point, substituting a new width when determining the mass of the foundation.

Such repeated calculations are performed until the increase in the width of the tape is less than 5 cm. For those who are a little confused, we will give a small example.

An example of a simplified calculation of a strip foundation.

Let us determine the minimum permissible width of the base of a recessed strip foundation for a 2-story brick house (see figure) measuring 10x8 meters with one load-bearing partition in the middle of the long side. The height of the walls is 5 m, the height of the gables is 1.5 m. The thickness of the walls is 380 mm (one and a half bricks), the basement and interfloor ceilings are made of hollow-core slabs, the roof is metal tiles. The load-bearing soil is loam with an estimated freezing depth of 1.1 meters.

I) Based on the depth of freezing, we assume the depth of laying the tape at the wound is 1.6 meters. To begin with, take the width of the tape equal to 20 cm.

II) Calculate the weight of the house:

1. The total area of the walls of the house together with the gables and the internal load-bearing partition (also folded into one and a half bricks) minus the window and door openings in our case will be equal to 212 m², and their mass will be 212 × 200 × 3 = 127,200 kg.

2. The total area of the basement and interfloor ceilings 160 m², and their weight taking into account the operating load is 160 × (350+210) = 89,600 kg.

3. The roof in our example has an area of about 185 m². Its mass with a metal tile roof and snow load for central Russia will be equal to 185 × (30 + 100) = 24,050 kg.

4. We sum up the obtained figures and get 240,850 kg.

III) The weight of the foundation itself with a height of 1.6 m, a total length of the tape of 44 m and with a previously accepted width of 0.2 m will be equal to 1.6 × 44 × 0.2 × 2500 = 35,200 kg.

The total weight of the house will be 276,050 kg.

IV) Taking the Ro value for loam equal to 3.5 kg/cm² and converting the total length of the foundation strip into centimeters, we calculate the required width:

H = 1.3 × 276,050 / (4400 × 3.5) = 23.3 cm

V) We see that the resulting value does not exceed the initially accepted 20 cm by more than 5 cm. Therefore, the calculation can be completed at this point and the minimum possible width of the foundation base can be taken in rounded form as 24 cm.

Conclusion: By making the width of the base of the foundation more than 24 cm, we can expect that this soil will support the house in terms of its load-bearing capacity.

Now, in a nutshell, what would happen if the bearing capacity of the soil was, for example, 2 kg/cm². Then the width of the tape would be equal to 40.8 cm. After this we return to point III. The mass of the tape becomes equal to 71,800 kg, therefore the total weight of the house is 312,650 kg, and the adjusted width of the tape is B = 1.3 × 312,650 / (4400 × 2) = 46.2 cm.

We see that the discrepancy with the previous value of 40.8 cm was again more than 5 cm, so we return to point again III, we calculate the mass of the foundation, the entire house and an even more precise width of the foundation strip. By the way, this time it will be equal to 47.6 cm. The discrepancy with the previous value is only 1.4 cm, so the calculation can be stopped and the minimum possible width of the foundation base taken as a rounded figure is 48 cm.

Please note that 48 cm is the width of the sole, not the entire tape. It can be narrowed, up to 20 cm (depending on the thickness of the wall and the design of the floors), and only an expansion is made at the bottom (see pictures below). Using the same principle, heavily loaded prefabricated foundations are made from FBS blocks. First, wide foundation pads are laid, and then narrower foundation blocks are placed on them.

At the beginning of the article it was mentioned that almost any low-rise building can be built on a recessed strip foundation, but this is not always advisable. Let's see - why? Let's take a small example wooden house for which the foundation was calculated in the article and let's try to calculate the tape for it. It turns out that its minimum allowable width will be only 7.1 cm. And you will have to do at least 20 cm. The overconsumption of concrete alone will be almost 200%, not to mention all the associated materials and work. Obviously, a columnar foundation in this case would be a better choice.

Thus, we have more or less figured out the calculation, now let’s talk directly about the technology itself.

Stages of construction of a buried monolithic strip foundation.

1) What to dig - trenches or pit?

Sometimes the answer to this question is very simple. For example, if you are going to build a house with a basement, it is obvious that you need to dig a foundation pit. But if the ground floor is not planned, what then?

And then you need to take into account the features of your project, your construction site, the possibility of mechanizing the work and decide on your own (or on the advice of a more experienced builder friend). What you need to pay attention to:

Type of soil on the site, especially its flowability - you must agree, it is problematic to dig a trench with smooth walls that do not crumble at the slightest touch in dry sandy soil. In addition, with great depth and manual work, this simply becomes an unsafe activity.
Fertile layer thickness- this is especially true if you are going to make floors on the ground. The fertile layer will need to be completely removed, because... it tends to decrease in volume over time due to decay processes. And due to the fact that in some regions of our country this layer is very thick, digging a pit and then filling it with non-heaving material (sand) becomes inevitable.
Required width of tape sole- it’s one thing if the calculation requires a width of 20-30 cm, another if it’s 50-60 cm. Filling the entire tape to such a width is a rather expensive undertaking. It can be made with an extension at the base, but for this it is necessary to construct formwork. Fiddling with formwork in a narrow, deep trench is extremely inconvenient, so sometimes it’s actually easier to dig a pit.

2) Preparation and marking of the site.

Before construction begins, it is necessary to take measures to drain surface rainwater from the construction site. You should not pour concrete into soil that has become soggy after rain, and no one is immune from bad weather. Considering the terrain, dig small drainage trenches if necessary.

Before excavating, try to bring the necessary building materials to the site in advance. The shorter the cycle time foundation works(up to the construction of the blind area), the better.

The marking of the site will be discussed in detail in a separate article.

3) Further order of work depends on whether we are going to pour concrete directly “into the ground” or into the formwork.

When pouring into a trench you must:

level and clean the bottom of the trench;
lay insulation if foundation insulation is required;
cover the trench with a layer of rolled waterproofing;
make concrete preparation - pour at least 5 cm of lightweight concrete to the bottom of the trench and let it harden (this prevents damage to the waterproofing layer by reinforcement and protects it from corrosion due to contact with the ground);
install a reinforcement cage on the set concrete preparation, lay household communications;
build leveling formwork for the base;
pour concrete.

When pouring into formwork, the sequence is different:

level and clean the bottom of the trench or a section of the bottom of the pit under the future foundation;
install formwork;
make concrete preparation;
install a reinforcement frame, lay household communications;
pour concrete;
disassemble the formwork;
waterproof the foundation;
insulate the foundation;
backfill the soil.

In the near future, each main stage of foundation construction, such as formwork, reinforcement, will be devoted to a separate detailed article, because they all require special personal attention. And now, in conclusion, a few more general recommendations:

carefully level and compact the base under the foundation strip, especially if this is done after the excavator has been working. The sole must be level and strictly horizontal. If you don’t have a construction level, check with a hydraulic level (costs a penny, sold at any hardware store);
for insulation, use extruded polystyrene foam (EPS) with a thickness of 50-100 mm. Polystyrene foam is not suitable for these purposes. When laying insulation in a trench, you can attach it to the side walls, for example, with plastic umbrellas (fungi) or simply pieces of thick wire, sticking it into the ground through the EPS. For temporary fixation before pouring concrete, this is quite enough;
When covering the trench with waterproofing, make sufficient overlaps (about 20 cm). An extra roll will not save much money;
When installing the reinforcement cage, use tie wire or plastic clamps. Welding in this case is not recommended;
the formwork must be strong and reliable. The recessed strip foundation is quite high and when pouring concrete it will experience enormous pressure. Cases of formwork rupture are not that uncommon in construction, especially when the concrete is compacted with a good industrial vibrator;
Fill the tape using a mixer. A recessed strip foundation is a very massive structure, so in the example discussed above (the foundation for a 2-story house 24 cm wide), the volume of the concrete mixture will be almost 17 m³. It is simply unrealistic to pour them yourself using a conventional concrete mixer so that unacceptable layer-by-layer hardening does not occur;
When pouring, it is advisable to use a concrete vibrator; in extreme cases, do the bayonet with a pointed piece of reinforcement. Also, for better air removal, you can hit the formwork with a small sledgehammer, unless of course you are confident in its strength;
You can remove the formwork and do waterproofing approximately 3-7 days after pouring (depending on the weather - the hotter and drier, the faster).
backfilling of a buried strip foundation can be done with native previously removed soil with layer-by-layer compaction. The use of coarse sand here, as in the construction shallow foundation, no longer important;
try not to delay the construction of the blind area.

Let's leave it at that for now. We will be glad to see your questions and especially your personal experience in the comments.

Structurally for reinforced concrete beams the minimum width is allowed 15 cm, and for shallow strip foundations (which are free-lying beams on an elastic foundation), a width of at least 25 cm for light garden buildings, and a width of at least 30 cm For country houses. The width of a shallow strip foundation cannot be less than the width of the wall resting on it.

However, in addition to design restrictions, there are also specified requirements bearing capacity of soils underlying shallow strip foundations. The specific load from the building per unit area should not exceed 70% on the bearing capacity of the soil. The magnitude of the load can be adjusted using the area of support of the foundation on the ground. The larger the support area, the smaller the specific load transmitted to the ground.

Methodology for approximate calculation of the minimum sufficient width of a shallow strip foundation. This method for determining the minimum sufficient width of a shallow strip foundation is based on the idea that the value of the specific load per unit area of the soil underlying the foundation should be less than the bearing capacity (design resistance of the foundation) of the soil underlying the foundation. The difference between the load from the house and the bearing capacity of the soil should be at least 30 percent greater in favor of the bearing capacity of the soil (safety factor for concrete structures, cast on the construction site with a specific weight of less than 1600 kg/m3). In order not to tire the most impatient readers who are rushing to finally find out, without unnecessary sentimentality, the minimum sufficient width of a shallow strip foundation, we publish a table based on data from the British government building codes Building Regulations Approved Document A: 2010, 2E3, Table No. 10. The British architectural department calculated everything for us in advance, and we tried to average and adapt the presented data a little to make the data presentation more convenient:

Now, the more adventurous and less curious readers can run off to cast their own shallow monolithic strip foundation, and the rest can find out how the British obtained this data and make their own more accurate calculation for their own home, so as not to get into trouble.

To determine the minimum sufficient width of a shallow strip foundation based on the bearing capacity of the underlying soils, it is necessary to solve the equation:

Dead weight of the building- this is the sum of the weights of all building elements of the house structure. To calculate them you need to use the following tables:

Wall material	kgf/m 2
Wooden frame-panel, 150 mm thick with mineral wool insulation
From cellular concrete blocks with a density of 500-600 kg/m3 of solid masonry, thickness, mm: 200 250 300 350	100-120 125-150 150-180 175-210
Made of sawdust concrete, 350 mm thick
Made of expanded clay concrete, 350 mm thick
Made of slag concrete, 400 mm thick
Made from effective brick, thickness, mm: 380 510 640	500-600 650-750 800—900
From solid brick of continuous masonry, thickness, mm: 250 380 510	450-500 700-7501 900- 1000

Table No. 6Load from 1 m 2 floors with a span of up to 4, 5 m

Table No. 7Table of the amount of timber per m 3 of lumber

Table No. 8Table of the number of boards per cubic meter of lumber

Board size (mm)	Number of boards with a length of 6 m per cubic meter of lumber	Volume of one board 6 m long (m 3)

Table No. 9Tableroofing weight values

View roofing material	Weight 1 m 2 (kg)
Rolled bitumen-polymer roofing
Bitumen-polymer soft tiles

Metal tiles
Corrugated sheet, Galvanized steel, Seam roofing
Cement-sand tiles
Ceramic tiles

Slate roofing

Green roof

Table No. 11 Load from 1 m 2 horizontal projection of the roof

Any structure must have a foundation. In order for a building to serve for many years, it is very important to correctly calculate the parameters of the foundation. And in order to do everything right, you need to know certain characteristics.

Width calculation

To lay the foundation of a building, it is important what kind of soil is on the site, at what level the groundwater is, the weight of the building itself, and how much the ground freezes.

All design work is based on engineering calculations. These are complex calculations, so average load values are usually used for calculations.

Roof:

slate - 40-50 kg/m2;
roofing felt - 30-50 kg/m2;
tiles - 60-80 kg/m2;
sheet steel - 20-30 kg/m2.

Walls:

brick - 200-270 kg/m2;
reinforced concrete - 300-350 kg/m2;
wood - 70-100 kg/m2;
frame with insulation - 30-50 kg/m2.

Sopor ≥ Rdom/ Qn.sp, Where:

Sosupport– lower support area;

Rdom- weight of the building;

Qn.sp- bearing capacity of the soil

Soil bearing capacity- the ability of the soil to withstand the load by 1 cm area.

For a two-story house

Shf– foundation width,

WITH– value of soil resistance;

IN– value that takes into account the smallest soil weight.

Height calculation

According to SNiPs, the foundation must protrude at least 20 cm above the ground, however, in practice, when taking into account the main parameter - the depth of soil freezing, this value increases to 30-35 cm.

The deeper the level of freezing, the greater the height of the foundation should be. When freezing up to 3 m, the height of the foundation can reach up to 1 m.

For a two-story building, the choice of the height of the foundation protrusion above the ground is completely unimportant; the number of storeys in no way affects the stability or strength of the building. During construction, they are guided by the convenience of constructing the entrance to the building.

According to the standards, there must be at least 3 steps at the entrance, and this is possible with an optimal value of 35-40 cm. Such a protrusion performs another function - it protects structures from the constant influence of soil and precipitation. Also, to ensure that water does not have a destructive effect on the foundation of the house after construction is completed, it is advisable to do it around the building.

The minimum value is considered to be the height above the ground - 35-40 cm, but if the foundation is higher than these values, then this is acceptable. The only condition is that the height of the protrusion should not exceed the width of the foundation.

To summarize: the foundation is the main part of the structure, on which the durability of the building depends. Therefore, it is necessary to approach its construction responsibly, making accurate calculations and adhering to existing norms and rules in construction.

Only in this case will the building being constructed be reliable, last a long time and become a reliable shelter.

Construction of a house begins with the construction of the foundation. The strength and durability of the house built on it will depend on the quality of the foundation. The cost of this lower part of the future structure is approximately a quarter of the entire cost of the house. In addition, this is a labor-intensive process and requires a clear understanding of its ultimate goal. Errors made during calculations and during the construction of the foundation will be very expensive when corrected. Often the amount for reworking a defect even exceeds the initial costs. Avoiding mistakes when constructing a foundation can only be done by competent calculation and execution.

The strip foundation is universal. This type of foundation is suitable for houses with any wall structure.
In cross section, the strip foundation of the house forms a rectangle located vertically. The upper part of this rectangle should take into account the slope of the construction site and protrude with its edge approximately 100 mm above the plane of the adjacent soil surface. In addition, the upper edge of the foundation, depending on the design of the walls of the house, may be wider than the thickness of the wall.

When constructing a residential building with 1-3 floors, the transverse dimensions of the strip foundation usually do not differ much. This can be explained by the fact that the loads from the house on the ground are insignificant, while the area of the supporting base of the foundation will always be approximately 2-3 times larger than required by calculation.

So, depending on the material used to create the strip foundation, the average width for rubble foundations is 600 mm; for concrete or reinforced concrete and rubble concrete 400-600 mm; for foundations made of brick - this is 500-550 mm. This width of the base of the strip foundation ensures dressing vertical seams stones and is convenient to use, reducing unnecessary labor costs.

If the soil at the construction site is weak or heterogeneous, then most likely the pressure of the house with its weight on such soil will exceed the norm (for central Russia this is 1-1.5 kg/cm²). In this case, it is necessary to increase the width of the foundation base. This can be done by creating ledges along the height of the foundation every 300-600 mm. In addition, you can create a “cushion” in the lower part of the foundation by placing a reinforced concrete slab or compacting coarse sifted sand, with grains of sand 1-2 mm in size, in a layer 200-300 mm thick.

Terms, definitions and parameters for calculating a strip foundation for a house

Foundation- the lower part of the house, located underground, designed to transfer and distribute the load from the building to the ground.
Foundation sole- the foundation plane directly interacting with the ground.
Foundation depth- the distance from the base of the foundation to the ground surface.
Underlying soil layer (base)- a layer of soil on which the base of the foundation rests.
Estimated soil freezing depth- the position of the frost line relative to the ground level, accepted as a calculated value (SNiP standards).
Ground water level- the position of the groundwater table relative to the ground level in a conditionally open pit (well).
Compressible soil thickness- a deformable part of the soil that takes the load from the foundation.

The procedure for calculating a strip foundation for a house

Determining the depth of the strip foundation

The depth of the foundation depends on the depth of soil freezing, the groundwater level and the underlying soil layer. Typically, the foundation depth is chosen below the freezing depth, but not less than 500 mm.

The depth of freezing is determined by the climatic conditions of the region and corresponds to the greatest value of freezing of wet soil without snow cover at the lowest recorded temperatures. It can be determined from the table:

In addition, it must be taken into account that when living in a house all year round, when in winter it warms up due to heating, the estimated depth of soil freezing decreases by 15-20%.

Also, the estimated depth of soil freezing can be determined by the formula:

df=kn× d fn,

kn- the coefficient taking into account the influence of the thermal regime of the structure is taken according to table 1 SNiP 2.02.01-83 Foundations and foundations;
dfn- The standard depth of seasonal soil freezing is taken from the climate map “Zonation of the territory of the Russian Federation by mass of snow cover.”

When calculating the depth of the foundation relative to groundwater, they strive to ensure that the base of the foundation transfers the load to a durable layer of soil. This layer is located above the groundwater level, which depends on the geological conditions of the area where the construction site is located.

When calculating the depth of the foundation, taking into account the presence of the design features of the house, the depth of the basement is also taken into account.

Thus, the depth of the foundation is equal to the maximum value, consisting of values calculated from climatic (depth of soil freezing), geological (depth of groundwater) and structural (presence of a basement) features.

Calculation of the dimensions of the foundation base

Determining the geometric parameters, the main one of which is the area of the foundation base, is also a critical stage of the calculation. Since the reliability of further operation of the house depends on the correctness of its definition. If the support area turns out to be less than necessary, this will lead to uneven subsidence of the structure and its deformation. Excessive area of the foundation base is an unjustified additional expense.

There are two options for calculating the area of the foundation base using limit states. The first calculation option is based on the bearing capacity of the base, the second is based on permissible deformations of the structure.

Calculation of the dimensions of the foundation base based on the bearing capacity of the foundation

It consists of assessing the resistance of the underlying soil layer to the effects of the weight of the structure. Under the influence of operational loads due to compaction of the foundation soil, a deformation shift of the soil layers occurs and foundation settlement occurs. The depth of settlement depends both on the strength of the foundation soil and on the magnitude of the force pressing on the soil. There is a formula for calculation:

S>ϒnF/ϒcR O ,

Where:
S- area of the foundation base (cm 2);
F- design load on the base (total weight of the house with additional operational load) (kg);
ϒ n - reliability coefficient (equal to 1.2);
ϒ c - working conditions coefficient;
R o - conditional design resistance of the foundation soil for the foundation.

The total weight of the house with additional operating load is the sum of the following values:

the mass of the walls of the house with gables and internal partitions;
weight of the basement and interfloor floors;
roof mass taking into account roofing material and snow load.

There are convenient online calculators for calculating the weight load on the foundation that you can use. For an approximate calculation total weight You can use this table at home:

The operating conditions coefficient has the following values:

The conditional design resistance of the foundation soil for the foundation is determined from the tables:

Calculation of the dimensions of the foundation base based on permissible deformations of the structure

Unlike the first option for calculating the area of the foundation, this option allows you to eliminate deformation of the house structure from uneven subsidence of the foundation. This is ensured by assessing the compliance of the real and permissible level of deformation of the structure under the influence of operational loads.

There are the following types of structural deformations:

Deflection and camber- occurs due to uneven settlement of the foundation base.

occurs when the foundation settles on one side.

occurs when the flexural rigidity of a structure (high-rise structure or element) is high.

Skew- occurs when the foundation settles unevenly in one of the areas.

Horizontal offset- occurs in foundations, in basement walls, in retaining walls in areas loaded with horizontal forces.

The permissible values of deformation of structures depend on the design features and materials used in construction are given in the table:

Relative unevenness of settlement - the maximum ratio of the difference in settlement of two sections of the foundation to the distance between them. Based on this value, it is possible to determine the minimum value of the distances between areas with uneven settlement, and determine the estimated area of the foundation base.

Example:

Let's pretend that two-storey house made of brick was deformed in the form of a deflection in the center by 1 cm.

The distance along the length of the foundation between the measurement points of the deflection section is 600 cm. With a building length of 12 m, the relative unevenness of settlement is 1/600 = 0.0017. The permissible uneven settlement of such a structure according to the table is 0.002. Therefore, a draft of 1 cm is acceptable.

Calculation of the height of the base

The height of the plinth is calculated taking into account the climatic characteristics of the area of the construction site in terms of the amount of snow cover and the necessary characteristics of the rigidity of the cross-section of the foundation.

Calculation of the amount of materials and the total cost of the strip foundation

Calculation of reinforcement for strip foundations

Reinforcement of the strip foundation is necessary to increase the resistance to deformation and operational loads of the structure. To reinforce the foundation, you will need to calculate the diameter and number of reinforcement bars.

From practice and technical documentation it follows that the amount of reinforcement should be at least 0.1% of the cross-sectional area of the base.

Calculation of reinforcement diameter:

For light structures - 8 mm.
For medium structures - 10-12 mm.
For heavy buildings - 14 mm.

Calculation of the number of reinforcement bars:

To calculate the total footage of horizontal rods, it is necessary to multiply the perimeter of the entire foundation by 4.
To calculate the number of jumpers, you need to divide the total length of the foundation by the planned length between the jumpers and also multiply by 4.
If the foundation is deep and includes two frames, then in the calculations all results are simply doubled.
When calculating the number of rods, you can proceed from the standard length of reinforcing rods - 6 m.

Example:

For a strip foundation for house 6× 8 m with two partitions of 6 m and 4 m has a perimeter of (6+8)×2=28 m. Taking into account the partitions, the total perimeter will be 28+6+4=38 m. This means the total footage of the reinforcement will be 38 × 4 = 152 m. Taking into account the length of the reinforcing rod - 6 m, on the 8 m section you will need another 2 m of rod, for which the remains of a 4-meter partition are suitable. Thus it turns out (4+4) X 2=8 joints. Taking into account the overlap of the rods at the joints of 0.5 m in both directions, 152 + 8 = 160 m of reinforcement will be needed. In pieces it will be 160/6 = 26.6, rounded up to 27 pieces of reinforcing rod. For lintels with a knitting distance of 0.5 m, with a foundation length of 38 m, for vertical and horizontal rods you will need 38/0.5 ×4=304 pieces. With a frame height of 0.5 m and a width of 0.25 m, 304/2 × will go horizontally0.25=38 m, and vertically 304/2 × 0.5=76 m of reinforcing rod. The number of rods for the jumpers is (38+76)/6=19 pieces.

Calculation of concrete for strip foundation

To calculate the amount of concrete for the construction of a strip foundation, there is a simple formula:

TO bΣ= Sh f× IN f×( D 1+D 2),

TO bΣ - required amount concrete;
Sh f - foundation width;
IN f - foundation height;
D 1 - length of the inner side of the structure;
D 2 - length of the outer side of the structure.

Calculation of the cost of a strip foundation

To calculate the cost of a strip foundation, it is necessary to clarify the current prices for the materials used and multiply by the values obtained from the calculations described above.

Concrete will cost the most. Its cost will be 25% of all foundation costs. Then 15% to 20% of the cost, depending on size and quality, will go to fittings. Formwork will require 10% of the total cost. Next 5% of the cost is transportation costs. Other, not the smallest 40-45% of expenses will require sand, wire for knitting, waterproofing material, brick, fasteners, tools, etc.

There are also convenient online calculators for a complete calculation of the strip foundation and videos with detailed instructions.

Video on the topic

The decision on the foundation parameters is made on the basis of certain indicators that characterize not only the building itself, but also the surrounding area.

In the project, already prepared with all the calculations, the thickness of the entire foundation, the depth of the foundation, and so on are accurately indicated. Such indicators depend on the nature of the soil and hydrogeological conditions, that is, on the groundwater level, the nature of the soil, the depth of its freezing during severe frosts, and so on. In the most common case, the decision on the foundation is made on the assumption that the soil is averagely unkempt.

To determine the nature of the soil you need to dig a hole with the following indicators:

The length must be more than a meter;
The width must be more than a meter;
Depth from 2.5 to 3 meters.

To determine the content of clay particles in the soil, you need to pour a little earth into a glass of water up to half a glass.

Next you need to let this mixture brew. After that, all that remains is to measure the layers: the layer that is below is clean soil, the layer that is above is clay rocks. After measuring the layers, you just need to figure out their relationship to each other.

This whole procedure for determining clay content is needed in order to avoid troubles during further construction and operation of the building. The whole point is that clay soil tends to swell from moisture, which can lead to a change in the bearing capacity of the foundation.

The optimal soil for the foundation can be called loamy rocks, which have very low humidity, that is, almost dry rocks. Also, the bearing capacity and water permeability of clay rocks changes when the admixtures of sand and gravel in them change.

Also a good option would be sandy loam, which is also in a dry state, but if it is damp, then it becomes mobile.

Soil types

For these reasons, you need to carefully select the soil on which to build, and based on this, determine the required thickness of the foundation.

Heaving soils

In heaving soils, in which the groundwater level is more than two meters from the maximum freezing depth, the foundation should be laid to a depth equal to the freezing depth multiplied by 0.75, but this figure should not be less than 70 centimeters.

Sandy soil

Based on grain size, sandy soils are divided into:

Gravelly;
Large;
Average;
Small;
Dusty.

Silty and fine sands cannot serve as reliable support for the foundation, especially if they contain an admixture of clay or silt. Coarse-grained sands are less susceptible to heaving and have low water absorption, so such soils serve as an excellent base.

If groundwater flows at a depth of 2 meters from the freezing level, then the foundation level is approximately 6.5 meters.

If the groundwater level is at a fairly close distance from the surface of the earth, then the foundation needs additional protection, for example, its expansion, the use of a pile structure, and so on.

Peaty soil

Such soil cannot be a good foundation for a foundation. This is due to the fact that peaty soils have a high percentage of moisture.

Types of foundation and its dimensions

In practice, most people use two main types of foundations - strip, or solid and columnar. It is better to use strip foundations when they are laid shallow, as well as in cases where the walls of the building are made of heavy materials.

As a rule, in heaving soils, in order to save money, the trench is filled first with sand, then with a layer of crushed stone and a layer of gravel. Each layer is watered generously and compacted. The height of such a pseudo-foundation should not exceed half the height of the main foundation.

Depending on the thickness of the wall, you can determine the thickness of the foundation. But it is worth noting that it should not be less than 35 centimeters.

In order to reduce the subsidence of the foundation by reducing the pressure on the soil, it is thickened from below. This is done by constructing two or three steps.

The height of such steps should be between 30 and 40 centimeters, and the width should be about 15-25 centimeters. The foundation edge must be placed above ground level.

Rubble; Usually in private construction the following types of strip foundations are used:

Brick;
Rubble concrete;
Concrete;
Monolithic;
Prefabricated.

For rubble and rubble concrete strip foundations, the thickness is chosen to be approximately 10 centimeters wider than the wall. Such foundations are built from granite, sandstone, which is very dense. The height of the ledge must have at least 2 rows of masonry.

The following mortar compositions are used for masonry:

For low-moisture soils, a cement-lime composition is used, which consists of 1 part of cement of a grade not lower than M400, 2.1 parts of lime paste, 15 parts of sand;
For wet rocks, the same components are used, but in a ratio of 1k0.7k8;
For saturated soils - 1 to 6 without lime mixture.

Before laying the first row, a solution about five centimeters thick is poured onto the base. Then you need to lay stones on top and compress them tightly.

When constructing a rubble concrete foundation, the filler material can be stone, crushed stone, brick, broken brick, and so on.

If the foundation walls are completely vertical and the depth does not exceed one meter, then a compactor is placed at the bottom of the trench, that is, cement mortar, about five centimeters thick, and one of the above-mentioned fillers is placed on it.

If there are several layers, then every 15-20 centimeters they need to be separated by a layer of cement mortar.

In turn, each layer must be shed with liquid cement mortar. If the height of the foundation is more than a meter or the width of the pit is greater than the width of the foundation itself, then wooden panels are placed on its sides, which serve as formwork. It can be removed only after two weeks in hot weather.

The thickness of precast foundations is determined by prefabricated elements, such as blocks. They can have a thickness of 30, 40,50, and 60 centimeters.

Since it is rare where you can find slabs of smaller thickness. In this case, the calculation of the thickness of the foundation slab is made based on data about the walls, soil, and so on. The blocks should be laid with the seams tied vertically. In order to expand the base, reinforced concrete slabs are used. In this case, the minimum thickness slab foundation will be equal to 60 centimeters.

The second type of foundation is columnar foundations. Such a foundation is much more economical than a strip foundation. It can be used when the walls of a building will be erected from relatively lightweight building materials.

The distance between the pillars should not exceed 2-2.5 meters. This applies to corners and where walls intersect. When the pillars are ready, reinforced concrete slabs or lintels must be laid on top.

Such jumpers are called rand beams. The walls of the building will be located on them. In turn, the pillars can be erected from stone, brick, or be rubble or concrete.

According to the general classification, they are also divided into monolithic and prefabricated.

If the pillars are made of brick, then only red, well-burnt brick should be used. The minimum thickness of pillars for the foundation is 50 centimeters.

However, there are exceptions. For example, if a one-story building is being built frame house, then you can make the thickness of the pillar equal to 38 by 38 centimeters for the corner post, and 38 by 25 centimeters for the intermediate post.

The minimum thickness of the pillars, and accordingly the foundation itself, which are made of rubble stone is about 60 centimeters in both directions.

In order to make a monolithic pillar, it is necessary to make formwork, or a special foundation pit. Concrete should have a density of about 1.8 tons per cubic meter, that is, it should be very heavy. The minimum width for such a foundation is 40 centimeters in both directions.

As a ceiling, rand beams are placed on the pillars, which can be made of prefabricated or monolithic reinforced concrete. In order to ensure free settlement of the rand beams, given that the building gives a general settlement, and also to protect against ground heaving, a sand layer about 25-50 millimeters thick is poured under the beams.

Calculation of foundation width

The calculation process consists of several stages:

At the first stage, you should decide on the type of soil, as discussed above;
Next, using a special table, you need to determine how much load can be given on one centimeter of a square pillar that stands on different soils.

You need to grind it and determine the volume using a measuring cup. Then the following formulas are used: A=1-P1/P; Р1=П/В0;Р=П/В1.

In these formulas, P and P1 are the volumetric weight of the earth in a natural and compacted state, P is the weight of one unit volume of soil, B0 and B1 are the volumes of soil in a natural and compacted state.

And now again, using a special table, we select the soil resistance. For example, sandy loam has a resistance of 3 kilograms per square centimeter with a coefficient of 0.5.

Next, taking into account the bearing capacity of the soil where the building stands, you can calculate the number of pillars, or rather the step at which they should be installed. This will also help to calculate the cross-section of each pillar.

For example, you can take the following data from the table

Wooden or frame-panel walls, the thickness of which is 15 centimeters, exert a pressure of approximately 40 kilograms of force per square meter.

Thus, we calculate the total weight of the walls and use another table to look at the load from the floor. For example, a reinforced concrete basement floor exerts a load of 300-500 kilograms of force per square meter of foundation. One square meter a wall made of cellular blocks with a density of 600 kilograms per cubic meter exerts a pressure of approximately 120 kilograms of force per square meter.

Again we calculate the total load from the entire floor. Then we calculate the load from the roof in the same way. After this calculation, it is necessary to add up all the resulting masses and obtain the total mass of the entire house or building.

When the weight is found, you can add to it the weight of the decoration and furniture. Then we simply divide the entire weight by the number of pillars and get the mass that falls on one pillar. After this, we divide the mass by the area of support of one pillar.

For example, let the resulting mass per column be about 10,000 kilograms. Let the pillar have a square cross-section with a side of approximately 1 meter. Then its support area will be 1 times 1 - 1 square meter, that is, 10,000 square centimeters.

From these calculations it is not difficult to calculate the weight per square centimeter of soil under the pillar, that is, divide 10,000 by 10,000, and we get exactly 1 kilogram per square centimeter.

Then we look from the table in the first paragraph to see what bearing capacity the soil on which the building stands has. If it is less than this figure, then the area of the pillar or the number of pillars should be increased, but if it is larger, then all the foundation data have been selected correctly.

When calculating the weight of a house, it is allowed not to take into account the weight of the interior decoration and furniture, just like the people in the house. This is not due to the fact that this weight is small, but to the fact that when calculating the mass of the walls, openings were not taken into account, that is, doors, windows, arches, and so on.