What is the sole of the foundation. Based on what conditions determine the dimensions of the sole of the foundation of a shallow foundation
Column Foundation
The base of the most common monolithic strip foundation is a reinforced concrete platform, which is needed so that the load from both the foundation itself and the building that stands on it is evenly distributed to the ground. As a rule, the width of the sole of the strip foundation or the base of the foundation should be twice the width of the foundation itself.
The construction of the base of the foundation comes from the calculation of data that characterize the soil.
The height of such a sole, as a rule, is made no more than thirty centimeters, and the width of the sole of the foundation is made at the level of sixty centimeters. For the most part, such foundations are reinforced with several rows of reinforcement, one rod of which has a diameter of twelve millimeters.
Sometimes it happens that the width of the sole exceeds the width of the foundation several times. This is due to the fact that some types of soils simply cannot hold large masses that arise during the construction of fairly large objects.
Construction stages
Before you start construction, you need to mark the exact location of the foundation in the pit, that is, outline the corners and intersections of the walls, and so on. If before the start of work, surveyors worked on this site, then the marking is not difficult. It remains just to pull the cord between the poles (special flags). Landmarks, as a rule, are installed even before the moment they began to dig a pit.
Also in this case, a plumb line is used. It helps to set new flags. For convenience, pieces of reinforcement can be used as such flags - then when pouring the foundation, they will not need to be removed, but poured along with them. The flags must be set at a distance that exactly corresponds to the length of the wall that will stand on this section of the foundation.
After two flags are set, you need to set two more, that is, in the remaining two corners. You can do this diagonally. It consists in the fact that with the help of simple mathematical calculations the diagonal of the building is accurately calculated based on knowledge of the length and width of the building.
Knowing the length of the diagonal and the dimensions of the foundation, you can easily and most importantly accurately determine the position of the other two flags. It is done like this:
The width of the sole of the strip foundation is often greater than the width of the foundation itself
- Two people hold the beginning of the tape measure at the points already marked;
- Another person crosses the two free ends of the tape measures at the mark that shows the length of the wall;
- At the intersection point, another flag is driven into the ground.
After the markup is made, it must be fully checked to eliminate possible errors. It's easy to check. All you need to do is simply measure the lengths of all sides, and if they correspond to the construction plan, then the markup is done correctly.
Foundation formwork
After marking and checking it, if successful, formwork should be prepared for the future foundation. For it, you can use ordinary boards that are about 30 centimeters wide and at least three in thickness. This is due to the fact that when pouring concrete, it will exert a very large lateral pressure on the formwork, and thin boards can simply bend, which will lead to a curvature of the foundation.
To fasten the boards together, it is necessary to drive U-shaped metal rods into the ground, while the horizontal bar of such a rod should be no more than the width of the foundation. Such elements must be placed from each other at a distance not exceeding 70 centimeters.
The boards themselves must be positioned so that the wall is exactly in the center of the foundation.
The work begins with the fact that two boards of the specified size are fastened together at an angle of ninety degrees. Such a structure will serve as an outer corner. Further, this angle is set at a certain distance from the cord.
After that, using U-shaped brackets, we install the inner walls of the formwork, which must be installed exactly parallel to the outer walls. So there is a gradual advance from one corner of the formwork to the second and third. All brackets that fix the formwork can be placed on straight sections at a distance of about 110-120 centimeters.
At the junction, the boards should be knocked together with nails, which should be driven in at an angle in order to nail two boards with one nail. On the sides of the joint, one fixing bracket must be installed.
If the boards have slightly curved ends, then in order not to get a gap between them, another board is nailed up, from the outside, which closes this gap. If any board turned out to be a little longer than all the others, then you can not cut it, but simply nail it on top of the second board.
backfilling
The width of the foundation is calculated depending on the load of the building and the bearing capacity of the soil
After complete installation formwork, some places should be strengthened. This can be done using backfill. It is necessary to sprinkle with earth those places where there is a potential weakness, for example, the place of the joints of the formwork boards, or the place where there is no way to drive in a latch, and so on. Such places need to be sprinkled with earth to the very top of the boards. In addition, you can sprinkle the entire foundation around the perimeter, but with less land. This will prevent the formwork from being lifted and pushed out of its position when the ground is very wet, such as during rain.
Setting the foundation level
You can set the level of the edge of the foundation using a theodolite. There are two main rules for using this tool:
- It must have a strictly horizontal arrangement;
- Must be placed at the exact specified depth.
In order not to remeasure later, the level marks can be fixed by means of small nails. Hammering nails is only half their length in increments of about 0.5-1 meter. Nails are hammered from the inside of all formwork boards. Later, when concrete begins to be poured into the formwork, such nails will serve as a measuring line along which you need to navigate so that the foundation is not poured higher in one place and lower in another.
pouring concrete
Trench for strip foundation
The concreting of the pit begins with the most inaccessible places. If it turns out that there is no way to approach some places at all, then their filling happens like this:
- First, we begin to fill in the place that is located next to the hard-to-reach;
- With a shovel, we rake concrete into a hard-to-reach place until it reaches the level marked with nails.
Foundation reinforcement
After the concrete has been poured, the reinforcement of the concrete can begin. It is better to strengthen the foundation with reinforcement with a diametrical section of 12-12.5 millimeters. To do this, the reinforcement bars must be laid out on liquid concrete, at a distance of about fifteen to twenty centimeters from each formwork wall. The rods need to be pushed under the U-shaped clamps.
After the bars are laid, they should be sunk into the concrete. This can be done with bayonet shovels. It is necessary to produce recession to a depth of about twenty centimeters, that is, two-thirds of the length of the shovel bayonet.
When the bars are completely immersed in concrete, in order to avoid air getting there, you need to make a tracing from above with a shovel, that is, repeatedly push the shovel into the concrete and stick it out, so that the shovel bayonet is perpendicular to the reinforcement bar.
Foundation grout
Now that the reinforcement is laid, you need to slightly raise the U-shaped fixed elements. It is not worth raising them completely, but to a height of about 5-10 centimeters. This is necessary in order to perform the grouting of the edge of the concrete surface in order to smooth it. In turn, smoothing is necessary in order to facilitate subsequent work on the construction of the basement or walls, as well as to simplify the process of removing dirt from the foundation.
Keyway cutting
Such a groove is needed to ensure a reliable connection between the foundation and the plinth or wall of the building. An extrusion is performed along the entire center line of the upper foundation edge. There are no standards for the size of the groove, but it is usually made quite wide. For example, as one of the dimensions of such a groove, there may be dimensions:
In general, such indicators can range from 2.5 to 5 centimeters, and from 6 to 10 centimeters, respectively.
Indentation is best done with a long wooden block with a rectangular section, and, as a rule, the width of the groove is determined by the width of the beam.
It is best to make a groove device after the concrete has already hardened a little. This fact will allow the groove to keep its rectangular shape and not swim. However, if the concrete is already too hard, then when the beam is pressed in and then removed, the walls of the keyway may crumble.
Grooves should only be placed in straight sections. They should not be made at the corners, moreover, the grooves should not reach corners of the order of 50-80 centimeters.
Formwork cleaning
After the foundation concrete has gained about 80 percent of its strength, which is achieved after a week in hot weather, then you can remove the formwork. Before removing the boards, you first need to carry out some work. For example, by drawing all corners. This is done as follows:
- First, we take a ruler and on each outer formwork board at the corner we mark a distance of ten to fifteen centimeters;
- Further, drawing directly along the foundation, we draw lines from the points parallel to the walls;
- Place a dot where the lines intersect.
As a result of such simple work, it turns out that we have drawn a square, one corner of which is the outer corner of the foundation.
Such work is needed in order to then know exactly where the outer corner of the foundation is, since it often happens that it is chipped during the construction process, and it becomes unclear in which place of the foundation to display the corner of the wall.
Column Foundation
A columnar foundation is used when it is necessary to build a building that will have a relatively small weight, for example, such a building can be a frame house.
Structurally, such a foundation consists of ordinary pillars and floor slabs. Poles can be made of various materials:
- Brick;
- Stone;
- tree.
You can use other materials.
The width of one column depends mainly on the bearing capacity of the soil on which it is installed, and on the mass of the entire building. It is very easy to calculate this.
First of all, you need to find out what type of land you plan to build on. Further, according to the reference data, you can find what kind of bearing capacity this type has. For example, we learned that no more than 2.5 kilograms of force per centimeter can be exerted on the ground. square square soil.
Then further we measure the mass of the planned building. This can also be done according to special reference data, based on the characteristics of each building material. For example, if it is known that construction will take place in foam blocks, then it is not difficult to calculate how many pieces of such blocks are needed and how much they will all weigh. In the same way, we find out the mass of the ceiling and the roof.
The mass of the finish can be ignored, as well as the people inside the building. This weight has already been taken into account, since there was no deduction for all niches, that is, windows and doors.
After all calculations of the mass have been made, and it has become known, it is necessary to calculate the area on which all this mass will stand. They do it this way: first calculate the number of pillars, then the area of contact with the ground of each pillar, that is, the width of the pillar is multiplied by the length of the pillar. After that, you can calculate the total area of \u200b\u200bthe support, as the number of pillars multiplied by the area of \u200b\u200bthe support of one column.
After this calculation is made, then you need to find out with what force the house will press on one centimeter square of the support area. To do this, you need to divide the entire weight by the entire area. We get the pressure per square centimeter. For example, the entire mass turned out to be 100,000 kilograms, and the entire area is 50,000 square centimeters, respectively, 2 kilograms of force will be exerted on one square centimeter.
A monolithic strip foundation is not reinforced only during the construction of small and non-critical buildings - garages, utility sheds, garden arbors. In the case of the construction of residential buildings, public, industrial, commercial buildings, especially in difficult soil conditions, reinforcement is mandatory.
Reasons why you need to reinforce a reinforced concrete foundation
In a reinforced concrete structure, each component - concrete or reinforcement - performs a different function. Concrete in tension is able to elongate only a fraction of a millimeter. At high tensile loads and transverse shear forces in an unreinforced concrete structure, deformations can occur, leading to cracking and the appearance of other defects, up to destruction.
The steel frame members of reinforced concrete can take tensile loads ten times greater than those that concrete can take. Plastic rolled steel, having the ability to elongate without breaking by 5-25 mm, works in tension, preventing the development of deformations in the structure beyond the allowable limits.
A monolithic foundation tape is a system of beams connected to each other at corners and intersections, lying on a solid elastic soil base. Soils are constantly affected by climatic factors - they freeze in winter and thaw in spring, are moistened by surface or groundwater, while increasing or decreasing in volume.
The forces arising in this case from below are transferred to the foundation, and with a constant load from the building from above, compressive and tensile forces arise in the structure. At the same time, compression and tension can experience different zones sections of monolithic beams that make up the strip foundation.
Therefore, the main scheme for reinforcing a strip foundation is a three-dimensional frame with the location of rolled steel products at the top and bottom of the cross section. If the width of the sole of the tape exceeds the width of the wall by more than 600 mm, then the sole is additionally reinforced with flat meshes.
What reinforcement is used to reinforce strip foundations
Reinforcement of the strip foundation is carried out by means of spatial frames and flat meshes, in which rolled steel products are divided into workers, which perceive the main tensile forces, and constructive, which serve to fix the working rods.
Consider which steel rods can be used for a strip foundation. As a working one, corrugated steel of class A3 is used, according to another classification A400, produced according to GOST 5781-82* or A500C according to GOST R 52544-2006. Corrugated rolled products contribute to better adhesion of working rods to concrete. Reinforcement of the strip foundation with A500C rolled products allows welding frames and meshes. As a constructive, rods with a smooth surface of class A1 or, according to another designation, A240 are used.
Reinforcement of a periodic profile
We wrote about the use of working reinforcement of classes A3 and A500C, the differences between them, the benefits of using A500C, the features of installing frames and grids in the article “Strip foundation: from earthworks and pillows to pouring concrete and removing formwork”.
All reinforcement work must be carried out following the instructions of the technical documents. SP 52-101-2003 "Concrete and reinforced concrete structures without prestressing reinforcement", SNiP 52-01-2003 "Concrete and reinforced concrete structures", using which you can reinforce the strip foundation with your own hands.
Calculation of the reinforcement diameter and the number of working bars for the tape
Diameter round bars for a strip foundation is determined on the basis of a calculation that takes into account the loads carried by the foundation. The load is collected from all load-bearing walls in the application for 1 linear meter along the length of the foundation. The total load includes:
- self-weight of wall structures made of different materials masonry, lightweight concrete blocks, wooden, monolithic reinforced concrete, etc.;
- own weight of floors - reinforced concrete or wood, collected from 1 m 2 and half of the span between the bearing walls;
- the weight of people, furniture, partitions, equipment, etc., acting on floors, collected from 1 m 2 and half of the floor span. Accepted by SNiP 2.01.07-85* "Loads and impacts";
- the weight of the coating and roof structures, collected from 1 m 2 and half of the span;
- weight of snow cover in winter, taken according to SNiP 2.01.07-85*.
After collecting the loads, the width of the belt structure is calculated taking into account the bearing capacity of the base. We gave examples of how to correctly collect loads, calculate the width of the tape and the thickness of the anti-heap pillow in the article “Shallow-deep strip foundation: depth calculation, base preparation, do-it-yourself reinforcement and calculation calculator”.
There are also tables for collecting loads for different types walls and ceilings, values of design resistances various types soils that can be used in the calculation of any strip foundations intended for low-rise buildings. For a quick calculation, a calculator is provided on the article page.
The calculation of reinforcement is carried out taking into account the accepted dimensions of the foundation structure - the width of the sole and the height of the section according to the method SNiP 2.03.01-84* "Concrete and reinforced concrete structures". In order to correctly calculate the reinforcement of the strip foundation according to SNiP, you should contact professional designers.
And we will give a simplified calculation method.
Simplified calculation of strip foundation reinforcement
A simplified calculation of rolled steel for a strip foundation consists in selecting the number of working rods, as well as their diameter, according to the main indicator - the minimum percentage of reinforcement.
According to requirements p.5.11 Table 5.2 Benefits to SP 52-101-2003 the total area of working rods that can absorb tensile forces should not be less than 0.1% of the cross-sectional area of the calculated reinforced concrete structure.
Since a monolithic tape has the form of a beam, which is affected by multidirectional forces, the stretched zones can be both at the top and at the bottom of its cross section.
Thus, the main condition for the calculation is the presence in both sections of the section of the structure of longitudinal working rods with a total area of at least 0.1% total area sections.
Formula for calculating the percentage of reinforcement by clause 5.11 of the Manual to SP 52-101-2003:
Pr – unit equal to 100%;
A s ; - the desired total area of the working rods, mm 2;
b – tape width, mm;
h - working height of the cross section, in mm.
From this formula, you can find the required minimum area of the rods:
When calculating, it is necessary to take into account the rules for reinforcing the strip foundation set forth in Allowances for SP 52-101-2003 in the "Guidelines for the design of concrete and reinforced concrete structures from heavy concrete (without prestressing)".
According to clause 5.17 of the Manual to SP 52-101-2003 the minimum diameter of each of the working rods is limited to 12 mm.
Initial data: monolithic strip foundation for external walls with a section of 600 mm (b - width) by 500 mm (H - full height);
First, we determine h0, which will be equal to the height of the section without a protective concrete layer.
The protective layer that must be maintained for the lower rods on the bottom of the tape, laid on sand or gravel preparation - 70 mm. But for the upper reinforcement, the protective layer is 30 mm, so we take the average value - 50 mm:
h0 = H - 50 = 500 - 50 = 450 mm
We determine the cross-sectional area of \u200b\u200bthe tape that will be used in the calculations:
b x h0 \u003d 600 x 450 \u003d 270,000 mm 2
The required minimum area of the working rods As in each sectional zone will be equal to:
As \u003d b x h0 x 0.001 \u003d 270,000 x 0.001 \u003d 270 mm 2
To select the diameters of the working rods and their number according to the minimum required area, we present Table 1.
According to the table, we find the nearest values for a minimum diameter of 12 mm, provided that 3 rods are installed. The value will be between columns with 2 (226 mm 2) and 3 rods (339 mm 2), we accept more - 339 mm 2 for 3 rods.
As a result, we finally accept 3 working rods with a diameter of 12 mm in both cross-sectional zones.
Strip foundation reinforcement schemes
Here are two main schemes for reinforcing a monolithic reinforced concrete foundation, which can be used in low-rise construction.
Scheme 1 - if the width of the tape is equal to the width of the wall
Reinforcement scheme 1
Scheme 2 - if the width of the tape exceeds the width of the wall
Reinforcement scheme 2
In both cases, the tape is reinforced along the length with a spatial frame, the working rods of which, located in both zones of the cross section of the structure, perceive and compensate for tensile forces.
If the tape protrudes beyond the edge of the base by more than 0.5 m, tensile forces will occur in the sole area perpendicular to its axis. In order to compensate for these forces, the reinforcement of the bottom of the tape is additionally used in the transverse direction to the axis of the wall.
The optimal solution in this case is knitting a mesh consisting of working and structural rods and laying it before installing a spatial frame.
When constructing spatial frames, in addition to longitudinal working rods, transverse reinforcement is used, which serves not only to connect longitudinal rolled products into one structure, but also to perceive transverse, cutting loads on the tape. Transverse reinforcement also counteracts the formation of cracks in the structure and prevents lateral buckling of the working rods.
As part of the spatial frames, the transverse rolled products are used in the form of clamps that cover the longitudinal working rods along the perimeter of the frame. For clamps, fittings with a smooth surface of class A1 are used, having a diameter of 6-8 mm.
Spatial frame clamps
In a whitepaper SP 52-101-2003 "Concrete and reinforced concrete structures without prestressing reinforcement" reinforcement diameters are determined at different conditions reinforcement, which are given in Table 2.
In addition to the requirements for the use of reinforcing bars of a certain diameter and class for various elements of spatial frames and flat meshes, the standards provide for a number of rules for the reinforcement of monolithic structures.
Rules for reinforcing a monolithic strip foundation
In the production of tape reinforcement, the following regulatory rules must be observed:
- working rods installed in the longitudinal direction of frames and grids must have the same diameter. In the case of using reinforcement with different diameters, rods with b O the larger diameter must be located in the lower zone of the tape;
- with a belt width exceeding 150 mm, the number of longitudinal working elements placed in one level should not be less than 2;
- the distance in the frame between the longitudinal elements installed at the same level is not allowed less than 25 mm in the lower row of the frame and less than 30 mm in the upper row. When constructing spatial frames, it is also necessary to provide places for the passage of deep vibrators. In these places, the clearance should not be less than 60 mm;
- the step of the rolled products in the strip foundation, provided for the installation of clamps or transverse elements, must be within ¾ of the height of the structure and not more than 500 mm;
- the protective layer of concrete provided for the working reinforcement of frames or meshes located at the sole of the tape should be 35 mm for concrete preparation, 65 mm for preparation from sand or crushed stone;
- protective concrete layer on the lateral and upper sides of the structure - 40 mm, for clamps or transverse rods - 10 mm.
Fabrication of frames and meshes
In the case of using ordinary rolled products of classes A1, according to another classification A240, and A3 (A400), reinforcement is knitted under a strip foundation, for which special knitting wire is used. Welding of reinforcing elements is possible only when using rolled products of class A400C or A500C.
Knitting wire is made of low-carbon steel, has a diameter of 0.8-1.4 mm and is designed specifically for the manufacture of elements of the supporting frame of reinforced concrete structures. When knitting frames and nets, lengths of 30 cm are used, which are pre-cut.
Consider how to knit reinforcement for a strip foundation. To perform this type of work, a special tool is used: hand hooks or nozzles for a screwdriver, knitting guns, pliers, tongs and wire cutters.
Hook for manual knitting of fittings
A loop is made from the pieces of knitting wire, which is passed around the junction of the reinforcing bars, then the ends are twisted manually using a knitting hook or mechanically using a screwdriver nozzle or a gun.
Reinforcement knitting methods
Since rebar frames and meshes have a limited length, the question may arise: how to tie reinforcement for a strip foundation. Along the length, frames and meshes are joined using: overlap without welding or welding in the case of using rolled products of class A400C or A500C.
Rebar tying gun
When welding with an overlap, the length of the rods of the reinforcement to be connected should not be less than 10 diameters.
In the case of overlapping, the length of the bypass of reinforcing bars must be at least 20 diameters of the elements to be joined and at least 250 mm.
Knitting of reinforcement in a mechanized way
To calculate the total volume of material, you can use the reinforcement calculator for the strip foundation, located on this page.
Reinforcement of corners and joints
At the junctions and corner joints of the tape, the greatest concentration of stresses occurs, so these nodes must be further strengthened.
For reinforcement, the installation of additional rods is used according to the following schemes:
Strengthening the corner with additional rods
When reinforcing the angle of the tape, additional L-shaped and trapezoidal rods are installed, which are attached to the working rods in the upper and lower levels of the connected frames.
Reinforcement of the T-shaped intersection
When strengthening the T-shaped intersection, additional trapezoidal rods are installed in the upper and lower levels of the connected frames.
Strengthening the intersection of walls
When strengthening the mutual intersection, trapezoidal rods are installed.
Reinforcement of the corners of the strip foundation can also be carried out according to the following schemes:
Reinforcement of the corner with U-shaped elements
Option to reinforce the corner with L-shaped clamps
Option to strengthen the T-shaped abutment with U-shaped and L-shaped clamps
Calculation of the amount of reinforcement
Initial data: a low-rise building measuring 10 x 12 m with an average load-bearing wall located on the long side. Tape section 400 x 400 mm. Reinforcement - a spatial frame of 6 bars of working reinforcement with a diameter of 12A3. Clamps made of smooth rolled products with a diameter of 6A1 are located with a step of 400 mm.
Determine the total length of the tape:
10 x 2 + 12 x 3 = 56 m.p.
The length of the working rods will be equal to:
Length of one clamp:
0.4 x 4 / 1.15 \u003d 1.39 m (1.15 is the coefficient for converting the perimeter of the tape section into the length of the clamp)
Clamp rod length:
140 x 1.39 = 194.6 m.p.
We increase the calculation results by 5% - this is a margin that takes into account the cutting of reinforcement and waste.
Working fittings: 336 x 1.05 = 353 m.p. or 352 x 0.888 = 313 kg
Clamps: 194.6 x 1.05 = 204 m.p. or 204 x 0.222 = 46 kg
For a quick calculation of the amount of materials, you can use the reinforcement and formwork strip foundation calculator located here.
Methods and techniques for reinforcing a strip foundation
The above two main schemes, according to which it is possible to reinforce a strip foundation, as well as schemes for reinforcing corners and intersections for low-rise buildings, have been repeatedly used and tested in real construction in difficult soil conditions - with foundations made of subsidence and heaving soils. Therefore, I recommend using these diagrams and the information provided on the selection of steel rods and the design of frames for houses 1-2 floors high under any soil conditions.
When building more complex and critical structures for the design of the foundation, you should contact professional designers.
GOST 5781-82* “Hot-rolled steel for reinforcing reinforced concrete structures;
GOST R 52544-2006 “Welded rebar of ribbed sections of A500C and B500C classes for reinforcing reinforced concrete structures”;
SP 52-101-2003 "Concrete and reinforced concrete structures without prestressing reinforcement";
SNiP 52-01-2003 "Concrete and reinforced concrete structures";
SNiP 2.01.07-85* "Loads and impacts";
SNiP 2.03.01-84* "Concrete and reinforced concrete structures";
Manual to SP 52-101-2003 “On the design of concrete and reinforced concrete structures made of heavy concrete without prestressing reinforcement”;
"Guidelines for the design of concrete and reinforced concrete structures from heavy concrete (without prestressing").
How to properly reinforce a strip foundation: do-it-yourself knitting of reinforcement, what diameter to use, calculator and reinforcement diagram
Information about the reinforcement of strip foundations with your own hands
In accordance with SNiP2.02.01-83, the condition for carrying out deformation calculations (for the second limit state) is the limitation of the pressure averaged over the base of the foundation p the value of the calculated resistance R:
p £ R, (6.4)
Where p- average pressure under the base of the foundation, kPa;
R is the design resistance of the base soil, kPa.
This condition must be carried out with underload: for monolithic foundations - £5%, for prefabricated foundations - £10%.
The fulfillment of the condition is complicated by the fact that both parts of the inequality contain the desired geometric dimensions of the foundation, as a result of which the calculation has to be carried out by the method of successive approximations over several iterations.
The following sequence of operations is proposed when selecting the dimensions of the foundation:
Þ are given by the shape of the base of the foundation:
If the foundation is tape, then a section of the tape 1 m long and wide b.
If the foundation is rectangular, then they are given by the ratio of the sides of the rectangle in the form h=b/l= 0.6…0.85. Then A=bl=b 2 /h, Where A is the area of the rectangle, l- length, b is the width of the rectangle. From here. A special case of a rectangle is a square, in which case
Þ calculate the preliminary area of \u200b\u200bthe foundation according to the formula:
Where NII- the sum of loads for calculations for the second group of limit states, kPa. In the case of strip foundations, this is a linear load, in the case of rectangular and square foundations, it is a concentrated load;
R0- tabular value of the design resistance of the soil, where the base of the foundation is located, kPa;
g¢ II- averaged calculated value of the specific gravity of soils lying above the base of the foundation, kN / m 3;
d1- the depth of laying the foundations of non-basement structures or the reduced depth of laying the external and internal foundations from the basement floor:
Where hs- the thickness of the soil layer above the base of the foundation from the side of the basement, m;
hcf- the thickness of the basement floor structure, m;
g cf- the calculated value of the specific gravity of the basement floor structure, kN / m 3;
Figure 6.6: Towards Determination of Foundation Depth
A- at d1<d; b - at d1>d; c - for slab foundations
1- outer wall; 2 - overlap; 3 - inner wall; 4 - basement floor; 5 - foundation
Þ according to the known form of the foundation, the width of the foundation is calculated:
in case of strip foundation b=A¢;
in the case of a square foundation;
in the case of a rectangular l=h/b.
After determining the required dimensions of the foundation, it is necessary to design the foundation body in the form of a sketch with dimensions in the explanatory note. At the same time, the dimensions of the foundation can be varied within small limits from the design considerations set out in clause 6.2.1. Only after clarifying all the dimensions of the foundation, you can proceed to the next paragraph.
Þ according to the formula (7) SNiP 2.02.01-83 calculate the design soil resistance of the base R:
Where g с1 And g c2- coefficients of working conditions, taking into account the peculiarities of the work of different soils at the base of the foundations and taken according to Table 6.14;
k- coefficient accepted: k=1 - if the strength characteristics of the soil ( With And j) determined by direct tests and k=1.1 - if they are accepted according to the tables of SNiP;
kz- coefficient accepted kz=1 at b<10м; kz=z 0 /b+0.2 at b³10m (here z0=8m);
b- width of the base of the foundation, m;
gII And g¢ II- average calculated values of the specific gravity of soils lying respectively below the base of the foundation (in the presence of groundwater, it is determined taking into account the weighing effect of water) and above the base, kN / m 3;
from II- the calculated value of the specific adhesion of the soil lying directly under the base of the foundation, kPa;
db- basement depth - the distance from the planning level to the basement floor, m (for structures with a basement width B£20m and depth over 2m accepted db\u003d 2m, with a basement width B>20m accepted db=0);
Mg, M q, Mc- dimensionless coefficients, taken according to Table 6.15;
d1- the depth of laying the foundations of non-basement structures or the reduced depth of laying the external and internal foundations from the basement floor (see the previous paragraph), m.
Table 6.14
Coefficient values g с1 And g c2
| soils | g с1 | g c2 for buildings and structures with a rigid structural scheme with a ratio of their length (or a separate compartment) to height L/H | |
| ³4 | £1.5 | ||
| Coarse-clastic with sandy filler and sandy, except for fine and silty | 1,4 | 1,2 | 1,4 |
| The sands are fine | 1,3 | 1,1 | 1,3 |
| Silty sands: low-moisture and wet saturated with water | 1,25 1,1 | 1,2 1,2 | |
| Dusty-argillaceous and coarse-grained with silty-argillaceous filler, with an index of soil or filler fluidity: I L£0.25 | 1,25 | 1,1 | |
| The same, at 0.25< I L£0.5 | 1,2 | 1,1 | |
| The same, at I L >0,5 |
Notes:
1. Buildings and structures are considered rigid, the structures of which are adapted to the perception of additional forces from deformations of the base.
2. In buildings with a flexible design scheme, g c2=1.
3. At intermediate values of the ratio of the length of a building or structure to height L/H coefficient g c2 determined by interpolation.
Table 6.15
Coefficient values Mg, M q And Mc
| j II, deg | Mg | M q | Mc | j II, deg | Mg | M q | Mc |
| 3,14 | 0,72 | 3,87 | 6,45 | ||||
| 0,03 | 1,12 | 3,32 | 0,84 | 4,37 | 6,90 | ||
| 0,06 | 1,25 | 3,51 | 0,98 | 4,93 | 7,40 | ||
| 0,1 | 1,39 | 3,71 | 1,15 | 5,59 | 7,95 | ||
| 0,14 | 1,55 | 3,93 | 1,34 | 6,35 | 8,55 | ||
| 0,18 | 1,73 | 4,17 | 1,55 | 7,21 | 9,21 | ||
| 0,23 | 1,94 | 4,42 | 1,81 | 8,25 | 9,98 | ||
| 0,29 | 2,17 | 4,69 | 2,11 | 9,44 | 10,80 | ||
| 0,36 | 2,43 | 5,00 | 2,46 | 10,84 | 11,73 | ||
| 0,43 | 2,72 | 5,31 | 2,87 | 12,5 | 12,77 | ||
| 0,51 | 3,06 | 5,66 | 3,37 | 14,48 | 13,96 | ||
| 0,61 | 3,44 | 6,04 | 3,66 | 15,64 | 14,64 |
Þ determine the actual stresses under the base of the foundation:
The reactive pressure of the soil on the sole of the hard centrally loaded foundation is assumed to be evenly distributed, kPa:
, (6.8)
Where NII- normative vertical load at the level of the edge of the foundation, kN;
G f II And G g II- the weight of the foundation and soil on its ledges (to determine the weight, it is necessary to determine the volume of the foundation or soil body and multiply it by the specific gravity), kN;
A- the area of \u200b\u200bthe sole of the foundation, m 2.
eccentrically loaded consider the foundation, in which the resultant of external loads does not pass through the center of gravity of the area of its sole. Such loading is a consequence of the transfer of a moment or a horizontal component of the load to it. When calculating the pressure along the base of an eccentrically loaded foundation, it is assumed that it changes according to a linear law, and its boundary values under the action of a moment of forces relative to one of the main axes are determined as for the case of eccentric compression:
, (6.9)
Where M x , M y- bending moments, relative to the main axes of the base of the foundation, kNm;
W x , W y- moments of resistance of the section of the base of the foundation relative to the corresponding axis, m 3.
The pressure diagram under the base of the foundation, obtained by this formula, must be unambiguous, i.e. over the entire width of the cross section, the stresses must be compressive. This is due to the fact that tensile stresses, if they occur, can lead to separation of the base of the foundation from the base and a special calculation will be required, which is not included in the scope of the course project.
Þ The "load-settlement" relationship for shallow foundations can be considered linear only up to a certain pressure limit on the foundation. As such a limit, the design soil resistance of the base is taken R. Condition fulfillment p=R corresponds to the formation in a homogeneous base under the edges of the foundation of insignificant, depth zmax@b/4, areas of ultimate stress state (areas of plastic deformations) of the soil, allowing, according to SNiP, the use of a model of a linearly deformable medium to determine the stresses in the base.
The applicability of the model of a linearly deformable medium is ensured by the fulfillment of the following conditions:
* For centrally loaded foundations:
p<R, (6.10)
* For eccentrically loaded foundations:
p<R,
pmax<1.2R(6.11)
* For eccentrically loaded foundations with bending moments in two directions:
p<R,
pmax<1.2R
p with max<1.5R(6.12)
In most cases, after the first iteration, this condition is not met with the required tolerance (exceeding R above p up to 5%). All operations must be completely repeated, substituting in the formula for A¢ instead of R0 design resistance value R. Calculate A, b, choose a foundation with a new value b, define a new value R, calculate p and check condition again p<R.
Usually, as a result of the second iteration, the condition p
With strip foundations, when the width of the slabs coincides with the calculated width, it is allowed to replace rectangular slabs with slabs with corner cutouts. In this case, the plates (of any shape) are laid in the form of a continuous tape. If the calculated width does not match the width of the slab, intermittent foundations are designed.
According to the established laying depth, shape and size of the base of the foundation, the foundation is constructed using prefabricated reinforced concrete and concrete foundation structures or monolithic concrete structures.
Accompany the calculations with the necessary sketches.
Features of the calculation of discontinuous foundations:
During the construction of buildings that do not require increased rigidity, on solid soils (dense and medium-density sands; hard, semi-hard, tough-plastic dusty-clayey) at a groundwater level below the base of the foundation, it is allowed to use discontinuous strip foundations, which are arranged from slabs located at some distance from each other. It is especially advisable to use such foundations in cases where the width obtained in the calculations is less than standard slabs.
Figure 6.7: Intermittent foundation
1 - soil surface; 2 - concrete blocks; 3 - foundation slabs; 4 - gaps between the plates filled with soil
Intermittent foundations made of rectangular slabs and with corner cutouts are not recommended:
* in soil conditions of type II in terms of subsidence;
* when loose sands lie under the base of the foundation;
* when the seismicity of the area is 7 points or more; in this case, it is necessary to use plates with corner cutouts, laying them in the form of a continuous tape;
* when silty-clayey soils with a fluidity index lie below the base of the foundation I L>0,5.
Due to the distributive ability of soils and the arch effect, the pressure under the sole of discontinuous foundations is leveled at a shallow depth and it can be considered that they work as solid foundations. Therefore, their width is determined, the design resistance is assigned, and the settlement is calculated as for continuous strip foundations without deducting the areas of the gaps.
Optimal interval between plates C are assigned from the condition of equality of the design resistance of the soil R obtained for a strip foundation with a width b, soil resistance obtained for discontinuous foundation R p with slab width b p, length l p, with the coefficient of working conditions k d:
, (6.13)
The coefficient of working conditions depends on the state of the soil (for intermediate values it is determined by interpolation):
* k d=1.3 - for sands with porosity coefficient e@0.55 and silty clay soils with flow index I L £ 0;
* k d=1 - for sands with porosity coefficient e@0.7 and silty clay soils with flow index I L=0,5;
From the operating conditions of the foundation soils and wall blocks, the interval between the slabs should be C£(0.9…1.2)m and not more than 0.7× l p, and the width of the slab should be b p£1.4 b. To make better use of intermittent foundations, the number of spacings can be increased by using shortened slabs (1180 and 780mm) if this does not entail an unjustified increase in labor costs.
3.1 Determining the depth of the foundation
Figure 1 - To determine the depth of the foundation
The building has a basement with a depth of 3 m, therefore, in any case, the base of the foundation will be below the freezing depth. Let us determine the minimum laying depth based on the standard freezing depth according to the formula:
where kh is the coefficient that takes into account the influence of the thermal regime of the building on the depth of soil freezing at the foundations of the outer walls, determined from table 13;
dfn is the normative freezing depth, determined from the map of normative freezing depths, for the city of Bykhov dfn= 1.05m
df=0.6∙1.05=0.63m
We assign the depth of the foundation, depending on paragraph 1 and paragraph 5 of Chapter 4. The mark of the finished floor, according to the assignment, at DL=-0.30 m will be equal to 62.80 m, the mark of the basement floor will be in this case equal to 62.8-3=59.8 m.
The mark of the bottom of the ceiling above the basement is 62.50 m. We accept the foundation structure of five blocks 0.6 m high and a pillow 0.3 m high. Thus, the mark of the base of the foundation will be 59.02 m.
d=62.5-59.2=3.3m
3.2 Sand cushion arrangement
Since soft-plastic loam cannot be a natural base, we put the foundation slab on a sand cushion with a thickness of 1m.
Let's set the characteristics that the sand cushion soil should have: ρds = 1.62 g/cm3 - required density; Wopt = 12% - optimal humidity for sand of medium size. Let us determine the physical characteristics of the cushion soil.
Porosity coefficient according to the formula (3):
where ρs is the density of solid particles of the soil, t/m3, for a sand cushion we take ρs=2.67 t/m3
Degree of humidity of soil of a pillow:
Thus, based on the obtained physical characteristics, we conclude that the material of the sand cushion is sand of medium size, medium density, low moisture content.
Let's determine the mechanical characteristics of this soil according to tables 4, 5: R0=500 kPa, Cn=1 kPa, φn=350, En=30 MPa
3.3 Determining the dimensions of the sole of the strip foundation
The dimensions of the base of the foundation mainly depend on the mechanical properties of the foundation soils and the nature of the loads transferred to the foundation, on the features of the load-bearing structures that transfer the load to the foundation. The dimensions of the foundation must be selected in such a way that the following condition is met:
those. design precipitation should not exceed the allowable.
According to the fulfillment of this condition, it is realized under the following condition:
PCP≤R,Pmax≤1.2R , Pmin≥0 (10)
The dimensions of the base of the foundation for a brick wall are determined by 1 linear meter of its length by the method of successive approximation.
The calculated value of the load Fv=120kN.
Figure 2 - Calculation scheme of the strip foundation
We determine the area of the sole of the strip foundation by the formula:
(11)
For a strip foundation, the width of the pillow is determined by the formula:
b=A/1m.p. (12)
b1=0.28m2/1m.p.=0.28m
We specify the calculated resistance according to the formula:
R= 
(13)
where gС1 and gС2 are the coefficients of working conditions, taking into account the peculiarities of the work of different soils at the base of the foundations and taken according to table 16, .
k – coefficient accepted: k=1.1 – because the strength characteristics of the soil are taken according to the normative tables;
kZ – coefficient accepted kZ=1 at b
Foundation called the underground part of the building, designed to transfer the load from the building to the foundation soils lying at a certain depth. outsole the foundation is called its lower surface in contact with the base; the upper plane of the foundation, on which the ground structures rest, is called sawed-off . Behind width foundation, the minimum size of the sole is taken b, but for length - its largest size l. Height foundation hf is the distance from the sole to the edge, and the distance from the surface of the layout to the sole is called depth d.
Shallow foundations include foundations that transfer the load to the foundation soils mainly through the sole. They are used in various fields and engineering-geological conditions, both in precast and monolithic versions (Table 6.2). Table 6.2
Areas of application for shallow foundations
With a central load, it is recommended to take the shape of individual foundations in terms of square, and for an eccentric load - rectangular (with an aspect ratio of 0.6 ... 0.85).
Regardless of the soil conditions (except for rocky soils), preparations with a thickness of 100 mm are arranged under the foundations: under monolithic - concrete, from concrete of class B3.5; and under the prefabricated - from sand of medium size. When erecting foundations on rocky soils, a leveling layer of concrete of class B3.5 is arranged on the soil base.
The calculation of a shallow foundation begins with a preliminary selection of its design and basic dimensions, which include the depth of the foundation, the dimensions and shape of the sole. Then, for the accepted dimensions of the foundation, the foundation is calculated for the limit states.
Determining the depth of the foundation. Obviously, the smaller the depth of the foundation, the less the amount of material expended and the lower the cost of its construction, so it is natural to strive to take the depth of the foundation as low as possible.
Rice. Soil stratification schemes with options for foundations: 1 - solid soil; 2-more durable soil; 3-weak soil; 4-sand pillow; 5-zone pinning
The minimum depth of foundations is assumed to be at least 0.5 m from the planned surface of the territory; the depth of the foundation in the bearing layer of soil should be at least 10 ... 15 cm.
The depth of seasonal freezing of soils. df=khdfn, where kh is the coefficient taking into account the effect of thermal
construction mode, dfn - normative depth of seasonal freezing of soils, m.
Determination of the shape and dimensions of the base of the foundations. The shape of the sole of the foundation is largely determined by the configuration. When calculating shallow foundations for the second limit state (deformations), the area of the foot can be preliminarily determined from the condition pP≤R, where pP is the average pressure along the base of the foundation, R is the calculated resistance of the base soil.
This condition must be met with underload: for monolithic foundations - £5%, for prefabricated foundations - £10%.
The fulfillment of the condition is complicated by the fact that both parts of the inequality contain the desired geometric dimensions of the foundation, as a result of which the calculation has to be carried out by the method of successive approximations over several iterations.
The following sequence of operations is proposed when selecting the dimensions of the foundation:
Þ are given by the shape of the base of the foundation:
If the foundation is tape, then a section of the tape 1 m long and wide b.
If the foundation is rectangular, then they are given by the ratio of the sides of the rectangle in the form h=b/l= 0.6…0.85. Then A=bl=b2/h, Where A is the area of the rectangle, l- length, b is the width of the rectangle. From here. A special case of a rectangle is a square, in which case
Þ calculate the preliminary area of \u200b\u200bthe foundation according to the formula:
Where NII- the sum of loads for calculations for the second group of limit states, kPa. In the case of strip foundations, this is a linear load, in the case of rectangular and square foundations, it is a concentrated load;
R0- tabular value of the design resistance of the soil, where the base of the foundation is located, kPa;
g¢II- average calculated value of the specific gravity of soils lying above the base of the foundation, kN / m3;
d1- the depth of laying the foundations of non-basement structures or the reduced depth of laying the external and internal foundations from the basement floor:
Where hs- the thickness of the soil layer above the base of the foundation from the side of the basement, m;
hcf- the thickness of the basement floor structure, m;
gcf- the calculated value of the specific gravity of the basement floor structure, kN / m3;
Þ according to the known form of the foundation, the width of the foundation is calculated:
in case of strip foundation b=A¢;
in the case of a square foundation;
in the case of a rectangular l=h/b.
After determining the required dimensions of the foundation, it is necessary to design the foundation body in the form of a sketch with dimensions in the explanatory note. At the same time, the dimensions of the foundation can be varied within small limits from the design considerations set out in clause 6.2.1. Only after clarifying all the dimensions of the foundation, you can proceed to the next paragraph.
Þ according to the formula (7) SNiP 2.02.01-83 calculate the design soil resistance of the base R:
Figure 6.6: Towards Determination of Foundation Depth
A- at d1d; c - for slab foundations
1- outer wall; 2 - overlap; 3 - inner wall; 4 - basement floor; 5 - foundation
Center loaded foundation. A foundation is considered to be centrally loaded if the resultant of external loads passes through the center of its sole area. Soil reactive pressure along the sole of a rigid centrally loaded foundation is assumed to be uniformly distributedpII=(NoII+GfII+GgII)/A, where NoII is the calculated vertical load at the level of the foundation edge; GfII and GgII - calculated values of the weight of the foundation and soil on its ledges; A is the area of the base of the foundation. In preliminary calculations, the weight of the soil and the foundation in the volume of the ABCD parallelepiped, at the base of which lies the unknown area of the sole A, is determined approximately from the expression GfII + GgII = γmAd where γm is the average value of the specific gravity of the foundation and soil on its ledges, d is the depth of the foundation, m.
A=NoII/(R-γmd). Having calculated the area of the sole of the foundation, find its width b. The width of the strip foundation, for which the loads are determined per 1 m of length. After calculating the value of b, the dimensions of the foundation are taken, taking into account the modularity and unification of structures, and the pressure is checked. The found value of pII should be as close as possible to the value of the calculated R.
Eccentrically loaded foundation. eccentrically loaded consider the foundation, in which the resultant of external loads does not pass through the center of gravity of the area of its sole. Such loading is a consequence of the transfer of a moment or a horizontal component of the load to it. When calculating the pressure along the base of an eccentrically loaded foundation, it is assumed that it changes according to a linear law, and its boundary values under the action of a moment of forces relative to one of the main axes are determined as for the case of eccentric compression:
, (6.9)
Where Mx, My- bending moments, relative to the main axes of the base of the foundation, kNm;
wx, wy- section modulus of the base of the foundation relative to the corresponding axis, m3.
The pressure diagram under the base of the foundation, obtained by this formula, must be unambiguous, i.e. over the entire width of the cross section, the stresses must be compressive. This is due to the fact that tensile stresses, if they occur, can lead to separation of the base of the foundation from the base and a special calculation will be required, which is not included in the scope of the course project.
A foundation is considered eccentrically loaded if the resultant of external loads does not pass through the center of gravity of its sole area. When calculating the pressure along the base of an eccentrically loaded foundation, it is assumed that it changes according to a linear law, and its boundary values \u200b\u200bunder the action of a moment of forces relative to one of the main axes. pmax \u003d (NII / A) (1 ± 6e / b), where NII is the total vertical load on foundation, including the weight of the foundation and soil on its ledges; A - the area of the sole of the foundation; e - eccentricity of the resultant relative to the center of gravity of the sole; b - the size of the base of the foundation in the plane of the action of the moment.
Since with eccentric loading relative to one of the central axes, the maximum pressure on the base acts only under the edge of the foundation, when selecting the dimensions of the sole; its foundation can be taken 20% more than the calculated and soil resistance, i.e. pmax≤1,2R At the same time, the average pressure along the base of the foundation, defined as pII=NII/A, must satisfy the condition pII≤R.
In those cases when the point of application of the resultant of external forces is displaced relative to both axes of inertia of the rectangular base of the foundation, the pressure under its corner points is found by the formula.pcmax=(NII/A)(1±6ex/l±6ey/b).
Since in this case the maximum pressure acts only at one point of the base of the foundation, it is allowed that its value satisfies the condition pcmax≤1.5R. 
Checking the pressure on the underlying layer of soft soil. In the presence and within the compressible thickness of the base, weak soils or soils with a design resistance less than the pressure on the bearing layer, it is necessary to check the pressure on them in order to clarify the possibility of using the theory of linear soil deformability in calculating the base. The latter requires that the total pressure on the roof of the underlying layer does not exceed its design resistance, i.e. σzp+ σzg≤Rz
Where σzp and σzg are vertical stresses in the soil at a depth z from the base of the foundation (respectively, the foundation is additional from the load and from the own weight of the soil); Rz is the calculated soil resistance at the depth of the roof of a weak layer, the value of Rz is determined both for a conditional foundation with a width bz and a depth of dz. The coefficients of working conditions γС1, γС2 and reliability k, as well as the coefficients Mq, Mc are found in relation to a layer of soft soil. The width of the conditional foundation is assigned taking into account the stress dissipation within the layer of thickness z. If we assume that the pressure acts on the sole of the conditional foundation AB, then the area of its sole should be Az=NoII/σzp. Knowing Az, we will find the width of the conditional rectangular foundation bz=(√Az+a2)-a, where a=(1-b) / 2 (1 and b are the length and width of the base of the designed foundation. For strip foundations bz = Az / 1.
Settlement calculation.
Calculation of foundations by deformations is carried out on the basis of condition (6.1):
S£ Su,
Where S- joint final deformation (settlement) of the base and structure, determined by the calculation according to the instructions of Appendix 2 of SNiP 2.02.01-83, the methodology of which is described below.
Su- the limit value of the joint deformation of the base and the structure, set according to the instructions of clause 6.1.
The calculation scheme of the base is applied in the form of a linearly deformable half-space with a conditional limitation of the depth of the compressible thickness Ns. The scheme of distribution of vertical stresses in a linearly deformed half-space is shown in Fig.6.9.
For calculation S the method of layer-by-layer summation of sediment is used, which can be used in cases where the pressure under the base of the foundation p does not exceed the design soil resistance of the base R.
The sequence of settlement calculation by the layer-by-layer summation method is as follows:
a) against the background of a geological section (made on a scale), show the contours of the designed foundation;
b) to the left of the foundation axis, construct a diagram of vertical stresses from the own weight of the soil (diagram szg) using the formula:
Where g¢- specific gravity of the soil located above the base of the foundation;
dn- the depth of the foundation;
gi, hi- specific gravity and thickness, respectively i-th layer of soil;
The specific gravity of soils lying below the groundwater level, but above the aquiclude, should be taken taking into account the weighing effect of water:
If there is a waterproof layer in the thickness of the base - hard, semi-hard, rigid clays, hard and non-fractured rocky loams rocks, then pressure from the overlying soil and groundwater is transferred to its roof. Then, on the roof of the aquiclude, a stress jump occurs by the value hwgw.
c) Divide the soil thickness from the bottom of the foundation down into elementary layers, the thickness of which is conveniently taken equal to 0.2 b or 0.4 b. When staking out, it is not necessary to pay attention to the boundaries of the layers of various soils and to the level of groundwater;
d) to the right of the axis from the level of the base of the foundation, construct a diagram of additional vertical stresses (diagram szp). Additional vertical stresses at depth z from the sole of the foundation, are determined by the formula:
szp=ap0, (6.19)
Where a- coefficient taken depending on the shape of the base of the foundation, the ratio of the sides of the rectangular foundation and the relative depth equal to x=2z/b;
p0=p-szg,0- additional vertical pressure on the base (for foundations with a width b³10m accepted p0=p);
e) determine the lower limit of the compressible thickness (LCST), which is at the level where the condition is met szp=0,2szg. It is convenient to determine the NGST graphically, for which it is enough to build a plot to the right of the axis 0.2szg on the same scale as the plot szp. Plot intersection point szp And 0.2szg determine the NGST;
Where b– dimensionless coefficient equal to 0.8;
szp,i is the average value of the additional vertical normal stress in i- that layer of soil, equal to half the sum of the indicated stresses on the top zi-1 and bottom zi layer boundaries along the vertical passing through the center of the base of the foundation;
hi, Ei are the thickness and modulus of deformation, respectively i- that layer of soil; if in i- th layer includes two geological layers, then Ei take according to the layer whose thickness is in i-that layer more;
n is the number of layers into which the compressible thickness of the base is divided. 
Figure 6.9: Scheme of the distribution of vertical stresses in a linearly deformable half-space:
DL- layout mark; NL- mark of the surface of natural relief; FL- mark of the sole of the foundation; WL- groundwater level; B.C- the lower boundary of the compressible thickness; d And dn- the depth of the foundation, respectively, from the level of planning and the surface of the natural relief; b- foundation width; p- average pressure under the base of the foundation; p0- additional pressure on the base; szg And szg,0- vertical stress from the own weight of the soil at a depth z from the sole of the foundation and at the level of the sole; szp And szp,0– additional vertical stress from external load at depth z from the sole of the foundation and at the level of the sole.
House Determination of preliminary dimensions of the sole of shallow foundations views - 391
Selection of types of bases and foundations based on a comparison of options
According to hydrogeological conditions during the construction and operation of the facility.
Groundwater does not directly affect the depth of foundations. It is recommended to lay foundations above the groundwater level to eliminate the extreme importance of dewatering or dewatering. When laying foundations below the groundwater level, provide for appropriate waterproofing and work methods that preserve the structure of the soil. When designing the foundations, the possibility of changing the hydrogeological conditions of the site during the construction and operation of the structure is taken into account.
So, after considering separately each condition that determines the depth of the foundation, the explanatory note indicates the absolute mark of the sole or notes that there are no restrictions.
Finally, the minimum value of the absolute mark of the base of the foundations is taken and the depth of the foundation is calculated:
The mark of the sole of the grillage is assigned according to the same conditions (with the exception of clause 3.3.3).
According to the design conditions, the height of the grillage is (h0 + 0.25) m, but not less than 30 cm, where h0 is the height of the pile embedded in it, which is taken at least 5 cm.
At the end of the section, it is extremely important to analyze the parameters of the future excavation. If the absolute marks of the soles of all the foundations of a structure differ slightly from each other, then it is possible to place all the foundations with a single absolute mark. This will reduce earthwork costs.
In the course project, the laying depth is determined for each foundation specified for calculation.
The choice of types of bases and foundations is made on the basis of a joint analysis of the initial data on the engineering-geological and hydrogeological conditions of the construction site and above-foundation structures.
Soils are in most cases used in their natural state. But if the upper relatively small thickness is composed of weak soils that are not capable of absorbing loads from foundations in their natural state, then special measures are provided (compacting, fixing or replacing with other soils with the necessary properties). If the thickness of weak soils is large, then measures for their artificial improvement or their replacement may turn out to be too expensive. Economically more expedient may be the foundation method, in which the load is transferred to dense layers lying at a considerable depth under a layer of soft soils. For this purpose, pile foundations are arranged (for example, piles, shell piles, pillar piles).
It is extremely important for the student to decide on the use of one of the two possible types of foundation - natural or artificially improved, and also to consider 2 options for foundations (shallow and deep).
Shallow foundations include separate (columnar), strip and in the form of a solid reinforced concrete slab.
Types of piles are distinguished by the material, the shape of the transverse and longitudinal sections, the method of manufacture and immersion in the ground. At the same time, driving clay soils of solid and semi-solid consistency with piles is allowed in exceptional cases. Piles cannot be used when there are boulders and other obstacles in the thickness. In these cases, foundations are made using the methods of a wall in the ground or a sinkhole.
When choosing foundation options, only expedient and competing options are considered.
There are different types of bases or foundations under one building. For example, the heavy part of the building can rest on a pile foundation, and the lighter part on shallow foundations (Fig. 5).
| Rice. 5. Type of foundations and foundations: a - foundations of different loads with the same soil foundation; b - foundations of the same load with different soil bases. |
The dimensions of the sole are determined by the method of successive approximation.
1. Calculate the sole area A in the first approximation
2. Choose the shape of the sole. It is known that the most optimal in terms of leading sediment is round, but it is laborious to use. For this reason, the sole of the foundation is taken to be square, and only the presence of a large moment makes it necessary to accept it as rectangular ().
3. Based on A1, calculate the width and length of the foundation at the accepted ratio. For example, for a square sole: , for a rectangular one: ; ; . Dimensions are taken in multiples of 10 cm.
4. Determine the design soil resistance of the base (appendices B10 and B11)
5. Calculate the sole area in the second approximation
6. Specify the size of the sole and. We can stop at this approximation by assuming .
7. The foundation is designed by assigning a certain number and sizes of steps (Fig. 6), and the average pressure under the base of the foundation is calculated
8. Check the following conditions:
a) the average pressure under the base of the foundation should not exceed the design soil resistance of the foundation, ᴛ.ᴇ. ;
b) under the action of a moment in one direction (Fig. 6, a), the pressure under the most and least loaded face of the foundation should be, respectively:
c) under the action of a moment in two directions, the pressure at the angular maximum loaded point (Fig. 6, b) should not exceed 1.5R, ᴛ.ᴇ. ;
If the above conditions are not met, then it is extremely important to do the following:
1) change the size ratio of the sole, ᴛ.ᴇ. give the sole an elongation in the plane of action of the greatest moment, but no more than;
2) increase the sole area by 20% or more;
3) shift the base of the foundation in the direction of the moment relative to the fixed column, then the eccentricity is equal to the distance from the center of the base to the point of intersection of the axis of the column with the base of the foundation (Fig. 7). At the same time, the sole area remains unchanged. The values and to check the above conditions are determined by the formula:
If all conditions are met, the preliminary calculation of the dimensions of the shallow foundation sole is considered completed.
The width of the sole of the strip foundation under the wall is determined similarly, based on the design conditions per 1 m of the length of the foundation (with l= 1 m).
Prefabricated strip foundations are designed intermittent.
In case of weak, subsidence and swelling soils, as well as in the presence of karst phenomena, in seismic regions and in undermined territories, monolithic reinforced concrete cross tapes or slab foundations under the entire structure are arranged to reduce the unevenness of building deformations. The main structural types of slabs are: a beamless slab with columns resting on prefabricated sleeves (Fig. 8, a), a beamless slab with monolithic sleeves (Fig. 8, b), a ribbed slab ͵ connected to the columns using monolithic sleeves or reinforcement outlets (Fig. 8, c), box-section plate (Fig. 8, d).
The dimensions of the plate in the plan are equal to the outer dimensions of the frame (the outer surfaces of walls or columns), increased by two thicknesses of the glass wall or retreating 10 ... 20 cm from the bearing walls. The thickness of the slab is determined by calculating it as a reinforced concrete element, and in the course project they take 40 ... 60 cm.
The foundation of a building is its main element. He provides existence. Whole house as a whole. So that the foundation does not begin to fall apart today, it should be filled with a very high quality.
We will help you competently carry out this task, for this you just need to follow our recommendations. 
Before you start pouring the foundation, you should make the necessary preparations. First of all, you will need to clearly determine the position of your home. Then carefully clean the area and level it well.
The breakdown of the building should not be carried out independently. It is better to entrust this matter to a professional. With the help of special devices and devices, he will accurately mark all the corners of the outer type with pegs. This is done to visualize the outer line of the foundation wall.
The main thing that you must complete is that it is mandatory to determine if your house is rectangular. 
This is easy enough to do. To do this, simply measure its diagonals. They must be identical, if not, then the house does not belong to the rectangular type.
After the mark of the external type of catch is completed, you can start driving in pegs. Three pegs should be driven in for each corner. The distance between them should be about 1 m from the marked foundation line. Then you should start nailing the boards.
This must be done so that their edge, which is considered the top, shows the level of the end of the foundation walls. The level will help you to carry out high-quality alignment. 
Next, you will need to stretch the cord. This should be done through the board top edges on a pair of opposite catches. To properly adjust the position of the cord, you will need a plumb line. This should be done so that the cord is directly below the mark that the professional will make. You will need to make notches in those.
Where the line touches the board, this is done in order to measure the position of the board. Remember that the notches you make must be completely identical to each other. The cords that you have pulled will help you in the next stages of construction. Namely, in determining the most even line of installation of house walls. During the digging process, you can remove the cord. This is where the notches that you previously made on the surface of the boards come in handy. 
They will always help you determine where the edges of the outer type of foundation walls are located. You will also need to define a load-bearing center beam. This will be required for the correct breakdown of the line of the external type of foundation. This is not that difficult to do, you just need to accurately measure the distance from the parts of the catch. Then it will be necessary to drive in pegs.
After that, you will need to lay horizontal type boards. Please note that they must be on the same level. It is very important. The next step is to place the cord. This must be done following previous recommendations. Corda you will be digging the pit directly under the foundation, you can optionally remove the pegs if they start to interfere with you. After all the steps taken, you can proceed to the implementation of the walls of the foundation and its soles.
The sole of the building and foundation
So you come to the creation of the soles of the building. We have prepared some tips for you to make this process as successful as possible. Remember that before you start the moment of digging a ditch for the foundation, you must remove a layer of earth. The removed layer should be from the entire surface at once. You will need to dig additional ditches. 
As for the dimensions of the ditches, they should be about half a meter. Remember the important information that the sole of the foundation should be about 10 cm thick. No less. In case the foundation is not of a very good level of bearing, you must expand it and also reinforce it. The wedge, which is located at the top of the foundation, performs important functions.
It helps the foundation wall withstand lateral loads. Such loads can occur in case of ground displacement. You may encounter irregularities in the excavation. In this type of situation, you should resort to leveling the pit with concrete. Never use soil that has already been excavated. 
You will undoubtedly need to lay the foundation for the pillars and columns. In order to easily determine the line on which the pillars are located, the main function of which is to support the beam of the bearing type, you need to use a cord.
On the plan of the house, you should find the coordinates for the placement of the pillars, as well as their dimensions. The foundations under them must be poured so that the parts that are on their surface are in the center of the foundation itself.
The size of the foundation will depend entirely on the pressure of the foundation itself, as well as the load. Usually, the dimensions of the foundation for pillars and columns are 60 by 60 for a building with one floor, and 80 by 80 with several. Be sure to consider this nuance. On the issue of soil density, it is better to consult directly with a professional.
He will give good advice. You must take into account that the smallest thickness of the foundation that could not be reinforced is 0.1 m for columns. You should take into account that the thickness of the foundation itself cannot be narrower than the distance between the edges of the foundation and the pillar. It is important to take into account the time of pouring the foundation for fireplaces, it must coincide with the time of pouring the chimney. I would like to say a few words about the stepped foundation.
These foundations are very common when the ground is sloped, or in houses in which there is a presence of different levels. Remember that the sole of the foundation and the vertical part should be poured at the same time. Of particular importance is the placement of the lower section of the sole. It will be better if it is placed on the basis without violations.
Concrete is perfect for connecting with a vertical type sole. Its thickness should be approximately 15 cm, and the width should fully correspond to the parameters of the base of the foundation. If you know about the presence of a sufficiently large slope, do not one step, but several.
This is an important point. Please note that the distance of the steps in the vertical position should not exceed 60 cm. This does not apply to the rock base. If the base is made of gravel or sand, the distance should not be higher than 40 cm. You need to follow our advice, and the pouring of the base of the building will be successful.
Also you can see video Beginning of construction. Excavation
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