Strip foundations are most common in the construction of private, low-rise buildings. Easy to implement, no special equipment or complex equipment required. All work can be done independently. The most important and difficult thing: perform the reinforcement correctly strip foundation 40 cm wide. What this is, how it affects the life of the building, we will consider below in more detail.
The strip foundation is the basis of the building. Its durability determines its service life, the need for repairs or additional strengthening. In order not to discover distortions in the walls in a year, two or five, not to watch how cracks “grow” under the windows, you should not neglect the reinforcement. This article will tell you how to do it correctly, what requirements need to be met.
How is reinforcement performed?
Before starting construction, you must familiarize yourself with the requirements of SNiP 2.03.01-84. It contains a direct indication that a strip foundation for a residential building cannot be without reinforcement. The width and height of the base and building do not matter.
There are two components at its core:
- concrete. Resistant to compression loads. But when the bending or tensile moment increases, the strip foundation is destroyed;
- reinforcement cage. Reduces the load on the concrete mass under the influence of bending or tensile forces. It consists of longitudinal tiers connected into a single structure by jumpers: transverse and vertical.
The number of tiers or belts directly depends on the height of the strip foundation:
- for shallow depths up to 1 meter high, 2 are enough;
- if the height exceeds 120 cm, an intermediate reinforcement belt is added.
Expert opinion
Sergey Yurievich
Ask a question to an expertThe width of the base is not taken into account. You don't have to look at her.
For longitudinal belts and lintels, the optimal material is corrugated reinforcement with a diameter of 12-16 mm. Smooth, 8-10 mm in diameter, recommended only as lintels if a strip foundation is being installed
Expert opinion
Sergey Yurievich
Construction of houses, extensions, terraces and verandas.
Ask a question to an expertFor dressing, a special knitting wire with a diameter of 1-2 mm is used. Welding is not recommended: the metal gets very hot, and “weak” spots appear at the joints, which need to be looked after especially carefully during the process of pouring concrete. If damaged, the reinforcement will not perform its function. At the same time, wire ligation is a complex and lengthy process that requires special skills. Welding is much faster.
Reinforcing frame configuration
When calculating reinforcement, the requirements of SNiP 2.03.01-84 “Manual for designing foundations for buildings and structures” must be taken into account:
- longitudinal frame elements strip base located at a distance of 10 cm or less;
- between tiers of the frame - 50 cm or less;
- transverse vertical jumpers are located at a distance of 30 cm or less;
- from the lintels, frame contour to the formwork - at least 5 cm. Otherwise, the destruction of the concrete belt and the release of reinforcement to the surface of the strip foundation is possible;
- The lower belt should not lie on the ground. If a backfill of sand and crushed stone has not been made beforehand, a single brick or special plastic stands are placed under the tier, depending on the condition of the soil and its homogeneity.
Calculation of reinforcement for reinforcing a strip foundation 40 cm wide
It is better to calculate the required volumes before starting work, so as not to stop and look for where to urgently buy several rods or a coil of wire. In the above calculation, a conditional strip foundation with the following parameters is taken as a basis: height 70 cm, width 40 cm. The perimeter of the building is 50 meters.
Expert opinion
Sergey Yurievich
Construction of houses, extensions, terraces and verandas.
Ask a question to an expertFor a base 70 cm high, two reinforcing belts are sufficient.
Each tier has 3 rods. For connection, reinforcement with a diameter of 12 mm is used, the pitch is 30 cm.
Quantity calculations:
- laying 3 rods in 2 tiers will require 300 meters;
- 167 jumpers are planned for the entire house, placed in increments of 30 cm;
- for a vertical jumper the length is 60 cm, for a transverse one - 30 cm. Each joint requires 2 vertical and 2 horizontal jumpers.
Total: for vertical lintels you will have to purchase 200.4 meters of reinforcement, for horizontal ones - 100.2 meters. In total, the building requires at least 600.6 meters of reinforcing bars with a diameter of 12 mm. This number is not final. When placing an order, please provide a reserve in case of defects and reinforcement of corners. Take into account such parameters as the length and width of the facade, the number of meters in one rod. If possible, purchase rods that are pre-cut to size to reduce waste.
How is reinforcement performed?
For straight sections, it is important to choose whole rods. The fewer joints and connections, the stronger the strip foundation. When forming corners, overlapping of elements located perpendicularly is not allowed. The reinforcement should be bent in a “P” or “G” shape.
The frame can be assembled both directly on site, in the pit, and outside it. The first may not be very convenient due to the small space. In the second case, it is important to accurately observe all the dimensions, so as not to subsequently have to rework the frame for the strip foundation.
It is difficult, but possible, to bend the reinforcement at the required angles at home. To do this, you will need a section of channel in which holes are cut with a grinder strictly on the same line. The reinforcement rod is placed in the grooves. A steel pipe is placed on the long end and used as a lever. Bending requires a lot of effort, but allows you to do without purchasing a sheet bender. The ligation of the rods is done with wire.
The rods prepared for reinforcement are laid in the trench in accordance with the requirements described above after the formwork is installed. The tiers are strictly horizontal to the ground. At the next stage, when all the belts are installed and tied, you can proceed to pouring concrete. It is important to ensure that the reinforcement remains in place and does not move. For a private low-rise building, the optimal grade of concrete is M200. After curing in accordance with building regulations, the strip foundation will gain strength and will be ready for further use. Concrete should be covered with an opaque film for 28 days, protected from direct sunlight and periodically moistened with water.
Video about reinforcing strip foundations
The price of 1 cubic meter of “factory-made” concrete is from 1,600 to 3,600 rubles (depending on the characteristics of the mixture and the impudence of the supplier), and pouring it into manufactured formwork will cost 1,000 rubles per 1 m 3 or more. And that is not all! Here it is worth adding the development of soil, backfilling with sand, crushed stone, and even materials “lost” by migrant workers.
Installing a strip foundation with your own hands will allow you to save up to 140%, and at the same time, doing it is as simple as 2+2. The main problem along the way is reinforcement. Here, mistakes can be very costly, so it is necessary to consider the entire process in detail. Your attention step-by-step instruction installation of formwork and reinforcement with visual materials and useful tips.
Basic rules for reinforcing strip foundations
Let's start with the main mistakes of beginners, SNiP rules and general recommendations that must be followed. Let's consider those that can significantly affect the quality of the foundation and the fate of your future structure.
- To reinforce the strip foundation of a 1-2-story house, rods of 10-24 mm are used. It is advisable to take the average value; a smaller diameter is unacceptable.
- Welded joints are prohibited, only tying of reinforcement is allowed. Welding overheats the metal, reducing its tensile strength by 1.6-2.5 times in places of perpendicular fastening.
- If the soil has a uniform density along the entire perimeter, we use thinner reinforcement (10-14 mm). If the density is uneven, the rods should be thick (16-24 mm).
- It is not recommended to use “smooth” reinforcement when reinforcing the foundation with your own hands. The adhesion of the material (adhesion to concrete) will be significantly less than that of corrugated rods, making calculations even more difficult. Smooth reinforcement is allowed for transverse connections - the load on them will be significantly less.
- Longitudinal reinforcement inside the foundation should be placed no closer than 5 centimeters from the formwork. The rule also applies to the base and top of the foundation. Otherwise, the foundation may crumble at the edges, and the reinforcement itself will rust.
- The distance between the crossbars can be from 25 to 45 centimeters, it is not advisable to violate this range during low-rise construction.
- Reinforcing the corners of a strip foundation differs from laying reinforcement along a trench; it must be done according to a special scheme (we will discuss it later in the article).
- Longitudinal rods are laid every 40 centimeters of the height of the foundation. For example, at a height of 120 centimeters it is necessary to lay 3 layers of longitudinal reinforcement
Expert advice: all data must comply with SNiP foundations 2.02.01-83 and 2.03.01-84; it would be better to read the requirements in detail before starting work. Do not rely on the “eye”; all work must be performed in accordance with the instructions and standards.
Do-it-yourself formwork installation
A properly made frame for the foundation will not only save a lot of money on the purchase of concrete, but also significantly simplify the reinforcement process. If you already have it, you can skip this stage and go straight to the reinforcement; if you don’t, then consider the step-by-step instructions.
STEP 1 : choice of material. With a small foundation height, choosing a material is quite easy: plywood, wood, OSB boards, fiberboard. The main requirement is the strength of the material. For high foundations (from 150 cm) only metal is used, since the pressure of concrete will be significant and wood can fail. httpsv://www.youtube.com/watch?v=Gial8rI0FzM
STEP 2: preparation of the base. Let's say we have a trench that meets the standards, now we need to compact the base. Pour a 150 mm layer of sand, fill it with water, then 4-5 centimeters of concrete to level the surface. Don’t forget about communications; they need to be done at the stage of formwork and reinforcement of the foundation of the house, we must not forget about the embedded pipes in the right places, otherwise you will have to work with a hammer for a long time and violate the solidity of the foundation.
STEP 3: having a construction project, we install pegs along the perimeter of the trench exactly to the width of the future foundation, they will hold the bottom of the formwork, we place spacers at least 70% of the height of the structure (the spacer itself is 2 times the height of the foundation). It wouldn’t hurt to make 3-4 control slats for the width of the foundation, which can be used to measure width deviations.
Expert advice: you need to drive the nails from the inside of the formwork into the spacers and pegs, so that later you can remove everything without any problems. Otherwise you will have to break it wooden beams or sawing off the hats as the protruding nails are concreted. If the foundation is 150 cm or higher, it is recommended to tie the formwork together with wire in a checkerboard pattern every 1 m.
The formwork should be as rigid as possible, the gaps between structural elements no more than 0.3 cm, so that when pouring the foundation, the liquid mixture does not leak out, otherwise the strength of the foundation will decrease in arithmetic progression.
It must withstand heavy loads and retain its shape after pouring cement, this is the only way to do it monolithic foundation for a home that will last for decades.
Important: the inside of the formwork must be lubricated with technical oil or other petroleum products (working off will do) so that it can more easily peel off from the concrete after it has hardened. You can use it many times.
Photos of strip foundation formwork
Formwork with spacers 
Correct formwork 
Panel formwork
Installation of reinforcement along the entire perimeter of the strip foundation
The formwork is ready, now you can move on to the most important process - reinforcing the foundation with your own hands. They use steel and fiberglass reinforcement; we will focus on the first option, since it will be much cheaper. We will need to purchase the following materials:
- longitudinal reinforcement with a thickness of 14-18 mm (average value, your project may be different);
- transverse and vertical rods with a diameter of 10-12 mm;
- knitting steel wire;
- good pliers or pliers for manipulating wire (or very strong hands).
Important: it is necessary to fasten the reinforcement with knitting steel wire, since it has a low stretch coefficient and is quite strong. This will significantly simplify the assembly of the structure, but the wire does not affect the strength of the foundation; it only fixes the reinforcement before pouring the foundation.
STEP 1 : We make calculations and purchase materials. It is very easy to calculate how much materials you need. Transverse rods are placed at a distance of about 30 centimeters (small errors are not a big deal), longitudinal paired reinforcement every 40 centimeters of height (don’t forget the first section), and vertical reinforcement every 60 cm. Divide the length of the wall by the number of cross members and the number of “tiers” of longitudinal reinforcement . Let's look at the example of a foundation 10x10 meters and 120 cm high:
- 1000 cm: 30 cm = 33 (number of transverse rods on 1 tier);
- 33 x 3 = 99 (number of transverse rods per side);
- 99 x 4 = 396 (all rods on 4 sides).
Now we multiply 396 by the width of the foundation (let it be 70 cm): 396 x 70 = 27720 cm. 277 meters of rods must be purchased. We carry out similar calculations for longitudinal reinforcement:
- 1000 x 2 = 2000 (one tier);
- 2000 x 3 = 6000 (side);
- 6000 x 4 = 24000 cm (you need to purchase 240 meters).
And, of course, vertical elements. We will install them on both sides of the foundation with a frequency of one transverse jumper, that is, every 60 cm:
- 2 x 17 = 34 (pieces per side);
- 34 x 4 = 136 (pieces for the entire base);
- 136 x 120 cm = 16320 cm or 163 meters.
We substitute the parameters of your building as an example and get the correct calculation of the elements for reinforcing the strip foundation of the house. Don’t forget 5-8% for “every firefighter”.
STEP 2: Do you already have 5-6 cm of concrete at the bottom of the trench for leveling? Skip this step. If not, we fill in 15 cm of sand, then 5 cm of concrete, level everything, do not forget about communications and the place for them. If you don’t want to mess around, you can simply put a thick PVC film on the bottom. The main purpose of this step is to level the ground and hold back a little water that will appear after pouring the concrete.
STEP 3: knitting reinforcement for strip foundations. It can be done in a trench or nearby if it is inconvenient to turn around there or the trench itself is too narrow according to the design. During “remote” assembly, you will immediately need to think about ways to lower the metal down so as not to damage the structure. Let's look at how to make foundation reinforcement with your own hands:
- We start with the lower crossbars. We lay them out in 30 cm increments, put 2 longitudinal reinforcements on top of them, and at the “crossroads” we knit them together with wire.
- Let's move on to vertical jumpers. We place the vertical element through 1 transverse one and tie it together.
- We attach 2 more tiers, retreating 40 cm up.
Examples of incorrect tying of reinforcement
Rope is prohibited 
Small overlap 
Welding is prohibited
Important: leave 20 cm after each connection, since the reinforcement may move slightly when pouring the foundation under its load. No need to clamp tightly tying wire, you can leave it to “play” a little, it will be more correct.
4. We lower the frame into the trench (if the assembly was not in it), retreating 5 centimeters from the formwork, and fix it in any convenient way.
Correct foundation reinforcement scheme and examples of reinforcement tying
Twisting with a tool 
Wire tie 
Nice slap 
Crochet 
Twisting diagram
You will end up with 4 “blocks” that will be the length and width of one side minus 5 cm on all sides. Next, we’ll look at how to properly fasten them together and reinforce the corners where most of the total load falls.
Reinforcement of strip foundation corners
The corners are subject to multidirectional compressive forces, since the soil tends to expand or contract depending on the season, sag under the influence of the heavy weight of the building, or be washed away by groundwater. You cannot make mistakes when reinforcing corners, since you may end up with 4 different foundations that will live their own lives. Cracks and faults will be difficult to avoid and such a design will be as ineffective as foundations without reinforcement. There are several ways to strengthen corners.
- Special welded mesh. A ready-made structure is used to strengthen the lower and upper tier of the foundation. Cell no more than 200 x 200 mm, The thickness of the reinforcement depends on the weight of the structure, most often 12 mm. Every 50-60 cm, tiers of metal mesh are connected by vertical reinforcement. Bypass to corners – from 80 centimeters.
- Reinforcement of strip foundations with separate reinforcing bars. It is generally accepted that this is The best way, since it is not advisable to allow welded joints in the reinforcement belt. It can be divided into several categories:
- reinforcement of corners with L-shaped longitudinal reinforcement with overlapping legs of 60 centimeters;
- reinforcement of right angles and junctions with U-shaped elements;
- strengthening junctions with L-shaped products.
Below are a few examples of how NOT to reinforce corners.
No gain 
There is no overlap
The foundation is the basis of any building. Exactly from good foundation The quality of operation of the house and its durability will depend. The foundation usually consists of a hardened concrete mixture. To impart rigidity to the concrete base, it must be reinforced with special steel rods according to certain rules.
There is a scheme for reinforcing a strip foundation, following which the builder will lay a strong and durable foundation for the house.
What is a strip foundation and why reinforce it?
Strip foundation is one of the most common concrete foundations. It is a ribbon structure made around the perimeter of the house, as well as in the areas of partitions.
The strip foundation has the following advantages:
- withstands heavy loads of various buildings made of brick, stone, blocks;
- provides for the arrangement of the basement;
- Suitable for heterogeneous soils where there is a risk of subsidence and swelling.
Thus, the foundation experiences a double load: heavy walls press from above, and soil tension acts from below. The last fact is especially relevant for any building. Indeed, as a result of winter freezing, wet soil increases in volume. If the foundation is not rigid enough, its integrity may be compromised, leading to cracks in the walls and the risk of the house collapsing.
But concrete itself copes well with these loads. So why do we need to add additional steel bars?
This is due to the fact that the load at different points of the foundation is different. After all, the condition of the soil is not the same in different places, just like the pressure of the house, the load on the foundation will be different.
To equalize this physical indicator, steel rods are placed in the concrete base, which ensure uniform distribution of the load over the entire area of the foundation.
Type and quantity of reinforcement to strengthen the foundation
There are 2 types of reinforcing rods: steel and composite. Metal fittings are used more often, as their effectiveness has been proven over the years.
The use of fiberglass is used for those buildings where the requirements for limiting radio interference, magnetic fields, and chemical exposure are increased.
Metal reinforcement can be rod or wire. For a strip foundation, use bar reinforcement of a periodic profile of class A-3 or according to GOST A400. This building material has good adhesion to concrete and the lower and upper parts of the frame are knitted from it.
The vertical and transverse parts of the frame are made from smooth rods with a cross-section of up to 1 cm. Smooth rods are called mounting rods. Transverse reinforcement should be selected class A-1 or according to GOST A240.
Usually the mesh frame is made in 2-3 rows. The step between the vertical bars: 40-70 cm, and the horizontal ones - 30-60 cm. If the buried foundation has a height of less than 1 m, then it will need 2-3 longitudinal levels.
For example, consider a foundation 60 cm high and 30 cm wide. This basis laid under a building whose length and width are 5 m each.
In this case, a two-tier mesh is made with a step of 0.5 m. For 4 longitudinal lines of 20 m each, 80 linear meters of working reinforcement will be required. The calculation of mounting vertical rods is taken taking into account indentations from the surface of 5 cm. If the number of intersections = 51, then we get the total length of the rods: 1.4 m * 51 = 71.4 m. It is recommended to buy material with a margin of 10%.
Thus, by adding numbers, we obtain the total amount of required reinforcement: 80 + 71.4 + 10% ~ 170 linear meters.
Video on how to properly reinforce the spatial frame of a shallow strip foundation:
Rules for laying steel rods in a concrete base
Before making a metal frame, the iron rods should be cleaned and their quality checked.
The technology for reinforcing the strip base is performed according to the following algorithm:
- A sand-crushed stone cushion 5 cm thick is poured into the dug trench. This is necessary to prevent corrosion of the iron rods.
- Formwork is carried out and a thin layer of concrete is poured.
- Transverse rods are laid on top in increments of 80 cm.
- When forming the frame, longitudinal rods are laid perpendicular to the previous rods in 2 rows. The intersections are connected. The lower level of the frame is ready.
- Vertical smooth rods are installed at the joints. It is important to maintain perpendicularity.
- The upper tier of the frame is attached to the vertical rods. It is a frame in which the rods are fixed at intervals of 20 cm.
- The upper tier is equipped with longitudinal rods, which are fastened to the remaining rods with clamps or wire.
- The reinforcement skeleton is rigidly fixed to the formwork. The gap between the iron structure and the formwork should be 3-5 cm.
- Monitor the quality of fastenings and remove excess debris.
The most important thing when making a frame is to securely fasten the rods together, especially in the corners of the foundation. It is important to maintain even angles and perpendicularity. There are 2 ways to combine rods: welding and wire knitting.
Welding in private construction is undesirable, since this method does not provide the proper quality of the perpendicular structure. Builders often neglect the requirements of the standards and weld manually using resistance welding rather than arc welding.
The preferred method of connecting the rods is knitting with wire, with a diameter of 0.8-3 mm. This is done using a crochet hook. The quality of this connection is much higher than in the previous version. The disadvantages of the method are: the process is more labor intensive and has low rigidity compared to a welded structure.
Figure 1 shows a diagram of strengthening the foundation under cottage, size 10x6 m.
As longitudinal rods, class A-3 rods with a diameter of 12 mm are used; The transverse rods are reinforcement with a diameter of 8 mm, class A-1.
The spacing of the floors is 0.6 m, and in the area of corners - 0.2 m. Corners and T-shaped intersections are reinforced with haunches - class A-3 reinforcement, with a diameter of 12 mm. In the area of abutments, the haunches are overlapped, which is equal to: 50*d, where d is the diameter of the rod.
Reinforcement of corners and T-shaped joints can be done using special tabs. They are peculiar corners, with the length of the shelves equal to: 50*d, where d is the diameter of the reinforcement. For example, if the diameter of the reinforcement is 10 mm, then the bend of the legs is 500 mm. An example of such fastening is shown in Figure 2.
To summarize, we can highlight the basic rules for foundation reinforcement:
- The diameter of the working rods must be at least 12 mm.
- Longitudinal (working) rods together with transverse reinforcement form a frame, the elements of which are welded or tied.
- For average size foundation, 3-4 longitudinal rods are needed.
- The diameter of the transverse rods is 6-8 cm, which are laid in increments of 200-600 mm.
- The thickness of the tape base is taken to be at least 300 mm.
- Corners and T-shaped intersections are reinforced with special haunches or paws. The diameter of these fasteners must be equal to the diameter of the working rods.
A metal frame in the foundation design is the key to a strong house and comfortable stay in him.
How the French make a strip foundation:
A shallow strip foundation (hereinafter referred to as MZLF) is one of the types of strip foundations, which is characterized by a shallow depth, significantly less than the depth of soil freezing, and a relatively small consumption of concrete mixture. This article discusses the main advantages and disadvantages of MZLF, the most common mistakes during their construction, a simplified calculation method suitable for private developers (not professionals), recommendations for constructing a foundation with your own hands.
The main advantages of MZLF are:
- economical - concrete consumption is significantly lower than during the construction of a conventional strip foundation. It is this factor that most often determines the choice of this technology in low-rise construction;
- reduced labor costs - less excavation work, less volume of prepared concrete (this is especially important when it is not possible to pour the finished mixture from a mixer);
- smaller tangential forces of frost heaving due to the reduced area of the lateral surface of the foundation.
However, during the construction of the MZLF, it is necessary to strictly adhere to the technology; a frivolous attitude to the process can lead to the appearance of cracks, and then all of the above advantages, as they say, will go down the drain.
The most common mistakes made when installing MZLF:
1) selection of the main working dimensions of the foundation without any (even the most simplified) calculation at all;
2) pouring the foundation directly into the ground without covering it with non-heaving material (sand). According to Fig. 1 (on the right) we can say that in winter time years, the soil will freeze to the concrete and, rising, drag the tape upward, i.e. the tangential forces of frost heaving will act on the foundation. This is especially dangerous if the MZLF is not insulated and a high-quality blind area is not equipped;
3) improper reinforcement of the foundation - choosing the diameter of the reinforcement and the number of rods at your discretion;
4) Leaving the MZLF unloaded for the winter - it is recommended to carry out the entire cycle of work (construction of the foundation, erection of walls, and arrangement of the blind area) one construction season before the onset of severe frosts.
Calculation of a shallow strip foundation.
The calculation of the MZLF, like any other foundation, is based, firstly, on the value of the load from the weight of the house itself and, secondly, on the calculated soil resistance. Those. the soil must withstand the weight of the house transmitted to it through the foundation. Please note that it is the soil that supports the weight of the house, and not the foundation, as some believe.
If an ordinary private developer can calculate the weight of a house if desired (for example, using our online calculator located), then it is not possible to determine the calculated soil resistance on your site on your own. This characteristic is calculated by specialized organizations in specialized laboratories after conducting geological and geodetic surveys. Everyone knows that this procedure is not free. Mostly, architects who design a house resort to it, and then, based on the data received, they calculate the foundation.
In this regard, it makes no sense to provide formulas for calculating the size of the MZLF within the framework of this article. We will consider the case when a developer carries out construction on his own, when he does not conduct geological and geodetic surveys and cannot accurately know the calculated soil resistance on his site. In such a situation, the dimensions and design of the MZLF can be selected according to the tables below.
The characteristics of the foundation are determined depending on the material of the walls and ceilings of the house and its number of storeys, as well as on the degree of heaving of the soil. How you can determine the latter is described
I. MZLF on medium and highly heaving soils.
Table 1: Heated buildings with walls made of lightweight brickwork or from aerated concrete (foam concrete) and with reinforced concrete floors.
Notes:
— the number in brackets indicates the pillow material: 1 — medium-sized sand, 2 — coarse sand, 3 — a mixture of sand (40%) with crushed stone (60%);
— this table can also be used for houses with wooden floors, the safety margin will be even greater;
— see below for foundation design options and reinforcement options.
Table 2: Heated buildings with walls made of insulated wooden panels ( frame houses), logs and beams with wooden floors.
Notes:
— the numbers in brackets mean the same as in Table 1;
- above the line value for walls made of insulated wooden panels, below the line - for log and timber walls.
Table 3: Non-buried foundations of unheated log and timber buildings with wooden floors.
Notes:
- above the line values for log walls, below the line - for walls made of timber.
Design options for MZLF on medium- and highly heaving soils, indicated in the tables by letters, are shown in the figures below:
1 - monolithic reinforced concrete foundation; 2 - sand filling of sinuses; 3 — sand (sand-crushed stone) pillow; 4 — reinforcement frame; 5 - blind area; 6 7 — waterproofing; 8 - base; 9 — ground surface; 10 - sand bedding; 11 - turf.
Option a.— the upper plane of the foundation coincides with the surface of the earth, the base is made of brick.
Option b.- the foundation protrudes above the surface by 20-30 cm, forming a low base or being part of the base.
Option c.- the foundation rises 50-70 cm above the ground, while it also serves as a base.
Option d.- non-buried foundation-basement; Table 3 shows that such foundations are used for unheated wooden buildings.
Option d.- used instead of options b. or V. when the width of the base of the foundation significantly exceeds the thickness of the wall (more than 15-20 cm).
Option e.- a shallow strip foundation on a sand backfill is used quite rarely on weak (peaty, silty) soils when high level groundwater for wooden buildings. Depending on the size of the building, bedding is done either under each strip or under the entire foundation at once.
Reinforcement of a shallow strip foundation.
MZLF reinforcement is made with meshes of working reinforcement and auxiliary reinforcing wire. The working reinforcement is located in the lower and upper parts of the foundation, and it must be immersed in the thickness of the concrete by about 5 cm. The lower mesh works to deflect the foundation tape downward, and the upper mesh works to deflect the tape upward. There is no point in placing the working reinforcement in the middle of the tape (as can sometimes be seen on the Internet).
Table 4: Options for foundation reinforcement.
MZFL reinforcement schemes are shown in the following figure:
A.— a mesh with two working reinforcement rods; b.— a mesh with three working reinforcement rods; V.— T-shaped joint; G.— L-shaped corner joint; d.— additional MZLF reinforcement with a large sole width, when the sole is more than 60 cm wider than the base (the additional mesh is located only in the lower part.
1 — working fittings (A-III); 2 — auxiliary reinforcing wire ∅ 4-5 mm (Вр-I); 3 — vertical reinforcement rods ∅ 10 mm (A-III), connecting the upper and lower mesh; 4 — reinforcement for strengthening the corner ∅ 10 mm (A-III); 5 — connection with wire strands (twisting length is at least 30 diameters of the working reinforcement); 6 — additional working fittings ∅ 10 mm (A-III).
II. MZLF on non-heaving and slightly heaving soils.
Shallow strip foundations on non-heaving and slightly heaving soils do not have to be made only from monolithic concrete. You can also use other local materials, for example, rubble stone, red ceramic brick. MZLF is laid at 0.3-0.4 meters without a sand cushion. Moreover, for wooden buildings and one-story brick (or aerated concrete) foundations, they don’t even need to be reinforced.
For 2- and 3-story houses with walls made of stone materials, MZLF is reinforced. Concrete foundations are reinforced according to the 1st reinforcement option (see Table 4 above). Foundations made of rubble or brick are reinforced with masonry mesh made from BP-I reinforcement ∅ 4-5 mm with a cell size of 100x100 mm. The nets are placed every 15-20 cm.
MZLF structures on non-heaving and slightly heaving soils are shown in the figure below:
1 - foundation; 2 - base; 3 - blind area; 4 — waterproofing; 5 — subfloor (shown conditionally); 6 - mesh made of wire reinforcement, 7 — reinforcement according to option 1 (see table 4)
Options a. and b.- for wooden and one-story brick (aerated concrete) buildings.
Options c. and Mr.— for two- and three-story brick (aerated concrete) buildings.
The width of the sole b is determined depending on the number of storeys of the building and the material of the walls and ceilings.
Table 5: Values of the width of the sole of the MZLF on non-heaving and slightly heaving soils.
Stages of construction of a shallow strip foundation and recommendations.
1) Before starting construction of the foundation, if necessary, it is necessary to ensure high-quality drainage of surface rainwater from neighboring areas from the building site. This is done by cutting out drainage ditches.
2) The foundation is marked and trenches are torn out. It is recommended to begin excavation work only after all necessary materials have been delivered to the construction site. It is advisable to organize the process of cutting out the trench, filling the tape, backfilling the sinuses and constructing the blind area as a continuous process. The less it is extended in time, the better.
3) The dug trenches are covered with geotextiles. This is done so that the sand cushion and sand filling of the sinuses do not become silted over time by the surrounding soil. At the same time, geotextiles allow water to pass through freely and do not allow plant roots to grow.
4) A sand (sand-crushed stone) cushion is poured layer by layer (in layers of 10-15 cm) with careful compaction. They use either manual rammers or area vibrators. Tamping should not be taken lightly. Shallow foundations not as powerful as foundations poured to the entire freezing depth and therefore freebies here are fraught with the appearance of cracks.
5) The formwork is laid out and the reinforcement frame is knitted. Do not forget to immediately provide water and sewerage to the house. If the foundation is also a plinth, remember about the vents (does not apply to buildings with floors on the ground).
6) Concrete is poured. Filling of the entire tape must be done continuously, as they say, in one go.
7) After the concrete has set (3-5 days in summer), the formwork is removed and made vertical.
8) The sinuses are backfilled with coarse sand with layer-by-layer compaction.
9) A blind area is being constructed. It is advisable (especially with a small height of the foundation tape) to make the blind area insulated. This measure will further reduce the forces of frost heaving affecting the MZLF in winter. Insulation is made with extruded polystyrene foam.
As already mentioned at the beginning of the article, it is not allowed to leave the MZLF unloaded or underloaded (the building is not fully built) for the winter. If this happens, the foundation itself and the soil around it must be covered with any heat-saving material. You can use sawdust, slag, expanded clay, straw, etc. There is also no need to clear the snow on the construction site.
It is highly not recommended to build a shallow strip foundation in frozen soil in the winter.
In the comments to this article, you can discuss with readers your experience in the construction and operation of MZLF or ask questions that interest you.
Any building, regardless of its purpose, is unthinkable without a reliable foundation. Construction of the foundation is one of the most important and natural tasks of the entire construction cycle as a whole, and this stage, by the way, is often one of the most labor-intensive and costly - often up to a third of the estimate is spent on it. But at the same time, any simplifications, unreasonable savings on the quality and quantity of necessary materials, and neglect of current rules and technological recommendations must be absolutely excluded.
Of all the variety of foundation structures, it enjoys maximum popularity as the most universal, suitable for the majority of houses and commercial structures being built in the field of private construction. Such a base is highly reliable, but, of course, with high-quality execution. And the key condition for strength and durability is well-planned and correctly carried out reinforcement of the strip foundation, drawings and basic principles of construction of which will be discussed in this publication.
In addition to the diagrams, the article will provide several calculators that will help the novice builder in performing this rather difficult task of creating a strip foundation.
Important features of a strip foundation
General concepts. Advantages of a strip foundation
So, in short, a few general concepts about the construction of a strip foundation. By itself, it represents a continuous concrete strip, without breaks into door or gate openings, which becomes the basis for the construction of all external walls and major internal partitions. The tape itself is buried to a certain calculated distance into the ground and at the same time protrudes from above with its base part. The width of the tape and the depth of its placement, as a rule, are maintained the same throughout the entire foundation. This form contributes to the most uniform distribution of all loads falling on the base of the building.
Strip foundations can also be divided into several varieties. So, they are not only poured from concrete, but also made prefabricated, using, for example, special foundation reinforced concrete blocks, or using rubble filling. However, since our article is devoted to reinforcement, in the future only the monolithic version of the foundation strip will be considered.
Strip foundations can be classified as a universal type of foundation. This scheme is usually preferred in the following cases:
- When constructing houses from heavy materials - stone, brick, reinforced concrete, building blocks and the like. In a word, when you need to evenly distribute a very significant load on the ground.
- When does the developer plan to have a full-fledged basement or even ground floor- only a tape circuit can allow this.
- During the construction of multi-level buildings, using heavy interfloor slabs.
- When a building site is characterized by heterogeneity of the upper layers of the soil. The only exceptions are completely unstable soils, when creating a strip foundation becomes impossible or unprofitable, and it makes sense to turn to another scheme. Strip foundations are also impossible in regions with permafrost.
A monolithic strip foundation has a considerable number of other advantages, which include durability estimated for many decades, relative simplicity and clarity of construction, wide possibilities in terms of laying engineering communications and organization of insulated floors on the first floor. In terms of its strength qualities it is not inferior monolithic slabs, and even surpasses them, while requiring less material costs.
However, one should not think that a strip foundation is an absolutely invulnerable structure. All of the listed advantages will be valid only if the parameters of the foundation being built for the house correspond to the conditions of the construction area, the design load, and have a built-in strength reserve. And this, in turn, means that the design of a foundation (any foundation, by the way) is always subject to special requirements. And tape reinforcement occupies one of the key positions in a series of these problems.
The width of the foundation strip and its depth
These are two key parameters on which the reinforcement scheme of the future foundation strip itself will depend.
Prices for fittings
fittings
But the degree of penetration into the ground strip foundations can be divided into two main categories:
- A shallow strip foundation is suitable for the construction of frame structures, small country houses and outbuildings, provided there is sufficiently stable, dense soil on the site. The base of the tape is located above the soil freezing line, that is, it usually does not fall below 500 mm without taking into account the base part.
- For buildings constructed from heavy materials, as well as in areas where the soil condition is not stable, a deep tape is required. Its base already drops below the soil freezing level by at least 300÷400 mm, and if there is also a basement in the construction plans, even lower.
It is clear that the height of the foundation strip as a whole, including its depth, are by no means arbitrary values, but parameters that are obtained as a result of carefully carried out calculations. When designing, a whole array of initial data is taken into account: the type of soil on the site, the degree of their stability both in the surface layers, and the change in structure as they deepen; climatic features of the region; presence, location and other features of groundwater aquifers; seismic characteristics of the area. Plus, the specifics of the building planned for construction are superimposed - the total load, both static, created only by the mass of the structure (naturally, taking into account all its constituent elements), and dynamic, caused by both operational loads and all kinds of external influences, including wind, snow and others.
Based on the above, it would be appropriate to make one important remark. The fundamental position of the author of these lines is that the calculation basic parameters foundation tape - does not tolerate an amateurish approach.
Despite the fact that on the Internet you can find many online applications for carrying out such calculations, it would still be better to entrust the issue of foundation design to specialists. At the same time, the correctness of the proposed calculation programs is not at all disputed - many of them fully comply with the current SNiP and are capable of truly producing accurate results. The problem lies on a slightly different plane.
The bottom line is that any, even the most advanced calculation program, requires entering accurate initial data. But in this matter it is impossible to do without special training. Agree that it is correct to assess the geological features of the construction site, take into account all the loads falling on the foundation strip, and with their decomposition along the axes, provide for all possible dynamic changes– it’s simply beyond the capabilities of a non-professional. But every initial parameter matters, and underestimating it may well then “play a cruel joke.”
True, if you plan to build a small country house or outbuilding, then inviting a specialist designer may seem like an excessive measure. Well, at your own peril and risk, the owner can build a shallow strip foundation, using, for example, the approximate parameters given in the table below. For light buildings, a deeply buried tape is not required (large deepening can even play a negative role, due to the application of tangential forces during frost heaving of the soil). As a rule, in such cases they are limited to a maximum depth of the sole of 500 mm.
| Type of building being constructed | Barn, bathhouse, outbuildings, small garage | Single storey country house, including - with an attic | One- or two-story cottage designed for permanent residence | Two or three story mansion |
|---|---|---|---|---|
| Average soil load, kN/m² | 20 | 30 | 50 | 70 |
| TYPES OF SOIL | RECOMMENDED DEPTH | TAPE LAYINGS | (EXCLUDING BASEMENT | FOUNDATION PARTS) |
| Pronounced rocky soil, opoka | 200 | 300 | 500 | 650 |
| Dense clay, loam that does not disintegrate after compression with the force of the palm | 300 | 350 | 600 | 850 |
| Packed dry sand, sandy loam | 400 | 600 | Professional foundation calculation required | |
| Soft sand, silty soil or sandy loam | 450 | 650 | Professional foundation calculation required | Professional foundation calculation required |
| Very soft sand, silty soil or sandy loam | 650 | 850 | Professional foundation calculation required | Professional foundation calculation required |
| peat bog | A different type of foundation is required | A different type of foundation is required | A different type of foundation is required |
Let us emphasize once again that these are only average values that cannot be considered as the ultimate truth. In any case, if an amateur builder uses such sources, he takes a certain risk on his own responsibility.
Now - about the width of the foundation strip.
This also has its own characteristics. Firstly, to ensure the rigidity of the foundation structure, it is customary to adhere to the rule that the total height of the tape should be at least twice its width - but this rule is not difficult to follow. And secondly, the width of the tape in the area of the sole should be such that the distributed load is less than the calculated soil resistance parameters, naturally, also with a certain design margin. In a word, a foundation strip with a full load should stand stably, without sagging into the ground. In order to save materials, the base of the strip foundation is often made wider to increase the support area.
There is probably no point in presenting here formulas and tabulated values of soil resistance for carrying out independent calculations. The reason is the same: not so much the difficulty in performing calculations, but problems with the correct determination of the initial parameters. That is, again, it is better to turn to professionals on such issues.
Well, if you are building a light structure or a country house, then you can be guided by the fact that the width of the tape should be at least 100 mm greater than the thickness of the walls being built. As a rule, when independently planning a foundation, round values are taken that are multiples of 100 mm, usually starting from 300 mm and above.
Foundation tape reinforcement
If a specialist is engaged in designing a strip foundation, then finished drawing will, of course, include not only the linear parameters of the concrete belt itself, but also the characteristics of the reinforcement - the diameter of the reinforcing bars, their number and spatial location. But in the case when a decision is made to independently erect a foundation for a building, when planning the structure it is necessary to take into account certain rules established by the current SNiP.
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What fittings are suitable for these purposes?
For proper planning, you need to have at least a little understanding of the range of reinforcement.
There are several criteria for classifying reinforcement. These include:
- Production technology. Thus, the reinforcement can be wire (cold-rolled) and rod (hot-rolled).
- According to the type of surface, reinforcing bars are divided into smooth and having a periodic profile (corrugation). The profiled surface of the reinforcement ensures maximum contact with the poured material.
- The reinforcement can be designed for conventional or prestressed concrete structures.
To create a reinforcing structure for a strip foundation, as a rule, reinforcement is used that is produced in accordance with GOST 5781. This standard includes hot-rolled products intended for reinforcing conventional and preloaded structures.
In turn, these fittings are distributed into classes, from A-I to A-VI. The difference mainly lies in the types of steel used for production and, therefore, in the physical and mechanical properties of the products. If low-carbon steel is used in elementary-class fittings, then in high-class products the metal parameters are close to alloy steels.
It is not necessary to know all the characteristics of reinforcement classes when building independently. And the most important indicators, which will influence the creation of the reinforcement frame, are given in the table. The first column shows the classes of reinforcement according to the two designation standards. So, in brackets there is a designation of classes, digital designation which shows the yield strength of the steel used for the production of reinforcement - when purchasing the material, such indicators may appear in the price list.
| Valve class according to GOST 5781 | steel grade | Rod diameters, mm | Allowable bending angle in a cold state and the minimum radius of curvature when bending (d – diameter of the rod, D – diameter of the mandrel for bending) |
|---|---|---|---|
| A-I (A240) | St3kp, St3sp, St3ps | 6÷40 | 180º; D=d |
| A-II (A300) | St5sp, St5ps | 10÷40 | 180º; D=3d |
| -"- | 18G2S | 40÷80 | 180º; D=3d |
| AC-II (AC300) | 10GT | 10÷32 | 180º; D=d |
| A-III (A400) | 35GS, 25G2S | 6÷40 | 90º; D=3d |
| -"- | 32G2Rps | 6÷22 | 90º; D=3d |
| A-IV (A600) | 80C | 10÷18 | 45º; D=5d |
| -"- | 20ХГ2Ц, 20ХГ2Т | 10÷32 | 45º; D=5d |
| A-V (A800) | 23Х2Г2Т, 23Х2Г2Ц | 10÷32 | 45º; D=5d |
| A-VI (A1000) | 22Kh2G2AYu, 20Kh2G2SR, 22Kh2G2R | 10÷22 | 45º; D=5d |
Pay attention to the last column, which indicates the permissible bending angles and curvature diameters. This is important from the point of view that when creating a reinforcing structure, you come to make bent elements - clamps, inserts, legs, etc. When manufacturing jigs, mandrels or other devices for bending, it is necessary to focus on these values, since reducing the bending radius or exceeding the angle can lead to the reinforcement losing its strength properties.
Rods class A-I Available in smooth version. All other classes (with some exceptions, which, however, depend more on individual requirements customer) – with a periodic profile.