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, briefly, 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 the developer plans to have a full-fledged basement or even a ground floor at his disposal, only a strip scheme 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 over many decades, relative simplicity and clarity of construction, ample opportunities in terms of laying utilities and organizing insulated floors on the first floor. In terms of its strength properties, it is not inferior to 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 of the basic parameters of the foundation strip 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 simply beyond the capabilities of a non-professional to correctly assess the geological features of the construction site, take into account all the loads falling on the foundation strip, and with their distribution along the axes, and provide for all possible dynamic changes. 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 | One-story country house, including one 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 strip reinforcement
If a specialist is designing a strip foundation, then the 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 that 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. Thus, in brackets there is a designation of classes, the digital designation of 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 qualities.
Class A-I rods are available in a smooth version. All other classes (with some exceptions, which, however, depend more on the individual requirements of the customer) are with a periodic profile.
When building a house on a strip foundation, the question of reinforcement arises. Reinforcement is placed in a concrete structure to increase its flexural strength, since concrete has a very low moment capacity. To prevent problems with the tape poured with your own hands, in the future it is necessary to thoroughly study such an issue as the reinforcement scheme for the strip foundation.
The rods embedded in concrete differ in purpose:
- Longitudinal horizontal(working fittings). They are located along the belt and absorb bending loads. The diameter is selected by calculation. For any structure whose thickness is 15 cm or less, the reinforcement is laid in one layer. For elements with a thickness of more than 15 cm (strip foundations), a reinforcement cage is used, which most often consists of lower and upper reinforcement. In a strip foundation, the diameters of the longitudinal rods for making frames may differ, but the lower ones are always taken to be larger or equal (for small loads) in diameter.
- Transverse horizontal(clamps). They ensure the joint operation of longitudinal reinforcement and connect the reinforcement cage into a single whole. When building with your own hands, they are assigned for design reasons (without calculation).
- Vertical(clamps). When the thickness of the structure is more than 15 cm, it is necessary to tie not only the longitudinal rods located at the same horizontal level, but also the upper and lower parts of the reinforcement frame. The function is taken over by vertical clamps. The diameter and pitch are determined for design reasons.
For each type of reinforcement, the following is considered separately:
- diameter;
- number of rods.
- steel grade;
- reinforcement class;
- protective layer.
Selection of reinforcement material
Basic documents to follow:
- (clauses 6.2 and 11.2);
- GOST 5781-82* for steel.
Types of marking of reinforcement products:
- A - rod (hot rolled);
- Вр – wire (cold-deformed);
- K - rope (high strength).
For reinforcement frames of strip foundations, rods of class A400 yield strength are used. There is an outdated marking that is still used by builders - All. When purchasing, it is important to be able to “by eye” distinguish between rods belonging to different classes. It is worth noting that reinforcement cages can be knitted from rods belonging to higher classes, but this is impractical and expensive. To eliminate the possibility of accidentally purchasing a material with a lower yield strength, you need to remember:
- class A240 (Al) has a smooth surface;
- class A300 (All) - periodic profile, ring pattern;
- needed for reinforcing the A400 (Allll) tape, it has a periodic profile with a crescent-shaped pattern (outwardly reminiscent of a herringbone pattern).
When laying reinforcement with your own hands, you should pay attention to the grade of steel. According to GOST, reinforcing bars belonging to class A400 should be made of steel 5GS, 25G2S, 32G2Rps. If steel is purchased in large quantities directly from the plant, then the required grade is indicated in the application. If it is not available, in accordance with GOST, the choice is made by the manufacturer.
Protective layer of concrete
Under this phrase lies the distance that the rods should not reach the outer surface of the product, that is, concrete protects the rods from external adverse influences. According to the document “Guide to the design of concrete and reinforced concrete structures made of heavy concrete without prestressing,” the protective layer provides:
- conditions for the joint work of concrete and reinforcement frame;
- anchoring and the possibility of making joints of frame elements;
- protection of steel from corrosion and other negative external influences;
- protection from high temperatures and direct exposure to fire.
Plastic clamp to create a protective layer of concrete on the sides of the foundation.
According to the above manual, the minimum values of the thickness of the protective layer can be summarized in a table.
In this case, the thickness of the protective layer is taken to be no less than the diameter of the rods.
A plastic cube to create a protective layer of concrete underneath the foundation.
Working reinforcement
When building a house with your own hands, it is not necessary to perform complex calculations on limit states in order to determine the cross-section and number of reinforcement cage bars. As a guide to calculations, use the “Manual for the design of concrete and reinforced concrete structures made of heavy concrete without prestressing” and.
According to these documents, using table 5.2 of the manual and clause 10.3.6 of the joint venture, the total cross-section of all longitudinal bars of the reinforcing frame is calculated:
- when the side of the tape is less than 3 meters - 0.1% of the cross-sectional area of the foundation, the diameter of the rods is at least 10 mm;
- when the side of the tape is more than 3 meters - 0.1%, the diameter of the rods is at least 12 mm.
Requirements for the minimum diameter of rods, depending on the length, are presented in the manual “Reinforcement of elements of monolithic reinforced concrete buildings”.
The use of rods with a diameter of more than 40 mm is not allowed. The rods are distributed evenly in the upper and lower layers, guided by the assortment of reinforcement. If rods of different diameters are used for work (when using leftovers), those with a larger diameter are placed at the bottom. In this case, the pitch requirements presented in paragraph 10.3.5 and paragraphs 5.9-5.10 of the design manual are taken into account.
The longitudinal rods of the reinforcement frame are placed according to the table.
Important! If laying a large number of rods is required, it is allowed to arrange them in bundles; the distance between them is determined from their total cross-section.
Providing a protective layer and distance between the upper and lower reinforcement is achieved through the use of clamps. To secure individual rods of the lower layer, round-shaped plastic clamps are most often used. The top layer is held in place by vertical clamps. Sometimes they resort to using “chairs” or “frogs” for reinforcement.
The rods are available in standard lengths - 6 and 12 meters. If it is necessary to reinforce longer structures, extensions are made along the length. In this case, the amount of overlap is taken to be at least 20 rod diameters, but not less than 250 mm.
Horizontal cross clamps
When building a foundation with your own hands, these rods are assigned structurally and do not depend on the cross-section. It is necessary to take into account the load from the building elements (for massive ones it is better to provide a reserve). According to the same documents as for longitudinal reinforcement, the minimum diameter of transverse rods is 6 mm, but not less than 0.25 of the diameter of the working reinforcement.
The pitch of the rods is assigned to at least 20 diameters of the working rods. For example, with a cross-section of longitudinal elements of 14 mm, the pitch of horizontal clamps must be at least 280 mm. For ease of installation with your own hands, take a rounded value of 300 mm.
The length of the rods depends on the width of the tape and the required protective layer. Fastening is carried out on top of the working reinforcement. Length joining is usually not required.
Vertical clamps
The diameter is determined depending on the height of the tape:
- less than 800 mm - from 6 mm;
- more than 800 mm - from 8 mm, but not less than 0.25 of the diameter of the working rods.
When building foundations with your own hands for massive buildings, it is recommended to lay the rods with a margin. The step is assigned in the same way as for transverse reinforcement. The length of the rods is selected by subtracting the amount of the protective layer at the top and bottom from the height of the foundation strip.
Reinforcement of corners and junctions
According to clause 8.9, monolithic foundations for all walls are rigidly connected to each other and combined into a system of cross strips. In the joint zone, the pitch of transverse reinforcement usually changes and reliable fastening of working rods running in different directions is ensured. There are several methods of reinforcement.
Corner connections
Rigid overlap and “foot”
The free ends of the reinforcement in one direction are bent at right angles and tied to perpendicular rods. In this case, the external ones are connected to each other, and the internal ones are tied to the external one.
The length of the curved section of the “foot”, with the help of which the overlap is ensured, is taken to be 35-50 diameters of the working reinforcement. The pitch of the clamps is set to 3/8 of the height of the foundation strip.
Scheme of reinforcement of the “foot” corner.
L-shaped clamps
To ensure a reliable connection of the working rods, the external rods work together due to an L-shaped clamp placed on them with an overlap of at least 50 diameters of the longitudinal rods. The internal rods are tied to the external ones, as in the previous case:
a. bend the working rods at an angle of 90 degrees, the length of the bend (“foot”) is 50 diameters;
b. attach the legs to the outer rods.
The pitch of the clamps (horizontal and vertical) is 0.75 from the height of the foundation strip.
Reinforcement of the corner with a G-clamp and paws.
U-shaped clamps
In this case, additional reinforcement products are used, bent in the shape of the letter P. For one corner, two such clamps with a length of 50 diameters of the longitudinal rods are required. With this connection, the internal working rods have the same length as the external ones. In the place where the U-shaped clamps overlap, an additional frame of vertical and transverse reinforcement is installed.
Corner reinforcement with P-clamps.
Reinforcement of obtuse angles
Perform with overlap. The outer rod is bent at the required angle, and the inner ones are connected to the outer ones with an overlap of at least 50 diameters. An additional vertical clamp is provided at the bend point of the outer rod.
Scheme of obtuse angle reinforcement.
Wall connections
Lap joint
The reinforcement of the adjacent wall is bent, the bend length is 50 diameters. Both rods from the adjacent tape are attached to the outer rod of the perpendicular wall. In the connection area, the pitch of vertical and transverse clamps is set to 0.375 times the height of the monolithic tape.
The abutment reinforcement is “legs”.
L-shaped clamp
Clamps bent at right angles are attached to the rods of the adjacent wall. The rod is bent so that each side is equal to 50 diameters of the working reinforcement. The first side is connected to the rods of the adjacent wall, and the second to the outer working rod of the perpendicular tape. The pitch of the clamps (vertical, transverse) at the junction is reduced by half compared to the entire long strip.
Reinforcement of the junction with G-clamps.
U-shaped clamp
The connection is made to the external rod of the working reinforcement with a “claw”. Additional reliability is provided by a rod curved in the shape of the letter P, 2 times the width of the foundation strip.
Reinforcement of the junction with P-clamps.
Common Mistakes
1) knitting rods at right angles;
2) use of longitudinal bent reinforcement without anchoring;
An example of incorrect corner reinforcement.
3) connection of longitudinal rods with viscous crosshairs;
4) lack of connection between external and internal rods.
Another example of incorrect corner reinforcement.
Knitting frames
When building a foundation with your own hands, it is extremely important to ensure that all frame elements are securely fastened together. For convenience, possible questions are summarized in a table.
| What and how? | For binding, annealed knitting wire with a diameter of 0.8-1.0 mm is used. To work you will also need a crochet hook. For large volumes of work, special machines for tying reinforcement (knitting gun) are used. |
| Why is it better to knit? | When constructing foundations with your own hands It is recommended to use knitting. Welding is primarily used for large prefabricated frames. This is due to the fact that in the conditions of a construction site there is a possibility of burning through working reinforcement. In addition, when using welding, the help of a qualified worker will be required, which will increase the cost of construction. In addition, the welding site is a potential point of accelerated corrosion. |
| When can knitting be replaced by welding? | Knitting provides greater reliability under construction site conditions (this does not apply to factory-made welded frames), so it makes sense to replace it only if you have a welding machine and experience. Replacing knitting with welding (performed directly at the construction site) is recommended to be done only in straight sections. More details on this issue can be found in GOST 14098-91, Appendix 2 “Assessment of the performance qualities of welded joints under static load”. In this table, one immediately notices a large number of compounds marked ND (unacceptable) or NC (inappropriate). |
When designing and constructing foundations, many questions arise. Each of them should be treated carefully to avoid complications during operation.
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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 in their construction, a simplified calculation method suitable for private developers (not professionals), and 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 the winter season 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 aerated concrete (foam concrete) and 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 timber 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, silted) soils with a high groundwater level 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 are not as powerful as foundations poured to the full depth of freezing, and therefore freezing here is 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.
The foundation is an important supporting structure of a building, the quality of which determines its durability and operational safety. Depending on the characteristics of the house and the characteristics of the foundation under it, one chooses from several types of structures, each of which requires calculations and competent design. Strip foundation is one of the most commonly used for private construction. When constructing it, various technologies are used, one of which is reinforcement.
Strip foundation reinforcement is used for both prefabricated and monolithic structures. If, when using factory-produced foundation blocks, it is not possible to regulate the percentage of reinforcement and the diameters of the rods, then in monolithic structures the process of selecting reinforcement and placing it in the thickness of the tape is important. Why is reinforcement needed and how is it carried out?
Why and when is reinforcement of strip foundations necessary?
A feature of concrete, which is used in the production of many types of foundations, is that they do not react equally to different types of loads. Each building element of a building is subject to varying degrees of compressive, tensile, twisting, and bending forces. Concrete reacts differently to them. For example, if it can withstand a certain compressive load without problems, then the same tensile force can lead to cracking and destruction of the material.
To solve the problem, reinforcement is used. It consists of installing steel rods in stretched areas, which are connected to concrete into one structure and absorb tensile loads. When calculating, you need to keep in mind that stretching is possible in different parts of the foundation, depending on the acting forces. There are also conditions that first compressive forces arise in one zone, and then (during heaving or subsidence) tensile forces.
To reinforce a monolithic strip foundation, rolled steel rods are used. The fittings are divided into classes, depending on their features (A-I, A-II, A-III). The following is used directly in the concrete:
- individual rods;
- reinforcing mesh;
- reinforcement frames.
The mesh is a flat structure of longitudinal and transverse rods connected to each other. The frame is a volumetric element, which consists of the same longitudinal and transverse rods connected into a three-dimensional structure. The choice of the type of reinforcement scheme for a strip foundation is justified during design depending on the value of the loads and the characteristics of the foundation.
Necessary materials
For reinforcement, rods made of rolled steel of several classes are used. For strip foundations the following are most often used:
- A-III with a diameter of 10-16 mm as a working one, which absorbs tensile loads;
- VR-1 with a diameter of 4-5 mm for transverse reinforcement (smooth wire).
A knitting wire is also needed to connect the rods together into a single frame or mesh. Knitting occurs using a special hook, which is made from a steel rod or purchased in hardware stores.
To increase the service life of the reinforcement and protect it from atmospheric influences, it is necessary to ensure a distance from the outer surface of the concrete to the edge of the rod. This distance is called the protective layer. For the foundations of private houses it is at least 30 mm.
To provide a protective layer, different methods are used. Basically this is laying supports from different materials. To do this, use scraps of reinforcement, pieces of steel, or purchase special stands from hardware stores.
Reinforcement scheme for buried foundations
Reinforcement of a buried strip foundation is carried out with longitudinal reinforcement bars tied into a frame. Where are the working rods located? Since they perceive tensile forces, they are located in areas of such loads. In a strip foundation, these are the upper and lower parts of the structure. At the same time, tensile forces do not occur in these two parts of the foundation, but during design it is not always possible to determine with high probability that the load will occur only in the upper or only in the lower zone.
The foundation acts as a beam, which is subject to the weight of the building and possible heaving forces. If the structure is calculated and built correctly, then the house evenly presses on the entire strip. In this case, tensile forces may not occur. But if subsidence or heaving occurs, then the loads increase unevenly. This is why reinforcement is carried out.
The frame is positioned in such a way that the working reinforcement is protected by a layer of concrete. For it, rods of grade A-III are used. The diameter is selected by calculation, but more often it is 12-14 mm. The transverse rods are made from 4-5 mm BP-1 wire in increments of 150-200 mm.
In the cross section, two working rods are most often placed in the lower and upper zones, but with a large belt width it is possible to use three. This option is possible if the tape width is 400 mm and above.
The corners of the foundation, as well as the joints of the walls, are reinforced with reinforcing diagonal bars, the edges of which are brought around the turn and hooked onto the frame or stack on each side.
Since fittings are often sold in lengths of 6 or 12 mm, before purchasing it is important to calculate the dimensions so that they can be cut into the required lengths immediately upon purchase.
Reinforcement scheme for shallow foundations
The difference between shallow foundations is that their base is not located below the depth of seasonal freezing of the soil. The principles of reinforcement and technology of work are the same as for buried ones. There are options for placing the tape directly on the top surface of the soil. The lack of deepening leads to the fact that it is especially necessary to take into account the effects of heaving forces in winter.
Basically, the reinforcement of shallow strip foundations is carried out with meshes. Their location depends on the degree of heaving of the soil. On non-heaving and slightly heaving soils, reinforcement is not always used. This makes the design cheaper.
The meshes consist of longitudinal reinforcement bars of class A-III of different diameters. For small buildings, rods with a diameter of 10 mm are more often used. In the cross section of the tape, two rods are most often placed at the top and bottom of the foundation. For transverse reinforcement, BP-1 wire with a diameter of 4 mm is used.
Reinforcement of the corners of a strip foundation of this type is carried out using the same methods as for a recessed one - by installing rods to reinforce the corner joint. The joints of the walls also need strengthening.
Stages of work
Let's take a closer look at the stages of work for reinforcing a strip foundation with your own hands. Before starting work, it is important to correctly calculate the amount of reinforcement and select a diagram. You can also use standard solutions, but there is always a possibility that the soil characteristics on your site require the use of special measures to strengthen the foundation and the proposed average frame parameters are not suitable in terms of strength characteristics.
Another option is possible when the base is strong and does not require the use of as much material as is proposed for use. There is overspending and an increase in the price of the house. To avoid these troubles, it is recommended to carry out a professional design calculation with the involvement of a civil engineer.
If the scheme has been developed and materials have been purchased, installation begins. The work is carried out in the following order:
- It is necessary to mark the dimensions of the foundation on the ground surface. It is important to observe the dimensions and angles exactly.
- Mark the trenches. Their width should be such that it is convenient to assemble the formwork. More often, a margin of 15-20 cm on each side is enough for this.
- Dig trenches for the tapes. The depth of the trench consists of the height of the foundation and sand backfill. The thickness of the sand cushion depends on the heaving of the soil. In low-heaving clay soils, 10 cm is enough, and in highly heaving clay soils, 60 cm of bedding is provided. It is better to select the exact value of the layer thickness by calculation or from tables in the regulatory literature. Recessed foundations are located below the depth of soil freezing in the region.
- Install the formwork. Wooden boards or sheet materials (OSB, steel) are used for it. Most often, softwood boards 25-40 mm thick are used. The height of the formwork should be 5-10 cm above the top of the tape itself. Before installation, the boards are knocked down into panels of the required width. The shields are sequentially installed in the trench and secured.
- Prepare the fittings. If meshes are used, then vertical rods from scrap reinforcement are driven into the prepared formwork. A distance of 50-100 mm is arranged between them. The distance from the pins to the formwork is selected in such a way as to provide a protective layer of reinforcement (from 30 mm). The frames are knitted either directly in the formwork or outside it, and then transferred.
- The reinforcement bars are knitted together with a special wire using a hook.
- Corners and intersections of walls are reinforced with additional rods.
- After creating the frames or fixing the meshes in the formwork, their reliability is checked and the foundation is concreted.
The construction of walls can begin after achieving the required strength. This period depends on the quality of the concrete mixture and the ambient temperature. On average, the design strength of concrete is achieved in 28 days.
Reinforcement of the base of the strip foundation, if any, is carried out with a reinforcing mesh, which is connected to the frame of the strip itself before concreting.
Strip foundations are the most popular in private construction. It is ideal for the construction of small houses, garages, bathhouses and other outbuildings. All construction work can be done with your own hands, and the relatively low consumption of materials and the minimum amount of excavation work can reduce the price and production time. Of course, for everything to go as expected, you need to know how to properly reinforce the foundation.
Before telling you how to properly reinforce a strip foundation, it’s worth saying a few words about the choice of reinforcement.
- If you need to reinforce the foundation for a one-story or two-story house, as well as lighter buildings, you should take reinforcement with a diameter of 10-24 millimeters. Thicker material will cost too much and its high strength will not be used. Less thick reinforcement may not withstand the load.
- It is advisable to use special corrugated fittings. It provides a better connection to concrete, guaranteeing its high strength and reliability. The smooth analogue costs a little less, but is not suitable for use due to low adhesion. The only exception is cross connections. The load on them is significantly less.
- If the soil is homogeneous over the entire area of the foundation, then you can use material with a cross-section of 10-14 millimeters. With heterogeneous soil, the load on the base increases, so it is advisable to invest in rods with a diameter of 16-24 millimeters.
Of course, buying thick corrugated fittings is quite expensive. But if you decide to reinforce the strip foundation with your own hands, then the amount of work is not too large. So, you will have to overpay a maximum of a few hundred rubles - this is fully compensated by the high durability and reliability of the finished structure.
When independently calculating and selecting reinforcement for the reinforcement frame of a strip foundation, there is a high probability of error. In the future, it may cause the destruction of the house, so the best solution would be to order a foundation reinforcement project from a designer, and carry out the binding of the frame yourself according to the drawing.
How much reinforcement is needed?
Before you go to the store to buy material, you need to find out how much it will be needed to reinforce the strip foundation. To do this, you should think in advance which reinforcement scheme for a strip foundation will be the best choice and carry out calculations for a specific object.
An example of frame reinforcement for a foundation
When building small houses, garages and bathhouses, the following frame configuration is usually used:
- 2 belts: upper and lower;
- each belt consists of 3-4 reinforcement rods;
- the optimal distance between the rods is 10 centimeters. Please note that the distance from the reinforcement to the edges of the future foundation should be at least 5 centimeters;
- The connection of the belts is carried out using clamps or pieces of reinforcement at a pitch of 5-30 centimeters, depending on the area of reinforcement.
This scheme is optimal. Now, knowing the dimensions of the future building, it is not at all difficult to carry out the appropriate calculations.
Let's say you want to build a spacious frame or wooden cottage with an area of 150 square meters with external walls with a perimeter of 50 meters. Let's carry out calculations based on this. When reinforcing the strip foundation, we use SNiP and the characteristics described above.
We have two belts with three rods in each. Total - 6 multiplied by 50 = 300 meters of main reinforcement. We take into account the number of jumpers, which are laid in increments of 30 centimeters. To do this, divide 50 meters by 0.3. We get 167 pieces. The transverse jumpers in this base will have a length of 30 centimeters, and the vertical ones - 60 centimeters. For vertical jumpers you will need 167x0.6x2=200.4 meters. On horizontal ones - 167x0.3x2=100.2 meters. In total, 300 meters of thick corrugated reinforcement and 300.6 meters of thinner, smooth reinforcement will be required. Having received these numbers, you can safely go to the store for material - a strip foundation without reinforcement will not last long. Some experts recommend taking reinforcement with a margin of 10-15%. After all, a certain amount of material will be needed to strengthen the corner parts of the strip foundation and will be used for joining.
How to knit the frame?
The rules for reinforcing strip foundations force one to abandon the use of welding in favor of knitting, since when welding is used, in the places of welded joints, metal rods lose strength up to 2-2.5 times. In addition, this is where corrosion most often appears, which can damage the reinforcement within a few years, significantly reducing the reliability and durability of the foundation. Only connection by knitting is permissible. This is a rather complicated stage, which will take a lot of time for an inexperienced user. However, a lot here depends on what tool you use.
Reliable knot for tying reinforcement with wire
The classic tool for tying reinforcement in strip foundations is a special crochet hook. Using it, experienced craftsmen can produce up to 12-15 knots per minute (of course, if the knitting wire is prepared and cut in advance). The main advantage of this option is its affordability - a hook can be bought in many stores for a hundred rubles or even cheaper. The downside is that the speed of working with it is not great even for experts. Please note that you will have to make many hundreds of ties even if you have to reinforce small foundations.
Wire and hook for knitting the frame
If you want to finish the job quickly, you can use a special knitting gun. Working with it, even an inexperienced user can easily produce 25-30 knots per minute. That is, productivity will increase at least 2 times. Alas, the cost of such equipment is not low - from 50 thousand and above. In addition, to work with it you need a special wire - a regular one may not be suitable. This further increases the cost. But if you have the opportunity to rent a knitting gun for a few hours or a day, feel free to agree to such an offer, just don’t forget to find out the maximum diameter of the reinforcement that it can tie. Working with high-quality tools, you will spend a maximum of a day assembling the frame - correct reinforcement of the strip foundation becomes much easier and faster. When working manually, this process can take a week or more.
How to make a frame correctly?
Before proceeding with the reinforcement of the strip foundation, drawings of suitable frames should be studied. After all, the strength of the frame determines whether the foundation will serve for many decades or will become covered with cracks in the first spring due to seasonal fluctuations in soil level.
In order not to make mistakes during manufacturing, you need to remember a few rules:
- The overlap (the distance from the place of knitting to the edge of the rod) should be at least 5 centimeters.
- At corner joints, perpendicularly running rods must be connected to each other - in no case should you use two separate blocks that are not connected to each other. The ideal solution would be corners made from bent reinforcement - this type of foundation reinforcement scheme is the most reliable. But for this you need to have special equipment; if the reinforcement has a diameter of 14 millimeters or more, smaller diameters can be bent at home.
- Connections using wire must be tight - if you use a crochet hook, then tighten the wire until it stops so that there is no free space between the clamp and the main reinforcement. Also check with your hand; if the clamp moves when touched, you should make an additional tie with wire.
- The overlap during reinforcement should be equal to 40-50 diameters of the reinforcement. There must be a separation between adjacent connecting rods and the top and bottom layers, according to the design.
- The reinforcing frame must stand level in the formwork. It is also necessary to take care of a protective layer of concrete for the reinforcement, done according to the requirements of the drawing. It should be remembered that the minimum protective layer is equal to the diameter of the reinforcement.
Bending of all elements for foundation reinforcement is performed cold. Do not heat the reinforcement under any circumstances, as this will lead to a loss of its strength.
As you can see, the rules are as simple as possible. But some inexperienced builders are unaware or forget about their existence. This leads to the fact that the reinforcement technology of the strip foundation is disrupted and its service life is significantly reduced.
Excavation and preparatory work
One of the advantages of a strip foundation is the relatively small amount of excavation work. A couple of people, working every day with short breaks, can easily dig a pit of a suitable size on normal soil. When the foundation pit is ready, you can begin to arrange it.
The first step is making the foundation cushion. Thanks to it, the negative impact of groundwater on the foundation is reduced, and the load from the foundation itself and the entire building is distributed as evenly as possible over the ground. Here you can use different materials. Most often sand or crushed stone is used. They do their job well - the main thing is that the thickness of the pillow is at least 15-20 centimeters.
But some experts recommend a concrete pad. Yes, it costs the most. Expensive cement and the need to reinforce the cushion greatly increase the cost and time of construction. But as a result, you get the most reliable foundation for the foundation, guaranteeing that it will last for many years. Therefore, we can say with confidence that this money will not be thrown away.
An example of a reinforced strip foundation
If work is carried out on weak, heaving soil, or if you plan to build a heavy brick house, but the use of a monolithic foundation is undesirable for some reason, then you can use a strip foundation with a sole. The widening (glass) can significantly reduce the load on the ground. Of course, do not forget about the reinforcement of the foundation shell - on heaving soils it will regularly withstand significant tensile and bending loads. It is very important to provide it with sufficient strength.
When using a foundation with a sole, the volume of excavation work increases. In addition, you will have to spend additional money on reinforcing the base of the strip foundation - if it fails, this will lead to the rapid destruction of the entire structure.
Formwork is installed on top of the finished cushion. When choosing the width, keep in mind that the finished foundation should be 10-15 centimeters thicker than the external load-bearing walls.
The next step is to install waterproofing. Some builders use roofing felt, but this is a rather expensive material. And the heavy weight makes the installation process difficult. Therefore, it is quite possible to use construction polyethylene. Yes, it is less durable. But it is only needed for a few days - so that the cement laitance does not go into the sand. Therefore, cheap and lightweight polyethylene is quite suitable. It is laid on top of the formwork. At the joints, make a larger overlap - at least 10-15 centimeters - and glue with wide tape.
This concludes the preparatory work. Now we’ll tell you about pouring and reinforcing the foundation with your own hands.
We install the frame, pour concrete
It is best to assemble a frame from reinforcement directly in a prepared pit - this allows the elements to be most firmly fixed. But if we are talking about reinforcing a buried strip foundation or if the pit is too narrow for work to be carried out directly in it, then you can assemble the frame from the outside above the trench, and then carefully lower it into place. Problems usually do not arise here and step-by-step instructions are not needed.
The last and one of the most critical stages is pouring the foundation.
Pouring a strip foundation with a concrete pump
It is advisable to use concrete grade M200 or higher for this. It has high strength to withstand significant loads, and also has sufficient frost resistance.
It should be said right away that the work will require a large amount of material. Carry out all the necessary calculations in advance - you need to pour concrete in one go, avoiding delamination and other separations. Otherwise, the strength of the foundation will significantly decrease, and this will affect the safety of the house. For the same reason, it is advisable to rent a concrete mixer. Today many companies offer such a service. In addition, renting cheap models is relatively inexpensive - less than a thousand rubles per day. If you work intensively during this time, it is quite possible to cope with the work. In addition, the presence of a concrete mixer allows you to increase productivity - you just need to throw in sand, cement and pour in water, soon getting a finished product that just needs to be poured onto the frame installed in the formwork. Working with shovels, such productivity cannot be achieved.
After pouring concrete, you must wait 28 days. During this time, the concrete will gain sufficient strength and it will be possible to begin building a house, garage or bathhouse.
We recommend watching a video where an experienced civil engineer will talk about the important nuances of foundation reinforcement. What should you pay attention to when performing work first, so that the foundation of the house is reliable.
Now you know how to reinforce a strip foundation with your own hands. To do this, it is not at all necessary to have highly specialized skills or buy expensive equipment. It is enough to know, at least in theory, how to reinforce a foundation. Experience will come with the process, and all tools can be replaced with cheap analogues or rented, saving money and time.
