Reinforcement of strip foundation. Subtleties of the strip foundation reinforcement process Reinforcement of strip foundations of low-rise buildings
The foundation is constantly subject to various influences: soil movement, the weight of the house, frost. Such actions lead to deformation and destruction of the base, and if it deteriorates, then the bathhouse itself will not last long. For a building to last for many years, it is important that it is done correctly. One of the stages of foundation installation is reinforcement. Let's take a closer look at what reinforcement is, why it is needed, and how to properly knit reinforcement for a strip foundation.
Strip foundation reinforcement: what is it and why is it needed?
Foundation reinforcement is a method of increasing the load-bearing capacity of a structure by installing a frame. When building a bathhouse, you need to take into account all the requirements that the foundation must meet, specified in SNiP. Reinforcement of the strip foundation is necessary to increase the service life of the foundation and the entire structure as a whole.
The basis of the strip foundation is concrete. As is known, such a material is non-plastic, i.e., when stretched, it can easily crack. The metal frame is characterized by higher stability and reliability; when reinforced, it takes part of the load upon itself, thereby preventing stretching and reducing the risk of destruction of the foundation; tying reinforcement for a strip foundation is necessary.
What material is best to make a reinforcement cage for a strip foundation?
Progress does not stand still; new building materials are constantly appearing. If previously the knitting of reinforcement was strictly made of metal, today you can see plastic reinforcement, fiberglass options and composite parts. So what is the best way to knit reinforcement? Let's look at the materials in more detail.
Despite the emergence of new and more modern materials, time-tested metal fittings remain popular. Its service life is about 50 years. The fittings are distinguished by their high weight, strength and reliability. Its main disadvantage is that it begins to rust when exposed to moisture.
To make the binding of reinforcement for the foundation, hot-rolled construction reinforcement of a periodic profile, mechanically strengthened or heat-treated, is suitable.
When choosing reinforcement, you need to take into account the value of the yield strength. For the installation of longitudinal rows bearing the main load, rods of class A400 (AIII) are suitable; it is permissible to use materials of a higher class. But keep in mind that they will cost more.
It is important that the profile is ribbed, because it adheres better to concrete and evenly transfers the load.
For lintels, smooth reinforcement of class A1 is suitable; this will help reduce the cost of the foundation, while it will have sufficient strength.
For a strip foundation, reinforcement with a diameter of 6-12 mm is suitable.
If the side of the tape is less than 3 m, then it is permissible to use rods of at least 10 mm for horizontal clamps; if more than 3 m, then at least 12 mm. For vertical clamps with a foundation height of less than 80 cm, reinforcement with a diameter of 6 mm or more is suitable, for more than 80 cm - from 8 mm.
Fiberglass reinforcement is 2 times stronger and 9 times lighter than metal. An important advantage of this material is that, unlike metal, it is not subject to corrosion. Fiberglass reinforcement has a service life of about 70 years. Moreover, its price is cheaper than the metal version.
You need to choose the material for reinforcement carefully. If the reinforcement is of poor quality, then cracks may appear on the foundation, which will gradually expand, and as a result, the structure will serve you much less than it could.
You can knit a frame from reinforcement yourself or buy ready-made reinforcement mesh. The second option will be more expensive, but the question of how to knit the reinforcement frame will not bother you, since these concerns will be taken on by specialists. In addition, the advantage of the finished mesh is that the rods are firmly connected to each other in the factory. This ensures that the frame will not fall apart over time, as can happen with a homemade one. In addition, in this case it will be easier to reinforce the foundation: you just need to lay the frame in two parallel layers.
How to properly reinforce a strip foundation with your own hands?
Strong and properly connected reinforcement retains its shape perfectly when pouring concrete for the foundation. But in order for the structure to be of high quality and reliable, you need to know how to properly knit reinforcement for the foundation and choose the right materials.
According to the rules, any reinforcement must be protected from external factors; for this, it is poured with concrete. Moreover, it is necessary that the upper part of the frame is buried in concrete by 5 cm, and the lower part by 7 cm.
If you want to know how to properly reinforce a foundation, then do not forget to study SNiP, where everything is described in detail. In particular, in accordance with SNiP, the total area of the frame must be at least 0.1% of the cross-sectional area of the foundation.
Tension zones can appear in both the upper and lower parts of the foundation, so reinforcement should be done on both sides.
If you are making a shallow foundation, then the step between the horizontal bars during reinforcement should be 30 cm.
Strip foundation reinforcement scheme
First, preparatory work is carried out to clean the area. Next you need to prepare the trench.
The pattern of tying reinforcement for a strip foundation begins with the installation of formwork.
Do-it-yourself foundation reinforcement is carried out according to the following scheme:
- Metal rods are driven in an amount equal to the depth of the base, at a distance of 5 cm from the formwork. 40-60 cm should be left between the rods.
- Stands are installed at the bottom, 2-3 threads of the bottom row of reinforcement are placed on them. Different materials are used as stands, for example, bricks or stones are suitable. Thanks to them, the reinforcement is located higher, and after pouring concrete it will be protected from all sides.
- The rows of reinforcement are attached to the perpendicularly installed pins with jumpers.
- The joints are strengthened by welding or tying with wire.
After you have made the reinforcement frame, you need to prepare holes for ventilation and pour concrete.
Foundation reinforcement schemes may be different. When choosing, you need to take into account the size of the foundation, its load-bearing capacity and personal preferences.
The best option is a foundation with a square or rectangular frame, since in this case a strong frame is obtained.
How to properly reinforce a strip foundation? You can knit the structure in the trench, as described above, or first prepare the reinforcement cage and then lower it into the trench. The difference between these schemes for tying reinforcement for the foundation is that in the second case everything can be done independently, but if you mount the frame directly in a trench, you will need the help of another person.
How to reinforce a strip foundation? To make a bundle or weld?
Horizontal and vertical rods must form a single structure. They can be connected in two ways: by tying or welding.
The disadvantage of welding is that such a procedure deteriorates the physical properties of the metal, it becomes more brittle, so this method is rarely used.
If you still decide to fasten the rods by welding, then try to reduce the number of seams.
Another option is a bunch. It requires a soft and thin wire with a diameter of 0.5 mm and a length of about 30 cm. It should be folded in half and placed at the junction of the rods, and a hook should be threaded into the resulting loop. Next, twist it to form a knot.
Do not twist the wire too much, it may break!
How to knit reinforcement? Methods
Tying reinforcement for the foundation can be done manually or using tools. If you are not a professional builder and are building a bathhouse for yourself, then there is no need to buy special tools - it will be a waste of time. You can do the job just fine manually.
Methods of tying reinforcement:
- Wire knitting.
This is the simplest method that does not require special skills. You need to take a wire with a diameter of 12 mm, cut it into pieces 1.8-2 m long, and bend it in half. Next, you need to fasten the elements with wire in such a way that there are free ends 0.3-0.5 m long. After that, you need to hold the free ends with one hand, and with the other, insert the hook into the loop with rotational movements and twist the wire with both hands.
- Knitting with reinforcement clips.
Paper clips can be purchased at the store. This mounting option is suitable for hard-to-reach places. To tie reinforcement under a strip foundation, you need to fix a paper clip along the diameter on one rod, and press the adjacent reinforcement with the other end. The advantage of this type of fastening is that the reinforcement process is faster than other methods.
- Lapped knitting.
This method is relevant in the case of tying reinforcement at the junction of foundation walls and for lengthening the reinforcement cage. The length of the overlap should be approximately 30 times the diameter of the reinforcement. Moreover, the reinforcement must be installed so as not to touch the formwork. This is necessary to protect it from moisture, high and low temperatures.
All these methods have in common that the first loop must be placed along the spiral grooves so that the fastener holds better.
Reinforcement of strip foundation corners
The corner of the reinforced concrete foundation is subject to stronger pressure compared to the walls; it is the most vulnerable point in the entire structure. If the reinforcement of the corners of the walls is done incorrectly, this will lead to cracks, which will cause the foundation to quickly collapse.
Some people believe that it is enough to lay the rods at right angles, but this opinion is wrong. This design does not allow making a single rigid frame, so the base will not last long.
How to reinforce the foundation in the corners? It is best to use bent L- or U-shaped rods. They strengthen the joints in the corners of the foundation. It is also necessary to reduce the step for tying the rods by 2 times. An effective option is to bend the corners and overlap them using clamps at a distance of 7 cm from the corner.
L-shaped rods are the most reliable option for connecting foundation corners. They need to be laid out so that one end of the rod goes towards one wall, and the other end towards the other. L-shaped rods ensure solidity and high strength of the foundation.
How to knit reinforcement for a foundation? Watch the features of knitting corners in this video:
The strip foundation has a non-standard geometry: its length is tens of times greater than its depth and width. Due to this design, almost all loads are distributed along the belt. A concrete stone cannot compensate for these loads on its own: its bending strength is not enough. To give a structure increased strength, not just concrete is used, but reinforced concrete - this is a concrete stone with steel elements located inside - steel reinforcement. The process of laying metal is called strip foundation reinforcement. It’s not difficult to do it with your own hands, the calculations are elementary, the diagrams are known.
The quantity, location, diameters and grade of reinforcement - all this must be specified in the project. These parameters depend on many factors: both on the geological situation on the site and on the mass of the building being constructed. If you want to have a guaranteed strong foundation, you need a project. On the other hand, if you are building a small building, you can try, based on general recommendations, to do everything yourself, including designing a reinforcement scheme.
Reinforcement scheme
The location of the reinforcement in the strip foundation in cross section is a rectangle. And there is a simple explanation for this: this scheme works best.
Reinforcement of a strip foundation with a strip height of no more than 60-70 cm
There are two main forces acting on the strip foundation: heaving forces press from below during frost, and the load from the house from above. The middle of the tape is almost not loaded. To compensate for the action of these two forces, two belts of working reinforcement are usually made: above and below. For shallow and medium deep foundations (up to 100 cm deep) this is enough. For deep belts, 3 belts are already required: too high a height requires reinforcement.
For most strip foundations, the reinforcement looks like this
To ensure that the working fittings are in the right place, they are secured in a certain way. And they do this using thinner steel rods. They do not participate in the work, they only hold the working reinforcement in a certain position - they create a structure, which is why this type of reinforcement is called structural.
To speed up work when knitting a reinforcing belt, clamps are used
As can be seen in the strip foundation reinforcement diagram, the longitudinal reinforcement bars (working) are tied with horizontal and vertical supports. They are often made in the form of a closed loop - a clamp. It’s easier and faster to work with them, and the design is more reliable.
What fittings are needed
For strip foundations, two types of rods are used. For longitudinal ones that bear the main load, class AII or AIII is required. Moreover, the profile is necessarily ribbed: it adheres better to concrete and transfers the load normally. For structural lintels, cheaper reinforcement is used: smooth first class AI, 6-8 mm thick.
Recently, fiberglass reinforcement has appeared on the market. According to manufacturers, it has better strength characteristics and is more durable. But many designers do not recommend using it in the foundations of residential buildings. According to the standards, it must be reinforced concrete. The characteristics of this material have long been known and calculated; special reinforcement profiles have been developed that ensure that metal and concrete are combined into a single monolithic structure.
Reinforcement classes and their diameters
How concrete will behave when paired with fiberglass, how firmly such reinforcement will adhere to concrete, how successfully this pair will resist loads - all this is unknown and has not been studied. If you want to experiment, please use fiberglass. No - take iron fittings.
Do-it-yourself calculation of strip foundation reinforcement
Any construction work is regulated by GOSTs or SNiPs. Reinforcement is no exception. It is regulated by SNiP 52-01-2003 “Concrete and reinforced concrete structures”. This document specifies the minimum amount of reinforcement required: it must be at least 0.1% of the cross-sectional area of the foundation.
Determination of reinforcement thickness
Since the strip foundation in section has the shape of a rectangle, the cross-sectional area is found by multiplying the lengths of its sides. If the tape has a depth of 80 cm and a width of 30 cm, then the area will be 80 cm * 30 cm = 2400 cm 2.
Now you need to find the total area of the reinforcement. According to SNiP it should be at least 0.1%. For this example it is 2.8 cm 2. Now, using the selection method, we will determine the diameter of the rods and their number.
For example, we plan to use reinforcement with a diameter of 12 mm. Its cross-sectional area is 1.13 cm 2 (calculated using the formula for the area of a circle). It turns out that in order to provide recommendations (2.8 cm 2), we will need three rods (or they also say “threads”), since two are clearly not enough: 1.13 * 3 = 3.39 cm 2, and this is more than 2.8 cm 2, which are recommended by SNiP. But it will not be possible to divide three threads into two belts, and the load on both sides will be significant. Therefore, they stack four, laying a solid margin of safety.
In order not to bury extra money in the ground, you can try to reduce the diameter of the reinforcement: calculate it at 10 mm. The area of this rod is 0.79 cm 2. If we multiply by 4 (the minimum number of working reinforcement bars for a strip frame), we get 3.16 cm 2, which is also enough with a margin. So for this version of the strip foundation, you can use class II ribbed reinforcement with a diameter of 10 mm.
Reinforcement of a strip foundation for a cottage is carried out using rods with different types of profiles
Installation step
There are also methods and formulas for all these parameters. But for small buildings it’s simpler. According to the recommendations of the standard, the distance between horizontal branches should not be more than 40 cm. This parameter is used as a guide.
How to determine at what distance to lay reinforcement? To prevent steel from corroding, it must be embedded in concrete. The minimum distance from the edge is 5 cm. Based on this, the distance between the rods is calculated: both vertically and horizontally it is 10 cm less than the dimensions of the tape. If the width of the foundation is 45 cm, it turns out that between the two threads there will be a distance of 35 cm (45 cm - 10 cm = 35 cm), which corresponds to the standard (less than 40 cm).
The reinforcement step of a strip foundation is the distance between two longitudinal bars
If our tape is 80*30 cm, then the longitudinal reinforcement is located one from the other at a distance of 20 cm (30 cm - 10 cm). Since medium-level foundations (up to 80 cm in height) require two reinforcement belts, one belt from the other is located at a height of 70 cm (80 cm - 10 cm).
Now about how often to install jumpers. This standard is also in SNiP: the installation step of vertical and horizontal dressings should be no more than 300 mm.
All. We calculated the reinforcement of the strip foundation with our own hands. But keep in mind that neither the mass of the house nor the geological conditions were taken into account. We relied on these parameters to determine the size of the tape.
Corner reinforcement
In the design of a strip foundation, the weakest point is the corners and the junction of the partitions. In these places loads from different walls are combined. In order for them to be successfully redistributed, the reinforcement must be properly tied. Simply connect it incorrectly: this method will not ensure load transfer. As a result, after some time, cracks will appear in the strip foundation.
The correct scheme for reinforcing corners: either bends are used - L-shaped clamps, or longitudinal threads are made 60-70 cm longer and bent around the corner
To avoid this situation, when reinforcing corners, special schemes are used: the rod is bent from one side to the other. This “overlap” should be at least 60-70 cm. If the length of the longitudinal rod is not enough to bend, use L-shaped clamps with sides also at least 60-70 cm. Schemes of their location and fastening of the reinforcement are shown in the photo below.
The abutments of piers are reinforced using the same principle. It is also advisable to take the reinforcement with a reserve and bend it. It is also possible to use L-shaped clamps.
Reinforcement diagram for adjacent walls in a strip foundation (to enlarge the picture, right-click on it)
Please note: in both cases, in the corners, the installation step of the transverse jumpers is reduced by half. In these places they already become workers - they participate in the redistribution of the load.
Reinforcement of the base of a strip foundation
On soils with not very high bearing capacity, on heaving soils or under heavy houses, strip foundations are often made with a sole. It transfers the load to a larger area, which gives greater stability to the foundation and reduces the amount of subsidence.
To prevent the sole from falling apart under pressure, it also needs to be reinforced. The figure shows two options: one and two belts of longitudinal reinforcement. If the soils are complex, with a strong tendency to winter baking, then two belts can be laid. For normal and medium-heaving soils, one is enough.
Reinforcement rods laid lengthwise are working. They, as for the tape, are taken in second or third class. They are located at a distance of 200-300 mm from each other. They are connected using short pieces of rod.
Two methods of reinforcing the base of a strip foundation: on the left for foundations with normal bearing capacity, on the right for not very reliable soils
If the sole is not wide (rigid design), then the transverse sections are constructive and do not participate in load distribution. Then they are made with a diameter of 6-8 mm, bent at the ends so that they cover the outer rods. They are tied to everyone using a binding wire.
If the sole is wide (flexible), the transverse reinforcement in the sole is also working. She resists the soil’s attempts to “collapse” her. Therefore, in this version, the soles use ribbed reinforcement of the same diameter and class as the longitudinal one.
How much rod do you need?
Having developed a strip foundation reinforcement scheme, you know how many longitudinal elements you need. They are laid around the entire perimeter and under the walls. The length of the tape will be the length of one reinforcement rod. By multiplying it by the number of threads, you get the required length of the working reinforcement. Then add 20% to the resulting figure - a margin for joints and overlaps. This is how much in meters you will need working reinforcement.
You count how many longitudinal threads according to the diagram, then calculate how many structural rods are needed
Now you need to calculate the amount of structural reinforcement. Calculate how many crossbars there should be: divide the length of the tape by the installation pitch (300 mm or 0.3 m, if you follow the recommendations of SNiP). Then you calculate how much it takes to make one lintel (add the width of the reinforcement cage with the height and double it). Multiply the resulting figure by the number of jumpers. You also add 20% to the result (for connections). This will be the amount of structural reinforcement to reinforce the strip foundation.
Using a similar principle, you calculate the amount needed to reinforce the sole. Putting everything together, you will find out how much reinforcement is needed for the foundation.
Technologies for assembling reinforcement for strip foundations
Reinforcing the strip foundation with your own hands begins after installing the formwork. There are two options:
- The entire frame is assembled directly in a pit or trench. If the tape is narrow and high, it is inconvenient to work.
According to one technology, the reinforcement is knitted directly in the formwork
Both options are imperfect and everyone decides how it will be easier for him. When working directly in a trench, you need to know the procedure:
- The longitudinal rods of the lower reinforced belt are laid first. They need to be raised 5 cm from the edge of the concrete. It is better to use special legs for this, but pieces of bricks are popular among developers. The reinforcement is also 5 cm away from the formwork walls.
- Using transverse pieces of structural reinforcement or molded contours, they are fixed at the required distance using tying wire and a hook or a tying gun.
- Then there are two options:
- If contours formed in the form of rectangles were used, the upper belt is immediately tied to them at the top.
- If during installation you use cut pieces for crossbars and vertical posts, then the next step is tying up the vertical posts. After they are all tied, a second belt of longitudinal reinforcement is tied.
There is another technology for reinforcing strip foundations. The frame turns out to be rigid, but there is a large consumption of rods for the vertical posts: they are driven into the ground.
The second technology for reinforcing a strip foundation is to first drive in vertical posts, tie longitudinal threads to them, and then connect everything with transverse ones
- First, vertical posts are driven in at the corners of the tape and at the junction of horizontal rods. The racks should have a large diameter of 16-20 mm. They are placed at a distance of at least 5 cm from the edge of the formwork, checking horizontal and vertical, and driven into the ground 2 meters.
- Then vertical rods of the calculated diameter are driven in. We determined the installation pitch: 300 mm, in the corners and at the junctions of the walls it is half as much - 150 mm.
- The longitudinal threads of the lower reinforcement belt are tied to the posts.
- At the intersection of the racks and longitudinal reinforcements, horizontal jumpers are tied.
- The upper reinforcement belt is tied, which is located 5-7 cm below the upper surface of the concrete.
- Horizontal jumpers are tied.
It is most convenient and quick to make a reinforcing belt using pre-formed contours. The rod is bent to form a rectangle with the specified parameters. The whole problem is that they need to be made identical, with minimal deviations. And a large number of them are required. But then the work in the trench moves faster.
The reinforcing belt can be knitted separately, and then installed in the formwork and tied into a single whole on site
As you can see, reinforcing a strip foundation is a lengthy and not the easiest process. But you can cope even alone, without helpers. It will take a lot of time, though. It’s easier to work with two or three people: both carry the rods and set them out.
Reinforcement of a strip foundation with your own hands: diagrams, calculation of the diameter of the reinforcement, location in the corners and in the sole
How to calculate and make the reinforcement of a strip foundation, choose the thickness of the rod, the distance between the threads, reinforcement schemes for corners and junctions, assembly technology - you will find all this here.
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.
Making a reinforcement cage for a strip foundation is not difficult. It is worth studying the basic requirements for the reinforcement that will be used and the technology for reinforcing a strip foundation. If all conditions are met, the finished frame will be strong and durable.
Strip foundations have many advantages over other foundations. It is suitable for the construction of the foundation of both multi-story and single-level buildings on any type of soil. That's why he's so often used in individual construction. Knowing the basic principles of work and following them exactly, with correctly performed calculations, you can easily build a foundation with your own hands.
During the operation of the building, settlement often appears. The soil under the base becomes denser. And the greater the pressure on the foundation, the faster this process occurs. If the calculations are performed correctly and the load is evenly distributed on the ground, then cracks and chips on the sides do not appear on the strip base. But in fact, the opposite often happens.
In this case, an inexperienced builder may encounter the problem of how to properly reinforce a strip foundation. After all, it depends on how long the building will be in use. Therefore, it is worth taking a closer look at the choice of material and the technological sequence itself.
When soils freeze, they swell and this causes additional forces to be exerted on the foundation.
Selection of fittings
The strength of the frame depends on the choice of reinforcement.
Existstwo main types:
- steel (metal);
- composite (fiberglass)
The latter type arose in the 50s of the last century, but was not widely used in individual construction, despite the fact that it has a number of advantages over metal ones.
Steel reinforcement, in turn, can be:
- rod;
- wire
To perform reinforcing work on a strip foundation, bar reinforcement of a periodic profile is used, as a working one (another name is longitudinal) and smooth (transverse) as an auxiliary one.
Longitudinal reinforcement must provide good attachment to concrete. Therefore most often choose periodic corrugated profiles. It also differs in strength classes. In Soviet times, in accordance with GOST, class A-3 was most often used in private construction, which, as indicated in modern construction literature, corresponds to the marking A400.
For mounting frame parts in the transverse direction, steel rods of class A-1, or its modern analogue A240, are used. But there is not much difference between them.
Features of fittings
They are like that:
- the diameter of the longitudinal rods must be at least 12 mm;
- the number of working rods in the frame must be at least four (maybe six);
- The pitch of transverse reinforcement is from 200 to 600 mm. The cross-section of steel rods is 6-8mm;
- The strip foundation should be at least 300 mm thick.
In the upper part of the foundation, which is subject to compressive forces, reinforcement of a smaller diameter can be laid than in the stretched lower part. After all, part of the loads at the top will be taken over by concrete
T-junctions and places that may be subject to deformation must be reinforced with embedded products(for example, reinforcing haunches or paws). They must match the diameter of the working rods.
Foundation reinforcement scheme
Before starting work, it is necessary to carefully consider the diagram of the reinforcement cage for the strip foundation and draw up a drawing. For example, if the house consists of one floor with a 10x6 attic, it will look like this.
Six metal rods of class A3 with a diameter of 12 mm are taken as working rods, and transverse reinforcement will be performed using clamps made of class A1 rods with a diameter of 8 mm. The clamps are installed in increments of 200 mm in corners and T-shaped intersections, and 600 mm in the rest.
Weak spots are reinforced using corner and diagonal haunches using class A3 rods with a diameter of 12 mm. In the places where the haunches adjoin the working rods, an overlap of 50 diameters (50x12mm=600 mm) is installed.
In this case, joining along the length of the longitudinal rods is planned to be done with an overlap along a length of similar length (600 mm). Such places should be reinforced with clamps with a smaller pitch (200 mm). The reinforcement rods are planned to be 11.7 m long. The fewer connections the better, so it is better to take steel rods of maximum length.
You can also reinforce corners and T-shaped intersections with the help of so-called legs, which, in fact, are L-shaped bends of working rods with a diameter of 50.
Often during the period of operation, steel can be subject to corrosion, so when reinforcing the foundation it is better to carry out additional work in order to provide a protective layer for the reinforcement.
For a strip foundation, the size of this layer is approximately 40 mm at the side and top edges. If the sole is made of B2-5 concrete, the thickness of which is 100 mm, the protective layer must be at least 40 mm, but can be increased to 70 mm.
The connecting clamp is installed in increments equal to 3/8 of the height of the strip foundation, which is a minimum of 25 cm. According to SNiP, the spacing between the working reinforcement bars in a strip foundation must be no less than 25 cm and no more than 40 cm. Transverse reinforcement is installed in increments equal to 1/2 the height of the working section, but not more than 0.3 m.
The protective layer for the reinforcement is formed from concrete. Its task is to prevent moisture from reaching the metal of the frame
How to make a reinforcement cage for a strip foundation
The base frame resembles a regular square or rectangle.
The principle of reinforcement is as follows:
- brick rows are laid at the bottom of the trench, the height of which is at least 5 cm. This is done so that a gap is formed between the frame and the lower part of the base;
- rods of the required length for vertical reinforcement are cut according to the sample;
- Metal rods are laid longitudinally on the rows of bricks. It is better if they are of maximum length;
- The working rods are connected using wire with jumpers at a distance of 30 cm from each other. They should be 10 cm long less than the thickness of the base (retreating 5 cm on each side);
- rods are installed vertically in each cell in the corners. Their length should be 10 cm less than the height of the base;
- vertical rods are connected to longitudinal rods and fixed with jumpers.
Particular attention should be paid to the reinforcement of corners, because These places bear the most loads.
When reinforcing corners, the following rules must be observed:
- the rods at the corners are bent so that their ends are recessed into the base wall with a minimum overflow of 40 cm (for rods with a diameter of 10 mm);
- the places where they overlap each other must be strengthened with vertical and transverse rods;
- if the length of the rod is not sufficient to bend it onto the wall, then it is necessary to strengthen such places with L-shaped rods;
- The clamps at the corners are connected in steps of 2 times smaller.
If these requirements are met, the corners will remain in their original form longer.
Knitting reinforcement cage
To tie the reinforcement, use a knitting wire with a diameter of 0.8-1.2 mm, which is cut into pieces of 10-20 cm. The minimum number of connections should be half the number of intersections.
In private housing construction, the reinforced frame is often welded instead of knitting to speed up the process
You can knit reinforcement in several ways:
- using pliers. To do this, fold the wire in half, twist and fix the ends with blunt-nose pliers;
- You can use a hook and a screwdriver with a special attachment. To do this, the pieces of wire are folded in half and looped onto the hook. The ends are wrapped at the intersection of the reinforcement, and then put back into the hook. Then, turning the hook, rotate it. You can use a screwdriver for these purposes;
- using paper clips, clamps, brackets, etc.;
- tying gun.
The last method is considered the fastest and most effective. To do this, the nozzle of the gun is placed at the intersection of the reinforcement, and the knitting is completed with the tool.
Foundation reinforcement is considered correct if the the following building rules:
- the base of the upper and lower surfaces has been reinforced;
- the longitudinal tiers of the frame are made of horizontally located rods connected by transverse and vertical rods;
- the working reinforcement consists of A3 class rods, the diameter of which is 10-16 mm, and the clamps for the connection are made of BP-1 class reinforcement with a diameter of 4-5 mm;
- the frame is installed inside the base at a distance of at least 5 cm from its surface;
- longitudinal rods are installed in increments of 25 to 40 cm, and connecting clamps - 30 cm;
- the corners of the base of the reinforcement are laid to form an overlap of 40 cm;
- the diameter of the wire for knitting is 0.8-1.2 mm;
- the minimum number of knittings is equal to half the number of intersections in the frame.
If these conditions are met, the frame will be quite strong and stable. Experts recommend using specially designed tools for knitting. This will significantly reduce the knitting time. If you lack experience in this area, it is better to turn to specialists.
For more information on how to make a reinforcement cage for a strip foundation with your own hands, you can watch the video:
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 strip in the future, it is necessary to thoroughly study the issue of reinforcing 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. Appointed 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).
It is worth paying 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
These rods are designated 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, the rounded value is 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.
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, it is extremely important to ensure that all frame elements are securely fastened to each other. 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, 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.
