Do-it-yourself reinforcement of the strip foundation: diagrams, calculation of the diameter of the reinforcement, location in the corners and in the sole

The strip foundation has a non-standard geometry: its length is tens of times greater than the depth and width. Due to this design, almost all loads are distributed along the belt. On its own, a concrete stone cannot compensate for these loads: its bending strength is not enough. To give the 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 is not difficult to make it with your own hands, the calculation is elementary, the schemes are known. 

The quantity, location, diameters and grade of reinforcement – all this should be spelled out 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 erected. If you want to have a guaranteed strong foundation, a project is required. On the other hand, if you are building a small building, you can try to do everything yourself, based on general recommendations, 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.

Two main forces act on the strip foundation: from below, in frost, heaving forces press, from above, the load from the house. At the same time, 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: top and bottom. For shallow and medium-depth foundations (up to 100 cm deep), this is enough. For deep belts, already 3 belts are required: too high a height requires reinforcement.

You can read about the depth of the foundation here.

In order for the working reinforcement to be in the right place, it is fixed in a certain way. And they do it with the help of 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.

As can be seen in the reinforcement scheme of the strip foundation, the longitudinal bars of the reinforcement (workers) are tied up with horizontal and vertical supports. Often they are made in the form of a closed loop – a collar. It is easier and faster to work with them, and the design is more reliable.

What fittings do you need

For the strip foundation, two types of bar are used. For longitudinal, which carry 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 fittings are taken: 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 regulations, it should be reinforced concrete. The characteristics of this material have long been known and calculated, special reinforcement profiles have been developed that contribute to the fact that metal and concrete are combined into a single monolithic structure.

How concrete will behave 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 the reinforcement of the strip foundation

Any construction work is standardized by GOSTs or SNiPs. Reinforcement is no exception. It is regulated by SNiP 52-01-2003 “Concrete and reinforced concrete structures”. This document indicates the minimum amount of reinforcement required: it must be at least 0,1% of the cross-sectional area of ​​uXNUMXbuXNUMXbthe foundation.

Determination of reinforcement thickness

Since the strip foundation in the 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 cm².

Now you need to find the total area of ​​u0,1bu2,8bthe reinforcement. According to SNiP, it should be at least XNUMX%. For this example it is XNUMX cm². Now, using the selection method, we determine the diameter of the bars and their number.

For example, we plan to use rebar with a diameter of 12 mm. Its cross-sectional area is 1.13 cm² (calculated using the formula for the area of ​​a circle). It turns out to provide recommendations (2,8 cm²) we will need three rods (or they also say “threads”), since two are clearly not enough: 1,13 * 3 = 3,39 cm², which is more than 2,8 cm²recommended by SNiP. But three threads cannot be divided into two belts, and the load will be significant on both sides. Therefore, four are laid, 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 under 10 mm. The area of ​​this bar is 0,79 cm². If we multiply by 4 (the minimum number of bars of working reinforcement for a strip frame), we get 3,16 cm², 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.

We figured out how to calculate the thickness of the longitudinal reinforcement for a strip foundation, you need to determine with what step to install vertical and horizontal lintels.

Installation step

For all these parameters, there are also methods and formulas. But for small buildings, it’s easier. According to the recommendations of the standard, the distance between horizontal branches should not be more than 40 cm. They are guided by this parameter.

How to determine at what distance to lay reinforcement? In order for steel not to corrode, it must be embedded in concrete. The minimum distance from the edge is 5 cm. Based on this, the distance between the bars 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 there will be a distance of 35 cm between the two threads (45 cm – 10 cm = 35 cm), which corresponds to the standard (less than 40 cm).

If we have a tape of 80 * 30 cm, then the longitudinal reinforcement is located one from the other at a distance of 20 cm (30 cm – 10 cm). Since two reinforcement belts are required for medium-level foundations (up to 80 cm high), one belt from the other is located at a height of 70 cm (80 cm – 10 cm).

Now about how often to put jumpers. This standard is also in SNiP: the installation step of vertical and horizontal dressings should be no more than 300 mm.

All. Do-it-yourself reinforcement of the strip foundation was calculated. But keep in mind that neither the mass of the house nor the geological conditions were taken into account. We were based on the fact that these parameters were based on when determining the dimensions of the tape.

Corner reinforcement

In the design of the strip foundation, the weakest point is the corners and the junction of the walls. In these places, loads from different walls are connected. In order for them to be successfully redistributed, it is necessary to correctly bandage the reinforcement. Just connect it incorrectly: this method will not ensure the transfer of the load. As a result, after some time, cracks will appear in the strip foundation.

To avoid this situation, when reinforcing corners, special schemes are used: the bar is bent from one side to the other. This “overlap” should be at least 60-70 cm. If the length of the longitudinal bar for bending is not enough, use L-shaped clamps with sides also at least 60-70 cm. The diagrams of their location and fastening of the reinforcement are shown in the photo below.

By the same principle, the adjoining walls are reinforced. It is also advisable to take the reinforcement with a margin and bend it. It is also possible to use L-shaped clamps.

Please note: in both cases, in the corners, the installation step of the transverse jumpers is halved. In these places, they are already becoming workers – they are participating in the redistribution of the load.

Reinforcement of the sole of the 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 over a large area, which gives greater stability to the foundation and reduces the amount of subsidence.

So that the sole does not fall apart from 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. With normal and medium soils, one is enough.

Reinforcement bars laid in length are working. They, as for the tape, take the second or third class. They are located from each other at a distance of 200-300 mm. They are connected using short pieces of rod.

If the sole is not wide (rigid scheme), then the transverse segments are constructive, they do not participate in the distribution of the load. Then they are made with a diameter of 6-8 mm, bent at the ends so that they cover the extreme bars. Tied to everyone with a knitting wire.

If the sole is wide (flexible), the transverse reinforcement in the sole is also working. She resists the ground’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 fit around the perimeter and under the walls. The length of the tape will be the length of one reinforcing bar. Multiplying it by the number of threads, get the required length of the working reinforcement. Then add 20% to the resulting figure – the margin for joints and “overlaps”. That’s how much in meters you will need working fittings.

Now you need to calculate the amount of structural reinforcement. Consider how many cross jumpers should be: divide the length of the tape by the installation step (300 mm or 0,3 m, if you follow the recommendations of SNiP). Then you calculate how much it takes to make one jumper (add the width of the reinforcing cage with the height and double it). Multiply the resulting figure by the number of jumpers. Add 20% to the result (for connections). This will be the amount of structural reinforcement for reinforcing the strip foundation.

By a similar principle, consider the amount that is needed to reinforce the sole. Putting it all together, you will find out how much reinforcement you need for the foundation.

You can read about the choice of brand of concrete for the foundation here. 

Technologies for assembling reinforcement for a strip foundation

Do-it-yourself reinforcement of the strip foundation begins after the formwork is installed. There are two options:

• The entire frame is assembled directly in the pit or trench. If the tape is narrow and high, it is so inconvenient to work.
  

• Near the pit, segments of the frame are being prepared. They are transferred in parts and installed in their intended place, linking them into a single whole. It is more convenient to work this way, except that the associated structures made of reinforcement are very inconvenient and difficult to transfer.

Both options are not ideal and everyone decides how it will be easier for him. When working directly in the trench, you need to know the procedure:

• The longitudinal bars of the lower armored 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 with developers. Reinforcement is also spaced 5 cm from the formwork walls.
• Using transverse pieces of structural reinforcement or molded contours, they are fixed at the required distance using a tie wire and a hook or a tie gun.
• Next, there are two options:
  • If contours shaped in the form of rectangles were used, the upper belt is immediately tied to them at the top.
  • If during installation cut pieces are used for transverse jumpers and vertical racks, then the next step is to tie up the vertical racks. After they are all tied, the second belt of longitudinal reinforcement is tied.

There is another technology for reinforcing a strip foundation. The frame turns out to be rigid, but there is a large consumption of the bar for vertical racks: they are driven into the ground.

• First, vertical posts are driven in at the corners of the tape and at the junctions of the horizontal bars. 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 the horizontal and vertical, they are driven into the ground by 2 meters.
• Then vertical bars of the calculated diameter are hammered. We determined the installation step: 300 mm, in the corners and at the junction of the walls it is half as much – 150 mm.
• Longitudinal threads of the lower reinforcement belt are tied to the racks.
• At the intersection of racks and longitudinal reinforcements, horizontal jumpers are tied.
• The upper reinforcement belt is tied up, which is located 5-7 cm below the upper surface of the concrete.
• Attached horizontal jumpers.

It is most convenient and fastest to make a reinforcing belt using preformed contours. The rod is bent, forming a rectangle with the given parameters. The whole problem is that they must be made the same, with minimal deviations. And it takes a lot of them. But then the work in the trench moves faster.

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, however, a lot of time. It is more convenient to work together with two or three people: both to carry the bars and to expose them.