Location and calculation of reinforcement in a strip foundation

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 type 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.

You can read about the depth of the foundation here.

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 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.

Regulatory framework for the production of construction reinforcement

In 2016, interstate GOST 34028-2016 was adopted. It was developed to replace GOST 5781-82 and GOST 10884-94, which regulate the production of the entire range of construction reinforcement. The introduction of the new regulatory document was planned from 01/01/2018, but was delayed by a year.

It is unknown whether this date will be met, since GOST 34028-2016 has not yet been signed by Kazakhstan and Belarus (the countries of the Customs Union), as well as some CIS countries. Therefore, the answer to the question of what reinforcement to use for the foundation from a documentation point of view may soon change.

However, this issue is of greater concern to designers who are responsible for the compliance of technical and working documentation for construction with the requirements of current regulatory documents. The process of transition to a new regulatory framework will also create certain inconveniences for consumers of valves.

For example, those who work in construction will continue to use an array of technical documentation and literature, which will retain the same terms, designations and reference standards. Therefore, after switching to GOST 34028-2016, we will have to get used to what kind of reinforcement to use for the foundation of a house and other structures.

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 cm2.

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 cm2. Now, using the selection method, we will determine the diameter of the rods and their number.

Quotes from SNiP that relate to reinforcement (to enlarge the picture, right-click on it)

For example, we plan to use reinforcement 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 that in order to provide the 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, and this is more than 2.8 cm2, which SNiP recommends. 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 cm2. If we multiply by 4 (the minimum number of working reinforcement bars 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.

Reinforcement of a strip foundation for a cottage is carried out using rods with different types of profiles

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

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.

What types of reinforcement can be used when pouring the foundation?

It's no secret that the foundation is poured from a cement composition - concrete. And despite the high durability and strength of this material, it is very fragile, and therefore special reinforcement is used to strengthen it. Even at the design stage, specialists determine the class of reinforcement for the foundation.

If previously only metal rods were used when pouring foundations, today this is far from the only option. To strengthen the foundation base, two types of reinforcement are used today:

· Metal - a classic type of reinforcement, consisting of rods made of steel. Their most common option are rods with a round cross-section. To improve the strength characteristics of such rods, a helical ribbed surface is applied to their surface.

· Relatively recently they began to produce reinforcement from fiberglass. Despite the fact that the invention of composite rods dates back to the 70s of the last century, they began to be actively used only in recent years. And today such products are gradually replacing their metal counterparts. They are made from high-strength fiberglass, which provides the main advantages of such rods, which include reliable corrosion resistance.

Which type of fittings is better

With the advent of the fiberglass analogue, many people began to wonder: which reinforcement is better? In fact, the ideal option has not yet been invented, and therefore it is not possible to answer this question unambiguously: both types of rods have their drawbacks. And one of the main disadvantages of fiberglass reinforcement is the relatively recent start of its use. Therefore, it is still difficult to talk about its strength and durability.

When deciding which reinforcement to choose, you must first pay attention to the diameter of the rods:

1. For metal options, the cross-section can be in the range of 5-32 mm;

2. Fiberglass rods are usually made in diameters of 4-20 mm.

To ensure the construction has the necessary strength characteristics, the correct diameter of the reinforcing bars should be selected. In this case, it is necessary to take into account the size and weight of the building, the type of foundation, the presence of seasonal deformations, the type of soil, etc.

For a private home, steel rods with a diameter of 10-16 millimeters are most often chosen. Such rods are strong enough to withstand the load exerted by a building of one or two floors.

Metal rods can have either a smooth or ribbed surface. Rods of the first type are most often used as connecting jumpers, and therefore they do not experience the main loads from the building. Ribbed options are designed for areas where tensile loads are present.

Also, when choosing reinforcement, you should take into account the difference in steel grade. For example, rods can be made of low-alloy or carbon steel.

Corner reinforcement

In the design of a strip foundation, the weakest point is the corners and the junction of the walls. 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.

Types and brands

When choosing a suitable material for the frame, consider certain indicators:

• type of steel rods;
• metal class;
• grade of steel alloy.

In the construction industry, three types of reinforcement are used:

• hot rolled (A);
• cold-worked (VR);
• ropeway (K).

Hot-rolled metal, manufactured in accordance with GOST, is used for the foundation. Wire (cold-deformed reinforcement) is used much less frequently; before its use, preliminary calculations are carried out for its limiting state. Rope elements have a high strength index, and reinforcing the foundation with such reinforcement is considered economically infeasible.

It is worth mentioning the class of reinforcing bars. If you are deciding what kind of reinforcement is needed for the foundation of a house, buy metal whose class starts from A 400 (A III).

Class A 240 or A 300 reinforcement is not used for pouring the foundation.

Based on their location, the following types of foundation reinforcement are distinguished:

• working. The rods are located taking into account the type of foundation. For a strip foundation, the reinforcement is represented by rods located longitudinally, the laying of which is carried out in one or two rows in the upper and lower parts of the structure. For a slab foundation, reinforcement is performed with a mesh or rods placed in two directions. The number of rows is determined by the thickness of the base. When it does not exceed fifteen centimeters, the rods are laid in a slab foundation in one row; in other cases, at least two such rows are arranged;

• transverse. It is represented by horizontal clamps that ensure the joint operation of the working elements. If you don’t know what kind of reinforcement is used for a strip foundation, pay attention to this option. When constructing a monolithic slab, this type is not used;
• vertical. Clamps located between a pair of rows ensure their operation. If the thickness of the base is less than one hundred and fifty millimeters, such a layout of reinforcement is not used.

It is recommended to draw up reinforcement drawings and detailed diagrams in advance in order to determine the minimum diameter of the rods, their number and location in the foundation before installation.

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 segments are structural 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.

What is reinforcement and why is it needed?

Tensile and compressive forces act on the foundation of an already built house. Under the load of the house itself, the lower part is subject to tension, and the upper part is subject to compression. In winter, when the soil freezes, it also puts pressure on the foundation and lifts it, and in the warm season on the site, the soil moves and presses both from below and from the side. These forces deform and destroy the foundation, which changes the geometry of the upper structures. This, in turn, causes cracks in the walls, gaps between window frames and the walls of the house, and distortions in the floor level. This results in a draft when windows and doors are closed, swelling of the floor covering, dampening of the plaster, and in the worst case, damage to electrical wires and water supply and sewerage pipes inside the house.

Foundation reinforcement is long round rods made of steel or fiberglass that strengthen concrete structures, including the foundation. The reinforcement tied into the frame takes on part of the vertical load and helps to evenly distribute the pressure of the house on the soil. A steel reinforcement bar can withstand tens of times more tension than unreinforced concrete. For comparison, the tensile strength of unreinforced concrete is 5.5 MPa, and that of reinforced concrete is in the range of 215-400 MPa. Reinforced concrete resists compression 8-10 times better than unreinforced concrete. In mass housing construction, it is impossible to build anything from concrete without strengthening it properly, since this process is regulated by SNiPs (Building Norms and Rules). Ignoring them, firstly, is dangerous for the lives of people in the building, and secondly, it means criminal liability for the head of a construction company. In private housing construction, to save budget and time, they try to build a foundation using only the lower part of the frame. This is explained by the fact that the tensile load under the weight of the house falls on the “bottom” of the foundation. But when the soil freezes and increases in volume, the lower part only contracts, and the upper, unsupported part stretches. In this case, deformation and destruction of the foundation, and then the house, cannot be avoided.

To avoid such consequences, the foundation must be strengthened with a frame made of reinforcement. Today in Russia, 90% of concrete structures are built on steel or metal reinforcement, SNiPs have been developed and used for this type of reinforcement, it has been produced in Russia by metallurgical plants since Soviet times and is considered as a traditional and proven material for strengthening concrete.

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.

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

Features of steel reinforcement

Steel is a strong and relatively ductile iron-based alloy that allows you to reliably strengthen the foundation , protecting it from cracking. A correctly calculated frame allows you to compensate for mechanical overloads that occur during seasonal soil deformations (they are associated with the movement of groundwater, climate change and other factors). For the construction of the foundation, 12 mm corrugated reinforcement is most often used.

The main problem of inexpensive steel grades is their tendency to corrosion, and most existing methods to increase corrosion resistance lead to a sharp increase in the cost of the material. Concrete does not have very high hygroscopicity, but during long-term use, water penetrates into its structure, which is why the steel reinforcement begins to collapse. Traditionally, this problem is solved by maintaining a certain gap between the reinforcement and the edge of the foundation (6 cm or more).

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
• Frame sections are prepared close to the pit. They are transferred in parts and installed in their designated place, linking them into a single whole. It’s more convenient to work this way, except that it is very inconvenient and difficult to move connected structures made of reinforcement.

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.

Installation rules

If all the selection points have been determined, then you can proceed to the installation of reinforcement in the foundation. First of all, you need to decide how the steel bars will be connected to each other. There are three options: welding, wire and overlap. Everything is clear with the first ones, but the third position requires clarification.

Essentially, this is laying reinforcement into the foundation in the form of a grid without fastening. The rods are stacked on top of each other alternately, which creates a fairly strong cage. This connection method can only be used when constructing a slab foundation, when the reinforcing frame is a single-row mesh. Let's face it, it's not the most reliable frame for a foundation. It is better to use wire or welding for connection.

As for the installation of reinforcement for the foundation itself, it all depends on the type of foundation structure.

1. When pouring the slab model, the rods are assembled directly in place. They are laid out on the surface of the prepared pillow, where they are either tied or welded together.
2. If the strip foundation is buried, then it is better to make a reinforcing frame outside the trench, and then simply lower the finished structure.
3. If it is shallow, then assembly can be carried out at the installation site. Although it will be easier to assemble it on the side.
4. The frame is installed ready-made in the well for a columnar foundation. This is a structure of three or four rods tied with horizontal clamps.

Please note that the frame must be laid in the body of the foundation. Therefore, stands are installed under it. For example, solid brick, metal profiles, stones and other durable objects and fixtures. You cannot use rusty fittings or other metal products for this.

There is one operation during the installation process that involves installing reinforcement in the corners of the foundation structure. Here the rods can be connected to each other, as in all other cases, or the rods can be bent at 90°, which will ensure the integrity of the frame. Because the fewer joints, the more reliable the system. Therefore, it is important to know how to bend foundation reinforcement.

Features of plastic fittings

Plastic began to be used for this purpose relatively recently, so it has not yet become as widespread as steel. Builders are wary of using it because of its increased ductility and fluidity, which in the long term can lead to a loss of foundation strength . However, fiberglass is still considered a promising material for this purpose.

The main advantages of plastic and fiberglass:

• high moisture resistance and durability;
• chemical inertness towards many chemicals;
• Low weight and the ability to wind into coils greatly simplify transportation.

Nowadays plastic reinforcement is used mainly for reinforcing lightly loaded foundations . And in the case of solid structures and large brick houses, builders still prefer to reinforce the foundation with a frame made of steel rods.

Classification of reinforcement by strength

Depending on the requirements for reliability and load capacity, steel rods are classified on a scale from C1 to C8 . The higher the number, the higher the strength of the material. This usually means a higher quality and more expensive grade of steel using expensive alloying methods. The strength class of the reinforcement is necessarily taken into account when designing a building.

Existing standards make it possible to calculate the required class of reinforcement for the selected type of foundation without any problems. The calculations take into account the total mass of the building, the characteristics of the soil and landscape, the presence of groundwater and seasonal changes in its level. Different types of foundations have their own characteristics in terms of the choice of material for reinforcement.

Comparison of valve characteristics

If we compare the technical parameters, it should be noted that composite reinforcement is superior to steel in the following components:

• resistance to corrosion and deformation;
• does not conduct electricity;
• has a longer service life.

At the same time, it is worth noting that in Russia, in the construction of various objects, according to tradition, steel reinforcement is mainly used.

How to choose the right reinforcement for building your home

The only correct way to choose reinforcement for construction is to entrust the design to professionals. They will create a complete building design with a detailed description of the foundation and all the necessary calculations.

The project allows you to quickly purchase all the necessary materials and begin construction.

Independent calculation of the required materials, unfortunately, cannot guarantee the correct selection of the type and diameter of the reinforcement, as well as the determination of its required quantity.

If you have no experience, then it is better to refuse independent design. Contact the Render House company - we are engaged in the construction of turnkey houses according to standard and individual projects, we carry out all calculations in advance and draw up an estimate.

Connection methods

Welding can lead to subsequent corrosion of the metal.
During the installation of reinforcing structural units, horizontal and vertical rows should be connected to each other by two well-known methods: bonding and welding.

The reinforcement should be tied with metal wire.

There are devices to simplify the work: pliers, a gun, a hook.

The knitting process itself takes a long time, even with the use of a tool.

If you have never encountered this type of work before, then it is better to entrust it to professionals; the strength of the foundation will depend on the quality of good knitting.

Welding is not the best method of connecting reinforcement, since the protective layer on the rod is damaged in the molten zones and rust can form; moreover, the work requires specialized welding skills. An important disadvantage can also be considered the fact that the weld is prone to destruction during the filling of the formwork with concrete. For more information about knitting metal rods for the foundation, see this video:

The joints of the reinforcement by welding have high strength, but the foundation itself is not able to cope with the load from the action of the soil, which will lead to the formation of pressure in the tape and its destruction.

It is best to use reinforcement tying when constructing the foundation frame. Before carrying out this process, you should purchase a special wire.

Tools for tying reinforcement

When connecting transverse and longitudinal elements, the wire is tightened with the following devices:

• pliers;
• special hooks;
• screwdriver.

Special guns are also used, which significantly speed up the work process.