The specifics of individual construction are determined by the influence of a huge number of factors, from the availability of roads and electricity to a pond or hill in the neighborhood. All these factors influence the choice of foundation and the method of its installation. What formwork should be used, what concrete mixtures to use, how to reinforce the foundation - these are the first questions at the start of building a reliable and durable house.
Reinforcement is an essential element of a reliable foundation Source newspasky.ru
To avoid problems
When laying the foundation of a private house, it is difficult to foresee what loads it will be subjected to in the future. Perhaps the owner will need to install a massive lathe or arrange a dance hall in the house, a water supply will break, or a powerful structure will be erected in the neighborhood, causing a rise in groundwater or a new underground flow. The loads will change, the foundation, not designed for dramatic changes in loads, will burst and sag, followed by the collapse of the building.
If the developer considers the installation of a strip foundation to be the most appropriate option, then reinforcement is necessary to guarantee its reliability. And you should know at least approximately how the foundation should be reinforced, the calculation of reinforcement, laying and tying should be done, even if other people will build your house.
Source ar.aviarydecor.com
Calculation with reserve
Foundation reinforcement is the installation of a metal frame in its mass, designed to create a single, indestructible structure from concrete.
Making an accurate engineering calculation of the foundation of a small individual building is difficult and expensive; it requires geological surveys, comparisons with promising development projects for the area as a whole, soil and groundwater characteristics, and solving a long list of issues.
Based on this, private developers are guided by two basic rules for constructing foundations:
The base of the foundation should lie below the soil freezing depth according to the maximum indicators of the development region.
Reinforcement of the strip foundation is mandatory and is carried out in accordance with the general recommendations of the standards with a margin of safety.
Basic rules for performing reinforcement Source sevparitet.ru
Arrangement of the foundation frame
The monolithic foundation is reinforced in the form of a single frame throughout its entire height. The distance between individual reinforcing bars should ensure free passage of the concrete mixture between them. In other words, if crushed stone of fraction 20-40 is used in concrete, then the gap between the frame rods should be at least 4 cm.
The use of rubble stone is much cheaper than conventional concrete mixture, but to create a unified structure, it is necessary to bandage the foundation along the entire perimeter. The reinforcement cage is incompatible with large stones; in such cases, the technological solution is to install reinforced belts at the bottom and top of the foundation.
Even the largest fractions of concrete should pass freely through the frame Source earny.ru
On our website you can find contacts of construction companies that offer foundation design and repair services. You can communicate directly with representatives by visiting the “Low-Rise Country” exhibition of houses.
Stages of work on arranging a reinforcement frame
The base for the foundation is made of a layer of sand of at least 10 cm, the sand is covered with a layer of crushed stone of fraction 2-5, then the sand-crushed stone base is compacted, and only then should the laying and tying of the reinforcement frame begin.
Reinforcing rods, cut along the length of the foundation strip on one side, are laid out at a distance of 20-30 cm from each other along the bottom of the foundation. At the corners they are screwed with soft knitting wire to the vertical rods, as well as to each other when overlapping.
To create the vertical corner supports of the frame, the horizontal bottom rods of the frame are bent at an angle of 90 degrees. Extended by overlapping joints and fastening with wire.
To facilitate the work on reinforcing the corners of the foundation, the installation of anchors is allowed; the work is similar to the installation of grillages or armored belts. At all corners of the foundation, 4 metal rods are driven into the ground, covered with bitumen resin on the bottom for waterproofing. They act as anchors for fastening the frame. In cross-section, the driven anchor pins should form a square with sides parallel to the foundation strip.
Anchors driven into the ground on which the frame is attached Source sjthemes.com
Vertical reinforcing rods equal to the height of the foundation are screwed or tacked to the anchors for fixation. All vertical rods are tied or welded together along the perimeter, forming a pillar structure.
In order to avoid contact between the metal and the sand-crushed stone base, brick halves are placed under it along the entire length of the rod at intervals of 1 m.
Rods are cut for transverse laying of reinforcement. Their length should be 10 cm less than the width of the monolithic tape, that is, the crossbars should be completely covered with poured concrete with a distance of 5 cm from the outer wall of the foundation.
The pitch of the foundation reinforcement with transverse rods is 50 cm along the entire length of the longitudinal reinforcement.
All reinforcement connections are twisted with binding wire.
Depending on the length of the foundation side, the distance between the vertical rods ranges from 30 to 80 cm.
Only two longitudinal rows may be enough: top and bottom.
Each horizontal row is parallel to the bottom one and similar to it.
It is quite possible to assemble the frame close to the foundation, and then simply lower it into a trench or formwork.
The frame does not have to be assembled inside a hole prepared for the foundation - installation can be done outside, and then the entire structure can be lowered down Source newspasky.ru
Of course, this method is only possible if there is a flat area for assembly, otherwise it will be difficult to achieve accurate work.
Fastening with binding wire
Two common methods of connecting reinforcement in a frame structure are welding and knitting, and knitting is considered more reliable. When filling a foundation with a concrete mixture, welded joints often cannot withstand the weight of the concrete.
The knitting wire, cut into 40-50 cm lengths, is doubled, inserted from below at the intersection of the rods, and twisted with pliers.
The version with twisting using a hook is simpler and faster: the wire is wound freely with a gap around the junction of the reinforcement, its ends are twisted by hand one or two turns, a hook is inserted into the gap between the reinforcement and the wire, and by turning it the wire is tightened.
Hooks are sold in hardware stores, but bending a cleaned welding electrode is sufficient for this purpose.
For large volumes of fastening reinforcement with wire, there is a special knitting gun. It is very effective in places that are easily accessible, but where access is difficult, and these are usually corner joints, a simple hook is again more useful.
Using a knitting gun significantly speeds up the process of tying foundation reinforcement Source dvamolotka.ru
Plastic clamps are often used instead of wire. This significantly speeds up and facilitates the work process, but at low temperatures such fasteners lose their elasticity and strength.
How fittings work
In construction, reinforcement is usually called rods of various diameters and shapes to counteract compressive and tensile loads, internal and external. The division into types, classes and groups depends on the properties and characteristics specified for the reinforcement.
The division into groups of reinforcement depends on the characteristics:
material of manufacture;
profile shape;
method of use;
installation technique;
appointment.
In the construction of foundations, the spatial arrangement of the reinforcement is important. Longitudinally oriented reinforcement elements work to minimize the formation of cracks by redistributing the load on the surface of longitudinally oriented structures.
Transverse reinforcement connects concrete in the compression zone with longitudinal reinforcement, redistributing and reducing loads.
Why do you need reinforcement: just a concrete beam on top, and reinforced underneath Source rmnt.mirtesen.ru
When studying the markings of reinforcing steel, the designations C and K after the numerical value of the yield strength are of practical importance for a private developer
The C index indicates the possibility of welding the reinforcement; the absence of this index means that welding is undesirable due to the fragility of the joint. The designation K indicates the increased resistance of the reinforcement to corrosion.
Diagram and calculation of metal
The need for metal and estimated costs can be easily calculated if you draw a reinforcement diagram for a strip foundation with all longitudinal, transverse and vertical rods. The reinforcement is sold by weight, so when calculating, you should take into account the diameter of the reinforcement; it is possible to complete the frame with metal of different diameters and types of corrugation.
To reinforce foundations, reinforcement of various brands and diameters is used; the most common are reinforcing bars with a diameter of 10-14 mm, smooth and ribbed. For laying transverse connections, it is permissible to use round smooth reinforcement of a smaller diameter.
The most common types of reinforcement for foundations are made of steel grades M35GS and M25GS, rod lengths up to 12 m, diameter from 10 to 40 mm.
Correct selection of reinforcement cross-section is the key to foundation strength Source armsetka16.ru
Alternative fittings
The difficulty of transporting steel reinforcement due to its length and many problems in working with metal forced developers to pay attention to alternative solutions.
One of them could well be fiberglass reinforcement.
It has many advantages, but to appreciate them, it is worth remembering the original purpose of foundation reinforcement. In essence, the reinforcement cage should protect the concrete foundation from stretching. The modulus of elasticity of metal rods is significantly lower than that of similar plastic rods. This means that the low elasticity threshold of plastic rods will lead to deformation, and therefore destruction of the foundation, much faster than a metal one. And the meaning of replacing metal with a plastic composite disappears.
The second very unpleasant drawback concerns specifically individual developers who do not have special conditions for aligning plastic reinforcement rolled into coils.
The disadvantages of modern plastic reinforcement relate only to the undesirability of using it in monolithic strip foundations. There are many areas of application where this type of reinforcement will perform better than steel reinforcement, but not in foundations.
Conclusion
Reinforcement prevents concrete from collapsing when shear or torsion stresses occur in it, therefore reinforcement is a mandatory stage in the construction of a concrete foundation. The rules for reinforcement are quite simple, but require strict adherence - in this case, your house will receive a reliable foundation.
Under the thesis " strip foundation reinforcement» understand the construction process, the main purpose and task of which is to enhance the durability of the structure, as well as significantly increase its service life.
In order to achieve the greatest efficiency, it is necessary to make a careful, thoughtful calculation of the amount of required reinforcement, as well as select the best samples of previously declared construction reinforcement.
Why is it necessary to reinforce a strip foundation?
Reinforcement of the strip foundation helps the structure to withstand very heavy loads. Moreover, with its help, you can be guaranteed to obtain a strong position even for houses with a complex configuration.
A strip foundation is a monolithic structure made of a metal frame and concrete. Moreover, the first element fulfills the main load-bearing capacity of the entire building. Therefore, competent implementation of the reinforcement frame and knitting of reinforcement under the strip foundation play a key role in the construction process. Let's look at these components in more detail.
Reinforcement cage for your strip foundation
The base frame is a regular rectangle or square. The commonly used reinforcement principle is quite simple:
- Brick rows are placed at the bottom of the trench. Their height should be on average at least 5 cm. This stage is important in order to create the most important gap between the frame, as well as the lower part of the base.
- Rods are cut to the required length to perform vertical reinforcement.
- Metal rods are laid out longitudinally on the existing rows of bricks. In this case, it is better that they are of maximum length;
- Using wire, the working rods are connected by jumpers at the required distance - 30 cm from each other. Their length should be 10 cm less than the thickness of the base (and a 5 cm indentation on each side must be maintained);
- Rods are installed vertically in each cell in the corners. Moreover, their length should be 10 cm less than the existing height of the base;
- Vertical rods are connected using longitudinal rods and are also fixed using jumpers.
Particular attention should be paid to such a stage as corner reinforcement. After all, it is these elements bear the greatest load.
Reinforcement frame
Rules for reinforcing corners
Rules for corner reinforcement:
- The rods at the corners should be bent in such a way that their ends can be buried in the wall with a minimum overflow of 40 cm (this is relevant when the diameter of the rods is 10 mm);
- The places where they overlap each other must be strengthened using both transverse and vertical rods;
- In the case when the length of the rod is not sufficient to bend it onto the wall, additional strengthening of such places is required with the help of L-shaped rods;
- The clamps in the corners must be connected to the next step - 2 times less.
If these simple rules and requirements are followed, the corners will be able to remain in their original form longer.
Reinforcement of joints with amplifiers
Knitting reinforcement for a strip foundation
For the purpose of tying reinforcement under a strip foundation, knitting wire with the following diameter is mainly used - from 0.8 to 1.2 mm. It is cut into pieces ranging in size from 10 to 20 cm. The minimum number of connections should be half of the available number of intersections.
However, in the case of private housing construction, the reinforcement frame is often welded instead of using wire. This is done to speed up the process. In this case, there are several ways to actually tie reinforcement:
- Using pliers. For this purpose, the wire is folded in half. And its ends are twisted and fixed using pliers with blunt teeth.
- In addition, a screwdriver with a special attachment, as well as a hook, can be used. In this case, the pieces of wire are also folded in half and looped onto the hook. At the intersection of the reinforcement being processed, the ends are wrapped and then laid back on the hook. After this, turning the hook, they rotate. You can also use a screwdriver for these purposes.
Foundation reinforcement
There are 2 main types of reinforcement used in the construction process:
1. Steel, which, in turn, is divided into 2 subspecies:
- Rod;
- Wire.
2. Composite. Today, this option is used quite rarely due to important disadvantages.
To reinforce a strip foundation, a rod type of reinforcement is used as the main working material, as well as smooth reinforcement as an additional one.
The main property of working reinforcement is its ability to quickly and perfectly adhere to concrete. Such products are produced with a periodic profile and are divided into classes regarding strength indicators.
Steel reinforcement
Basic requirements for strip foundations and reinforcement
During the reinforcement process, the type is established, as well as the main controlled values regarding the quality of the reinforcement. According to existing standards, hot-rolled construction reinforcement with a periodic type profile, as well as heat-treated or mechanically strengthened products, are allowed for use.
The choice of reinforcement class takes place with mandatory consideration of the so-called guaranteed value of the yield strength at the maximum possible loads. In addition to important tensile characteristics, the standards are also subject to ductility, resistance to corrosion processes, weldability, as well as resistance to low temperatures, permissible elongation before the onset of negative destructive processes, as well as relaxation resistance.
Working rods must have a minimum diameter of 12 mm.
The number of longitudinal rods used must be at least 4 (and at best 6). Moreover, the longitudinal rods are connected into a spatial frame using wire knitting or welding.
The pitch in the case of transverse reinforcement should be from 20 to 60 cm, and the diameter of the reinforcement, in turn, should be from 6 to 8 mm.
Regarding places with an increased level of possible settlement, as well as at T-shaped intersections, reinforcement reinforcement should be used using reinforcing haunches and legs (and their diameter should be equal to that used for longitudinal bars).
The average thickness of the strip base should be approximately 30 cm.
What to look for when choosing fittings
Please note the following possible symbols:
- Index C confidently indicates that the reinforcing bars are of the weldable type;
- The K index indicates to us that the reinforcement will be resistant to the well-known processes of corrosion cracking, which can become an unpleasant consequence of pressure on the foundation.
If there are no such designations on the packaging, it would be prudent not to purchase such a product.
Calculation of volume and diameter of reinforcement
Calculation of reinforcement for a strip foundation is required in order to give the necessary strength to the foundation by thinking through the amount of reinforcement required, as well as its diameter. The thickness of the reinforcement for a strip foundation may vary.
For example, in the case of constructing a foundation for a private house, products with a diameter of 10 to 20 mm are used as longitudinal ones. As a transverse option, as well as a vertical one, ribbed or smooth reinforcement with a round cross-section, as well as with a diameter of 6 and up to 12 mm, is often found. Thus, it is clear that the diameter of the reinforcement for a strip foundation can also be different. It depends on the design, as well as the weight of the foundation.
What is the optimal distance between reinforcement in a strip foundation?
In accordance with the standards in the construction industry, the minimum distance (between the rods of the reinforcement being erected) is determined depending on the following important factors:
- Diameter of rods;
- The location of the existing structure relative to the direction of concreting;
- Concrete aggregate size;
- Selected most optimal installation method;
- Type of compactor for concrete.
In addition, it is believed that the distance between the placed longitudinal reinforcement bars should not be less than 25 cm and more than 40 cm. As for the step in the case of transverse reinforcement, the most optimal value is considered to be no more than 30 cm. In this case, the step should be half the height working section.
It is clear that the size of the reinforcement for a strip foundation may vary in each specific case. There is no universal recipe for how to calculate reinforcement for a strip foundation. However, you can follow the above recommendations in order to most optimally and competently determine how much reinforcement is needed for a strip foundation.
Reinforcement laying step
For the purpose of reinforcing slab foundations, it can vary within the following limits - 20-30 cm. At the same time, it should be borne in mind that in the case of a heavy house and difficult soil, the step should be smaller.
In general, the amount of rod required is determined based on operating conditions and dimensions. At the same time, there are 2 reinforcement belts - lower and upper.
The main load falls on the horizontal guides. In the case of ribbons with a width of 30-40 cm, there are enough of them, 2 at the bottom and at the top. In the case of a larger width, use 3 or 4 rods in one row at once.
In most cases, you can find 2 horizontal rows in strip foundations:
- Located 5 cm below the top edge;
- It is located 5 cm above the lower one.
At the same time, they are connected using transverse dressings into a single structure. The step in this case will be approximately from 30 to 50 cm. It all depends on the soil and the mass of the building under construction.
In the case of columnar bases, the arrangement of the rods will depend on the existing diameter of the pillars. The most important condition is that the distance from the rods to the edge of the post should be at least 5 cm. Horizontal dressings, for their part, should also be spaced approximately every 50 cm.
Selecting a strip foundation reinforcement scheme
It is necessary to clearly determine what the possible minimum content of reinforcement should be present in the strip foundation. In addition, it is necessary to correctly determine the number of required longitudinal rods, as well as their minimum diameter.
The reinforcement scheme for a strip foundation can be completely different. The main rule is to choose a scheme that would be the simplest in each case and at the same time reliable. In some cases, a scheme is chosen that involves reinforcing only the upper or lower part of the foundation. However, this is not an entirely correct and competent decision.
A simple scheme for tying reinforcement (introductory diagram)
After all, on the one hand, the weight of the entire house will put a load on the tape. In this case, it is necessary to take into account that the important resistance of concrete to compression will be many times greater than its possible resistance to the so-called tension, which can manifest itself at the bottom of the tape. On the other hand, the forces of so-called frost heaving of the soil can cause tensile stress in the upper part of the foundation. For this obvious reason it is more prudent to reinforce both the upper and lower parts.
At the same time, it is believed that it is not at all necessary to reinforce the middle part. After all, it is practically not subject to tensile stresses. However, in the case when the calculated amount of reinforcement is such that it cannot fit in exactly 2 rows (lower and upper), it is quite possible to arrange additional tiers in the middle of the frame.
Another recommendation: it is more prudent to choose a strip foundation reinforcement scheme that will include simple geometric shapes (rectangle, square). In this case, it will be easiest to make the frame axes correct. The strength of the foundation will also increase.
Fig. 1 possible version of the strip foundation reinforcement scheme: 1 – longitudinal type reinforcement; 2 - vertical reinforcement of vertical type (or clamp); 3 – formwork; 4 - transverse type reinforcement (or clamp).
Great care should be taken in choosing the correct installation schemes for reinforcement at the junction of walls, as well as at corners. Below you can consider reinforcement diagrams that will help you correctly connect the reinforcement cage in these places.
Scheme of reinforcement of strip foundation junctions
Typical mistakes in reinforcing a strip foundation with your own hands
Regarding the corners. The main problem is usually laying the corner bars crosswise. In this case, various cracks often appear.
Regarding waterproofing material. Often, when carrying out work, they forget about the use of waterproofing. In this case, water can wash away the cement over time and make the existing concrete less strong and stable. In addition, the appearance of so-called shrinkage cracks can be an unpleasant consequence. Therefore, it is recommended to carefully attach the waterproofing layer to the formwork in order to eliminate the formation of unwanted folds, as well as depressions in the foundation.
Regarding pouring concrete. In many cases, the concrete mixture does not reach right to the edges when filling a strip foundation. Topping up is often done only after a couple of days. Moreover, this technology for reinforcing a strip foundation no longer involves a monolithic structure. In this case, the design is similar to two ordinary beams with so-called single-layer reinforcement, joined together using layers of existing concrete mixture, as well as transverse reinforcement. In general, concrete pouring should be continuous (with a maximum interval of two hours).
Regarding ventilation. Typical errors also occur during the installation and operation of cold underground ventilation products. They are performed using pipes with a diameter of 10 cm. The minimum area required for ventilation should be approximately 0.05 m2 (20 by 25 cm). Moreover, these vents should not be closed in winter. Indeed, in this case, the ventilation process will be disrupted, and the structures will begin to rot.
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.
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When carrying out construction activities for the construction of residential buildings and industrial facilities, various types of foundations are used to ensure the stability of the structure being constructed. Foundations made along the perimeter of the building are widely used. To strengthen this structure, tape reinforcement is performed.
The need to reinforce a strip foundation is due to the properties of concrete, which maintains its integrity under the influence of compressive loads, but at the same time is prone to cracks under the influence of bending moments and tension. This serious drawback of the concrete monolith can be compensated for by the reinforcement of a monolithic strip foundation, which increases the stability and service life of the buildings being built.
The foundation of the building absorbs significant loads associated with the reaction of the soil, the mass of the structure and other factors. The reinforcement frame is exposed to increased stress concentrations, ensuring the integrity of the concrete mass. Errors in foundation reinforcement associated with the destruction of the zero level can cause fatal consequences.
The foundation is the basis of a building for any purpose; it is the most important part of any building.
That is why we will consider in detail how to properly reinforce a strip foundation, and dwell on the criteria for choosing reinforcement and the technology for reinforcing a strip foundation.
Settlement stage
At the design stage, it is important to skillfully calculate what kind of reinforcement is needed for a strip foundation. This will create a reliable foundation that will ensure the strength characteristics of the building under construction with a long service life. When performing calculations at the preparatory stage of work, many factors should be analyzed:
- soil characteristics in the conditions of a specific construction site;
- acting loads, which the reinforcement frame perceives;
- the mass of the building due to the design features and materials used;
- climatic conditions in the construction area;
- soil reaction associated with the proximity of groundwater and soil freezing at negative temperatures.
The rules for reinforcing strip foundations provide a special approach to the selection of base material
Based on the results of design work, the diameter of the reinforcement for the strip foundation is determined and a decision is made on the degree of penetration of the foundation into the ground:
- To a depth limited to 0.5 m for hard soils that are not prone to heaving.
- To an immersion depth increased below the soil freezing level for problem soils.
The options don't end there. After all, construction science does not stand still; new supporting structures with increased strength are being developed. A new base option has been introduced and tested in operation, when a monolithic reinforced slab is poured onto a pre-made reinforced strip frame. Which base design is better is determined at the design stage, taking into account the specific conditions of the actual terrain. Depending on the characteristics of the base chosen according to the project, designers decide whether to reinforce the tape or reinforce the foundation slab, as well as which reinforcement is best to use for the foundation.
Reinforcement selection criteria
Proper reinforcement of the strip foundation determines the strength characteristics of the supporting structure. When deciding whether to reinforce a slab located on a strip base, or to reinforce a standard base, focus on the features.
Reinforcement of a monolithic strip foundation requires compliance with certain rules
Reinforce the base with steel rods having the following characteristic features:
- the presence of the index “C” in the designation of steel rods indicates the possibility of using electric welding equipment to combine elements with a common frame;
- the presence of a capital letter “K” in the abbreviation confirms the resistance of the rods to corrosion that occurs when concrete is saturated with moisture;
- product class designation A2 and A3, which allows the use of steel rods fixed in a common frame with wire, while maintaining the strength of each of the connected elements. The use of electric welding to fix such rods is not permitted.
Foundation reinforcement made from steel rods with a cross section of 10–12 mm has the required operational strength. The optimal diameter of the reinforcement for a strip foundation is determined according to calculations that take into account specific operating conditions, soil characteristics and the values of the operating loads.
About the need for strengthening
To what extent is it necessary to strengthen a concrete mass with steel wire? After all, concrete has fairly high strength characteristics. Indeed, concrete has increased resistance to compressive loads, but requires reinforcement against the destructive effects of tensile forces.
The greatest likelihood of stretching is on the surface of the base, this is where the reinforcement should be placed
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 connected 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.