Reinforcement of a strip foundation is a mandatory procedure, without which it is impossible to guarantee the high-quality construction of a future building. The work of reinforcing the foundation is very important, but it is not very difficult to perform, and if you understand all the subtleties and nuances, then you can do it without any problems with your own hands.
Materials and tools
To reinforce a strip foundation, both steel rods and fiberglass reinforcement are used; we will focus on steel rods (read how to choose steel reinforcement here), because fiberglass is, firstly, an expensive pleasure, and secondly, it is very rarely used for country house construction in a number of reasons.
We will prepare the following tools:
- A container for mixing concrete or a concrete mixer;
- Construction mixer;
- Bulgarian;
- Shovel;
- Pliers;
- Gloves.
Preparatory work
First: It is necessary to calculate and purchase fittings and binding wire. There is nothing complicated in calculating the required amount of reinforcement. Horizontal guides, for which 12mm reinforcement is used, are usually laid in increments of 30 - 60 cm. Transverse and vertical sections are formed with 8mm reinforcement, in increments of 40 - 70cm. Knowing these data, it is very easy to estimate how many linear meters of reinforcement you need to buy specifically for your purposes, plus take a small reserve and purchase 10% more than you need.
Important: For a ribbon with a height of less than 90 cm, a two-row frame is usually used; for a height of more than 90 cm, a three or more tier frame is knitted.
As for the knitting wire, everything is even simpler; it takes about 25 - 30 cm for each connection.
Second: After the material is delivered to the construction site, it must be carefully inspected and cleaned of dirt and rust. Many people neglect this operation, but it must be remembered that extraneous “inclusions” can, although insignificantly, still worsen the performance characteristics of concrete.
Step-by-step instructions for reinforcing a strip foundation
Step 1: We form a concrete base. To do this, pour sand at the bottom of the trench, 20 -30 cm thick, tamp it down and fill it with concrete with a layer of 5 - 10 cm. This way we will protect the lower reinforcement from corrosion.
Advice: in order to save money, you can not “bother” with filling the “sole”, but waterproof the trench with ordinary dense polyethylene film.
Step 2: We install the formwork. We will not stop at this stage because we have an article on the topic “how to install formwork for a foundation,” where everything is described in detail.
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Step 3: We begin to knit the reinforcement. This work can be done both directly in the trench and next to it. It is more convenient, of course, to tie the individual sections close to the installation site, and then install them in the proper place. The general scheme will be as follows.
Important: It is highly not recommended to use welding to connect reinforcement to each other; with this method of fastening, the joints will begin to rust very quickly.
The assembly of the structure should begin with the lower transverse rods (8mm), they are laid out in increments of no more than 80cm from each other. Then we lay working reinforcement (12mm) longitudinally on them; the distance between the longitudinal rods should not exceed 40 cm, but if it is more than 40 cm, then we add another rod to the structure. The joints of the transverse and longitudinal rods are secured with knitting wire.
So, we previously formed the lower level of the frame, then we should fix the vertical jumpers (8mm). This is done like this - in the places where the longitudinal and transverse reinforcement rods come into contact, we install another rod vertically and connect it with wire to the main structure, thus installing all the necessary vertical elements.
Important: Be careful and when installing, make sure that the vertical reinforcement is fixed in relation to the longitudinal reinforcement exactly at 90 degrees.
The next stage of assembling the frame will be the installation of the upper transverse and longitudinal rods. All actions are the same, to the vertically fixed reinforcement using a knitting wire with an overlap at the edges of at least 20 cm, we first attach the transverse and then the longitudinal elements of the reinforcement.
Using the method described above, we assemble the required number of sections, install them in a trench if the assembly was not carried out in it, and using spacers we rigidly fix the frame in relation to the formwork, leaving a gap between them of 3-5 cm.
The main part of the work has been completed, but the most important stage remains, the reinforcement of the strip foundation in the corners.
Step 4: Fastening reinforcement in the corners. Here you need to be extremely careful and do the work as efficiently as possible, because the corners of the foundation take on the greatest concentrated stress. To reinforce the foundation at the corners, U- or L-shaped reinforcement techniques are used. See below for how to do this job correctly.
For right angles:
For angles greater than 160 degrees:
Well, the crosshairs are reinforced like this:
The work is all finished, good luck with your construction.
How to properly reinforce a strip foundation with your own hands?
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, be guided by the marking features of the reinforcing bars.
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
Strip foundation reinforcement technology
The reinforcement of a strip foundation is a key factor in its reliability and durability (you can also read separately about the reinforcement of a pile-grillage foundation). According to the provisions of SNiP No. 2.03.01-84, unreinforced foundations are not approved for use in residential construction.
Strip foundation reinforcement
From this article you will learn how to properly reinforce a strip foundation. We will look at methods for calculating reinforcement and study reinforcement diagrams, as well as get acquainted with the technology of doing the work ourselves.
1 How to reinforce the foundation strip?
The foundation structure consists of two components - a concrete body and a reinforcement frame embedded inside it. Concrete, as a material, is highly resistant to deformation loads in compression, but it is weak in tension and bending, under the influence of which the tape can collapse. These loads are absorbed by the reinforcement frame, which resists deformations in the zone of increased external influence.
Reinforcement of a strip foundation is carried out using a spatial frame consisting of longitudinal reinforcement belts connected to each other by transverse and vertical jumpers. The number of longitudinal belts is selected based on the height of the tape:
- shallow foundations are reinforced with a frame in two longitudinal belts - upper and lower;
- recessed foundations, the height of which exceeds 120 cm, are reinforced with a frame with a middle reinforcement belt.
The longitudinal belt of the frame is made of corrugated rods fittings with a diameter of 12-16 mm, class A3 rods are used. Jumpers are made from sections of rods of similar diameter or from smooth profile reinforcement bent into rectangular clamps with a diameter of 8-10 mm.
Reinforced frame with two longitudinal belts
Assembly of the reinforced frame is carried out using tying wire or welding. The first method does not require the use of special equipment, however, it is quite labor-intensive to implement, while welding is a faster way to install the frame. For knitting, steel wire with a diameter of 1-2 mm is used.
The frame configuration is determined by the provisions of SNiP No. 2.03.01-84 “Manual for designing foundations for buildings and structures.” The following distances must be maintained:
- the pitch between the constituent elements of the longitudinal belt is no more than 10 cm (determines the number of rods in the belt);
- the step between the longitudinal belts in the vertical plane is no more than 50 cm;
- the step between the transverse and vertical connecting jumpers is no more than 30 cm;
Transverse diagram of reinforced frame
When installing the frame, it is necessary to provide a protective layer of concrete - a distance of 5 centimeters between the contours of the frame and the walls of the concrete body. The size of the reinforcement skeleton is selected based on the dimensions of the foundation so that the above rule is observed. Laying of reinforcement on the bottom of the formwork is done using plastic mushroom stands, which raise the rods to the required height.
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1.1 Calculation of reinforcement for strip foundations
The consumption of reinforcement must be determined at the foundation design stage in order to subsequently know exactly the amount of material purchased. Let's look at how to calculate reinforcement for a strip foundation using the example of a shallow foundation 70 cm high and 40 cm thick.
Read also: how to reinforce a floor screed, and what kind of mesh is required for this?
First you need to determine the frame configuration. It will consist of an upper and lower chord, 3 reinforcement bars in each. The distance between the rods of 10 cm + 10 cm goes to the protective layer of concrete. The connection will be made by welding sections of reinforcement of similar sizes in increments of 30 cm. The diameter of the reinforcement for the strip foundation is 12 mm, class A3.
Reinforced frame of strip foundation
We determine the required amount of reinforcement:
- To find out the consumption of rods for the longitudinal belt, you need to calculate the perimeter of the foundation. Let’s take a conventional building with a perimeter of 50 m. Considering that there are 6 reinforcement bars in two belts (3 in each), its consumption will be: 50 * 6 = 300 m.
- Next, we calculate how many connections will need to be made to join the belts. To do this, we divide the perimeter by the step between the jumpers: 50/0.3 = 167 pcs.
- Taking into account the required thickness of the protective layer (5 cm), the length of the vertical jumper will be 60 cm, and the transverse one - 30 cm. The number of each type of jumper for each connection is 2 pieces.
- We determine the consumption of rods for vertical jumpers: 167 * 0.6 * 2 = 200.4 meters.
- We calculate the material consumption for the cross lintels: 167*0.3*2 = 100.2 m.
In total, the calculation of the reinforcement for the strip foundation showed that the total consumption of A3 rods with a diameter of 12 mm will be 600.6 m. This quantity is not final, the material must be taken with a margin of 10-15%, since it will be necessary to use additional reinforcement to strengthen the corner parts of the foundation strip.
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1.2 Reinforcement of a strip foundation (video)
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2 Work technology
After the amount of reinforcement has been determined, a strip foundation reinforcement scheme must be selected, according to which the reinforced frame will be assembled. Straight sections of the structure are made of solid rods, while in corner areas additional reinforcement is required with reinforcement curved in a U or L-shape. The use of perpendicular overlap of individual reinforcement bars at corners and junctions is not permitted.
The correct reinforcement of the corners of a strip foundation is shown in the diagram:
Reinforcement of foundation corners
Scheme of reinforcement of strip foundations at junctions:
Reinforcing a strip foundation with your own hands involves assembling the frame in a convenient place and then placing it inside the formwork. The technology requires bending reinforcement into rectangular clamps, which can be easily done at home using a homemade device.
On the 20th channel, you need to cut out grooves with a grinder, into which the reinforcement is subsequently inserted, and a section of steel pipe is placed on the rod, which is used as a lever. The finished rings must be fastened by welding or tied with wire. For rods with a diameter of 10-15 mm, 1.2-1.5 mm wire is used.
Read also: how to reinforce a column so that it stands for many years?
The length of the rods on the longitudinal belt should be equal to the length of the side of the house. The rods are threaded inside the ring and fixed with knitting wire at the corners of the clamp and in its central part. The step between the clamps is 30 cm. At the exit you should have 4 components of the frame - 2 equal to the length and 2 smaller ones equal to the width of the house. Next, the frames are laid in the trench and connected with reinforcement bars bent at an angle in accordance with the diagram presented above.
Bending of clamps from reinforcement
When installing the frame inside the trench, the following rules must be observed:
- the frame must be raised 5 cm above the bottom of the trench using stands - SNiP requirements do not allow the use of brick fragments for this purpose;
- installation must be carried out strictly at a horizontal level;
- the frame must be fixed relative to the side walls of the trench using pins driven into its walls so that the reinforcement does not move during concreting.
Read also: how is brick and aerated concrete masonry reinforced, and is it necessary to do it?
The reinforcement of a strip foundation according to the execution technology is identical for shallow and recessed foundations. After installing the reinforced frame, the concreting stage begins - for pouring it is used concrete grade M200. You can determine the required amount of concrete based on the volume of the foundation - you need to multiply the length, width and perimeter of the tape.
Reinforced frame of the foundation tape
Note that the technology for constructing a strip foundation requires the obligatory arrangement at the bottom of the trench of a compacting cushion made of layers of sand and crushed stone of equal thickness (thickness from 10 to 20 cm each). The cushion is used to protect the foundation from vertical heaving loads, which is especially important when constructing a shallow foundation located in a layer of freezing soil.
Reasons why you need to reinforce a reinforced concrete foundation
In a reinforced concrete structure, each component - concrete or reinforcement - performs different functions. Concrete, when stretched, can elongate by only fractions of a millimeter. Under large tensile loads and transverse shearing forces, deformations may occur in an unreinforced concrete structure, leading to cracking and the appearance of other defects, including destruction.
The steel elements of a reinforced concrete frame can withstand tensile loads ten times greater than those that concrete can withstand. Ductile rolled steel, having the ability to elongate without breaking by 5-25 mm, works in tension, preventing the development of deformations in the structure beyond acceptable limits.
A monolithic foundation strip is a system of beams connected to each other at corners and intersections, lying on a continuous elastic soil foundation. Soils are constantly exposed to climatic factors - they freeze in winter and thaw in spring, they are moistened by surface or groundwater, while increasing or decreasing in volume.
The forces arising from below are transferred to the foundation, and with a constant load from the building above, compression and tension forces arise in the structure. In this case, compression and tension can be experienced by different cross-sectional zones of the monolithic beams that make up the strip foundation.
Therefore, the main reinforcement scheme for a strip foundation is a three-dimensional frame with rolled steel products located at the top and bottom of the cross section. If the width of the base of the tape exceeds the width of the wall by more than 600 mm, then the base is additionally reinforced using flat mesh.
When designing, it is determined what reinforcement is needed for the strip foundation.
What kind of reinforcement is used to reinforce strip foundations
The reinforcement of the strip foundation is carried out using spatial frames and flat meshes, in which rolled steel products are divided into working ones, which absorb the main tensile forces, and structural ones, which serve to secure the working rods.
Let's consider what steel rods can be used for a strip foundation. As a working material, corrugated rolled steel of class A3 is used, according to another classification A400, produced according to GOST 5781-82* or A500S according to GOST R 52544-2006. Corrugated steel promotes better adhesion of working rods to concrete. Reinforcing the strip foundation using rolled A500C allows you to weld frames and meshes. As a structural rod, rods with a smooth surface of class A1 or, according to another designation, A240 are used.
We wrote about the use of working reinforcement of classes A3 and A500C, the differences between them, the benefits of using A500C, and the features of installing frames and meshes in the article “”.
All reinforcement work must be carried out following the instructions of technical documents SP 52-101-2003 “Concrete and reinforced concrete structures without prestressing reinforcement”, SNiP 52-01-2003 “Concrete and reinforced concrete structures”, using which you can reinforce the strip foundation with your own hands.
Calculation of the diameter of the reinforcement and the number of working rods for the tape
The diameter of the round bars for a strip foundation is determined based on a calculation that takes into account the loads carried by the foundation. The load is collected from all load-bearing walls per 1 linear meter along the length of the foundation. The total load takes into account:
- dead weight of wall structures made of different masonry materials, lightweight concrete blocks, wood, monolithic reinforced concrete, etc.;
- own weight of floors - reinforced concrete or wood, collected from 1 m 2 and half the span between load-bearing walls;
- the weight of people, furniture, partitions, equipment, etc., acting on the floors, collected from 1 m 2 and half the span of the floor. Accepted by SNiP 2.01.07-85* “Loads and impacts”;
- the weight of the covering and roof structures collected from 1 m2 and half the span;
- weight of snow cover in winter, taken according to SNiP 2.01.07-85*.
After collecting the loads, the width of the belt structure is calculated taking into account the bearing capacity of the base. We gave examples of how to correctly collect loads, calculate the width of the tape and the thickness of the anti-heave cushion in the article “”.
There are also tables for collecting loads for different types of walls and floors, the values of the calculated resistances of various types of soil, which can be used when calculating any strip foundations intended for low-rise buildings. For quick calculations, a calculator is provided on the article page.
Reinforcement calculations are carried out taking into account the accepted dimensions of the foundation structure - the width of the base and the height of the section according to the method SNiP 2.03.01-84* “Concrete and reinforced concrete structures”. To correctly calculate the reinforcement of a strip foundation in accordance with SNiP, you should contact professional designers.
And we will give a simplified calculation method.
Simplified calculation of strip foundation reinforcement
A simplified calculation of rolled steel for a strip foundation consists of selecting the number of working rods, as well as their diameter according to the main indicator - the minimum percentage of reinforcement.
According to requirements clause 5.11 Table 5.2 Manuals for SP 52-101-2003 the total area of the working rods that can absorb tensile forces should not be less than 0.1% of the cross-sectional area of the reinforced concrete structure being calculated.
Since a monolithic tape has the form of a beam, which is subject to multidirectional forces, stretched zones can be at the top and bottom of its cross section.
Thus, the main condition for the calculation is the presence in both zones of the cross-section of the structure of longitudinal working rods with a total area of at least 0.1% of the total cross-sectional area.
Formula for calculating the percentage of reinforcement by clause 5.11 of the Manual to SP 52-101-2003:
$$\quicklatex(size=25)\boxed(\mu_s = \frac(A_s)(b \times h_0) \times Pr )$$
Where:
Pr—unit equal to 100%;
A s; – the required total area of the working rods, mm 2;
b – tape width, mm;
h 0 ; – working height of the cross section, in mm.
From this formula you can find the required minimum area of the rods:
$$\quicklatex(size=25)\boxed(As = b \times h_0 \times 0.001)$$
When calculating, you need to take into account the rules for reinforcing strip foundations set out in Manuals for SP 52-101-2003 in the “Guide to the design of concrete and reinforced concrete structures made of heavy concrete (without prestressing)”.
According to clause 5.17 of the Manual to SP 52-101-2003 The minimum diameter of each of the working rods is limited to 12 mm.
Initial data: monolithic strip foundation for external walls with a section of 600 mm (b - width) by 500 mm (H - total height);
First, we determine h0, which will be equal to the height of the section without a protective concrete layer.
The protective layer that must be maintained for the lower rods on the sole of the tape laid on sand or crushed stone preparation is 70 mm. But for the upper reinforcement the protective layer is 30 mm, so we take the average value of 50 mm:
h0 = H – 50 = 500 – 50 = 450 mm
We determine the cross-sectional area of the tape that will be used in the calculations:
b x h0 = 600 x 450 = 270,000 mm 2
The required minimum area of working rods As in each section zone will be equal to:
As = b x h0 x 0.001 = 270,000 x 0.001 = 270 mm 2
To select the diameters of the working rods and their number according to the minimum required area, we provide Table 1.
Using the table, we find the closest values for a minimum diameter of 12 mm, provided that 3 rods are installed. The value will be between columns with 2 (226 mm 2) and 3 rods (339 mm 2), we take the larger one - 339 mm 2 for 3 rods.
As a result, we finally accept 3 working rods each having a diameter of 12 mm in both cross-sectional zones.
Strip foundation reinforcement schemes
We present two main reinforcement schemes for a monolithic reinforced concrete foundation that can be used in low-rise construction.
Scheme 1 - if the width of the tape is equal to the width of the wall
Scheme 2 - if the width of the tape exceeds the width of the wall
In both cases, the tape is reinforced along its length with a spatial frame, the working rods of which, located in both zones of the cross-section of the structure, perceive and compensate for tensile forces.
If the tape protrudes beyond the edges of the base by more than 0.5 m, tensile forces will occur in the area of the sole perpendicular to its axis. In order to compensate for these forces, reinforcement of the sole of the tape is additionally used in the transverse direction to the axis of the wall.
The optimal solution in this case is to knit a mesh consisting of working and structural rods and lay it before installing the spatial frame.
When constructing spatial frames, in addition to longitudinal working rods, transverse reinforcement is used, which serves not only to connect longitudinal rolled products into one structure, but also to absorb transverse, cutting loads on the strip. Transverse reinforcement also prevents the formation of cracks in the structure and prevents lateral buckling of working rods.
As part of spatial frames, transverse rolled products are used in the form of clamps that cover longitudinal working rods along the perimeter of the frame. For clamps, reinforcement with a smooth surface of class A1, having a diameter of 6-8 mm, is used.
In the technical document SP 52-101-2003 “Concrete and reinforced concrete structures without prestressing reinforcement” The diameters of the reinforcement were determined under different reinforcement conditions, which are given in Table 2.
In addition to the requirements for the use of reinforcing bars of a certain diameter and class for various elements of spatial frames and flat meshes, the standards provide for a number of rules for the reinforcement of monolithic structures.
Rules for reinforcing a monolithic strip foundation
When producing tape reinforcement, the following regulatory rules must be observed:
- working rods installed in the longitudinal direction of frames and meshes must have the same diameter. In the case of using reinforcement with different diameters, rods with b O the larger diameter must be located in the lower zone of the belt;
- with a belt width exceeding 150 mm, the number of longitudinal working elements placed in one level should not be less than 2;
- the distance in the frame between longitudinal elements installed at the same level is not allowed to be less than 25 mm in the bottom row of the frame and less than 30 mm in the top row. When constructing spatial frames, it is also necessary to provide places for the passage of deep vibrators. In these places, the clearance should not be less than 60 mm;
- the pitch of rolled products in the strip foundation, provided for the installation of clamps or transverse elements, must be within ¾ of the height of the structure and not more than 500 mm;
- the protective layer of concrete provided for the working reinforcement of frames or meshes located at the bottom of the tape should be 35 mm when preparing with concrete, 65 mm when preparing from sand or crushed stone;
- protective concrete layer on the sides and top of the structure - 40 mm, for clamps or transverse rods - 10 mm.
Manufacturing of frames and meshes
In the case of using conventional rolled products of classes A1, according to another classification A240, and A3 (A400), the reinforcement for the strip foundation is knitted, for which a special knitting wire is used. Welding of reinforcing elements is possible only when using rolled products of class A400C or A500C.
The knitting wire is made of low-carbon steel, has a diameter in the range of 0.8-1.4 mm and is designed specifically for the manufacture of load-bearing frame elements of reinforced concrete structures. When knitting frames and meshes, 30 cm long sections are used, which are pre-cut.
Let's look at how to knit reinforcement for a strip foundation. To perform this type of work, a special tool is used: hand hooks or screwdriver attachments, knitting guns, pliers, tongs and wire cutters.
A loop is made from pieces of knitting wire, which is passed around the junction of the reinforcing bars, then the ends are twisted manually using a crochet hook or mechanically using a screwdriver attachment or a gun.
Since frames and meshes of reinforcement have a limited length, the question may arise: how to tie the reinforcement for a strip foundation. Along the length, frames and meshes are joined using: overlap without welding or welding in the case of using rolled products of class A400C or A500C.
When overlapping welding, the length of the reinforcement bars being connected should not be less than 10 diameters.
In the case of an overlap connection, the length of the bypass of the reinforcing bars must be at least 20 times the diameter of the elements being connected and at least 250 mm.
To calculate the total volume of material, you can use the reinforcement calculator for strip foundations located on this page.
Reinforcement of corners and joints
At the junctions and corner joints of the tape, the greatest concentration of stress occurs, so these nodes must be further strengthened.
For reinforcement, additional rods are installed according to the following schemes:
When strengthening the angle of the tape, additional L-shaped and trapezoidal rods are installed, which are attached to the working rods in the upper and lower levels of the connected frames.
When strengthening the T-shaped intersection, additional trapezoidal rods are installed in the upper and lower levels of the connected frames.
When increasing mutual intersection, trapezoidal rods are installed.
Reinforcement of the corners of a strip foundation can also be carried out according to the following schemes:
Calculation of the amount of reinforcement
Initial data: a low-rise house measuring 10 x 12 m with a middle load-bearing wall located along the long side. The section of the tape is 400 x 400 mm. Reinforcement – spatial frame of 6 working reinforcement rods with a diameter of 12A3. Clamps made of smooth rolled steel with a diameter of 6A1 are located at intervals of 400 mm.
Determine the total length of the tape:
10 x 2 + 12 x 3 = 56 m.p.
The length of the working rods will be equal to:
56 x 6 = 336 m.p.
Length of one clamp:
0.4 x 4 /1.15 = 1.39 m (1.15 is the conversion factor of the perimeter of the tape section to the length of the clamp)
Number of clamps:
56 / 0.4 = 140 pcs.
Length of rods for clamps:
140 x 1.39 = 194.6 m.p.
We increase the calculation results by 5% - this is a margin that takes into account the cutting of reinforcement and waste.
Working reinforcement: 336 x 1.05 = 353 m.p. or 352 x 0.888 = 313 kg
Clamps: 194.6 x 1.05 = 204 m.p. or 204 x 0.222 = 46 kg
To quickly calculate the amount of materials, you can use the reinforcement and formwork strip foundation calculator located here.
Methods and techniques for reinforcing strip foundations from an expert on the website portal
The above two main schemes, according to which a strip foundation can be reinforced, as well as schemes for reinforcing corners and intersections for low-rise buildings, have been repeatedly used and tested during real construction in difficult soil conditions - with foundations made of subsidence and heaving soils. Therefore, I recommend using these diagrams and the information provided on the selection of steel rods and the design of frames for houses 1-2 floors high under any ground conditions.
When constructing more complex and critical structures, you should turn to professional designers to design the foundation.
The foundation is the basis of the entire structure, the guarantor of its durability and strength. Strip foundation reinforcement, drawings and diagrams must be prepared and calculated in advance.
The need for reinforcement
The foundation is a monolithic reinforced concrete structure. Concrete itself is a material that can withstand significant loads only in compression. To increase the tensile and bending strength of the foundation, it is necessary to make a metal frame. Moreover, strength indicators increase only due to the arrangement of working longitudinal rods. Therefore, thick profiled rods with a diameter of 10÷14 mm are used as them. Transverse elements are designed to create the spatial geometry of the foundation; they do not allow the longitudinal bars to move when pouring concrete mortar, and keep them in the right place. For these purposes, you can use smooth or profiled rods with a diameter of 6÷9 mm.
Requirements of regulatory documents for reinforcement materials
You can learn how to properly reinforce a strip foundation by reading the regulatory documents. The requirements that must be observed when reinforcing a monolithic strip reinforced concrete foundation are set out in SNiP (No. 52-01-2003).
- hot-rolled smooth or profiled (Ø=3÷80 mm);
- thermomechanically strengthened profiled (Ø=6÷40 mm);
- cold-rolled profiled or smooth (Ø=3÷12 mm).
Important! Compared to smooth ones, profiled rods have better adhesion to concrete: this can significantly increase the reliability of the structure.
Requirements for the arrangement of rods in the reinforced frame for a strip foundation:
- the distance between the longitudinal rods should be no more than 0.4 m;
- the pitch of the transverse fixing elements should be no more than 0.5 m.
The minimum relative content of working longitudinal rods in a reinforced concrete element must be at least 0.1%.
For tying elements of reinforced concrete structures, only special baked steel wire (Ø=0.8÷1.2 mm) should be used.
Example of calculation of foundation reinforcement
Let's try to calculate how much materials will be needed to arrange the reinforcement of a specific strip foundation with drawings. Let's say we are building a small country house from building blocks (0.4 m wide) with overall (external) dimensions of 5 × 8 m. The nature of the soil on our site allows us to make the height of the strip 0.9 m, its width 0.4 m, which corresponds to the width of the building material of the walls. We will use longitudinal working rods with a diameter of 12 mm and □-shaped transverse clamps made from rods with a diameter of 8 mm.
Reinforcement of a shallow strip foundation:
The photo shows that the distance between the working longitudinal rods (0.4 m) and the pitch of the □-shaped transverse clamps (0.5 m) were chosen in accordance with the requirements of regulatory documents.
We check the relative content of longitudinal working rods in our reinforced concrete structure. To do this, we will use the following terms and notations:
- h – foundation height (900 mm);
- w – foundation width (400 mm);
- Sₒ – cross-sectional area of the foundation;
- Sₐ – total cross-sectional area of longitudinal rods (6 pieces);
- r is the radius of the longitudinal rod (6 mm), which is equal to d/2, where d is the diameter of the rod (in our case d=12 mm);
- D – relative content of working rods in the “body” of the foundation.
Sₒ = h∙w = 900∙400 = 360000 mm²
Sₐ = 6∙π∙r² = 6∙3.14∙6² = 678.24 mm²
D = (Sₐ∙100)/ Sₒ = (678.24∙100)/360000 = 0.1884 ≈ 0.19% (which is 1.9 times the minimum permissible value, that is, we chose the strip foundation reinforcement scheme correctly) .
Calculation of the number of longitudinal rods
In order to determine how many standard longitudinal rods (6 m) we need, we use the following values:
- L – foundation length (8000 mm);
- W – foundation width (5000 mm);
- P – perimeter;
- N – number of longitudinal elements (in our case 6 pieces);
- X is the total length of the longitudinal rods.
P = (L+ W)∙2 = (8000 + 5000)∙2 = 26000 mm = 26 m
X = P∙N = 26∙6 = 156 m
To the obtained value it is necessary to add 20% (material for the manufacture of L-shaped or U-shaped elements to ensure correct and sufficient overlap when joining elements).
Xadd = X∙0.2 = 156∙0.2 = 31.2 m
The final total length of the longitudinal reinforcing bar:
Xok = X + Xadd = 156 + 31.2 = 187.2 m
The standard length of a reinforcing bar is 6 m. It remains to calculate how many such bars are needed: Xok/6 = 187.2/6 = 31.2 ≈ 32 pieces.
Manufacturing of transverse elements and calculation of the amount of material
Laying reinforcement in a strip foundation is impossible without installing transverse (vertical) elements. Typically, □-shaped clamps are used for these purposes. Clamp options:
As can be seen from the presented photo, all three options differ in manufacturing technology, but the rod consumption in all cases is approximately the same. Length of rod (Ø=8 mm) required for the manufacture of one clamp: (800+300)∙2+250 = 2450 mm.
Option #1
- We measure approximately 120 mm and, using a bending device, bend this part of the future clamp in the form of a hook.
- At a distance of 800 mm from the hook, bend the rod at an angle of 90˚.
- We measure 300 mm from the resulting angle and bend the second hook.
Option No. 2
- We measure 250 mm from the end of the workpiece and, using a device, bend this part by 90˚.
- We set aside 800 mm from the result and bend the rod at an angle of 90˚.
- We measure 300 mm and make another 90˚ bend.
- We set aside 800 mm from this angle and bend the rod 90˚.
Attention! We fasten the overlap of the rods with spot welding or 2÷3 twists of wire.
Option No. 3
- We cut two blanks from the rod, each 860 mm long and two 360 mm long.
- We fold them into a rectangle (the protrusion on each side is 30 mm).
- We fasten the corners of the clamp or twisted wire.
Now let's calculate how many clamps are needed to reinforce our foundation:
Q = P/T (P is the perimeter of the foundation strip, T is the pitch of the transverse clamps)
Q = 26/0.5 = 52 pieces
Plus, we will need additional clamps to strengthen the frame in the corners (2 pieces on each side of all four corners, that is, an additional 16 clamps). It is necessary to make 68 □-shaped transverse clamps for the strip foundation.
The length of the workpiece for one element is 2450 mm, that is, we can make only 2 clamps from one standard rod. The required number of rods (Ø=8 mm) is 34 pieces.
5 / 5 ( 1 voice )
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
Foundation reinforcement is a very important part of building the foundation of a house. Reinforcement creates strength for foundations of various designs, made from a variety of materials.
This primarily applies to strip foundations, in which a large mass of concrete presses on the soil. The design of a foundation with reinforcement is a classic example of how simply and effectively the properties of a building structure can be improved if you use theoretical knowledge and practical experience.
Why is concrete reinforced?
Reinforcement began to be used quite a long time ago - since the 18th century. First, concrete structures were reinforced by placing metal on top of concrete, and then a method of strengthening the reinforcement by internal laying was invented. Why is concrete reinforced? Concrete is a very strong material in compression, but very brittle when stretched. The tensile strength index is 10-30 less than the compressive strength. Reinforcing steel, and now various composite reinforcing materials, make it possible to build structures of foundations and walls that compensate for the shortcomings of one or another material.
Thus, a reinforced strip foundation can withstand:
- from below – tensile pressure;
- the upper part of the foundation resists the great pressure of the walls and roof from above;
- From the sides and below the foundation is affected by the force of frost heaving of the soil, the pushing force of which can exceed the weight of the house.
How is the reinforcement installed?
The reinforcing mesh is laid longitudinally and transversely in the concrete formwork. Longitudinal reinforcement takes on the heaviest loads, and therefore it is placed below and above the foundation being poured. If the foundation height is not lower than 15 centimeters, then transverse reinforcement bars are also installed. Usually. in case of choosing a metal reinforcing product, use steel rods with a diameter of 5 to 8 mm.
During the construction of the reinforcing mesh, it is fastened together, creating a single reinforcing frame structure. Tying the reinforcement into one frame eliminates the possibility of improper redistribution of loads. Thus, the reinforcing frame creates powerful resistance to the weight of the house, as well as to the forces that lift the foundation or try to test it in tension. The distance between the longitudinal reinforcement bars is fixed at 400-500 mm. The pitch of reinforcement that is installed transversely should not exceed 300 mm.
The step width and reinforcement density must be calculated taking into account:
- the reinforced concrete structure element used;
- element dimensions in width and height;
- a calculated value that ensures the effective involvement of concrete and reinforcement in maintaining the rigidity of the structure;
- in longitudinal reinforcement, the distance between the rods should not exceed twice the sectional height of the concrete element.
Strip foundation: subtleties of reinforcement
A monolithic strip foundation is reinforced not only along the walls of the structure. A reinforcing mesh is placed at the base of the foundation on a sand cushion, and then formwork, stones, gravel, broken bricks are placed, and then the whole mixture is poured with concrete.
A reinforced monolithic foundation is very resistant to soil heaving. If such a reinforced foundation is laid below the freezing point, then you can expect the construction of a strong and durable foundation. If the foundation walls are higher than 50 centimeters, then they will experience a powerful one-sided lateral load of the soil and therefore such foundations must be constructed with reinforcement.
When the strip foundation contains a full frame of reinforcement, the reinforcement cannot be directly connected to the open ground and formwork elements. This can cause rust, which is unsafe for steel reinforcement structures, which can therefore rot and crumble. The safe layer to protect the frame should be at least 45-75 centimeters.
Foundation reinforcement angles are a special concern for builders. It is the corner structures that experience increased stress. To create angular bends, it is necessary to bend wire corners at the site where the frame is created. If reinforcement is carried out by simply grasping straight reinforcing steel bars with wire, then the strength of the structure will be orders of magnitude lower, and it will not be able to become a monolithic frame. In fact, in this case, it is possible to obtain, considering the situation from an engineering point of view, several individual beams, and not a common monolithic reinforced mass. This sharply reduces the possibility of resistance of the foundation structure not only to compression, but also to the most dangerous stress for concrete structures - tension, lateral forces.
If reinforcement is carried out for an obtuse angle of the foundation, then the structure of the reinforcement frame is strengthened by additional adhesion to the external reinforcing structure, as well as by installing clamps in the transverse fabrication.
Reinforcement calculation
Before you start reinforcing, it is important to calculate the amount of materials needed for it. To do this, you need to determine the required cross-section of the rods used for reinforcement.
If you are building an outbuilding to store equipment or accommodate a small workshop, then reinforcement with a cross-section of up to 10-12 millimeters is quite suitable. If a strip concrete foundation is to be reinforced, then a larger cross-section of reinforcement is required - from 15-20 millimeters. In addition, the reinforcement should preferably have periodic surface profiling. This creates additional strength of the reinforcing frame. Rods for additional reinforcement, including for vertical installation, can be thinner - from 10 millimeters in cross-section. If a lower longitudinal row is installed, then the distance to the upper row is usually at least 30 centimeters.
To simplify the calculations, they suggest starting from this indicator: if the length of one part of a reinforced concrete element is 3 meters or a little more, then the smallest diameter of the reinforcing bar should be 12 mm. If we take into account all the loads and distribute them evenly over the element, then it is necessary to make two reinforcement belts with a rod with a cross-section of 12 mm. What will be the diameters of the transverse reinforcement? If the reinforcement frame is no more than 80 centimeters, then the minimum cross-section of the reinforcement is 8 mm.
All these estimates are indicative only. As we have already said, they can be used to estimate the work required, and not for a specific house, since there are many features of the house design that need to be taken into account. First of all, the features of the walls, the weight of the roof, internal sections, and type of floors.
Reinforcement technology
When reinforcing, it is important to remember one important construction rule - concrete, the mortar for pouring the body of a monolithic strip foundation, covers the reinforcement cage on all sides by at least 50 millimeters. That is, if the cross-section of the foundation is 400 by 400 millimeters, then the cross-section of the frame will be 300 by 300 millimeters.
The assembly of the frame is carried out after preparing the required rod with the required cross-section and surface:
- the step in the reinforcement strip foundation usually does not exceed 30-50 centimeters;
- steel stiffeners are placed on reinforcing bars;
- reinforcement is fixed at the corners of each of the ribs;
- fastening is carried out with twists or special fixing one-piece elements, including installation of the frame by welding;
- the cross-section of the frame must be quadrangular;
- after installing the frame, it is placed in place for laying the foundation;
- when laying, a distance of 50 mm from the bottom and walls of the trench is maintained;
- Broken brick, stone or concrete fragments are poured under every fourth rib to prevent bending.
Strengthening frame joints
In order to connect frame elements, several methods are used.
- an overlap connection involves connecting reinforcement along outlets with a length of at least 50 centimeters;
- using the lining method, the reinforcement is connected using scraps of reinforcement, bent and U-shaped clamps with a length of connection to the reinforcing bars of 50 cm on each side.
The reinforcing belt can be fastened
- directly;
- in an angular way;
- T-shape.
In order to connect the reinforcement, a strong knitting wire is used, which has a cross-sectional diameter of at least one millimeter. The metal ribs of the reinforcement frame are fixed on top of the twisted wires, put on the frame before connecting the reinforcement. If the reinforcement is connected with an overlap, the dimensions of the ribs are slightly increased.
If welding is used in the connection, the joints may experience strength problems due to intense heat and giving them the properties of a brittle hardened iron surface. This in turn can lead to destruction of the reinforcement. Therefore, welding fastening is not recommended for significant loads on the foundation structure.
Metal and concrete in one harness
Reinforcing the foundation, creating a frame from metal or a composite rod, allows you to create a strong and durable connection between concrete and metal or composite. Such connections can withstand enormous loads, last for more than 100 years and can create a strong foundation for any type of home.
In the construction of a reinforced foundation, it is important to know the properties of materials and carry out the correct calculations and apply consistent technology for laying the frame. Without the use of all this professional knowledge, foundation reinforcement will be ineffective and will not be able to ensure the strength of the entire building structure. Foundation reinforcement has made it possible for several centuries in construction to use all the best properties of the two most common building materials - concrete and metal.
