Overlap of reinforcing bars. Reinforcement overlap during knitting - connection standards according to SNiP. Individual design requirements
Reinforcement overlap during knitting (SNiP 52-101-2003) is the simplest and at the same time a truly reliable option for connecting reinforcement, which guarantees long-term operation of a foundation or other concrete structure.
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Sanitary norms and rules of 2003 indicate all types of connection that exist today. These include mechanical and welded butt-type joints, as well as overlap joints performed without welding. Mechanical ones are made by threaded or pressed couplings using special units, welding ones involve welding, and overlap joints can be of three types:
- rods with straight ends with mounting or welding on the overlap of the rods (transverse);
- profile periodic rods with straight ends;
- rods with paws, hooks, loops (they are called bends).
Overlapping is recommended to connect reinforcement with a cross section of not more than 40 millimeters. In the document, which is an analogue of the Sannorm we are interested in (ACI 318–05), a section of rods of no more than 36 millimeters is allowed. This limitation is due to the fact that there were practically no tests of the reliability of the connection of larger diameter reinforcement, which means that there are no confirmed data in this regard.
It is forbidden to connect the fittings in places of maximum stress of the rods and areas of application of (concentrated) load on them. It is allowed to carry out the connection both without a knitting wire, and with it. In the latter case, the wire is used for. Experts advise using crimped joints or screw couplings when working with rods with a cross section of more than 25 mm. This is because in this case:
- the level of safety of the structure increases (the volume of the concrete mixture is limited at the joints);
- financial costs for reinforcement are reduced (overlaps, as a rule, require a considerable overrun of reinforcement - up to 20-25%).
The distance between the reinforcing bars, which are overlapped, in the horizontal and vertical direction must be from 25 mm and above. Compliance with this condition makes it possible for concrete to penetrate into the "secluded" places of the frame without problems. For reinforcement with a cross section of more than 25 mm, it is recommended to select the value of the specified distance, which is identical to the diameter of the rods. But the greatest distance between the reinforcement elements along the width of the foundation tape should be up to eight sections of reinforcing parts.
In cases where a binding wire is used, the distance between the rods is often assumed to be zero, since in this situation it depends solely on the height of the profile ledges. In this case, the largest distance between the reinforcement elements is selected in such a way that it is not higher than four diameters of the reinforcing bars. The distance between pairs of joints located next to each other, with an overlap connection, is taken not less than 30 mm (not less than two diameters of the rods).
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It is necessary to space apart the connections located in the neighborhood. Moreover, in such a way that up to 50 (no more) percent of the rods are connected simultaneously in one section.
The calculated (one) section, which must be determined to determine the number of joined rods, is understood to be an area with a length of 130% of the total length of the overlap (measurement is carried out along the rods). It is important to understand here: when designing, reinforcing joints are considered as lying in one section, provided that their centers are located exactly in the specified area. The smallest distance (in length) between the joints according to SNiP should be 610 millimeters.
ACI 318-05 and the Sanctions we are considering advise loose (i.e. loose) bar connections to be made in structures that are not prestressed. The recommendation is quite logical, because concrete with this connection option fills the rods from all sides, which guarantees ultra-reliable fixation of each bar, which is unrealistic to achieve when pouring an incomplete circumference of a reinforcing element connected to an adjacent rod with a knitting wire. Note that the overlap along its length cannot be less than 25 centimeters.
Another important provision of the Sanitary Norms is that in one section (in the design) the connection can have no more than 50 percent of the metal bars embedded in the foundation tape. In addition, it is allowed to join the described method of welded meshes and individual reinforcing elements without a mandatory run-up. But such a permit is valid only in cases of using reinforcement for reinforcement of a non-working (so-called constructive) type.
If the reinforcement functions both in compression and in tension, a 30-centimeter (and higher) overlap is possible. In the construction documentation of most foreign countries, the overlap is set at the level of 40 diameters of the connected elements for reinforcement. In domestic practice, this indicator is equal to 50 diameters (we are talking about class A400 fittings).
Also, the indicator of the recommended overlap depends on which brand of concrete is used to fill the structure. So, for example, for a mixture of M300, it is 35 diameters, M250 - 40 diameters, M200 - 50. But for reinforcement A-II and A-I, the overlap is always selected at the rate of 40 diameters. True, it is important to understand that all these statements are true for calculated indicators. In practice, real (not the minimum recommended) overlap rates are usually several times higher.
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The length of the overlap of the bars during anchoring is set, focusing on the fact that the force acting in the reinforcing structure is perceived by the resistance forces of the metal elements and the adhesion forces of concrete and reinforcement, which are observed along the entire length of the connection. SNiP for overlapping reinforcement during knitting gives the following recommended overlap lengths (all values in millimeters):
- 1090 for fittings with a section of 36;
- 960 for 32;
- 860 for 28;
- 760 for 25;
- 680 for 22;
- 580 for 18;
- 480 for 16;
- 380 for 12;
- 300 for 10 mm bars.
The sanitary codes also contain tables with recommended anchoring overlap lengths for different brands concrete for reinforcement elements working in compression and tension. According to these tables, the minimum overlap length for M450 concrete with an A400 rod section of 6 millimeters is 20 centimeters. But for a concrete mix M250 and a rod with a cross section of 40 mm, the length indicator is already 158 cm.
Let's add a few more important points of SNiP, which were discussed in this article:
- at the place of anchoring of the structure with an overlap, transverse additional reinforcement should be mounted (this condition is mandatory);
- spacing of anchoring of adjacent bars is not allowed less than 61 centimeters;
- cross-shaped overlaps should be connected with clamps or clamps made of plastic materials or annealed viscous wire.
When performing measures related to the reinforcement of concrete structures, it becomes necessary to connect reinforcing bars to each other. When performing work, it is necessary to know what overlap of the reinforcement, how many diameters according to SNiP is the amount of overlap of the bars. From the correctly selected length of the overlap, taking into account the area cross section reinforcement, depends on the strength of the foundation, or armored belt. Properly performed calculation of reinforced concrete elements, taking into account the type of connection, ensures the durability and strength of construction objects.
Types of connections between reinforcing elements
Wanting to deal with possible options joining reinforcing bars, many craftsmen turn to the requirements of existing normative documents. After all, a well-executed connection provides the required margin of strength in compression and tension. Some developers are trying to find an answer according to SNiP 2 01. Others are studying building codes and regulations under the number 52-101-2003, containing recommendations for the design of structures made of reinforced concrete reinforced with non-stressed steel reinforcement.
In accordance with the requirements of the current regulatory documents, steel reinforcement is used to reinforce non-stressed elements, in contrast to stressed structures, where reinforcing ropes of classes K7 and above are used for reinforcement. Let us dwell on the methods used for fixing reinforcing bars.
The current building codes and regulations (SNiP) describe in detail the fastening of reinforcement by all currently existing methods
The following options are possible:
- overlapping knitted rods without welding. Fixation is carried out using additional bent steel bars, repeating the configuration of the reinforcing joint. It is allowed, according to SNiP, to overlap straight rods with transverse fastening of elements using knitting wire or special clamps.
Reinforcement overlap during knitting depends on the diameter of the bars. Concrete-filled structures made of knitted rods are widely used in the field of private housing construction. The developer is attracted by the simplicity of technology, ease of connection and reasonable cost of building materials;
- fixing reinforcing bars using household electric welding equipment and professional units. The technology of connecting reinforcement using welding machines has certain limitations. Indeed, significant internal stresses arise in the welding zone, which negatively affect the strength characteristics of reinforcing cages.
It is possible to overlap reinforcing bars using electric welding using fittings of certain brands, for example, A400C. Steel reinforcement welding technology is mainly used in the field of industrial construction.
Building codes and regulations contain an indication of the need to strengthen the concrete mass with at least two solid reinforcing loops. To implement this requirement, steel rods are connected to the overlap. SNiP allows the use of rods of various diameters. In this case, the maximum size of the cross-section of the rod should not exceed 4 cm. SNiP prohibits overlapping the rods using knitting wire and welding in places where a significant load is applied along or across the axis.
These include mechanical and welded butt-type joints, as well as overlap joints performed without welding. Fixing reinforcing bars by electric welding
Docking fittings using electric welding is used in the areas of industrial and special construction. When connecting using electric welding, it is important to achieve a minimum distance between the rods and fix the elements without a gap. Increased load capacity of the connection zone, stretched from the action, is achieved when using reinforcing bars marked A400C or A500C.
Professional builders pay attention to the following points:
- the inadmissibility of the use for welded joints of common fittings marked A400. As a result of heating, strength is significantly reduced and susceptibility to corrosion is increased;
- increased probability of breaking the integrity of the rods under the influence of significant loads. The current rules allow the use of electric arc welding for fixing reinforcement with a diameter of up to 25 mm;
- the length of the weld and the class of bars used are interrelated. The table of the normative document contains all the necessary information about the fixation of rods using electric arc welding.
The normative document allows the use of electrodes with a diameter of 0.4-0.5 cm when performing welding activities and regulates the amount of overlap exceeding ten diameters of the rods used.
It is forbidden to connect reinforcement in places of maximum stress of the rods and areas of application of (concentrated) load on them Reinforcement overlap without welding during installation of armored belt
Using A400 AIII-marked rods popular in construction, it is easy to overlap the reinforcement using annealed knitting wire.
- connection with an overlap of the straight ends of the reinforcing bars;
- overlapping bars fixation using additional reinforcement elements;
- tying rods with ends curved in the form of peculiar loops or hooks.
With the help of knitting wire, it is allowed to connect reinforcement of a profile section with a diameter of up to 4 cm. The amount of overlap increases in proportion to the change in the diameter of the rods. The amount of bar overlap increases from 25 cm (for bars with a diameter of 0.6 cm) to 158 cm (for rods with a diameter of 4 cm). The amount of overlap, according to the standard, should exceed the diameter of the bars by 35-50 times. SNiP allows the use of screw couplings along with knitting wire.
The distance between the reinforcing bars, which are overlapped, in the horizontal and vertical direction must be from 25 mm and above Requirements of regulatory documents for reinforcing joints
When connecting rods with a knitting method, it is important to consider a number of factors:
- mutual arrangement of reinforcement in a spatial frame;
- features of the placement of sections with overlap relative to each other;
- the length of the overlap section, determined by the section of the rod and the brand of concrete.
When a section with overlapped rods is located in the zone of maximum load, the overlap should be increased to 90 with the diameter of the connected rods. Building codes clearly indicate the dimensions of the docking sections.
The length of the joint is affected not only by the diameter of the cross section, but also by the following points:
- the magnitude of the current load;
- brand of concrete mix used;
- class of steel reinforcement used;
- placement of butt joints in a spatial frame;
- purpose and scope of reinforced concrete products.
It should be noted that the amount of overlap decreases with increasing brand of concrete used.
In cases where a binding wire is used, the distance between the rods is often assumed to be zero, since in this situation it depends solely on the height of the profile ledges Consider the change in the amount of overlap that perceives compressive loads for class A400 reinforcement with a diameter of 25 mm:
- for concrete grade M250, the rods are fixed with a maximum overlap of 890 mm;
- concreting of the reinforcing grid with a solution of the M350 brand allows to reduce the overlap to 765 mm;
- with an increase in the brand of concrete used up to M400, the overlap of the bars decreases to 695 mm;
- pouring the reinforcing cage with M450 concrete mortar allows to reduce the overlap to 615 mm.
For reinforcements of the tensioned zone of the reinforcement cage, the overlap for the specified reinforcement is increased and amounts to:
- 1185 mm for concrete M200;
- 1015 mm for concrete M350;
- 930 mm for concrete M400;
- 820 mm for concrete M450.
When carrying out measures related to reinforcement, it is important to correctly position the overlapping areas, and take into account the requirements of building codes and regulations.
- evenly distribute the connections throughout the reinforcing cage;
- withstand minimum distance between joints not less than 610 mm;
- take into account the brand of concrete solution and the cross section of reinforcing bars.
Compliance with the requirements of building codes guarantees the strength and reliability of concrete structures reinforced with a reinforcing cage. Having studied in detail the recommendations of SNiP, it is easy to independently select the required amount of reinforcement overlap, taking into account design features reinforced concrete product. Recommendations of professional builders will help to avoid mistakes.
Good morning!
Today at Unsolicited advice I will continue the topic of working seams of concreting and joining of reinforcement. More precisely, we have already talked about seams, now let's talk about docking.
Reinforcement of the required length does not always get to the construction site, as a result, the question arises that it needs to be joined. As with the issue of concrete joints, many designers try to ignore this problem and leave the decision to builders. Anyone who does this puts the design at risk.
The builder is not required to know where to join the reinforcement. He will dock it in the most convenient place for him, but at the same time - in the most dangerous place for the structure. In the "Recommendations for the use of reinforcing bars according to DSTU 3760-98 in the design and manufacture of reinforced concrete structures without prestressing reinforcement", the requirements are well described (see clause 2.3.3), a couple of especially important ones, I will give here:
1. “Overlap joints of working reinforcement are not recommended to be located in the tension zone of bending and eccentrically tensioned elements in the area of maximum effort and in places where the reinforcement is fully used. Lap joints of working reinforcement are not allowed in linear elements, the section of which is fully stretched. Let me explain a little. We must clearly convey to the builder where he can join the reinforcement. It is impossible to dock in the tension zone: that is, the lower working reinforcement in the slab, for example, cannot be joined in the middle of the span, and the upper one - above the supports (for multi-span slabs). It is there that the slab is stretched, and the diagram of moments tells us about this, and even just an attempt to imagine how the floor will bend during loading: which of its surfaces will try to stretch, and which ones will shrink. It's very easy to draw something like this:
I gave an example for a floor slab, but similar schemes can be made for any structure in which reinforcement is ordered running meters. Sometimes the designer immediately sets the layout of rods of a certain length, indicating the joining points. Here there is a risk of drowning in correspondence on the coordination of all new docking points, because builders may have rebar of completely unpredictable lengths. The values of L/4 and L/3 are taken from a specific calculation and may differ from those given by me.
2. “The joints of welded meshes and frames, as well as stretched rods of knitted frames and overlapped meshes, should be spaced apart. In this case, the cross-sectional area of the working rods, joined in one place or at a distance less than the bypass length l l, should not exceed 50% of the total cross-sectional area of the tensile reinforcement.
The rods should be located as far as possible without a gap, the maximum clear distance between the joined rods should not exceed 4d or 50mm.
Clear distance between joints located in different places along the length of the element, it must be at least 0.5 l l, or at least 1.5 l l in the axes of the joints.
Adjacent overlap joints should be located at a clear distance of at least 2d and at least 30 mm. How to convey all this to the builder? I advise you to take Figure 6 of the "Recommendations ..." as a basis and show the following diagram in the drawing:
Please note that the amount of overlap for the working reinforcement in the upper and lower zones of the slab is different (see the coefficient from Table 12 of the "Recommendations ..."). In the example, I gave a diagram for reinforcement with a diameter of 12 mm.
Always pay attention to the fact that in one section there should be no more than 50% of the joints of the tension bars of the reinforcement. Sometimes this requirement is very difficult to meet, especially in cramped circumstances, and it is necessary to change the diameters of the rods and their number.
In general, I advise you to study the recommendations far and wide before proceeding with the design of the overlap in a particular design.
I also want to write about the joining of reinforcement in columns. This is a specific topic, the solution of which has not yet been found for me. How before, before the introduction of rolled products according to DSTU 3760, were reinforcing bars joined according to GOST 5781? Here is a drawing from the "Reinforced Concrete Design Guide":
It is clear from the figure that half of the protrusion rods come out of the overlap by the length of the overlap, the second half - by two lengths of the overlap. This ensures the spread of the joints - no more than 50% in one section. But in GOST fittings there were completely different overlap lengths - several times less (!) than for fittings according to DSTU 3760. For example, let's take a look: for a rod according to DSTU with a diameter of 20 mm in concrete B25, the overlap is 1630 mm (according to the calculation according to the "Recommendations …”). Two overlap lengths is already 3260 mm (sometimes, this is less than the height of the floor!). What to do with it, the rules are silent. What do designers do with it? Either all the rods are released by the same amount of overlap (I will not say that this is true), or they choose the method of joining by welding with overlays or by crimping. But all these options need to be agreed with the customer - after all, his money and his capabilities.
Perhaps, I will talk about the features of joining reinforcement in columns in the next issue. Good luck with your design!
Sincerely, Irina.
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Comments
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0 #33 Irina
During the reinforcement of the foundation or the manufacture of any type of armored belt, almost every person has a question about what the length of the overlap should be and how to properly perform it. Indeed, it has great importance. Properly executed joining of steel bars makes the reinforcement connection more durable. The structure of the building becomes protected from various kinds deformations and destruction. The impact on the foundation is minimized. As a result, the trouble-free service life is increased.
Overlapping reinforcement during knitting is the simplest and at the same time a truly reliable option for connecting reinforcement
Connection types
The current building codes and regulations (SNiP) describe in detail the fastening of reinforcement by all currently existing methods. To date, such methods of joining reinforcing bars as:
- Overlap joints made without welding:
- overlap when joining with curved parts (loops, paws, hooks).
- overlap in the joints of straight reinforcement bars with transverse fixation;
- overlap of the straight ends of the rods.
- Mechanical and welded types of butt joints:
- using welding machines;
- with the help of professional mechanical units.
The requirements of SNiP say that in concrete base at least 2 continuous reinforcement cages must be installed. They are performed by fixing the reinforcing bars with an overlap.
The option of interweaving overlapping rods is popular in private construction. And there is an explanation for this - this method is available, and necessary materials have a low cost. It is possible to join the overlap of reinforcement bars without the use of welding using a knitting wire.
Industrial construction often uses the second option for connecting reinforcing bars.
building codes it is allowed to use rods of different sections (diameters) during the overlapping of reinforcement. But they should not exceed 40 mm due to the lack of technical data confirmed by research. In those places where the loads are maximum, it is forbidden to fix the overlap both when knitting, and in the case of using welding.
Connection of rods by welding
Reinforcement overlap using welding is allowed only with rods of grades A400C and A500C. Reinforcement of this class is considered to be weldable. But the cost of such rods is quite high. The most common class is A400. But its use is unacceptable, since when it is heated, the strength and resistance to corrosion are noticeably reduced.
It is forbidden to weld places where there is an overlap of reinforcement, regardless of the class of the latter. There is a possibility of rupture of the rods when exposed to large loads. So say foreign sources. The Russian rules allow the use of electric arc welding of these places, but the size of the diameters should not exceed 2.5 cm.
It is forbidden to connect reinforcement in places of maximum stress of the rods and areas of application of (concentrated) load on them
The length of welds and reinforcement classes are directly related. We use electrodes with a cross section of 4-5 mm. The length of the overlap during welding is less than 10 diameters of the rods used, which meets the requirements of the regulatory GOSTs 14098 and 10922.
Installation of an armored belt without the use of welding
When carrying out the installation of overlapping joints during knitting, rods of the most popular brand - A400 AIII are used. The places where the overlap is made are tied with a knitting wire. SNiP impose special requirements when choosing this method of ligament.
How many options are there for fixing rods without welding?
Rebar connection:
- overlap of end rods;
- overlap of rods with straight ends with welding of transverse rods;
- with curved ends.
If the rods have a smooth profile, only the 2nd or 3rd options can be used.
Reinforcement connection should not be placed in places of concentrated load application and places of greatest stress
Essential connection requirements
During the knitting of joints by the overlap method without the use of welding, the rules determine some parameters:
- Lining length.
- Features of the location of nodes in the structure.
- The location of the overlaps in relation to each other.
As already mentioned, it is forbidden to place overlapped reinforcement in places of highest load and maximum stress. They should be located in those places of the reinforced concrete product where there is no load, or it is minimal. If there is no such technological possibility, the size of the connection is selected from the calculation - 90 sections (diameters) of the joining rods.
Technical standards clearly regulate what the dimensions of such connections should be. However, their value may depend not only on the cross section. It is also affected by the following criteria:
- degree of load;
- brand of concrete used;
- reinforcement class;
- the location of the connection nodes in the structure;
- place of application of reinforced concrete products.
In cases where a binding wire is used, the distance between the rods is often taken to be zero.
The fundamental condition for choosing the length of the overlap is the diameter of the reinforcement.
The following table can be used to conveniently calculate the dimensions of the joining of rods when knitting without using the welding method. As a rule, their size is adjusted to 30 times the cross section of the reinforcement used.
| Reinforcement section, cm | Overlap size | |
| In centimeters | In millimeters | |
| 1 | 30 | 300 |
| 1,2 | 31,6 | 380 |
| 1,6 | 30 | 480 |
| 1,8 | 32,2 | 580 |
| 2,2 | 30,9 | 680 |
| 2,5 | 30,4 | 760 |
| 2,8 | 30,7 | 860 |
| 3,2 | 30 | 960 |
| 3,6 | 30,3 | 1090 |
There are also minimized values for overlapping bars. They are assigned based on the strength of concrete and the degree of pressure.
The distance between the reinforcing bars, which are overlapped, in the horizontal and vertical direction must be from 25 mm and above
In the compressed zone of concrete:
| Reinforcement cross section (class A400), cm | Concrete class (strength) | |||
| IN 20 | B/25 | B/30 | B/35 | |
| Concrete grade | ||||
| M/250 | M/350 | M/400 | M/450 | |
| Overlap size (in centimeters) | ||||
| 1 | 35,5 | 30,5 | 28 | 25 |
| 1,2 | 43 | 36,5 | 33,5 | 29,5 |
| 1,6 | 57 | 49 | 44,5 | 39,5 |
| 1,8 | 64 | 55 | 50 | 44,5 |
| 2,2 | 78,5 | 67 | 56 | 54,5 |
| 2,5 | 89 | 76,5 | 69,5 | 61,5 |
| 2,8 | 99,5 | 85,5 | 78 | 69 |
| 3,2 | 114 | 97,5 | 89 | 79 |
| 3,6 | 142 | 122 | 115,5 | 98,5 |