Reinforced concrete has found widespread use in modern construction. The material is characterized by high strength, resistance to external influences, durability. When erecting buildings from piece materials (brick, cinder block, aerated concrete blocks), it becomes necessary to block door and window openings. For these purposes, a special type of reinforced concrete products is used - lintels.
Destination
Lintels are reinforced concrete beams resting on the masonry walls and transferring to them the load of the masonry and ceilings overlying the opening. Structurally, the lintel consists of a reinforcing cage and heavy concrete. In practice, prefabricated reinforced concrete products of factory production or monolithic, manufactured on site are used. In this case, the brands of the products used and the area of their support should be determined by calculation, taking into account the collected loads.
Standards
The main parameters of prefabricated products are determined by GOST "Reinforced concrete lintels for buildings with brick walls". This standard defines the classification, brands, sizes and characteristics. Jumperssubdivided into:
- PB - bar, having a width of up to 250 mm.
- PP - plate, with a base width of 250 mm.
- PG - beam, a quarter is cut in the section.
- PF - facade, for openings with a quarter with a layer of the protruding part of the masonry from 250 mm.
For the production of standard products, a set of standard working documentation has been developed - 1.038.1-1, including working drawings.
Design
The reinforcement frame of the lintel ensures its strength. It consists of longitudinal and transverse reinforcement. The norms provide for the use of both prestressed and non-prestressed reinforcement. The diameter and pitch of the rods is determined by calculation or by standard projects.
When pouring lintels, heavy concrete with a density of 2200-2500 kg/m3 is used. The required strength of concrete is determined by calculation, the brand for water permeability and frost resistance - taking into account the environment.
Slinging holes are made in the structure for lifting or singing, as required by GOST. Reinforced concrete lintels are allowed to be made with reinforcement outlets for construction in seismically hazardous zones.
The range of commercially available jumpers is shown in the table of GOST 948-84. The standard provides for PB bar lintels with a length of 1030 to 5950 mm with prestressed or non-stressed reinforcement. Their section is divided into 10 groups - from 125 x 65 (h) mm to 250 x 290 (h) mm.
PP lintels are available in lengths from 1160 to 2980 mm with prestressed or non-stressedfittings. Their section is represented by 10 groups - from 380 x 65 (h) mm to 510 x 220 (h) mm.
Beam lintels with a quarter of type PG are provided only with non-tensioned reinforcement from 1550 to 5960 mm long. 8 section options are possible - from 250 x 290 (h) mm to 510 x 440 (h) mm.
Front lintels PF are produced in length from 770 mm to 4280 mm with a cross section of eight groups - from 90 x 90 (h) mm to 290 x 90 (h) mm.
Designation
Grades of serial products consist of two or three groups and fully characterize reinforced concrete lintels: dimensions, load-bearing capacity, section, etc. length expressed in tens of centimeters. For example, 8PB25 - bar lintel, section No. 8 from the GOST 948-84 table, length - 2460 mm.
The second group shows the average allowable load (kN/m), reinforcement class. For example, 71-AtV: load 70, 61 kN/m and AT-V reinforcement.
The third marking group contains information about the presence of rebar outlets, embedded parts, sling loops, as well as various characteristics depending on external conditions.
Additional marking uses symbols:
- "a" - anchor outlets for installation of balcony slabs;
- "p" - sling loops;
- "С" - for seismically dangerous areas over 7 points;
- "P" - high-density concrete, or "O" - extra-dense.
Full marking of reinforced concrete lintels may look like this:10PB21-27-ap - bar lintel size 2070 mm, section number 10, allowable load 27, 26 kN/m, with sling loops and anchor outlets of reinforcement for mounting balcony slabs.
Insulation
The outer walls of the house must provide the level of thermal protection provided for by the norms. This level is characterized by the value of heat transfer resistance. The larger it is, the higher the thermal protection of the building and, accordingly, the lower the heat consumption and heating fees. Unfortunately, reinforced concrete is an excellent conductor of heat. The lintels form heat-conducting inclusions on the surface of the walls - cold bridges. They lead not only to additional heat losses, but also in some cases to dew. To avoid this negative effect, it is necessary to insulate the jumpers. If the building has additional insulation according to the ventilated facade system or according to the "wet technology" - no questions arise. But if the outer walls do not require additional insulation, then the jumpers are installed indented from the outer surface of the wall. An insert made of effective insulation (for example, stone wool) is mounted in this indentation and covered with plaster or other finishing material under the plane of the wall.
Jump brand selection
For the correct choice of the jumper brand, in addition to the purpose, it is necessary to determine the bearing capacity. The permissible load of the product of each brand is indicated in GOST "Reinforced concrete lintels". After completingcollection of loads and having decided on the length of the span, you can use the tables of the 1.038.1-1 series, where each brand is compared: the estimated span, the length of support and loads. When choosing reinforced concrete lintels, dimensions and sections, it is necessary to take into account the seismic hazard of the construction area.
Jumper calculation
The lintel is the supporting structure of the building, and its calculation should be carried out by a specialist with the appropriate education and work experience. However, for small projects, you can choose reinforced concrete lintels yourself. The dimensions of the products are determined, guided by the SNiP "Stone structures". It is necessary to calculate the height of the masonry above the lintel and compare it with the calculated span of the lintel. If the height of the masonry is greater than the calculated span, then the carrier jumper is not needed. It is easy to explain this: at a certain height, the wall above the opening has enough of its own bearing capacity, then it does not need the help of a jumper. Knowing the specific gravity of the masonry, it is easy to determine the brand of the jumper. The specific gravity values of the most popular masonry materials are given below:
- brickwork - 1400-1900 kg/m3;
- foam concrete blocks - 900-1400 kg/m3;
- aerated concrete blocks - 400-1200 kg/m3
It is necessary to take into account the density of the masonry material of a particular manufacturer.
Installation of jumpers
Non-bearing lintels of door and window openings with a span of up to two meters are allowedlay manually, over two meters - using lifting mechanisms. The support depth is taken according to the series, but usually not less than 200 mm in partitions and not less than 250 mm in walls. Support pads are checked for level.
Reinforced concrete lintels for brick walls should be installed on masonry, additionally reinforced with meshes. To fill the entire thickness of the walls, the jumpers are installed in a package of several pieces. End products should not protrude beyond the plane of the walls. When installing jumpers, their design orientation should be observed. It is impossible to adjust the jumpers along the length by trimming, because their reinforcement is uneven and is designed for the span length indicated in the product brand.
Self-made jumpers
In the case when it is not possible to deliver finished reinforced concrete products to the construction site, they can be made directly on site. First, it is necessary to make formwork of the appropriate size from lumber. It is convenient if the formwork is designed for the manufacture of several products at the same time. A frame is made of reinforcement, the longitudinal one is used with a diameter of 12-14 mm, the transverse one is 4-6 mm. For longitudinal reinforcement, the rods are installed in two levels, the transverse reinforcement is installed in increments of 3/4 of the height of the product. At a length of 1/6 of the span length from the support zone, the pitch of the transverse reinforcement is reduced. For large-sized jumpers, it is necessary to lay mounting loops. In the manufacture of the frame, welding or knitting wire is used.
The assembled frame is installed in the formwork. To create a protectivethe reinforcement layer is lifted using plastic supports such as "rack" or "high chair". Then the formwork is poured with heavy concrete, followed by vibration compaction. After pouring the lintels, reinforced concrete doors and windows must be cured for at least 24 days.
Production of lintels in span
In private housing construction, reinforced concrete lintels for openings can often be made on site. This method is not very technologically advanced, but it eliminates the need for lifting and installation, which is very convenient in the absence of lifting mechanisms. When casting directly on the wall, it is possible to make arched reinforced concrete lintels.
The dimensions of this design are determined by an individual project. Before casting the lintel, the masonry of the walls is brought to the required mark. The support platforms are checked for level and leveled with concrete footing. Formwork is mounted from lumber in the opening. The lower part of the formwork is made of a fairly thick board and reinforced with props. The step and section of the supports is selected in accordance with the span length and the weight of the lintel, assuming a density of heavy concrete of 2500 kg/m3. The supports are fixed to each other and to the wall structure. Reinforced concrete window lintels require a quarter in the formwork structure to install a window block.
Often when building a house from blocks, masonry material suppliers produce special U-shaped blocks. They are placed on a support in the opening and performfixed formwork functions. Such elements have the thickness of ordinary blocks in the masonry and do not highlight the jumper on the surface of the wall.
The reinforcing cage is mounted in the formwork, mounting loops are not needed in this case. After that, the formwork is poured with heavy concrete, compacting it with vibrotamping. The filled jumper is left for 24 days, after which the formwork is dismantled.
