Reinforced concrete has found widespread application 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), there is a need to overlap door and window openings. For these purposes, a special type of reinforced concrete products is used - jumpers.
Appointment
The lintels are reinforced concrete beams resting on the masonry of the walls and transferring to them the load of the masonry and ceilings overlying the opening. Structurally, the jumper consists of a reinforcing cage and heavy concrete. In practice, prefabricated reinforced concrete products of factory production or monolithic, made on the spot, are used. In this case, the brands of 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 classification, grades, sizes and specifications. Jumpers are divided into:
- PB - squared, having a width of up to 250 mm.
- PP - plate, with a base width of 250 mm.
- PG - beam, a quarter is cut in cross 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 is developed - 1.038.1-1, including working drawings.
Design
The reinforcing frame of the jumper provides its strength. It consists of longitudinal and transverse reinforcement. The standards provide for the use of both prestressed and non-prestressed fittings. The diameter and pitch of the rods is determined by calculation or by standard designs.
When pouring lintels, use heavy concrete with a density of 2200-2500 kg / m3. The necessary strength of concrete is determined by calculation, the brand for water permeability and frost resistance - taking into account the environment.
For lifting, the construction made sling holes or sang, as required by GOST. Reinforced concrete lintels may be made with releases of reinforcement for construction in earthquake-prone zones.
The nomenclature of commercially available jumpers is reflected in the table of GOST 948-84. The standard provides for PB jumpers with a length of 1030 to 5950 mm with tensioned or non-tensioned reinforcement. Their cross section is divided into 10 groups - from 125 x 65 (h) mm to 250 x 290 (h) mm.
PP plate jumpers are available in lengths from 1160 to 2980 mm with tensile or non-tensile fittings. Their cross section is represented by 10 groups - from 380 x 65 (h) mm to 510 x 220 (h) mm.
Jumper beams with a quarter type PG are provided only with non-tensile reinforcement with a length of 1550 to 5960 mm. There are 8 possible cross-sections - from 250 x 290 (h) mm to 510 x 440 (h) mm.
Facade jumpers PF are produced in lengths 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
Brands of serial products consist of two or three groups and fully characterize reinforced concrete lintels: dimensions, bearing capacity, cross section, etc.
The first group begins with a figure defining the cross section (according to the table), the letter name and length, expressed in tens of centimeters. For example, 8PB25 - a jumper block, section No. 8 from the table of GOST 948-84, length - 2460 mm.
The second group indicates the average permissible load (kN / m), the class of reinforcement.
For example, 71-AtV: load 70.61 kN / m and AT-V fittings.
The third group of marking contains information about the availability of releases of reinforcement, embedded parts, sling loops, as well as various characteristics depending on external conditions.
In the additional marking, the following symbols are used:
- "a" - anchor releases of fittings for the installation of balcony slabs;
- "p" - sling loops;
- “C” - for seismically hazardous areas over 7 points;
- “P” - concrete of increased density, or “O” - extra-dense.
Full marking of reinforced concrete lintels may look like this:
10PB21-27-up - a bar bridge with a size of 2070 mm, cross-section number 10, permissible load 27.26 kN / m, with sling loops and anchor outlets for the installation of balcony slabs.
Warming
The external walls of the house must provide the level of thermal protection provided for by the standards. 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 consumption of heat and heating fees. Unfortunately, reinforced concrete is an excellent conductor of heat. Lintels form heat-conducting inclusions on the surface of the walls - cold bridges. They lead not only to additional heat loss, but also in some cases to dew. To avoid this negative effect, it is necessary to insulate jumpers. If the building has additional insulation using a ventilated facade system or wet technology, no questions arise. But if the outer walls do not require additional insulation, then jumpers are installed indented from the outer surface of the wall. In this indentation, an insert from an effective insulation (for example, stone wool) is mounted and closed with plaster or other finishing material under the wall plane.
Jumper brand selection
For the correct choice of the brand of the jumper, in addition to the destination, it is necessary to determine the bearing capacity. The permissible load of the product of each brand is specified in the GOST "Reinforced concrete lintels". After collecting the loads and determining the length of the span, you can use the tables of the series 1.038.1-1, where each brand is associated with: estimated span, bearing length and load. When choosing reinforced concrete lintels, sizes and sections, it is necessary to take into account the seismic hazard of the construction area.
Jumper calculation
The jumper is the supporting structure of the building, and it should be carried out by a specialist with the appropriate education and experience. Nevertheless, for small projects, you can independently choose reinforced concrete lintels. The dimensions of the products are determined, guided by SNiP "Stone structures". It is necessary to calculate the height of the masonry above the jumper and compare it with the calculated span of the jumper. If the masonry height is greater than the calculated span, then the carrier jumper is not needed. It is easy to explain: at a certain height, the wall above the opening has its own bearing capacity, then the jumper does not need help. Knowing the specific weight 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.
Jumper mounting
Bearing lintels of door and window openings with a span of up to two meters can be laid manually, more than two meters - using lifting mechanisms. Depth of support is taken in series, but usually not less than 200 mm in partitions and not less than 250 mm in walls. Reference sites are checked by level.
Reinforced concrete lintels for brick walls should be installed on masonry, additionally reinforced with nets. To fill the entire thickness of the walls, the jumpers are installed in a package of several pieces. Extreme products should not protrude beyond the plane of the walls. When installing jumpers, their design orientation should be observed. You can not adjust the jumpers along the length of the trim, because their reinforcement is uneven and is designed for the span indicated in the product brand.
Self-made jumper
In the case where it is not possible to deliver finished reinforced concrete products to a 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 to produce several products at the same time. A frame is made from the reinforcement, longitudinal is used with a diameter of 12-14 mm, transverse - 4-6 mm. For longitudinal reinforcement, the rods are installed in two levels, the transverse reinforcement is set in increments of 3/4 of the height of the product. At a length of 1/6 of the span from the bearing area, the pitch of the transverse reinforcement is reduced. For large jumpers, it is necessary to lay mounting loops. In the manufacture of the frame using welding or knitting wire.
The assembled frame is installed in the formwork. To create a protective layer of concrete, the reinforcement is lifted using plastic supports of the “rack” or “stool” type. Then the formwork is poured with heavy concrete, followed by compaction by vibration. After filling the lintels, the reinforced concrete door and window must be kept for at least 24 days to harden.
Span jumper production
In private housing construction, quite often reinforced concrete lintels for openings can be made on site. This method is not very technological, but it eliminates the need for lifting and installation, which is very convenient in the absence of load-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. Reference sites are checked according to the level and aligned with a concrete block. From lumber in the opening, formwork is mounted. The lower part of the formwork is made of a sufficiently thick board and is reinforced by props. The pitch and cross-section of the supports is selected in accordance with the span and weight of the bridge, assuming the density of heavy concrete is 2500 kg / m3. The supports are fixed to each other and to the wall structure. Reinforced concrete window lintels require a quarter of the formwork design for installing the window unit.
Often when building a house out of blocks, suppliers of masonry material produce special U-shaped blocks. They are laid on a support in the opening and perform the functions of a fixed formwork. Such elements have the thickness of ordinary blocks in the masonry and do not emit a jumper on the smooth surface of the wall.
A reinforcing cage is mounted in the formwork, mounting loops in this case are not needed. Then the formwork is poured with heavy concrete, compacting it with a ram rammer. The fused jumper is left for 24 days, after which the formwork is dismantled.