Make an opening in the concrete ceiling. Basic rules for constructing monolithic floors

The most reliable (but not always appropriate) option for interfloor slabs is a monolithic slab. It is made of concrete and reinforcement. Read about the rules for installing monolithic floors in this article. Analysis of the characteristics of types and applications, installation of monolithic floors.

In what cases is it necessary to install monolithic floors?

Monolithic reinforced concrete flooring is the most reliable, but also the most expensive of all existing options. Therefore, it is necessary to determine the criteria for the feasibility of its design. In what cases is it advisable to install monolithic floors?

1. Impossibility of delivery/installation of prefabricated reinforced concrete slabs. Subject to conscious refusal of other options (wooden, lightweight Terriva, etc.).
2. Complex configuration in plan with an “unfortunate” location interior walls. It, in turn, does not allow decomposition sufficient quantity serial floor slabs. That is, it is required a large number of monolithic areas. Costs for crane, and the formwork is not rational. In this case, it is better to immediately move on to the monolith.
3. Unfavorable operating conditions. Very heavy loads, extremely high values humidity that cannot be completely solved by waterproofing (car washes, swimming pools, etc.). Modern stoves Floors are usually pre-stressed. Tensile steel cables are used as reinforcement. Their cross section is very high strength The stretch is very small. Such slabs are extremely vulnerable to corrosion processes and are characterized by a brittle rather than ductile nature of destruction.
4. Combining overlap functions with function monolithic belt. Supporting precast concrete slabs directly on lightweight block masonry is generally not permitted. A monolithic belt is required. In cases where the cost of the belt and prefabricated floor is identical to or exceeds the price of a monolith, it is advisable to focus on it. When resting it on masonry with a depth equal to the width of the belt, the installation of the latter is usually not required. An exception may be difficult soil conditions: type 2 subsidence, seismic activity, karst formation, etc.

Determining the required thickness of a monolithic floor

For bendable slab elements, decades of application experience reinforced concrete structures, the value was determined experimentally - the ratio of thickness to span. For floor slabs it is 1/30. That is, with a span of 6m optimal thickness will be 200mm, for 4.5mm - 150mm.

An underestimation or, conversely, an increase in the accepted thickness is possible based on the required loads on the floor. At low loads (this includes private construction), it is possible to reduce the thickness by 10-15%.

VAT of floors

For determining general principles When reinforcing a monolithic floor, it is necessary to understand the typology of its operation through analysis of the stress-strain state (SSS). The most convenient way to do this is with the help of modern software systems.

Let's consider two cases - free (hinged) support of the slab on the wall, and pinched one. Slab thickness 150mm, load 600kg/m2, slab size 4.5x4.5m.

Deflection under the same conditions for a clamped slab (left) and a hinged slab (right).

The difference is in the moments of Mx.

The difference is in Mu's moments.

The difference is in the selection of upper reinforcement according to X.

The difference is in the selection of upper reinforcement according to U.

The difference is in the selection of lower reinforcement according to X.

The difference is in the selection of lower reinforcement according to U.

Boundary conditions (nature of support) are modeled by imposing the corresponding connections at the support nodes (marked in blue). For hinged support, linear movements are prohibited; for pinching, rotation is also prohibited.

As can be seen from the diagrams, when pinched, the work of the supporting section and middle region slabs are significantly different. IN real life any reinforced concrete (prefabricated or monolithic) is at least partially clamped in the body of the masonry. This nuance is important when determining the nature of the reinforcement of the structure.

Reinforcement of a monolithic floor. Longitudinal and transverse reinforcement

Concrete works great in compression. The reinforcement is tensile. Combining these two elements we get composite material. Reinforced concrete, which involves strengths each component. Obviously, the reinforcement must be installed in the tensile zone of concrete and absorb tensile forces. Such reinforcement is called longitudinal or working. It must have good adhesion to the concrete, otherwise it will not be able to transfer the load to it. For working reinforcement, periodic profile rods are used. They are designated A-III (according to the old GOST) or A400 (according to the new one).

The distance between reinforcing bars is the reinforcement pitch. For floors it is usually taken equal to 150 or 200 mm.
In case of pinching, a supporting moment occurs in the support zone. It generates tensile force in the upper zone. Therefore, working reinforcement in monolithic floors is placed both in the upper and lower zones of concrete. Special attention should be paid to the lower reinforcement in the center of the slab, and the upper reinforcement at its edges. And also in the area of ​​support on internal, intermediate walls/columns, if any, this is where the greatest stresses arise.

To ensure the required position of the upper reinforcement during concreting, transverse reinforcement is used. It is located vertically. It can be in the form of supporting frames or specially bent parts. In lightly loaded slabs they perform a structural function. Under heavy loads, transverse reinforcement is involved in the work, preventing delamination (cracking of the slab).

In private construction, transverse reinforcement in floor slabs usually performs a purely structural function. The supporting shear force ("shear" force) is absorbed by the concrete. The exception is the presence of point supports - racks (columns). In this case, it will be necessary to calculate the transverse reinforcement in the support zone. Transverse reinforcement is usually provided with a smooth profile. It is designated A-I or A240.

To support the upper reinforcement during concreting, bent U-shaped parts are most widely used.

Pouring the floor with concrete.

Calculation of a monolithic floor example

Manual calculation of the required reinforcement is somewhat cumbersome. This is especially true for determining deflection taking into account crack opening. The standards allow the formation of a crack in a tensile concrete zone with a strictly regulated opening width. They are completely invisible to the eye, we are talking about fractions of a millimeter. It is easier to simulate several typical situations in a software package that performs calculations strictly in accordance with current building codes. How to calculate the installation of monolithic floors?

The following loads were taken into account in the calculation:

1. Self-weight of reinforced concrete with a calculated value of 2750 kg/m3 (with a standard weight of 2500 kg/m3).
2. The weight of the floor structure is 150 kg/m2.
3. The weight of the partitions (average) is 150 kg/m2.

General view of the calculation scheme.

Scheme of deformation of slabs under load.

Diagram of Mu moments.

Diagram of moments Mx.

Selection of upper reinforcement according to X.

Selection of upper reinforcement according to U.

Selection of lower reinforcement according to X.

Selection of lower reinforcement according to U.

The spans were assumed to be 4.5 and 6 m. The longitudinal reinforcement was specified:

• class A-III fittings,
• protective layer 20mm

Since the area of ​​support of the slab on the walls was not modeled, the results of selecting reinforcement in the outer plates can be ignored. This is a standard nuance of programs using the method finite elements for calculation.

Pay attention to the strict correspondence of the spikes in moment values ​​with the spikes of the required reinforcement.

Monolithic floor thickness

In accordance with the calculations performed, we can recommend, for the installation of monolithic floors, in private houses, a floor thickness of 150 mm, for spans up to 4.5 m and 200 mm up to 6 m. It is not advisable to exceed the span of 6m. The diameter of the reinforcement depends not only on the load and span, but also on the thickness of the slab. The often installed fittings with a diameter of 12 mm and a pitch of 200 mm will form a significant reserve. Usually you can get by with 8mm at 150mm pitches or 10mm at 200mm pitches. Even this reinforcement is unlikely to work to the limit. The payload is assumed to be 300 kg/m2 - in a home it can only be formed by a large closet completely filled with books. Actual load in residential buildings, as a rule, significantly less.

The total required amount of reinforcement can be easily determined based on the average reinforcement weight coefficient of 80 kg/m3. That is, to install a floor with an area of ​​50 m2 with a thickness of 20 cm (0.2 m), you will need 50 * 0.2 * 80 = 800 kg of reinforcement (approximately).

In the presence of concentrated or more significant loads and spans, the diameter and pitch of the reinforcement specified in this article cannot be used for constructing a monolithic floor. Calculations for the corresponding values ​​will be required.

Video: Basic rules for constructing monolithic floors

Monolithic floors

When redevelopment involves combining rooms vertically, as well as when installing communications between floors, it becomes necessary to construct and strengthen openings in the ceilings.

This type work is potentially dangerous for the structures of the house and living in it, therefore it requires development and approval project documentation based on a technical opinion on the possibility of redevelopment.

All work on dismantling and subsequent strengthening of the opening in the floor slab should only be carried out qualified specialists, having in construction.

How to make an opening in the ceiling?

When creating openings in floors, you should use special equipment that does not transmit shock vibrations and vibrations to the concrete. Using professional ones allows you to avoid the formation of cracks that occur when working with jackhammers and rotary hammers.

A universal joint cutter is usually used for cutting floors. This device consists of a frame with a trolley and a motor with a belt drive that rotates cutting disc at a given speed. It has variable cutting depth and does not need to be fixed on special guides. The joint cutting machine uses a disk coated with segments of industrial diamonds as a working surface.

Sometimes an opening in the ceiling is made in a different way - using large-diameter diamond core bits.

Typically, the dismantling process itself goes like this: the marked opening is removed in small parts, carefully lowering each with a winch and steel cable onto bags or tires. In some cases, the required area is removed entirely.

When making openings in floors, reinforcement with metal structures is required according to the design in order to compensate for the decrease in their strength and the increase in load on them.

Strengthening openings in ceilings

Before the beginning dismantling works on the ceiling it is unloaded with the help of temporary supporting supports.

When strengthening a small opening in a monolithic reinforced concrete slab, channel framing is used around the perimeter. The metal is welded to the reinforcement protruding from the ceiling and caulked with mortar.

When strengthening the opening big size metal supports are used, attached to the lower load-bearing walls (channels, I-beams or corners). This structure is mounted before cutting the hole. On brick walls, reinforcement beams are inserted at both ends into grooved grooves, and they are attached to monolithic ones with special locks. Interval between metal elements and the overlap is caulked with mortar.

If for some reason it is not possible to attach metal structures to the load-bearing walls, permanent columns are installed under the affected floor.

When reinforcing a small opening in panel slabs, a channel or other type of profile is brought in from below and connects the slab to the opening with intact slabs. Another metal jumper is placed on top and pulled together with pins from the bottom.

Example of an opening in a ceiling:

Depending on the layout of the house, the staircase may be located in a specially designated area staircase or directly in the residential and utility rooms of the house. If the staircase is located in a stairwell (Fig. 31), then the beams supporting landings, are usually installed after the walls are built. To do this, grooves or wide niches are left in the brick walls. When installing them in a wall, the ends of wooden beams are made with a bevel and wrapped in rolls waterproofing materials, for example, with roofing felt or coated with hot bitumen mastic. Waterproofing is treated only side surfaces beams, the end of the beam must be open and not touch the wall! Bevelling the beam is done to better release water vapor, and waterproofing the side surfaces is done to prevent the beam from getting wet from the walls. To level the beams to the design level, wooden leveling pads can be installed under their ends. They will also prevent the wood of the beams from collapsing, increasing the area of ​​their support on the wall. Leveling pads are completely covered with antiseptics, for example, they are pre-bathed in a bitumen primer - a mixture of molten bitumen and diesel fuel. After installing and aligning the beams to the design position, the niches (or grooves) are filled with bricks and rubbed with mortar. The lateral waterproofing of the beam should protrude from the plane of the wall; it is better to trim it slightly than to allow the end of the beams to rot due to contact of unprotected wood with brick walls.

Rice. 31. Installation of wooden beams in a staircase made of brick or other piece wall materials

If the opening is located in the center of the room, there is a need to secure the hanging ends of the cut beams framing the opening with other load-bearing beams ceilings Cropped wooden beams secured with two short paired cross beams. Cross beams must have a thickness and height equal to the corresponding parameters of the main beams, and are attached to them with bolts and angles or special metal profiles. The beams framing the staircase opening are installed in pairs, short beams forming the opening are attached to them, and cut floor beams are already attached to them (Fig. 32). On one side of the staircase opening, it is allowed to cut no more than two floor beams.

rice. 32. Construction of a staircase in wooden floor

If the opening is located in the ceiling next to stone wall, the transverse beams are embedded at one end into the wall. Beam support unit brick wall is solved similarly to the support node for a platform beam. Seat subsequently plastered.

Unlike an opening in a wooden ceiling, which can be cut into an already finished design, the opening in the floor made of reinforced concrete slabs must be left in advance, even during the process of manufacturing this floor.

The contour of the opening is framed by steel profiles: channels, I-beams or a structure made of corners. To create monolithic sections formed on one or both sides of the contour of the staircase opening, steel beams are placed along the floor slabs, similar to beams in a wooden floor. They are supported on opposing walls, and two transverse beams are inserted between them, forming an opening (Fig. 33). The steel beams are fastened to each other by welding. The steel frame thus obtained rests on opposite walls in the same way as all other floor slabs. An opening for the stairs is left inside this frame, and reinforced monolithic sections are made along the edges. The direction of the shelves of the longitudinal beam profiles is best done inward monolithic area, this simplifies the production of a concrete monolith. The location of the shelves of the cross beam profiles does not matter, but when decorating an opening with wood, sometimes it is better to direct them inside the staircase opening.

rice. 33. Construction of a staircase opening in a ceiling made of reinforced concrete slabs

The entire steel frame needs to be raised 20–30 mm relative to the lower plane of the floor slabs, then when constructing a monolithic section, cement laitance will flow under the profiles and hide the metal. To prevent this layer of cement from subsequently falling off and exposing the steel profile, wire shorts must be welded onto its lower flange and with their help, the plaster mesh must be secured to the beams.

Sometimes to save money steel profiles Instead of a structure with longitudinal load-bearing beams made of channels or I-beams, a beamless design is used. In this design, there are no longitudinal beams, and the opening is formed with steel corners, resting their shelves on adjacent floor slabs. This design partially transfers the weight of the monolithic section and the stairs to the adjacent floor slabs. After checking by calculation bearing capacity floor slabs, this design can be used in small monolithic areas. For wide devices stairwells It's better not to do it.

Reinforcement of monolithic sections is assigned according to the project or calculation. The lower formwork panel is made on the ground and pulled with ropes to the installation site. Where it is attached with wire twists to the beams carrying the formwork. Boards mounted on edge or thick reinforcing bars or crowbars can be used as beams. Wire loops are thrown over these beams, inserted between the branches of the mounting wire and the wire begins to twist. Thus, the formwork panel is attracted and pressed against the adjacent floor slabs. To prevent laitance from leaking out, the shield is covered plastic film or glassine. Carry out reinforcement of monolithic sections and filling concrete mixture. The twisted wires remain forever in the body of the concrete. When stripping, their ends protruding from the monolith are cut down or trimmed with a grinder.

For an opening in the ceiling under the stairs, even during the construction of the building, a gap in width is left standard plate made of reinforced concrete. Since the opening for the stairs on stringers and bowstrings usually occupies a much smaller area than a standard reinforced concrete slab, the space remaining after the opening is equipped is subsequently filled with concrete.

Installation of metal beams for the opening in the ceiling under the stairs

Arranging an opening under the stairs, along the slabs interfloor covering steel beams are placed. They are installed in the same way as when making a staircase opening in a wooden ceiling. Metal beams welded together. The metal frame thus obtained should rest on the walls of the building, just like reinforced concrete slabs interfloor covering. When the frame of the profiles is installed in its place, they begin to reinforce the areas to be poured with a monolith. The lower surface of the formwork is formed by a shield, which is made on the floor of the lower floor and lifted to the installation site using ropes. Already at the installation site, this shield is attached to the beams supporting the formwork. Such beams can be made from boards placed on edge, or from thick reinforcing bars.

Wire loops are placed on the beams, and mounts are inserted between their branches. After this, they begin to twist the wire, thereby attracting and pressing the formwork panel to the adjacent floor slabs. To prevent the possibility of laitance leakage, the shield is covered with plastic film. When the formwork is secured, they begin to reinforce and pour the concrete mixture. The assembly wire twists of the formwork are left inside the concrete monolith.

Installation of a metal frame for an opening in the ceiling under the stairs

When making a metal frame from profiles, it is recommended that their “horns”, that is, the shelves of the profiles lying lengthwise, be placed in the middle of the ceiling. Then it will be easier to produce a monolithic section. For transversely lying profiles, it does not matter where the horns are directed. But if the opening in the ceiling under the stairs is planned to be finished with wood or other material, then it is also better to direct these horns inside the areas being poured with concrete.

To hide the metal frame, it must be raised relative to the bottom surface of the floor slabs by twenty to thirty millimeters. Then the cement poured into the formwork will flow under metallic profile, closing the steel frame. To ensure that the cement does not begin to fall off over time, it is recommended to weld several short pieces of metal to the bottom flange of the profile and attach a special plaster mesh to them.

Construction of a beamless structure for an opening in the ceiling under a staircase

There are also more economical option devices for staircase openings, when instead of a welded structure, a so-called beamless structure is used. It does not include longitudinal beams, and the opening itself is framed with metal corners. These corners rest on the edges of adjacent floor slabs with their shelves. In this case, the entire weight of the monolithic section and the staircase itself will be transferred directly to the interfloor slabs. This method This method is only suitable for fairly narrow staircases, and this method is not suitable for constructing a wide staircase opening.

The technique for making openings in the ceiling along the stairs on bowstrings and on stringers is almost identical. That is, the openings themselves, the options for supporting the bowstring on the lower and upper beams are the same as for stairs on stringers.