Framing openings in a monolithic ceiling. Openings in floors
Stairs are an important structural element of a multi-storey building. It, of course, should be as reliable and safe as possible during operation. To increase its load strength, it is imperative to reinforce the stairwell (march). This will increase the stability of the staircase itself, protect the structure from deflection and stretching, and prevent chips and cracks.
Before starting work, you need to pay attention to the type of design of the future staircase and take into account the level of loads that it will receive daily. Under intense loads, the structure will begin to experience pressure from above, where the concrete will begin to compress, while tension will occur from below, which will lead to a loss of concrete strength. Therefore, reinforcement of the lower part of the march is especially important. When reinforcing a stairway, staple-bending products and flat reinforcing cages are used, less often - reinforcing meshes, but their effectiveness in these structures is practically zero. When assembling frames and laying reinforcement, additional elements are used - channels laid on the sides of the formwork, and reinforcing corners for reinforcing the edges of the steps.
It is necessary to reinforce the stairway in the direction from top to bottom, because the main pressure is from above, which means that you need to strengthen the stairs from the back side. The dimensions of the flat frames of both the upper and lower must match. Simple monolithic stairs that do not have platforms in their design do not need to reinforce the lower part, only the top will be enough. The fittings will increase the rigidity of the stairs and protect it from possible shocks and damage. Double-flight stairs must be securely fastened to the walls, since their monolithic platforms take on huge loads from the weight of the same staircase. For this, special reinforced concrete crowns are used. If the house is being built using the technology of monolithic casting, the construction and reinforcement of the stairs should be taken up in the process of building the walls themselves.
To calculate the amount of reinforcement required, you need to compare such indicators as the length of the flight of stairs, the distance between the bars, the minimum height of the work plates, the diameter of the reinforcing bars. It is worth entrusting the calculations to professionals, people who know this business. Even if you decide to do the reinforcement yourself, you cannot do without a well-designed reinforcement project-scheme.
The technology of reinforcing openings in monolithic reinforced concrete slabs is rather sparingly covered in domestic regulatory documentation. In the design manual "Reinforcement of elements of monolithic reinforced concrete buildings" (Moscow, 2007), in the section Reinforcement in the places of holes, it is said: Holes of significant dimensions (more than or equal to 300 mm) in monolithic reinforced concrete walls and slabs should be bordered with additional reinforcement with a cross section not less than the cross section of the working reinforcement (of the same direction), which is required by the calculation of the slab as solid. Holes up to 300 mm are not bordered with special rods. Knitted working and distribution fittings thicken around such holes - two extreme rods are placed with an interval of 50 mm. When reinforcing the slab with welded meshes, it is recommended to cut holes up to 300 mm in the reinforcement in place, while it is advisable to bend the cut rods into the body of the slab.
In the Guidelines for the design of reinforced concrete structures with beamless ceilings (Moscow, 1979) in paragraph 3.13. says: Single holes with a maximum size of up to 700 mm are arranged in the ceiling without local thickening of the slab. The weakening of the slab by the hole should be compensated by additional reinforcement laid along the edges of the hole. If concentrated forces are applied to the edge of the slab adjacent to the hole, and also in cases where the prefabricated slab is significantly weakened by holes (by 50% or more), it is recommended to reinforce the slabs along the edges of the holes with rigid reinforcement or provide for thickening of the slabs, or border the holes with ribs. The rigidity of the bordering ribs must be no less than the rigidity of the section of the plate section occupied by the hole. Thickening (strengthening) of the part of the heel adjacent to the hole is recommended to be performed under the condition of equality of the stiffness of the section weakened by the hole, and without taking into account the weakening. With rectangular holes, at the corners of these holes in the slab, 2 - 4 reinforcing bars with a diameter of 10 - 14 mm should be laid, placing them in plan at an angle of 45 ° to the sides of the hole.
The requirement for indirect reinforcement of opening corners to absorb longitudinal loads in slabs and prevent cracking is contained in the Reinforced Concrete Design Guide (S. N. Sinha Handbook of Reinforced Concrete Design, 2008). Round openings in slabs are also subject to indirect reinforcement.
Foreign regulatory documents (Swedish building codes VVK 04, Polish building codes PN-B-03264) provide the following requirements for the reinforcement of holes and openings in monolithic reinforced concrete slabs:
Holes and openings with a diameter (side) of 150 mm or less do not require reinforcement. Holes from 150 to 450 mm require reinforcement with U-shaped clamps (transverse reinforcement) along the perimeter of the opening, connecting two layers of reinforcement. In foreign sources, the length of the clamps is defined as three plate thicknesses, and in domestic sources as two plate thicknesses (SP 63.13330.2012 Concrete and reinforced concrete structures. Basic provisions. Updated edition of SNiP 52-01-2003, clause 10.4.9). Holes (openings with a diameter) (linear) 450 mm to 900 mm - require framing the opening with double thickened reinforcement around the perimeter and laying indirect corner double reinforcement. Holes or openings with a side of more than 90 cm require reinforcement of the slab with internal hidden beams or retaining beams.
According to various sources, the maximum opening size can be up to 1/4 of the largest side of the slab, or no more than 1/3 of the smallest side of the slab. Minimum allowable thickness
During redevelopment with the combination of premises vertically, as well as during the installation of communications between floors, it becomes necessary to arrange and strengthen openings in the ceilings.
This type of work is potentially dangerous for the structures of the house and living in it, therefore it requires the development and approval of 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 be carried out only by qualified specialists who are in construction.
How to make an opening in the ceiling?
When organizing openings in ceilings, special equipment should be used that does not transmit shock vibrations and vibrations to concrete. The use of professional ones allows you to avoid the formation of cracks that occur when working with jackhammers and perforators.
For cutting floors, a universal joint cutter is usually used. This machine consists of a frame with a trolley and a belt-driven motor that rotates the cutting disc at a given speed. It has a variable cutting depth and does not need to be fixed on special guides. As a working surface in the joint cutter, a disk coated with segments of technical diamonds is used.
Sometimes an opening in the ceiling is made in a different way - it is done with the help of large-diameter diamond crowns.
Usually the dismantling process itself goes like this: the marked opening is removed in small parts, carefully lowering each winch with a steel cable onto bags or tires. In some cases, the required area is removed entirely.
When making openings in ceilings, it is necessary to reinforce them with metal structures according to the project in order to compensate for the decrease in their strength and the increase in the load on them.
Reinforcement of openings in ceilings
Before starting dismantling work on the floor, it is unloaded with the help of temporary supporting supports.
When strengthening a small opening in a monolithic reinforced concrete slab, a channel frame around the perimeter is used. The metal is welded to the reinforcement protruding from the ceiling and caulked with mortar.
When strengthening a large opening, metal supports are used that are attached to the lower load-bearing walls (channels, I-beams or corners). This design is mounted before cutting the hole. On the brick walls, the reinforcement beams are wound with two ends into the slotted grooves, and they are attached to the monolithic ones with special locks. The gap between the metal elements and the overlap is minted with a solution.
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 strengthening a small opening in panel ceilings, 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 laid on top and pulled together with studs from the bottom.
An example of an opening in a ceiling:
Passage of ventilation ducts and communications through reinforced concrete floor slabs.
GOST 9561-91. Reinforced concrete multi-hollow floor slabs for buildings and structuresclause 1.2.9. In cases stipulated by the working drawings of a particular building (structure), the slabs may have embedded products, reinforcement outlets, local cuts, holes and other additional structural details.
A guide to the design of residential buildings.
Issue. 3
(to SNiP 2.08.01-85)
clause 6.15. In floor slabs, it is recommended to arrange channels for hidden electrical wiring. The diameter of the channels in solid slabs is recommended to be no more than 30 mm.
The sealing of through technological and communication holes in floor slabs is recommended to be provided with a mortar on expanding cement or gypsum. Moscow 1989 .
clause 6.6. In slabs with holes or cutouts for the passage of sanitary communications, the reinforcement of the grids crossing them is usually cut. To compensate for it, shortened rods or flat frames with a section equivalent in strength to the cut reinforcement should be installed along the contour of holes or cutouts.
The rods should be wound beyond the edge of the hole or cutout to a distance equal to 50 diameters, and if the hole is located near the support, to the edge of the grid above the support. If the position of the hole or cutout is asymmetrical with respect to the center of the slab, most of the compensating rods should be located: at the slabs supported along the contour, closer to the center, at the slabs supported on three sides; closer to the free edge of the slab (Fig. 16).
clause 6.7. If it is necessary to replace design reinforcement with reinforcement of a different diameter or class, it is not necessary to maintain the bar spacing specified by the project, it is only necessary to comply with the requirements of regulatory documents regarding the distance between the bars (see clause 6.4) and the adequacy of the strength of the replaced reinforcement. At the same time, the value of the design force in the replacement (n) reinforcement per 1 m of the slab section must be no less than in the replaced (b), i.e.
Rice. 16. Scheme of arrangement of compensating rods along the perimeter of cutouts (holes)
Schemes of input and operational quality control of construction and installation worksPart 1 Issue 2
Installation of prefabricated reinforced concrete structures of residential buildings, installation of light enclosing structures
Plates must have:
- steel embedded parts, reinforcement outlets and other structural elements intended for connection with adjacent building structures;
- channels for concealed electrical wiring, sockets for junction boxes and sockets, plastic boxes with anchors for attaching fixtures;
- openings and openings for the passage of engineering communications.
Design guide
Moscow, 2007
Reinforcement at holes
Holes of significant size (more than or equal to 300 mm) in monolithic reinforced concrete walls and slabs should be bordered with additional reinforcement with a cross section not less than the cross section of the working reinforcement (in the same direction), which is required by the calculation of the slab as solid (Fig. 3.26, a).
Holes up to 300 mm are not bordered with special rods.
Knitted working and distribution fittings thicken around such holes - the two extreme rods are placed with an interval of 50 mm (Fig. 3.26, b).
Figure 3.26 - Reinforcement of plates in the places of holes
A, b - holes, respectively, more than 300 and up to 300 mm (with working and distribution knitted fittings); 1 - reinforcement bars of the plate: 2 - special reinforcement bars bordering the hole
When reinforcing with welded meshes, holes up to 300 mm in the reinforcement are recommended to be cut in place, while it is advisable to bend the cut rods into the body of the slab.
Design Guide for Concrete and Reinforced Concrete Structures from Heavy Concrete(without prestressing)
Moscow, 1978
Holes in slabs
3.141. Large holes in reinforced concrete slabs, panels, etc. should be bordered by additional reinforcement with a cross section of at least the section of the working reinforcement (of the same direction), which is required by the calculation of the slab as a solid one (Fig. 108, a).
Holes up to 300 mm in size are not framed with special rods. The knitted working and distribution reinforcement of the slab thickens around such holes - two rods are placed with a gap of 50 mm (Fig. 108, b). When reinforcing the slab with welded meshes, it is recommended to cut such a hole in the reinforcement in place.
Holes (openings), if necessary by calculation, are framed with reinforced ribs. The dimensions and reinforcement of these ribs depend on the size, shape, location in the plan relative to the floor beams, the purpose of the opening, and in each individual case are decided by the designer based on the calculation.
In reinforcing drawings, special rods for reinforcing the slab within the size of the hole, with the exception of bordering ones, as a rule, are not given, and a note should be placed on the drawing: within the hole, the rods are cut in place and bent into the body of the slab.
When reinforcing the ceiling with welded meshes, holes up to 500×500 mm in size are not taken into account when laying out the meshes, and a note is given on the drawing: cut the hole in place.
For large mesh openings, they are laid out taking into account the holes, however, in the area of the opening, it is recommended to reinforce the slab with separate rods without violating the unification of the meshes.
Additional reinforcement bordering the holes must be inserted beyond the edges of the hole to a length not less than the overlap value lн in accordance with clause 2.46 of this Guide.
Rice. 108. Reinforcement of plates in places of holes
a - holes larger than 300 mm; b - holes up to 300 mm in size; 1 - reinforcement rods of the plate; 2 - fringing rods formed by thickening of the plate reinforcement; 3 - rods of special fittings bordering the hole
4.3. The outlines of reinforced concrete structures designed for the loads specified in clause 4.1 should be taken simple: without sharp changes in elevations, without breaks in elements and a sharp change in cross sections. In places of conjugation of structural elements (for example, a crossbar with a column), as well as a change in the sections of elements by more than 1.5 times, as a rule, haunches, rounding of incoming corners, etc. should be arranged. Holes are recommended to be arranged round, and if necessary, the device of a rectangular hole, its corners should be rounded.
Reinforced concrete spatial structures of roofs and ceilingsSP 52-117-2008
Part 1
Methods of calculation and design
6.5 Openings and openings
6.5.1 In thin-walled spatial structures, it is allowed to design holes and openings of various shapes within the distance between diaphragms or stiffeners, as well as a larger value, but with verification by calculation. Light openings can also be arranged using a difference in the surfaces of the coating or expansion of shells, folds or arches.
With large openings in the slab, it is recommended to arrange spacers and braces, which, together with the bordering ribs, form a frame or truss capable of absorbing normal or tangential forces, or only normal forces. It is possible to use special metal structural elements that provide strength and rigidity of reinforced concrete elements of shells with holes and support translucent panels.
6.5.2 Holes in the slab of shells and folds with a side size (or diameter) of not more than 15δ may be made without special thickening of the edge of the slab, but with the installation of structural reinforcement surrounding the hole with a diameter of at least 8 mm at δ > 30 mm.
Holes are recommended to be designed round, oval or polygonal with corners rounded with a radius r ≥ 2δ (Fig. 6.7).
1 - fittings; 2 - reinforcement joint with a bypass of 30d or an equal-strength welded joint
Figure 6.7 - Fringing reinforcement of holes
6.5.3 In the zone of slab openings larger than 15δ, the edges of the shelves and walls must have thickening and reinforcement, taken according to the calculation. The thickening must have a height ≥ 3δ, a width ≥ 2δ and an area of concrete and reinforcement not less than the area of concrete and reinforcement in the cross section of the cut out part of the slab. Holes arranged in tensioned shelves or walls must have a sufficient amount of reinforcement in the ribs to absorb the force that falls on the cut-out part of the shelf or wall.
Guidelines for the design of reinforced concrete structures with flat slabs
Moscow, 1979
1.10. When arranging openings or openings in ceilings to pass utilities, elevator shafts, stairwells, etc. they should be placed within the slab part of the floor. The device of openings within capitals, as a rule, is not recommended. If necessary, within the limits of the capital, it is allowed to arrange holes with a diameter of not more than 200 mm.
Between the capitals, in the above-column strip of a monolithic beamless ceiling, it is recommended to place holes so that they occupy no more than 0.5 of the width of this strip, i.e., no more than 0.5 of the width of the capital.
In prefabricated beamless ceilings, it is advisable to provide special slabs with holes, and do not install slabs in places where openings are formed. In the ceilings, in the areas adjacent to the openings, additional slabs and capitals can be used, and in case of emergency, semi-capitals. In some cases, when holes are formed, it is allowed to arrange monolithic sections of the floor.
The presence of holes and openings in the ceilings should be taken into account by the calculation.
Rice. 21. Examples of designing floor slabs in the places of holes
a - with single holes with dimensions up to 700 mm; b and c - when the plate is weakened by holes by 50% or more or with concentrated forces applied to the edges of the plate at the hole
3.11. The horizontal reinforcement of the walls of the glass of the capital must be continuous, closed along the inner and outer perimeters of the glass; the vertical reinforcement of these walls should be securely anchored in the capital plate and the bottom of the cup.
3.12. In order to reduce the opening of cracks at the contact of monolithic concrete with prefabricated concrete, it is recommended to install reinforced rods at the top of the interface between the capital and the column at the edge of the column (Fig. 15).
3.13. Single holes with a maximum size of up to 700 mm are arranged in the ceiling without local thickening of the slab (Fig. 21, a). The weakening of the slab by the hole should be compensated by additional reinforcement laid along the edges of the hole.
If concentrated forces are applied to the edge of the slab adjacent to the hole, and also in cases where the prefabricated slab is significantly weakened by holes (by 50% or more), it is recommended to reinforce the slabs along the edges of the holes with rigid reinforcement (see Fig. 21 b) or provide thickening of the plates, or bordering the holes with ribs (see Fig. 21 c).
The rigidity of the bordering ribs must be no less than the rigidity of the section of the plate section occupied by the hole.
Thickening (strengthening) of the part of the heel adjacent to the hole is recommended to be performed under the condition of equality of the stiffness of the section weakened by the hole, and without taking into account the weakening.
With rectangular holes, at the corners of these holes in the slab, 2 - 4 reinforcing bars with a diameter of 10 - 14 mm should be laid, placing them in plan at an angle of 45 ° to the sides of the hole.
The bearing capacity of slabs with holes is determined by calculation.