Economic comparison of floors of different types. Precast concrete floor. Monolithic cover. Frame wood flooring. Frame ceiling on beams from LVL timber. Monolithic floor technology Advantages and disadvantages

Pouring a monolithic floor slab is not the easiest, but a truly universal and time-tested method. In this article, we will talk about the main structural features and stages of the floor installation, as well as the types of formwork, including fixed formwork.

Building typology and scope

The main scope of monolithic floors are buildings with load-bearing walls made of brick, block masonry or concrete panels, as well as domed houses. The requirements for the solidity of the overlap may be due to:

• non-standard building plan;
• the need to significantly increase the bearing capacity of the floor;
• increased requirements for hydro and noise insulation;
• the need to provide free planning;
• reducing the cost of interior decoration.

Filling is carried out, as a rule, after the completion of the construction of the walls of the first floor. However, there are options for pouring monolithic ceilings already in buildings with a roof, if weather or other conditions so require. In this case, I-beams are mounted on the masonry of the lower floor and a crown is poured along the perimeter of the bearing walls to the height of the ceiling. Also, to strengthen the mechanical bonds, embedded reinforcement is produced from the inside of the crown by 40-50 cm. Its total cross section cannot be less than 0.4% of the cross section of the longitudinal section of the crown.

Design calculations of the supporting structure

When choosing the span length, it should be related to the thickness of the slab as 30:1. However, with independent design, there is practically no point in making an overlap thicker than 400 mm, since the bearing capacity of the structure increases along with its own weight and static stresses. Therefore, the permissible load on self-made ceilings rarely exceeds 1500-2000 kg / m 2.

The situation can be corrected by including in the supporting structure I-beams steel beams laid on a concrete-leveled masonry surface of the bearing walls. Another way to increase the length of the span while maintaining relative freedom of layout is to support the ceiling on columns. With a thickness of a monolithic structure up to 400 mm and a span in four directions from the columns up to 12 meters, the cross-sectional area of ​​​​the support is 1-1.35 m 2, provided that the cross-section of the embedded reinforcement in the column is at least 1.4%.

Calculation of reinforcement of a monolithic slab

In general, the thickness of the plate is determined by the amount of reinforcing steel that is embedded in it. Reinforcement density, in turn, depends on the maximum allowable load and resistance to cracking. Avoiding special cases, we can give a general example of a design that demonstrates full compliance with regulatory requirements with a sufficiently high margin of safety.

In private construction, reinforced concrete is reinforced with reinforcement with a periodic profile of class A400, aka A-III.

Diameter of rods in slabs thick:

• up to 150 mm - not less than 10-12 mm;
• from 150 to 250 mm - not less than 12-14 mm;
• from 250 to 400 mm - at least 14-16 mm.

The reinforcement is laid in two meshes with a mesh size of 120-160 mm, the thickness of the concrete protective layer from the edges of the slab is at least 80-120 mm, and at least 40 mm above and below. The direction of laying four rows of reinforcement, starting from the bottom: along, across, across, along. For ligation, galvanized wire with a thickness of at least 2 mm is used.

Installation of formwork of various types

Formwork must withstand a load of 500-1100 kg/m 2 , including the dynamic impact of falling concrete. To create a formwork plane, you can use:

1. Plastic sheets of reusable formwork.
2. Moisture resistant plywood 17-23 mm thick.
3. OSB 20-26 mm thick.

The edges of the slabs should fit snugly against the walls; it is not allowed to use formwork with gaps at the joints of more than 2 mm, unless it is planned to cover the surface with a waterproofing film.

Sometimes it is reasonable to make the formwork permanent, using profiled sheets for this, orienting them with a narrow shelf down. They are placed along the slab so that the waves form numerous stiffeners during pouring. The calculation of the thickness is carried out from the lower edge, thus saving the concrete mixture is 20-25%. In this case, the height of the ridge should not exceed a third of the total thickness of the slab. If the formwork is not planned to be removed, self-tapping screws with a rubber washer are screwed into it and tied with a thin wire to the reinforcement.

Formwork installation begins with the placement of racks: these can be either steel telescopic racks with a tripod and a unifork, or wood without defects with a cross section of at least 100 cm 2. Each post must be connected to two adjacent inch board slanted braces. The racks are mounted along the lines of the beams, the distance between which, depending on the thickness of the plate 150-400 mm, is:

• 190-240 cm with plywood thickness up to 20 mm;
• 210-260 cm with plywood thickness from 21 cm.

In this case, the distance between the uprights of one beam, depending on the gap between them, is:

• from 140 to 200 cm with a span of up to 150 cm;
• from 120 to 180 cm with a span of 160-210 cm;
• from 100 to 140 cm with a span of 210-250 cm.

The main beams, as a rule, are made of timber 100x100 mm. On them across with a step of 500-650 cm, secondary beams are laid, which have a cross section of 50% of the main ones. If the formwork is made of profiled sheet, the step of the secondary beams is equal to 3.5 distances between the waves.

Vertical formwork is mounted from retaining panels attached to the outer wall of the building. Often, aerated concrete blocks 80-100 mm thick are laid around the perimeter to hide the floor belt.

Reinforcement and strapping

After the formwork is installed, it is lubricated with an anti-adhesive compound and the laying of the reinforcement begins. On the crowns and supporting ribs, the rods are tied into a square, keeping the minimum allowable protective layer on all sides. The main array of overlap is reinforced with mesh. The bottom layer is laid on plastic "crackers" that control the preservation of the lower protective layer. The mesh is tied up at the intersection of every third rod.

After tying the lower mesh, intermediate clamps are installed on it every 100 cm in a checkerboard pattern. To strengthen the support on the walls, end clamps are mounted. These elements help maintain the design distance between two reinforcement planes.

The mounted upper mesh is connected to the lower connecting brackets. After completion of the installation, the reinforcing structure should be as one piece and easily take the load from people walking on it.

Pouring concrete

Monolithic floors are poured with concrete grade B20-B30, prepared in factory conditions. Filling monolithic ceilings should be carried out in one stage, so filling the space with small doses is not recommended. If it is impossible to perform the entire amount of work at once, sections of the slab must be cut with a mesh with a cell of 8-10 mm.

The supply of the mixture to the ceiling can be carried out by a concrete pump or a bulk bucket lifted by a crane. After feeding to the top, the mixture is evenly distributed, seated by vibration and left to harden.

Further actions

Concrete gains sufficient strength after 4 weeks, all this time it needs periodic wetting and protection from rain for the first 2 days. After drying, the formwork can be removed and the walls can be erected.

Overlapping from monolithic reinforced concrete is performed where buildings with non-traditional geometry layout are designed. This allows not to "adjust" the walls of the house and its internal layout to the dimensions of the prefabricated floor slabs.

If construction is envisaged in an urban area with cramped conditions, where it is impossible to use large-sized construction equipment, then this dictates the implementation of a monolithic reinforced concrete floor.

In terms of strength characteristics, bearing capacity, the floor made of monolithic reinforced concrete is superior to the prefabricated version, since it is a cast structure that works as a single unit.

In addition, the surface of the bottom of the floor does not need such a thorough finish as the prefabricated version, which requires sealing the joints between the panels and their further finishing.

Consider the disadvantages of monolithic overlap:

• Greater labor intensity of work compared to the prefabricated option, since all work is performed at the construction site. Whereas the prefabricated ceiling - brought, unloaded, mounted or mounted directly from the "wheels".
• Significant formwork costs - lumber, Finnish plywood, metal formwork and other types.
• Long concrete hardening time, which leads to a delay in the implementation of the following work according to the technology. This factor increases the duration of construction.

Types of monolithic reinforced concrete floor

beam ceiling is a slab and beams (ribs). For large spans (more than 6 m), intermediate supports are required, which are made in the form of girders or columns made of monolithic reinforced concrete.

Caisson floors - one of the varieties of beam ceiling. It consists of a plate and two mutually perpendicular beams located in the lower zone. This design creates rectangular recesses from below, called caissons.

In short, when calculating this type of floor, the reinforcement and concrete are redistributed in the structure (slab - ribs). This allows you to save material, to carry out the overlap of large spans. But this is a topic for another article.

Caisson ceilings are mainly used abroad in the construction of public buildings with false ceilings.

Monolithic beamless reinforced concrete floors - this is a solid slab, based on walls or columns, which are located at a distance of 5 - 6 meters from each other.

The thickness of the plate is taken according to the calculation and varies between 120 - 250 mm. The use of these reinforced concrete slabs supported by columns makes it possible to achieve a much greater variety of space-planning solutions.

Balcony slabs, made together with a monolithic ceiling and being part of it, have greater strength and durability compared to their prefabricated counterparts.

All elements of both types of floors are interconnected. The cross-sectional dimensions of each element, the required amount of reinforcement is determined by calculation in each individual case.

Monolithic reinforced concrete flooring technology

Let us consider in more detail the most common beamless monolithic reinforced concrete floors today. This type of overlap has found wide application in multi-storey housing construction, in the construction of buildings and structures in areas with high seismicity.

The frames of such buildings, consisting of columns and a reinforced concrete slab, have increased strength and durability. Recently, this type of overlap has become increasingly used in the construction of cottages and private houses.

Formwork installation

The formwork system must ensure its rigidity and geometric stability during the entire process of building construction. Its installation is carried out in accordance with the project for the production of works. Before starting work, geodesy is carried out on the breakdown of axes, installation sites.

It can be made from edged boards, waterproof plywood with a thickness of 18 mm or more, from metal inventory boards. Waterproof plywood is most convenient for decking (flooring) due to its relatively low weight, the presence of a protective coating and repeated turnover.

To support the formwork, special supporting racks are used, which are fastened together.

It is installed strictly horizontally, its surface is lubricated (with emulsol, engine oil waste, etc.). The gaps in it before concreting must be sealed to prevent leakage of cement milk through them, as this reduces the quality of concrete and damages the formwork.

In the construction of multi-storey buildings, it is advisable to use an inventory form of multiple use, which is rearranged from floor to floor. Its cost pays off due to the high turnover.

It is she who has gained the most popularity today. With proper handling and proper care (cleaning, lubricating the surface in contact with concrete), the number of revolutions of such formwork can reach several tens.

Reinforcement of a monolithic floor

The reinforcement of the structure is carried out according to the project, which indicates the diameter of the reinforcement, the size of the cells, the amount of overlap between the reinforcing bars when joining them along the length.

Reinforce the ceiling of monolithic reinforced concrete should be frames or meshes made at the factory. At the construction site, it is allowed to make only additional fittings or connections between frames.

Replacement of fittings by class, brand, assortment is carried out only with the approval of the design company. The displacement of reinforcing products during their installation in the formwork is not allowed by more than 1/5 of the largest diameter of the rods and 1/4 of the installed rod.

Permissible deviations from the design of the thickness of the protective layer of the concrete mix should not be more than:
- With a layer thickness of 15mm and less than 3mm;
- With a layer thickness of more than 15mm 5mm.

After installing the fittings, an act for hidden work should be drawn up, which must be signed by a representative of technical supervision. The act is accompanied by a certificate for reinforcing products, electrodes, a copy of the certificate of welders, other replacement documents agreed with the design institute (if any).

Concrete laying

After signing the act for hidden work on the installation of reinforcement, it is allowed to proceed to concreting. In order to obtain a high quality monolithic reinforced concrete floor, it is important to carry out the concreting process continuously and lay the entire volume of concrete during one work shift.

If for some reason this does not work out, then concreting seams (working seams) are arranged, which should not fall on the load-bearing columns, but should be located between them. In slabs, they are performed in the middle of the slab span.

Concrete is laid into the structure in horizontal layers, without gaps and of the same thickness. Since the reinforced concrete floor is a very important structure, the concrete mixture for it should be ordered at mortar concrete plants, units.

This is dictated by the fact that they are responsible for the manufactured products, they supply concrete during continuous concreting strictly according to the hour in the application. In addition, the organization provides a passport for its products, laboratory quality control is carried out.

Ready-mixed concrete is fed to the ceiling in special containers by a crane or pumped with a concrete pump. Compaction of the concrete mixture is carried out by vibrators, the type of which depends on its thickness.

After concreting is completed, proper care is important, especially in hot weather. It consists in watering reinforced concrete with water, covering it with wet sawdust or other materials that prevent water from evaporating from the body of concrete.

The formwork is removed after the strength of reinforced concrete has been cured (three to four weeks), the period is specified by the project. Then the plate is accepted.

Basic acceptance requirements:

• Full compliance of the accepted design with working drawings;
• The quality of concrete for strength (if provided by the project, then water resistance, frost resistance, etc.);
• The presence of holes, expansion joints, embedded parts and other things in accordance with the project;
• Presence of a journal of concrete works;
• Laboratory tests of concrete cubes.

The overlap of monolithic reinforced concrete is accepted and formalized by an acceptance certificate for a critical structure or an act for hidden work.

Ceilings consist of a bearing part, which transfers the load to walls or individual supports, and an enclosing part, which includes floors and ceilings. According to the material of the bearing part, reinforced concrete floors, wooden and steel beams, as well as reinforced silicate and ceramic floors are distinguished. The cost of ceilings and floors in the total cost of the house reaches 20% of its total value.

The main material for flooring in modern construction is reinforced concrete. Reinforced concrete floors are divided into prefabricated and monolithic, concreted in the formwork. In recent years, mainly prefabricated and monolithic ceilings have been used.
Ceilings must meet the requirements of strength, rigidity, fire resistance, durability, sound and heat insulation, if they separate heated rooms from unheated rooms or from the outside environment. Ceilings in rooms with wet processes must be waterproof, and in rooms with gas emissions - gas-tight.

In country houses with brick walls, ceilings made of reinforced concrete panels with round voids are used, the length of which is from 4800 mm to 6980 mm, width from 1000 to 2400 mm, height 220 mm, and also with flat ones - 2700-4200 mm long with a gradation of 300 mm , width 1200, 1500 mm, thickness 120 and 160 mm. The panels are laid (fig. 1) on a layer of freshly laid masonry mortar 10 mm thick with embedding on supports of at least 120 mm. Through one panel (step 2400-3000 mm) they are connected to the walls with anchors with a diameter of 8-10 mm, which are attached to the hinges and led into the masonry 250 mm from the end of the panel, ending with a bend at an angle of 90 ° horizontally by 380 mm.

The seams between the panels are filled with cement mortar composition 1: 4 (by volume). Panels are installed using truck cranes.

Reinforced concrete floors

Such floors have a number of valuable qualities, the main of which are great strength, durability and fire resistance. When designing the structures of elements of prefabricated reinforced concrete floors, it is necessary to strive to enlarge them in order to reduce the number of installation operations and butt joints.

Precast concrete floors

Prefabricated reinforced concrete floors are divided into three main groups: in the form of flooring (slabs), large-panel and beam. Overlappings in the form of floorings consist of flat or ribbed elements of the same type, laid close; connect them by filling the gaps with cement mortar. Such floors consist of a bearing reinforced concrete part (usually textured from below), a sound or thermal insulation layer and a floor structure. The supports for the flooring are walls and girders. The most common are hollow decks with a height of 160 mm with spans up to 4 m and 220 mm with spans over 4 m. The decks have longitudinal voids of circular cross section (Fig. 2, a).

In the manufacture of floorings with vertical voids, the consumption of concrete is reduced by up to 15% compared to round-hollow ones. Vertical round voids are formed using pipe liners (the liners are welded to the channels). Floorings that can cover entire rooms are called large panels. The absence of joints in the floor panels within the room increases their sound insulation and provides a higher quality ceiling finish.
To ensure standard soundproofing properties from airborne noise, single-layer structures of interfloor panel ceilings, made of heavy concrete, must have a mass exceeding 300 kgf / sq.m.

When installing separate type ceilings, which use the soundproofing capacity of the air gap between the upper and lower communication floor panels, as well as when installing layered ceilings, it is possible to ensure the normative soundproofing ability with the weight of the floor less than 300 kgf/sq.m.
By design, interfloor large-panel reinforced concrete floors can be with a layered floor, a separate type (with a separate floor, ceiling or from two separate load-bearing panels) and with a layered floor and a separate ceiling (Fig. 3). All these floor structures have a relatively small mass (less than 300 kgf / sq.m.); normative sound insulation is provided by a layered floor structure or the presence of a continuous air gap in the thickness of the ceiling.
Floor panels are made solid, hollow (with round voids) and tented. The load-bearing single-layer panel (Fig. 4, a) is a reinforced concrete slab of constant cross section with a lower surface ready for painting and an even upper surface.

Solid single-layer reinforced concrete panels with a thickness of 140 mm cover spans up to 3.6 m. To cover large spans (6-6.6 m), mainly solid single-layer prestressed reinforced concrete panels with a thickness of 14-16 cm or expanded clay-reinforced concrete with a thickness of 18 cm are used.

The hipped panel (Fig. 4, b) has the form of a slab framed along the contour with ribs facing downwards in the form of a cornice. Interfloor floors are also arranged from flat reinforced concrete panels with a thickness of 14-16 cm.

Prefabricated reinforced concrete interfloor floors ( fig. 5) beam type consist of tee beams and filling between them. The filler here is a roll of gypsum concrete or lightweight concrete slabs 80 mm thick and 395 mm long, reinforced with wooden slatted or bar frames, and in attic floors - lightweight concrete slabs 90 thick and 395 mm long, reinforced with welded steel mesh. The seams between the beams and slabs are filled with cement mortar and rubbed. Attic and basement floors must be insulated, interfloor soundproofing. For this, expanded clay or sand bedding, layered coatings with elastic gaskets are used. At the same time, it is desirable that heat and sound insulation be carried out not due to an increase in the weight of building structures.
Since the elements of beam ceilings are relatively light in weight, they are used in buildings equipped with low-capacity cranes (up to 1 t).
When constructing reinforced concrete floors in sanitary facilities, a waterproofing layer is included in the floor structure. To do this, 1-2 layers of roofing material are usually glued on bituminous mastic over decking or panels.

Monolithic floors

Monolithic ceilings are performed according to the established formwork. By transferring loads from the floor to the load-bearing walls, monolithic ceilings serve as an additional rigid frame of the building. Their device requires a certain professional skill and should be carried out according to the project under the guidance of a specialist builder. Making floors in place has its advantages. It does not require special transport and lifting equipment. Small-scale mechanization is enough to lift and move concrete. Monolithic slabs are based on the Monier slab, in which the reinforcement is placed in places of tension, that is, in the lower part of the slab. This is because steel has 15 times the tensile strength of concrete. The reinforcing frame of the slab should be located at a distance of at least 3-5 cm from the formwork walls so that concrete can fill this space. The length of the span covered by monolithic slabs should not exceed 3 m. For plumbing pipelines, special metal or vinyl sleeves with an inner diameter larger than the pipeline being laid are installed in the ceiling. The gap between the sleeve and the pipeline is minted with tarred tow.

The disadvantages of monolithic ceilings include the need to install wooden formwork over almost the entire area of ​​\u200b\u200bthe house. However, this does not mean that the formwork must be set all at once. Overlapping can be done in separate spans, transferring the formwork as the concrete sets.
The bearing capacity of monolithic ceilings is provided by reinforcement, the diameter of which must be at least 8-12 mm. In this case, intermediate joints of the rods along the entire length of the floor are undesirable. The minimum layer of concrete on the outer side of the ceiling must be at least 2 cm. The span must be concreted in one working cycle.

Sometimes in private housing construction, this type of flooring is used - monolithic reinforced concrete, based on metal beams (paired channels, I-beams, square pipes, etc.).

The advantages of such an overlap is that due to the fairly often located beams (from 1 m to 2.5 m on average), the overlap itself can be made quite thin (but not less than 50 mm). Such an overlap is reinforced in one layer, which also gives considerable savings.

The main disadvantage is that, according to fire safety requirements, metal structures must be coated with a special fire retardant composition, and this is an expensive pleasure.

In this article, we will consider two questions: how to make a reinforced concrete floor and how to choose metal beams.

Where should you start? From the floor plan analysis. Let's say we have a floor of 4x8 m. It is more rational to place the beams along the short side of the slab, i.e. the length of the beams will be 4 meters (not counting the depth of support on the walls). The shorter the beam, the less metal we will spend on it, and the less often these beams can be placed. Of course, this is not a hard and fast rule, just rational advice.

The load from the weight of the partitions (it is advisable to place the beams under the partitions to avoid excessive load on the lightweight floor),

The own weight of the floor.

Then you need to set the pitch of the metal beams. Here, a monolithic overlap comes to the fore. If we make the step of the beams too frequent, we risk causing an overrun of both metal and reinforced concrete. If the distance between the beams, on the contrary, is too large, this will cause an increase in the reinforcement in the slab, an increase in the thickness of this slab (in this case, the load on the beams will increase significantly), which means that the section of the beams will also increase. Therefore, always before starting the calculation, it is necessary to analyze and select the optimal distance between the floor beams. The calculations below are applicable under the following conditions: there must be the same distance between all beams; the condition L 1/L 2 > 2 must be satisfied, where L 1 is the length of the beam, L 2 is the distance between adjacent beams.

In principle, there are several ways to calculate this type of overlap.

The first way (more time-consuming, especially without sufficient experience, but sometimes necessary). You can specify the profile of metal beams (for example, you already have metal of a specific profile); then, given the thickness of the floor and the step of the beams, you can collect the loads and perform the calculation of the beam. At the same time, performing the calculation, in several approaches you can determine the maximum allowable distance between the beams, at which the conditions of strength and deformability are met. After that, you can proceed to the calculation of the floor and determine its thickness and reinforcement. If everything went well, fine. If the thickness turned out to be greater than you specified, the calculation will need to be repeated from the beginning - until all parts of the problem converge.

Second way. The calculation begins with a reinforced concrete floor. We set the step of the beams and the thickness of the slab, collect the loads and perform the calculation of the slab. If necessary, we adjust the spacing of the beams and the thickness of the slab to the most economical results. We collect the load on the beam from the resulting span and select the section of the beams.

We will consider the second way with an example.

The calculation is carried out for conditionally a dedicated strip of slab 1 m wide.

It is necessary to block the room with a plan size of 6x10 m. Above the ceiling there will be living rooms - a temporary load of 150 kg / m 2. Slab materials: class B15 concrete, design concrete resistance Rb = 7.7 MPa, hot-rolled rebar of a periodic profile of class A400C, design resistance of rebars s Rs = 365 MPa.

The minimum floor thickness must be greater than L /35, where L is the distance between the beams.

We set the step of the beams - 2.5 m, the direction of the beams - along the short side of the room, the thickness of the reinforced concrete. ceilings - 80 mm (which is more than 2.5 / 35 \u003d 0.071 m \u003d 71 mm), the distance from the bottom edge of the slab to the working reinforcement is 35 mm.

We collect loads on 1 m 2 of overlap.

Type of load:	normative, kg / m 2	safety factor	calculated, kg / m 2
Partition weight load (average)
Floor own weight 2500*0.08

To calculate the overlap, it is necessary to find the maximum bending moment that occurs in the extreme span of the slab, and is equal to: М = qL 2 /11 ( see formula 6.169 reference book "Design of reinforced concrete structures”, Golyshev A.B.).

In our case, q = 6241 m, L = 2.5 m - the distance between the beams, then M = 624 * 2.5 2 / 11 \u003d 355 kg * m.

For slabs reinforced with mesh in the lower zone(without top reinforcement), the following condition must be met:

αm > αR (See Section 3.18 of the Concrete Design Guide and reinforced concrete structures made of heavy and light concrete without prestressing). α R value find from tables s 18 allowances. For class AIII (A400C) reinforcement and class B15 concrete αR = 0.440.

We find α m \u003d M / R b bh 0 2 \u003d 355 / (770000 * 1 * 0.045 2) \u003d 0.228, where

b = 1 m - strip width s floor for which the calculation is performed;

h 0 \u003d 0.08 - 0.035 \u003d 0.045 m - the distance from the upper zone of the plate to the center of gravity of the working reinforcement.

Condition αm = 0.228< αR = 0,440 выполняется. Из таблицы 20 пособия при αm = 0,228 находим значение

Find the area of ​​the working reinforcement of the plate:

As \u003d M / (R s * ζ * h 0) \u003d 355 / (36500000 * 0.87 * 0.045) \u003d 0.000248 m 2 \u003d 2.48 cm 2. We accept rebar with a diameter 8mm in increments of 200 mm (5 rods per 1 meter of slabs s, with an area of ​​​​2.52 c m 2).

For self-examination, you can use the table from the reference book Linovich L.E. for the selection of thickness and reinforcement of the floor, depending on the load. This table shows the results for single-span slabs. Our slab is considered multi-span (the number of spans is equal to the number of beam steps), and it works much better due to multi-span. Therefore, the results of the calculation according to the example should be better (more economical) than in the table from the reference book.

We proceed to the calculation of the beam (see the book Ya.M. Likhtarnikov "Calculation of steel structures” pp. 60-61 or the book Vasiliev A.A. "Metal structures" §24). First of all, it is necessary to determine the linear load on each beam. The load on 1 m 2 of the floor turned out to be 540 (624) kg / m 2, and we took the step of the beams 2.5 m. Then the load per 1 linear meter of the beam is:

standard 540*2.5 = 1350 kg/m;

calculated 624*2.5 = 1560 kg/m.

The span of the beam in the clear is 6 m. The depth of support on each side is 0.2 m. Then the estimated length of the beam is 6 + 2 * 2 * 0.2 / 3 = 6.3 m.

Let's find the maximum moment in the beam section according to the formula M = qL 2 /8, where q is the load per 1 linear meter of the beam, L is the estimated length of the beam.

Standard moment M n \u003d 1350 * 6.3 2 / 8 \u003d 6698 kg * m,

design moment M p \u003d 1560 * 6.3 2 / 8 \u003d 7740 kg * m.

Determine the required moment of resistance:

W tr \u003d M p / 1.12R \u003d 7740 / (1.12 * 21) \u003d 329 cm 3. According to the assortment (for example, a reference book by Ya.M. Likhtarnikov Calculation of steel th structures, Appendix VI), we select an I-beam No. 27 (modulus W = 371 cm 3, moment of inertia I = 5010 cm 4).

We check the strength of the beam from the condition:

σ= M/1.12W \u003d 7740 / (1.12 * 371) \u003d 18 kN / cm 2, which is less R \u003d 21 kN / cm 2 - the condition is provided.

Let's check the stiffness of the beam.

M n * L / (10EI) \u003d 6698 * 630 / (10 * 21000 * 5010) \u003d 0.004 \u003d 1/250 - the condition is met (although at the limit).

Thus, the selected beam passes according to the calculation. But it turned out to be quite powerful. To reduce the beam section, you need to specify a smaller step of the beams and recalculate the problem from the beginning. The smaller the step of the beams (the distance between the beams), the less load they have, which means the smaller the cross section will be.

The diagram of the overlapping device, the characteristics of which we determined during the calculation, see the figure below.

Attention! For the convenience of answering your questions, a new section "FREE CONSULTATION" has been created.

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Comments

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0 #65 Irina 23.01.2013 09:25

Quoting Dmitry:

what ceilings are possible during the construction of walls from aerated concrete blocks. And is it possible to replace metal beams with something else when installing monolithic ceilings.

Any overlap is possible (prefabricated, monolithic, wooden, etc.), the main thing is to avoid large spans and perform wall calculations in accordance with SNiP (DBN) "Stone and reinforced masonry structures".
You can not make beams at all, but simply make the floor monolithic, only thicker and with more powerful reinforcement. You can also make a ribbed monolithic floor, or use prefabricated reinforced concrete. beams.

0 #71 Maxim 05.03.2013 20:36

Quoting Irina:

It would be better not according to calculations, but according to SNiP (DBN) "Loads and Impacts" - it is different in different cities. And it is given for 1m2, which greatly simplifies the calculation.
And yet, if your ceiling is on an extension to a higher part of the house, then a snow bag may form, this must also be taken into account. If so, please describe in detail.

For my 4th zone, the Middle Urals, the load is already 240 kg per square meter !! and how I counted these 800 kg of my calculated ones ...)

The building is separate.

0 #76 Irina 18.04.2013 11:04

Quoting Andrew:

Hello, I need the opinion of an experienced person.
Overlapping is made, the room is 6 * 6.5 wide, materials: I-beam height 120mm laying step 1200mm (overlapped in width, i.e. 6m), between I-beams concrete to the full height (120mm) in concrete reinforcement 10mm with a step of 120mm in length, (across between I-beams cage through 400mm reinforcement 10mm) from the bottom there was formwork plywood and aged on racks for a month.
Question: what load can this flooring withstand (payload)
Thanks for the answer.

Andrey, I have experience in design, and not in the scientific analysis of what was done wrong. For a span of 6m, I-beam No. 12 is clearly not enough. Its shelves for supporting reinforcement of 10mm are also not enough (you need a minimum of 100mm). Reinforcement pitch 400mm is too large, no more than 200mm is needed. With such initial data, the calculation will not give an answer, because calculation assumes that everything is designed correctly.

0 #77 Andrey 24.04.2013 23:01

Quoting Irina:

Andrey, I have experience in design, and not in the scientific analysis of what was done wrong. For a span of 6m, I-beam No. 12 is clearly not enough. Its shelves for supporting reinforcement of 10mm are also not enough (you need a minimum of 100mm). Reinforcement pitch 400mm is too large, no more than 200mm is needed. With such initial data, the calculation will not give an answer, because calculation assumes that everything is designed correctly.

Hello! Thanks for the answer.
Reinforcement pitch 400mm is transverse between I-beams (support 25mm), parallel to the I-beams is reinforcement with a pitch of 120mm with a diameter of 10mm from one wall to another (10 pieces in each span)
http scheme
The overlap is standing, there are no cracks, but I'm interested in what kind of load it can withstand.

The online store https://www.site presents projects, the construction of which is carried out using the most modern building technologies and materials. One of the examples of the application of modern innovative technologies are often ribbed prefabricated monolithic ceilings. The technology of prefabricated monolithic floors came to Russia from Europe, where the mass construction of individual houses using this technology has been going on for over 25 years. The most well-known European and domestic construction technologies in Russia and the CIS countries using often-ribbed prefabricated monolithic slabs are, first of all, large-span slabs of the German ALBERT system, Polish slabs TERIVA (TERIVA), Belarusian slabs DAH, Russian prefabricated monolithic ceilings "Marco". Let's take a closer look at this advanced, economical and reliable slab construction technology.

Often-ribbed precast-monolithic floors (what is it)

Frequently ribbed prefabricated monolithic floors consist of light reinforced concrete beams made in the form of a spatial steel reinforcement cage and a reinforced concrete base (beam) of rectangular cross section, hollow blocks and cast-in-situ concrete poured at the facility.

Hollow blocks (liners) laid on reinforced concrete beams can be ceramic, gas silicate, polystyrene concrete or concrete. Such ceilings have excellent soundproofing and thermal properties, and communications, including electrical wiring, are placed in the channels available in the blocks without any problems. It is also important that the floors in question can be successfully used in the construction of low-rise buildings using the "Build it yourself" method. Practice shows that only two or three people are able to lay reinforced concrete beams on walls erected using any technology, liners are placed on them, and then the resulting base (fixed formwork) is poured with concrete. In Russia, the most modern, economical and affordable technology is MARCO prefabricated monolithic ceilings. That is what we will consider in more detail.

Frequently ribbed prefabricated-monolithic ceilings of the MARKO system

The MARKO system is produced with two types of floor beams. These are beams with a reinforcement cage 150 mm high and beams with a reinforcement cage 200 mm high. The dimensions of the concrete bar of the beams are 40x120 mm, the concrete class is not lower than B20.
To ensure the necessary bearing capacity of the beam ceiling, the lower reinforcement chord of the beam can be reinforced with additional longitudinal reinforcement, which is installed during the manufacture of the beam. Diameter of additional fittings from 6 to 16 mm. The strength class of additional reinforcement is A500.

Reinforcement of beams of precast-monolithic floors in the area of ​​load-bearing walls

The system provides two options for reinforcing beams in the area of ​​load-bearing walls.
In the first variant the upper and lower longitudinal reinforcement of the beams does not extend beyond the concrete element. This floor variant, by analogy with floor slabs, is designed for the free support of beams on load-bearing walls.
In the second variant the beams are provided with reinforcing outlets, the length of which is specified by the design documentation. This option is designed to pinch beams in a monolithic belt of a load-bearing wall, which is recommended when building frame houses, as well as from aerated concrete and foam concrete blocks. The calculation of the bearing capacity of floors shows that the interfloor floor in this case can carry a large payload, but the design of the floor nodes becomes much more complicated. Reinforced concrete floor slab, obtained after pouring concrete, binds the walls and increases the seismic resistance and reliability of the building.

Production of reinforcing cages and floor beams

Triangular reinforcing cages (trigons) are produced on high-performance welding equipment of the well-known Austrian company Filzmoser (Filzmoser) from high-strength reinforcement class B500. The equipment is fully automated and provides high quality reinforcement preparation and frame welding.

For the manufacture of floor beams, a special vibration stand is used. The metal form of the stand consists of 12 separate elements. Beam manufacturing time - 10-12 hours. The stand allows to produce beams up to 12 meters long. The productivity of one stand is 280 linear meters of floor beams per day.

Liner blocks in a frequently ribbed prefabricated monolithic slab

For the installation of a frequently ribbed prefabricated monolithic floor of the MARKO system, insert blocks 150 and 200 mm high are used.

The radial and trapezoidal units are designed to be used as ceiling elements. Such a ceiling covering in terms of bearing capacity is not inferior to the usual one. All blocks of prefabricated monolithic floors are made of polystyrene concrete with a density of less than 400 kg/m 3 . The weight of the blocks does not exceed 6 kg. Blocks and beams in a ribbed slab perform the functions of a fixed formwork for the slab and take on the loads that occur during pouring of concrete.

IMPORTANT! The technical documentation for blocks and beams has been approved by the Research, Design and Technological Institute of Concrete and Reinforced Concrete NIIZhB and registered by GOSSTANDART. According to the results of certification tests, the blocks are classified as low-hazard non-combustible materials with low smoke-generating ability. A positive sanitary and epidemiological conclusion was obtained for MARKO polystyrene concrete.

For the production of blocks, a high-performance vibrating stand is used. Vibrostand performance - 3000 blocks per shift. This allows you to complete blocks of 350 m 2 floors.

The thickness of the often ribbed prefabricated monolithic ceiling MARKO

The system of prefabricated-monolithic ceiling Marco provides four options for the thickness of the ceiling. On fig. 1. A diagram of the thinnest ceiling of the MARCO SMP-200 system is presented.

In construction practice, to increase the bearing capacity of a prefabricated monolithic floor There are two options for solving this problem. First option when higher blocks weighing up to 18 kg are used to increase the bearing capacity of the floor. These are the recommendations of Polish and Belarusian manufacturers of often ribbed prefabricated monolithic ceilings. Unfortunately, this solution has a significant drawback, namely, with this option, the own weight of the ceiling reaches 450 kg/m 2, which is quite comparable with the weight of a monolithic reinforced concrete slab.
The Russian system of prefabricated-monolithic floor MARKO provides an alternative option increase the bearing capacity of the floor. To solve this problem, additional floor slabs made of foam are used. The slabs have a thickness of 50 mm for covering the SMP-300 and 100 mm for covering the SMP-350.

The slabs are glued to the top surface of the blocks with any cement-based tile adhesive. The use of additional slabs allows the use of a single nomenclature of blocks for all types of floors.
The most powerful in the range of manufactured products of prefabricated monolithic floors of the Marco system are SMP-350 floors. In the construction of this type of flooring, an additional slab with a thickness of 100 mm is used. This option allows you to use a prefabricated monolithic floor for spans up to 10 meters.
The use of the SMP-350 system for basement floors significantly reduces the heat loss of the building (as is known, about 30% of heat loss in houses without basements occurs through basement floors). The "constructive cake" of the SMP-350 floor, in which the bonding layer is made of expanded clay concrete, additional floor slabs are made of foam plastic, and the cement screed is replaced by a polystyrene concrete screed, best solves the problem of warming the floor and the floor of the first floor of the house.
A similar design can be used for the attic floor, if the insulation of the roof of the house is not provided.

Frequently ribbed monolithic-prefabricated floors in houses made of cellular concrete

The use of MARCO prefabricated monolithic ceilings makes it possible to abandon the mandatory installation of a separate monolithic belt (seismic belt) on walls made of low-bearing materials (aerated concrete, foam concrete, expanded clay concrete, MARCO polystyrene concrete, etc.). Due to simple technological methods, a monolithic belt is formed simultaneously with the concreting of the floor slab. To do this, floor beams are hung above the wall on inventory racks with a gap of 40-50 mm. After filling the gap with concrete, a full-fledged monolithic belt will form on the wall. Such a method of installing fixed formwork for overlapping and seismic belt significantly reduces the cost of construction and reduces the time. The resulting monolithic reinforced concrete membrane holds walls together and significantly increases the strength of buildings. A correctly made monolithic belt evenly distributes the load along the entire perimeter of the walls and prevents the formation of cracks in case of uneven shrinkage of the foundation.
When installing the ceiling of houses on walls made of materials with a high bearing capacity (brick, concrete), attention should be paid to the possibility of reducing the number of beams by mounting the blocks directly on the wall. This technique reduces the consumption of bonding concrete and reduces the cost of overlapping.

Technology for the device of often ribbed prefabricated monolithic floors

Structurally, a often ribbed monolithic prefabricated floor, after pouring with concrete, becomes an analogue of a ribbed reinforced concrete floor slab. Each rib consists of a beam and a concrete core formed during the pouring of concrete. It is important to note that high adhesion must be provided between the beam and the concrete core. Only in this case the element will work as monolithic reinforced concrete. The cross-sectional area of ​​concrete elements is an I-beam floor and significantly depends on the thickness of the floor.
As we have already noted, it is quite possible to assemble and install a frequently ribbed floor during the construction of an individual residential building by the developer himself, together with two physically strong assistants. By following step by step the recommendations of the installation instructions for the installation of floors, even an unprepared person will be able to assemble a fixed formwork floor. The beams are laid on the walls with a step of 600 mm. The weight of a running meter of a beam does not exceed 17 kg. This allows, in most cases, the installation of beams without the use of a crane. Blocks are manually placed between the beams. The weight of the block is not more than 6 kg. The blocks are covered with a reinforcing mesh with cells measuring 100x100 mm from wire with a diameter of 4-6 mm.

In some cases, additional reinforcement of floors may be required, for example, for balconies. Of course, building a second floor or third floor ceiling may require the use of a crane.

The prefabricated ceiling structure prepared in this way performs the function of a fixed ceiling formwork, on which a bonding layer of monolithic concrete of class B20 (M250) is poured. Concrete is poured taking into account weather and temperature conditions. Concrete is compacted with a vibrating screed or by baying. Consumption of concrete mixture is 0.07-0.12 m 3 per square meter of floor. The weight of one square meter of the finished often ribbed prefabricated monolithic floor is 230-348 kg. For comparison, the weight of a square meter of a 200 mm thick monolithic floor is 480-500 kg. Compared to monolithic ceilings, the volume of reinforcing and preparatory work at the construction site is also significantly reduced.
If necessary, beams and floor blocks are easy to modify directly on the construction site. This possibility is often used for the construction of bay windows and rooms with complex wall configurations. Production makes it possible to ensure the accuracy of manufacturing a floor beam within one centimeter, but the low accuracy of wall erection often leads to the need to refine the beams at the construction site.

The fire resistance limit of the ceiling is REI 60 (60 minutes), and when using two layers of drywall for finishing the ceilings, 120 minutes. For comparison, the same indicator for overlapping on corrugated board does not exceed 30 minutes.
Calculations show that the thermal insulation of MARKO floors is higher than that of other types of floors. This is primarily due to the fact that the composition of the prefabricated ceiling includes blocks of polystyrene concrete, which have increased heat-shielding characteristics.

Objects with often ribbed prefabricated monolithic ceilings

There are circumstances and construction projects in which there is simply no other solution than the installation of a prefabricated monolithic floor.

Let's highlight the most characteristic of them:
Objects for which the reconstruction project provides for the replacement of interfloor and attic wooden or weakened floors without dismantling the roof or repair of floors.
Objects for which the determining factor is the weight of the floor or its thickness
Objects for which the bearing capacity of the floor is decisive
Objects for which the heat-shielding or sound-proofing parameters of the floor are decisive
Objects with walls of complex configuration (bay windows, ledges)
Objects where it is impossible or impractical to use a crane or other lifting equipment
Objects for which transport for one reason or another cannot enter the construction site.

Of particular interest is the experience of mounting prefabricated-monolithic floors when replacing floors on wooden beams. In this case, the task of strengthening the floors (increasing the bearing capacity) is often posed. As a rule, the thickness of the monolithic slabs obtained as a result of the reconstruction is even less than the thickness of the original wooden slab. A monolithic floor (reinforced concrete floor slab) is connected to the load-bearing walls and strengthens them. Before the advent of often ribbed prefabricated monolithic ceilings in such cases, as a rule, they used ceilings on metal beams, the total thickness of which is 30-40% higher than the thickness of prefabricated monolithic ceilings. The weight of a linear meter of an I-beam metal beam with a height of 220 mm is 33.1 kg. This is 2.5 times heavier than the beams of prefabricated monolithic floors. In addition, the thermal insulation of the ceiling on metal beams is much less than the thermal insulation of prefabricated monolithic ceilings.

Finishing ceilings from prefabricated monolithic ceilings

For finishing ceilings from prefabricated monolithic ceilings, you can use drywall on a metal or wooden frame, plastic panels, plaster, suspended ceilings such as Amstrong, wooden lining and other finishing materials.

Efficiency of using often-ribbed prefabricated monolithic slabs

The use of often ribbed prefabricated monolithic ceilings allows:
- Reduce the weight of interfloor slabs in comparison with hollow core slabs by 30% and two times in comparison with reinforced concrete monolithic slabs
- Carry out the installation of ceilings without using a crane
- Exclude the device of a separate monolithic belt on the walls of low-bearing building blocks
- Eliminate the screed device for leveling the subfloor
- Replace wooden and weakened floors with concrete
- Overlap rooms of complex shape with bay windows and ledges
- Carry out installation in hard-to-reach places, including in existing premises
- Reduce the cost of building floors by 30-40%
- Increase the bearing capacity of the floor up to 1000 kg/m2
- Ensure high performance of monolithic floors of buildings in terms of thermal protection and sound insulation
- Refine the elements of the floor at the construction site: cut, shorten, give the necessary shape
- Use voids in ceilings for laying communications
- Use beams to build powerful load-bearing lintels
- Deliver to the construction site 250 sq.m. prefabricated floors with one machine
- Beamed ceilings of the system are well combined with walls made of any building materials.

Projects of houses with often ribbed prefabricated monolithic ceilings

I 165-6	I 183-6	K 263-0

K 305-0	K 247-3-1	I 237-5