Wooden and metal floors are always assembled on special supports. The latter are called beams. Concrete floors can be laid in the spans of the building box without the use of such supporting elements. After all, slabs of this type in themselves are distinguished by increased strength and excellent bearing capacity.
A bit of history
Beamed ceilings were first used in the construction of a building in 1902 in the United States by engineer Orlano Norkors. In Russia, such designs were also used at the beginning of the last century. The first such house in our country was built in Moscow in 1908. It was a four-story building for a warehouse of dairy products. It was built under the guidance of engineer A.F. Lopeit. A feature of buildings of this type was that the columns in them had an expanded top. Thus, the contact area of supports and plates increased and the reliability of installation increased. Therefore, at the beginning of the century, overlappings of this type were called "mushroom-shaped".
Where are used
Such ceilings can be equipped in buildings of almost any type. Very often, beam-free structures can be seen, for example, in residential urban slab high-rise buildings. Also, in many cases, it is in this way that floors are made in production shops, in warehouses, in garages, etc.
In particular, such structures are often equipped at food industry enterprises. This can be, for example, dairies, workshops for the production of semi-finished products, etc. That is, most often flush-mounted ceilings are installed where there are increased requirements for hygiene.
In private housing construction, interfloor structures of this type are rarely used. But sometimes suburban residential buildings are built in this way.
In construction, there are only three types of such ceilings:
- national teams;
- prefabricated monolithic.
The first variety of designs consists of two parts: a plate located above the column, and capitals. Beam prefabricated floors have a relatively simple configuration. The plate in this case relies on special shelves arranged above the column. The latter, in turn, are held on to capitals and are interconnected by welding.
Monolithic and precast monolithic structures
The second type of beam-free ceilings is monolithic. They are used where smooth ceilings are needed. For example, they are very widely used in underpasses and the subway. These are overlapping flat inextricable slabs, which are supported by columns. The latter in this case also have capitals.
A feature of prefabricated monolithic beam-free ceilings is that they are designed with a square or rectangular grid of columns. Most often, the supports are installed according to the 6x6 m scheme. Such ceilings are laid on prefabricated, span and over-column panels.
Such a variety of designs among builders is also quite popular. In this case, the floor elements are based directly on the pylons and columns of the frame. Plates in such designs most often have a constant thickness.
Such ceilings during the construction of buildings began to be used in 1940. A feature of beam-free structures of this type is the reduced area of support of plates on columns. In this case, for the perception of cutting forces, the technique of transverse reinforcement of beam-free ceilings is additionally used. Steel rods significantly increase the strength of the plates in the area where they adjoin the supports.
Also, when designing buildings of this type, large diameter columns may be provided. When using such elements, the contact area of the supports and plates increases. Consequently, the loads cannot destroy the overlap in the area of the columns.
Types of Frames
Buildings with beam-free ceilings can be built using various technologies. Frameworks of such houses are:
In the systems of the first variety, the main load-bearing functions along the floors are performed by columns and crossbars mounted in two directions. Frame elements in such buildings are rigid frames. The latter perceive all the loads acting on the building - both vertical and horizontal.
In communication frames, the main loads are on the column and diaphragm systems, also called pylons. The role of the ceilings themselves in such buildings is greatly increasing. In addition to the actual vertical loads, these structures in this case perceive horizontal, and then transfer them to the diaphragms.
Combined frame-link frames are usually used in load-bearing structures made of steel and reinforced concrete. In this case, diaphragm systems perceive 85-90% of horizontal loads. At the same time, with a small gain, they can withstand them completely, at 100%.
Compared with conventional, bezel-less floors have a number of unconditional advantages. The advantages of such designs include primarily:
- low complexity of finishing work;
- decrease in height and cubic capacity of the building;
It’s much easier to trim smooth bezel-less floors than regular ones. In this case, you do not even have to bind the ceiling. All that is needed to finish such a floor is plastering the surface and further painting. However, both of these operations will not take too much time.
The thickness of the beam-free reinforced concrete floors are usually less than traditional. Accordingly, with the same cubic capacity, the building will be lower.
What other advantages are there
Caring for the surface of beams is much easier. Indeed, in this case, the design of the ceiling or floor does not have gaps where debris or dust could clog. Accordingly, pathogens of various kinds do not start in such ceilings. That is why it is customary to equip structures of this type in food shops or, for example, in hospitals.
What are the disadvantages
Of course, such overlaps also have disadvantages. The main disadvantage of structures of this type, in comparison with beam, is the large weight. Supports for floors of this type have to be installed as durable as possible.
Also a minus of beam-free structures is a limited span. The distance between the supports for the slabs of such floors should not be too large. Reinforced concrete - the material is very durable. But with a significant area and serious load, such a plate will still begin to bend and may even collapse.
It is considered economically feasible only to equip beam-free ceilings in spans with a width of not more than 5x6 meters with a load of 5 kN / m 2 . In this case, the designs are usually obtained quite reliable.
Designing beam-free floors - the procedure is quite complicated and very responsible. This work can only be done by an experienced, highly qualified engineer. Difficulties in the preparation of drawings, of course, can also be attributed to the disadvantages of such designs.
Features of calculating flush-free floors
Designing this type of floor, therefore, should be as thorough as possible. In conventional constructions of this type, a lot of fairly short lags take the load. Plates have a large area, and therefore can bend more.
How do the calculation of beam-free floors? As already mentioned, similar designs are most widely used in construction, mounted over spans up to 5-6 m. If the distance between the supports is greater, designers usually have difficulty in ensuring the strength of the slabs for punching.
Destruction in this way overlap begins around the column. Concrete in this place loses its integrity, which can lead to instant collapse of the slab. There are several ways to increase the bursting strength of a structure:
- by increasing the working thickness of the plate;
- by increasing the area of bearing;
- by installing transverse reinforcement.
There are several methods for calculating beamed ceilings of monolithic, prefabricated or precast monolithic. For example, in construction, the technology for calculating the total bending moment is often used.
Also, designing beamed monolithic ceilings can be carried out using more accurate and modern technologies. For example, one of these techniques is called moment estimation.
This technique for performing the calculation when installing beams is nowadays used quite often. In this case, the first thing engineers take as the basis is that the forces on the capitals are distributed along the triangle. For the calculated span of the panel, the distance between the centers of gravity of the latter is taken. The total total bending moment in this case can be calculated by the following formula:
- M = 1/8 WL (1-2c / 3L) (1-2c / 3L).
Here W is the full load on the cell of the bezel-less floor slab, L is the distance between the columns, c are the sizes of the capitals.
This formula was derived by J. Nichols in 1914. Already in 1917, it was adopted as one of the ACI building codes. This formula is used to calculate floor slabs.
This somewhat more modern technique was developed on the basis of both experiments and theoretical data. In our country, V.I. Murashov and A.A. Gvozdev were engaged in its improvement in the 30s of the last century.
For a square panel, the formula in this case uses this:
- M0 = 1/8 WL (1-2c / 3L) (1-2c / 3L).
To determine the moments in the design sections and when designing the reinforcement, the floors using this technique are divided into spans and over-column strips in plan. Moreover, they do this in such a way that the width of each such part is equal to half the distance between the axes of the columns in all directions.
In each such strip during the operation of the building, negative and positive moments arise. At the same time, they are usually larger in the column elements than in the span elements. The width of the strip moments are determined by the curves. However, in practice, their stepwise measurement is used. In this case, along the width of the bands, the moments are taken constant.
With various kinds of plastic deformations, the redistribution of M can also occur. Therefore, the moments in the four calculated sections of the plates are determined so that their sum is ultimately equal to the beam M0.
Features of the installation of plates
The assembly technology of beam-free ceilings depends primarily on their variety. When using reinforced concrete slabs, the construction procedure is as follows:
- plate production at the enterprise;
- loading them onto vehicles and delivery to the construction site of the facility;
- unloading plates with a truck crane at the construction site;
- installation of plates on columns and walls of the building with a truck crane.
It is believed that the length of reinforced concrete slabs cannot exceed 9 m.
Installation of monolithic flooring
Such structures are poured into pre-assembled wooden formwork. The bottom of this shape is also made boardwalk. From below it is supported by special telescopic supports. After that, fill as follows:
- install reinforcement on special fungi-stands;
- concrete mixture is poured into the formwork.
Mortar is prepared at the enterprises with exact observance of all the required technologies in terms of proportions and uniformity. It is fed into the formwork using a hose from a tanker truck.
The form is removed from the overlap thus removed after about 2 weeks. All this time, the plate is poured daily with water from a hose in order to prevent the appearance of surface cracks. The further construction of the building is started no earlier than two weeks later. In order for concrete to gain sufficient strength, it takes at least a month.