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 are themselves distinguished by increased strength and excellent bearing capacity.
A bit of history
Beamless floors were first used in the construction of a building in 1902 in the USA by engineer Orlano Norcors. 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 dairy products warehouse. 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 extended top. Thus, the area of contact between supports and plates increased and the reliability of installation increased. Therefore, at the beginning of the century, ceilings of this type were called "mushroom-shaped".
Where used
Such floors can be equipped in buildings of almost any type. Very often, beamless structures can be seen, for example, in residential urban slab high-rise buildings. Also, in many cases, floors are made in this way in production workshops, warehouses, garages, etc.
In particular, such structures are often equipped at food industry enterprises. These can be, for example, dairies, workshops for the production of semi-finished products, etc. That is, most often beamless ceilings are mounted where there are increased hygiene requirements.
In private housing construction, interfloor structures of this type are rarely used. But sometimes suburban residential buildings are built in this way.
Main varieties
In construction, there are only three types of such floors:
- national teams;
- monolithic;
- precast-monolithic.
The first type of structure consists of two parts: a slab located above the column and a capital. Beamless prefabricated floors have a relatively simple configuration. The slab in this case rests on special shelves arranged above the column. The latter, in turn, are kept on the capitals and are interconnected by welding.
Monolithic and precast-monolithic structures
The second type of beamless floors is monolithic. They are used where smooth ceilings are needed. For example, they are widely usedin underground passages and the subway. Such ceilings are flat inseparable slabs supported by columns. The latter in this case also have capitals.
A feature of prefabricated monolithic beamless ceilings is that they are designed with a square or rectangular grid of columns. Most often, in this case, the supports are installed according to the 6x6 m scheme. Such floors are laid on prefabricated, span and above-column panels.
Capitalless ceilings
This kind of constructions among builders is also quite popular. In this case, the floor elements rest directly on the pylons and columns of the frame. Slabs in such structures most often have a constant thickness.
Such ceilings in the construction of buildings began to be used in 1940. A feature of beamless structures of this type is the reduced area of supporting plates on columns. For the perception of shear forces in this case, the technique of transverse reinforcement of beamless floors 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, columns of large diameter can be provided. When using such elements, the area of \u200b\u200bcontact between the supports and plates increases. And consequently, the loads cannot destroy the overlap in the area of the columns.
Types of frames
Buildings with beamless ceilings can be built using different technologies. The frames of such houses are:
- frame;
- liaison;
- frame-liaison.
In the systems of the first variety, the main bearing functions on the ceilings 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 tie frames, the main loads fall on the systems of columns and diaphragms, also called pylons. The role of the floors themselves in such buildings is greatly increasing. In addition to the actual vertical loads, in this case, these structures also perceive horizontal ones, after which they transfer them to the diaphragms.
Combined braced frames are usually used in load-bearing structures made of steel and monolithic reinforced concrete. In this case, diaphragm systems perceive 85-90% of horizontal loads. At the same time, with a small increase, they can withstand them completely, at 100%.
Benefits
Compared to conventional, beamless floors have a number of unconditional advantages. The advantages of such structures include in the first place:
- low labor intensity of finishing work;
- reducing the height and cubic capacity of the building;
- improving sanitation.
Finishing smooth beamless floors is much easier than regular ones. In this case, you do not even need to perform the filing of the ceiling. All that is needed to finish such an overlap is plastering the surface and further painting. Moreover, both of these operations will not take too muchtime.
Beamless reinforced concrete slabs are usually thinner than traditional ones. Accordingly, with the same cubic capacity, the building will be lower.
What other advantages are there
Care for the surface of beamless floors is much easier. Indeed, in this case, the design of the ceiling or floor does not have slots where debris or dust could clog. Accordingly, various kinds of pathogenic microorganisms 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
The disadvantages of such overlaps, of course, also exist. The main disadvantage of structures of this type, in comparison with beam structures, is their heavy weight. Supports for floors of this type have to be installed as strong as possible.
Also, the limited span width is considered a disadvantage of beamless structures. The distance between the supports under the slabs of such floors should not be too large. Reinforced concrete is a very durable material. But with a large area and a serious load, such a plate will still begin to bend and may even collapse.
Economically feasible is only the arrangement of beamless floors in spans no more than 5x6 meters wide at a load of 5 kN/m2. In this case, the designs usually turn out to be quite reliable.
Designing beamless floors is a rather complicated and very responsible procedure. Only an experienced person can do this job.highly qualified engineer. Difficulties in drawing up drawings, of course, can also be attributed to the disadvantages of such structures.
Features of the calculation of a beamless floor
Design floors of this type, therefore, should be as careful as possible. In conventional structures of this type, the load is taken by many fairly short lags. Plates, on the other hand, have a large area, and therefore can bend more.
How do the calculation of beamless floors? As already mentioned, such structures 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 ensuring the strength of the plates for punching.
The ceiling begins to collapse in this way around the column. The concrete in this place loses its integrity, which can lead to an 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 bearing area;
- by installing transverse reinforcement.
There are several methods for calculating beamless slabs, monolithic, prefabricated or prefabricated-monolithic. For example, in construction, the technology of calculating the total bending moment is often used.
Also, the design of beamless monolithic slabs can be carried out using more accurate and modern technologies. For example, one of these methods is calledmoments.
Old technology
This technique for performing calculations when installing beamless floors is used quite often in our time. In this case, the first thing engineers take as a basis is that the forces on the capitals are distributed over a triangle. In this case, the distance between the centers of gravity of the latter is taken as the calculated span of the panel. The total total bending moment in this case can be calculated using the following formula:
M=1/8 WL(1-2c/3L)(1-2c/3L)
Here W is the total load per cell of a beamless floor slab, L is the distance between columns, c is the dimensions of the capitals.
This formula was developed by J. Nichols in 1914. Already in 1917 it was adopted as one of the ACI building codes. This formula is used to calculate floors with capital columns.
Estimation of moments
This slightly more modern technique was developed based on both experimental 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 is:
M0=1/8 WL(1-2c/3L)(1-2c/3L)
To determine the moments in the design sections and in the design of reinforcement, floors using this technique are divided into span and over-column strips in plan. Moreover, they do it 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.
Beach such strip during the operation of the building there are negative and positive moments. At the same time, they are usually larger in over-column elements than in span elements. From the width of the bands, the moments are determined from the curves. However, in practice, their stepwise measurement is used. In this case, the moments are assumed to be constant over the width of the strips.
With various kinds of plastic deformations, redistribution of M can also occur. Therefore, the values of the moments in the four design sections of the plates are determined so that their sum is ultimately equal to the beam M0.
Features of plate installation
The assembly technology of beamless slabs depends primarily on their variety. When using reinforced concrete slabs, the construction technique is as follows:
- production of plates at the enterprise;
- loading them on vehicles and delivery to the construction site;
- unloading slabs by truck crane at the construction site;
- installation of plates on the 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 a monolithic ceiling
Such structures are poured into pre-assembled wooden formwork. The bottom of this form is also made plank. From below it is supported by special telescopic supports. After that, fill as follows:
- install fittings on special fungus-stands;
- concrete mixture is poured into the formwork.
The mortar is prepared at enterprises with strict observance of all required technologies in terms of proportions and uniformity. It is fed into the formwork using a hose from a tank truck.
The form is removed from the overlap filled in this way after about 2 weeks. All this time, the plate is daily watered with water from a hose in order to prevent the appearance of surface cracks. Further construction of the building will start no earlier than in another two weeks. It takes at least a month for concrete to gain sufficient strength.
