The appearance of reinforced concrete structures was a real breakthrough in the construction of the twentieth century. Thanks to reinforced concrete, our ideas about strength, durability and reliability have radically changed. With the advent of this material, modern cities ceased to be flat and low-rise; skyscrapers, symbols of the power and modernity of the new era, came into fashion. This material has dramatically changed our world, making it safer and more accessible.
So what are reinforced concrete and reinforced concrete structures?
Reinforced concrete is a combination of reinforcement and concrete, which together form a single whole, and based on the totality of their physical characteristics, they provide maximum strength to this material.
As is known, concrete has high compressive properties, but at the same time low tensile strength (tensile strength of concrete is 10-15 times less than compressive strength). Therefore, concrete (non-reinforced) structures are practically not used. To improve the physical properties of concrete, steel wire was added to its structure, which, as is known, works well in tension. Thus, reinforced concrete was created - an effective material in which compressive stresses are absorbed by concrete, and tensile stresses are absorbed by steel reinforcement.
Reinforced concrete products began to be patented at the end of the 19th century, and since then this material has gone through a long road of evolution, which is more than 150 years, but we can say with confidence that the improvement of reinforced concrete products is not yet finished. Modern reinforced concrete structures are reinforced not only during tension and bending, but also during torsion, shear, eccentric and axial compression. In these cases, working reinforcement is installed to reduce the cross-sectional dimensions of elements and reduce the dead weight of structures, as well as to ensure greater reliability.
Today, along with conventional reinforcement, special, pre-stressed reinforcement is also made. Prestressing makes it possible to effectively use stronger reinforcing steels and high-grade concrete, which is not possible in conventional reinforced concrete. In prestressed reinforced concrete structures, the reinforcement is subjected to pre-tension, and the concrete is subjected to compression. Prestressing of reinforced concrete structures significantly increases crack resistance and reduces deformation of structural elements, since it creates preliminary compression of concrete in those parts that, under operational load, work in tension.
Types of reinforced concrete structures
According to the method of execution, reinforced concrete structures can be prefabricated, monolithic or precast-monolithic.
Precast concrete structures
Prefabricated reinforced concrete structures are more common, since their use makes it possible to industrialize and maximize the mechanization of construction. When manufacturing prefabricated structures in factory conditions, the most advanced technology for preparing, laying and processing concrete mixtures can be widely used, production can be automated, and construction work can be significantly simplified.
Monolithic reinforced concrete structures
Monolithic reinforced concrete structures are widely used in structures that are difficult to divide and unify, for example, in some hydraulic structures, heavy foundations, swimming pools, in structures made in mobile or sliding formwork (shells) coverings, silos, etc.).
Prefabricated monolithic reinforced concrete structures
Precast monolithic reinforced concrete structures are a combination of prefabricated elements and monolithic concrete laid at the construction site.
Advantages and disadvantages of reinforced concrete products
Durability. Reinforced concrete is distinguished by its exceptional durability due to the reliable preservation of the reinforcement enclosed in concrete. Reinforced concrete resists atmospheric influences well, which is especially important in the construction of open engineering structures (overpasses, masts, pipes, bridges, etc.).
Strength reinforced concrete not only does not decrease over time, but may even increase.
Fire resistance. Concrete structures have high fire resistance. Practice has shown that a protective layer of concrete 1.5–2 cm thick is sufficient to ensure the fire resistance of reinforced concrete structures during fires. In order to further increase fire and heat resistance, special fillers are used (basalt, diabase, sha-mot, blast furnace slag etc.) and increase the thickness of the protective layer to 3-4 cm.
Seismic resistance. Reinforced concrete products, due to their monolithic nature and greater rigidity compared to structures made from other materials, are characterized by very high seismic resistance.
High performance. Reinforced concrete can easily be given any appropriate structural and architectural forms. Operating costs for maintaining structures and caring for reinforced concrete structures are very low. In terms of time spent on manufacturing and installation, prefabricated reinforced concrete structures can compete with steel ones, especially when manufacturing reinforced concrete structures using the rolling method, the cassette method, when mounting from wheels and using other progressive methods of manufacturing and installation.
Disadvantages of reinforced concrete structures
The disadvantages of reinforced concrete structures include relatively large dead weight, high heat and sound conductivity, the possibility of cracks appearing before the application of operational load (from shrinkage and self-stresses in reinforced concrete for technological reasons), as well as from the action external loads due to the low tensile strength of concrete.
Basic physical and mechanical properties of concrete, reinforcing steel and reinforced concrete products
Concrete for concrete and reinforced concrete products must have sufficiently high strength, good adhesion to the reinforcement and density, which ensures the safety of the reinforcement from corrosion and the durability of the structure. Sometimes it is additionally required to provide: water-permeability, water resistance, frost resistance, increased fire resistance and corrosion resistance, low weight, low heat and sound conductivity. For prestressed structures, concrete with increased strength and density, limited shrinkage and creep is used.
The physical and mechanical properties of concrete depend on the composition of the mixture, the type of binders and fillers, the water-binding ratio, methods of preparation, laying and processing of the concrete mixture, hardening conditions (natural hardening, steaming, autoclave processing), age of concrete, etc. All this should be taken into account when choosing materials for concrete, designating its composition and methods of preparation. The most widely used in construction are ordinary heavy concretes with a density of 2200-2500 kg/m3 inclusive, prepared using ordinary dense aggregates. Concrete with a density of more than 2500 kg/m3 is considered particularly heavy; they are used, for example, for radiation protection.
With a concrete density of more than 1800 kg/m³ to 2200 kg/m³, concrete is classified as lightweight, and with a density of 1800 kg/m³ and below - lightweight.
Lightening the weight of concrete is achieved by using porous aggregates. Cellular concrete is a mixture of binders, water, finely ground aggregate and vapor-forming substances. Concretes based on porous aggregates and cellular concretes, compared to heavy concretes, are distinguished not only by their lower own weight, but also by their reduced sound and thermal conductivity. However, they are prone to increased deformation under load, are characterized by higher shrinkage and creep, and their adhesion to reinforcement is worse than conventional concrete. In some cases, these concretes require anti-corrosion coating of reinforcement.
Concrete for structures operating in special conditions, must meet the relevant specific requirements. Thus, for hydraulic structures (hydraulic concrete), in addition to sufficient strength, concrete must have increased waterproofness, water resistance, frost resistance, and for massive parts of structures - low heat generation during hardening (low exothermicity).
Ordinary concrete, when exposed to high temperatures for a long time, is destroyed due to dehydration of the cement stone, its severe shrinkage and reduction in strength, differences in temperature deformations of the cement stone and aggregates, and other reasons. In this regard, ordinary concrete with a cement binder is allowed for use in structures exposed to prolonged exposure to temperatures not exceeding 50°C.
For operation of structures at higher temperatures, heat-resistant concrete should be used. Concrete for structures exposed to aggressive environments must have sufficient corrosion resistance. To protect concrete from the penetration of aggressive substances, the surface of structures is shotcrete, rubbed, coated liquid glass, plastic films, bituminous materials, varnishes and paints, or lined with acid-resistant ceramic tiles, etc.
Improving the properties of concrete was achieved by introducing polymers into its composition. Such concretes are called plastic concretes or polymer concretes. Used as polymer binders various types thermoplastics, rubbers and thermosetting resins. Concrete made with polymer-mineral binders has increased resistance to aggressive environments, however, their corrosion resistance is selective and depends on the type of polymer.
Other positive properties of concrete with additions of thermoplastics and rubbers include increased impact strength and abrasion resistance. Such concretes are used for lining tanks, pipes, channels, for covering roads and airfields, etc.
For concrete and reinforced concrete structures made from ordinary heavy concrete, the following classes of compressive strength are provided: B3.5; B5; B7.5; B10; B12.5; B15; B20; B25; B30; B35; B40; B45; B50; B55; B60.
For reinforced concrete structures made of heavy concrete, concrete of class below B7.5 is not allowed. For repeated loads, concrete of class B15 or higher is recommended. For reinforced concrete compressed rod elements, concrete of a class not lower than B15 should be taken, and for heavy loads (for example, for columns of the lower floors of multi-story buildings or with significant crane loads) - not lower than B25.
The average density grade of concrete corresponds to the average density of concrete in the dried state in kg/m3. For lightweight concrete based on porous aggregates, concrete grades in terms of density lie within D 800 - D 2000 with an interval of 100. At a density above 2000 to 2200 kg/m³, concrete is classified as lightweight, and at more than 2200 kg/m³ - heavy.
The frost resistance grade of concrete characterizes the number of cycles of alternate freezing and thawing in a water-saturated state that the samples can withstand. For heavy concrete, the following frost resistance grades are established: P50; P75; P100; ISO; P200; P300; P400; P500.
The grade of concrete for water resistance depends on the degree of water permeability of the concrete. As brands increase, the values of filtration coefficients Kf decrease. The following grades of concrete have been established for water resistance: W2, W4, W6, W8, W10, W12.
Types and mechanical properties of steel reinforcement reinforced concrete products
As noted above, reinforced concrete acquired its unique qualities through the use of reinforcement, therefore the properties of reinforced concrete structures directly depend on its quality, quantity and filling.
According to manufacturing technology, steel reinforcement is divided into hot-rolled rod and cold-drawn wire. Depending on the nature of the surface, the reinforcement can be smooth or of a periodic profile, i.e. with a notch (to improve adhesion to concrete). The mechanical properties of reinforcing steels depend on the manufacturing technology of the reinforcement and chemical composition steel.
Hot-rolled steel bars of periodic profile have found the greatest application as reinforcement for reinforced concrete structures. The shape of a periodic profile (with notches) improves the adhesion of reinforcement to concrete, which reduces the width of cracks in concrete during tension and allows one to avoid a number of constructive measures for anchoring reinforcement. Rod reinforcement is divided into classes: hot-rolled classes A-1, A-I, A-Sh, A-1U, A-U and A-VI, thermally and thermomechanically hardened classes At-Sh, At-1U, At-U, At -U1, At-UN, reinforced with class A-Shv hood.
For the reinforcement of reinforced concrete structures, ordinary reinforcing wire of class VR-1 (corrugated) with a diameter of 3-5 mm, obtained by cold drawing, is widely used. High-strength reinforcing wire is also produced by cold drawing classes V-I and Vr-P - smooth and periodic profile with a diameter of 3-8 mm with a conditional yield strength wires V-I- 1500-1100 MPa and Vr-I - 1500-1000 MPa.
Reinforcement of reinforced concrete structures is selected taking into account its purpose, class and type of concrete, manufacturing conditions of reinforcement products and operating environment (risk of corrosion), etc. As the main working reinforcement of conventional reinforced concrete structures, steel of classes A-Sh and Vr-1 should be used. In prestressed structures, predominantly high-strength steel of classes V-I, Vr-N, A-VI, At-U1, A-U, At-U- and At-UC is used as prestressing reinforcement.
Elements of buildings and structures made of reinforced concrete and their combinations. They are widely used in many areas of construction, in some cases they are more appropriate and economical than structures made from other materials.
Reinforced concrete structures- the main type of structures for the construction of industrial and warehouse buildings, silos, bunkers and tanks, water supply and sewerage systems. structures, overpasses, foundations for rolling mills and dynamically driven machines. loads, high chimneys, retaining walls etc. Reinforced concrete structures are widely used in the construction of bridges, hydraulic engineering. structures, thermal power plants, during underground work, construction of airfields, roads, supports and poles for power lines, communications, lighting, overhead roads, etc. In the construction of residential and public buildings, buildings, prefabricated reinforced concrete elements are increasingly being used, including large-panel. Reinforced concrete structures are the basis of long-term defense structures. Significant progress has been made in the construction of reinforced concrete floating docks and ships. At nuclear power plants, reinforced concrete barriers against radiation are installed.
Modern reinforced concrete structures are very diverse. In accordance with two main types of reinforced concrete are distinguished reinforced concrete structures made of ordinary and prestressed reinforced concrete. Conventional reinforced concrete structures are classified according to three criteria - method of execution, type of reinforcement and type of concrete; In addition, all reinforced concrete structures differ in the type of stress state.
Monolithic reinforced concrete structures, carried out directly on the construction site, have in many cases given way to more industrial prefabricated reinforced concrete structures made in factories. Monolithic structures are used when elements are non-standard and have low repeatability, under particularly heavy loads, as well as in structures that are difficult to divide (swimming pools, foundations for rolling equipment, etc.). Finally, they are appropriate when they can be carried out using industrial methods using inventory forms - sliding, adjustable (silos, factory pipes, etc.), mobile (certain shells), etc.
Prefabricated reinforced concrete structures are increasingly used in construction, especially residential, civil and industrial. The design of precast concrete elements is significantly influenced by their manufacturing and installation methods. The production of reinforced concrete elements in a factory way is undergoing significant development - in cassette forms, by the method of vibratory rolling, vibration stamping, etc., with which a high production speed and a reduction in their weight are achieved. Prefabricated monolithic concrete structures are a combination of prefabricated elements with monolithic concrete, providing a reliable connection between them.
Conventional reinforced concrete structures are made mainly with flexible reinforcement in the form of sections. rods or welded mesh and frames" Welded reinforcement, due to its better anchoring, allows the use of steel of higher strength; This method of reinforcement is more industrial.” Reinforced concrete structures with load-bearing reinforcement (rolled profiles or spatial welded frames) are used relatively rarely and only in monolithic reinforced concrete. In this case, concreting is carried out in suspended formwork using reinforcement as a supporting structure; increased steel consumption is required.
Heavy concrete (volume weight more than 1800 kg/m3) is widely used in monolithic and prefabricated residential buildings. Cement-sand concrete, prepared by vibration mixing, is used for thin-walled structures. Reinforced concrete structures made of lightweight and cellular concrete are used in Ch. arr. in order to obtain lightweight structures, and in housing and civil. (and industrial) construction, their heat-shielding properties become of great importance. Reinforced concrete structures made from heat-resistant concrete are increasingly being introduced into the construction of metallurgy, oil and chemical structures. industry; their use provides significant savings in metal, simplifies construction methods and eliminates the need for expensive refractories.
In reinforced concrete structures, reinforcement usually serves to absorb tensile forces, in the direction of which the reinforcing bars are located, but in some structures, the reinforcement perceives compressive forces together with concrete. The simplest reinforced concrete structures in which tensile forces arise during bending are a slab and a beam of rectangular cross-section. In beams, the supports are located along the same line along the axis, in slabs - along the entire width, and often along the entire contour. With a side ratio of more than 2:1, the slab is called a beam slab; with a ratio of less than 2:1 and supports along the entire perimeter, it is called a contour-supported slab.
Slabs and beams can be simply supported, with embedded supports, continuous, or cantilevered. In a slab (beam) lying freely on two supports and uniformly loaded, the bending moments, equal to zero at the supports, gradually increase towards the middle, reaching a maximum there; At the same time, tensile stresses in the lower zone of the slab increase towards the middle. To avoid destruction of the slab due to the low tensile strength of concrete, the reinforcement is located in the tension zone, near the lower surface of the slab. A slab (beam) with embedded ends, other things being equal, can have a smaller cross-section than a simply supported one. In accordance with the diagram of bending moments in the middle part of such a slab, the lower fibers are subject to tension, and the upper fibers are subject to compression; in places of embedding, on the contrary, tensile stresses act in the upper zone, and compressive stresses act in the lower zone. Therefore, in a slab with embedded ends, the reinforcing bars are located
both below and above. In practice, the most appropriate here is curved reinforcement, which can withstand tensile stresses in both the lower and upper zones. In multi-span continuous slabs and beams, the reinforcement is located in accordance with the diagram of the largest positive and negative moments. In a cantilever slab (beam), tensile forces arise in the upper part of the section, where the reinforcement is placed. In accordance with the increase in moment, the slab at the support is usually made thicker.
Reinforcing bars that absorb the main forces are called working bars. The beam slabs also contain distribution bars to keep the working bars at a certain distance, to counteract the formation of cracks due to concrete shrinkage and temperature fluctuations and to better distribute the (concentrated) load.
Reinforcement designed to absorb compressive forces can be located in two ways. In the first case, the working rods are located in the direction of the compressive forces. This reinforcement works together with concrete directly in compression. In beams, compression reinforcement is used when the cross-sectional dimensions are limited. In columns and racks this arrangement of reinforcement is common; In addition to the longitudinal rods, transverse connections are installed in them - clamps, which prevent the longitudinal rods from bulging during compression and thereby increase the overall resistance of the element to compression. According to the second method, the reinforcement to strengthen the compressed concrete is located perpendicular to the direction of the compressive force. Such reinforcement prevents the transverse expansion of concrete and thereby forces it to work under conditions of all-round compression, when the resistance of concrete to compression greatly increases. Transverse reinforcement, also called indirect, is laid in the form of a spiral of round steel or separate. rings. In eccentrically compressed elements (frame posts, arches, vaults, etc.), the reinforcement on one side of the section works in tension, on the other - in compression, but often the reinforcement works in compression on both sides of the section.
Monolithic reinforced concrete structures. In foreign construction practice, monolithic ribbed and beamless floors are widespread; in the USSR they are used less frequently, mainly in industry. building If part of the concrete located in the tensile zone of a thick slab and serving mainly for the connection between the tensile reinforcement and the compressed section zone is removed, leaving concrete only directly above the rods, which are brought into groups, then a ribbed slab will be obtained. This structure works as a slab or beam of rectangular cross-section, having the width of the ribbed slab B and its total height h. A ribbed slab is more economical and has a lower dead weight than a rectangular slab and is therefore subject to less bending moment for the same payload. Thin parts of the slab in the spaces between the ribs also experience bending in the other direction under load and must be provided with reinforcement perpendicular to the ribs.
In a ribbed floor, beams usually go in two directions: the main ones - along the lines of the columns; secondary ones, resting on the main beams, are perpendicular to them; the slab covering the beams is monolithically connected to them. The reinforcement is concentrated in the ribs, where it is much stronger than in a solid slab. Tangential and main tensile stresses, distributed in an ordinary slab over a large area and not playing a serious role, here in beams (ribs) are of great importance, since they are perceived by a smaller section of concrete. This requires strengthening the ribs with transverse reinforcement in the form of clamps and bent rods when reinforcing the sections. rods or with welded frames in the form of transverse rods. Depending on the width of the beam, one, two or three flat frames are installed (rarely more). In a regular ribbed ceiling monolithic slab is a beam and when reinforcing it, it is separated. the number of rods is taken from 5 to 14 per 1 linear. m. Plates are often reinforced with welded mesh, and the reinforcement can be continuous or separate.
In places in the slab, stresses arise mainly perpendicular to them. In such slabs, as in beam slabs, they are used welded mesh, which greatly simplifies and speeds up reinforcement.
Ribbed floors can have slabs supported along the contour if the aspect ratio of the slabs formed by the intersection of the beams is less than 1.5. The reinforcement of such slabs in both directions will be working. In this case, rods parallel to the beams are located less often near them than in the middle of the slab, since in these the concrete slab is monolithically connected directly to the columns, upper part(capital) for this purpose expands like a mushroom (abroad, such ceilings are called mushroom-shaped). Sometimes, to obtain a smooth ceiling, reinforced concrete capitals are replaced with rigid reinforcement hidden in the slab. Depending on the placement of the columns, the floors have square or rectangular panels. Square panels are more economical. Spans rarely exceed 6 flights.
Among monolithic reinforced concrete structures, various kinds of thin-walled spatial coverings are of great importance. Some of them (cylindrical, shed) are easily carried out in inventory mobile formwork. In sliding and adjustable formwork, monolithic wasp forces, glass-type water towers, factory-made chimneys, television towers and other high-rise buildings.
Prefabricated reinforced concrete structures. On the production and use of prefabricated reinforced concrete in the USSR. Reinforced concrete for industrial and housing construction. Catalogs of unified prefabricated housing complexes are periodically published.
Basic Prefabricated reinforced concrete elements - slabs, beams and columns - differ in shape and design from monolithic ones. Along with small slabs, large panels are widely used, the use of which contributes to the maximum industrialization of the country and better use of lifting and transport mechanisms. Prefabricated reinforced concrete beams are manufactured in various sections - rectangular, T-beams with a flange at the top or bottom, hollow, I-beams, U-shaped, etc. The most common are single-span beams; continuous ones are used for dynamic amplifiers. loads and seismic building Precast beams are usually reinforced with welded frames or prestressed reinforcement. The maximum weight of elements of mass use is limited by the lifting capacity of cranes: in residential construction it is usually 1.5, 3 and 5 /p, in industrial construction - up to 10 tons, and in some cases - up to 40 tons or more. Dept. elements, as a rule, are connected using electric arc welding of metal embedded parts (steel sheets, angles, channels or I-beams) or reinforcement rods, followed by concrete coating.
Two building schemes are used - with column spacing of 6 and 12 columns with spans from 6 to 36 columns. Reinforced concrete racks come in rectangular and I-beam sections, and at particularly high heights - two-branch (paired). The foundations for prefabricated racks are made of reinforced concrete stepped ones - monolithic or prefabricated glass type. Reinforced concrete prestressed gable beams are most often used as transverse load-bearing structures for spans from 12 to 24 m, and reinforced concrete trusses for spans up to 36 m. Segmental trusses with a prestressed bottom chord have become more widespread. For buildings with a flat roof, reinforced concrete trusses with parallel chords are used, in which the lower chord and stretched lattice elements are subjected to prestress. Pre-stressed ribbed panels measuring (3 and 1.5) m X 6 m and (3 and 1.5) m X 12 m or double-cantilever panels 3 m X 12 m are laid on beams or trusses. With a column spacing of 12 m, they are often installed in the longitudinal direction reinforced concrete rafter structures for supporting intermediate transverse load-bearing trusses on them.
Along with planar coating systems, spatial thin-walled systems are used, which are technically and economically advantageous - various types of shells and folds, prefabricated or prefabricated monolithic. In Fig. 7, a and b show diagrams of two prefabricated long-span shells - a flat shell of double curvature, covering an area of 40 m X 40 l or more, as well as coverings in the form of barrel vaults with a span of up to 100 m. The main feature of these shells is prefabricated elements (flat or curved) factory-made, from which the coatings are assembled on site. In addition to long-span shells, prefabricated and precast-monolithic shells of various sizes are being developed and implemented - cylindrical long and short, shells in the form of hyperbolic shells. paraboloids, wavy ones made of reinforced cement, etc. For external walls with a column spacing of 6 m, panels measuring 6 m X 1.2 and 6 l * X 1.8 l are used; at a pitch of 12 m, it is advisable to install prestressed panels 12 m long. In buildings with overhead cranes with a lifting capacity of 5 to 30 tons, reinforced concrete prestressed crane beams with a span of 6 or 12 m are used.
In prefabricated multi-storey industrial buildings. In buildings, depending on the nature of production, loads and manufacturing conditions of structures, beam or beamless floor schemes are used. For most industrial buildings, a grid of bmHbm and 9mHbm columns is installed; at the same time, with a load of up to 1000 kg/m2, the primary use of a 9l* X 6 m mesh is recommended; at loads of 2000 and 2500 kg/m2 - 6zhH 6 m.
In beam-type buildings with a reinforced concrete frame and self-supporting walls, the foundations, as a rule, are three-stage reinforced concrete monolithic, the columns are of square or rectangular section with consoles, on which prefabricated purlins of T-section are laid or, for better use of floor heights, rectangular section with side shelves; Ribbed or hollow-core flooring is laid along the purlins. In buildings without beams, square capitals are installed on columns of rectangular (or round) cross-section with small consoles, on which over-column slabs-beams with quarters are laid on four sides, and on them - medium square slabs. Slabs - beams and square slabs - solid or multi-hollow. Weight dep. elements up to 5 tons. During installation, the capitals are connected to the columns by welding embedded parts. Consolidation with concrete ensures their reliable connection. The beam slabs are connected to the capitals by welding steel parts.
Precast monolithic reinforced concrete structures are less industrialized than prefabricated ones. It is advisable to use them under large dynamic impacts from installations, when it is necessary to divide a structure into large elements, if the use of powerful cranes is unprofitable, if there are many openings and holes in the ceilings that make it difficult to use standard prefabricated elements, and in a number of other cases.
Often in prefabricated monolithic reinforced concrete structures, the tensile zone is formed by prefabricated elements that serve as formwork, and the compressed zone is formed by ordinary monolithic concrete or reinforced concrete. In winter, concreting prefabricated monolithic buildings, like monolithic ones, is associated with some difficulties. An example of a prefabricated monolithic interfloor slab. From the prefabricated T-iron of the floor, clamps are released upward and after laying the prefabricated ribbed flooring and additional rods - limit states: in terms of bearing capacity (strength or stability), in terms of deformations (stiffness) and in terms of the formation of cracks or their maximum opening. The task of the calculation is reduced to providing for a given structure guarantees against the occurrence of one or another calculated limit state in it during operation.
In connection with new views on the strength of structures and their limit states and the need to unify methods for calculating structures made of different materials, the method of calculating reinforced concrete in the USSR was revised. With the previously used method of calculation by stage of destruction with a single general safety factor, possible fluctuations in actual loads, strength characteristics of materials, section sizes, etc. could not be taken into account. These deviations, and therefore the load-bearing capacity of the structure, are more correctly assessed when calculating according to limit states. There are three resistances, which are standard resistances multiplied by the corresponding coefficients of homogeneity of materials and coefficients of operating conditions of concrete and reinforcement, as well as the coefficient of operating conditions of the structure t and the geometric characteristics of the section S. When calculating by bearing capacity, the limit state, for example, of bending elements is characterized by the perception of full forces of tensile reinforcement with full use of the resistance of concrete and reinforcement of the compressed zone. The diagram of compressive stresses in concrete is assumed to be rectangular at stresses equal to the design resistance of concrete to compression during bending RK and stress in the reinforcement equal to its design resistance. The magnitude of the calculated forces (M, N and Q) in the strength characteristics of materials (Ra and i?a). The magnitude of the design forces in the sections of elements in most cases should be determined taking into account plasticity. deformations. However, the application of this calculation is still limited and in many cases static. the calculation is made assuming elastic operation of the structure.
Abroad, calculations of reinforced concrete structures are usually carried out using the “elastic reinforced concrete” method, i.e., according to permissible stresses. However, in socialist countries and in some capitalist countries. (Austria, England, Brazil, USA) in some cases, calculations based on the stage of destruction are also used. The limit state calculation method was introduced, for example, in Hungary.
Lit.: Murashev V.I., Sigalov E.E., Baykov V.N., Reinforced concrete structures. General course, ed. P. L. Pasternak, M., 1962; Sakhnovsky K.V., Reinforced concrete structures, 8th ed., M., 1959; Pasternak P.L., Antonov K.K., Dmitriev S.A., Reinforced concrete structures, M., 1961; Designer's Handbook, ed. V. I. Murasheva, [vol. 5], M., 1959; Gvozdev A. A., Calculation of the load-bearing capacity of structures using the limit equilibrium method, M., 1949; Instructions for the calculation of statically indeterminate reinforced concrete structures taking into account the redistribution of forces, M., 1960; Berg O. Ya., Physical foundations of the theory of strength of concrete and reinforced concrete, M., 1961; SN and P, part 2, section. B, ch. 1 - Concrete and reinforced concrete structures. Design standards, M., 1962.
Reinforced concrete, compared to other building materials, appeared relatively recently and almost simultaneously in Europe and America. Its history goes back no more than 150 years. However, by now it has become the most widespread in construction, has its own history and its own outstanding figures.
Reinforced concrete structures are load-bearing elements of buildings and structures made from reinforced concrete, and combinations of these elements.
The appearance of reinforced concrete structures is associated with the great growth of industry, transport and trade in the second half of the 19th century, when it was necessary to build new factories, factories, ports and many other capital structures. By this time, the cement industry and ferrous metallurgy were developed. They were preceded by centuries of experience in construction with stone, unreinforced concrete, wood and two centuries of experience in construction with metal.
Studies of the coatings of the Tsarskoye Selo Palace showed that Russian craftsmen used reinforced concrete back in 1802, but they did not consider that they had received a new one building material, and did not patent it.
The first product made of reinforced concrete was a boat built by Lambeau in France in 1850. The first patents for the manufacture of reinforced concrete products were received by Monier in 1867... 1870. In 1892, the French engineer F. Gennebic proposed monolithic reinforced concrete ribbed floors and a number of other rational building structures, and all subsequent reinforcement drawings are drawn conditionally, as if the concrete is transparent and the reinforcement is clearly visible throughout the entire thickness of the concrete. In Russia, reinforced concrete began to be used in 1886 for floors on metal beams.
In 1885 in Germany, engineer. Weiss and Prof. Bauschinger conducted the first scientific experiments to determine the strength and fire resistance of reinforced concrete structures, the safety of iron in concrete, the adhesion forces of reinforcement to concrete, etc. At the same time, for the first time, engineer. M. Koenen suggested, confirmed by experiments, that the reinforcement should be located in those parts of the structure where tensile forces can be expected.
In 1886, M. Koenen proposed the first calculation method reinforced concrete slabs, which contributed to the development of interest in the new material and the wider distribution of reinforced concrete in Germany and Austria-Hungary.
In 1891, the most talented Russian builder, prof. N. A. Belelyubsky was the first to conduct a series of tests of reinforced concrete structures: slabs, beams, arches, tanks, grain silos, a bridge with a span of 17 m, which, in terms of test methods and results obtained, were in many ways superior to the work of foreign scientists and served as the basis for the widespread use of reinforced concrete in construction. In 1911, the first technical conditions and standards for reinforced concrete structures were published in Russia.
The time of appearance of F. Hennebique's proposals, i.e. late XIX century, can be considered the beginning of the first stage in the development of reinforced concrete, characterized by the appearance in practice of various kinds of reinforced concrete rod systems. Since that time, the method of calculating concrete structures based on permissible stresses, based on the laws of resistance of elastic materials, has come into widespread practice. The development of reinforced concrete during this period was greatly influenced by the works of scientists N. M. Abramov (on the calculation of reinforced concrete) and I. G. Malyugi, A. A. Baykov, N. A. Zhidkevich, M. Belyaev and others (on the development fundamentals of concrete technology).
In 1904, in Nikolaev, according to the design of engineers N. Pyatnitsky and A. Baryshnikov, the world's first sea lighthouse was built from monolithic reinforced concrete, 36 m high, with walls 10 cm thick at the top and up to 20 cm at the bottom. Around the same time, beamless interfloor slabs were installed for a dairy products warehouse in Moscow. The priority for the creation of these structures belongs to the Russian engineer, later an outstanding scientist, prof. A.F. Loleytu. However, in pre-revolutionary Russia there were no conditions for genuine progress in the development of reinforced concrete.
The idea of prestressing tensile elements was first put forward and implemented in 1861 by Russian artillery engineer. A.V. Gadolin in relation to the manufacture of steel barrels of artillery guns.
The issue of using prestressed reinforcement in reinforced concrete structures was raised in 1928 in the works of E. Freissipe, and then in the works of German engineers F. Dischinger, E. Heuer, U. Finsterwalder and others, which served as the beginning of the practical use of prestressed reinforced concrete structures .
After the revolution, reinforced concrete construction in Russia received a scale unprecedented in the world. The need to save material as much as possible and reduce the cost of reinforced concrete structures forced the Soviet school to take into account all the most advanced in European and American practice and widely develop its own theoretical and experimental research in the field of reinforced concrete. For these purposes, soon after the revolution, a number of research institutes and laboratories were created for the theoretical and experimental study of the physical and mechanical properties of concrete and reinforced concrete. Departments of building structures were organized in construction and transport universities. All this made it possible to train highly qualified reinforced concrete specialists in a short time. This, in turn, contributed to a significant expansion of the use of reinforced concrete in hydraulic engineering and housing and civil construction.
In 1925... 1932 Soviet scientists V. M. Keldysh, A. F. Loleit, A. A. Gvozdev. P. L. Pasternak and others, based on extensive experimental work, developed general methods calculation of statically indeterminate rod systems (arches and frames), which made it possible to design and build many public and industrial buildings made of reinforced concrete that were unique for their time: the Central Telegraph, the Izvestia House, the buildings of the Ministries of Light Industry and Agriculture in Moscow, the post office and the House of Industry in Kharkov, the House of Soviets in Leningrad, Minsk, Kyiv and a number of other large buildings.
In hydraulic engineering construction, reinforced concrete was first used during the construction of the Volkhov hydroelectric power station (1921... 1926), the largest at that time. The dam was built on reinforced concrete caissons, transported to the installation site afloat. The main station building is reinforced concrete frame, with reinforced concrete arcades supporting the 130-ton track overhead crane. Reinforced concrete was also widely used in the main substation and in all secondary substations. Volkhovstroy was the first big practical school Soviet specialists in reinforced concrete. Following the Volkhov hydroelectric power station, the Dnieper hydroelectric power station (1927... 1932) and the Nizhne-Svirskaya hydroelectric power station (1928... 1934) were built, in which concrete and reinforced concrete were used even more widely.
Around 1928, reinforced concrete began to be widely used in the construction of thin-walled spatial structures: various shells, warehouses, tents, vaults and domes. Soviet scientist V. Z. Vlasov was the first to develop a general practical method for calculating shells, significantly ahead of foreign science in this area. In 1937, the world's first “Instructions for the calculation and design of thin-walled coverings and floors” was published, compiled on the basis of theoretical and experimental work carried out under the leadership of A. A. Gvozdev.
The first thin-walled dome of significant diameter (28 m) was built in 1929 in Moscow for a planetarium, and the largest smooth dome at that time with a diameter of 55.5 m was built in 1934 over the auditorium of the theater in Novosibirsk. The design of the dome was developed by engineer. B.F. Mothers according to the idea and under the leadership of P.L. Pasternak.
The use of frame and thin-walled spatial systems in construction using their rigidity and solidity should be considered the second stage in the development of reinforced concrete.
In 1936, prestressed reinforced concrete was first used in the USSR for the manufacture of cable network supports in the Transcaucasian railways. The widespread introduction of prestressed reinforced concrete structures was greatly facilitated by the work of scientists V.V. Mikhailov, A.A. Gvozdev, S.A. Dmitriev and others.
Enormous work on studying and creating the theory and practice of reinforced concrete structures and on developing the most progressive solutions is carried out by the Research Institute of Concrete and Reinforced Concrete (NIIZhB) and many other research and design institutes.
Based on an in-depth study of the physical and elastoplastic properties of reinforced concrete, as well as experimental data, A.F. Loleit, A.A. Gvozdev and others (1931...1934) created a theory for calculating reinforced concrete based on destructive forces. It was the basis for the standards (OST 90003-38), according to which all industrial and civil buildings and structures were calculated.
The widespread industrialization of reinforced concrete construction, the development of prestressed structures, the introduction of high-strength materials and the development of a new method for calculating reinforced concrete structures should be considered the beginning of the third stage in the development of reinforced concrete structures. An outstanding example of the third stage is the tower of the Big Moscow Television Center, built in 1965, with a total height of 522 m. The lower part to a height of 385 m is made of monolithic prestressed reinforced concrete. The diameter of the tower at the bottom is 18.0 m, and at the top - 8.5 m with wall thicknesses of 46 and 30 cm, respectively. At around 65 m, the tower trunk turns into a conical base with a bottom diameter of 61 m. At an altitude of 360 m there is a restaurant for 420 people and observation decks for 600... 700 people. The lower part of the conical base is made in the form of supporting structures (legs) with a height of 17.3 m. At the 42 m mark, the shell of the conical base has a diaphragm ring that absorbs the force from anchoring the prestressed reinforcement ropes.
Soviet scientists and engineers carried out fruitful scientific and design research in all areas of the theory and practice of reinforced concrete. The accumulated experience and powerful construction industry are a solid foundation that ensures further progress of reinforced concrete structures in our country.
The construction of modern facilities is not complete without reinforced concrete structures. Such structures have many advantages. The iron frame is protected on all sides by concrete, which has long term work and is not afraid of rain, snow, heat, or frost. Iron and concrete are a great tandem! Reinforced concrete products are consolidated both during tension, compression and bending, and during twisting and shearing. The metal frame helps to achieve stability, strength and hardness of the structure, and serves to reduce the size and weight of the device. Using various technologies, they produce monolithic, prefabricated, precast-monolithic concrete and reinforced concrete structures with non-prestressed and prestressed reinforcement.
Reinforced concrete structures have found application in the construction of residential buildings, industrial buildings and engineering buildings. Prefabricated reinforced concrete is most often used, but monolithic and precast-monolithic are also found. In order to obtain a product with the smallest mass, as far as technology allows, to reduce labor and materials costs, high-quality concrete mortar and high-strength reinforcement are used for reinforced concrete structures.
The main types of reinforced concrete products are used in construction, where the temperature does not exceed fifty degrees Celsius and does not drop to minus seventy degrees. Reinforced concrete structures are used more often than steel or stone ones in the construction of the following objects:
- airfields;
- nuclear reactors;
- bunker;
- high chimneys;
- various massive structures;
- warehouse buildings;
- roads;
- foundations;
- offshore structures;
- factory buildings.
Often reinforced concrete products are the basis for the construction of industrial facilities and residential buildings. The advantages of reinforced concrete structures are:
- strength, which only increases over time;
- durability;
- resistance to fire;
- relatively affordable price;
- possibility of making it yourself;
- resistance to seismic activity;
- the ability of reinforced concrete to take various architectural forms.
Disadvantages include:
- formation of cracks;
- heavy weight;
- additional insulation is required;
- thermal conductivity.
Main types of structures
Based on the type of production, they are distinguished:
- Prefabricated. They are very popular due to the most mechanized construction.
- Monolithic. They are used in the construction of monolithic structures, for example, hydraulic structures, heavy foundations.
- Prefabricated monolithic. Prefabricated monolithic elements are connected both by concrete and welding.
By area of use there are:
- for residential buildings;
- for industrial buildings;
- for public buildings and structures.
Reinforced concrete products can be: non-stressed and. The most popular reinforced concrete products used for construction:
- panels;
- foundations;
- beams;
- floor slabs.
Panels
A common type of reinforced concrete structures are panels, which are used in the construction of buildings and structures for residential and industrial purposes. The panel has a flat rectangular shape, which can have openings for doors and windows, as well as projections for window sills.
When transporting panels, they are installed in a vertical position at an angle of ten degrees. When transporting several panels at once, it is necessary to prevent their contact, so pads are laid between them.
Farms
Reinforced concrete trusses are used for floors in industrial buildings and cultural buildings. They look like a flat rectangular structure with gratings. When transporting products, they are given a vertical position.
Trusses made of reinforced concrete have high strength, rigidity, fire-fighting properties and frost resistance. Products are made from heavy, light or structural concrete, mainly aggloporite concrete and expanded clay concrete. When using a reinforced concrete truss, you should carefully approach its installation. An accurate calculation of the load-bearing capacity of the building is carried out. They check the quality of the elements, dimensions and prepare the place of support.
Beams and crossbars
Beams and crossbars are used in the construction of foundations and roofs; they act as load-bearing elements for the installation of crane mechanisms. Beams are produced single-pitched, double-pitched or rectangular. During transportation of the beam, the crossbars are installed vertically in the vehicle. To support the beams, they use pads installed under the lower plane of the products. Depending on the length of the structure, the distance between the pads is determined. The sides of the beams and crossbars are fastened along their entire height. Transportation of beams is allowed only in a vertical position; horizontal transportation is prohibited, as there is a risk of destruction of the products. When transporting several elements at the same time, separators with a thickness of more than ten centimeters are laid between them.
Piles
Reinforced concrete structures in the form of piles are used for the foundations of industrial and residential buildings. Piles are used to erect structures on unstable soils. When transporting piles, they are given a horizontal position, ensuring support on special supports. It is allowed to lay piles on a vehicle when transporting in tiers.
They are highly resistant to chemicals and corrosion, waterproof and frost-resistant. Piles are easy to install with special equipment and can provide the structure being built with durability, high strength and reliability.
Racks for power transmission line supports. Reinforced concrete racks or power line racks provide a supporting element for lighting fixtures and power lines. During transportation, it is allowed to transport the racks together in one group, ensuring a horizontal position. During transportation, support for the racks should be prepared in the form of a special lining.
The main purpose of reinforced concrete racks is the ability to reliably hold electrical wires at the required distance from the surface of the earth or water. The reliability and strength of the supports is achieved by using a reinforcement cage and a special type of concrete mortar in the design of the products. Individually, each power line rack differs in purpose and design. There are end, intermediate, corner and anchor supports made of reinforced concrete. Single-chain and multi-chain are also produced.
Columns
It is a load-bearing element of residential, cultural, industrial and domestic buildings. Columns are made of rectangular shape and two-branch, which is designed for heavy crane loads. The elements are transported in a stack, where the first row of columns is placed on the cargo space of the vehicle, and subsequent rows are laid on the previous one, covered with special linings.
Found application in the construction of public and residential buildings. They are almost finished building elements with a hollow thin-walled rectangular prism and with openings for doors and windows.
Volumetric blocks can have insulating and insulating panels. When transporting volumetric blocks, they are given a vertical position, while ensuring that the elements are supported at four corners on the loading platform. Volumetric blocks made of reinforced concrete are sensitive to dynamic overloads that are formed during transportation. These building products made of reinforced concrete have the peculiarity of shifting the center of gravity from the geometric center in the transverse and longitudinal direction. To avoid the block moving during transportation, special thrust protrusions are installed on the loading area.
Sanitary cabins
Sanitary cabins are used in the construction of public and residential buildings. They are represented by volumetric elements with large mass and dimensions. When transporting elevator shafts and sanitary cabins, a vertical position is allowed, supported by a loading platform with two spacers. Elevator shafts with a height of up to 140 centimeters can be transported in 2 tiers in height, while installing wooden linings between the rows in height of more than 10 centimeters.
Conclusion
Reinforced concrete structures are used in the construction of various buildings and structures, and not only. Varieties of reinforced concrete products (panels, volumetric blocks, trusses, plumbing cabins), due to their dimensions, weights and conditions that must be observed during transportation, require narrow specialization of rolling stock.
Transportation of beams, columns, supports and racks of power lines, crossbars and piles has similar requirements for the transportation process, so the rolling stock layouts for their transportation may be the same.