What can be cooked from squid: quick and tasty

Building structures are very diverse in their purpose and application. Nevertheless, they can be combined according to some common features of certain properties, i.e. classify, while clarifying some concepts.

Various approaches to the classification of structures are possible.

Having the calculation of structures as the main ultimate goal of the textbook, it is most expedient to classify them according to the following criteria:

1) according to the geometric feature of the structure, it is customary to divide into arrays, beams, slabs, shells (Figure 1.1) and rod systems (Figure 1.3):

Rice. 1.1. Geometric classification of structures: a) array; b) timber; c) plate; d) shell

An array is a structure in which all dimensions are of the same order, for example, the dimensions of the foundation can be as follows: a = 1.8 m; B - 1.2 m; I = 1.5 m. Sizes can be different, but their order is the same - meters;

A bar is an element in which two sizes are many times smaller than the third, i.e. they are of different order: b "/, A" /. For example, for a reinforced concrete beam, they can be as follows: L - 20 cm, I = 40 cm, a / = 600 cm, i.e. they can differ from each other by an order of magnitude (10 or more times).

A beam with a broken axis is usually called the simplest frame, and with a curved axis - an arch (Fig. 1.2, a, b);

A slab is an element in which one size is many times smaller than the other two: And "a, And" /. As an example, we can cite a ribbed reinforced concrete slab (more precisely, the slab field), in which the thickness of the I slab itself can be 3-4 cm, and the length and width of the order of 150 cm. The slab is a special case of more general concept- a shell, which, unlike a slab, has a curvilinear outline (Fig. 1.1, d). Considering shells is beyond the scope of our course;

Rod systems are geometrically unchangeable systems of rods that are hinged or rigidly connected to each other. These include building trusses (girders or cantilevers) (Fig. 1.3).

Rice. 1.2. Varieties of bars: a) frame; b) arch

Rice. 1.3. Examples of the simplest rod systems: a) girder girder; b) console farm

The dimensions in all examples are given as a guide and do not exclude their variety. There are cases when it is difficult to attribute a structure to one type or another on this basis. Within the framework of this textbook, all constructions fit well into the given classification;

2) from the point of view of statics, structures are divided into statically definable and statically indefinite. The former include systems (structures), the forces or stresses in which can be determined only from the equations of statics (equations of equilibrium), the latter - those for which the equations of statics alone are not enough. This tutorial focuses primarily on statically definable constructs;

3) according to the materials used, structures are divided into steel, wood, reinforced concrete, concrete, stone (brick) ",

4) from the point of view of the stress-strain state, i.e. arising in structures of internal forces, stresses and deformations under the action of an external load, they can be conditionally divided into three groups: the simplest, simple and complex (Table 1.1). This division is not generally accepted, but it allows you to bring into the system the characteristics of the types of stress-strain states of structures, which are widespread in construction practice and will be discussed in the textbook. It is difficult to reflect all the subtleties and features of these states in the presented table, but it makes it possible to compare and evaluate them as a whole. More details about the stages of stress-strain states will be discussed in the corresponding chapters.

Building structures are very diverse in their purpose and application. Nevertheless, they can be combined according to some signs of commonness of certain properties, i.e. classify, while clarifying some concepts. Various approaches to the classification of structures are possible.

Having the calculation of structures as the main ultimate goal of the textbook, it is most expedient to classify them according to the following criteria:

I) geometrically structures are usually divided into arrays, beams, slabs, shells (Fig. l.l) and rod systems (Fig. 1.3):

array- a structure in which all dimensions are of the same order, for example, the dimensions of the foundation can be as follows: a= 1.8 m; b = 1.2 m; h = 1.5 m. Sizes may be different, but their order is the same - meters;

bar- an element in which two sizes are many times smaller than the third, i.e. they are of different order: b "l, h" l . For example, for a reinforced concrete beam, they can be as follows: b = 20 cm, h = 40 cm, and l = 600 cm, i.e. they can differ from each other by an order of magnitude (10 or more times).

A beam with a broken axis is usually called the simplest frame, and with a curved axis - an arch (Fig. 1.2, a, b)

plate- an element in which one size is many times smaller than the other two: h "a, h" l. An example is a ribbed reinforced concrete slab (more precisely, a slab field), in which the thickness of the slab itself h it can be 3-4 cm, and the length and width are about 150 cm. The slab is a special case of a more general concept - a shell, which, unlike a slab, has a curvilinear outline (Fig. 1.1, d). Considering shells is beyond the scope of our course;

rod systems are geometrically unchangeable systems of rods, hinged or rigidly connected to each other. These include construction trusses (girders or cantilevers) (Fig. 1.3).

The dimensions in all examples are given as a guide and do not exclude their variety. There are cases when it is difficult to attribute a structure to one type or another on this basis. Within the framework of this tutorial, all constructions fit well into the given classification;

2) from the point of view of statics constructions are divided into statically definable and statically undefined. To the first are systems (structures), the forces or stresses in which can be determined only from the equations of statics (equations of equilibrium), to the second - those for which the equations of statics alone are not enough. This tutorial focuses on statically definable constructs;

3) by materials used constructions are divided into steel, wood, reinforced concrete, concrete, stone (brick);

4) from the point of view of the stress-strain state, those. arising in structures of internal forces, stresses and deformations under the action of an external load, they can be conditionally divided into three groups: protozoa, simple and complex(Table 1.1). This division is not generally accepted, but it allows you to bring into the system the characteristics of the types of stress-strain states of structures, which are widespread in construction practice and will be discussed in the textbook. It is difficult to reflect all the subtleties and features of these states in the presented table, but it makes it possible to compare and evaluate them as a whole. The stages of stress-strain states will be discussed in more detail in the corresponding chapters.

Structural supporting structures of industrial and civil buildings and engineering structures structures are called, the dimensions of the sections of which are determined by calculation. This is their main difference from architectural structures or parts of buildings, the cross-sectional dimensions of which are assigned according to architectural, thermal engineering or other special requirements.

Modern building structures must meet the following requirements: operational, environmental, technical, economic, production, aesthetic, etc.

In the construction of gas and oil pipelines, steel and prefabricated reinforced concrete structures are widely used, including the most progressive ones - prestressed. Recently, structures made of aluminum alloys are being developed, polymer materials, ceramics and other effective materials.

Building structures are very diverse in their purpose and application. Nevertheless, they can be combined according to some signs of commonness of certain properties and it is most expedient to classify them according to the following main features:

1) according to the geometric feature of the structure, it is customary to divide into arrays, beams, slabs, shells (Fig.1.1) and rod systems:

Array - a construction in which all dimensions are of the same order;

timber - an element in which two dimensions determine transverse section, many times less than the third - its length, i.e. they are of different order: b "I, h" /; a bar with a broken axis is usually called the simplest frame, and with a curved axis - an arch.

slab - an element in which one dimension is many times smaller than the other two: h "a, h" I. A slab is a special case of a more general concept - a shell, which, unlike a slab, has a curvilinear outline;

rod systems are geometrically unchangeable systems of rods that are hinged or rigidly connected to each other. These include building trusses (beam or cantilever) (Fig. 1.2).

the nature design scheme constructions are divided into statically definable and statically indefinite. The former include systems (structures), the forces or stresses in which can be determined only from the equations of statics (equations of equilibrium), the latter - those for which the equations of statics alone are not enough and for the solution requires the introduction of additional conditions - the equations of compatibility of deformations.

according to the materials used, structures are divided into steel, wood, reinforced concrete, concrete, stone (brick);

4) by the nature of the stress-strain state (SSS), i.e. arising in structures of internal forces, stresses and deformations under the action of an external load, it is conditionally possible to divide them into three groups: the simplest, simple and complex (Table 1.1).

This division allows you to bring into the system the characteristics of the types of stress-strain states of structures, which are widespread in construction practice. In the presented table, it is difficult to reflect all the subtleties and features of these states, but it makes it possible to compare and evaluate them as a whole.

Building structure is called an enlarged building element of a building, structure or bridge, made of building materials and products.

Building structures are classified by purpose and building material.

By appointment there are:

1. Carriers - those structures of buildings and structures that can withstand power loads. They ensure their stability and strength, as well as allow safe operation of the building. These include: load-bearing walls, columns, foundations, floors and coverings, etc.

2. Fencing - structures that limit the volume of the building and divide it into separate functional rooms. Divided into: external (protect from weathering) and internal (to ensure sound insulation and dividing the internal space). The enclosing structures include partitions, self-supporting walls, filling openings, etc.

By material, building structures are divided into:

Concrete and reinforced concrete;

Steel structures;

Wooden;

Stone and reinforced stone;

Plastic;

Complex (combine several types of materials).

The main requirements for building structures:

1. Reliability. This concept includes three components: strength, rigidity and stability.

Strength is the ability of a structure to withstand all loads without destruction;

Stiffness is a property that allows a building structure to deform under the action of loads within acceptable limits;

Stability - the ability of a structure to maintain a constant position in space under the action of loads.

2. Ease of use- this is the ability to use buildings and structures for their intended purpose. It is necessary that the structures are designed in such a way that it is possible to easily inspect, repair, reconstruct and strengthen them.

3... Profitability... When designing, it is necessary to make sure that there is no overspending of building materials and try to ensure the minimum labor costs when installing the structure.

9.2. Reinforced concrete structures and products

Reinforced concrete structures and products, elements of buildings and structures made of reinforced concrete, and combinations of these elements.

High technical and economic indicators of reinforced concrete structures, the ability to relatively easily give them the required shape and dimensions while maintaining the specified strength, have led to their widespread use in almost all branches of construction. Modern reinforced concrete structures (reinforced concrete structures) are classified according to several criteria: by the method of execution (monolithic, prefabricated, precast-monolithic), the type of concrete used for their manufacture (from heavy, light, cellular, heat-resistant, etc. concretes), the type of stress state ( ordinary and pre-stressed).

Monolithic reinforced concrete structures, performed directly on construction sites, are usually used in buildings and structures that are difficult to divide, with non-standard and low repeatability of elements and with especially heavy loads (foundations, frames and floors of multi-storey industrial buildings, hydraulic engineering, reclamation, transport and other structures).

In some cases, they are advisable when performing work by industrial methods using inventory formwork - sliding, movable (towers, cooling towers, silos, chimneys, multi-storey buildings) and mobile (some thin-walled coating shells).

The erection of monolithic reinforced concrete structures is technically well developed. There are also significant advances in the application of the prestressing method in the production of monolithic structures. A large number of unique structures (television towers, industrial pipes of great height, reactors of nuclear power plants, etc.) are made in monolithic reinforced concrete. In modern construction practice in a number of foreign countries (USA, Great Britain, France, etc.), monolithic reinforced concrete structures have become widespread, which is mainly due to the absence in these countries of a state system for unifying parameters and typing structures of buildings and structures. In the USSR, monolithic structures predominated in construction until the 30s.

The introduction of more industrial prefabricated structures in those years was held back due to the insufficient level of mechanization of construction, the lack of special equipment for their mass production, as well as high-capacity assembly cranes. The share of monolithic reinforced concrete structures in the total volume of reinforced concrete production in the USSR is approximately 35% (1970).

Precast concrete structures and products- the main type of structures and products used in various sectors of construction: housing and civil, industrial, agricultural, etc.

Prefabricated structures have significant advantages over monolithic ones, they create ample opportunities for the industrialization of construction. The use of large-sized reinforced concrete elements allows the bulk of the work on the construction of buildings and structures to be transferred from construction site to a plant with a highly organized technological production process. This significantly reduces the construction time, ensures a higher quality of products at the lowest cost and labor costs; The use of prefabricated reinforced concrete structures allows the widespread use of new efficient materials (lightweight and cellular concrete, plastics, etc.), and reduces the consumption of timber and steel, which are necessary in other branches of the national economy. Prefabricated structures and products must be technological and transportable, they are especially advantageous with a minimum number of standard sizes of elements, repeated many times.

With the growth of production and use of prefabricated reinforced concrete in construction, the technology of its manufacture has improved. The unification of the basic parameters of buildings and structures for various purposes was also carried out, on the basis of which standard designs and products for them were developed and introduced.

Depending on the purpose in the construction of residential, public, industrial and agricultural buildings and structures, the following most common prefabricated reinforced concrete structures are distinguished:

For foundations and underground parts of buildings and structures (foundation blocks and slabs, panels and blocks of basement walls);

For building frames (columns, girders, girders, crane beams, rafter and rafter beams, trusses);

For outdoor and interior walls(wall and partition panels and blocks);

For intermediate floors and building coverings (panels, slabs and decking); for stairs (flights of stairs and landings);

For sanitary devices (heating panels, ventilation and waste chute blocks, sanitary cabins).

Prefabricated reinforced concrete structures are manufactured mainly at mechanized enterprises and partially at equipped landfills. The technological process for the production of reinforced concrete products consists of a number of sequentially performed operations: concrete mix, manufacture of reinforcement (reinforcing cages, meshes, bent rods, etc.), reinforcement of products, molding of products (laying of concrete mixture and its compaction), heat and moisture treatment, providing the necessary strength of concrete, finishing of the front surface of products.

In modern technology of precast concrete, 3 main methods of organizing the production process can be distinguished: the aggregate-flow method of manufacturing products in movable forms; conveyor production method; bench method in non-movable (stationary) forms.

With the aggregate-flow method all technological operations(cleaning and lubrication of forms, reinforcement, shaping, hardening, stripping) are carried out at specialized stations equipped with machines and installations that form a production line. Molds with products are sequentially moved along the technological line from one station to another with an arbitrary time interval depending on the duration of the operation at a given station, which can vary from several minutes (for example, mold lubrication) to several hours (hardening of products in steaming chambers). This method is advantageous to use at medium-sized factories, especially when producing designs and products of a wide range.

Conveyor method used in factories of high power when producing the same type of products of a limited range. With this method, the technological line operates on the principle of a pulsating conveyor, i.e. the forms with products move from station to station after a strictly defined time required to perform the longest operation.

A variation of this technology is vibratory rolling method used for the manufacture of flat and ribbed plates; in this case, all technological operations are performed on one moving steel belt. With the bench method, the products during their manufacture and until the concrete hardens remain in place (in a stationary form), while the technological equipment for performing individual operations moves from one form to another. This method is used in the manufacture of large products (trusses, beams, etc.). For molding products of complex configuration (flights of stairs, ribbed slabs, etc.), matrices are used - reinforced concrete or steel forms that reproduce the imprint of the ribbed surface of the product. In the case of the cassette method, which is a kind of bench method, products are made in vertical forms - cassettes, which are a series of compartments formed by steel walls. On the cassette installation, the products are molded and hardened. The cassette installation has devices for heating products with steam or electric current, which significantly accelerates the hardening of concrete. Cassette method usually used for mass production of thin-walled products.

Finished products must meet the requirements of applicable standards or specifications. The surfaces of products are usually made with such a degree of factory readiness that no additional finishing is required at the construction site.

During installation, prefabricated elements of buildings and structures are connected to each other by monolithing or welding of embedded parts, designed for the perception of certain force effects. Much attention is paid to reducing the metal consumption of welded joints and their unification. Prefabricated structures and products are most widely used in housing and civil construction, where large-scale housing construction (large-panel, large-block, volumetric) is considered as the most promising. Precast reinforced concrete is also used for mass production of products for engineering structures (so-called special reinforced concrete): bridge spans, supports, piles, culverts, trays, blocks and tubing for lining tunnels, slabs of roads and airfields, sleepers, contact supports networks and power lines, fencing elements, pressure and non-pressure pipes, etc.

A significant part of these products is made of prestressed reinforced concrete by bench or flow-aggregate method. For shaping and compacting concrete, very effective methods are used: vibrocompression (pressure pipes), centrifugation (pipes, supports), vibration stamping (piles, trays).

The development of prefabricated reinforced concrete is characterized by a tendency towards further enlargement of products and an increase in the degree of their factory readiness. So, for example, for covering buildings, multilayer panels are used, supplied for construction with insulation and a layer of waterproofing; blocks of 3x18 m and 3x24 m in size, combining the functions of the supporting and enclosing structures. Combined roofing slabs made of lightweight and aerated concrete have been developed and successfully applied. In multi-storey buildings, prestressed reinforced concrete columns are used to a height of several floors. For the walls of residential buildings, panels are made in sizes for one or two rooms with a variety of external finishes, equipped with window or door (balcony) blocks. Significant prospects for the further industrialization of housing construction have a way of erecting buildings from volumetric blocks. Such blocks for one or two rooms or for an apartment are manufactured at the factory with full interior decoration and equipment; assembling houses from these elements takes only a few days.

Precast-monolithic reinforced concrete structures are such a combination of prefabricated elements (reinforced concrete columns, girders, slabs, etc.) with monolithic concrete, which ensures reliable joint operation of all components.

These constructions are mainly used in ceilings multi-storey buildings, in bridges and overpasses, during the construction of certain types of shells, etc.

They are less industrial (in terms of construction and installation) than prefabricated ones. Their use is especially advisable at high dynamic (including seismic) loads, as well as when it is necessary to divide large-sized structures into component elements due to the conditions of transportation and installation. The main advantage of prefabricated monolithic structures is lower (compared to prefabricated structures) steel consumption and high spatial rigidity.

Most of the reinforced concrete and concrete products are made of heavy concrete with an average density of 2400 kg / m 3. However, the share of products made of structural, heat-insulating and structural lightweight concrete on porous aggregates, as well as from aerated concrete of all types, is constantly increasing. Such products are used mainly for enclosing structures (walls, coatings) of residential and industrial buildings.

Bearing structures made of high-strength heavy concrete of C30 / 35 and C32 / 40 classes and lightweight concrete of C20 / 25 and C25 / 30 classes are very promising. A significant economic effect is achieved as a result of the use of structures made of heat-resistant concrete (instead of piece refractories) for thermal units in metallurgical, oil refining and other industries; for a number of products (for example, pressure pipes), the use of tensioning concrete is promising.

Reinforced concrete structures and products are mainly made with flexible reinforcement in the form of individual rods, welded meshes and flat frames. For the manufacture of stress-free reinforcement, it is advisable to use contact welding, which provides a high degree of industrialization of reinforcement work. Structures with bearing (rigid) reinforcement are used relatively rarely and mainly in monolithic reinforced concrete when concreting in suspended formwork. In bending elements, longitudinal working reinforcement is installed in accordance with the diagram of maximum bending moments; in columns, longitudinal reinforcement predominantly perceives compressive forces and is located along the perimeter of the section. In addition to longitudinal reinforcement, distribution, assembly and transverse reinforcement (clamps, bends) are installed in reinforced concrete structures, and in some cases, the so-called. indirect reinforcement in the form of welded meshes and spirals.

All these types of reinforcement are interconnected and provide the creation of a reinforcing cage that is spatially unchanged during the concreting process. For prestressed reinforcement of prestressed reinforced concrete structures, high-strength bar reinforcement and wire, as well as strands and ropes from it, are used. In the manufacture of prefabricated structures, the method of tensioning the reinforcement on the stops of stands or forms is mainly used; for monolithic and precast-monolithic structures - the method of tensioning the reinforcement on the concrete of the structure itself.

The wide form-building and technical capabilities of reinforced concrete structures had a huge impact on the world architecture of the 20th century. On the basis of reinforced concrete structures, new scales, architectonics and spatial organization of buildings and structures have developed. Rectilinear frame structures give buildings a strict geometrism of forms and a measured rhythm of divisions, a clear structure. Horizontal floor slabs rest on thin supports, a light wall, being deprived of a load-bearing function, often turns into a glass curtain screen. Uniform distribution of static forces creates tectonic equivalence of the building elements. Curvilinear structures (especially thin-walled shells of various, sometimes bizarre outlines), with their complex tectonics of forms (sometimes approaching sculptural ones) and continuously changing rhythm of elements, have great plastic and spatial expressiveness. Curvilinear structures allow to overlap huge halls without intermediate supports and create volumetric-spatial compositions of unusual shape. Some modern reinforced concrete structures (for example, lattice) have ornamental and decorative qualities that form the appearance of facades and coatings. Plastically meaningful modern reinforced concrete structures give aesthetic expressiveness not only to residential and civil buildings, but also to engineering and industrial structures (bridges, overpasses, dams, cooling towers, etc.).

Bearing structures.

Reinforced concrete columns:

Rice. 9.1. Column two-branch middle row

Rice. 9.2. Two-branch column of the extreme row

Rice. 9.3. ... Columns of a girderless frame

Rice. 9.4. Column of one-story industrial buildings

a) Column of the middle row with two consoles

Rice. 9.5. Single-branch column of the middle row

b) Column of the extreme row with one console

Rice. 9.6. Single-branch column of the extreme row

Rice. 9.7. Column of the middle row, single-branch for multi-storey buildings

Rice. 9.8. One-branch column of administrative buildings

Rice. 9.9. Single-branch column of warehouse buildings

Rice. 9.10. Single-branch columns of multi-storey administrative buildings

Rice. 9.11. Reinforced concrete ledger with shelves

Rice. 9.12. Reinforced concrete crossbar

Crossbars are designed for frames of multi-storey buildings, industrial, administrative and household purposes, industrial enterprises, residential buildings and shopping and entertainment complexes.

Frost resistance not lower than F50.

Rice. 9.13. Reinforced concrete T-section beams

Rice. 9.14. Reinforced concrete T-section beams

The beams are designed for the frames of multi-storey buildings, industrial, administrative and residential buildings of industrial enterprises, residential buildings and shopping and entertainment complexes.

Frost resistance not lower than F50.

Classification of building structures

Structures are called structural load-bearing structures of industrial and civil buildings and engineering structures, the dimensions of the sections of which are determined by calculation. This is their main difference from architectural structures or parts of buildings, the cross-sectional dimensions of which are assigned according to architectural, thermal engineering or other special requirements.

Modern building structures must meet the following requirements: operational, environmental, technical, economic, production, aesthetic, etc.

In the construction of gas and oil pipelines, steel and prefabricated reinforced concrete structures are widely used, including the most progressive ones - prestressed. Recently, structures made of aluminum alloys, polymeric materials, ceramics and other effective materials are being developed.

1 ) on a geometric basisstructures are usually divided into arrays, beams, slabs, shells (Fig.1.1) and rod systems:

– array- a design in which all dimensions are of the same order;

bar- an element in which two dimensions defining the cross-section are many times smaller than the third - its length, i.e. they are of different order:b« I, h« /; a bar with a broken axis is usually called the simplest frame, and with a curved axis - an arch.

plate- an element in which one size is many times smaller than the other two: h« a, h“I.A slab is a special case of a more general concept - a shell, which, unlike a slab, has a curvilinear outline;

rod systemsare geometrically unchangeable systems of rods, hinged or rigidly connected to each other. These include building trusses (beam or cantilever) (Fig. 1.2).

by the nature of the design schemestructures are divided into statically definableand statically undefined.The former include systems (structures), the forces or stresses in which can be determined only from the equations of statics (equations of equilibrium), the latter - those for which the equations of statics are not enough and for the solution requires the introduction of additional conditions - the equations of compatibility of deformations.

by materials usedstructures are divided into steel, wood, reinforced concrete, concrete, stone (brick);

4) by the nature of the stress-strain state(VAT),those. arising in structures of internal forces, stresses and deformations under the action of an external load, it is conditionally possibledivide them into three groups: protozoa, simpleand complex(Table 1.1).

This division allows you to bring into the system the characteristics of the species stress-strain states of structures, which are widespread in construction practice. In the presented table
it is difficult to reflect all the subtleties and features of these states, but it makes it possible to compare and evaluate them as a whole.

Concrete

Concrete is an artificial stone material obtained in the process of hardening a mixture of binder, water, fine and coarse aggregates and special additives.

The composition of the concrete mixture is expressed in two ways.

In the form of ratios by weight (less often by volume, which is less accurate) between the amounts of cement, sand and crushed stone (or gravel) with the obligatory indication of the water-cement ratio and cement activity. The amount of cement is taken as a unit, so the ratio between the constituent parts of the concrete mixture is 1: 2: 4. It is permissible to establish the composition of the concrete mixture by volume only in small construction, but the cement should always be dosed by weight.

At large facilities and central concrete plants, all components are dosed by weight, while the composition is indicated as the consumption of materials per 1 m3 laid and compacted concrete mix, for example:

Cement 316 kg / m 3

Sand 632 kg / m 3

PAGE_BREAK--

Crushed stone ……………………………………… .1263 kg / m 3

Water 189 kg / m 3

Total weight of materials 2400 kg / m 3

To ensure reliable operation of load-bearing elements under given operating conditions, concretes for reinforced concrete and concrete structures must have certain predetermined physical and mechanical properties and, first of all, sufficient strength.

Concrete is classified according to a number of characteristics:

by appointmentdistinguish between structural, special (chemically resistant, heat-insulating, etc.);

by the type of binder- based on cement, slag, polymer, special binders;

by type of placeholder- on dense, porous, special aggregates;

by structure- dense, porous, cellular, large-porous.

Concrete is used for different types building structures manufactured at precast concrete factories or erected directly at the site of their future operation (monolithic concrete).

Depending on the area of application of concrete, a distinction is made between:

normal- for reinforced concrete structures (foundations, columns, beams, floors, bridge and other types of structures);

hydraulic engineering- for dams, sluices, lining of canals, etc .;

concrete for building envelopes(lightweight concrete for building walls); for floors, sidewalks, road and airfield surfaces;

special purpose(heat-resistant, acid-resistant, for radiation protection, etc.).

Strength characteristics of concrete

Compressive strength of concrete

Compressive strength of concrete V is called the ultimate strength (in MPa) of a concrete cube with an edge of 150 mm, manufactured, stored and tested under standard conditions at the age of 28 days, at a temperature of 15–20 ° C and a relative humidity of 90–100%.

Reinforced concrete structures differ in shape from cubes, therefore compressive strength of concreteRvncannot be directly used in strength calculations for structural elements.

The main characteristic of the strength of concrete in compressed elements is prismatic strengthRf, - temporary resistance to axial compression of concrete prisms, which, according to experiments on prisms with the side of the baseaand height hwith regard hla= 4 is approximately 0.75, where R: – cube strength, or ultimate compressive strength of concrete,found when testing a sample in the form of a cube with an edge of 150 mm.

The main characteristic of the strength of concrete in compressed elements and compressed zones of bent structures is prismatic strength.

To determine the prismatic strength, the sample - the prism is loaded in a press with a stepped compressive load until fracture, and the deformations are measured at each loading step.

The dependence of the compressive stresses is plotted afrom relative deformations e, which is non-linear, since in concrete, along with elastic, inelastic plastic deformations also occur.

Experiments with concrete prisms with a square base aand height hshowed that the prismatic strength is less than the cubic strength and decreases with an increase in the ratio hla(fig. 2.2).

Continuation
--PAGE_BREAK--

Cubic strength of concrete R(for cubes size 150 NS150 NS150 mm) and prismatic strength Rh(for prisms with a height-to-base ratio hla> 4) can be associated with a certain dependence, which is established experimentally:

The prismatic strength of concrete is used when calculating bending and compressed concrete and reinforced concrete structures (for example, beams, columns, compressed elements of trusses, arches, etc.)

As a characteristic of the strength of concrete in the compressed zone of bending elements, it is also taken Rh. Axial tensile strength of concrete

Concrete strength under axial tensionR/, 10–20 times lower than when compressed. Moreover, with an increase in the cubic strength of concrete, the relative tensile strength of concrete decreases. The tensile strength of concrete can be related to the cube strength by the empirical formula

Classes and grades of concrete

The control characteristics of the quality of concrete are called classesand stamps.The main characteristic of concrete is the class of concrete in terms of compressive strength B or grade M. The class of concrete is determined by the value of the guaranteed compressive strength in MPa with a security of 0.95. Concrete is divided into classes from B1 to B60.

The class of concrete and its grade depend on the average strength:

compressive strength class of concrete, MPa; average strength that should be ensured during the production of structures, MPa;

the coefficient characterizing the security of the concrete class adopted in the design is usually taken in constructiont= 0,95;

coefficient of variation of strength, characterizing the homogeneity of concrete;

concrete grade by compressive strength, kgf / cm 2 ... To determine the average strength (MPa) for the class of concrete (with a standard coefficient of variation of 13.5% and t= 0.95) or according to its brand, the formulas should be applied:

In the regulatory documents, concrete is used, however, for some special structures and in a number of current standards, the concrete grade is also used.

In production, it is necessary to ensure the average strength of the concrete. Exceeding the specified strength is allowed by no more than 15%, as this leads to an overconsumption of cement.

For concrete and reinforced concrete structures, the following are used classes of concrete for compressive strength:heavy concrete from B3.5 to B60; fine-grained - from B3.5 to B60; lungs - from B2.5 to B35; cellular - from B1 to B15; porous from B2.5 to B7.5.

For tensile structures, a concrete class is additionally assigned axial tensile strength- only for heavy, light and fine-grained concrete - from VDZ to V ? 3,2.

An important characteristic of concrete is the grade frost resistanceIs the number of cycles of alternating freezing and thawing that water-saturated concrete samples withstood at the age of 28 days without a decrease in compressive strength of more than 15% and a weight loss of no more than 5%. Denoted -F ... For heavy and fine concrete ranges from F 50 to F 500, for lightweight concrete - F 25- F 500, for aerated and porous concrete - F 15- F 100.

Waterproof gradeWIt is assigned for structures that have requirements for limiting permeability, for example, reinforced concrete pipes, tanks, etc.

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Waterproofness is the property of concrete not to let water pass through itself. She is estimated filtration coefficient- the mass of water that has passed per unit time under constant pressure through a unit area of the sample at a certain thickness. The following grades have been established for heavy, fine-grained and lightweight concrete:W 2, W 4, W 6, W 8, W 10, W 12. The number in the stamp means the water pressure in kgf / cm 2 , at which its seepage through samples of 180 days of age is not observed.

Self-stress markS p means the value of prestress in concrete, MPa, created as a result of its expansion. These values range fromS p 0.6 to S p 4.

When determining the own weight of structures and for heat engineering calculations great importance has the density of concrete.Concrete grades by average densityD (kg / m 3 ) installed with a graduation step of 100 kg / m 3 : heavy concrete - D = 2300-2500; fine-grained - 88

D = 1800-2400; lungs - D = 800-2100; cellular - D = 500-1200; porous - D = 800–1200.

Armature

The reinforcement of reinforced concrete structures consists of individual working rods, meshes or frames, which are installed to absorb the acting forces. The required amount of reinforcement is determined by calculating structural elements for loads and impacts.

The fittings installed by calculation are called working;installed for design and technological reasons - assembly room.

Work and assembly fittings are combined into reinforcement products -welded and knitted meshes and frames, which are placed in reinforced concrete elements in accordance with the nature of their work under load.

Reinforcement is classified according to four criteria:

depending on the manufacturing technology, a distinction is made between rod and wire reinforcement. In this classification, a rod is meant reinforcement of any diameter withind= 6–40 mm;

depending on the method of subsequent hardening, hot-rolled reinforcement can be thermally hardened, i.e. heat treated, or hardened in a cold state - by drawing, drawing;

by the shape of the surface, the reinforcement is of a periodic profile and smooth. Ribbed protrusions on the surface of periodic bar reinforcement, reefs or dents on the surface of wire reinforcement significantly improve adhesion to concrete;

– According to the method of application, when reinforcing reinforced concrete elements, prestressing reinforcement is distinguished, i.e. pre-tensioned and non-tensioned

Hot-rolled bar rebar, depending on its main mechanical characteristics, is divided into six classes with a symbol:A- I, A-P, A-Sh, A- IV, A- V, A- Vi.The main mechanical characteristics of the used fittings are given in table. 2.6.

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Bar reinforcement of four classes is subjected to thermal hardening; hardening in its designation is marked by an additional index "t": Am-Sh, Am- IV, At- V, At-VI.An additional letter C indicates the possibility of joining by welding, and the letter K indicates increased corrosion resistance. Cold Drawn Reinforcement class A-Sh marked with an additional index B.

Each class of reinforcement corresponds to certain grades of reinforcement with the same mechanical characteristics, but different chemical composition... The designation of the steel grade reflects the content of carbon and alloying additives. For example, in grade 25G2S, the first digit denotes the carbon content in hundredths of a percent (0.25%), the letter G - that the steel is alloyed with manganese, the number 2 - that itthe content can reach 2%, the letter C is the presence of silicon (silicon) in the steel.

The presence of others chemical elements, for example, in brands 20ХГ2Ц, 23Х2Г2Т, it is designated by letters: X - chromium, T - titanium, C - zirconium.

Bar reinforcement of all classes has a periodic profile with the exception of round (smooth) reinforcement of the classA- I.

Reinforcement products used for the manufacture of reinforced concrete structures

For the reinforcement of reinforced concrete structures, they are widely used ordinary reinforcing wire of class Br-I(grooved) with a diameter of 3-5 mm, obtained by cold drawing low-carbon steel through a system of calibrated holes (dies). The smallest value of the conventional tensile yield strength of the wire Вр-I with a diameter of 3–5 mm it is 410 MPa.

High-strength reinforcing wire is also produced by cold drawing. classes B-P and VR-I - smooth and periodic profile (Fig. 2.8,G)with a diameter of 3–8 mm with a conventional yield point wire V-P- 1500-1100 MPa and VR-P - 1500-1000 MPa.

The reinforcement of reinforced concrete structures is selected taking into account its purpose, class and type of concrete, conditions for the manufacture of reinforcing products and the operating environment (risk of corrosion), etc. As the main working reinforcement of conventional reinforced concrete structures, steel of classes А-Ш and Вр-I ... In prestressed structures, predominantly high-strength steel is used as prestressing reinforcement. classes B-I, Vr-P, A- VI, At - VI, A- V, At- VandAt-VII.

Reinforcement of prestressed structures with solid high-strength wire is very effective, however, due to the small cross-sectional area of wires, their number in the structure increases significantly, which complicates reinforcement work, gripping and tension of reinforcement. To reduce the complexity of reinforcement work, ropes pre-twisted in a mechanized way, bundles of parallel wires and steel cables... Non-twisting steel ropes of class K are manufactured mainly with 7- and 19-wire ropes (K-7 and K-19).

Strength conditions of eccentrically compressed T-and I-profile elements

When calculating the elements of the T and I-profile, two cases of the location of the neutral axis can be encountered (Fig. 2.40): the neutral axis is located in the shelf and the neutral axis crosses the rib. With a known reinforcement, the position of the neutral axis is determined by comparing the forceNwith the effort perceived by the shelf.

If the condition is met: N< Rbb" fh" f , then the neutral axis is located in the shelf. In this case, the calculation of the T-section or I-section is performed as for an element of a rectangular profile with a widthbj- and height h.

It should be noted that the strength calculation of T and I-profile elements is very laborious. The problem of checking the strength of normal sections with known reinforcement is relatively simple to solve, and the calculation of longitudinal reinforcement is much more difficult, especially when several loadings with moments of different signs are applied.

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Example 2.5. It is required to check the strength of the column section. Column section b= 400 mm; h= 500 mm; a = a "= 40 mm; heavy concrete class B20 (Rb= 11.5 MPa, Eb= 24000 MPa); fittings of class А-Ш (Rs= Rsc= 365 MPa); sectional area of reinforcement As= A ^= 982 mm (2025); calculated length Iq= 4.8 m; longitudinal force n= 800 kN; bending moment m =200 kN m; humidity environment 65%.

Strength conditions for tensile members

Under tension conditions, the lower chords of trusses and lattice elements, tightening of arches, walls of round and rectangular tanks and other structures work.

For tensile members, it is effective to use high-strength prestressed reinforcement. When designing stretched elements, special attention should be paid to the end sections, where reliable transfer of forces must be ensured, as well as to the joining of reinforcement. Rebar joints are usually welded.

Calculation of centrally stretched members

When calculating the strength of centrally tensioned reinforced concrete elements, it is taken into account that cracks normal to the longitudinal axis appear in the concrete and all the force is taken up by the longitudinal reinforcement.

Calculation of eccentrically stretched members at low eccentricities

If the strength Ndoes not go beyond the boundaries outlined by fittings Asand A" s, with the appearance of a crack, the concrete is completely turned off from work and the longitudinal force is perceived by the reinforcement Asand L.

Calculation of eccentrically stretched members at large eccentricities

If the strength Ngoes beyond the reinforcement As, then a compressed concrete zone appears in the element. For an element of rectangular cross-section, the strength conditions have the form

N -e< R bbx (h– NS/2) + RscA & h– a"),

N= RsAs- Rbbs~ RscA^.

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When using relative values £, = xlh^ andaT= 2; (1 - 1/2) strength conditions are converted to the form

N-e< R bambhl + RscA ^ (h– a"),

N = RSAS-R £ bh-Rsc4.

Static calculation of the transverse frame of a one-story industrial building

It is required to perform a static calculation of the transverse frame of a single-storey two-span industrial building by the displacement method and determine the bending moments, longitudinal and lateral forces in the characteristic sections of the columns according to the original data.

Structural elements buildings and initial data for the calculation, take the previous practical lesson.

When calculating by the method of displacements, angular or linear displacements of the frame nodes are taken as unknowns.

Fundamentals of calculation of building structures for limit states

For a building, structure, as well as foundations or individual structures, limiting states are those states in which they cease to meet the specified operational requirements, as well as the requirements specified during their construction.

Building structures are calculated in two groups limit states.

Calculation by the first group of limit states(for serviceability) provides the required bearing capacity of the structure - strength, stability and endurance.

The limiting states of the first group include:

general loss of shape stability (Fig. 1.4, a, 6);

loss of position stability (Fig. 1.4, c, d);

brittle, ductile or other nature of destruction (Fig. 1.4, e);

– destruction under the joint influence of force factors and unfavorable influences of the external environment, etc.

Calculation by the second group of limit states(according to suitability for normal operation) is produced for structures, the magnitude of deformations (displacements) of which may limit the possibility of their operation. In addition, if, according to the operating conditions of the structure, the formation of cracks is unacceptable (for example, in reinforced concrete tanks, pressure pipelines, during the operation of structures in corrosive environments, etc.), then a calculation is made on the formation of cracks. If it is only necessary to limit the width of crack opening, the calculation is performed for crack opening, and in prestressed structures, in some cases, for their closure.

The method of calculating building structures by limiting states is intended to prevent the occurrence of any of the limiting states that may arise in a structure (building)during their operation during the entire service life, as well as during their construction.

The idea of calculating structures by first limit statecan be formulated as follows: the maximum possible force effect on the structure from external loads or actions in the section of the element -Nshould not exceed its minimum design bearing capacity F:

N<Ф { R ; A},

where R - design resistance of the material; A - geometric factor.

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Second limiting statefor all building structures is determined by the values of ultimate deformations, when exceeding which the normal operation of structures becomes impossible:

Drawing up the layout diagram of the pumping shop building for the pumping station

As far as possible, the building is designed from standard elements in compliance with building design standards and a single modular system. The grid of columns can be, for example, 6NS9; 6 NS12; 6 NS18; 12 NS12; 12 NS18 m.

In order to preserve the uniformity of the coating elements, the columns of the extreme row are positioned so that the alignment axis of the row of columns passes at a distance of 250 mm from the outer edge of the columns (Figure 1.16) with a column pitch of 6 m or more.

The columns of the extreme row with a step of 6 m and cranes with a lifting capacity of up to 500 kN are positioned with a zero reference, aligning the axis of the row with the outer edge of the column. The extreme transverse alignment axes are displaced from the axis of the end columns of the building by 500 m. With a large length in the transverse and longitudinal directions, the building is divided by expansion joints into separate blocks. Longitudinal and transverse expansion joints are performed on paired columns with an insert, while at longitudinal expansion joints the axes of the columns are displaced relative to the longitudinal centerline axis by 250 mm, and at transverse expansion joints by 500 mm relative to the transverse centerline axis

Foundation structures

Distinguish between shallow foundations; pile; deep-laid (drop wells, caissons) and foundations for machines with dynamic loads.

Shallow foundations

Reinforced concrete foundations are widely used in engineering oil and gas structures, industrial and civil buildings. They are of three types (fig. 4.19): separate- under each column; tape- under the rows of columns in one or two directions, as well as under load-bearing walls; solid- under the entire structure. Foundations are most often erected on natural foundations (they are mainly considered here), but in some cases they are also performed on piles. In the latter case, the foundation is a group of piles, united on top by a distribution reinforced concrete slab - a grillage.

Separate foundations are arranged with relatively low loads and a rather rare arrangement of columns. Strip foundations under rows of columns are made when the soles of individual foundations are close to each other, which is usually the case with weak soils and high loads. It is advisable to use strip foundations in case of heterogeneous soils and external loads that are different in value, since they level out uneven foundation settlements. If the bearing capacity of the strip foundations is insufficient or the deformation of the base under them is more than permissible, then solid foundations are arranged. They even out the sediments of the base to an even greater extent. These foundations are used for weak and heterogeneous soils, as well as for significant and unevenly distributed loads.

Foundation depth d\ (the distance from the leveling mark to the base of the foundation) is usually assigned taking into account:

geological and hydrogeological conditions of the construction site;

climatic features of the construction area (freezing depth);

–Constructive features of buildings and structures. When assigning the depth of the foundation, it is necessary

also take into account the peculiarities of the application and the magnitude of the loads, the technology of work during the construction of foundations, foundation materials and other factors.

The minimum depth of the foundations during construction on dispersed soils is taken at least 0.5 m from the planning surface. When building on rocky soils, it is enough to remove only the upper, heavily destroyed layer - and the foundation can be made. The cost of foundations is 4–6% of the total cost of the building.

Separate column foundations

According to the manufacturing method, foundations are prefabricated and monolithic. Depending on the size, the prefabricated foundations of the columns are solid and composite. Dimensions (edit) solid foundations(Figure 4.20) are relatively small. They are made of heavy concrete of classes B15-B25, installed on sand and gravel compacted preparation with a thickness of 100 mm. In the foundations, reinforcement is provided, located along the sole in the form of welded nets. The minimum thickness of the protective layer of the reinforcement is assumed to be 35 mm. If there is no preparation under the foundation, then the protective layer is made at least 70 mm.

Prefabricated columns are embedded in special sockets (glasses) of foundations. Embedment depth d2 take equal to (1.0-1.5) - multiple of the larger cross-sectional dimension of the column. The bottom plate of the nest must be at least 200 mm thick. The gaps between the column and the walls of the glass are taken as follows: at the bottom - not less than 50 mm; top - not less than 75 mm. During installation, the column is installed in the socket using spacers and wedges or a conductor and straightened, after which the gaps are filled with concrete of class B 17.5 on a fine aggregate.

Prefabricated foundations of large sizes, as a rule, are made up of several mounting blocks (Fig. 4.21). They consume more materials than solid ones. At significant moments and horizontal spacers, the blocks of composite foundations are connected to each other by welding of outlets, anchors, embedded parts, etc.

Monolithic separate foundations are arranged for prefabricated and monolithic frames of buildings and structures.

Typical structures of monolithic foundations mated with precast columns are designed for unified dimensions (multiples of 300 mm): foot area - (1.5 x 1.5) - (6.0 x 5.4) m, foundation height - 1.5 ; 1.8; 2.4; 3.0; 3.6 and 4.2 m (Fig.4.22).

The foundations include: an elongated sub-column, reinforced with a space frame; foundation slab with an overhang to thickness ratio of up to 1: 2, reinforced with a double welded mesh; highly placed reinforced podkolnok.

Monolithic foundations, mated with monolithic columns, are stepped and pyramidal in shape (stepped formwork is simpler). The total height of the foundation is taken such that it is not required to reinforce it with clamps and bends. The pressure from the columns is transmitted to the foundation, deviating from the vertical within 45 °. This is guided by when assigning the dimensions of the upper steps of the foundation (see Fig. 4.23, v).

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Monolithic foundations, like prefabricated ones, are reinforced with welded meshes only along the sole. When the side of the sole is more than 3 m, in order to save steel, non-standard welded nets are used, in which half of the rods are not brought to the end by 1/10 of the length (see Fig.4.23, e).

For connection with a monolithic column, reinforcement with a cross-sectional area equal to the design cross-section of the column reinforcement at the foundation edge is produced from the foundation. Within the foundation, the outlets are connected with clamps into a frame, which is installed on concrete or brick pads. The length of the outlets from the foundations must be sufficient for the arrangement of the reinforcement joint in accordance with the existing requirements. The release joints are made above the floor level. Column reinforcement can be connected with overlap outlets without welding according to the general rules for the design of such joints. In columns, centrally compressed or off-center compressed at small eccentricities, the reinforcement is connected to the outlets in one place; in columns, eccentrically compressed at large eccentricities - at least at two levels on each side of the column. If at the same time there are three rods on one side of the column section, then the middle one is connected first.

It is better to connect the armature of columns with outlets by arc welding. The joint design should be convenient for installation and welding

If the entire section is reinforced with only four rods, then the joints are only welded.

Strip foundations

Under the load-bearing walls, strip foundations are performed mainly prefabricated. They consist of cushion blocks and foundation blocks (Fig. 4.24). Pillow blocks can be of constant and variable thickness, solid, ribbed, hollow. Lay them close or with gaps. Only the cushion is calculated, the protrusions of which act as cantilevers loaded with reactive soil pressure. R(excluding the mass of weight and soil on it). The section of the cushion reinforcement is selected by the moment

M = 0.5p12 ,

where / is the console departure.

Thickness of solid cushion h set based on lateral force Q= pi, appointing it such that it does not require the setting of transverse reinforcement.

Strip foundations under the rows of columns are erected in the form of separate ribbons of the longitudinal or transverse (relative to the rows of columns) direction and in the form of cross ribbons (Fig. 4.25). Strip foundations can be prefabricated and monolithic. They have a T-shaped cross-section with a shelf at the bottom. For soils of high cohesion, a T-profile with a shelf on top is sometimes used. At the same time, the volume of excavation and formwork is reduced, but mechanized excavation is more complicated.

Brand shelf protrusions work like consoles, pinched in a fin. The shelf is assigned such a thickness that, when calculating for the shear force, it does not require reinforcement with transverse bars or bends. For small overhangs, the shelf is assumed to be of a constant height; at large - variable with a thickening to the edge.

A separate foundation strip works in the longitudinal direction for bending like a beam, which is under the influence of concentrated loads from the columns from above and distributed reactive soil pressure from below. The ribs are reinforced like multi-span beams. Longitudinal working reinforcement is assigned by calculation for normal sections for the action of bending moments; transverse bars (clamps) and bends - by calculation of inclined sections for the action of shear forces.

Solid foundations

Solid foundations are: slab bezelless; slab-and-beam and box-shaped (Fig. 4.26). The greatest rigidity is possessed by box foundations. Solid foundations are made with especially large and unevenly distributed loads. The configuration and dimensions of the solid foundation in the plan are set so that the resultant of the main loads from the structure passes in the center of the sole

In buildings and structures of great length, solid foundations (except for end sections of small length) can be approximately considered as independent strips (tapes) of a certain width, lying on a deformable foundation. Solid slab foundations of multi-storey buildings are loaded with significant concentrated forces and moments in places where stiffness diaphragms are described. This should be taken into account when designing them.

Beamless foundation slabs reinforced with welded meshes. The grids are taken with working fittings in one direction; they are stacked on top of each other in no more than four layers, joining without overlap - in the non-working direction and overlapping without welding - in the working direction. The upper nets are laid on the support frames.

Basic information about the soils of the foundations of oil and gas structures

Soils are any rocks, both loose and monolithic, lying within the weathering zone (including soils) and being the object of human engineering and construction activities.

Most often, unconsolidated, loose and clayey soils are used as bases, less often, since they rarely come to the surface, rocky soils. The classification of soils in construction is adopted in accordance with GOST 25100–95 “Soils. Classification ".

Knowledge of the building classification of soils is required to assess their properties as bases for the foundations of buildings and structures. Soils are divided into classes according to the general nature of structural bonds. Distinguish: the class of natural rocky soils, the class of natural dispersed soils, the class of natural frozen soils, the class of technogenic soils.

Rocky soils consist of igneous, metamorphic and sedimentary rocks with structural cohesion, high strength and density.

The magmatic ones include granites, diorites, quartz porphyries, gabbros, diabases, pyroxenites, etc .; to metamorphic- gneisses, schists, quartzites, marbles, rhyolites, etc .; To sedimentary- sandstones, conglomerates, breccias, limestones, dolomites. All rocky soils have a very high strength, structurally rigid bonds and make it possible to erect almost any oil and gas facilities on them.

To loose soils, called in GOST 25100-95 dispersed, includes soils consisting of individual elements formed in the process of weathering of rocky soils. Transfer of individual particles of loose soil by water currents, wind, sliding under its own weight, etc. leads to the formation of large masses of loose soils. The bonds between individual particles are weak. Loose or dispersed soils do not always have sufficient bearing

ability, therefore, the placement of structures on such soils should be justified. A thorough study of the properties of the soil in its natural state is required, as well as their change under the influence of the load from the structures.

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One of the main characteristics of loose soils is the size of individual particles and their cohesion with each other. Depending on the size of individual particles, soils are subdivided into coarse, sandy and clayey. Coarse soils contain more than 50% by mass of particles with a particle size of more than 2 mm; sandy bulk soils dry contain less than 50% by weight of particles with a particle size of more than 2 mm; clayey soils have the ability to significantly change properties depending on the saturation with water.

According to the size of individual particles, clay and sandy soils are subdivided into more differentiated types: loam, silty loam, sandy loam.

Determination of the size of the soles of foundations carried out on dispersed soils

As already noted, for foundations on dispersed soils it is considered normal when the settlement of the foundation does not exceed the limiting value, in this case, the pressure on the ground under the base of the foundation usually does not exceed the calculated soil resistance R(see § 4.1.4.2).

The size of the base of the foundation depends on its draft (deformation). Deformation analysis refers to the second group of limiting states, and, accordingly, the calculations of the dimensions of the foundation base should be carried out according to the loads adopted for the calculation of the second group of limit states - iVser (service load). The service load is taken to be equal to the standard load or is determined approximately through the design load divided by 1.2 - the average safety factor for loads:

Nser= Nn or Nser= N/1 ser assembled up to the top edge of the foundation, therefore, when determining the dimensions of the base of the foundation, it is necessary to take into account the load from its own weight and the weight of the soil located on the ledges of the foundation Nf as they also put additional pressure on the ground. Load Nf can be roughly defined as the product of the volume occupied by the foundation and the soil on its edges, V =Afd1 , on the average specific gravity of concrete and soil atT= 20 kN / m3 (Fig.4.35); Af- the area of the foot of the foundation.

The pressure under the sole of the foundation is determined by the formula

P= N+ N/ A= (4.32)

Equating the pressure under the base of the foundation to the calculated resistance of the soil p= R, you can derive a formula to determine the required area of the footing of the foundation (4.33)

To check the sufficiency of the area of existing or designed foundations, use the formula

With a horizontal bedding of soil layers (homogeneous, evenly and not strongly compressible soil) for buildings and foundations of a conventional structure, it can be assumed that the dimensions of the foundation soles selected in this way (according to formula (4.33)) (or a verified existing foundation (according to formula (4.34)) satisfy the requirements of the calculation for deformations (4.34) and the calculation of the foundation settlement can be omitted (for more details see paragraph 2.56 of SNiP 2.02.01–83 *).

The calculation of the area of \ u200b \ u200bthe foot of the foundation is usually performed in the following sequence.

Having established according to the tables (see tables 4.6, 4.7) the value of the calculated soil resistance Rq, we determine the approximate value of the area of the base of the foundation according to the formula (4.35)

then we assign the dimensions of the foundation sole and, having determined the mechanical characteristics of the soils (specific adhesion spi, the angle of internal friction fp (see Tables 4.4, 4.5), we determine the updated value of the design soil resistance R according to the formula (4.14), according to which, in turn, we specify the required dimensions of the base of the foundation according to the formula (4.33), and finally accept the base of the foundation.

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Before calculating the reinforcement, make sure that the dimensions of the foundation do not intersect with the faces of the punching shear pyramid. To determine the cross-section of the mesh of the lower step, bending moments are calculated in each step (Figure 4.36).

Bending moment in section I – I is equal to

MI = 0.125 / p gr (l-lk) 2b, (4.36)

and the required cross-sectional area of the reinforcement

A= MI / 0.9Rsh. (4.37)

For section II – II, respectively

MII= 0.125 RURgr(1- l1 ) 2 b; (4.38)

AsII= MII/0,9 Rs(h- hI). (4.39)

The choice of fittings is carried out according to the maximum value Asi, where i= 1–3.

The foundations are reinforced along the bottom with welded meshes made of rods of a periodic profile. The diameter of the rods must be at least 10 mm, and their pitch must be no more than 200 and no less than 100 mm.

Calculation of foundations for extreme columns

With the combined action of vertical and horizontal forces and moments, i.e. under eccentric loading, the foundations are designed as rectangles in the plan, elongated - in the plane of the moment.

The dimensions of the foundation in the plan should be assigned so that the greatest pressure on the ground at the edge of the sole from the design loads does not exceed l, 2 R. Preliminarily, the dimensions can be determined using the formula (4.35), as for a centrally loaded foundation.

The maximum and minimum pressure under the edge of the foundation is calculated using the eccentric compression formulas for the least favorable loading of the foundation under the action of the main combination of design loads.

For the load diagram shown in Fig. 4.34, 4.35:

N= N+ GCT+ ymdIAf, (4.41)

where M, N, Q- design bending moment, longitudinal and transverse forces in the column section at the level of the top of the foundation, respectively; GCT- design load from the weight of the wall and foundation beam. For foundations of building columns equipped with overhead traveling cranes with a lifting capacity Q> 750 kN, as well as for foundations of columns of open crane trestles, it is recommended to take a trapezoidal stress diagram under the base of the foundation with a ratio of> 0.25, and for foundations of columns of a building equipped with cranes with a lifting capacity Q< 750 kN, the condition must be met pmin> 0; in buildings without cranes, in exceptional cases, a diagram is allowed (Fig. 4.37). In this case e> 1/6.

It is desirable that from constant, long-term and short-term loads, the pressure is, if possible, evenly distributed over the sole.