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Classification of External Forces (Loads) Resistance

External forces in resistance to materials are divided into active and jet(bond reactions). Loads Are active external forces.

Loads by Application Method

By application method load there are voluminous(own weight, forces of inertia), acting on each infinitesimal element of volume, and superficial. Surface loads are divided into concentrated loads and distributed loads.

Distributed loads are characterized by pressure - the ratio of the force acting on a surface element along the normal to it, to the area of ​​this element and are expressed in the International System of Units (SI) in pascals, megapascals (1 PA = 1 N / m2; 1 MPa = 106 Pa), etc. and in the technical system - in kilograms of force per square millimeter, etc. (kgf / mm2, kgf / cm2).

In sopromat it is often considered surface loads distributed along the length of the structural member. Such loads are characterized by intensity, usually denoted by q and expressed in newtons per meter (N / m, kN / m) or in kilograms of force per meter (kgf / m, kgf / cm), etc.

Loads by the nature of change over time

By the nature of the change over time, there are static loads- growing slowly from zero to their final value and subsequently not changing; and dynamic loads causing large forces of inertia.

Assumptions of resistance

Assumptions of Sopromat Sopromat

When constructing a theory of strength, stiffness and stability analysis, assumptions are made related to the properties of materials and to the deformation of the body.

Material property assumptions

Consider first assumptions related to material properties:

assumption 1: the material is considered homogeneous (its physical and mechanical properties are considered the same at all points;

assumption 2: the material completely fills the entire volume of the body, without any voids (the body is considered as a continuous medium). This assumption makes it possible to apply the methods of differential and integral calculus in the study of the stress-strain state of the body, which require the continuity of the function at each point of the volume of the body;

assumption 3: the material is isotropic, that is, its physical and mechanical properties at each point are the same in all directions. Anisotropic materials - the physical and mechanical properties of which change depending on the direction (for example, wood);

assumption 4: the material is perfectly elastic (after removing the load, all deformations disappear completely).

Deformation Assumptions

Now let's look at the main body deformation assumptions.

assumption 1: deformations are considered small. From this assumption it follows that when composing the equilibrium equations, as well as when determining the internal forces, the deformation of the body can be ignored. This assumption is sometimes referred to as the principle of initial sizes. For example, consider a rod embedded with one end into a wall and loaded at the free end by a concentrated force (Fig. 1.1).

The moment in the seal, determined from the corresponding equilibrium equation by the method of theoretical mechanics, is equal to:. However, the rectilinear position of the bar is not its equilibrium position. Under the action of force (P), the bar will bend, and the point of application of the load will shift both vertically and horizontally. If we write down the equilibrium equation of the bar for the deformed (bent) state, then the true moment arising in the embedment will be equal to: . Assuming that the deformations are small, we assume that the displacement (w) can be neglected in comparison with the length of the rod (l), that is, then . The assumption is not possible for all materials.

assumption 2: the displacements of the points of the body are proportional to the loads causing these displacements (the body is linearly deformable). For linearly deformable structures, the principle of independence of the action of forces ( superposition principle): the result of the action of a group of forces does not depend on the sequence of loading the structure by them and is equal to the sum of the results of the action of each of these forces separately. This principle is also based on the assumption that the loading and unloading processes are reversible.

Classification of external loads acting on structural members.

General classification of structural elements.

Technical objects and structures consist of separate parts and elements, which are very diverse in shape, size, other parameters and characteristics. From the standpoint of engineering calculations, it is customary to distinguish four main groups of structural elements: rods, plates, shells, and arrays.

Rods- these are straight or curved structural elements in which one dimension (length) significantly exceeds two other dimensions (in a spatial orthogonal coordinate system), see Figure 20. Examples of structural elements such as rods: legs of a chair or table, a column of a building structure, a lifting rope cars, gearshift lever of a car, etc.

Z Curved bar

Straight bar

Figure 20. Schemes of structural members of the type of bars

t (plate thickness)

Figure 21. Schematic of a structural element of the plate type

Figure 22. Scheme of a structural element of the shell type (cylindrical)

Rice. 23. Scheme of a structural element of an array type

Plate- these are flat structural elements in which one size (thickness) is significantly less than the other two. Examples of plates: table top; walls and ceilings of buildings, etc., see Figure 21, from which it can be seen that the thickness of the plate is significantly less than its two dimensions in plan.

Shells- these are non-planar thin-walled structural elements in which one size (wall thickness) is much smaller than other sizes. Examples of shells: pipelines for transporting liquid and gaseous products (cylindrical shells); cylindrical, spherical or combined containers for liquids; conical bins for bulk materials; non-planar coatings of various structures, etc., see Figure 22, which shows a cylindrical shell (thin-walled cylindrical pipe), in which the wall thickness is much less than its diameter and length.

Arrays- these are structural elements in which all three sizes are commensurate. Examples of arrays: foundation blocks of machines, machines and building structures; massive bridge supports, etc., see Figure 23.

The courses "Mechanical Engineering" and "Resistance of Materials" focus on the fundamental study of structural elements such as rods. Plates, shells and arrays are studied in advanced courses "Resistance of Materials" and in special courses.

Focused Forces- these are the forces applied to a structural element on the site of its surface, the dimensions of which, in comparison with the dimensions of the entire surface of the structural element, can be neglected. As a rule, concentrated forces are the result of action on a given body (structural element) of another body (in particular, another structural element). In many practically important cases, concentrated

forces can be considered as applied to a structural element at a point without noticeable damage to the accuracy of engineering calculations. Units of measurement of concentrated forces N (Newton), kN (kilonewton), etc.

Volumetric forces- these are forces applied throughout the volume of a structural element, for example, distributed gravity forces. Units of distributed volumetric forces are N / m 3, kN / m 3, etc. The total force of gravity (N, kN) of any structural element is often conventionally taken into account in calculations as a concentrated force applied at a point called its center of gravity.

Distributed forces (loads)- these are the forces applied on a part of the area (or length) of the deformable body, commensurate with the dimensions of the whole body. There are superficially distributed forces (loads), the units of which are N / m 2, kN / m 2, etc. (for example, distributed snow loads on building coatings), as well as linearly distributed loads (along the length of structural elements), the units of which are N / m, kN / m, etc. (for example, the distributed pressure forces of slabs supported by structural beams).

Static forces (loads)- these are forces (loads) that do not change (or insignificantly change) their value, position and direction of action during the operation of the structure.

Dynamic forces (loads)- these are forces (loads) that significantly change their value, position and / or direction in short periods of time and cause vibrations of the structure.

Rated loads- these are normal maximum loads arising during the operation of the structure.

Control questions:

1) What is studied in the course "Strength of Materials"? What is its relevance to highly skilled technicians?

2) What are external loads and internal forces?

3) Explain the concepts of deformation, strength, stiffness and stability.

4) Explain the concepts of uniformity, continuity, isotropy and anisotropy.

5) Give the classification of structural elements.

6) Give a classification of external loads acting on structural members.

1. Alexandrov A.V. and others. Resistance of materials. Textbook for universities - M .: Higher. shk., 2001 .-- 560 p. (p. 5 ... 20).

2. Stepin P.A. Strength of materials. - M .: Higher. school, 1983 .-- 303 p. (p. 5 ... 20).

3. Handbook on the strength of materials / Pisarenko GS. and others - Kiev: Naukova Dumka, 1988. - 737p. (p. 5 ... 9).

Control tasks for the IWS- with the help of educational literature to expand information on the following issues:

1) what are elastic forces?

2) what is the essence of the principle of the absence of initial internal efforts in the body (pp. 9-10)?

3) what are the principles of schematization of external loads acting on structural elements used in engineering calculations (, pp. 8-11)?

4) explain the principle of independence of the action of forces (, p. 18-20;, p. 10)?

5) explain the principle of Saint-Venant (, pp. 10-11);

6) what is the difference between deformation and movement (, p. 17-18; p. 13-14) ?;

7) the general concept of the section method (, p. 13-16;, p. 14-17);

8) the general concept of stresses in a deformable body, designations of normal and tangential stresses (, pp. 13-15; pp. 17-20).

9) classification of external loads acting on structural members (see clause 5.3).

Lecture 6. Topic 6. "Central tension-compression of straight rigid rods"

The purpose of the lecture- to outline the introductory provisions on the topic, the essence and application of the section method for determining the internal forces in the rods under central tension-compression; to give an initial understanding of the diagrams of internal efforts.

Statistical loads do not change over time or change very slowly. Under the action of statistical loads, a strength calculation is carried out.

Repeated variables loads change their meaning or value and sign many times. The action of such loads causes metal fatigue.

Dynamic loads change their value in a short period of time, they cause large accelerations and inertial forces and can lead to sudden destruction of the structure.

It is known from theoretical mechanics that, according to the method of applying the load, there can be focused or distributed on the surface.

In reality, the transfer of the load between the parts does not occur at a point, but at a certain site, that is, the load is distributed.

However, if the contact area is negligible compared to the size of the part, the force is considered to be concentrated.

When calculating real deformable bodies in the resistance of materials, the distributed load should not be replaced by a concentrated one.

The axioms of theoretical mechanics in the strength of materials are used to a limited extent.

You cannot transfer a pair of forces to another point of the part, move a concentrated force along the line of action, you cannot replace the system of forces with a resultant one when determining displacements. All of the above changes the distribution of internal forces in the structure.

In the process of construction and operation, the building experiences various loads. External influences can be divided into two types: power and non-power or exposure to the environment.

TO power impacts include various types of loads:

permanent- from the own weight (mass) of the building elements, soil pressure on its underground elements;

temporary (long-term)- from the weight of stationary equipment, long-term storage of goods, dead weight of permanent elements of the building (for example, partitions);

short-term- from the weight (mass) of moving equipment (for example, cranes in industrial buildings), people, furniture, snow, from the action of the wind;

special- from seismic impacts, impacts as a result of equipment failures, etc.

TO non-force relate:

temperature influences that cause changes in the linear dimensions of materials and structures, which in turn leads to the occurrence of force effects, as well as affecting the thermal regime of the room;

exposure to atmospheric and ground moisture, as well as vaporous moisture contained in the atmosphere and in the air of the premises, causing a change in the properties of the materials from which the building structures are made;

air movement causing not only loads (in case of wind), but also its penetration into the structure and premises, a change in their humidity and thermal conditions;

exposure to radiant energy the sun (solar radiation) causing, as a result of local heating, a change in the physical and technical properties of the surface layers of material, structures, a change in the light and thermal conditions of the premises;

exposure to aggressive chemical impurities contained in the air, which in the presence of moisture can lead to the destruction of the building structure material (corrosion phenomenon);

biological influences caused by microorganisms or insects, leading to the destruction of structures made of organic building materials;

exposure to sound energy(noise) and vibration from sources inside or outside the building.

At the place of application of efforts load divided into focused(e.g. equipment weight) and evenly distributed(own weight, snow).

By the nature of the action, the load can be static, i.e. constant in value over time and dynamic(drums).

In direction - horizontal (wind pressure) and vertical (dead weight).

That. the building is affected by a variety of loads in terms of magnitude, direction, nature of action and place of application.

Rice. 2.3. Loads and influences on the building.

You may get such a combination of loads in which they all act in the same direction, reinforcing each other. It is these unfavorable load combinations that building structures are counting on. The normative values ​​of all efforts acting on the building are given in DBN or SNiP.

5. Centrally stretched steel elements: scheme of work, application, strength calculation

Center Stretched Elements- these are elements in the normal section of which the point of application of the longitudinal tensile force N coincides with the point of application of the resultant forces in the longitudinal reinforcement.

Centrally-tensioned elements include tightening arches, lower chords and downward truss braces and other elements (Fig. 51).

Centrally stretched members are generally designed to be prestressed.

Basic principles for the design of centrally stretched elements:

Rod working fittings without prestressing are connected along the length by welding;

Lap joints without welding are allowed only in slab and wall structures;

Stretched prestressed reinforcement in linear elements should not have joints;

In the cross-section, the prestressed reinforcement is placed symmetrically (to avoid eccentric compression of the element);

Off-center stretched elements- these are elements that are simultaneously stretched by longitudinal force N and bend in the moment M, which is equivalent to eccentric stretching by force N eccentric e o relative to the longitudinal axis of the element. In this case, 2 cases are distinguished: when the longitudinal tensile force N applied between the resultant forces in tensioned and compressed reinforcement, and the position when the force is applied beyond a given distance.

The eccentrically stretched elements include the lower chords of bevel trusses and other structures.

When constructing buildings, it is very important to take into account the degree of influence of external factors on its structure. Practice shows that neglect of this factor can lead to cracks, deformations and destruction of building structures. This article will consider a detailed classification of loads on building structures.

General information

All effects on a structure, regardless of their classification, have two meanings: standard and calculated. Loads that arise under the weight of the structure itself are called constant, since they continuously act on the building. The impact on the structure of natural conditions (wind, snow, rain, etc.), the weight distributed to the floors of the building from the accumulation of a large number of people, etc. -or interval may change their values.

The standard values ​​of permanent loads from the weight of the structure are calculated based on the design measurements and the characteristics used in the construction of materials. The calculated values ​​are determined using standard loads with possible deviations. Deviations can appear as a result of changes in the original dimensions of the structure or when the planned and actual density of materials does not match.

Classification of loads

In order to calculate the degree of impact on a structure, you need to know its nature. The types of loads are determined according to one basic condition - the duration of the effect of the load on the structure. Load classification includes:

• permanent;
• temporary:
  • long-term;
  • short-term.
• special.

Each item that includes the classification of structural loads should be considered separately.

Constant loads

As mentioned earlier, permanent loads include the impact on a structure, which is carried out continuously throughout the entire period of operation of the building. As a rule, they include the weight of the structure itself. Suppose, for the strip type of the base of the building, the constant load will be the weight of all its elements, and for the floor truss, the weight of its belts, struts, braces and all connecting elements.

It should be borne in mind that for stone and reinforced concrete structures, permanent loads can be more than 50% of the design load, and for wooden and metal elements, this value usually does not exceed 10%.

Temporary loads

Temporary loads are of two types: long-term and short-term. Long-term loads on the structure include:

• the weight of specialized equipment and tools (machines, apparatus, conveyors, etc.);
• load arising from the construction of temporary partitions;
• the weight of other content located in warehouses, attics, building archives;
• pressure of the contents of pipelines supplied and located in the building; thermal effects on the structure;
• vertical loads from overhead and overhead cranes; weight of natural precipitation (snow), etc.

• the weight of personnel, tools and equipment during the repair and maintenance of the building;
• loads from people and animals on the floor in living quarters;
• the weight of electric cars, forklifts in industrial warehouses and premises;
• natural loads on the structure (wind, rain, snow, ice).

Special loads

Special loads are of a short-term nature. Special loads are referred to a separate clause of the classification, since the probability of their occurrence is negligible. But still, they should be taken into account when erecting a building structure. These include:

• building loads due to natural disasters and emergencies;
• load arising from a breakdown or malfunction of equipment;
• structural loads arising from deformation of the soil or the base of the structure.

Classification of loads and supports

A support is a structural element that absorbs external forces. There are three types of supports in beam systems:

1. Articulated fixed support. Fixation of the end part of the beam system so that it can rotate but cannot move.
2. Pivot-movable support. This is a device in which the end of the beam can be rotated and moved horizontally, but the beam remains stationary vertically.
3. Rigid termination. This is a rigid fastening of the beam, in which it can neither turn over nor move.

Depending on how the load is distributed to the beam systems, the load classification includes concentrated and distributed loads. If the impact on the support of the beam system falls on one point or on a very small area of ​​the support, then it is called concentrated. The distributed load acts on the support evenly, over its entire area.

As practice shows, the topic of collecting loads raises the greatest number of questions from young engineers starting their professional career. In this article I want to consider what permanent and temporary loads are, how long-term loads differ from short-term ones and why such a separation is necessary, etc.

Classification of loads by duration of action.

Depending on the duration of the action, the loads and impacts are divided into permanent and temporary . Temporary load in turn are subdivided into long-term, short-term and special.

As the name suggests, constant loads operate throughout the entire operation. Temporary loads appear in separate periods of construction or operation.

The approximate densities of some basic materials are given in table. 1. Approximate weights of some roll and finishing materials are given in table. 2.

Table 1

Density of basic building materials

table 2

Weight of roll and finishing materials

Temporary loads subdivided into long-term, short-term and special.

- the load from people, furniture, animals, equipment on the floors of residential, public and agricultural buildings with reduced standard values;

- loads from vehicles with reduced standard values;

- weight of temporary partitions, gravies and footings for equipment;

- snow loads with reduced standard values;

- weight of stationary equipment (machines, motors, containers, pipelines, liquids and solids filling the equipment);

- pressure of gases, liquids and bulk solids in tanks and pipelines, overpressure and air pressure arising from ventilation of mines;

- loads on floors from stored materials and racking equipment in warehouses, refrigerators, granaries, book depositories, archives of similar premises;

- temperature technological influences from stationary equipment;

- weight of the water layer on water-filled flat surfaces;

- vertical loads from bridge and overhead cranes with a reduced standard value, determined by multiplying the full standard value of the vertical load from one crane in each span of the building by the coefficient:

0.5 - for groups of operating modes of cranes 4K-6K;

0.6 - for the group of operating mode of cranes 7K;

0.7 - for the group of operating mode of cranes 8K.

Groups of valve modes are accepted in accordance with GOST 25546.

- the weight of people, repair materials in the areas of equipment maintenance and repair with full standard values;

- loads from vehicles with full standard values;

- snow loads with full standard values;

- wind and ice loads;

- equipment loads arising in start-up, transient and test modes, as well as during its rearrangement or replacement;

- temperature climatic influences with full standard value;

- loads from mobile lifting and transport equipment (forklifts, electric cars, stacker cranes, telphers, as well as from bridge and overhead cranes with a full standard value).

- seismic impacts;

- explosive effects;

- loads caused by abrupt disturbances in the technological process, temporary malfunction or equipment breakdown;

- impacts caused by deformations of the base, accompanied by a radical change in the structure of the soil (when soaking subsidence soils) or its subsidence in areas of mine workings and in karst.