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Loads acting on structures and structures: classification and combinations. Glossary. Classification of external loads acting on structural elements Classification of loads according to the method of application

Impacts experienced by the stand from the hand bent (see Fig. 42), the board from the load (see Fig. 44), the cylindrical rod of the bolt when screwing the nut wrench(see Fig. 45), etc., represent external forces or loads. The forces arising in the places where the rack is secured and the board is supported are called reactions.

Rice. 42

Rice. 44

Rice. 45

According to the method of application, loads are divided into concentrated and distributed (Fig. 49).

Types and classification of loads:

Concentrated loads transmit their effect through very small areas. Examples of such loads are the pressure of the wheels of a railway car on the rails, the pressure of a hoist trolley on a monorail, etc.

Distributed Loads act relatively large area. For example, the weight of the machine is transmitted through the frame to the entire area of ​​​​contact with the foundation.

Based on the duration of action, it is customary to distinguish between constant and variable loads. An example of a constant load is the pressure of a plain bearing - the support of shafts and axles - and its own weight on the bracket.

Variable load It is mainly the parts of periodic action mechanisms that are affected. One such mechanism is a gear transmission, in which the teeth in the contact zone of adjacent pairs of gears experience a variable load.

By the nature of the action loads may be static And dynamic. Static loads remain almost unchanged during the entire operation of the structure (for example, the pressure of trusses on supports).

Dynamic loads and last for a short time. Their occurrence is associated in most cases with the presence of significant accelerations and inertial forces.

Dynamic loads are experienced by parts of impact machines, such as presses, hammers, etc. Parts of crank mechanisms also experience significant dynamic loads during operation from changes in the magnitude and direction of speeds, that is, the presence of accelerations.

As practice shows, the topic of load collection raises the greatest number of questions among young engineers starting their professional activity. 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 action, loads and impacts are divided into permanent And temporary . Temporary loads are in turn divided into long-term, short-term And special.

As the name itself suggests, permanent loads valid throughout the entire period of operation. Live loads appear during certain periods of construction or operation.

include: own weight of load-bearing and enclosing structures, weight and soil pressure. If prefabricated structures (crossbars, slabs, blocks, etc.) are used in the project, the standard value of their weight is determined on the basis of standards, working drawings or passport data of the manufacturing plants. In other cases, the weight of structures and soils is determined from design data based on their geometric dimensions as the product of their density ρ and volume V taking into account their humidity under the conditions of construction and operation of structures.

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

Table 1

Density of basic building materials

Material	Density, ρ, kg/m3
Concrete: - heavy - cellular	2400 400-600
Gravel	1800
Tree	500
Reinforced concrete	2500
Expanded clay concrete	1000-1400
Brickwork with heavy mortar: - made of solid ceramic bricks - made of hollow ceramic bricks	1800 1300-1400
Marble	2600
Construction waste	1200
River sand	1500-1800
Cement-sand mortar	1800-2000
Mineral wool thermal insulation boards: - not subject to load — for thermal insulation of reinforced concrete coverings — in ventilated facade systems — for thermal insulation of external walls followed by plastering	35-45 160-190 90 145-180
Plaster	1200

table 2

Weight of rolled and finishing materials

Material	Weight, kg/m2
Bituminous shingles	8-10
Plasterboard sheet 12.5 mm thick	10
Ceramic tiles	40-51
Laminate 10 mm thick	8
Metal tiles	5
Oak parquet: — 15 mm thick — thickness 18 mm — thickness 22 mm	11 13 15,5
Roll roofing (1 layer)	4-5
Sandwich roofing panel: — 50 mm thick — thickness 100 mm — thickness 150 mm — thickness 200 mm — thickness 250 mm	16 23 29 33 38
Plywood: - 10 mm thick — 15 mm thick — 20 mm thick	7 10,5 14

Live loads are divided into long-term, short-term and special.

relate:

— 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, grouts 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 granular bodies in containers and pipelines, excess pressure and air rarefaction that occurs during 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 coverings;

— vertical loads from overhead 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 7K crane operating mode group;

0.7 - for the 8K crane operating mode group.

Groups of crane modes are accepted according to GOST 25546.

relate:

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

— loads from vehicles with full standard values;

— snow loads with full standard values;

— wind and ice loads;

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

— temperature climatic influences with full standard value;

- loads from movable lifting and transport equipment (forklifts, electric vehicles, stacker cranes, hoists, as well as overhead and overhead cranes with full standard values).

relate:

— seismic impacts;

— explosive effects;

- loads caused by sudden disturbances technological process, temporary malfunction or breakdown of equipment;

- 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 mining and karst.

When solving problems of structural strength, external forces, or loads, are called the forces of interaction of the structural element under consideration with the bodies associated with it. If external forces are the result of direct, contact interaction of a given body with other bodies, then they are applied only to points on the surface of the body at the point of contact and are called surface forces. Surface forces can be continuously distributed over the entire surface of the body or part of it. The amount of load per unit area is called load intensity, is usually denoted by the letter p and has the dimensions N/m2, kN/m2, MN/m2 (GOST 8 417-81). It is allowed to use the designation Pa (pascal), kPa, MPa; 1 Pa = 1 N/m2.

The surface load reduced to the main plane, i.e. the load distributed along the line, is called the linear load, is usually denoted by the letter q and has the dimensions N/m, kN/m, MN/m. The change in q along the length is usually shown in the form of a diagram (graph).

In the case of a uniformly distributed load, the diagram q is rectangular. Under the action of hydrostatic pressure, the diagram q is triangular.

The resultant of the distributed load is numerically equal to the area of ​​the diagram and is applied at its center of gravity. If the load is distributed over a small part of the surface of the body, then it is always replaced by a resultant force, called concentrated force P (N, kN).

There are loads that can be represented in the form of a concentrated moment (pair). Moments M (Nm or kNm) are usually designated in one of two ways, or in the form of a vector perpendicular to the plane of action of the pair. Unlike the force vector, the moment vector is depicted as two arrows or a wavy line. The torque vector is usually considered to be right-handed.

Forces that are not the result of contact of two bodies, but are applied to each point of the volume of the occupied body (own weight, inertial forces) are called volumetric or mass forces.

Depending on the nature of the application of forces over time, static and dynamic loads are distinguished. A load is considered static if it increases relatively slowly and smoothly (at least over a few seconds) from zero to its final value, and then remains unchanged. In this case, we can neglect the accelerations of the deformed masses, and therefore the forces of inertia.

Dynamic loads are accompanied by significant accelerations of both the deformable body and the bodies interacting with it. The inertial forces arising in this case cannot be neglected. Dynamic loads are divided from instantly applied, impact loads into recurrent ones.

The instantly applied load increases from zero to maximum within a fraction of a second. Such loads occur when the combustible mixture in the engine cylinder is ignited. internal combustion, when starting off a train.

An impact load is characterized by the fact that at the moment of its application, the body causing the load has a certain kinetic energy. Such a load occurs, for example, when driving piles using a pile driver, in the elements of a forging hammer.

Classification of loads.

Statistical load (Fig. 18.2 A) do not change over time or change very slowly. When subject to statistical loads, strength calculations are carried out.

Re-variables loads (Fig. 18.26) repeatedly change value or value and sign. The action of such loads causes metal fatigue.

Dynamic loads (Fig. 18.2c) change their value in a short period of time, they cause large accelerations and inertia forces and can lead to sudden destruction of the structure.

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

In reality, the transfer of load between parts occurs not at a point, but at a certain area, i.e. the load is distributed.

However, if the contact area is negligibly small compared to the dimensions of the part, the force is considered concentrated.

When calculating real deformable bodies in the resistance of materials, it is not necessary to replace the distributed load with 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 on a part, move a concentrated force along the line of action, and you cannot replace a system of forces with a resultant when determining displacements. All of the above changes the distribution of internal forces in the structure.

Shapes of structural elements

All the variety of forms is reduced to three types based on one characteristic.

1. Beam- any body whose length is significantly greater than other dimensions.

Depending on the shape of the longitudinal axis and cross sections, several types of beams are distinguished:

Straight permanent beam cross section(Fig. 18.3a);

Straight stepped beam (Fig. 18.35);

Curved beam (Fig. 18.Sv).

2. Plate- any body whose thickness is significantly less than other dimensions (Fig. 18.4).

3. Array- a body that has three sizes of the same order.

Control questions and tasks

1. What is called strength, rigidity, stability?

2. By what principle are loads classified in the resistance of materials? What type of damage does repeated variable loads lead to?

4. What body is called a beam? Draw any beam and indicate the axis of the beam and its cross section. What bodies are called plates?

5. What is deformation? What deformations are called elastic?

6. At what deformations is Hooke’s law satisfied? Formulate Hooke's law.

7. What is the principle of initial sizes?

8. What is the assumption of the continuous structure of materials? Explain the assumption of homogeneity and isotropy of materials.

LECTURE 19

Topic 2.1. Basic provisions. External and internal loads, section method

Know the method of sections, internal force factors, stress components.

Be able to determine types of loads and internal force factors in cross sections.

Structural elements are tested during operation external influence, which is estimated by the magnitude of the external force. External forces include active forces and reactions of supports.

Under the influence of external forces, internal elastic forces arise in the part, striving to return the body to its original shape and size.

External forces must be determined by methods of theoretical mechanics, and internal forces must be determined by the main method of strength of materials - the method of sections.

In the resistance of materials, bodies are considered in equilibrium. To solve problems, equilibrium equations obtained in theoretical mechanics for a body in space are used.

The coordinate system associated with the body is used. More often, the longitudinal axis of a part is designated z, the origin of coordinates is aligned with the left edge and placed at the center of gravity of the section.

Section method

The method of sections consists of mentally dissecting a body with a plane and considering the equilibrium of any of the cut-off parts.

If the whole body is in balance, then each part of it is in balance under the influence of external and internal forces. Internal forces are determined from equilibrium equations compiled for the body part in question.

We dissect the body across the plane (Fig. 19.1). Let's look at the right side. External forces act on it F 4; F 5 ; F 6 and internal elastic forces q to, distributed over the section. The system of distributed forces can be replaced by the main vector Ro , placed at the center of gravity of the section, and the total moment of forces.

Main point It is also customary to represent them in the form of moments of pairs of forces in three projection planes:

M x- torque relative to Oh;M y - torque relative to O y, M z - torque about Oz.

The resulting components of elastic forces are called internal power factors. Each of the internal force factors causes a certain deformation of the part. Internal force factors balance the external forces applied to this element of the part. Using six equilibrium equations, we can obtain the magnitude of the internal force factors:

From the above equations it follows that:

N z - longitudinal force, Oz external forces acting on the cut-off part of the beam; causes tension or compression;

Q x - shear force, equal to the algebraic sum of projections onto the axis Oh

Q y - shear force, equal to the algebraic sum of projections onto the axis OU external forces acting on the cut-off part;

forces Q x and Q y cause a shear of the section;

M z - torque, equal to the algebraic sum of the moments of external forces relative to the longitudinal axis Oz-, causes the beam to twist;

M x - bending moment, equal to the algebraic sum of the moments of external forces relative to the Coolant axis;

M y - bending moment, equal to the algebraic sum of the moments of external forces relative to the Oy axis.

The moments M x and M y cause the beam to bend in the corresponding plane.

Voltages

Section method allows you to determine the value of the internal force factor in the section, but does not make it possible to establish the law of distribution of internal forces over the section. To assess strength, it is necessary to determine the magnitude of the force at any point in the cross section.

The intensity of internal forces at a cross-section point is called mechanical stress. Stress characterizes the amount of internal force per unit cross-sectional area.

Consider a beam to which an external load is applied (Fig. 19.2). By using section method let's cut the beam with a transverse plane, discard the left part and consider the equilibrium of the remaining right part. Select a small area on the cutting plane ΔA. The resultant of the internal elastic forces acts on this area.

Voltage direction p avg coincides with the direction of the internal force in this section.

Vector p avg called full tension. It is customary to decompose it into two vectors (Fig. 19.3): τ - lying in the section area and σ - directed perpendicular to the site.

If the vector ρ - spatial, then it is divided into three components:

Properly dosed physical activity has a beneficial effect on the body. They allow you to achieve an ideal figure, increase muscle tone and even strengthen the human immune system. However, to get the desired result, you need to correctly compose a set of exercises and choose their optimal intensity. We will tell you in our article what types of physical activity exist and for what purposes they are most suitable.

Load classification

Sports are performed for a specific purpose. This could be maintaining muscle tone, losing weight, recovering from an injury, or preparing for sports. In each case, the types of physical activity and their intensity will differ, so they are usually divided according to the following classification:

• aerobic;
• anaerobic;
• interval;
• hypoxic.

Our body is exposed to some of these loads every day, while others may be completely beyond the capabilities of a novice athlete. Let's look at what the differences are between each type and for what tasks you should choose one or another option.

Aerobic exercise group

Aerobic exercise (or cardio exercise) is a set of simple exercises that are aimed at enriching cells required quantity oxygen, increasing the body's defenses and training its resistance.

Our body is exposed to these loads every day: while going to the store, while cleaning the apartment, on the way to work and while walking. This also includes:

• cycling;
• water sports;
• skiing, skating, roller skating;
• daily gymnastics;
• walking up the stairs;
• dance classes, etc.

This group includes almost all options for active pastime. This is an ideal look to keep your body in good shape. physical activity.

Aerobic exercise is considered the safest. People can do them of different ages regardless of the level of training. For patients who have suffered severe injuries and have chronic diseases, such loads are recommended. However, in this case, the intensity of exercise and the body’s reaction should be strictly controlled by the attending physician.

Anaerobic exercises and how to do them

The anaerobic group of exercises includes types of physical activity characterized by increased severity and intensity. These include those performed by athletes to increase muscle mass, and training the body's endurance.

Exercises are performed using heavy dumbbells, barbells and various exercise machines. Their main essence is a short-term movement of gravity without body movement. The final result is a significant increase in volume muscle tissue and high strength indicators. However, you should know that in the process of rapidly increasing muscle volume, their elasticity decreases significantly.

Anaerobic exercise has contraindications and is not recommended for people over 40 years of age. However, you can perform exercises with moderate weights to keep your body in good physical shape: lift dumbbells up to 5 kg, use rubber or spring expanders.

Group of interval exercises: what are their features?

During training, athletes can alternate and combine different kinds physical activity (and its intensity). In this case, they talk about interval load, when classes include elements of the first and second types.

For example, young and healthy men involved in strenuous sports are required to perform aerobic exercise. That is, during their training, heavy exercises and light running alternate. At the same time, track and field athletes can additionally use large loads affecting a specific muscle group. In sports, types of physical activity alternate constantly, especially when it comes to professional training.

Hypoxic loads

They are used to train endurance of professional athletes. Hypoxic loads are considered heavy exercises, as they are performed in conditions of lack of oxygen, when a person is at the limit of his capabilities.

The main goal of this type of training is to minimize the process of acclimatization of the body in an unfamiliar environment. used for training respiratory system climbers who often stay in high altitude conditions, where

The principle of choosing types of physical activity (according to the nature of the impact)

The correct choice of optimal exercises is the key to obtaining the desired result. That is why before starting training you need to clearly define the final goal. It could be:

• rehabilitation after injuries, operations and chronic diseases;
• healing and recuperation, relieving stress after a hard day’s work;
• maintaining the body in its current physical shape;
• increasing endurance and increasing body strength.

Selecting the load in the second and third options usually does not cause difficulties. But choosing exercises for therapeutic purposes on your own is much more difficult. When thinking about what types of physical activity are most effective for recovery, one should take into account the person’s current condition and capabilities.

The same exercise can be very effective for an athlete in moderate physical shape and absolutely useless for a beginning athlete. Therefore, the choice of training program should be carried out according to the principle of threshold loads, and it is better if the coach is well aware of the athlete’s condition and capabilities.

Types of loads

In addition to the main classification of training, there is a division of exercises into several types. Each of them is aimed at developing a specific quality.

Based on the nature of the impact on the body, there are several main types of physical activity:

• power;
• expressways;
• for flexibility;
• to develop dexterity and coordination abilities.

To get the maximum benefit from your workouts, they should be performed in accordance with certain rules, which we will discuss below.

Strength exercises

Strength exercises help keep the body in good shape, slow down the aging process of tissues, and prevent the development of various cardiovascular diseases. It is important that everyone receives the load because inactive tissues are deprived of essential substances, which leads to their aging.

The positive effects of strength exercises are achieved if the load is gradually increased, but at the same time it corresponds to the person’s health status. The burden of loads and their repetition should also increase gradually. Exercises with an uncontrolled number of repetitions are absolutely ineffective for training endurance and strength.

In health-improving exercises, physical activity (the classification and types of which are prescribed by the doctor) is based on unlimited weights and a clearly established number of repetitions. This method of choosing loads allows you to achieve results and avoid injuries.

On initial stages training, you should use weights no more than 40% of the maximum possible according to the state of the body. Next, the load can be selected so that the maximum number of repetitions of the exercise is about 8-12 times. And for the muscles of the forearm, neck, lower leg and abdomen it would reach 15-20 times (with pauses between sets of 1-3 minutes).

High-speed loads

Such training does not require a person to have great endurance and great tension. They provide positive influence for both young and aging organisms. In the latter case, speed exercises are considered especially relevant. After all, the main sign of fading of the body is not only the fading of its motor functions, but also a slowdown in movements.

Speed ​​loads should not be carried out for longer than 10-15 seconds. Long exercises (30 to 90 seconds) should be performed with reduced power. It is these exercises, alternating with short time intervals for rest, that maximum degree help slow down the aging process of cells. In order to support the body in optimal shape Speed ​​exercises are recommended to be performed during every sport activity.

The benefits of elasticity of muscles, ligaments, joints

Flexibility exercises are the most popular types of loads in the world. They are included in the school classes of children of the most junior classes. Such loads help maintain flexibility and mobility of the joints and spine. In addition, the positive effects of such loads include:

• prevention of excessive wear of joints;
• preventing the development of arthritis;
• improvement of the condition of the joint capsule;
• prevention of osteochondrosis.

The elasticity of muscles, joints and ligaments significantly reduces the likelihood of injury and promotes rapid recovery of muscle tissue after physical activity. Flexibility exercises perfectly relax muscles and improve their tone.

The absence of such loads leads to tissue stiffening. Energy that could be used for recovery is wasted, and the muscle itself suffers from a lack of oxygen.

What other training is needed?

Dexterity and coordination abilities are no less important qualities required by a person throughout his life. In the absence of systematic training, these skills gradually decrease. What types of physical activity should be included in training to develop these abilities? Everything is as simple as shelling pears here. The most the best option There will be various sports games: tennis, table tennis, badminton, etc.

Light sports are great for training agility and are good prevention of cardiovascular diseases. There are no age restrictions for such loads, but they are very difficult to dose. For this reason, during training you need to control your own breathing and monitor your heart rate.

Agility training using sports games significantly increase the body’s adaptive abilities, and exercises that require constant attention train the mental reaction well. A person begins to accept faster complex solutions and acts faster in unexpected situations.

As we have seen, any type of physical activity can have a positive effect on a person. However, to achieve maximum results, training must be systematic and include several types of exercises simultaneously. In this way, you can ensure a high degree of resistance of the body to adverse factors, as well as constantly develop and improve new skills. The main thing is to remember, no matter what type of load you choose, it is important to always know when to stop!