1.4. Depending on the duration of the load, one should distinguish between permanent and temporary (long-term, short-term, special) loads.

1.5. Loads arising during the manufacture, storage and transportation of structures, as well as during the construction of structures, should be taken into account in the calculations as short-term loads.

Loads arising at the stage of operation of structures should be taken into account in accordance with clauses 1.6-1.9.

a) the weight of parts of structures, including the weight of load-bearing and enclosing building structures;

b) weight and pressure of soils (embankments, backfill), rock pressure.

The prestressing forces remaining in the structure or foundation should be taken into account in the calculations as forces from permanent loads.

a) the weight of temporary partitions, gravies and footings for equipment;

b) the weight of stationary equipment: machine tools, apparatus, motors, tanks, pipelines with fittings, support parts and insulation, belt conveyors, permanent lifting machines with their ropes and guides, as well as the weight of liquids and solids filling the equipment;

c) pressure of gases, liquids and bulk solids in tanks and pipelines, overpressure and rarefaction of air arising during ventilation of mines;

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

e) temperature technological influences from stationary equipment;

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

g) weight of industrial dust deposits, if its accumulation is not excluded by appropriate measures;

h) loads from people, animals, equipment on the floors of residential, public and agricultural buildings with reduced standard values ​​given in table. 3;

i) 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 (see clause 4.2) in each span of the building by a factor: 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 crane operating modes are accepted in accordance with GOST 25546 - 82;

j) snow loads with a reduced standard value determined by multiplying the full standard value in accordance with the instructions in clause 5.1 by a coefficient: 0.3 - for snow region III: 0.5 - for region IV; 0.6 - for V and VI regions;

k) temperature climatic influences with reduced standard values ​​determined in accordance with the instructions in paragraphs. 8.2 - 8.6 provided =
=
=
=
=0,
=
= 0;

l) impacts caused by deformations of the base, not accompanied by a radical change in the structure of the soil, as well as thawing of permafrost soils;

m) impacts due to changes in moisture, shrinkage and creep of materials.

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

b) the weight of people, repair materials in the areas of equipment maintenance and repair;

c) loads from people, animals, equipment on the floors of residential, public and agricultural buildings with full standard values, except for the loads specified in clause 1.7, a, b, d, e;

d) 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);

e) snow loads with full standard value;

f) temperature climatic influences with full standard value;

g) wind loads;

h) ice loads.

a) seismic effects;

b) explosive effects;

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

d) 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.

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 a person's immunity. However, in order to get the desired result, you need to correctly compose a set of exercises and choose their optimal intensity. What types of physical activity exist and for what purposes they are most suitable, we will tell you in our article.

Classification of loads

Sports activities are performed for a specific purpose. This can include 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 be different, therefore, it is customary to divide them in accordance with the following classification:

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

Our body is exposed to some of these loads on a daily basis, while others may be completely beyond the power of a novice athlete. Let's take a look at what are the differences between each type and for what tasks one or another option should be chosen.

Aerobic exercise group

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

Our body is exposed to these loads every day: during a trip to the store, in the process of cleaning an apartment, on the way to work and while walking. Also, this can include:

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

This group includes almost all options for active pastime. This is the ideal form of exercise to keep the body in good shape.

Aerobic exercise is considered the safest. They can be performed by people of different ages, regardless of the level of training. Patients who have suffered severe injuries and have chronic diseases are recommended just such loads. However, in this case, the attending physician must strictly control the intensity of the exercise and the reaction of the body.

Anaerobic exercise and how to do it

The anaerobic group of exercises includes types of physical activity that are distinguished by increased severity and intensity. These include those performed by athletes to increase muscle mass and endurance training.

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

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

Interval exercise group: what are their features?

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

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

Hypoxic loads

They are used to train the endurance of professional athletes. Hypoxic loads refer to 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 body's acclimatization process in an unfamiliar environment. are used to train the respiratory system of climbers, who often stay in high altitude conditions, where

The principle of choosing the types of physical activity (by 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. This could be:

• rehabilitation after injuries, operations and chronic diseases;
• recovery and restoration of strength, relieving stress after a hard day;
• maintaining the body in the available physical form;
• increased endurance and increased body strength.

The choice of load in the second and third options is usually straightforward. But exercises with a therapeutic purpose on your own are much more difficult to choose. Thinking about what types of physical activity restore most effectively, one should take into account the current state and capabilities of a person.

The same exercise can be very effective for an athlete in moderate physical condition and completely useless for a beginner athlete. Therefore, the choice of a 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 condition and capabilities of the athlete.

Types of loads

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

By the nature of the effect on the body, several main types of physical activity are distinguished:

• power;
• high-speed;
• for flexibility;
• on the development of dexterity and coordination skills.

To get the most out of your workouts, they should be done according to certain rules, which we'll talk about 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, since inactive tissues are deprived of essential substances, which leads to their aging.

The positive effect of strength training is achieved if the load gradually increases, but at the same time it corresponds to the state of human health. Burdening and repetition of loads should also increase gradually. Exercises with uncontrolled repetitions are completely ineffective for training endurance and strength.

In recreational exercises, physical activity (the classification and types of which the doctor prescribes) is based on an unsatisfactory burden and a clearly established number of repetitions. This method of choosing loads allows you to achieve results and avoid injury.

At the initial stages of training, weights should be used no more than 40% of the maximum possible for the state of the body. Further, 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).

Speed ​​type loads

Such training does not require a great deal of endurance and strong tension from a person. They have a positive effect on both young and aging bodies. In the latter case, speed exercises are considered particularly relevant. After all, the main sign of the body's wilting is not only the extinction of its motor functions, but also the slowing down of movements.

High-speed loads should not be carried out longer than 10-15 seconds. Longer exercises (30 to 90 seconds) should be done with reduced power. It is these exercises, alternated with short time intervals for rest, that contribute to the maximum extent to slowing down the aging process of cells. In order to keep the body in optimal shape, it is recommended to perform speed exercises during every sport.

The benefits of elasticity of muscles, ligaments, joints

Flexibility exercises are the most popular types of loads in They are included in school activities for children of the very youngest grades. Such loads help to maintain flexibility and mobility of the joints and the spine. In addition, the positive effects of such loads include:

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

The elasticity of muscles, joints and ligaments significantly reduces the likelihood of injury, contributes to the early recovery of muscle tissue after physical exertion. Flexibility exercises perfectly relax muscles and improve their tone.

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

What other training is needed

Agility and coordination abilities are equally important qualities required by a person throughout his life. In the absence of systematic training, these skills gradually diminish. What types of physical activity should be included in training to develop these abilities? Everything is as easy as shelling pears here. The best option would be various sports games: tennis, table tennis, badminton, etc.

Light sports are excellent for training dexterity and are a good prevention of cardiovascular diseases. Such loads do not have age restrictions, however, it is very difficult to dose them. For this reason, during training, you need to control your own breathing and monitor your heart rate.

Agility training with the help of sports games significantly increases the adaptive abilities of the body, and exercises that require constant attention train the mental response well. A person begins to make difficult decisions faster and acts faster in unforeseen situations.

As we have seen, any type of physical activity can have a positive effect on a person. However, to achieve maximum results, training should be systematic and include several types of exercises at the same time. Thus, it is possible to ensure a high degree of the body's resistance to adverse factors, as well as to 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!

In the limit state method, all loads are classified depending on the probability of their impact on normative and calculated.

On the basis of the impact, the loads are divided into permanent and temporary. The latter can be of long-term and short-term exposure.

In addition, there are loads that are allocated to the category special loads and impacts.

Constant loads- own weight of supporting and enclosing structures, soil pressure, prestressing.

Temporary long-term loads- weight of stationary technological equipment, weight of stored materials in storage facilities, pressure of gases, liquids and bulk materials in containers, etc.

Short-term loads- standard loads from snow, wind, mobile lifting and transport equipment, the mass of people, animals, etc.

Special loads- seismic impacts, explosive impacts. Loads arising during the installation of structures. Loads associated with the breakdown of technological equipment, impacts associated with deformations of the base due to changes in the soil structure (subsidence soils, soil settlement in karst areas and above underground workings).

There is sometimes the term "payload". Useful are called loads, the perception of which is the integral purpose of structures, for example, the weight of people for a pedestrian bridge. They can be both temporary and permanent, for example, the weight of a monumental exhibition structure is a constant load on the pedestal. For the foundation, the weight of all overlying structures also represents the payload.

When several types of loads act on a structure, the forces in it are determined as with the most unfavorable combinations using the combination coefficients.

SNiP 2.01.07-85 “Loads and Impacts” are distinguished:

basic combinations consisting of permanent and temporary loads;

special combinations consisting of permanent, temporary and one of the special loads.

With the main combination, including one live load, the combination ratio. With a larger number of temporary loads, the latter are multiplied by the combination factor.

In special combinations, the live loads are taken into account with the combination factor, and the special load with the factor. In all types of combinations, the constant load has a coefficient.

loaded elements

The complex stress state in the design of metal structures is taken into account through the design resistance, which is established on the basis of tests of metal samples under uniaxial loading. However, in real structures, the material, as a rule, is in a complex multicomponent stressed state. In this regard, it is necessary to establish a rule for the equivalence of a complex stress state to a uniaxial one.

As a criterion of equivalence, it is customary to use the potential energy accumulated in the material during its deformation to external influences.

For the convenience of analysis, the deformation energy can be represented as the sum of work on changing the volume A o and changing the shape of the body A f. The first does not exceed 13% of the total work under elastic deformation and depends on the average normal stress.

1 - 2υ

A o = ---------- (Ơ Χ + Ơ Y + Ơ Ζ) 2(2.3.)

The second work deals with material shifts:

A f = ------- [(Ơ Χ 2 + Ơ Υ 2 + Ơ z 2 - (Ơ x Ơ y + Ơ y Ơ z + Ơ z Ơ x) + 3 (τ xy 2 + τ yz 2 + τ zx 2)] (2.4.)

It is known that the destruction of the crystal structure of building steels and aluminum alloys is associated with shear phenomena in the material (movement of dislocations, etc.).

The work of shaping (2.4.) Is invariant, therefore, with a uniaxial stress state Ơ = Ơ, we have А 1 = [(1 +) / 3Е] Ơ 2

Equating this value to expression (2.4) and extracting the square root, we get:

Ơ pr = = Ơ(2.5)

This relationship establishes the energetic equivalence of a complex stress state to a uniaxial one. The expression on the right is sometimes called reduced voltage Ơ pr, meaning the reduction to some state with uniaxial stress Ơ .

If the maximum permissible stress in the metal (design resistance) is set according to the yield strength of a standard sample Ơ T, then expression (2.5) takes the form Ơ pr = Ơ T and represents the condition of plasticity in a complex stress state, i.e. condition for the transition of a material from an elastic state to a plastic one.

In the walls of I-beams near the application of a transverse load

Ơ x 0. Ơ y 0. τ xy 0... the rest of the stress components can be neglected. Then the plasticity condition takes the form

Ơ pr = = Ơ T (2.6)

At points remote from the place where the load is applied, the local voltage can also be neglected. Ơ y = 0, then the plasticity condition will be even more simplified: Ơ pr = = Ơ T .

With a simple shear, of all stress components, only

τ xy 0... then Ơ pr = = Ơ T... From here

τ xy = Ơ T / = 0.58 Ơ T (2.7)

In accordance with this expression in SNiP, the ratio between the design shear and tensile strengths is adopted,

where is the calculated shear strength; - yield point.

The behavior under load of a centrally tensioned element and a centrally compressed element, provided its stability is ensured, fully corresponds to the work of the material under simple tension-compression (Figure 1.1, b).

It is assumed that the stresses in the cross section of these elements are evenly distributed. To ensure the bearing capacity of such elements, it is necessary that the stresses from the design loads in the section with the smallest area do not exceed the design resistance.

Then the inequality of the first limit state (2.2) will be

where is the longitudinal force in the elements; - net cross-sectional area of ​​the element; - design resistance, taken equal if the development of plastic deformations is not allowed in the element; if plastic deformations are permissible, then it is equal to the largest of the two values ​​and (here and are the design resistances of the material according to the yield point and ultimate strength, respectively); - the coefficient of reliability for the material when calculating the structure in terms of ultimate resistance; - coefficient of working conditions.

Verification by the second limiting state is reduced to limiting the elongation (shortening) of the bar from standard loads

N n l / (E A) ∆ (2.9)

where is the longitudinal force in the bar from standard loads; - the calculated length of the bar, equal to the distance between the points of application of the load to the bar; - elastic modulus; - gross cross-sectional area of ​​the bar; - the limiting value of elongation (shortening).

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.

28. Dynamic, cyclic loading, the concept of endurance limit.

Dynamic load is a load that is accompanied by the acceleration of particles of the considered body or parts in contact with it. Dynamic loading occurs when rapidly increasing forces are applied or in the case of accelerated motion of the investigated body. In all these cases, it is necessary to take into account the forces of inertia and the resulting movement of the masses of the system. In addition, dynamic loads can be subdivided into shock and re-variable loads.

Impact load (impact) - loading at which the acceleration of body particles sharply change their value in a very short period of time (sudden load application). Note that, although impact belongs to dynamic types of loading, in a number of cases, when calculating for impact, inertial forces are neglected.

Repeated-variable (cyclic) loading - loads that change in time in magnitude (and possibly in sign).

Cyclic loading is a change in the mechanical and physical properties of a material under the prolonged action of cyclically changing stresses and strains in time.

Endurance limit(also limit fatigue) - in the sciences of strength: one of the strength characteristics of a material that characterizes it endurance, that is, the ability to perceive loads that cause cyclic stresses in the material.

29. The concept of fatigue of materials, factors affecting resistance to fatigue failure.

Material fatigue- in materials science - the process of gradual accumulation of damage under the action of variable (often cyclic) stresses, leading to a change in its properties, the formation of cracks, their development and destruction material for the specified time.

Effect of stress concentration

In places of a sharp change in the lateral dimensions of the part, holes, grooves, grooves, threads, etc., as shown in clause 2.7.1, a local increase in stresses occurs, which significantly reduces the endurance limit compared to that for smooth cylindrical specimens. This decrease is taken into account by introducing into the calculations effective stress concentration factor, representing the ratio of the endurance limit of a smooth sample at a symmetric cycle to the endurance limit of a sample of the same dimensions, but having one or another stress concentrator:

.

2.8.3.2. Influence of part dimensions

It has been experimentally established that with an increase in the size of the test sample, the limit of its endurance decreases ( scale effect)... This is due to the fact that with an increase in size, the likelihood of inhomogeneity of the structure of materials and its internal defects (cavities, gas inclusions) increases, as well as the fact that during the manufacture of small-sized specimens, the surface layer is hardened (hardened) to a relatively greater depth than in specimens. large sizes.

The influence of the dimensions of parts on the value of the endurance limit is taken into account by the coefficient ( scale factor), which is the ratio of the endurance limit of a part of a given size to the endurance limit of a laboratory sample of a similar configuration, having a small size:

.

2.8.3.3. Influence of surface condition

Cutting tool marks, sharp risks, scratches are the source of fatigue microcracks, which leads to a decrease in the material's endurance limit.

The influence of the surface condition on the endurance limit in a symmetric cycle is characterized by coefficient surface quality, which is the ratio of the endurance limit of a part with a given surface treatment to the endurance limit of a carefully polished specimen:

.

2.8.3.4. Effect of surface hardening

Various methods of surface hardening (mechanical hardening, chemical thermal and heat treatment) can significantly increase the value of the surface quality coefficient (up to 1.5 ... 2.0 and more times instead of 0.6 ... 0.8 times for parts without hardening). This is taken into account in the calculations by introducing a coefficient.

2.8.3.5. Influence of asymmetry of the cycle

Fatigue failure of a part is caused by long-term alternating stresses. But, as experiments have shown, with an increase in the strength properties of the material, their sensitivity to the asymmetry of the cycle increases, i.e. the constant component of the cycle "contributes" to the reduction in fatigue strength. This factor is taken into account by the coefficient.

Constant loads.(q) Depending on the duration of the action, the load is divided into permanent and temporary. Constant loads are the weight of the bearing and enclosing structures of buildings and structures, the weight and pressure of soil, the effect of prestressing reinforced concrete structures.

Temporary loads. Continuous loads (P) ... These include: the weight of stationary equipment on the floors - machines, apparatus, engines, tanks, etc .; pressure of gases, liquids, bulk solids in containers; the weight of specific contents in warehouses, refrigerators, archives, libraries and similar buildings and structures; the part of the temporary load established by the norms in residential buildings, in office and household premises; long-term temperature technological effects from stationary equipment; loads from one overhead or one overhead crane multiplied by factors: 0.5, 0.6 ... depending on the type of crane

Short-term loads. (S) These include: the weight of people, parts, materials in the areas of equipment maintenance and repair - aisles and other areas free from equipment; part of the load on the floors of residential and public buildings; loads arising from the manufacture, transportation and installation of structural elements; loads from overhead and bridge cranes used in the construction or operation of buildings and structures; snow and wind loads; temperature climatic influences.

Special loads. These include: seismic and explosive effects; loads caused by malfunction or breakdown of equipment and a sharp disruption of the technological process (for example, with a sharp increase or decrease in temperature, etc.); the impact of uneven deformations of the base, accompanied by a radical change in the structure of the soil (for example, deformation of collapsing soils during soaking or permafrost during thawing), etc.

Standard loads... They are established by norms or by nominal values. The standard constant loads are taken according to the design values ​​of the geometric and structural parameters and according to the average density values. Standard temporary technological and installation loads are set at the highest values ​​provided for normal operation; snow and wind - according to the average of the annual unfavorable values ​​or unfavorable values ​​corresponding to a certain average period of their repetitions.

Design loads. Their values ​​when calculating structures for strength and stability are determined by multiplying the standard load by the safety factor for the load γf, usually greater than unity Factor of reliability under the weight of concrete and reinforced concrete structures γ f -1> 1. The coefficient of reliability under the action of the weight of structures, used in the calculation of the stability of the position against floating, overturning and sliding, as well as in other cases when a decrease in mass worsens the operating conditions of a structure, is adopted γ f = 0.9. When calculating structures at the construction stage, the calculated short-term loads are multiplied by a factor of 0.8. When calculating structures for deformations and displacements (for the second group of limiting states), the calculated loads are taken equal to the standard values ​​with the coefficient γt = 1.

Combination of loads. Structures should be designed for various combinations of loads or corresponding forces, if the calculation is carried out according to the inelastic state scheme. Depending on the composition of the considered loads, the following are distinguished: basic combinations, including constant, long-term and short-term loads or efforts from them; special combinations, including permanent, long-term, possible short-term and one of the special loads or efforts from them.

In the basic combinations, taking into account at least two temporary loads, their calculated values ​​(or the corresponding efforts) are multiplied by the combination coefficients equal: for long-term loads f1 = 0.95; for short-term f2 = 0.9. Taking into account the same temporary load f1 = f2 = l. The norms allow, when taking into account three or more short-term loads, their calculated values ​​can be multiplied by the combination coefficients: f 2 = l- for the first in order of importance short-term load; f 2 = 0.8 - for the second; φ2 = 0.6 - for the rest.

In special combinations for long-term loads, f1 = 0.95, for short-term loads, f 2 = 0.8, except for cases stipulated in the design standards for buildings and structures in seismic regions.