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 calculations as short-term loads.
Loads arising during the operation stage of structures should be taken into account in accordance with paragraphs 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, backfills), rock pressure.
The forces from prestressing remaining in the structure or foundation should be taken into account in calculations as forces from permanent loads.
a) the weight of temporary partitions, grouting and footings for equipment;
b) the weight of stationary equipment: machines, apparatus, motors, containers, pipelines with fittings, supporting parts and insulation, belt conveyors, permanent lifting machines with their ropes and guides, as well as the weight of liquids and solids filling equipment;
c) pressure of gases, liquids and granular bodies in containers and pipelines, excess pressure and rarefaction of air that occurs during ventilation of mines;
d) loads on floors from stored materials and shelving equipment in warehouses, refrigerators, granaries, book depositories, archives and similar premises;
e) temperature technological influences from stationary equipment;
f) the weight of the water layer on water-filled flat surfaces;
g) the 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 overhead 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 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 operating modes are accepted according to 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 the coefficient: 0.3 - for snow region III: 0.5 - for region IV; 0.6 - for regions V and VI;
k) temperature climatic influences with reduced standard values, determined in accordance with the instructions of paragraphs. 8.2 - 8.6 provided = 
=
=
=
=0,
=
= 0;
l) impacts caused by deformations of the base, not accompanied by a fundamental change in the structure of the soil, as well as thawing of permafrost soils;
m) impacts caused by changes in humidity, shrinkage and creep of materials.
a) loads from equipment arising in start-up, transition and test modes, as well as during its rearrangement or replacement;
b) the weight of people, repair materials in equipment maintenance and repair areas;
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 vehicles, stacker cranes, hoists, as well as from overhead and overhead cranes with full standard values);
e) snow loads with full standard value;
f) temperature climatic effects with full standard value;
g) wind loads;
h) ice loads.
a) seismic impacts;
b) explosive effects;
c) loads caused by sudden disruptions in the technological process, temporary malfunction or breakdown of equipment;
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 mining areas and karst areas.
Statistical loads do not change over time or change very slowly. When subject to statistical loads, strength calculations are carried out.
Re-variables loads repeatedly change value or value and sign. 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, 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.
During construction and operation, the building experiences various loads. External influences can be divided into two types: power And non-force or environmental influences.
TO forceful impacts include different kinds loads:
permanent– from the own weight (mass) of the building elements, soil pressure on its underground elements;
temporary (long-term)– on the weight of stationary equipment, long-term stored cargo, dead weight of permanent building elements (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 wind;
special– from seismic impacts, impacts resulting from equipment failures, etc.
TO non-forceful relate:
temperature effects causing changes linear dimensions materials and structures, which in turn leads to the occurrence of force effects, as well as affecting the thermal conditions of the room;
exposure to atmospheric and ground moisture, and vaporous moisture contained in the atmosphere and indoor air, causing a change in the properties of the materials from which the building’s structures are made;
air movement causing not only loads (in wind), but also its penetration into the structure and premises, changing their humidity and thermal regime;
exposure to radiant energy sun (solar radiation) causing, as a result of local heating, a change in the physical and technical properties of the surface layers of materials, structures, changes 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 material of building structures (the phenomenon of corrosion);
biological effects 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.
Where the effort is applied loads are divided into concentrated(e.g. weight of equipment) and evenly distributed(own weight, snow).
Depending on the nature of the load, they can be static, i.e. constant in magnitude over time and dynamic(drums).
In direction - horizontal (wind pressure) and vertical (own weight).
That. a building is subject to a variety of loads in terms of magnitude, direction, nature of action and location of application.
Rice. 2.3. Loads and impacts on the building.
There may be a combination of loads in which they will all act in the same direction, reinforcing each other. It is these unfavorable combinations of loads that building structures are designed to withstand. The standard values of all forces acting on the building are given in DBN or SNiP.
5. Centrally tensioned steel elements: operation scheme, application, strength calculation
Centrally 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 stretched elements include arch stringers, lower chords and downward braces of trusses and other elements (Fig. 51).
Centrally tensioned elements are designed, as a rule, to be prestressed.
Basic principles for the design of centrally tensioned elements:
The rod working reinforcement without prestressing is connected along its length by welding;
Lap joints without welding are allowed only in slab and wall structures;
Tensile prestressed reinforcement in linear elements should not have joints;
IN cross section prestressed reinforcement is placed symmetrically (to avoid eccentric compression of the element);
Eccentrically stretched elements- these are elements that simultaneously stretch longitudinal force N and bend with a moment M, which is equivalent to eccentric stretching by force N with eccentricity e o relative to the longitudinal axis of the element. In this case, two cases are distinguished: when the longitudinal tensile force N applied between the resultant forces in tension and compression reinforcement, and the position when the force is applied beyond a given distance.
Eccentrically tensioned elements include the lower chords of braced trusses and other structures.
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 muscle tissue volume 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 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, 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. Here are the exercises with therapeutic purpose It’s much more difficult to choose on your own. 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 on the basis 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.
At the initial stages of 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 maximum amount repetitions of the exercise were 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 strong 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 withering 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 precisely such exercises, alternating with short time intervals for rest, that contribute to the maximum extent to slow down the aging process of cells. In order to keep 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. They include school lessons children of the youngest grades. 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 - 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 result, training should be systematic and include several types of exercises simultaneously. In this way, it is possible to ensure a high degree of resistance of the body to unfavorable 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!
When constructing buildings, it is very important to take into account the degree of influence of external factors on its design. Practice shows that neglecting this factor can lead to cracks, deformations and destruction of building structures. This article will discuss a detailed classification of loads on building structures.
General information
All impacts on a structure, regardless of their classification, have two meanings: normative and design. The loads that arise under the weight of the structure itself are called constant, since they continuously act on the building. Impacts on the structure are considered temporary natural conditions(wind, snow, rain, etc.), weight distributed on the floors of the building from accumulation large quantity people, etc. That is, temporary loads are loads on a structure that can change their values over a period of time.
Standard values of permanent loads from the weight of the structure are calculated based on design measurements and characteristics used in the construction of materials. Design values are determined using standard loads with possible deviations. Deviations may appear as a result of changes in the original dimensions of the structure or a discrepancy between the planned and actual density of materials.
Load classification
In order to calculate the degree of impact on a structure, it is necessary to know its nature. Types of loads are determined by one basic condition - the duration of the impact of the load on structures. 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 impacts on a structure that occur continuously throughout the entire period of operation of the building. As a rule, these include the weight of the structure itself. Let's say that for a strip-type building base the constant load will be the weight of all its elements, and for a floor truss - the weight of its chords, racks, braces and all connecting elements.
It is worth considering 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%.
Live loads
There are two types of temporary loads: long-term and short-term. Long-term loads on the structure include:
- weight of specialized equipment and tools (machines, apparatus, conveyors, etc.);
- load arising during the construction of temporary partitions;
- the weight of other contents located in warehouses, attics, and archive compartments of the building;
- 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 repair and maintenance of the building;
- loads from people and animals on floors in residential premises;
- weight of electric cars, forklifts in industrial warehouses and premises;
- natural loads on the structure (wind, rain, snow, ice).
Special loads
Special loads are short-term in nature. Special loads are included in a separate classification point, since the likelihood of their occurrence is negligible. But still they should be taken into account when constructing building structure. These include:
- loads on the building due to natural disasters and emergency situations;
- load resulting from breakdown or malfunction of equipment;
- loads on the structure resulting from deformation of the soil or foundation 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:
- Hinged-fixed support. Fixing the end part of a beam system so that it can rotate but cannot move.
- Articulated and movable support. This is a device in which the end of the beam can rotate and move horizontally, but the beam remains stationary vertically.
- Hard seal. This is a rigid fastening of the beam, in which it can neither turn over nor move.
Depending on how the load is distributed on beam systems, load classification includes concentrated and distributed loads. If the impact on the support of a beam system occurs at one point or over a very small area of the support, then it is called concentrated. The distributed load acts on the support evenly over its entire area.
External forces in strength of materials are divided into active And reactive(connection reactions). Loads are active external forces.
Loads by application method
By application method loads there are voluminous(own weight, inertial forces) acting on each infinitesimal element of volume, and surface ones. 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 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. etc., and in the technical system - in kilograms of force per square millimeter, etc. (kgf/mm2, kgf/cm2).
In compromising materials, they are often considered surface loads, distributed along the length of the structural element. Such loads are characterized by intensity, usually denoted q and expressed in newtons per meter (N/m, kN/m) or in kilograms of force per meter (kgf/m, kgf/cm), etc.
Loads according to the nature of changes over time
Based on the nature of changes over time, they are distinguished static loads- increasing slowly from zero to its final value and then not changing; And dynamic loads causing large inertial forces.
28. Dynamic, cyclic loading, the concept of endurance limit.
Dynamic load is a load that is accompanied by the acceleration of particles of the body in question or parts in contact with it. Dynamic loading occurs when rapidly increasing forces are applied or in the case of accelerated movement of the body under study. 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 divided into impact and repeated-variable loads.
Impact load (impact) is a loading in which the acceleration of body particles sharply changes its value in a very short period of time (sudden application of load). Note that, although the blow refers to dynamic views loading, in some cases, when calculating the impact, inertial forces are neglected.
Repeatedly variable (cyclic) loading – loads that change in magnitude (and possibly in sign) over time.
Cyclic loading is a change in the mechanical and physical properties of a material under the long-term action of stresses and strains that cyclically change over 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 absorb loads that cause cyclic stresses in the material.
29. The concept of fatigue of materials, factors influencing resistance to fatigue failure.
Material fatigue- in materials science - the process of gradual accumulation of damage under the influence 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 where there is a sharp change in the transverse dimensions of the part, holes, grooves, grooves, threads, etc., as shown in paragraph 2.7.1, a local increase in stress occurs, significantly reducing the endurance limit compared to that for smooth cylindrical samples. This reduction is taken into account by introducing into the calculations effective stress concentration factor, representing the ratio of the endurance limit of a smooth sample under 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 as the size of the test sample increases, its endurance limit decreases ( scale effect). This is explained by the fact that with increasing size, the probability of heterogeneity in the structure of materials and its internal defects (cavities, gas inclusions) increases, and also by the fact that when producing small-sized samples, hardening (hardening) of the surface layer takes place to a relatively greater depth than that of the samples 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 given dimensions to the endurance limit of a laboratory sample of a similar configuration having small dimensions:
.
2.8.3.3. Influence of surface condition
Traces of a cutting tool, sharp marks, scratches are the source of fatigue microcracks, which leads to a decrease in the endurance limit of the material.
The influence of the surface condition on the endurance limit in a symmetrical 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 thoroughly polished sample:
.
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 or more times instead of 0.6 ... 0.8 times for parts without hardening). This is taken into account in calculations by introducing the coefficient.
2.8.3.5. Impact of cycle asymmetry
The cause of fatigue failure of a part is long-acting alternating stresses. But, as experiments have shown, with an increase in the strength properties of a material, their sensitivity to cycle asymmetry 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.