Classification of loads in the resistance of materials. Strength of materials. The main tasks of the section. Classification of loads. Classification of external forces and structural elements

When constructing buildings, it is very important to take into account the degree of influence of external factors on its structure. Practice shows that neglecting 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 impacts on the structure, regardless of their classification, have two meanings: normative and design. Loads that arise under the weight of the structure itself are called constant, since they continuously affect the building. Temporary are the impacts on the structure of natural conditions (wind, snow, rain, etc.), the weight distributed on the floors of the building from the accumulation of a large number of people, etc. That is, temporary loads are loads on the structure, which during which -or interval can change their values.

The normative 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. 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 if the planned and actual density of materials does not match.

Load classification

In order to calculate the degree of impact on the structure, it is necessary to know its nature. The types of loads are determined according to one main condition - the duration of the impact of the load on structures. Load classification includes:

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

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

Permanent loads

As mentioned earlier, permanent loads include impacts on a structure that are carried out continuously throughout the entire period of operation of the building. As a rule, they include the weight of the structure itself. For example, for a tape type building foundation, 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 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%.

Live loads

There are two types of temporary loads: long-term and short-term. Long-term structural loads include:

weight of specialized equipment and tools (machines, devices, conveyors, etc.);
the load arising from the construction of temporary partitions;
the weight of other contents located in warehouses, attics, compartments, archives 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 the repair and maintenance of the building;
loads from people and animals on the ceiling in residential premises;
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 short duration. Special loads are referred to a separate classification item, 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 resulting from a breakdown or malfunction of equipment;
loads on the structure resulting from deformation of the soil or the foundation of the structure.

Classification of loads and supports

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

Hinged-fixed support. Fixation of the end part of the beam system, in which it can rotate, but cannot move.
Hinged-movable support. This is a device in which the end of the beam can rotate and move horizontally, but at the same time the beam remains stationary vertically.
Rigid closure. 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 the beam systems, the load classification includes concentrated and distributed loads. If the impact on the support of the beam system falls at one point or on a very small area of \u200b\u200bthe support, then it is called concentrated. The distributed load acts on the support evenly, over its entire area.

You decide, for example, to make yourself a house. Independently, without the involvement of architects and designers. And at some point in time, usually almost immediately, it becomes necessary to calculate the weight of this house. And here a series of questions begins: what is the magnitude of the snow load, what load should the ceiling withstand, what coefficient to use when calculating wooden elements. But before giving specific figures, you need to understand what is the relationship between the duration of the impact of the load and its magnitude.
Loads are generally divided into permanent and temporary. And temporary, in turn, into long-term, short-term and instantaneous. Surely an unprepared reader will have a question: what, in fact, is the difference how to classify the load? Take, for example, the load on the intermediate floor. The normative value of 150 kgf per square meter is prescribed in SNiP. Upon careful reading of the document, it is easy to see that 150 kgf / m² (full standard value) is used when classifying the load as "Short-term", but if we classify it as "long-term", then the load on the floor is already taken as only 30 kgf / m²! Why is this happening? The answer lies in the depths of probability theory, but for simplicity I will explain with an example. Imagine the weight of everything you have in the room. You may be a collector of cast-iron hatches from wells, but statistically, if you consider thousands of rooms of different people, then on average people are limited to half a ton of all kinds of items per room of 17 m². Half a ton is not enough for a room! But dividing the load by the area, we get only 30 kg / m². The figure is statistically confirmed and enshrined in SNiP. Now imagine that you (weighing 80 kg) enter the room, sit on a chair (weighing 20 kg) and your wife (weighing 50 kg) sits on your knees. It turns out that a load of 150 kg acts on a fairly small area. Of course, you can always move around the apartment in such a tandem, or simply weigh all 150 kg on your own, but you cannot sit still for 10 years. This means that you create a load of these 150 kg each time in a different place, while there is no such load elsewhere. Those. in the long run you won't go beyond the average 500kg per 17m², or 30kg/m², but in the short term you can create a load of 150kg/m². And if you are engaged in trampolining with a weight of 150 kg, then this will already be an "Instant" load, and its calculation is based on individual characteristics, because there are simply no statistics for such cases.

So, with the difference between the terms sorted out a little, now to the question: what's the difference for us, as designers? If you press on the board with a small mass for decades, it will still bend, and if you press harder and then release it, the board will return to its original state. It is precisely this effect that is taken into account by assigning load classes when calculating the strength of wood.

All information for this article is from SNiP 2.01.07-85 "Loads and impacts". Since I am a supporter of wooden housing construction, I will also refer to a special case of load classification in force for 2017, and also mention the EN 1991 Eurocode.

Classification of loads according to SNiP 2.01.07-85

Depending on the duration of the action of loads, it is necessary to distinguish between permanent and temporary loads.

Permanent loads

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

weight and pressure of soils (embankments, backfills), rock pressure;

hydrostatic pressure;

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

Live loads

Live loads are further divided into three classes:

1. Continuous loads

weight of temporary partitions, gravies and footings for equipment;

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

pressure of gases, liquids and loose bodies in containers and pipelines, overpressure and rarefaction of air that occurs during ventilation of mines;

floor loads from stored materials and racking equipment in warehouses, refrigerators, granaries, book storages, archives and similar premises;

temperature technological effects from stationary equipment;

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

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

loads from people with reduced standard values;

snow loads with a reduced standard value, determined by multiplying the full standard value by a factor:

0.3 - for III snow region,

0.5 - for the IV region;

0.6 - for V and VI districts;

temperature climatic impacts with reduced standard values;

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;

impacts due to changes in humidity, shrinkage and creep of materials.

2. Short-term loads

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

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

loads from people, animals, equipment for floors of residential, public and agricultural buildings with full standard values;

loads from mobile handling equipment (forklifts, electric cars, stacker cranes, hoists, as well as from overhead and overhead cranes with a full standard value);

snow loads with full standard value;

temperature climatic effects with full standard value;

wind loads;

ice loads.

3. Special loads

seismic impacts;

explosive impacts;

loads caused by sharp disturbances in the technological process, temporary malfunction or breakdown of equipment;

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

The normative loads mentioned above are given in the table:

In the version of this document updated for 2011, the reduced standard values of uniformly distributed loads are determined by multiplying their full standard values by a factor of 0.35.
Such a classification has been adopted for quite a long time and has already taken root in the minds of the "post-Soviet engineer". However, gradually, following the whole of Europe, we are moving to the so-called Eurocodes.

Load classification according to Eurocode EN 1991

According to the Eurocode, everything is a little more diverse and more complicated. All design actions should be taken in accordance with the relevant sections of EN 1991:

EN 1991-1-1 Specific gravity, permanent and temporary loads

EN 1991-1-3 Snow loads

EN 1991-1-4 wind effects

EN 1991-1-5 Temperature effects

EN 1991-1-6 Impacts during construction works

EN 1991-1-7 Special Impacts

In accordance with EN 1990 TCP, when considering impacts, the following classification is applied:

permanent effects G. For example, effects of own weight, fixed equipment, internal partitions, finishes and indirect effects due to shrinkage and/or settlement;

impact variables Q. For example, applied payloads, wind, snow and temperature loads;

special effects A. For example, loads from explosions and impacts.

If everything is more or less clear with a constant impact (we just take the volume of the material and multiply it by the average density of this material, and so on for each material in the construction of the house), then the variable impacts require explanation. I will not consider special impacts in the context of private construction.
According to the Eurocode, the magnitude of impacts is characterized by the categories of building use according to Table 6.1:

Despite all the information given, Eurocode implies the use of national annexes developed for each section of the Eurocode individually in each country using this Eurocode. These applications take into account the various climatic, geological, historical and other features of each country, allowing, nevertheless, to adhere to uniform rules and standards in the calculation of structures. There is a national annex to Eurocode EN1991-1-1, and in terms of load values, it refers entirely to SNiP 2.01.07-85, discussed in the first part of this article.

Classification of loads in the design of wooden structures according to Eurocode EN1995-1-1

For 2017 Belarus has a document based on the Eurocode TCH EN 1995-1-1-2009 "Design of timber structures". Since the document refers to the Eurocodes, the previous classification according to EN 1991 is fully applicable to wooden structures, however, it has an additional clarification. So, in calculations for strength and suitability for use, it is imperative to take into account the duration of the load and the influence of humidity!

Load duration classes are characterized by the effect of a constant load acting in a certain period of time during the operation of the structure. For a variable action, the appropriate class is determined based on an assessment of the interaction between the typical load variation and time.

This is a general classification recommended by the Eurocode, but the structure of the Eurocodes, as I already mentioned, implies the use of National Annexes developed in each country individually, and, of course, this annex is also available for Belarus. It slightly reduces the classification of duration:

This classification sufficiently correlates with the classification according to SNiP 2.01.07-85.

Why do we need to know all this?

Effect on wood strength

In the context of designing and calculating a wooden house and any of its elements, the classification of loads together with the class of operation is important and can more than double (!) Change the design strength of wood. For example, all calculated values of wood strength, among other factors, are multiplied by the so-called kmod modification factor:

As can be seen from the table, depending on the load duration class and operating conditions, the same grade I board is able to withstand a load, for example, 16.8 MPa in compression with a short-term exposure in a heated room and only 9.1 MPa with a constant load in fifth class operating conditions.

Influence on the strength of composite reinforcement

When designing foundations and reinforced concrete beams, composite reinforcement is sometimes used. And if the duration of the action of loads does not have a significant effect on steel reinforcement, then with composite reinforcement everything is very different. The coefficients of influence of the duration of the load for the automatic transmission are given in Appendix L to SP63,13330:

In the formula for calculating the tensile strength given in the table above, there is a coefficient yf - this is the reliability coefficient for the material, which is taken equal to 1 when calculating the limit states of the second group, and equal to 1.5 when calculating the first group. For example, in a beam in the open air, the strength of fiberglass reinforcement can be 800*0.7*1/1=560 MPa, but under continuous load 800*0.7*0.3/1=168 MPa.

Influence on the magnitude of the distributed load

According to SNiP 2.01.07-85, loads from people, animals, equipment on the floors of residential, public and agricultural buildings are accepted with a reduced standard value if we classify these loads as long-term. If we classify them as short-term, then we accept the full standard load values. Such differences are formed by probability theory and mathematically calculated, but in the Code of Practice they are presented in the form of ready-made answers and recommendations. The same effect of classification exists on snow loads, but I will consider snow loads in another article.

What should be counted?

We have already dealt a little with the classification of loads and understood that the loads on floors and snow loads are related to live loads, but at the same time they can apply to both long-term and short-term ones. Moreover, their value can differ significantly depending on which class we assign them to. Is it possible that in such an important issue the decision depends on our desire? Of course not!
In EN 1995-1-1-2009 "Design of Timber Structures" TCP there is the following prescription: if the load combination consists of actions that belong to different load duration classes, then the value of the modification factors that corresponds to the action of a shorter duration must be applied, for example for combination of self-weight and short-term load, the value of the coefficient corresponding to the short-term load is applied.
In SP 22.13330.2011 "Foundations of buildings and structures", the indication is as follows: loads on floors and snow loads, which, according to SP 20.13330, can be both long-term and short-term, are considered short-term when calculating the foundations for bearing capacity, and when calculating the deformations - lengthy. Loads from mobile handling equipment in both cases are considered short-term.

1.2. Classification of external forces and structural elements

External forces acting on structural elements, "as is known from the course of theoretical mechanics, are divided into active and reactive (reactions of bonds). Active external forces are commonly called. The origin and nature of the load are determined by the purpose, operating conditions and design features of the considered element. For example, for the drive shaft shown in Fig. 1.8, the loads are the forces acting on the teeth of the wheel, and the tension of the belt branches, as well as the gravity of the shaft itself and the parts mounted on it (gear and pulley).

For the truss rods of an overhead crane (Fig. 1.9), the main loads are the gravity forces of the lifted load and the trolley; farm gravity is less important.

The main load of the steam boiler drum is the pressure of the steam in it.

If the considered structural element moves with acceleration, then the number of loads acting on it also includes inertia forces.

The forces of gravity of this part of the structure and the forces of inertia arising from its accelerated movement are voluminous syalamv, i.e., they act on every infinitesimal volume element. Loads transmitted from one structural element to another are referred to as surface forces.

Surface sleeps are divided into concentrated into distributed. At the same time, it should be remembered that, of course, concentrated forces do not exist - this is an abstraction introduced for the convenience of technical calculations. The force is considered as concentrated if it is transmitted to the part along the area, the dimensions of which are negligibly small in comparison with the dimensions of the structural element itself. For example, the pressure force of a wagon wheel on a rail can be considered as concentrated, since although the wheel and rail are deformed at the point of contact, the dimensions of the area resulting from this deformation are negligible compared to the dimensions of both the rail and the wheel.

Loads distributed over a certain surface are characterized by pressure, i.e., the ratio of the force acting on a surface element normally to it, to the area of \u200b\u200bthe given element, and, therefore, are expressed in pascals (1 Pa \u003d \u003d 1 N / m ~), MPa, etc.

In many cases, one has to deal with loads distributed along the length of a structural element. for example, we can talk about the force of gravity per unit length of a beam, and if the section of the beam is not constant, then the force of gravity per unit of its length will be variable.

The load distributed along the length is characterized by intensity, usually denoted by q and expressed in units of force referred to units of length: N / m, kN / m, etc.

According to the nature of the change over time, there are: static loads, increasing slowly and smoothly from zero to its final value; having reached it, they do not change in the future. An example is centrifugal forces during the acceleration period and subsequent uniform rotation of a rotor;

reloads, repeatedly changing in time according to this or that law. An example of such a load are the forces acting on the teeth of gears;

short duration loads applied to the structure immediately or even with an initial velocity at the moment of contact (these loads are often referred to as dynamic or percussion). An example of shock is, for example, the load taken by the parts of a steam hammer during forging.

The question of bonds and their reactions is considered in sufficient detail in the course of theoretical mechanics. Here we restrict ourselves to a reminder of the most common types of connections.

Articulated support(single-connected support) is schematically depicted as shown in fig. 1.10, a. The reaction of such a support is always perpendicular to the support surface.

Hinged-fixed support(double-connected support) is shown schematically in fig. 1.10b. The reaction of the hinged support passes through. hinge center, and its direction depends on the acting active forces. Instead of finding the numerical value and direction of this reaction, it is more convenient to look for its two components separately.

In a tight box(three-connected support), a reactive pair of forces (moment) and a reactive force arise; the latter is more convenient to represent in the form of its two components (Fig. 1.11).

If the connection is a rod with hinges at the ends (Fig. 1.12), then the reaction is directed along its axis, that is, the rod itself works in tension or compression.

The shapes of structural elements are extremely diverse, but with a greater or lesser degree of accuracy, each of them can be considered in calculations either as a beam, or as a shell or plate, or as an array.

In the strength of materials, they mainly study methods for calculating the strength, stiffness and stability of a bar, that is, a body whose two dimensions are small compared to the third (length). Imagine a plane figure moving along a certain line in such a way that the center of gravity of the figure is on this line, and the plane of the figure is perpendicular to it. The body obtained as a result of such a movement is the beam (Fig. 1.13).

The flat figure, by the movement of which the beam is formed, is its cross section, and the line along which its center of gravity moved was the axis of the bar.

The axis of the beam is the locus of the centers of gravity of its cross sections. Depending on the shape of the axis of the beam and how its cross section changes (or remains constant), there are straight and curved bars with a constant, continuously or stepwise changing cross section (Fig. 1.14). As some examples of parts calculated as straight bars, one can indicate a drive shaft (see Fig. 1.8), any of the rods of an overhead crane truss (see Fig. 1.9); the hook of this crane is calculated as a curved beam.

Plate and shell(Fig. 1.15) are characterized by the fact that their thickness is small compared to other dimensions. The plate can be considered as a special case of the shell, so to speak, a "straightened" shell. Examples of parts considered as shells and plates are various tanks for liquids and gases, skin elements of ship hulls, submarines, and aircraft fuselages.

array a body is called, all three dimensions of which are quantities of the same order, for example, a foundation for a car, a ball or a roller of a rolling bearing.

By the nature of the application: concentrated and distributed.

According to the duration of actions in time: variable and constant.

By the nature of the action: static and dynamic.

Permanent loads:

The weight of a part of buildings and structures, including the weight of load-bearing and enclosing building structures;

Weight and pressure of soils, rock pressure;

The impact of prestressing in structures;

Live loads: Weight of temporary partitions; Weight of stationary equipment: machines, devices; Loads on floors of residential and public buildings with reduced standard values; Loads on residential floors in warehouses, refrigerators, granaries, archives, libraries and utility buildings and premises; Snow loads with a reduced design value;

Short-term loads : Loads on floors of residential and public buildings with full standard values; Snow loads with full design value; Loads from mobile handling equipment (overhead and overhead cranes, hoists, loaders); Loads arising from the manufacture, transportation and erection of structures, during the installation and rearrangement of equipment, as well as loads from the weight of products and materials temporarily stored at the construction site; Loads from equipment arising in start-stop, transitional and test modes; wind loads; Temperature and climatic influences;

Special loads: Seismic and explosive impacts; Loads caused by a sharp violation of the technological process, temporary malfunction or equipment breakdown; The impact of uneven deformations, accompanied by a change in the structure of the soil;

Work of centrally compressed columns under load and prerequisites for calculation of bearing capacity. Calculation of centrally compressed columns (racks).

Centrally compressed elements are called, the load on which acts along the center of gravity of the section (in columns with a symmetrical section, the center of gravity of the section is taken to coincide with the geometric center). The stress-strain state of centrally compressed columns and the nature of their destruction depend on many factors: material, size and shape of the cross section, length, methods of fixing the ends. With longitudinal or transverse bending, the destruction of the element occurs because the stresses in its extreme fibers reach the limit values, and the material is destroyed. All compressed elements are subject to buckling to some extent, its manifestation depends on their flexibility and the material from which the compressed element is made. Steel and wood columns tend to have small cross-sectional dimensions and are more flexible, while reinforced concrete and masonry columns have larger cross-sectional dimensions and are therefore less flexible. The norms take into account the safe values of buckling - this is the basis for the calculation of columns.

Calculation:

We select the calculation scheme of the column;

According to SNiP or a reference book, we find the calculated resistance: R y \u003d 24.5 Kn

Find the cross-sectional area: A

Determine the coefficient of buckling

Determine the estimated length of the rod: L ef = µ*L 0

According to the assortment, we determine the moments of inertia of the section relative to the main central axes: J x, cm 4; J y , cm 4

Find the minimum radius of gyration: i min = √ J min / √A

Determine the flexibility of the rod: λ = μ * L 0 / i min

The buckling coefficient (φ) is determined depending on the flexibility;

The bearing capacity is determined by the value of the permissible value of the compressive force.

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 of reinforced concrete structures.

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

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

Special loads. These include: seismic and explosive impacts; loads caused by a malfunction or breakdown of equipment and a sharp violation 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 fundamental change in the structure of the soil (for example, deformations of subsiding soils during soaking or permafrost soils during thawing), etc.

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

Estimated loads. Their values in the calculation of structures for strength and stability are determined by multiplying the standard load by the load safety factor γf, usually greater than one. Reliability factor under the action of the weight of concrete and reinforced concrete structures γ f-1>1. The safety factor under the action of the weight of structures, used in the calculation of the stability of the position against ascent, overturning and sliding, as well as in other cases when a decrease in mass worsens the working conditions of the structure, is adopted γ f=0.9. When calculating structures at the stage of construction, 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 limit states), the design loads are taken equal to standard values with a coefficient γt = 1.

combination of loads. Structures must be designed for various combinations of loads or their corresponding forces, if the calculation is carried out according to the inelastic state scheme. Depending on the composition of the considered loads, there are: basic combinations including permanent, 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 main combinations, taking into account at least two temporary loads, their calculated values (or the corresponding efforts) are multiplied by the combination coefficients equal to: for long-term loads f1 = 0.95; for short-term f2=0.9. When taking into account one temporary load f1=f2 = l. The norms allow, when taking into account three or more short-term loads, to multiply their calculated values by the combination coefficients: f 2 \u003d l- for the first in order of importance of the short-term load; f 2 = 0.8 - for the second; f2 = 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 specified in the design standards for buildings and structures in seismic areas.