Classification of Loads. Basic and Design Loads

Classification of Loads. According to their duration, loads are divided into dead and live. In turn, live loads may be subdivided into long-time, short-time and special loads.

Dead loads are loads which are related to the self-weight of the load-bearing and filler members of the structure, the mass and pressure of the soil, and the prestress in prestressed concrete structures.

Long-time loads include the weight of stationary equipment on floors (machines, apparatus, motors, tanks, etc.); the pressure of gases, liquids, and particulate materials in tanks; loads in storehouses, refrigerators, archives, libraries and the like; the specified part of the live load in civil buildings; long-time exposure to heat from stationary equipment; the load due to one hoist or traveling crane and the snow load for some climatic regions. The above values of crane, snow and some live loads con­stitute only a part of the total value; they are used when it is necessary to evaluate their long-time effect on displacement, deformation and cracking. The total values apply to short-time loads.

Short-time loads include the loads constituted by people, spare parts and materials in attendance and maintenance areas, aisles and other locations free from equipment; floor loads in residential and other civil buildings; loads arising during the manufacture, transportation and erection of structural members; loads presented by hoists and overhead cranes used in the erection and service of buildings and other structures; snow and wind loads; and climatic temperature factors.

Special loads are related to earthquake and explosion loads; loads caused by faulty equipment and sudden changes in temperature; nonuniform strain in foundations accompanied by a radical change in the soil structure (for example, strains caused by the soaking of settling soils or the thawing of permafrost soils); and so on.

Basic Loads. These are established by relevant standards according to a predetermined probability of exceeding average values or are set up according to rated values. For dead loads, the basic values are deduced from the design geometrical and structural variables and average densities. For live erection and service loads, they are deduced from the maximum safe values ensuring normal service; for snow and wind loads, they are deduced from the worst values averaged over a year or some other repetition period.

Load Combinations. Any structure should be designed for different load combinations or, in the case of plastic design, for the respective forces. According to the loads involved, there are basic load combinations consisting of dead, long-time and short-­time live loads or the appropriate forces, and special combinations which are com­posed of dead, long-time, likely short-time and one of special live loads or the respective forces.

In turn, basic load combinations are subdivided into two groups.

The first group includes dead, long-time and one short-time live load, the second contains dead, long-time and two (or more) short time live loads.

When structures are designed to resist special load combinations, short-time loads or the respective forces should be multiplied by a combination factor of 0.8, except the cases covered by specifications for structures in earthquake regions. The special load proper should be taken into account without any reduction.  

Vocabulary notes

Dead load – постійне навантаження    Live load – переменне навантаження Pressure - тиск

Prestressed concrete – попередньо напруженний бетон Storehouse - склад

Archive - архів

Total value – сумарна вартість Average value – середня вартість Rated value – нормована вартість To evaluate – оцінювати Displacement – переміщення Earthquake – землетрус

Exercises

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