Loads

Loads 

(in structural mechanics), force actions that produce changes in the stress-strain state of the components of buildings and structures. The following loads are distinguished according to the nature of their changes over time: static loads, whose place of application, direction, and intensity are assumed in calculations to be independent of time or to change so slowly that the resulting forces of inertia may be disregarded, and dynamic loads, whose magnitude, direction, and site of application change so rapidly that forces of inertia must be taken into account in calculations.

Static loads are divided into constant (or dead) loads, which in calculations of any given system are assumed to have constant action (the empty weight of structures, ground pressure, and so on), and intermittent (or live) loads, which may or may not be taken into account in calculations, depending on their significance for the structure in question. Intermittent loads are in turn divided into short-term traveling loads, which change their position (loads resulting from the congregation of people on the floors of buildings or from automobiles and trains on the spans of bridges), and long-term fixed loads (for example, the weight of stationary equipment and of racks and bins in warehouses).

The following loads are distinguished according to the nature of application to the body on which they act: concentrated loads, which are applied to a very small area (point), and spread loads, which are applied to an entire surface (line) or part of it. Spread loads are characterized by the intensity—that is, by the limiting ratio of the magnitude of the resultant load, distributed over a given surface or line, to the magnitude of the area or line on which it is acting, if the latter tends toward zero. A spread load of constant intensity is said to be uniformly spread. A spread load whose points of application occupy an entire surface or section is called a continuous, or distributed, load.

In calculations of structures by the method of limiting states, a distinction is made between rated loads, which are established by design standards and correspond to the conditions of normal use of the structure, and design loads, which are determined with reference to possible deviations from conditions of normal use of the structure. In cases of simultaneous action of several loads, the least favorable design load combination is determined; it corresponds to the critical value of the force or displacement arising in the elements of the structure or installation.

REFERENCE

Stroitel’nye normy ipravila. Part 2, sect. A, ch. 11: “Nagruzki i vozdeistviia: Normy proektirovaniia.” Moscow, 1962.

L. V. KASAB’IAN