Loads are forces that act or may act on a structure. For the purpose of predicting the resulting behavior of the structure, the loads, or external influences, including forces, consequent displacements, and support settlements, are presumed to be known. These influences may be specified by law, e.g., building codes, codes of recommended practice, or owner specifications, or they may be determined by engineering judgment. Loads are typically divided into two general classes: dead load, which is the weight of a structure including all of its permanent components, and live load, which is comprised of all loads other than dead loads.
The type of load has an appreciable influence on the behavior of the structure on which
it acts. In accordance with this influence, loads may be classified as static, dynamic, long duration, or repetitive.
Static loads are those applied so slowly that the effect of time can be ignored. All structures are subject to some static loading, e.g., their own weight. There is, however, a large class of loads that usually is approximated by static loading for convenience. Occupancy loads and wind loads are often assumed static. All the analysis methods presented in the following articles, with the exception of Arts, 3.52 to 3.55, assume that static loads are applied to structures.
Dynamic loads are characterized by very short durations, and the response of the structure depends on time. Earthquake shocks, high-level wind gusts, and moving live loads belong in this category.
Long-duration loads are those which act on a structure for extended periods of time.
For some materials and levels of stress, such loads cause structures to undergo deformations under constant load that may have serious effects. Creep and relaxation of structural materials may occur under long-duration loads. The weight of a structure and any superimposed dead load fall in this category.
Repetitive loads are those applied and removed many times. If repeated for a large
number of times, they may cause the structure to fail in fatigue. Moving live load is in this category.
