Foundations are subjected to static loads or dynamic loads or a combination of both static and dynamic loads. In addition to static loads, in some cases underlying soils are subjected to dynamic loads. Static loads produce no vibration and loads are imposed slowly. Dynamic loads produce vibration to foundation soil system.
Various sources produce dynamic loads to soils supporting foundations. They could be from vibratory motion of machines, vehicular movement, earthquakes, bomb blasts, impact of hammers, mine explosions, pile driving, winds, waves and so on. Foundations supporting machines such as reciprocating engines, radar tower, punch presses, turbines, large electric motors and generators etc are subjected to vibration caused by unbalanced machine forces as well as static weight of machine.
Soil Mechanics deals with soils subjected to static loads only. To deal with foundation supporting soils subjected to vibratory motion, knowledge of theory of vibration is required. Generally static loads imposed by machines are much more than the loads imposed by dynamic loads. As dynamic loads are comparatively smaller than static loads, the supporting soils will be in linear elastic state and hence theory of elasticity is used to solve machine foundation problems.
Machine foundation includes the studies of vibration of foundation-soil system transmitted by wave energy. The vibration puts soil in motion which should not be excessive and must be kept within the limits permissible to a particular machine unit. The wave energy transmitted through the underlying soil from the moving foundation must not cause harmful effect to machines, structures, or the people in the immediate vicinity. Thus the local soil condition and the foundation soil interaction must be evaluated and included as important factors in the design procedures.
In the design of machine foundations the following questions are to be answered.
- What are the numerical limits of failure?
- What are the relation between applied loads and quantities which are significant in the design?
- How do we evaluate the significant quantities and what are the errors involved?
- Finally, after the inaccuracies involved in evaluating these quantities are determined, what sort of factor of safety do we apply in the design process?