Figure. Structure and specified loading: a force in the vertical direction.
In structural optimization the term load case is used to define an independent set of boundary conditions (structural supports and loads). The term independent is used to highlight the fact that this set of boundary conditions is imposed on the structure exclusively and that it represents the whole set of boundary conditions acting on the structure at a time.
This means that when a structure is subjected to multiple load cases, it has to be loaded by all of the load cases, but only one at a time. From the numerical point of view, this means that one has to perform as many FEA computations as there are load cases. In other words, the computational effort in structural optimization is proportional to the number of load cases.
Preparing the set of load cases for topology optimization is a very delicate process that has to be performed with great care. Namely, topology optimization adapts the structure completely to the underlying boundary conditions. Therefore, each inaccuracy or lack of information in boundary conditions can have a serious negative influence on the final design.
Consider the example structure shown in the following figure. The specified load is a force acting in the vertical direction.
Figure. Structure and specified loading: a force in the vertical direction.
This structure could be optimized by considering only one load case containing the specified load. But this would be correct only if there is absolutely no possibility that the specified load might occasionally vary. If there is any chance of load variation, this must be reflected in additional load cases.
Figure. Structure and all possible load situations that might eventually occur must be considered in an optimization process.
IMPORTANT. Careful identification of all possible loading situations and adequate formulation of load cases is an absolute necessity to prevent bad results. It is by far better to have too many load cases than too few.