Most analysis problems involve the solution of how a model behaves in response to some action on this model--a force, a pressure, a temperature, or perhaps a magnetic field. In analysis terminology these actions are known as loads. Similarly, most models have certain conditions constraining their behavior. For example, an end of a cantilever beam fixed to a wall, or an adiabatic (non-conducting) boundary in a thermal problem. These constraints are referred to as boundary conditions. In this chapter loads and boundary conditions are frequently referred to as LBCs.

There is a great deal of similarity in both of these quantities. Both are applied to portions of your model, and some quantities may in fact be used as both loads and boundary conditions. For example, a fixed zero displacement in a structural model serves as a boundary condition, while an imposed non-zero displacement also has the effect of acting as a load. Hence, a common set of operations is used within Patran to create both loads and boundary conditions.

The specific loads and boundary conditions available to you depend upon the analysis program you are using with Patran. Both load and boundary conditions can be applied to either your geometric model or your finite element model. Both quantities have the important feature of being independent of the finite element model itself. A single analysis model may have many different states of loads and boundary conditions, or a series of loads applied at different times or frequency values in an analysis. This flexibility allows you to ask many different kinds of "what if?" questions using the same finite element model, and to simulate many kinds of complex behavior.

The Fields and Load Cases applications are used together with the Loads and Boundary Conditions application. The figure below shows the relationships between these three applications.

In addition to loads and boundary conditions, which remain constant over a period of time and over a particular region of the model to which they are applied, you can also define varying loads and boundary conditions. LBCs that vary temporally or spatially are defined using the Fields application. This application is described in detail later in this chapter.

In Patran, different individual sets of loads and boundary conditions are grouped into quantities known as load cases, which essentially constitute a separate analysis solution using the same model data. We will discuss load case capabilities in Patran later in this section.