Before beginning to analyse a structure, it is important to know what kind of structure it is. Different types of structures may need to be analysed using different methods. For example, structures that are determinate may be completely analysed using only static equilibrium, whereas indeterminate structures require the use of both static equilibrium and compatibility relationships to find the internal forces. In addition, real structures must be stable. This means that the structure can recover static equilibrium after a disturbance. There is no point analysing a structure that is not stable.
This section will explain the concepts of determinacy, indeterminacy and stability and show how to identify determinate, indeterminate and stable structures.