One of the main structural principles is that elements such as the roof, floor and walls must remain stationary. For this to happen, there needs to be an equilibrium of forces – when the forces acting on them are equal and opposite. Under loading, some deflection and deformation – in the form of bending and buckling – may occur, and if this movement is not allowed for then structural failure may be the result. Therefore, a principle of structures is that they be designed to maintain a state of equilibrium; resisting external loads without moving.

The study of the causes and effects of stationary forces acting on rigid objects is statics. When a structure is stationary or in equilibrium, it is a ‘static body’. For a structure to remain static, three basic equations must hold true: