# Overall form: Form-active, Semi-form-active and non-form-active structures

The form-active concept is an essential aid for assessing the efficiency of structural form. Using this concept, all buildings (and the structures that support them) can be placed into one of three categories which indicate the types of internal forces that will occur within them: these are form-active (axial only), semi-form-active (combined axial and bending) and non-form-active (bending only).

These categories have a direct bearing on structural efficiency (see form and efficiency) and therefore on embodied energy and carbon footprint.

Reference: Structure and Architecture, Chapter 4.