Before analysis and design there is crucial stage known as “investigation of structures stability”. It is necessary to determine that the structure we are going to design is stable or not.

Stability in simple words means that structure doesn’t undergo rigid body motion, that structure takes external forces and deforms.

There are two terminologies used in stability “Externally stable” and “Internally Stable” which are explained in this video below. Structure is said to be stable if and only if it is both externally and internally stable.

An experienced engineer will tell just by inspection weather a structure is stable or not and what steps are necessary to make an unstable structure stable.


video courtesy: Dr. Structure


  1. Picture 1 : Stable and unstable structures
    static system in the figure below left is unstable. Or , it`s just drawn incorrectly!