Prior to start analysis and design, the most vital stage belongs to "investigation of structures stability". So, it is very essential to find out whether the structure is durable or not.
In simple terms, stability means that structure doesn't sustain inflexible body motion, that structure absorbs external forces and deforms.
The stability in structure is categorized as "Externally stable" and "Internally Stable". The following video briefly explains these terms. Structure becomes durable when it is both externally and internally durable.
A knowledgeable engineer can easily detect the durability of a structure as well as suggests essential methods to transform an inconstant structure durable.