What is the primary objective of designing a structural system for ductility in a seismically active region?

The primary objective of designing a structural system for ductility in a seismically active region is to prevent catastrophic collapse and protect human life during a major earthquake. Ductility is the capacity of a structural element or material to undergo significant inelastic deformation—meaning to deform substantially beyond its elastic limit without losing its load-carrying capacity or fracturing suddenly. In a severe earthquake, intense ground shaking imposes significant forces and energy onto a structure. A ductile design allows the structure to absorb and dissipate this seismic energy through controlled, non-linear deformation in specific, engineered locations, often referred to as plastic hinges. A plastic hinge is a region within a structural member, like a beam or column, where inelastic bending occurs, allowing the member to yield and deform without breaking. This energy dissipation process, where the structure deforms plastically instead of failing abruptly and elastically, prevents a brittle failure mode that would lead to sudden collapse. By undergoing these controlled deformations, the structure effectively "bends but does not break," reducing the peak forces that must be resisted and ensuring that the building remains standing, even if severely damaged and irreparable. This mechanism directly achieves the paramount goal of safeguarding occupants by providing them time to evacuate and preventing the fatal collapse of the building.