In the context of seismic retrofitting, what is meant by the term 'capacity design'?

In the context of seismic retrofitting, 'capacity design' refers to a design philosophy where specific elements of a structure are intentionally designed to be weaker than other elements, creating a predetermined hierarchy of strength. The weaker elements are designed to yield and dissipate energy in a controlled manner during an earthquake, while the stronger elements are designed to remain elastic and protect the structure from collapse. The goal of capacity design is to force plastic hinges to form in predetermined locations, typically in beams rather than columns, and to prevent brittle failure modes, such as shear failure or connection failure. This is achieved by carefully calculating the forces that can develop in the weaker elements when they reach their capacity and then designing the stronger elements to resist these forces with a sufficient margin of safety. In seismic retrofitting, capacity design principles are used to identify and strengthen critical elements, such as columns or connections, to ensure that they can resist the forces transferred from yielding elements. This approach helps to improve the structure's overall ductility and energy dissipation capacity, reducing the risk of collapse and enhancing its seismic performance. For instance, strengthening column-beam joints to ensure beams yield before columns.