The process of evaluating and designing the retrofit of existing buildings differs from the conventional structural design of new buildings. The current state-of-the-art analysis and design approach for the seismic evaluation of existing buildings is founded on a performance-based philosophy. There are two parts to a performance-based analysis and design.
First, there is the establishment of a performance objective. This answers the question for the designer and the owner, “What degree of damage to the building am I willing to tolerate in the event of an earthquake?” It is not economically feasible to design all buildings to a performance objective that limits all damage or allows the building to remain fully operational and allow immediate occupancy following an earthquake. Therefore, performance objectives exist that allow a certain degree of damage to occur while still protecting life safety and preventing building collapse.