In performance-based seismic design, what is the primary difference in acceptable damage between the Immediate Occupancy (IO) and Life Safety (LS) performance objectives?

In performance-based seismic design (PBSD), the primary difference in acceptable damage between the Immediate Occupancy (IO) and Life Safety (LS) performance objectives lies in the extent and nature of damage allowed after an earthquake. The Immediate Occupancy (IO) performance objective aims for very limited damage, allowing the building to be used almost immediately after the seismic event. This means that structural elements should have minimal damage, and non-structural elements (like ceilings, walls, and equipment) should remain largely intact to ensure safe occupancy. The goal is to maintain essential functions of the building with only minor repairs needed. In contrast, the Life Safety (LS) performance objective allows for more significant damage, provided that the building maintains its structural integrity and prevents collapse, ensuring the safety of occupants during and immediately after the earthquake. While some structural damage is acceptable, it must not compromise the overall stability of the building. Non-structural damage can be extensive, but it should not pose a direct threat to life safety, such as falling debris blocking escape routes. Therefore, while both objectives prioritize safety, IO aims for minimal disruption and quick return to function, while LS focuses primarily on preventing loss of life, even if the building requires extensive repairs or demolition afterward. For example, a hospital designed for IO would need to remain operational after a quake, whereas a warehouse with an LS objective could sustain substantial damage as long as occupants can safely evacuate.