Structural-1 Immediate Occupancy Performance Level
Structural Performance Level S-1, Immediate Occupancy, means the post-earthquake damage state in which only very limited structural damage has occurred. The basic vertical- and lateral-force-resisting systems of the building retain nearly all of their pre-earthquake strength and stiffness. The risk of life-threatening injury as a result of structural damage is very low, and although some minor structural repairs may be appropriate, these would generally not be required prior to reoccupancy.
Seismic building performance is defined by specifying the maximum allowable building damage state that is acceptable. This damage state can be divided into a structural damage state and nonstructural damage state. When an acceptable damage state is coupled with a seismic hazard level (design demand) a performance objective has been defined that permits the design team to develop strategies for providing adequate resistance and toughness in the design to achieve the objective.
Structural-2 Damage Control Performance Range
Structural Performance Range S-2, Damage Control, represents the continuous range of damage states between the Life Safety Structural Performance Level (S-3) and the Immediate Occupancy Structural Performance Level (S-1). Design for the Damage Control Structural Performance Range may be desirable to minimize repair time and operation interruption, as a partial means of protecting valuable equipment and contents, or to preserve important historic features when the cost of design for immediate occupancy is excessive.
The Performance Based Seismic Design approach can thus permit selection of uniquely acceptable damage states that address the needs of a range of building owners/operators/occupants. Possible selections of acceptable damage state selections can be viewed in the window to your left. On the right are visual characterizations** of various levels of structural and nonstructural component damage.
Structural-3 Life Safety Performance Level
Structural Performance Level S-3, Life Safety, is the post-earthquake damage state in which significant damage to the structure has occurred, but some margin against either partial or total structural collapse remains. Some structural elements and components are severely damaged, but this has not resulted in large falling debris hazards, either within or outside the building. Injuries may occur during the earthquake; however, the overall risk of life-threatening injury as a result of structural damage is expected to be low. It should be possible to repair the structure; however, for economic reasons this may not be practical. While the damaged structure is not an imminent collapse risk, it would be prudent to implement structural repairs or install temporary bracing prior to reoccupancy.
Structural-4 Life Safety Performance Range
Structural Performance Range S-4, Limited Safety reflects the continuous range of damage states between the Life Safety Structural Performance Level (S-3) and the Collapse Prevention Structural Performance Level (S-5).
Structural-5 Collapse Prevention Performance Level
Structural Performance Level S-5, Collapse Prevention is the post-earthquake damage state in which the building is on the verge of partial or total collapse. Substantial damage to the structure has occurred, potentially including significant degradation in the stiffness and strength of the lateral-force-resisting system, large permanent lateral deformation of the structure, and - to a more limited extent - degradation in vertical-load-carrying capacity. However, all significant components of the gravity-load-resisting system must continue to carry their gravity load demands. Significant risk of injury due to falling hazards from structural debris may exist. The structure may not be technically practical to repair and is not safe for reoccupancy, as aftershock activity could induce collapse.
Nonstructural-A Performance Level
At this level, most nonstructural systems required for normal use of the building - including lighting, plumbing, HVAC, and computer systems - are functional, although minor cleanup and repair of some items may be required. This Nonstructural Performance Level requires considerations beyond those that are normally within the sole province of the structural engineer. In addition to assuring that nonstructural components are properly mounted and braced within the structure, it is often necessary to provide emergency standby utilities. It also may be necessary to perform rigorous qualification testing of the ability of key electrical and mechanical equipment items to function during or after strong shaking.
Nonstructural-B Performance Level
Minor window breakage and slight damage could occur to some components. Presuming that the building is structurally safe, occupants could safely remain in the building, although normal use may be impaired and some cleanup and inspection may be required. In general, components of mechanical and electrical systems in the building are structurally secured and should be able to function if necessary utility service is available. However, some components may experience misalignment or internal damage and be nonoperable. Power, water, natural gas, communications lines, and other utilities required for normal building use may not be available. The risk of life-threatening injury due to nonstructural damage is very low.
Nonstructural-C Performance Level
Nonstructural Performance Level C, Life Safety, is the post-earthquake damage state in which potentially significant and costly damage has occurred to nonstructural components but they have not become dislodged and fallen, threatening life safety either inside or outside the building. Egress routes within the building are not extensively blocked, but may be impaired by lightweight debris. HVAC, plumbing, and fire suppression systems may have been damaged, resulting in local flooding as well as loss of function. While injuries may occur during the earthquake from the failure of nonstructural components, overall, the risk of life-threatening injury is very low. Restoration of the nonstructural components may take extensive effort.
Nonstructural-D Performance Level
Nonstructural Performance Level D, Hazards Reduced, represents a post-earthquake damage state in which extensive damage has occurred to nonstructural components, but large or heavy items that pose a high risk of falling hazard to a large number of people - such as parapets, cladding panels, heavy plaster ceilings, or storage racks - are prevented from falling. The hazards associated with exterior elements along portions of the exterior of the building that are available for public occupancy have been reduced. While isolated serious injury could occur from falling debris, failures that could injure large numbers of persons - either inside or outside the structure - should be avoided. Nonstructural components that are small, lightweight, or close to the ground may fall, but should not cause serious injury. Larger nonstructural components in areas that are less likely to be populated may also fall.
