Performance level of structures against earthquakes describes limiting damage condition that assumed to be satisfactory for a given building and a given ground motion.
Moreover, building damages, danger to life safety of occupants in the building due to the damage, and post-earthquake serviceability of the building describe and control the limiting damage condition.
Added to that, building performance level against earthquakes is a combination of the performance of both structural and nonstructural components.
Lastly, performance levels of building structures against earthquake will be presented in the following sections.
Performance levels of buildings against earthquakes are as follows:
- Operational performance level
- Immediate occupancy performance level
- Life safety performance level
- Collapse prevention performance level
Operational performance level
- This performance level associates with functionality of the structure. Generally, all systems important to normal operation are operational.
- Damage to the building is limited, so the overall damage is very light and hence immediate occupancy is not questionable.
- The structure does not experience permanent drift.
- The building retains original strength and stiffness considerably.
- facades, partitions, and ceilings as well as structural elements suffer minor cracking only.
- Nonstructural component damage is negligible.
- The structure requires minor repairing which can be done without important disruption to occupants.
- Finally, power and other utilities are available; possibly from standby sources.
Immediate occupancy performance level
- The structure experience light damages
- There is no permanent drift.
- The building retains original strength and stiffness substantially.
- Minor cracking of facades, partitions, and ceilings as well as structural elements.
- Elevators can be restarted.
- Fire protection operable.
- The building space and systems are anticipated to be fairly usable. however, equipment and contents are generally secure but may not operate due to mechanical failure or lack of utilities.
- Concrete frame experience minor hairline cracking, limited yielding at few locations, and no crashing (strain of concrete less than 0.003)
- Steel moment frames experience minor local yielding at few locations. No buckling, fracture, and observable distortion of members.
- Lastly, braces of Braced steel frame structure suffer minor yielding or distortion
Life safety performance level
- This level intended to obtain a damage condition that presents a substantially low probability of danger to life safety. Whether the danger is due to structure damage or fallen of nonstructural components of a building.
- The building experiences moderate overall damage
- All stories of a structure retain some residual strength and stiffness left in.
- Gravity-loadbearing elements function.
- There will be no out of plane failure of walls or tipping of parapets.
- However, the structure undergoes some permanent drift.
- partitions suffer damage.
- Building may be beyond economical repair.
- Falling hazards mitigated but many architectural, mechanical, and electrical systems are damaged.
- Concrete frame beams damage extensively, shear cracking and cover spall off occur in ductile columns, and minor cracking develops in nonductile columns.
- Hinges create in steel moment frames. In addition to local buckling of some beams, serious joint distortion, and fracture of isolated moment connection. However, shear connection would remain sound and few elements might suffer partial fracture.
- Lastly, in braced frames, majority of bracing buckle or yield but do not fail entirely, and several connections may fail as well.
Collapse prevention performance level
- This level of building performance mainly relates to the vertical load carrying system and the structure need to stable under vertical loads only.
- Generally, the building damage is severe
- The structure retains little residual stiffness and strength.
- however, load bearing columns and walls function.
- The building suffers Large permanent drifts.
- Some exits blocked.
- Infills and unbraced parapets failed or at incipient failure.
- Building is near collapse.
- Nonstructural components damage extensively.
- In concrete frames, hinges and extensive cracking develop in ductile elements, nonductile columns experience splice failure and limited cracking, and short columns damage seriously.
- Beams and columns distort heavily in steel frames. Added to that, several moment connections fracture but shear connections remain intact.
- Finally, braces yield and buckle extensively in braced frames, and even many of them along with their connections could fail.