Limit States (Eurocodes)

According to EN1998-3 section 2.1, performance requirements refer to the state of damage in the structure defined through three limit states, namely Near Collapse (NC), Significant Damage (SD) and Damage Limitation (DL).

Limit State of Near Collapse (NC)
The limit state of Near Collapse (NC) may be selected, according to EN 1998-3, where the target state of damage in the structure is near collapse and would probably not survive another earthquake, even of moderate intensity. The structure is heavily damaged with low residual lateral strength and stiffness, although vertical elements are still capable of sustaining vertical loads. Most non-structural components have collapsed and large permanent drifts are present. The appropriate level of protection is achieved by choosing a seismic action with a return period of 2.475 years corresponding to a probability of exceedance of 2% in 50 years.

Limit State of Significant Damage (SD)
The limit state of Significant Damage (SD) may be selected, according to EN 1998-3, where the target state of damage in the structure is significant and can sustain after-shocks of moderate intensity, although it is likely to be uneconomic to repair. Some residual lateral strength and stiffness, and vertical elements are capable of sustaining vertical loads. Non-structural components are damaged, although partitions and infills have not failed out-of-plane. Moderate permanent drifts are present. The appropriate level of protection is achieved by choosing a seismic action with a return period of 475 years corresponding to a probability of exceedance of 10% in 50 years.

Limit State of Damage Limitation (DL)
The limit state of Damage Limitation (DL) may be selected, according to EN 1998-3, where the target state of damage in the structure is insignificant and does not need any repair measures. The structure is only lightly damaged, with structural elements prevented from significant yielding and retaining their strength and stiffness properties. Non-structural components, such as partitions and infills may show distributed cracking, but the damage could be economically repaired. Permanent drifts are negligible. The appropriate level of protection is achieved by choosing a seismic action with a return period of 225 years corresponding to a probability of exceedance of 20% in 50 years.

The Eurocodes National Annexes specify whether to employ all three Limit States, two of them, or just one.