Insights

For a hospital, communications centre, or other critical facility, backup power can't just work eventually after an earthquake. It has to work immediately. That's the standard behind Importance Level 4 (IL4): the seismic classification for buildings and infrastructure that must remain operational right after a major disaster.

A fuel storage tank supplying backup generators in one of these facilities needs to meet the same standard as the building itself. If the tank fails or shifts during an earthquake, the generator it's feeding is compromised at exactly the moment it matters most.

Clause A3 of the New Zealand Building Code defines the importance of a structure by its Importance Level (IL), based on the consequences if that structure fails. Five levels are defined, and the required seismic performance increases with each:

• Level 1 (IL1): Low-hazard structures, such as walkways, outbuildings, fences, and walls.
• Level 2 (IL2): Normal structures not covered elsewhere, such as timber-framed houses, car parking buildings, or offices.
• Level 3 (IL3): Structures that may hold crowds or high-value contents, or pose risk to large numbers of people nearby, such as conference centres, stadiums, and airport terminals.
• Level 4 (IL4): Buildings that must remain operational immediately after an earthquake or other disaster, such as hospital operating theatres, triage centres, emergency shelters, and other critical post-disaster infrastructure.
• Level 5 (IL5): Structures whose failure would pose a catastrophic risk to a large area or population, such as dams, nuclear facilities, or biological containment centres.

In Australia, the equivalent seismic loading requirements are set out in AS1170.4, the Australian earthquake loading standard.

IL4 compliance touches a wide range of people involved in a critical infrastructure project, each with their own stake in getting it right: architects and building designers specifying compliant equipment, project managers and builders coordinating installation, engineering firms validating seismic performance, local government bodies and test certifiers signing off on compliance, and property developers responsible for the finished building meeting code.