Importance Levels – Understanding the Criteria for Buildings IL1 to IL5

Mike Neale – Managing Director,
NAI Harcourts Hamilton.

This is a question that comes up periodically, but randomly I received three calls last week around this topic. For those seeking strong seismic resilience in Hamilton for buildings IL3 and above, it’s a pretty thin wedge when seeking premises options that require superior NBS standards.

Ensuring the safety and resilience of buildings is a top priority for authorities, especially in the wake of natural disasters and changing climatic conditions. The seismic performance of buildings is critical to safeguarding lives and preserving infrastructure during earthquakes. To achieve this, New Zealand has a stringent classification system for buildings based on their Importance Level (IL1 to IL5) and corresponding building regulations and criteria. Let’s delve into the criteria and regulations governing these importance levels.

Importance Level (IL1 to IL5) Classification:
Importance Levels (IL) are assigned to buildings based on their significance concerning safety and functionality during earthquakes. They are determined by considering the building’s function, occupancy, and consequences of failure during a seismic event. The IL classification ranges from IL1 (lowest importance) to IL5 (highest importance). The criteria for each level are as follows:

• IL1: Buildings with a low level of importance, such as agricultural structures and minor utility buildings. Failure of IL1 buildings during an earthquake is less likely to result in significant consequences. Examples include ancillary buildings not for human habitation and minor storage facilities.
• IL2: Buildings with moderate importance, including most residential and commercial structures. The failure of IL2 buildings may cause some damage but is not expected to result in major life-threatening situations. The vast majority of commercial and office buildings in Hamilton fall into this category.
• IL3: Buildings with high importance, like hospitals, emergency facilities, and key infrastructure. The failure of IL3 buildings could have significant consequences for public safety and critical services. Examples include buildings where more than 300 people congregate in one area / primary schools, secondary schools, or daycare facilities with a capacity greater than 250 / buildings with tertiary education and a capacity greater than 500 / buildings generating power, water treatment and other public facilities not included in IL 4
• IL4: Buildings of essential importance, including lifeline infrastructure like emergency response centres and power stations. The failure of IL4 buildings could lead to severe societal impacts and disruption. Examples include hospitals / fire, rescue and police stations / aviation control towers / buildings intended to contribute to emergency preparedness and used for communication in an emergency.
• IL5: Buildings with critical importance, such as emergency response headquarters and essential national infrastructure. The failure of IL5 buildings would have catastrophic consequences for society. This includes major dams and extremely hazardous facilities.

Building Regulations and Criteria:
New Zealand has robust building codes and regulations in place to ensure that structures are designed and constructed to withstand seismic forces. These regulations are outlined in the New Zealand Building Code. The building regulations for IL1 to IL5 buildings include:

1.  Seismic Design: Buildings in high and critical importance categories (IL3 to IL5) must comply with stricter seismic design requirements. Engineers use sophisticated techniques and calculations to ensure these buildings can withstand the forces generated by earthquakes.
2. Material Standards: The New Zealand Building Code sets out specific standards for construction materials. For buildings in higher importance levels, stricter material requirements are imposed to enhance resilience.
3. Building Envelope: IL1 and IL2 buildings may focus more on functionality and aesthetics, while IL3 to IL5 structures prioritize maintaining the building envelope even during a seismic event.
4. Redundancy and Resilience: Buildings with higher importance levels must demonstrate greater redundancy and resilience to withstand earthquake-induced stresses.
5. Foundation Requirements: The foundation design for IL3 to IL5 buildings is more stringent, considering factors like soil conditions, liquefaction potential, and ground motion amplification.
6. Regular Inspections: High and critical importance buildings may require more frequent inspections and monitoring to ensure ongoing structural integrity.
7. Building Consent Process: Local authorities evaluate building consent applications based on the IL classification, ensuring compliance with the appropriate regulations.

The classification of buildings into Importance Levels (IL1 to IL5) in New Zealand is crucial for managing seismic risk and promoting public safety. There are building regulations which ensure that higher importance structures are designed and constructed to withstand seismic forces.