Design levels for earthquake resilience of school buildings

Engineers must meet specific design levels for structural assessment and school building design.

Building importance levels

The Australian/New Zealand Standard 1170.0:2002 Structural Design Actions sets out importance levels (IL) for New Zealand structures based on the likely outcome if a building fails structurally — for example, if all or part of it collapses. These levels are:

  • IL1 — buildings with a low risk to life and the environment, or a low economic cost, that is, storage facilities and ancillary buildings
  • IL2 — buildings with a normal risk to life and the environment, or a normal economic cost (that is, all buildings except those that fall in the other categories)
  • IL3 — buildings that can house large numbers of people, or are important to society; they include school buildings that can hold over 250 people, and buildings where over 300 people can congregate in one area (for example, gymnasia and halls)
  • IL4 — buildings with special functions in a disaster.

The design action loadings for events like earthquakes, wind and snow depend on the IL.

Purchase a copy of the Standard from Standards NZ.

AS/NZS 1170.0:2002 — Standards NZ website (external link)

Ministry policy on design levels

The Ministry’s policy on design levels for structural assessment and school building design extends the New Zealand Building Code requirements. The policy aims to achieve a consistent level of performance for buildings in earthquakes, including considering the likelihood of damage in moderate shaking.

The policy has been updated (since the last review in 2012), to take into account observations of:

  • actual building performance from the Canterbury earthquakes, and
  • destructive testing of classroom buildings.

Timber-framed school buildings' earthquake resilience

The main policy change is to revert to the Building Code recommendations for ILs for new buildings. This differs from the previous policy that required all new school buildings to be designed to IL3.

The revised policy aims to reduce the costs of new buildings where the actual seismic risk to occupants is considered to be lower than previously thought, due to the nature of buildings and their materials.

The revised policy is now contained in the Structural and Geotechnical Guidelines for School Design. These guidelines are mandatory when undertaking work on Ministry-owned school buildings from 1 July 2015.

Structural and Geotechnical Guidelines for School Design

New school buildings

The policy for new school building design requires that seismic design loads for serviceability limit state (SLS) and ultimate limit state (ULS) provisions must be as stated in the Building Code.

For buildings of more than 1 storey with concrete suspended floors, and heavy single-storey buildings (concrete or reinforced masonry-walled structures), designers must satisfy the SLS2 provisions of the Building Code, for earthquakes with a return period of 100/250 years (for IL2/IL3 buildings respectively). SLS2 performance criteria shall be generally as defined in clause 2.5.2 of NZS1170.5 for IL4 buildings — namely that all parts of the structure shall remain operational to the extent appropriate for school purposes. More advice will be provided on this in the coming months.

Assessing existing buildings

The IL policy is unchanged when assessing and strengthening existing school buildings, as shown in the table.

Table 2: Importance levels for assessment and strengthening of existing school buildings

Building category

Buildings of higher occupancy and/or heavy floor construction

Buildings of lower occupancy and not of heavy floor construction

Building use

Classroom buildings for >250 persons, assembly halls, gymnasia

Buildings of 2 or more storeys with suspended concrete floors and/or heavy roofs

Classroom buildings for <250 persons, administration blocks, other buildings (see note below)

Assessment

IL3

IL2

Current code

Current code

Strengthening

IL3

IL2

67% current code

67% current code

 

Large buildings’ and joint buildings’ design levels

Large buildings that can accommodate more than 250 people at one time must be designed and assessed as IL3. This includes assembly halls, gymnasia and large classroom blocks. Based on current student teacher ratios, this will be blocks with 9-10 classrooms for secondary schools and blocks with 8-9 classrooms for primary schools.

Single-storey classroom and administration buildings that are joined together may be assessed as IL2, reflecting the low risk they present.

Using school buildings in a civil emergency

In the past, schools were often used as civil defence emergency centres. This use is no longer Ministry policy and reflects changes in civil defence emergency planning.

However, following a major emergency, it's possible that school buildings may be used for civil defence and emergency management. Usually this involves supporting the community, such as by providing:

  • a temporary meeting place, and/or
  • a welfare centre.

Buildings to be used in a disaster, such as for disaster response coordination, generally have to meet the higher structural requirements of IL4. However, this isn't a requirement for buildings that may be used in this way. 

Having major school facilities, such as assembly halls and gymnasia, designed for IL3 allows for this possible use, without requiring all of the existing building stock to have unnecessary structural capacity. Lessons from Christchurch include having a more flexible approach to using existing buildings as post disaster facilities when the state of and access to existing buildings can be better assessed. 

We have consulted with the Ministry of Civil Defence and Emergency Management and the Ministry of Business, Innovation and Employment, and it isn't expected that school buildings with use as welfare centres during an emergency should meet the higher structural requirements of IL4. 

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