NZS1170.5 : CHANGES TO SEISMIC DESIGN REQUIREMENTS FOR SUSPENDED CEILINGS
By Hedda Maria Oosterhoff (Architectural Consultant, T&R Interior Systems), based on comments made by Dr Joe Bain (Principal Engineer, JSK Consulting Engineers)
Novenber 8 2016
The New Zealand Seismic Design Actions Standard (NZS 1170.5) has been amended and changes came into effect in September 2016. This has significant implications on the design and installation of ALL suspended ceilings. Since NZS 1170.5 is the design actions standard cited by Verification Method 1 to clause B1 of the NZ Building Code, it must be adhered to in order for an installed ceiling to comply.
Before the amendment, seismic design requirements for ceilings were the source of some confusion, namely around selecting the appropriate part category to apply. This was especially a problem with tile-and-grid suspended ceilings.
The previous version of the standard allowed ‘parts’ which weighed less than 10kg and which were less than 3m above floor level to be classified as part category P7, which only required design to a serviceability limit state (SLS) - design for no or minimal damage during a 1-in-25-year earthquake. Considering only the weight of individual components, rather than the total weight of the ceiling system, meant that ceilings were not designed for much larger ultimate limit state (ULS) events. Depending on the building importance level, ultimate limit state part categories can require design for up to a 1-in-2500 year event. It is obvious that selecting a Serviceability vs Ultimate limit state has a huge effect on the resulting seismic design.
The vagueness in the previous standard surrounding weight led some designers to treat all ceilings as P7 parts since individual components weighed less than 10kg while others considered the total weight of the ceiling system and designed for ultimate limit state events. A third group made a case-by-case assessment for each design, depending on the total weight per square meter of the ceiling components, the height above the floor and the location.
Supporting comments provided in the amended version of NZS 1170.5 resolves this confusion. The new standard lowers the weight threshold for ultimate limit state design to 7.5kg, and makes it clear that the total weight of the ceiling system needs to be considered - tiles, grid, luminaires and any other supported services. It is no longer possible to define ceilings as P7 simply because the weight of the individual components is below 7.5kg. Except for exceptionally small ceilings, they all now need to be designed for ultimate limit state events.
Given the performance of suspended ceilings in the Canterbury earthquakes, and reports that are emerging about the performance in the recent Hanmer Springs quakes (especially in Wellington), these changes are very timely. Collapsing ceilings can injure occupants and interfere with evacuation and reoccupation of a building.
What does this mean?
The most important requirement is to design all future ceilings for ultimate limit state events.
Manufacturers can be expected to issue updated seismic design guides and calculators that provide ULS design frameworks in the near future. Once these guides are available, it should be possible to use them with confidence.
In the mean time, for smaller projects, if a manufacturer’s seismic design guide provides a methodology for ultimate limit state design, then this can still be used and will give approximate compliance with the building code. Contact the manufacturer or a specialist engineer for clarity around this.
Generic methodologies for serviceability limit state design are no longer applicable. If a grid manufacturer does not provide an ultimate limit state design methodology, then specific design by a suitably qualified engineer is likely to be required during the transition period.
Standards Australia has recently begun work on a revision of AS/NZS 2785, the standard for suspended ceilings. At this stage it seems that the main area of revision is expected to be the seismic design requirements for ceilings, so clarification of the design requirements should be coming by the end of 2017.