Since NZS 1170.5:2004 Structural design actions - Part 5: Earthquake actions - New Zealand Commentary incorporating Amendment No. 1 was published in September 2016 there have been some questions raised over the interpretation of Table 8.1 Parts and Components, as applied to suspended ceilings. In particular should suspended ceilings be designed for the ultimate limit state (ULS) or the serviceability limit state (SLS) and how should Note 2 regarding mass and fall height be interpreted to determine whether or not an application is a life hazard risk.
Ultimate limit state or serviceability limit state
When life hazard risks exist, there have been instances where suspended ceilings have been designed to the SLS rather than the ULS requirements. This is not the intent of the standard and it is recognised that some of the previous Standard and Commentary provisions may have contributed to this by not having been expressed clearly enough. The amendment to NZS 1170.5 was specifically drafted to clarify these design requirements.
Section 8 of AS/NZS 1170.5 needs to cover all parts and components not just suspended ceilings, so the provisions reflect this scope. For suspended ceilings the intention is that designers look at all risk categories in Table 8.1 and design for the worst case. In most cases more than one risk category will apply. Where failure of the ceiling represents a hazard to human life or would impact the continuing function of the evacuation (after earthquake) and human life support systems within the structure, the ceiling is required to be designed to the ultimate limit state. This reflects the building code objective for Clause B1 Structure which is to safeguard people from injury caused by structural failure which means fatalities at the extreme.
Mass of individual items and fall height
Note 2(a) to Table 8.1 applies to risk category P1, P2, and P3 which represent a hazard to human life. The structure limit state for design in these categories is the ULS. Whether these risk categories apply to suspended ceilings depends on the weight of the component and the fall height.
Note 2 states: 'Parts that need not be considered in this category are as follows:
(a) A part that weighs less than 7.5kg and which, if it experienced a loss of gravity support or were to become decoupled from the structure, would fall less than 3 m provided that the loss of gravity support does not lead to release of hazardous material;'
A simple way to interpret this is:
If the part weighs 7.5kg or more it represents a hazard to human life regardless of fall height. If the part can fall more than 3 m it represents a hazard to human life regardless of weight.
This means a suspended ceiling system, as a whole, (because its mass will be significantly more than 7.5kg) is not exempt and needs to be designed to the ULS. However, the prevention of individual ceiling components falling, such as the tiles, (because they would normally weigh less than 7.5kg) needs to be considered at the SLS (SLS1 or SLS2 earthquake as appropriate) unless they can fall more than 3m in which case they should also be prevented from falling under ULS loads.