Image by AWCI (http://awci.org.nz/suspended-ceilings/)
The NZ earthquake standard (NZS 1170.5:2004 -Structural design actions – Part 5: Earthquake actions – New Zealand) was amended in September 2016 as a result of the Canterbury Earthquakes. There is now more guidance around the seismic bracing design of suspended ceilings.
Suspended ceiling systems are classified as “parts” as per NZS 1170.5. The “part” classification of a suspended ceiling is important as it determines whether the seismic bracing is designed to withstand an Ultimate Limit State (ULS) level earthquake or a Serviceability Limit State (SLS) level earthquake.
Before the amendment, many ceilings only had to be designed to a SLS level earthquake. Previously, only 3m or more above floor level and greater than 10 kg in mass required a ULS design. The amendment has brought in further guidance and stricter rules in this area, stating that a part may only be classified as P7, which corresponds to SLS level design, if the part is less than 3m above floor level and weighs less than 7.5kg. Ceilings weighing more than 7.5kg now fall into the P2/P3 part category, which require ULS level design. This classification is further reinforced in the amended supplement that supports 1170.5, which specifically states suspended ceilings should be classified as P2/P3.
The “part” and “element” definitions are given below as referenced from the standard. In our opinion, an important point in the “part” definition is the link with the definition of an “element”.
“Part: An element that is not intended to participate in the overall resistance of the structure to horizontal displacement under earthquake conditions, for the direction being considered.”
“Element: A physically distinguishable assembly of members that act together in resisting lateral actions, such as moment resisting frames, structural (shear) walls, and diaphragms.”
We believe it is quite hard to argue that a suspended ceiling is not an assembly of members that act together. Our interpretation of ceiling weight is therefore the ceiling grid and linings or tiles acting together as one part. For this reason we believe all ceilings will be above 7.5 kg in total weight and therefore fall into the ULS criteria P2/P3. We will be contacting the standards committee to further the discussion and see what their intent of the definition is.
The mass of a suspended ceiling must be taken as the total mass of both the tiles and ceiling grid. Virtually all ceilings therefore have a mass of greater than 7.5kg and will have to be classified as a P2/P3 part.
This means all suspended ceilings must now be designed to withstand Ultimate Limit State level earthquakes to comply with Clause B1/VM1 of the New Zealand Building Code.
Typically within the suspended ceiling industry, generic seismic design guides provided by ceiling manufacturers have been used to design the seismic bracing for smaller, lightweight ceilings. The design guides typically only design for SLS level earthquakes. With the changes to NZS 1170.5, these design guides are no longer best practice for compliance with B1/VM1.
For suspended ceilings to comply with the building code in the absence of compliant seismic design guides, specific engineering will be required by a chartered professional engineer (CPEng). BVT specialises in providing specific engineering design of suspended ceilings, and this will ensure your suspended ceiling is both safe and compliant with the New Zealand Building Code.