A Significant portion of the estimated 16 billion dollar loss incurred in the 22nd February 2011 Christchurch earthquake and the subsequent aftershocks can be attributed to non-structural components, of which suspended ceilings is a component of.
Although the demands from the induced ground motions were up to twice of what a building is currently designed for, some buildings suffered little damage to there structural systems but severe damage to the non-structural components.
While Hush Interiors have been busy repairing and replacing earthquake damaged suspended ceilings it is now more important than ever to ensure that ceilings are designed to meet earthquake standards.
For ceilings to meet the standard, design input from a structural engineer is required so all relevant factors are taken into account and the standards are met. An engineered designed ceiling will have a PS1 producer statement issued to ensure code compliance.
Factors that are taken into account such as soil conditions under the building, floor height within the building, weight of ceiling tiles, partition loadings, light fixture weights and even rivet sizing can all affect the performance of a suspended ceiling.
Anything installed behind the ceilings such as light fittings, skylights, sprinkler systems and air conditioning units need to be independently hung or mechanically fastened according to there weight and size.
Ceiling damage can be attributed to several reasons such as;
- Ceiling wires not installed correctly,
- Services within the ceiling space or connected to the ceiling grid not complying with current codes,
- Perimeter walls or bulkheads insufficient to receive the line loads of a ceiling,
- Partitions being connected to the ceiling system but not independently braced,
- A lighter gauge of ceiling grid being installed outside its non-structural capability.
Key standards used for the seismic design of suspended ceiling systems in New Zealand are:
- NZS 1170.5:2004 – Structural Design Actions – Earthquake Actions.
- AS/NZS 2785:2000 – Suspended Ceilings – Design and Installation.
- NZS 4219:2009 – Seismic Performance of Engineering Systems in Buildings.
Earthquake forces need to be considered for all suspended ceilings in New Zealand to comply with AS/NZS 2785:2000. Earthquake forces can act in the vertical and/or horizontal direction. The most common method of horizontal restraint is to fix the ceiling to the building structure around its perimeter. If perimeter fixing is not sufficient or appropriate, or if spans exceed the capacity of the ceiling tees, the ceiling may be back braced by fixing to the structure above. If ceiling systems bridge dissimilar structures, then seismic joints may be incorporated. Perimeter walls with insufficient capacity or large deflections under face loads may be strengthened with additional knee braces to the structure above, or have a seismic clip or channel installed at the perimeter connection.
The level of seismic demand on the ceiling system should be calculated in accordance with NZS 1170.5:2004 – Section 8, Requirements For Parts And Components. This classifies building parts under 7 categories, ranging from P1 to P7. Currently light suspended ceilings are included in the least important category P7, which only requires design to the lowest level of seismic demand - serviceability limit state (SLS1) earthquake. Where the ceiling has parts weighing more than 10 kg and able to fall more than 3.0m onto a publicly accessible area, then life safety is considered to be a hazard and the ceiling must be designed as category P3, or P2 for an Auditorium, which both require the ceiling to be designed for an ultimate limit state (ULS) earthquake.
A SLS1 earthquake has an annual probability of exceedance of 1/25, whereas a ULS earthquake has an annual probability of 1/500. This means that a category P7 ceiling is 20 times more likely to have an earthquake exceed its design level earthquake than a category P3 ceiling. The level of earthquake loading for a P3 ceiling in Christchurch, is 1.5 times higher than a P7 ceiling, with ductility taken into account. A ceiling designed to only P7 is likely to suffer severe damage or even collapse in an ULS earthquake, as has been the case with several ceiling collapses in Christchurch earthquakes. Clearly this can cause life safety risk, as well as severe business disruption.
A draft "Recovery Project Report" recently prepared by Canterbury University, in consultation with representatives from the industry including Hush Interiors, recommends that all ceilings be designed as category P3 for ULS earthquakes, rather than P7. For a relatively small increase in seismic loading, the risk of severe damage or collapse from an earthquake event can be significantly reduced. Often this results in only a small increase in seismic bracing. Hush Interiors have adopted this recommendation as standard practise for the seismic design of their ceiling systems and typically design all their ceilings as category P3 rather than P7.
Everyone needs to make sure that the ceiling in their workplace complies with safety codes and standards, so ask the questions of your landlord and/or employer and check what category the ceiling has been designed as. Has it been designed as category P3 for ULS earthquakes, or is it only category P7 for SLS1 earthquakes?
If you need any help with what questions to ask or if you would like Hush Interiors to assess your current workplace ceiling then please contact us.