NZSEE along with Sesoc with Engineering NZ recently issued an “Advice on Hollow – Core Floors”. This advice was prepared based on research carried out following the observed performance of hollow core units following the recent earthquakes particularly the Kaikoura earthquake which had been the subject of a presentation at the recent Sesoc conference by Frank Büker and Nic Brooke.
The advisory includes advice to structural engineers based on our current understanding of the behaviour of hollow-core floors for new buildings, for assessments and for retrofit and suggested messages to clients noting they should be factual and not alarmist. The advisory concludes with information on where to get further technical knowledge.
These are the suggested messages to clients included in the advisory:
- Our knowledge of structural engineering develops over time. We continue to learn from the Canterbury and Kaikōura Earthquakes.
Hollow-core
- Hollow-core units are known to be fragile and susceptible to brittle failure during earthquakes. Therefore, their use in new buildings is not considered to be good structural engineering practice and is not recommended.
- There is no known way to show that hollow-core beta units comply with NZS 3101.
- There is no known international standard consistent with the NZ application of hollow-core that mitigates its known failure mechanisms.
Retrofits
- Recent research [8] shows that hollow-core units with supplementary seating retrofits can still be susceptible to collapse. This is due to cracking occurring beyond the end of the angle retrofit and/or propagating at a shallow angle. This kind of damage can occur even where the maximum imposed drift (lateral displacement between a building’s floors) is small.
- Recent testing indicates that a new form of retrofit, referred to as a strongback system, will provide effective support for all known failure modes. Strongback systems are also insensitive to the uncertainties in the failure modes, such as the location of the cracks.
- The earthquake score of hollow-core floors previously retrofitted with supplementary seating may be lower than previously understood.
The link to the full advice is here.
