Changes of seismic requirements at the supports of precast hollowcore floors were introduced to Concrete Structures Standard NZS3101 in 2006 and amended in 2017. Concerns were raised about the impact of these changes on fire performance of hollowcore floor systems. The objective of this project was to investigate the raised concerns and other potential fire issues associated with hollowcore floors from a systems approach.
A fire test of a precast hollowcore floor system with a 600mm wide link slab, as required by the current concrete structures standard “NZS3101:2006”, was undertaken at BRANZ at the systems level. Premature web shear failure and partial collapse of the precast hollowcore unit next to the link slab occurred at 30 minutes into the test, and this is much shorter than the tabulated fire rating of hollowcore floors in NZS3101:2006.
An amended Verification Method B1/VM1 published in November 2023 removed a deemed to comply pathway for the design of the supports for hollowcore floor systems. This change applied immediately with no transition period.
While a national research project about the seismic performance of precast concrete floors, ReCast, was a major driver of this amendment, the research project Fire performance of hollowcore floors also provided evidence to help MBIE’s decision making.
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Fire performance of hollowcore floors
A preliminary BRANZ study into the fire performance of hollowcore concrete floors suggests that conventional fire design requirements for precast floors may need to be reviewed.
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ER74 ReCast Floors project
A series of papers prepared as part of the ReCast Floors project relating to the performance of precast concrete floors in New Zealand, including design issues and retrofitting. The papers provide guidance for structural engineers.
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06 May 2024