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11.3 fire rating requirements

Fire rating requirements are determined from Acceptable Solution C/AS1 “Fire Safety” as part of the Approved Document for the New Zealand Building Code. In general different levels within buildings constitute different fire cells and floors are required to act as fire separations. The fire rating of the floor must be adequate to comply with both the firecell rating (F) and the structural rating (S) requirements set down in Part 5 of the acceptable solutions.

(a) Firecell Rating F
Firecell ratings resist the internal spread of fire and apply to primary and secondary elements within a firecell, including walls and floors which are fire separations, together with their supporting elements, within the same firecell. Firecell ratings range between zero and 60 minutes (assuming no sprinklers).

(b) Structural Rating S
Structural ratings resist structural collapse close to a relevant boundary and fire spread through external walls. Structural Ratings apply to primary elements which, within firecells provide stability to an external wall not permitted to have 100% unprotected area. Structural ratings vary between zero and 240 minutes (assuming no sprinklers) depending on the purpose which the firecell is used for and the degree of openings to the firecell.

The fire engineering designer is responsible for determining the fire rating requirements of the floor system for both “F” and “S” ratings based on the overall structural model of the building.

The purpose or activity to which a building is proposed to be used for determines its Fire Hazard Category (FHC). An increasing FHC coincides with an increasing Fire Rating requirement.

A significant number of offices fall within Fire Hazard Cat 2 (FHC2) and have an “S” rating requirement varying between 60 - 180 minutes. Most factories and some warehouses (provided building constitutes only medium fire load) fall within FHC3 and have an “S” rating requirement typically between 90 - 240 minutes.

It can be seen that generally, applicable “S” rating requirements are more restrictive than “F” rating requirements. Accordingly where building building structural systems are chosen so that the floor does not need to satisfy an “S” rating then the amount of fire emergency reinforcement (FER) will be significantly reduced, since the maximum rating it will required to comply with under ” F” rating provisions is 60 minutes.