The National Construction Code 2019 (NCC) details the Fire Resistance Level (FRL) of structures, as well as the flammability of materials, however the two are different.
The FRL is a grading period, representing the amount of time (in minutes) a structural system must withstand a fire based on three criteria; Structural Adequacy / Integrity / Insulation.
As defined in AS 1530.4, structural adequacy refers to the ability of a structure to maintain its stability and load bearing capacity during a fire, while integrity is its ability to resist the transfer of flames and hot gases from part to another, and insulation is its ability to contain temperatures and stop heat transfer between zones.
For example, Figure 1 below shows a FRL of 90 / 90 / 90, meaning a failure would not be expected within 90 minutes for each of the three criteria.
The FRL requirements not only look at the individual components that make up the structure, but rather how they work together, and importantly, the detailing between them.
As for the flammability of materials, the NCC 2019 details that metal sheeting shall have a combustible surface finish not exceeding 1mm in thickness and a spread-of-flame index no greater than 0. As stated in AS 1530.3 a spread-of-flame index relates to, “the radiation emitted by a burning specimen under impressed ration that varies according to the time of ignition of the specimen”.
BlueScope Steel have undertaken testing at the CSIRO and it has been determined that all their steel products have a spread-of-flame index of 0, which means they can be considered as non-combustible materials.
The ignition resistance of BlueScope steel also makes it a suitable option for fire-resistant constructions in bushfire prone areas. Care needs to be taken to ensure all the elements promoting a bushfire risk are considered. By following the building design and construction techniques outlined in AS 3959 and confirming any material or element of the building satisfies the testing criteria in AS 1530.8, the effects from bushfire can be reduced.