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Basic Fire Resistance Levels

The National Construction Code (NCC) requires building elements be fire resistant so that the building does not facilitate the spread of fire whilst also delaying the onset of structural collapse in fire. To put it bluntly, fire rating of structure reduces the risk of structural failure during occupant evacuation.

So if FRLs are so fundamental to life safety, why are fire engineers almost always being asked to justify reduction of FRLs?

But first, the basics!

structure fire engineer

What are Fire Resistance Levels?

The NCC defines the Fire Resistance Level (FRL) as the period of time from ignition whereby the structure will be "fire resistant". FRLs are expressed in the following format:

X minutes / Y minutes / Z minutes

Where X, Y and Z are numbers ranging from 0 to 240 minutes. Each of the values define a particular part of the building element's ability to resist fire. With assistance from Australian Standard 1530.4, definitions are shown below.

  • Structural Adequacy: Ability for a load bearing element to continue to hold a specified load when subject to fire.

  • Integrity: Ability for the building element to resist passage of flames and hot gases to other areas of the building.

  • Insulation: Ability of the non-exposed surfaces to remain below a specific temperature.

The key part of these properties lies in how they are measured for consistent and reproducible results. Australian Standard 1530.4 defines a Standard Fire Test which is a reproducible furnace fire following the time-temperature curve below.

fire engineering graph
Standard Time vs Temperature Curve (Courtesy of Standards Australia)

This test was effectively devised to create a baseline for different building occupancies containing varying amounts of fire load so the fire resistance data has context.

What this means is buildings which are typically expected to have less combustible/flammable items within it have a lower FRL value when compared to occupancies which have a greater quantity of combustible/flammable items.

Extract from NCC Specification C1.1 shown below shows, for example, a Class 2/3 residential apartment/hotel building requires external walls be provided with a FRL of 90/90/90 whereas a Class 7/8 storage/process building requires those same walls to have a 240/240/240 FRL.

fire resistance level entry
Extract from NCC Specification C1.1 (Courtesy of ABCB)

Rationalising FRLs

So with the definition of FRLs a bit clearer, how do fire engineers justify the reduction of FRLs? The core principle revolving around reduction of FRLs is being able to quantify what the potential fire load is within the fire compartment and demonstrating that the structure can withstand the potential fire.

There are a few things to address:

  • Demonstrating that the fire burns out before the desired duration of FRL is achieved (time equivalence).

  • The structure does not reach critical failure temperatures (steel).

  • Flame impingement does not occur directly on the structure.

The above is heavily simplified from the technical approach however, it is not the intent of this post to outline the minute details (we have to keep some of the trade secrets).

Equivalent Fire Burn Out

The Standard Fire Test is not representative of every type of fire out there (or even many of them for that matter) and as such, the time value of FRLs is somewhat disconnected from what would happen in a real fire. Analysis starts by defining the design fire in terms relative to the Standard Fire Test. This is graphically summarised below.

fire structure engineering equivalence
Fire Test Equivalence (Courtesy of University of Sheffield)

Once the design fire is converted into a form equivalent to a Standard Fire Test, the duration of the design fire can be ascertained and compared. With the typical fire cycle comprising ignition, growth, flashover and decay, if the fire completely decays before the desired FRL time is reached, it may be demonstrated that a reduced FRL is satisfactory.

There are limitations to the above approach and some academics do not believe time equivalence calculations are a satisfactory approach on its own. It’s therefore strongly recommended that you discuss methodology with your fire engineer and the relevant fire authority prior to proceeding into PBDB territory.

Critical Failure Temperatures

This part of the analysis is really only applicable to steel structures. Australian Standard 4100 defines the requirements for steel structures within buildings and part of it includes definition of what temperatures the steel can withstand prior to failure. Although this value may vary based on servers factors including expected dead loads within the building, typical values may be in the order of 550 degrees Celsius.

Calculation of the fire temperatures can be quite complex and often involves Computational Fluid Dynamics (CFD) simulations to determine gas temperature profiles. From here, the thermal inertia of the steel section and associated surface temperatures can be used to arrive at a final value.

Fire engineering simulation

Flame Impingement

Flame contact with steel structure can have a detrimental effect on the structure’s load bearing capacity. There are several empirical equations which could be used here from sources such as the Society of Fire Protection Engineers Handbook.

Bottom line: If the flames don’t reach the structure then that’s good!


Hopefully as a non-fire practitioner, you have a bit more of an understanding about FRLs and how fire engineers work to justify reduction on those levels. Remember, an analysis on your building may show that the Deemed to Satisfy levels of FRLs are not satisfactory in which you’d be required to implement stricter levels of fire resistant construction within the building.

Interested to see what options are available for your project? Contact us for more information!


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