Steel Frames
Steel does not burn. That statement is true, and it is also incomplete, and the gap between those two facts is where a genuine understanding of fire resistant steel design actually lives. Non combustibility tells you steel will not fuel or spread a fire. It tells you nothing about whether a steel beam or column will still be standing, carrying its design load, twenty minutes into that fire. Those are two different engineering questions, governed by two different parts of the National Construction Code, and this guide is about the second one: the fire resistance rating of structural steel framing, what actually determines it, and how it is achieved in practice.
Non combustibility is a material property, tested under AS 1530.1, and steel passes it easily. Fire resistance is a structural performance property, tested under AS 1530.4, and it asks a completely different question: for how long, in minutes, can a loadbearing element continue to carry its design load once a standard fire has started acting on it. A steel column can be entirely non combustible and still fail structurally well before an equivalent concrete or masonry element, because the mechanism of failure for steel in fire is not burning, it is softening.
Steel has a thermal conductivity roughly 25 times that of concrete, which means an unprotected steel member heats up far faster than a concrete or masonry equivalent when exposed to fire. According to Promat’s specifier guidance on structural steel fire protection, structural steel begins to soften at around 425 degrees Celsius, and between 600 and 650 degrees Celsius it loses roughly half its strength and carries a real risk of failure. For context, a fully developed building fire can reach temperatures around 1000 degrees Celsius. Left completely unprotected, structural steel members can reach these critical temperatures within roughly 10 to 20 minutes of fire exposure, depending on the size and shape of the member and the intensity of the fire.
This is precisely why you almost never see structural steel left bare in a building required to have a fire resistance level. The steel itself does not need to resist burning, since it cannot burn, but it needs to be insulated from the heat long enough to keep doing its structural job for the period the building’s fire engineering requires.
Under Performance Requirement CP1 of the National Construction Code, a building’s structural elements must, to the degree necessary, maintain stability during a fire. The measure used to demonstrate this is the Fire Resistance Level, or FRL, expressed as three numbers in minutes, for example 90/90/90, corresponding to three separate criteria tested under AS 1530.4, the Australian standard for fire resistance testing of building elements.
FRL Criterion | What It Measures | Relevance to Structural Steel |
|---|---|---|
Structural adequacy | Ability of a loadbearing element to keep carrying its design load without collapse | The component structural steel fire protection is designed to achieve, expressed as the first number in an FRL, for example 60 in 60/60/60 |
Integrity | Ability of an element to resist the passage of flame and hot gases through cracks or gaps | Generally a property of the wall or floor system as a whole rather than the bare steel member itself |
Insulation | Ability of an element to limit temperature rise on the unexposed face | Also a whole of system property, achieved through linings such as fire rated plasterboard rather than the steel alone |
A structural steel beam or column is generally only being asked to deliver the first of these three numbers, since a structural member does not usually form part of a fire compartment boundary the way a wall or floor system does. That is why you will often see a structural steel FRL requirement written as something like 90/-/- rather than a full three part rating: the requirement is for structural adequacy alone, with integrity and insulation addressed separately by the surrounding construction. Required FRLs themselves vary by the building’s class and type of construction under the National Construction Code, generally increasing with the building’s height and the number of people who need to evacuate.
The design methodology for structural steel fire protection in Australia sits in Section 12 of AS 4100:2020, Steel structures. This section introduces the concept of Limiting Steel Temperature, the temperature at which a given steel member, carrying its actual design load, can no longer maintain structural adequacy. Section 12 also sets out how to calculate the Period of Structural Adequacy, the time in minutes for a member to reach that limit under the standard fire test, and requires that this period be equal to or greater than the FRL the element is required to achieve.
A detail that catches a lot of people out: the limiting temperature is not a fixed number for steel in general, it depends on how heavily loaded the member actually is. A steel member working at close to its full design capacity has less spare strength to lose before it fails, so its limiting temperature is lower, sometimes cited around 350 degrees Celsius for heavily loaded members. A member with more spare capacity relative to its load can tolerate a higher temperature before failing, sometimes cited well above 700 degrees Celsius. This is one reason two apparently identical steel sections in the same building can need meaningfully different amounts of fire protection.
Section factor, sometimes written as Hp over A, is the second major variable. It is the ratio of a member’s heated perimeter to its cross sectional area, and it is essentially a measure of how much surface a member exposes to heat relative to how much steel mass is there to absorb that heat before its temperature rises. A thinner, lighter section has a higher section factor and heats up faster than a heavier, chunkier section of the same steel grade, which means thinner members generally need more fire protection, not less, to achieve the same FRL. This has a direct, practical implication for light gauge steel framing, covered further down.
Where an FRL requirement applies to a steel member, three broad categories of passive fire protection are used to keep it below its limiting temperature for the required period.
For light gauge steel wall and floor framing specifically, the more common approach is different again: rather than coating individual studs, the fire resistance is achieved by the wall or floor system as a whole, most often through fire rated plasterboard linings fixed to both faces of a steel stud wall, with the lining system tested and certified to AS 1530.4 as a complete assembly rather than the steel stud being protected in isolation.
Most fire engineering discussion focuses on individual members, yet real buildings fail at their joints as often as along a clean span. Connections between differently sized or differently protected members need their own assessment under AS 4100 Section 12, since a connection can behave quite differently in fire to either of the members it joins. A particularly common scenario is a protected structural member meeting an unprotected one, such as a window or door frame connecting back into the main structure. Left as is, the unprotected element heats up quickly and can draw heat directly into the protected member at that junction, undermining its protection. The standard remedy is a coatback: extending the same protective material used on the protected member a short distance onto the adjoining unprotected element, slowing the rate heat can transfer across the junction. It is a small detail with a large effect on real world performance, and one worth raising directly with your fire engineer at the design stage rather than after steel has been ordered.
Everything above concerns structural fire resistance, meaning a fire that starts inside or against a building. Bushfire resistance is a separate, though related, engineering question: how a building’s external elements withstand radiant heat and ember attack from a bushfire in the surrounding landscape, governed by Bushfire Attack Level ratings under AS 3959. Steel’s non combustibility is genuinely valuable here too, and we have covered the practical detail of building to a Bushfire Attack Level in our guide to Residential Steel Framing Across Melbourne and Victoria, so we will not repeat that ground here. One detail worth knowing regardless: according to the Australian Steel Institute, bushfire compliant construction must follow either the AS 3959 pathway or the NASH Standard pathway for steel framed construction in bushfire areas, and the two cannot be mixed on the same building element, so a wall built to one pathway cannot simply be paired with a roof built to the other.
Light gauge steel, the thinner, cold formed steel used in residential and commercial wall framing, has a higher section factor than heavy structural steel, which means it heats faster in isolation. In practice this rarely means individually fireproofing every stud. Instead, light gauge steel wall and stud framing systems achieve their required FRL as a tested assembly, generally through fire rated plasterboard on both faces, with the specific lining, stud spacing and fixing detailed to match a system that has actually been fire tested to AS 1530.4 rather than assumed from the individual components. For a project team, the practical takeaway is straightforward: the FRL belongs to the tested wall or floor system as a whole, and confirming which certified system a project’s specification calls for, then building to that system exactly, matters more than any single material choice within it.
Structural steel is non combustible, meaning it will not ignite or fuel a fire, but on its own it is not automatically fire resistant to a specific rating. Unprotected steel loses structural strength as it heats, softening from around 425 degrees Celsius and losing roughly half its strength between 600 and 650 degrees Celsius, so achieving a specific Fire Resistance Level generally requires passive fire protection or a tested lining system.
The three numbers refer to minutes of structural adequacy, integrity and insulation respectively, tested to AS 1530.4. A structural steel member is generally only responsible for the structural adequacy figure, since integrity and insulation are usually properties of the surrounding wall or floor system rather than the bare steel member itself.
In the United States, AISC Design Guide 19, Fire Resistance of Structural Steel Framing, covers this topic in depth against American codes. For projects in Australia, the equivalent technical framework sits in Section 12 of AS 4100:2020, the National Construction Code’s Performance Requirement CP1, and the testing regime set out in AS 1530.4, which this guide has summarised in an Australian context.
Where a wall or floor is required to achieve a specific FRL, yes, though this is usually achieved through a tested lining system, most commonly fire rated plasterboard fixed to both faces of the steel stud wall, rather than by coating the individual steel studs. The system as tested, not the steel component alone, is what carries the certified rating.
If your project has a specific FRL to meet, our team can talk through how it applies to your light gauge steel framing and connect you with the right tested system for your build. Get in touch with CMC Steel Solutions on 1300 285 566, email info@cmcsteelsolutions.com.au, or contact us to discuss your project.
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