Structural fire protection is something of a mystery at many drawing boards. One of those things discussed in hushed tones, at the eleventh hour. One of those responsibilities that is chucked to the next person, before being rapidly exited from. On the surface, it does feel like a kind of technical wizardry. A dark art.
There are a lot of types of things people make structural frames from. Concrete. Increasingly, timber, – which is a discussion unto itself, a kind of meta-magic, if you will - and of course, steel.
Steel doesn’t burn, they say. Why is it so hard?
Well, we answer. Steel doesn’t burn, and it doesn’t usually melt (fun fact: it actually does at very high temperatures, but that’s a story for another day), but that doesn’t mean it can’t fail.
It can. Quite catastrophically, even. Before your client’s espresso machine even gets hot.
The moment a fire takes hold, you are no longer holding principles of architecture, but juggling laws of physics. If you drop one, it gets properly messy.
Heat. Time. Load. Creep. Temperature gradients. Insurance clauses. It’s a lot to get lost in.
The rise of steel
Steel has been climbing the ranks of UK construction for decades as the framing of choice. It’s bedded itself in as an established fixture of the construction industry.
Around 70–80% of multi-storey commercial buildings in the UK use steel frames. It’s no great mystery why - it’s strong, light, modular, fast. It doesn’t need curing. It gives architects the long spans and exposed elegance that concrete just can’t without provoking Brutalist flashbacks.
So on paper, it’s almost perfect. In a fire, though, steel turns out to have all the self-control of a rock star in a minibar. Left unprotected, though, it folds fast. And more quietly than you might think.
Why steel shines, and where it blinds
Structurally, steel is a dream. Beautifully predictable. With known mechanical properties, and catalogued sections. Its’ behaviour under load is reasonably quantifiable. Basically, structural engineering calculations can be run and will provide a fairly reliable idea of what you’re dealing with.
Until you add heat.
Steel starts losing strength and stiffness first. Then, the collapse mechanisms begin - often without much warning. Standard fire curves can reach those thresholds in less than 20 minutes (depending on size and exposure).
The challenge often is that steel conducts heat far more rapidly than concrete. That high thermal conductivity means that the heat doesn’t just graze the surface, but barrels through the section.
This challenge can be mitigated. If the steel is sufficiently protected, that is.
Where it matters the most
Steel is an increasingly popular material in structural applications, for all the right reasons. Speed. Cost. Repeatability. Performance. But every beam, every column, every connection that contributes to structural stability must maintain integrity for long to keep the building standing upright while occupants escape and fire services do their thing.
If the frame folds, so does the whole fire strategy. So does the building.
The backbone you may never see
In many cases, and many buildings, the entire fire strategy – that’s every escape stair, every compartment line – quite literally hinges on the frame still being there.
Fire strategies do not work in a vacuum. They tend to operate on the basis that the building is still held together. But if the structural integrity fails, much of it goes kaput. It’s the third act of a disaster film: the strategy collapses with the structure.
That’s why the first step in protecting people is not signage or alarms, it’s making sure the floor stays where it’s supposed to.
What happens to unprotected steel when fire hits
This is where we leave the architectural render. Some time ago, a project team led by British Steel (including the BRE, who were, at the time, still a nationalised laboratory) conducted some research into the behaviour of unprotected steel under fire conditions. It wasn’t pretty, but of what they found formed the basis of what we know today.
- Unprotected, steel can heat rapidly. It can reach critical temperature in as little as fifteen minutes, depending on the section size and exposure. Barely enough time to eat lunch.
- At 550 °C, expose structural steel retains about 60 to 70% of its capacity – and that’s assuming no added stress. If it’s under load, which it always is, strength is lost even faster.
- Young’s Modulus falls off at high temperatures. Around 300–400 °C, stiffness reduction begins. Which means even small loads can start causing large deflections. Think beam sag, local buckling, deformation. Pick your poison.
- Creep is a slow motion killer. Above 400 °C, creep can begin to deform unprotected structural members under load. This hard to detect in the aftermath, but capable of making a structure that looks okay dangerously compromised.
- Bolted and welded joints heat unevenly. It doesn’t matter how beefy the member is if the joint fails – and, insufficiently protected, this can happen early.
These are, of course, relative to the cases where steel structures are unprotected, which are rare in modern times. But the same risks still apply to insufficient or inconsistent applications of protective materials (notably paint or spray). Material selection is everything in this game.
Why it matters
Safety first, always: A protected frame gives occupants time to escape and firefighters time to quell the blaze. Unprotected steel doesn’t: it fails. When you’re designing a building, there’s a lot in your hands – make sure you’re taking care of them.
Compliance: Approved Document B sets the required outcomes, EN 1993-1-2 provides fire design methods, and the BS EN 13381 series covers the rest. Together, they require that structural members are demonstrated to resist fire for 30, 60, 90, 120, and even 240 minutes. Provably, certifiably, verifiably.
Liability and risk: If the structure fails in a fire situation, and protection wasn’t sufficient at design stage, that problem is coming back to the design team. Insurers notice, and so do courts.
Who you gonna call?
(Spoiler: it’s not ghostbusters)
Approved Document B tells us what is needed to achieve in order to protect escape routes. It provides a necessary time interval for integrity to be maintained, but it doesn’t tell you how to get there.
This is because there is no single answer, and that’s because every project is different. Section factor (A/V), fire duration, fixing method. Boarded, sprayed, or intumescent. Ambient or compartment fire. Wet or dry systems. There’s a lot at play.
This isn’t ‘coat it and hope’, it’s precision work.
The good news is that you don’t need to be an expert. You just need us. We’ve seen more edges than U2 and we know how to make protection work.
You want to talk to the people who live and breathe passive fire protection: and that’s us. You can skip the marketing gloss and ring the technical line. We’ll get it right.