Steel gradually loses strength and stiffness at high temperature. The temperature at which a member fails, the critical steel temperature, depends on the load and the degree of utilisation: the more heavily the section is loaded relative to its capacity, the lower that temperature. The structural engineer determines that value for each member. To keep a steel structure standing for 30, 60, 90 or 120 minutes, there are two common routes: a fire-resistant coating that foams up when heated, or board cladding that encases the profile. Both work. The choice comes down to cost, appearance, maintenance and the situation on site.
How they work
Fire-resistant coating is an intumescent paint. When heated, the layer expands into an insulating foam, often many times its original thickness, which keeps the heat away from the steel. The temperature at which this starts differs per product. The dry film thickness is determined per section on the basis of the section factor (Am/V): the ratio between the heated perimeter and the cross-sectional area of the section. Together with the critical steel temperature and the required duration, this gives the film thickness from the system's data. A slender section has a higher section factor, heats up faster and therefore usually needs a thicker layer than a heavy section for the same requirement.
Board cladding wraps the section in fire-resistant boards, usually based on calcium silicate or gypsum. The board physically insulates the steel. The required board thickness likewise follows from the section factor, the critical steel temperature and the required duration, according to the board system's data.
The comparison
| Fire-resistant coating | Board cladding | |
|---|---|---|
| Appearance | Profile stays visible, suitable for exposed work | Profile becomes a rectangular column or beam |
| Cost at 30-60 minutes | Often cheaper | Often more expensive |
| Cost at 90-120 minutes | Can rise sharply due to thick layers | Often more favourable |
| Installation time | Longer: multiple layers with drying time in between | Shorter: dry installation, immediately serviceable |
| Weather sensitivity | Sensitive to temperature and humidity during application | Barely sensitive |
| Susceptibility to damage | Damaged by impact, can be repaired locally | More robust, but repair requires board replacement |
| Complex connections | Follows any shape effortlessly | Difficult and labour-intensive around connections |
| Maintenance | Periodic checks for damage, the layer can be repaired | Little maintenance, visual check of the joints |
When do you choose coating?
- The steel stays exposed and the architecture calls for the silhouette of the profile.
- The requirement is 30 or 60 minutes; coating is often the most economical route there.
- The structure has many complex connections, lattice girders or angled connections that boards are hard to fit around.
- Protection must be applied retroactively in an existing building where space around the profile is limited.
When do you choose cladding?
- The requirement is 90 or 120 minutes; the required film thickness can then become so great that board cladding works out cheaper.
- The steel is out of sight, above a ceiling or in a shaft, where appearance plays no part.
- The schedule is tight: board installation has no drying times and is insensitive to site conditions.
- The environment is rough, for example a warehouse with forklift traffic, where a coating is quickly damaged.
- Work takes place in low temperatures or high humidity, where coating does not cure properly.
- The environment is damp or outdoors and there is no coating system assessed for that environment.
What is decisive for both
Whichever you choose, the protection stands or falls on three things. The section factor and the critical steel temperature must be determined for each section. A uniform film thickness across the whole structure is only correct if it is sufficient for every section; usually the required thickness differs per section. The connections to walls and floors must be included, otherwise heat still reaches the steel via the flanks. And the execution must be documented : which system, which film or board thickness, on which section, measured and documented. Without those records it is difficult to show during an inspection that the structure achieves the required time.
A combination is fine too
In practice a mixed solution is often the sharpest: coating on the exposed columns in the entrance hall, board cladding on the beams above the suspended ceiling. There is no reason at all to roll out one technique across an entire building when the situation differs per building section.
Would you like to know which route works out best for your structure? We assess the profiles, the requirement and the situation, and cost out both options. See fire-resistant steel cladding or request a quote.
From practice · Emmeloord
Fire-resistant cladding of the steel load-bearing structure in Emmeloord
Steel columns and edge beams boxed in with fire-resistant board cladding, running continuously from floor to ceiling.
View the project
Frequently asked questions
Often, yes, at 30 and 60 minutes. At 90 and 120 minutes, the required film thickness can become so great that board cladding works out cheaper. The section factor and accessibility help determine exactly where the tipping point lies.
Only if the underlying layer is suitable and sound and falls within the tested build-up of the system. Blasting or a suitable primer is often required. Having this assessed beforehand prevents the entire layer from detaching later.
There is no fixed service life. Indoors, under normal conditions and with a system suited to the environment, a properly applied layer can last a long time. What matters is not the age but the condition: damage from later building work is a common reason why the protection locally no longer complies. So have the layer visually checked periodically, and measure the film thickness if in doubt.
Usually, yes. The required dry film thickness follows from the section factor of that specific section, the critical steel temperature and the required fire resistance duration. A uniform thickness across the whole structure is only correct if it is sufficient for every section; otherwise part of it is too thin.
Want to know more about fire-resistant steel cladding?
Fire-resistant cladding or coating for steel beams and columns, so load-bearing capacity is retained for up to 120 minutes in a fire.
More about fire-resistant steel cladding
