Why steel needs fire protection
Steel is non-combustible, but when heated it rapidly loses strength and stiffness. It is also a good conductor of heat, so in a fire a thin-walled section heats up almost as quickly as the surrounding gases. A frame designed for normal loads can lose stability long before the building has been evacuated and fire crews have deployed. The job of fire protection is to delay that moment for as long as the design requires.
The required fire resistance rating is denoted by the letter R followed by the time in minutes: R 15, R 45, R 90, R 120 and so on. It is assigned by the designer on the basis of the building’s degree of fire resistance and the role of the member in the frame.
The key parameter: section massivity
The rate at which a steel member heats up depends on how much steel there is per unit of heated surface area. In CIS practice this is expressed as the reduced metal thickness — the ratio of the cross-sectional area to the heated perimeter, in millimetres. European and international documents use the reciprocal — the section factor Hp/A, in m⁻¹. The principle is the same: heavy sections heat up more slowly and need a thinner layer of protection, while thin angles and small-diameter tubes need a thicker one.
The heated perimeter depends on how the member is protected and from which sides it is exposed to fire. A beam under a cast-in-situ slab is heated on three sides, a free-standing column on four. With box encasement, the perimeter of the box is used, not the outline of the section. Getting the perimeter wrong is a common reason for an incorrect thickness.
Fire protection methods for steel
| Method | Strengths | Limitations |
|---|---|---|
| Thin-film intumescent paints | Low weight, architectural finish, shop or site application | Cost rises at high ratings; sensitive to application and service conditions |
| Epoxy intumescent coatings | High mechanical and weathering resistance; suitable for hydrocarbon and jet fire scenarios | Price, complex application, a skilled crew is needed |
| Plasters | Economical at high ratings; cope well with complex geometry | Weight, wet process, appearance |
| Boards and mats | Dry installation, controlled thickness, durability | Labour-intensive at complex details; accurate cutting and fixing required |
How to choose fire protection for structural steel
Step 1. Gather the input data
A schedule of members with their section sizes, the required fire resistance ratings for each group, the exposure conditions and the surface area. Without this, any price is only “approximate”.
Step 2. Define the fire scenario and the environment
An ordinary building means a standard fire; oil and gas or chemical plants mean a hydrocarbon fire and possibly a jet fire. Service conditions are a separate matter: a heated interior, an unheated warehouse, a canopy, an open-air process unit.
Step 3. Select the type of material
For exposed steel at low and medium ratings, paint is usually the best option. For concealed members and high ratings, plaster or boards. For industrial sites handling petroleum products, epoxy or cementitious systems.
Step 4. Obtain a thickness calculation from the manufacturer
The thickness is determined for each section from approved test data. It should not be inferred “by analogy” with another project or another material.
Step 5. Check compatibility with the corrosion protection system
The primer and topcoat must be on the fire protection manufacturer’s approved list. If the steel is primed by the fabricator, this information must be passed on in advance.
Common mistakes
- A single thickness “for everything”, regardless of the different sections.
- An incorrect heated perimeter — especially for beams under slabs and columns within walls.
- Fire protection paint applied over an unspecified primer of unknown thickness.
- Overlooked elements: bracing, connections, brackets and beam seats, which are also part of the load-bearing structure.
- No acceptance check of dry film thickness and no batch certificates for the material.
Materials from leading manufacturers
- Interchar 1120 — International (AkzoNobel) — water-based paint for exposed structural steel inside buildings.
- Interchar 3120 — International (AkzoNobel) — epoxy coating for cellulosic fire, for shop application.
- Interchar 212 — International (AkzoNobel) — modified epoxy coating with enhanced damage resistance.
- PPG STEELGUARD 951 — PPG — epoxy fire protection for cellulosic fire that also provides corrosion protection.
- PPG STEELGUARD 651 — PPG — water-based thin-film paint for interiors.
- FIRETEX FX5090 — Sherwin-Williams — water-based paint for steel in dry interiors.
- Thermo-Lag E100 — Carboline — epoxy coating for commercial and light industrial projects, interior and exterior.
- Nullifire SC902 — Nullifire (Tremco CPG) — fast-curing single-coat system.
- CAFCO 300 — Isolatek International — gypsum-based spray-applied fire protection for beams and columns.
- PROMATECT-H — Promat — calcium silicate board encasement for steel columns and beams.
- ROCKWOOL Conlit 150 — ROCKWOOL — mineral wool boards for load-bearing steel.
- Chartek 1709 — International (AkzoNobel) — epoxy system for oil and gas facilities.
- Jotun SteelMaster 1200WF — Jotun — water-based thin-film paint for steel, optimised for 120 minutes.
- Jotun SteelMaster 1200HPE — Jotun — solvent-free epoxy coating, including for exterior conditions.
- Promat VERMICULUX-S — Promat — calcium silicate board for boxed fire protection of steel beams and columns, 240 min.
Frequently asked questions
Does galvanised steel need fire protection?
Yes — zinc does not increase fire resistance. That said, not every fire protection product is compatible with galvanised surfaces: surface preparation and a primer approved by the manufacturer are required.
Can fire protection be applied over old paint?
Only after the existing coating has been assessed and with the agreement of the fire protection manufacturer. In most cases the old paint is removed, because its adhesion and compatibility are unknown.
How long does fire protection paint last?
The service life is stated by the manufacturer for specific service conditions. In practice it depends heavily on the environment, the quality of application and prompt repair of any damage.
What is Hp/A in the manufacturer’s tables?
It is the section factor — the ratio of the heated perimeter to the cross-sectional area. The higher it is, the faster the section heats up and the thicker the protection needs to be.