Building Regulation requirements demand fire resistance up to a maximum of two hours, but in certain cases higher ratings may be required – for instance by insurance firms.
Building owners and designers are usually faced with a choice of three generic types of fire protection if a steel framed structure has been selected:
- Cementitious spray
- Boarded solutions
- Intumescent coating
Cost determinants of fire protection measures
The thickness of the protection required, and hence the cost, is determined by:
- How long the protection is supposed to last in a fire
- How many sides of the steel member are exposed
- The type of steel member (universal beam, universal column, rectangular hollow sections and so on)
- The section size and weight, or section size and wall thickness
- Whether it is used as a column or beam.
The above factors determine a member's "section factor" – the ratio of surface area exposed to fire to mass or cross-sectional area. This determines the thickness of protective material required and, therefore the number of layers or coats.
Economics of fire protection
Cementitious sprayed systems are likely to be the cheapest available method of protecting steel members against fire collapse. However, the appearance of such systems frequently rules them out of consideration. Then, rule-of-thumb economics suggests an intumescent solution for up to 60-minutes fire rating and a boarded method for 90 or 120 minutes. However, continuing intumescent developments, including the need for reduced thicknesses of material, has enabled intumescent to become competitive at 90-minute protection.
The rates shown in the table are intended to represent typical averages across all the structural members treated. In practice, the rate can be different for virtually every member. Of the systems examined, there is generally no difference in the cost of providing 30- or 60-minute protection, except in the case of intumescent coating.
In general, beams, which usually require only three-sided protection, require a lower loading or thickness than columns, which need four-sided protection. Typically, columns may have 20% greater loading than beams and so attract a proportionately greater rate for fire protection.
Hollow sections generally require a greater thickness of protection than a universal beam or column section for the same period of fire resistance: a small circular hollow section (88.9 × 5 mm) may require six coats of intumescent base coat to achieve 60 minutes
fire-resistance compared with the three coats needed for a 152 × 152 × 30 mm universal column. As a result, intumescent painting rates for hollow section columns may typically be £20-30 per m2 of surface area for one-hour protection, rather than the average rates shown above.
On individual projects a wide range of rates may be experienced and small quantities may have a disproportionate effect on unit rates.
Sprays are either mineral fibre/cement (such as Rockcem) or vermiculite/cement (such as Mandolite).
- Relatively cheap
- Can be applied to complex shapes
- Assists thermal and acoustic insulation
- Up to four hours fire protection
- Only likely to be suitable for protecting members out of sight
- Surrounding areas may require masking
- Resistant only to minor knocks and abrasions without proprietary coatings or sealants
- Difficult to ensure uniform thickness
- Bonding agent may be required
- Wire reinforcement required for large members
Thicknesses of material required:
- 10 mm thickness for 30 minutes fire protection
- 10-17 mm for 60 minutes
- Generally 17-25 mm for 90 minutes
- Generally 20-34 mm for 120 minutes
- Generally 50-70 mm for 240 minutes
Boarded fire-protection solutions are promoted as a clean, dry process, that can provide a neat finish with the required functional fire protection in one. However, they are impractical for protection to castellated beams or the increasingly used cellular beams where holes are used for services penetrations.
The four common types of boarding are:
Mineral fibre board: Spiralite, Promalit, Conlit
This is the cheapest board solution but is generally only used in non-visual areas. Its properties are:
- Tissue or foil faced
- Fixed with noggings or soldiers and proprietary spring screws
- Up to two hours protection with screws only
- Up to four hours with adhesive
- Resistant to minor physical damage
- Available in 20-55 mm board thicknesses
- 20 mm board satisfies all requirements up to 60 minutes fire resistance
- 90 minutes resistance achieved with maximum 30 mm thickness
Glass-reinforced gypsum board: Glasroc S, Knauf Fireboard
- Denser, less liable to damage from knocks – recommended for high-impact resistance areas such as industrial buildings, sports halls and other high-traffic areas
- Smooth surface for direct decoration or plaster finish
- Proprietary screw fixed or stapled
- Up to two hours protection (some systems up to four hours)
Vermiculite silicate bonded board: Cryosil, Vicuclad
- Denser than mineral fibre boards but relatively easy to damage
- Smooth surface, easy to decorate
- Fixed with noggings and screws (noggings not required for columns)
- Up to 120 minutes protection with screws only
- Up to four hours with adhesive
- Available in pre-formed semi-circular sections for circular steelwork
Metal-faced boarding: Rockliner, Vicucase
- Galvanized, colour-coated or stainless steel-faced
- Provides decorative finish and mechanical strength
- Proprietary jointing system
- Up to four hours fire protection
- Available in square, rectangular or circular profiles
These products consist of a thin-film coating – generally less than 3 mm but up to 6 mm on hollow sections – that expands (at about 200°C) to approximately 50 times its original film thickness, producing an insulating carbonaceous char that reduces the rate of temperature increase of the steel. In exposed situations, a separate decorative sealer is subsequently applied that also provides moisture and abrasion resistance.
The cost of intumescent coatings has reduced in recent years as a result of improved technology and increased competition. As a result, intumescent coatings have made considerable inroads into the market for fire protection to structural steelwork at the expense of boarded solutions. Intumescent coatings can now provide protection to structural steelwork for up to two hours at competitive cost.
- Ideal where the exposed profile of the steelwork is a part of the design concept
- Only system suitable for castellated or cellular beams where the holes are used for services
- Minimal weight compared with other protective systems
- Off-site application is possible and enables greater quality control and removes a process from the on-site construction programme
- Decorative top coat available in a wide range of colours.
- Up to 10 times faster than boarded systems
- Primer compatibility must be ensured
- Cannot be applied where condensation conditions exist, unless external-quality material is used
- Skilled application required
- Quality control – specific quality control records are usually required
- Adjacent self-finished surfaces must be masked out to guard against spray drift
- Film thicknesses have to be measured at frequent intervals using an electronic gauge (at least at 500 mm centres along each plane of each member)
- Inspections required by manufacturer to obtain guarantee
- Difficult to assess thickness of covering around bolted connections
- Cannot be exposed to water without top sealer coat unless an external-quality base coat is used
- A minimum of four hours is required between coats. (up to six coats can be required in extreme situations)
- With off-site application, site touch-up required
Usually applied by airless spray but can be applied by brush. The spray can achieve a thickness per coat of up to 1.4 mm but a brush will only achieve 0.375 mm.
Water-based products are generally used for up to 60 minutes protection and solvent-based ones for 90 or 120 minutes. Only solvent-based products can be used for external use. Water-based products have the advantage of being odour-less and free of safety hazard during application. Water-based products require less loading and dry film thicknesses will not usually exceed 1.5 mm.
The cost of off-site application is usually higher than on-site application, in part due to the necessity for on-site touch up and treatment to connections after erection. Nevertheless, off-site application accounts for a growing proportion of the market as demand for greater pre-fabrication grows and the time-saving benefits become more appreciated.