Durability- Structural steel fire protection
If
structural steel is your frame of choice, you need to think about fire
protection. Peter Mayer of Building LifePlans runs through the various
options and what each one will cost.
Introduction
Steel-frame frame construction offers a fast, flexible and integrated
structural solution for many building types. The downside is that structural
steel members may need additional fire protection. The type of fire protection
selected depends on the fire resistance requirements, expected use, environment,
appearance and cost.
Fire protection needs three key properties. These are:
• Insulation – resistance to the transfer of heat
• Integrity – resistance to fire protection
• Loadbearing capacity – the resistance to collapse.
Confirmation that steel structures meet minimum standards is achieved
by fire testing:
• The British Standard fire testing series BS 476, in particular,
parts 20 – 23
• The European suite of fire testing standards: DD ENV 13381, EN
1363 and EN 1365.
The requirements for fire protection depend on building type, height,
depth below ground, volume and floor area. Fire resistance criteria are
measured in 30, 60, 90 or 120 minutes.
Guidance is given in the relevant national Building Regulation fire safety
requirements. BS 5950–8, the code of practice for fire-resistant
steelwork, offers further guidance. However, fire protection is a specialist
discipline and therefore specialist advice should be taken.
Specification options
The most common passive fire protection options for internal structural steel can provide up to and over 120 minutes fire resistance, generally dependant on the material thickness. Detailed guidance is available from the so-called “Yellow Book” - Fire Protection for Structural Steel in Buildings, published by the Association for Specialist Fire Protection, the Steel Construction Institute and Fire Test Study Group.
Boards
An enormous choice of fire-resisting boards is available including: fibre boards, cement based gypsum, vermiculite or calcium silicate boards. These are available either pre-finished or requiring decoration. Boards are cut and fitted around steel sections, which can be time consuming and costly, especially at complex junctions.
Sprays
Spray systems offer the lowest application costs and the benefit that complex details are readily covered. As this is a wet trade the on-site time may be high due to the need to mask areas and allow for drying out of the system. Typical sprays are based on gypsum or cement, with added vermiculite or mineral wool.
Thin film intumescent coatings
Intumescent systems have the advantage of being applicable off-site and also allow structural form to remain visible as an architectural feature. In a fire, the intumescent layer exapands up to 50 times its original thickness providing an insulating char layer. BS 8282 provides guidance on the use of intumescent coating. Intumescent systems typically comprise three coatings: primer, intumescent layer and top coat. Thin film systems are less than 5mm thick. The intumescent layer may be water- or solvent-based. The top coat protects the intumescent coat from adverse environmental agents as well as providing a decorative finish..
Capital cost issues
• The type of steel structure influences the protection offered.
Composite beam and slab construction comprising steel beams, permanent
steel profiled decking and concrete slab has an inherent fire resistance
of up to 60 minutes. Additional fire protection measures may not be needed,
thereby reducing capital costs.
• Off-site applied intumescent fire protection has a higher material
cost. But this cost may be offset by with reduced programme times, labour
and preliminary costs on site. Additional costs may arise from repairing
damage caused by installation impacts or mishandling.
• Fire safety engineering is an alternative approach of achieving
satisfactory fire resistance. BS 7974 provides guidance on this. Fire
engineering assesses buildings so the appropriate amount of fire protection
is provided according to risk. It is based on real fire behaviour, real
loading performance and the level of risk. This process enables designers
to leave a proportion of secondary beams unprotected where a 30- or 60-minute
fire resistance is normally required. Compensatory measures may need to
be specified such as increased reinforcement mesh to floor slabs.
• Use of active fire-protection measures such as sprinkler systems
may enable reduction of the passive fire resistance requirement by 30
minutes for non–domestic buildings less than 30m in height.
In–use cost issues
Maintenance of fire protection in an internal environment is generally not required beyond redecoration. Inspections should be carried out to ensure the integrity of fire protection. All systems of fire protection are at risk of impact and abrasion damage. Where there is risk of high humidity, condensation or pollution, intumescent coatings may require a maintenance programme.
Specification options
| Capital cost (£) |
Fire resistance minutes | |
| Unprotected steel (Only where fire risk is low, when fire safety engineered) | - | 30 |
| Board Mineral fibre board clipped and screwed 12 mm thick | 19 – 26 | 60 |
| Spray: cement based vermiculite min 16mm thick | 9 – 13 | 60 |
| Thin film fire protection system comprising primer, intumescent coat and top coat | 20 – 27 | 60 |
Table notes
• Fire resistance depends on steel section perimeter : cross sectional
area ratio and thickness of coating. The costs and component descriptions
are indicative. Fire-resistance performance should be confirmed by reference
to the Yellow Book or test data.
• A cost analysis based on project specific information is essential
for a realistic best value appraisal.
First published in Building 2005
Further information
Building LifePlans provides latent defects warranty
for all buildings.
BLP Construction Durability Database at www.componentlife.com
provides durability information for building components.
Further information contact peter.mayer@buildinglifeplans.com
or telephone: 020 7204 2441.
