Costs: Structural steel

Steel-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 penetration
• 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 is 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 of 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 the 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 expands 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 5 mm 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 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 30 m 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.