Boards are widely used for structural fire protection in the UK, whether the protection system is in full view or hidden. They can be applied on unpainted steelwork and offer the specifier a clean, boxed appearance. Application is a dry trade and so is less likely to disrupt other trades on site and, as boards are factory manufactured, thicknesses can be guaranteed.
There are broadly two families of board protection: lightweight and heavyweight. Lightweight boards are typically 150–250kg/m3 and are not usually suitable for decorative finishes. They are cheaper than heavyweight equivalents and typically used where aesthetics are not important. Heavyweight boards are usually 700–950kg/m3 and will generally accept decorative finishes. Both types of board may be used in limited external conditions but the advice of the manufacturer should be sought.
Spray protection is extensively used in the USA but is less common in the UK. It can be used to cover complex shapes and details. Costs do not rise significantly with increases in thickness, because much of the cost of application is in labour and equipment. Some materials can also be used in external and hydrocarbon fire applications. The downsides are that sprays are not suitable for aesthetic purposes, and as application is a wet trade this may impact on other site operations. A cost allowance may have to be made for prevention of overspray.
Flexible blanket systems
Flexible blanket systems have been developed in response to the need for an easily applied fire protection material that can be used on complex shapes and details, but where application is a dry trade. There are a limited number of manufacturers of these products.
Until the late 1970s, concrete was by far the most common form of fire protection for structural steelwork. However, the introduction of lightweight, proprietary systems, such as boards, sprays and thin-film intumescent coatings, has seen a dramatic reduction in its use. Traditional methods, such as blockwork encasement, are also used occasionally.
Concrete tends to be used where resistance to impact damage, abrasion and weather exposure are important – for example, in warehouses, underground car parks or external structures. The principal disadvantages are the higher cost in comparison to lightweight systems, the high space utilisation rate, as large protection thicknesses take up valuable space around columns, and weight.
Detailed guidance on the installation of coatings, boards and spray protection systems is available from the Association for Specialist Fire Protection. Information on the thickness of concrete encasement for specific periods of fire resistance is published by BRE. It can also be found in BS EN 1994-1-2.
Fire safety engineering
Fire safety engineering can be seen as an integrated package of measures designed to achieve the maximum benefit from the available methods of preventing, controlling or limiting the consequences of fire.
The simplest form of structural fire engineering is the use of codes to design individual elements of construction. Techniques for structural fire engineering on assemblies of elements of construction using a simplified method have also been developed, requiring little specialised knowledge on the part of the engineer. These can be used in the design of simplified sub-assembly models and make use of the understanding of the behaviour of composite construction in fire gained from the Cardington fire tests.
Use of these models, which combine the residual strength of steel-composite beams with the strength of the slab in fire, allows the designer to leave large numbers of secondary beams unprotected in buildings requiring 30–120 minutes’ fire resistance, resulting in significant cost savings, although some compensating features such as increased reinforcement in the slab may be required.
FIGURE 5: Fire engineered solution, with boards and unprotected secondary steelwork
FIGURE 6: Office building with a 90-minute fire resistance period and is provided with intumescent paint pigmented with a dark grey colour