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Legislative Background

Approved Document B

Approved Document B of the UK Building Regulations 2000 (Fire Safety) is considered to be the bible of fire protection requirements throughout the UK. Scotland and Ireland have different standards but are based on the same principles in respect of the structure, though it should be noted that the main consideration is still for life safety. (In Scotland, it is more difficult to achieve a fire-engineered solution to reduce the statutory 120 minute protection requirement).

The key to most requirements for passive fire protection in the UK can be found in the following notes, which are an interpretation of those parts of Approved Document B relevant to this specific sector.

In broad terms, the higher a building the greater is the degree of fire protection requirement. This is to allow more time for people to escape and the ‘control of a fire’. The latter is intended to allow time to bring the fire under control (and prevent the building from collapse) and for the safety of the fire brigade personnel.

In steel-framed buildings, it is important to prevent the metal structure from weakening under intense heat. The standard for fire protection (under BS476 part 20 & 21) is to protect the structure—for a specified amount of time—from reaching the ‘critical failure temperature’ of 550°C. This is the temperature at which general construction steel will start to lose its structural strength possibly causing the building to collapse. In some cases, such as suspension hangers, the metal used is not structurally sound at lower temperatures. Tests have therefore been carried out using the same parameters but with a failure lower than 550°C.

There are a number of ways in which a steel building can be protected.

These include the use of fire-rated insulation boards, spray materials, concrete, and intumescent paint. The increasing demand for an aesthetically pleasing building can limit the options open to an architect or designer (typically, in office atriums, shopping centres and other public buildings—where the architect prefers to use circular sections).

A risk analysis is carried out by the professional bodies in conjunction with the recommendations of Approved Document B (in this regard, you should specifically refer to Appendix A).

When the required fire resistance of a building reaches 90 minutes or over, the options for suitable fire protection materials reduce even further due to the limitations of thin-film intumescent. In England, Table B3 of the Approved Document details the recommended fire protection requirements for different types of buildings. Buildings over 30 m high require 120 minutes fire protection unless they can be fire engineered to show a lower standard will suffice. Buildings over 30 m high also require a sprinkler system, irrespective of the fire protection requirement.

However, this is only a recommendation. Highly sophisticated computer generated simulations can now be used to reduce the period of fire protection significantly. This is possible because a clause inserted in the Approved Document B3 Guidance (‘Performance’—paragraph a) allows the fire engineer to assess the likely damage that a specific fire temperature curve would produce over time. The fire engineer should also take into consideration any change in materials since the original standard of fire temperature curves was originally set.

A ‘Cellulosic Fire Curve’ is the measure of temperature over time and uses cellulosic materials to achieve the data. The temperature curve reaches 1,100°C. The building must maintain stability for a set period of time at this temperature. The ‘Hydrocarbon Fire Curve’ was developed some years later. The temperature curve in this instance increases much quicker and the temperature reaches a maximum of 1,300°C. It is this curve that is used in the petrochemical industry to set standards of fire protection to chemical plants and offshore oil platforms and was the basis for development of cast epoxy intumescent.

However, materials for office building, furniture and equipmenthave changed over time. Much of the materials used now contain substantial amounts of hydrocarbon materials not used when the cellulosic standard was set. Recent live testing of a typical furnished office has shown that the temperature curve was similar to the hydrocarbon curve at the outset, reaching temperatures of 1,300°C before dropping down to follow more closely the cellulosic curve. It is a concern that many of these materials have not been tested to withstand this new dimension in fire. (The initial intense heat may damage or weaken the material at an early stage, thus rendering it unable to withstand the temperature of a standard cellulosic fire as the fire increases in intensity).

BS476 Part 20 – 21 sets out the test criteria for fire protection testing. Beams and columns are to be loaded to represent the loading the steel would have in a typical construction. (It is important when assessing a material that the data is scrutinised to ensure the loaded test was carried out under the criteria of BS476.)

There is now a new standard set by the EU called the prEN standard. This standard sets a higher test regime with increased monitoring of the steel section and also the method by which the steel temperature is measured during testing. There are only a few companies that are committed to this standard so far. All materials will eventually have to be re-tested to meet the standard, although the EU has allowed a period of grace (thought to be approximately 5 years) by which time the standard must be met in full.

The current prEN standard requires the manufacturers of intumescent paint to increase their loading tables—and may increase the cost of a thin-film coating significantly.

All intumescents expand in a fire. The material, when exposed to high temperatures, produces a carbon matrix similar to a burnt meringue (char). The structure of thin film intumescent is quite weak. The expansion ratio is between 50 and 75 times the Dry Film Thickness (DFT)—nominally around 1 mm for one hours fire protection. The current prEN standard may require a thin-film DFT to increase substantially. This weakness in structure, along with the increase in DFT, means that a wire mesh could well be required to support the carbon structure during a fire. The exact requirement is yet to be finally determined.

However, this should not always be the case with epoxy intumescent. The expansion of epoxy intumescent is 5 – 8 times the DFT. This results in a much more stable char and will probably negate the need for a wire mesh, which may otherwise be required under the EU standard.

Most current test data on epoxy intumescent though is based on a wire mesh system to support the intumescent during a fire. The mesh is required to withstand hydrocarbon and jet fire tests—much more aggressive conditions than cellulosic (normal building) fires. The test data on epoxy intumescent has been adapted for cellulosic conditions but has used the same test regime as that for hydrocarbon (this explains why the wire mesh has to remain for the time being).