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Fire protection of structural steelwork

• Introduction
• Boards
• Sprayed non reactive coatings
• Sprayed reactive intumescent coatings
• Guidance

Introduction

• The fire protection of structural steelwork uses a number of well-understood materials and methods, but there is very little publicly-available guidance to help the specifier select the most appropriate system for a particular use. This page seeks to provide some basic guidance.
• An immense amount of useful information is available from the Association for Specialist Fire Protection [http://www.asfp.org.uk/]. There are a number of reputable manufacturers, supplying well-known products, who are not members of the ASFP but the generic information available from the ASFP website will assist in the evaluation of all products and systems.
• There are three generic types of material - boards, sprayed non reactive coatings, and sprayed reactive intumescent coatings.
• There are three methods of application - box, profile, and solid - as described in the ASFP "Yellow Book" Volume 1, paragraph 1.2.
• The Fire Protection Products matrix lists all the named products from the ASFP "Yellow Book" Volume 2, with an indication of the preferred methods of application and the appropriate NBS specification section/clause.

Boards

Gypsum-based boards

• Method of application : Generally using the "box" method as ASFP "Yellow Book" Volume 1, paragraph 1.2.
• Environment : For internal use only.
• Robustness : A variety of board types are available with varying impact resistance.
• Specification :
• Framed encasement systems : NBS clause K10/255.
• Frameless encasement systems : NBS clause K10/265.

Other board materials

• Materials : calcium silicate, vermiculite silicate, fibre-reinforced cement, fibre-reinforced gypsum, compressed resin-bonded mineral wool.
• Method of application : Generally using the "box" method as ASFP "Yellow Book" Volume 1, paragraph 1.2.
• Environment : Fibre-reinforced gypsum and resin-bonded mineral wool are not generally suitable for external environments.
• Robustness : Excepting fibre-reinforced gypsum boards, these materials are not suitable for use where there is any risk of mechanical damage.
• Specification : NBS clause K11/885 (frameless encasement systems).

Sprayed non reactive coatings

• Materials : Mineral wool, perlite, or vermiculite in a cement-based binder/filler.
• Method of application : Generally using the "profile" method as ASFP "Yellow Book" Volume 1, paragraph 1.2. A number of products are also suitable for the "box" method using a wire mesh cage to form the box.
• Environment : For internal use only.
• Robustness : Not suitable for use where there is any risk of mechanical damage. Residual risk of fibre migration when the ceiling void is used as an air plenum.
• Specification : NBS section M22.

Sprayed reactive intumescent coatings

Sprayed intumescent coatings generally

• Method of application : "Profile" method as ASFP "Yellow Book" Volume 1, paragraph 1.2. Sprayed intumescent coatings generally comprise three "coats" - a primer, an intumescent coat, and a topcoat/sealer - although the intumescent "coat" may have to be applied as a number of individual coats to achieve the required dry film thickness. It is not uncommon for manufacturers to include multiple coating types within one overall system so, for example, a water-based intumescent coat might be finished with a solvent-based topcoat/sealer, or vice versa. This has implications when selecting a system which is suitable for the desired method of application (on-site, off-site, etc.) and the environment in which the steelwork is to be installed (interior, exterior, etc.).
• Robustness : Most intumescent coatings will resist minor abrasion and light mechanical damage but all are susceptible to impact damage, such as may be caused when installing services trunking and ductwork. Manufacturers have proprietary remedial touch-up systems for damaged coatings but it may be difficult, or even impossible, to effect repairs in constricted areas.
• Restrictions on use:
• Care should be taken when intumescent coatings are used in "fire-engineered" buildings as other factors (sprinklers, ventilation, etc.) may prevent the steelwork reaching the temperature at which the coatings become effective.
• Care should be taken to ensure that the intumescent coating has adequate space to expand and function as intended. Examples of unsatisfactory detailing, which do not provide adequate space, include the use of clamp-type fixings for curtain walling restraint, and the direct fixing of the perimeter channels of LSFS studwork to the coated structural steelwork.
• Structural steel beams with sprayed intumescent coatings should not be used as the support for services suspended below. In these circumstances, services should always be fixed to the soffits and not the beams.
• Specification :
• NBS clauses G10/640, 650, 660.
• NBS section M61
• NBS sections G10 and M61 in combination in special circumstances.
• Do not use NBS section M60 for specifying intumescent coatings.

Water-based coatings

• Environment : Generally only suitable for dry, internal, conditions. Some products may be suitable for an external environment for a limited period of time.
• Application / Health & Safety : Water-based coatings do not have the toxic properties of solvent-based and epoxy-based coatings and are therefore more suitable for use on site where it may not be possible to provide adequate ventilation. They do, however, require that the building is dry and the temperature above a stated minimum. It is possible to apply water-based coatings off site but they require more stringent protection (than solvent- and epoxy-based coatings) during delivery, storage, and after erection.

Solvent-based coatings

• Environment : Generally suitable for internal use but some products may be suitable for external use. Solvent-based products are generally more tolerant of limited exposure to an external environment than water-based products.
• Application / Health & Safety : Solvent-based coatings contain chemicals (xylene, naptha, benzene, etc.) which have stringent COSHH requirements and are thus unsuitable for use on site (whatever the manufacturers may say) without adequate controlled ventilation. Solvent-based coatings should, therefore, generally be applied off-site under factory-controlled conditions. Whilst "just-in-time" delivery to site is desirable, some products are more tolerant of on-site storage than others.

Epoxy-based coatings

• Environment : Generally suitable for internal and external use.
• Application / Health & Safety : Generally as for solvent-based coatings, although epoxy-based coatings are generally less susceptible than solvent-based coatings to damage during transportation, storage, and erection.

Guidance

Hybrid systems

• For the purposes of this guidance, hybrid means:
• Using different systems to fire protect different elements of a frame, e.g. a sprayed intumescent coating on the columns in combination with a board material around the beams.
• Using different systems to fire protect different parts of an individual element, e.g. a board material around a column below the ceiling level in combination with an intumescent coating on the column above the ceiling level.
• Using more than one system to fire protect the same part of an individual element, e.g. a sprayed intumescent coating on, and a board material around, a column or a beam. Note: This combination must be avoided unless the systems have been tested and certified in the proposed combination. Do not specify untested combinations of different systems.
• The main problem with hybrid systems (as the first two bullet points above) is that it is highly unlikely that the junctions between the different systems will have been tested. Although the contractual design responsibility for the detailing can be passed to a specialist sub-contractor, this may not absolve the specifier/ designer of all responsibility if the fire protection system fails at a junction of different systems during a fire.
• Examples of the problems that can occur at junctions include the expansion of an intumescent coating dislodging the fixings of an adjoining board, or the close fitting of a board preventing the expansion of the intumescent coating.
• It is an unfortunate fact-of-life that many specialist contractors, and even some Building Control Officers, have chosen to ignore junctions between different fire protection systems as an issue despite some evidence that failure at the junctions may have been a contributory cause of the early failure of the protection system during a fire.

Workmanship

• It cannot be over-emphasised that a fire protection system is only as good as its "weakest link" and a high quality of workmanship is absolutely essential. This is even more important for longer protection times (60 minutes and over) where the inevitable movement and distortion of the structure will expose poor workmanship and inadequate fixings. There have been a number of well-documented examples of structures failing as a result of the fire protection system (usually boards) falling off the element that they were supposed to be protecting long before the end of the intended fire resistance period.
• Some damage during the construction process is almost inevitable and manufacturers provide proprietary remedial products for their systems. Ensuring that these products are applied as intended is crucial to maintaining the integrity of the system. It has been rumoured that poor workmanship in applying the remedial treatment at beam/ column junctions damaged during construction was a contributory factor to the early collapse of the World Trade Centre towers.

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