Section 4. Structural Fire Precautions

4.1 General

Structural fire precautions include the measures introduced to:

i) ensure that the safety of persons including fire fighters carrying out search and rescue is not threatened by a premature collapse of the structure

ii) provide protection to routes that persons may use in the event of a fire to reach a place of safety

iii) contain the fire to the area of origin, or to restrict its uncontrolled spread and thereby reduce the consequential losses in terms of damage to the structure, product damage or loss and disruption to ongoing production

iv) restrict the unseen spread of fire in cavities and voids

v) prevent the spread of fire in certain situations from one building to another, either on the same site or an adjacent sites.

The internal walls and ceilings of a single storey, temperature controlled structure may require no fire resistance for legislative purposes. This document indicates measures to address the commercial consequences of fire, in terms of business interruption and loss of stock etc., that are a concern to legislation.

This chapter is designed to inform the lay operator of the nature of both the regulatory and the optional aspects of structural protection and is primarily designed for those contemplating the construction of new premises or about to undertake a major refurbishment.

4.2 Protecting the Structural Frame

Certain walls and floors in buildings will often be designated as fire resisting, either for the purpose of forming protected routes, or for fire separating or compartmenting parts of the building from each other in order to restrict fire spread or to isolate risks. These elements often rely upon the structure for support, and it is vital in such cases that the structure does not collapse prematurely. Firefighting can also be severely hampered by an unstable structure. It is therefore important that the structure is sufficiently able to resist fire exposure and prevent premature collapse in the event of a fire by applied protection if necessary, both with respect to the subsequent protection to fire fighters and limitation of damage to property and contents. The ability to resist fire exposure is adjudged by evaluation the loadbearing capacity of the element by means of BS 476: Part 21.

In respect of life safety, predictable structural fire behaviour is more critical in multi-storey buildings than in single storey and, as a consequence, the recommendations for single storey buildings are much less stringent. When an external wall is near a site boundary, some or all of it is likely to need a degree of fire resistance in order to avoid fire spread to the adjoining site. It is important that the structural elements supporting this wall have sufficient fire resistance, whether inherently due to their method of construction, or by means of suitably applied protection.

For fire compartment walls that derive support from the structure, it is vital that these structural members are either adequately protected or are inherently stable.

In general, roof members are not required by legislation to have any level of fire resistance. However it may be desirable for roof coverings to be so constructed, that they collapse in a controlled manner in order to vent the fire. In addition, in the interests of business continuity, some protection to the structural frame and plant/equipment support structure may also be desirable. Protection for the structural frame may also be required when the frame members are used as part of the construction of internal mezzaines.

It should be noted that such movement can adversely affect the behaviour of cavity barriers, if they have not been designed to accommodate such movement. Failure of cavity barriers can be very serious in respect of both loss of life or damage.

Illustration shows robust shielding to the fire protection around a steel column.

Recommendations

• Regulations are not concerned with property protection, and a single storey structure conforming to regulations may be vulnerable to serious damage by a fire. Depending upon the property/business protection strategy, a degree of additional structural protection may be considered desirable.

• Any applied fire protection, i.e. boards, sprays or intumescent paint should be inspected and maintained regularly.

• Insulated sandwich panels should not be subjected to any load, either in the cold state or as a result of thermal distortions in the structure, unless designed to take them.

• Steelwork used for supporting suspended ceiling panels from the main structure would benefit from being provided with 30 minutes fire protection in order to reduce deflection of these panels.

• Any internal framework erected to support plant and equipment, e.g.. chillers and circulating fans, particularly above internal envelopes or which supports a mezzaine, should be fire protected for a 30 minute period.

4.3 Walls - Internal and External

4.3.1 Surface Spread of Flame

The surface of flame spread properties of wall and ceiling linings are controlled under the Building Regulations. The normal standard for rooms is class 1, and for circulation spaces it is Class O. Class 1 is determined by evaluating the construction against the conditions and criteria laid down in BS476:Part 1, and Class O by means of BS476:Part 6 and 7, according to criteria given in the appropriate national Building Regulations (see note). Class 1 lined panels can consist of a combustible core protected by a thin metal skin. If the core is exposed, e.g. by surface damage or poor joint details, the actual flame spread properties of the panel may be very significantly impaired.

Note: Class O can be achieved by any material or the surface of a composite produce which:
i) is composed of materials of limited combustibility
ii) class 1 materials which have an index performance 1 of not more than 12,
 and a sub index of not more than 6.

Recommendations

• No wall shall be replaced or re-lined with walls or facings with a poorer surface spread of flame rating than that recommended under building regulations.

• Any applied surface treatment should be inspected and maintained regularly.

• The design of panel joints should be taken into accounts, (i.e. whether combustible cores are visible or poorly sealed), when assessing the surface spread of flame characteristics of a panel lining.

• Care should be exercised to ensure that combustible cores are not left exposed following servicing or maintenance work.

4.3.2 Fire Resistance

The fire resistance of separating elements such as walls in adjudged by means of BS476:Part 22, in the case of non-loadbearing walls, and part 21 if loadbearing. There are two relevant failure criteria for separating elements, e.g. integrity, where flaming takes place or gaps open up, and insulation, where the unexposed face temperature exceeds a mean temperature rise of 140°C or a maximum temperature rise of 180°C.

There are very few walls that are legislatively recommended/required to be fire resistant in a single storey industrial or storage building. Those walls that include external walls, that are situated within the boundary zone (up to 25m), and which cannot be considered to be unprotected, or those which are constructed to provide a protected route for escape purposes, or those for separating ancillary or high risks areas from other areas.

Insurance considerations will generally require storage to be fire separated from food processing etc. by 'compartment' walls. From the point of view of the owner/manager of a food processing or even a storage building there may be some advantage in separating areas of risk away from the other areas by the provision of additional fire resiting barriers. Such protection would limit the area involved in the early stages of a fire, aid fire fighting, and hopefully contain the fire to the enclosure of origin.

The need to increase the fire separation should, however, strictly reflect the risk of fire occurring and the resultant consequences. If there is a low risk of fire occurring (as is the case in a cold store or chilled area), then the need for such separation is accordingly low.

A cooking area would generally be considered a high risk, but it may not need to be considered high risk, if all equipment is protected by its own well maintained fire suppression systems, is regularly cleaned and complies with the recommended 'equipment to structure' distance recommendations given in 3.3.4.

Similarly, if the areas that are separated from the risk has a low loss potential in respect of both life safety and product losses or production disruption, then the need for fire resistant separation is not so great. Each case needs to be assessed on its merits, and any recommendations from the company's fire insurers, fire prevention officers need to be considered.

Where internal envelopes or rooms contain hazardous processes, which do not comply with the recommended positioning/protection or suppression recommendations given in clause 3.3.4, then the bounding structure should be able to provide an integrity rating of at least 30 minutes.

The level of compliance with the insulation criteria on the unexposed side may however be relaxed from the levels given above, depending upon the risk of fire spread that exists in practice. This will be dependant upon the volume and proximity of combustible materials in the protected areas. It may be that the radiation emitted by the wall gives a truer measure of the fire risk. Any relaxation needs to be confirmed by a suitably qualified fire safety engineer.

Walls will only be effective in containing fire and/or smoke, if the methods of joining these elements to each other or to ceilings above to form the structure reflect the anticipated levels of distortion under the appropriate fire exposure conditions. Most sandwich panels would generate high levels of distortion regardless of their composition, primarily due to the different in temperature between the exposed and unexposed faces. Delamination of the hot face may reduce the differential movement, but may be just as serious a problem.

Fryer/oven location within an insulated structure

(see also recommendations in Paragraph 3.3.4)

All walls within 3m footprint (indicated by dashed line) to be metal faced and

inherently fire resistant. Also shown is the minimum distance of cooking equipment from walls and ceilings.

Recommendations

• Where the risk of fire is higher than normal, e.g. in areas where cooking takes place without adequate suppression or spatial separation, or where the consequences of fire spread are high, then sub-compartmentation or separation, above the levels called upon in regulations is strongly recommended.

• Where additional fire resisting walls are introduced in order to provide enhanced property protection, then the duration of fire resistance does not need to be that required by regulations, but should reflect the actual requirements.

• The criterion of failure may be different from those given in BS476:Part 22, but should be chosen on the basis of the risk of fire spread.

• The jointing of any sandwich panels, to each other, in plane or at right angles, should be taken into account the anticipated fire generated distortions. The panel jointing system should be designed to contain the fire or smoke when distorted. Expert advice may be available from the panel manufacturer.

• Insulating compartment walls as defined in 1.2 should be adequately supported by inner masonry wall or by fire protected structural steel frames to ensure that panel bowing does not compromise the fire compartmentation.

4.4 Ceiling Membranes

4.4.1 General

Many panelled ceilings are there solely for the purpose of maintaining the environment conditions and do not require any fire resistance rating. In some cases a fire rated ceiling will inhibit venting, whereas a non-fire rated ceiling may collapse, or partially collapse, reducing lateral spread. On the other hand, fire resisting ceilings can protect services and plant above, as well as provide protection to the buildings structure and prevent fire reaching roof cavities

4.4.2 Surface spread of flame

Where a void exists above a ceiling membrane it should not assist the spread of fire in this void. It is normal for this surface to have a class 'O' rating. The room surface of any ceiling panels, whether fire resisting or not, should have the same surface spread of flame as the walls forming the room or inner envelope.

Recommendations

• The recommendations in 4.3.1 also apply to ceiling membranes,
  but in addition: - Dust and combustible rubbish which could contribute to fire spread should not be allowed to build up on top of ceiling membranes.

4.4.3 Fire resistance

When fire in a high risk area is to be contained, either the walls bounding the area have to extend unbroken to the roof, or the area needs to be capped-off with a fire resisting ceiling. This ceiling may itself be vented to aid removal of fire effluent gases through purpose designed ducts. Such a ceiling needs to be through purpose designed ducts. Such a ceiling needs to be evaluated as a ceiling membrane according to BS476:Part 22, although if it is carrying heavy plant, it may be more appropriate to evaluate it as a loadbearing floor to BS476:Part 21. Ceiling membranes are not subject to any limiting deflection criteria when tested, although it may be sensible to set deflection limits that do not compromise the performance of any cavity barriers fitted in the void above, when specifying such ceilings. The suspension system may need to be fire protected in order to control deflection of the horizontal panels when exposed to a fire in the cavity above. If the ceiling membrane is to prevent a cavity fire spreading downwards, then the influent of light fittings and services will require expert consideration before specifying, as there are not appropriate standard tests.

There is always the likelihood of combustible materials being on the surface of horizontal panels, and, as a consequence, any proposed relaxation of the insulation criteria should only be undertaken with care.

Recommendations

• Fire resisting ceilings should be specified when a high fire risk or fire load is to be contained, and where protection may need to be provided to protect essential plant and equipment above.

• Non-loadbearing fire resisting ceilings should be tested as ceiling membranes to BS476:Part 22, and loadbearing fire resisting ceilings as floors to BS476: Part 21.

• Ceiling membranes may need to be selected, taking into account deflection if such deflection could render cavity barriers ineffective.

• Ceiling membranes may also need some fire separating ability when fire is in the cavity above.

• The suspension system may need to be fire protected in order to avoid excessive deflection in respect to a cavity fire.

• Lighting units and cabling, that do not compromise the fire performance of the ceiling membrane, shall be selected.

4.5 Openings Through Walls & Ceilings

4.5.1 General

Openings through walls and ceilings, for access, vision or the provision of services, can be a major route of fire and smoke spread, whether the wall/ceiling are fire resisting or not.

There is much to be gained in respect of reducing the rate of spread and containing the fire to the areas of origin for as long as possible. Ill-fitting fire resisting doors, non-fire resisting glazed openings and unprotected, unsealed holes around pipes and cables all contribute to the loss of containment and the risk of enhanced fire spread.

Recommendations

• When openings are made in the elements bounding a temperature controlled space, it is important to maintain the continuity of any vapour barriers. Further information on this subject is to be found in the IACSC Guide on the design and construction of insulated, temperature controlled envelopes.

4.5.2 Openings in Non fire rates Walls and Ceilings

There are two aspects of fire risk associated with openings in non-fire resisting walls and ceilings:

i) The spread of smoke during the developing stages of a fire development.

ii) In the case of walls containing significant volumes of combustible materials, (e.g. sandwich panels containing combustible cores such as polystyrene, polyurethane and cork), the ignition of the core material results in enhanced fire lads and fire spread in the cavities behind the outer facing.

In respect of ii) above, if the combustible core material is about to be treated, such that it becomes flame retardant, or is inherently flame retardant, then this reduces the risk of the core becoming ignited by small ignition sources.

Any penetration must not allow the fire to attack any combustible core within a panel prematurely.

Recommendations

• Any potentially combustible material used for the core of sandwich panel construction should be of flame retardant treated grade if possible. Panels made from flame retardant cores should be suitably marked.

• The joint between any panel, which is combustible throughout or has combustible facings, and any non-combustible door or window frame, duct, pipe etc. shall be sealed with a high temperature sealant (e.g.. silicone) to prevent the early release of combustible vapours.

• The aperture formed in a combustible cored panel to accept a combustible or low melting point component, e.g.. cable, plastic pipe or lighting unit, should be lined with a suitable non-combustible, insulating lining material, and the joint should be sealed with an instrumescent sealant that has demonstrated its ability to seal such a joint.

• Any damage to a wall lining is to be repaired immediately so that non of the combustible core is left exposed. Operators of mechanical handling equipment should be encouraged to report accidents immediately and accidental damage, so caused, should not normally be the subject of disciplinary action.

4.5.3 Openings in Fire-rated Walls and Ceilings

The fire resistance of any wall or ceiling is only as good as its weakest parts. Apart from the junction between the barrier and the adjacent structure, any failure is most likely to occur at door openings, glazed openings, or at positions where services penetrate the construction. It is important that all these components have an equivalent level of fire protection, or are sealed in such a manner, that the fire cannot exploit the junctions. If the barrier is of a sandwich construction, large deflections/distortions may be expected during fire exposure, and the fire resistance of the components and seals needs to be tested or assessed, as being able to provide the necessary level of fire separation under such levels of distortion.

Note: Most fire resisting doors and glazing will have been tested in brickwork or blockwork walls, which provide significant restraint to the door frame, but which will not be present in many panel walls. Any reduction in restraint may reduce the performance significantly, and needs to be taken into account when selecting/specifying doors and glazing.

Recommendations

• Any component, e.g.. window, door, lighting unit installed in a wall or ceiling designed to contain a fire, shall have a similar level of fire resistance expressed in terms of integrity and insulation.

• Where radiation from a glazed window is not going to cause ignition of adjacent materials, then glazed components need only satisfy integrity.

• All fire doors or glazing shall have demonstrated their fire resistance, in a similar form of construction (see note) or have been assessed by a competent body as being able to do so.

• Any pipe, duct or trunking penetrating these walls shall maintain the fire resistance and shall comply with the insulation criteria for at least 500mm on the protected side.

• Panels should incorporate blanked-off pre-protected openings to allow services to be added without the need to cut new holes, and these should be sited as close to floor levels as is commensurate with the routing of the service.

4.6 Cavity Barriers

Many cold stores and food processing buildings have large roof cavities to accommodate the cold air circulation systems. Cavities in buildings, particularly those with false ceilings and floors create a very real danger to fire fighters, and can lead to fast and unacceptable levels of fire spread. Once fire enters these voids it can spread unseen, and the void should be broken up by cavity barriers. Since they cannot readily be seen, especially during a fire, much trust has to be put into the ability of barriers to perform, and therefore their specification and installation needs to be carefully considered. These cavity barriers need to be able to accommodate the anticipated fire generated movements. Cavity barriers range from fire resisting fabrics, mineral wool quilts to solid walls.

Where it may be anticipated that a cavity may be reduced in a fire as a result of differential distortion in the adjacent structural membrane, then a flexible barrier may cope better than a rigid barrier. Where the gap may be expected to open up, then the flexible barrier will invariably be too weak to resist movement and may be rendered ineffective without 'ties' (structural tension members), which may prevent the gap from becoming larger.

Recommendations

• Large roof voids should be sub-divided by cavity barriers.

• Where the dimensions of the cavity are expected to change during a fire, due to distortion of structural members of differential expansion, then the barrier must be able to maintain the fire separation despite the movement.

• The risk of either the structure or other services igniting on the protected side of the cavity barrier will need to be considered, when specifying barriers that do not satisfy the insulation criterion, or which may be permeable when exposed to pressurised fire gases.

• The sealing between the barrier and the structure must also take into account anticipated movements. Fire protection of the support system may reduce the distortion.