B3 Internal fire spread (structure)
(1) A building shall be so designed and constructed that, in the event of fire, its stability will be maintained for a reasonable period.
(2) a. A wall common to two or more buildings shall be so designed and constructed that it offers adequate resistance to the spread of fire between those buildings.
b. A building shall be sub-divided with fire resisting construction where this is necessary to inhibit the spread of fire within the building.
(3) A building shall be so designed and constructed that the unseen spread of fire and smoke within concealed spaces in its structure or fabric is inhibited where necessary.
(4) For the purposes of sub-paragraph 2(a), a house in a terrace and a semi-detached house are each to be treated as being a separate building.
Performance
The requirements of B3 may be met:
a. if the structural elements of the building are capable of withstanding the effects of fire for an appropriate period without loss of stability,
b. if the building is sub-divided by elements of fire resisting construction into compartments,
c. if any openings in fire separating elements are suitably protected in order to maintain the fire integrity of the element, and
d. if any hidden voids in the construction are sealed and subdivided to inhibit the unseen spread of fire and products of combustion, in order to reduce the risk of structural failure and the spread of fire, in so far as they pose a threat to the safety of people in and around the building.
The extent to which any of these measures are necessary is dependent on the use of the building and, in some cases its size, and on the location of the element of construction.
3.0 Introduction to Provisions
3.0.1 Provisions for loadbearing elements of structure are given in sub-section 3.1. Sub-section 3.2 is concerned with the subdivision of a building into compartments, and sub-section 3.3 makes provisions about concealed spaces (or cavities). Sub-section 3.4 gives information on the protection of openings and on fire-stopping which relates to compartmentation and to fire spread in concealed spaces. Sub-section 3.5 is concerned with special provisions which apply to certain types of buildings. Common to all these sub-sections and to other provisions of this Document is the property of fire resistance.
Fire Resistance
3.0.2 T he fire resistance of an element of construction is a measure of its ability to withstand the effects of fire in one or more ways:
resistance to collapse, i.e. the ability to maintain loadbearing capacity (which applies to loadbearing members only);
resistance to fire penetration, i.e. an ability to maintain the integrity of the element (which applies to fire-separating elements);
resistance to the transfer of excessive heat, i.e. an ability to provide insulation from high temperatures (which applies to fire-separating elements).
Provisions Elsewhere in Technical Guidance Document B concerning Fire Resistance
3.0.3 T here are provisions in Section B1 concerning the use of fire-resisting construction to protect means of escape.
There are provisions in Section B4 for fire resistance of external walls to restrict the spread of fire between buildings.
There are provisions in Section B5 for fire resistance in the construction of fire-fighting shafts.
Appendix A gives information on methods of test and performance for elements of construction.
Appendix B gives information on fire-resisting doors.
Material Alteration
3.0.4 In the case of a material alteration of a building and where a material change of use of the building is not also involved, the provisions in relation to the fire resistance for loadbearing elements of Structure (See 3.1) contained in any Guide or C ode of Practice, published by the Department of the Environment, Heritage and Local Government for the purpose of section 18(2) of the Fire Services Act, 1981, for the type of premises to which the material alteration is carried out, may be used as an alternative to the relevant provisions of this Technical Guidance Document.
Definitions
3.0.5 The following definitions apply specifically to Section B3. O ther terms applicable more widely throughout this Document are given in Appendix D.
Appliance ventilation duct A duct provided to convey combustion air to a gas appliance.
Cavity barrier A construction provided to close a concealed space against penetration of smoke or flame, or provided to restrict the movement of smoke or flame within such a space.
Concealed space (cavity) A space enclosed by elements of a building (including a suspended ceiling) or contained within an element, but not a room, cupboard, circulation space, protected shaft or space within a flue, chute, duct, pipe or conduit.
Compartment A building or part of a building, comprising one or more rooms, spaces or storeys, constructed to prevent the spread of fire to or from another part of the same building, or an adjoining building.
Compartment wall or floor A fire-resisting wall or floor used in the separation of one fire compartment from another.
Fire stop A seal provided to close an imperfection of fit or design tolerance between elements or components, to restrict or prevent the passage of fire and smoke.
Pipe Includes: pipe fittings and accessories; excludes: a flue pipe and a pipe used for ventilating purposes (other than a ventilating pipe for an above ground drainage system).
Platform floor (access or raised floor) A floor supported by a structural floor, but with an intervening concealed space which is intended to house services.
Separating wall A compartment wall used to separate one building from another, which is the full height of the buildings and is in a continuous vertical plane.
Provisions meeting the Requirement
3.1 Loadbearing Elements of Structure
Introduction
3.1.1 Premature failure of the structure can be prevented by provision for loadbearing elements of structure to have a minimum standard of fire resistance, in terms of resistance to collapse or failure of loadbearing capacity.
The purpose in providing the structure with fire resistance is threefold:
to protect the occupants, some of whom may have to remain in the building for some time while evacuation proceeds, if the building is a large one;
to protect fire fighters who may be engaged in search or rescue operations (though this is limited and is not intended to cover fire-fighting operations generally);
to reduce the danger to people in the vicinity of the building who might be hurt by falling debris or by the impact of the collapsing structure on other buildings.
Fire Resistance Standard
3.1.2 Structural frames, beams, columns, loadbearing walls (internal and external), floor structures and gallery structures, should have at least the fire resistance given in Appendix A, Table A1.
Application of the Fire Resistance Standards for Loadbearing Elements
(see Appendix A, Tables A1 and A2)
3.1.3 The measures set out in Appendix A include provisions to ensure that where one element of structure supports or gives stability to another element of structure, the supporting element has no less fire resistance than the other element.
The measures also provide for elements of structure that are common to more than one building or compartment, to be constructed to the standard of the greater of the relevant provisions. Special provisions about fire resistance of elements of structure in single storey buildings are also given, and there are concessions in respect of fire resistance of elements of structure in basements where at least one side of the basement is open at ground level. See "Application of the fire resistance standard in Table A2" in Appendix A.
Exclusions from the Provisions for Elements of Structure
3.1.4 The following are excluded from the definition of elements of structure for the purposes of these provisions:
a. a structure that only supports a roof, unless the roof performs the function of a floor, e.g. for parking vehicles, or as a means of escape (see B1) or is essential for the stability of an external wall which is required to have fire resistance (see B4);
b. the lowest floor of the building; and
c. a platform floor.
Additional Provisions
3.1.5 Additional provisions are required if a loadbearing wall is also:
a. a compartment wall (see 3.2);
b. a wall between a house and a small garage (see 3.2, par. 3.2.4.2);
c. protecting a means of escape (see B1, par. 1.4.2);
d. an external wall (see B4, sub-section 4.1 and 4.2);
e. enclosing a firefighting shaft (see B5, sub-section 5.3).
If a floor is also a compartment floor, see sub-section 3.2.
Floors in Domestic Loft Conversions
3.1.6 In altering an existing two storey single dwelling house to provide additional storeys, the floor(s), both old and new, should have the full 30 minute standard of fire resistance shown in Appendix A, Table A1. However, where only one new storey not exceeding 50 m2 in area and containing one or two habitable rooms is added in the roof space of a two storey dwelling house, it would be reasonable to provide the existing first floor with a modified 30 minute standard of fire resistance where:
a. the floor only separates rooms; and
b. the provisions in 1.5.7 of Section B1 are met.
‘Modified 30 minute’ standard satisfies the test criteria for the full 30 minutes in respect of loadbearing capacity, but allows reduced performances for integrity and insulation (see Table A1, item 3(a)).
Raised Storage Areas
3.1.7 Raised floors (which may be supported by racking) are frequently erected in single storey industrial or storage buildings. W hether the structure is considered as a gallery or is of sufficient size that it is considered as a floor forming an additional storey, the provisions for fire resistance of elements of structure set out in Appendix A, Table A1, would apply to the structure.
In the case of automated storage systems in which people do not normally go onto any of the raised storage tiers, it may not be necessary to provide the storage structure with fire resistance to ensure the safety of occupants of the building.
Where people can go onto the storage tiers in the course of their normal use, reduced levels of fire resistance in the construction of the raised storage platforms (or even little fire resistance so as to permit an unprotected steel structure) may also be acceptable provided the following conditions are satisfied:
a. the structure has only one tier and is used for storage purposes or access to plant or machinery only;
b. the number of persons likely to be on the floor at any one time is low and does not include members of the public;
c. the layout is such that any persons on the floor would be readily aware of any fire starting at the lower level; and
d. at least one stairway serving the raised floors, platform or tier discharges within 3 m of an exit from the building.
Features of layout or design that would allow occupants to be aware of a fire starting at the lower level include the use of perforations in the floor of the structure, or leaving a space between the edge of the platform and the walls of the room housing it, to make the smoke and the sounds of the fire obvious. If the floor is more than 10 m in width or length, an automatic fire detection and alarm system should be used to provide sufficiently early warning.
3.2 Compartmentation
Introduction
3.2.1 The spread of fire within a building can be restricted by sub-dividing it into compartments separated from one another by walls and/or floors of fire-resisting construction. The object is twofold:
a. to prevent rapid fire spread which could trap occupants of the building; and
b. to reduce the chance of fires becoming large, on the basis that large fires are more dangerous, not only to occupants but to people in the vicinity of the building.
Compartmentation is complementary to provisions made under Section B1 for the protection of escape routes, and to provisions made under Section B4 against the spread of fire between buildings.
The degree of sub-division that is appropriate depends on:
a. the use of the building, which affects the potential for fires and the severity of fires, as well as the ease of evacuation;
b. the height of the top storey in the building, which is an indication of the ease of evacuation and the ability of the fire service to intervene effectively; and
c. the availability of a fire suppression system which affects the growth rate of the fire, and may suppress it altogether.
Forms of Compartmentation
3.2.2 Sub-division is achieved using compartment walls and compartment floors, and provisions for their construction are given in pars. 3.2.5 et seq. These construction provisions vary according to the function of the wall or floor.
Special forms of compartmentation to which particular construction provisions apply, are:
a. walls common to two or more buildings (separating wall);
b. walls dividing buildings into separated parts (in which the parts can be assessed independently for the purpose of determining the appropriate standard of fire resistance); and
c. walls protecting houses from attached or integral small garages.
Junctions and Protected Shafts
3.2.3 For compartmentation to be effective, there should be continuity at the junctions of the fireresisting elements enclosing a compartment, and any openings from one compartment to another should not present a weakness.
Spaces that connect compartments, such as stairways and service shafts, need to be protected to restrict fire spread between the compartments, and they are termed protected shafts.
Provision of Compartment Walls and Compartment Floors
3.2.4 Compartment walls and compartment floors should be provided in the circumstances described in pars. 3.2.4.1 to 3.2.4.7 below, with the provision that the lowest floor in a building does not need to be constructed as a compartment floor.
Provisions for the construction of compartment walls and compartment floors in different circumstances are given in 3.2.5.
Provisions for the protection of openings in compartment walls and compartment floors are given in 3.2.6 and 3.2.7.
Diagram 11 illustrates schematically various forms of compartmentation and indicates the paragraph references for the relevant guidance.
Diagram 11 Compartment walls and compartment floors
3.2.4.1 All purpose groups The following guidance applies to all buildings and all purpose groups:
a. A wall common to two or more buildings(separating wall) should be constructed as a compartment wall;
b. Compartment walls and/or compartment floors should be provided to separate parts of a building that are occupied mainly for different purposes (see 0.3.2 and T able 0.1), or by different tenancies, from one another;
c. Where part of a building is used for a purpose that is ancillary to another purpose (see 0.3.2), it need not be separated by compartment walls and/or floors, unless that part is a place of special fire risk (see 1.0.9);
d. Compartment walls and/or floors should be provided to separate places of special fire risk (see 1.0.9) from other parts of a building.
Note: Many of the codes of practice and other documents referred to in Section B1 (1.1) for the purpose of the means of escape, identify specific areas of ancillary accommodation that are regarded as places of special fire risk. These areas should also be separated by way of compartmentation in accordance with the recommendations contained in those documents.
3.2.4.2 Dwelling houses Any wall separating semi-detached dwelling houses, or houses in a terrace, should be constructed as a compartment wall, and the dwelling houses should be considered as separate buildings.
If a small garage is attached to (or forms part of) a dwelling house, the garage should be separated from the rest of the house as indicated in Diagram 12.
3.2.4.3 Flats and maisonettes In buildings containing flats (purpose group 1(c)) the following should be constructed as compartment walls or compartment floors:
a. any floor (unless it is within a maisonette, i.e.between one storey and another within one dwelling), and
b. any wall separating a flat from any other part of the building, and
c. any wall enclosing a refuse storage chamber.
Diagram 12 Separation between garage and dwelling house
3.2.4.4 Residential (Institutional) buildings The following walls and floors should be constructed as compartment walls and compartment floors in Residential (Institutional) buildings (Purpose Group 2(a)):
a. any floor, other than the floor of the lowest storey;
b. any wall needed to sub-divide a storey to observe the limits for compartment sizes outlined in Table 3.1;
c. any wall needed to sub-divide a storey into at least two compartments for the purpose of progressive horizontal evacuation (see 1.2.7).
3.2.4.5 Other residential buildings The following walls and floors should be constructed as compartment walls and compartment floors in Other Residential buildings (Purpose Group 2(b)):
a. any floor, other than the floor of the lowest storey;
b. any wall needed to sub-divide a storey to observe the limits for compartment sizes outlined in Table 3.1.
3.2.4.6 Non-residential buildings The following walls and floors should be constructed as compartment walls and compartment floors in buildings of a non-residential purpose group (i.e. Office, Shop, Shopping centre, Assembly & Recreation, Industrial, Storage or Other Non-
Residential):
(a) any wall/floor needed to sub-divide the building to observe the size limits on compartments given in Table 3.1;
(b) any floor if the building, or separated part of the building, has a storey with a floor at a height of more than 30 m above ground level;
(c) the floor of the ground storey if the building has one or more basements;
(d) any basement floor if the building, or separated part, has a basement at a depth of more than 10 m below ground level; and
(e) in the case of a shopping centre, any walls or floors required to comply with the provisions outlined in 3.2.4.7 below.
3.2.4.7 Shopping centres (Purpose Group 4(b)) Shopping centres require special provisions in relation to compartmentation, because of the nature of the occupancies and the arrangement of individual units that make up these buildings and the fact that enclosed pedestrian mall areas are generally provided between individual units. While the majority of units comprise of individual shops of varying sizes, other uses such as assembly and recreation or offices may also be provided in these buildings. In addition, certain ancillary accommodation, such as service and storage areas present particular high fire risk and require compartmentation.
Compartmentation in shopping complexes should comply with the recommendations outlined in section 5 of BS 5588 Fire precautions in the design, construction and use of buildings, Part 10 C ode of practice for shopping complexes.
Special provisions relating to shopping centres generally, concerning structural fire protection, are outlined in 3.5.3.
Table 3.1 Maximum area and cubic capacity of a building or compartment
Construction of Compartment Floors and Compartment Walls
3.2.5 Every compartment wall and compartment floor should:
a. form a complete barrier to fire between the compartments they separate;
b. have the appropriate fire resistance as indicated in Appendix A, Tables A1 and A2; and
c. be constructed in accordance with the relevant guidance in the following pars. 3.2.5.1 to 3.2.5.11.
The performance requirements for fire resisting elements of construction are indicated in Appendix A, paragraph A5.
3.2.5.1 Compartment floors in residential (Institutional) buildings In a building of Purpose Group 2(a), Residential (Institutional), all compartment floors should be constructed of noncombustible materials (see Appendix A, Table A8).
An exception to the requirement for noncombustible compartment floors may be made in the case of a material alteration of an existing Residential (Institutional) building.
3.2.5.2 Compartment floors in high buildings In a building of any purpose group, where the height of the top storey is 10 m or more (see Appendix C , Diagram 38), any compartment floor which is required to have a fire resistance of 60 minutes or more should be constructed of non-combustible materials (see Appendix A, Table A8), apart from any floor finish.
An exception to the requirement for noncombustible compartment floors may be made in the case of an existing floor in an existing building used for any purpose other than Residential (Institutional), Purpose Group 2(a).
Note: The non-combustibility requirement does not apply in the case of a material alteration of an existing Residential (Institutional) building (see 3.2.5.1).
3.2.5.3 Fire resistance of timber floors in existing buildings In an existing building it may be possible to increase the fire resistance of existing timber floors. The techniques generally adopted to upgrade the fire resistance of timber floors are as follows:
a. The addition of a fire-resisting layer, or layers, beneath the existing floor joists. There are many techniques and materials available for such purposes. In some cases it is also necessary to provide a protective layer on top of the existing floor-boards or between the floor joists.
b. Filling the voids between the existing floor surface and ceiling below, or between the floor joists, with a suitable material. T here are a number of proprietary systems available which are based on this method. These are often more appropriate than the method outlined at (a) above in buildings of historic or architectural interest, where existing plasterwork is to be retained.
Many of the techniques employed in upgrading timber floors involve the use of proprietary materials and systems. These must be capable of achieving the required performance in the situations for which they are adopted. Particular care and attention to detail in the execution of any such upgrading works is necessary to ensure the required performance.
Table 14 of Building Research Establishment Report "Guidelines for the construction of fire-resisting structural elements (BR 128, 1988)" provides guidance in relation to the construction of fire resisting timber floors. Guidance on fire-resisting timber floors is also available from the Timber Research and Development Association (TRADA) and in the trade literature produced by manufacturers of fire protection materials and products.
Guidance on upgrading the fire resistance of existing timber floors is provided in Building Research Digest number 208 "Increasing the fire resistance of existing timber floors (revised 1988)".
3.2.5.4 Compartment walls In a building of purpose group 2(a), Residential (Institutional) and in a building of any purpose group, where the height of the top storey is 10 m or more (see Appendix C, Diagram 38), a compartment wall which is required to have a fire resistance of one hour or more should be constructed of materials of limited combustibility (see Appendix A, Table A7), apart from any wall surface complying with the requirements of B2, internal fire spread (linings).
3.2.5.5 Separating walls Compartment walls that are common to two or more buildings (separating walls) should run the full height of the building in a continuous vertical plane and should be constructed of non-combustible (see Appendix A, Table A8) materials. Adjoining buildings should only be separated by walls, not floors. A compartment wall being used to divide a building into separate occupancies or uses would not be subject to this provision.
3.2.5.6 Separating walls in dwelling houses An exception to the requirement for separating walls to be constructed of non-combustible materials may be permitted in the case of a separating wall between buildings of Purpose Group 1(a) Residential (Dwellings) of one or two storeys only, where the wall is part of a timber frame construction system consisting only of a structural frame of combustible materials, and the design, materials and workmanship used in the manufacture and construction of the wall are in accordance with the provisions of Technical Guidance Document D (Materials and
Workmanship).
3.2.5.7 Accommodation of Services in Compartment Walls/Floors and Separating Walls The construction of compartment walls and floors, and separating walls, may be such that the achievement of the required fire resistance performance (see Paragraph 3.2.5 and Appendix A) relies primarily on the integrity of the linings of such constructions, e.g. framed structures with walls which incorporate plasterboard or similar fire resistant linings and an enclosed void, hollow floor construction with fire resistance provided by floor deck material and ceiling board. The integrity of the linings of such constructions should not be breached to allow for the installation of services, e.g. pipes, wires, flues (including manufactured flues), except where necessary to allow services pass through these compartment walls or floors. Services may be surface mounted or accommodated in service ducts or within service cavities created external to the unbreached linings of the fire resistant compartment wall or floor. Where services pass through these compartment walls or floors, they should be contained in fire resistant ducts and the opening of such ducts should be protected and fire-stopped in accordance with Section 3.4 of this Document. No services should pass through separating walls of this construction type.
3.2.5.8 Separated parts Compartment walls used to form a separated part of a building (see 3.2.2 (b)) should run the full height of the building in a continuous vertical plane.
3.2.5.9 Junction of compartment wall or compartment floor with other walls Where a compartment wall or compartment floor meets another compartment wall, or an external wall, the junction should maintain the fire resistance of the compartmentation.
There are provisions in Section B1 in relation to the junction of an external wall and the enclosure to a protected stairway (see 1.3.6.6).
Consideration may also be required in the case of junctions of compartment walls and re-entrant or opposing elevations of the same building (see 3.2.5.10).
3.2.5.10 Courtyards, light-wells and opposite elevations of the same building In the case of buildings which contains courtyards, light-wells or wings, the unprotected areas on opposing or adjacent elevations could contribute to the transfer of fire between compartments. An important area in this respect is where a compartment wall meets at the junction formed by the internal angle of two elevations to a building. In this situation, the external walls should be protected on one or both elevations for a sufficient distance to limit the potential for horizontal fire spread between the compartments. In other situations, to limit fire spread between different compartments, it may be necessary to consider the space separation between opposing elevations.
In the case of a Residential (Institutional) building, where the means of escape (see Section B1) have been designed on the basis of progressive horizontal evacuation, special consideration is required for the junctions between compartment walls and the external elevations of the building (see 3.5.4).
3.2.5.11 Junction of compartment wall and roof The junction between a compartment wall and the roof of a building should be capable of restricting fire spread between compartments. A compartment wall should be taken up to meet the underside of the roof covering or deck and fire stopped where necessary at the wall/roof junction. A compartment wall should not be stopped at ceiling level while using a cavity barrier in the roof space to continue the line of compartment to the roof. The construction of the wall, particularly between any ceiling and the roof, should not contain imperfections that would provide a route for fire penetration or premature failure of the fire resistance performance of the wall. T he gap between the wall and the underside of the roof should be as small as practicable (generally not greater than 50 mm) and be filled with suitable fire stopping material over the full width of the wall.
Where structural roof members such as beams, purlins and rafters are built into or carried across a masonry or concrete compartment wall, any openings for them should be as small as practicable and any gaps should be effectively fire stopped with non-combustible fire stopping material over the full width of the wall. Where a trussed rafter supporting a roof is carried across a compartment wall it should be designed so that failure due to fire in one compartment will not cause failure of any part of the truss in another compartment. No structural roof member should be carried across a compartment wall which is also a separating wall.
The design and detailing of the junction between a compartment wall and any roof valley, gutter or other roof configuration, should be carefully considered so as to ensure that a means is not provided at the junction for premature fire spread between compartments.
If a fire penetrates a roof near a compartment wall there is a risk that it will spread over the roof to the adjoining compartment. To reduce that risk, the junction between a compartment wall and a roof should be constructed in accordance with the following:
a. a zone of the roof at 1.5 m wide on both sides of the wall should have a covering of class BRO O F(t4) (European class) or AA, AB or AC (National class) (see table A5 to Appendix A) on a non-combustible (see table A8 to Appendix A) sub-strate or deck. T he compartment wall ,should be carried up to the underside of the roof deck and be fire stopped with resilient noncombustible fire stopping material over the full width of the wall (see Diagram 13(A)).
Note: where the roof deck comprises a double skin insulated sheet, the insulating material directly above the compartment wall should be of limited combustibility (see A18 and Table A7 of Appendix A) for a width of not less than 300 mm which forms a fire stopping seal to the cavity between the inner and outer skins. In the case of a non-insulated double skin roof deck, similar fire stopping should also be provided.
b. in buildings not more than 15 m high (see Diagram 37, Appendix C ) which are used as dwellings, residential (except institutional), offices, or assembly, timber tiling battens or combustible boarding not exceeding 25 mm in thickness used as a substrate to the roof covering may be carried over the compartment wall, provided that they are fully bedded in mortar or other suitable non-combustible fire stopping material for the full width of the wall, and any cavities within the thickness of the roof, above and below the sarking felt or similar membrane along the line of the wall and at the eaves are adequately fire stopped (see Diagram 13(B)).
As an alternative to (a) or (b) above the compartment wall may be extended above the line of the external roof surface by a height of not less than 375 mm to form a parapet wall (see Diagram 13(C )).
As an alternative to (a), (b) or (c) above for any building type, any other system which has been shown by test to be equally effective in restricting the spread of fire at a compartment wall/roof junction may be used.
Diagram 13 Junction of compartment wall with roof
Openings Between Compartments
3.2.6 Any door openings in compartment walls should be protected by means of fire doors, in accordance with the provisions outlined in Appendix B and Table B1. Openings for the passage of pipes, ducts and other services should be protected in accordance with the provisions outlined in sub-section 3.4.
3.2.6.1 Openings in separating walls Any openings in a wall which is common to two or more buildings should be limited to those for:
a. a door which is needed to provide a means of escape in case of fire and which has the same fire resistance as that required for the wall (see Appendix B, Table B1) and is fitted in accordance with the provisions of Appendix B; and
b. the passage of a pipe which meets the provisions in Section 3.4.
3.2.6.2 Openings in other compartment walls or in compartment floors Openings in compartment walls (other than those described in par. 3.2.6.1) or compartment floors should be limited to those for:
a. doors which have the appropriate fire resistance given in Appendix B, Table B1, and are fitted in accordance with the provisions of Appendix B; and
b. the passage of pipes, ventilation ducts, chimneys, appliance ventilation ducts or ducts encasing one or more flue pipes, which meet the provisions in sub-section 3.4; and
c. refuse chutes of non-combustible construction; and
d. protected shafts which meet the relevant provisions below; and
e. Atria designed in accordance with BS 5588 Part7.
Protected Shafts
3.2.7 Any stairway or other shaft passing directly from one compartment to another, should be enclosed so as to delay or prevent the spread of fire between compartments, and is termed a protected shaft.
If the protected shaft is a stairway, see also B1 for provisions relating to protected stairways and B5 if the stairway also serves as a fire-fighting stairway.
3.2.7.1 Construction of protected shafts The construction enclosing a protected shaft (see Diagram 14) should:
a. form a complete barrier to fire between the different compartments which the shaft connects;
b. have the appropriate fire resistance given inAppendix A, Table A1, except for glazed screens which meet the provisions of par. 3.2.7.3;
c. meet the requirements of section 3.2.5 for the construction of compartment walls; and
d. satisfy the provisions about their use, ventilation and the treatment of openings in the paragraphs below.
3.2.7.2 Use for protected shafts The uses of protected shafts should be restricted to stairways, lifts, escalators, chutes, ducts, pipes, and/or as sanitary accommodation and washrooms.
3.2.7.3 Glazed screens to protected shafts Where a protected shaft contains a stairway, but is not a fire-fighting shaft (see B5), and is entered from a protected corridor or lobby, the part of the enclosure between the shaft and the corridor or lobby may incorporate glass so as to provide at least 30 minutes fire resistance in terms of integrity only, if the principles in Diagram 15 and the provisions of Table A4 of Appendix A are met.
Diagram 14 Protected shafts (construction)
Diagram 15 Glazed screen separating protected shaft from lobby or corridor
3.2.7.4 Pipes for oil or gas in protected shafts If a protected shaft contains a stairway and/or lift, it should not also contain a pipe conveying oil or gas (other than hydraulic oil in the mechanism of a hydraulic lift) or contain a ventilating duct (other than a duct provided for the purposes of pressurizing the stairway to keep it smoke free).
3.2.7.5 Ventilation of protected shafts conveying gas A protected shaft conveying piped flammable gas should be adequately ventilated direct to the outside of the building.
3.2.7.6 Openings into protected shafts An external wall of a protected shaft does not normally need to have fire resistance, unless it is a fire-fighting shaft, (see B5), or if it contains a stairway, and the layout is such that it may expose persons to risk. Openings in other parts of the enclosure to a protected shaft should be limited as follows:
a. Where part of the enclosure to a protected shaft is a wall common to two or more buildings (i.e. a separating wall), only the following openings should be made in that wall:
a door which is needed to provide a means of escape in case of fire and which has the same fire resistance as that required for the wall (see Appendix B, Table B1) and is fitted in accordance with the provisions of Appendix B; and
the passage of a pipe which meets the provisions in sub-section 3.4.
b. Other parts of the enclosure (other than an external wall) should only have openings for:
doors which have the appropriate fire resistance given in Appendix B, Table B1, and are fitted in accordance with the provisions of Appendix B; and
the passage of pipes which meet the provisions in sub-section 3.4; and
inlets to, outlets from and openings for a ventilation duct, (if the shaft contains or serves as a ventilating duct) which meet the provisions in sub-section 3.4; and
the passage of lift cables into a lift motor room (if the shaft contains a lift). If the motor room is at the bottom of the shaft, the openings should be as small as practicable.
3.2.7.7 Buildings containing one or more atria Detailed guidance on all issues relating to the incorporation of atria in buildings is given in BS 5588 Part 7 Code of practice for incorporation of atria in buildings. However it should be noted that for the purposes of TGD B, the standard is relevant only where the atrium breaches any compartmentation.
3.3 Concealed Spaces (Cavities)
Introduction
3.3.1 Hidden voids in the construction of a building provide a ready route for smoke and flame spread. This is particularly so in the case of voids above other spaces in a building, e.g. above a suspended ceiling or in a roof space. As the spread is concealed, it presents a greater danger than would a more obvious weakness in the fabric of the building. Provisions are made to restrict this by interrupting cavities which could form a pathway around a barrier to fire, and sub-dividing extensive cavities.
It should be noted that cavity barriers should not be provided above compartment walls as these walls are required to be carried up full storey height, to a compartment floor or to the roof as appropriate, (see 3.2.5.10). The fire resistance standards for cavity barriers are lower than for a compartment wall, and it is important to use a compartment wall in this situation to maintain the standard of fire resistance.
Provision of Cavity Barriers
3.3.2 Cavity barriers should be provided in accordance with Table 3.2 in specified locations for different purpose groups.
The dimensions of concealed spaces should be in accordance with the provisions outlined in 3.3.3.
Diagram 16 illustrates the need for cavity barriers at the intersection of fire resisting construction and elements containing a concealed space.
Diagram 16 Interrupting concealed spaces (cavities) Par. 3.3.2
Table 3.2 Provision of cavity barriers
Diagram 17 Masonry cavity walls excluded from provisions for cavity barriers Table 3.2 Par. 3.3.3
Diagram 18 Roofspaces over protected stairway in dwelling houses of three or more storeys (alternative arrangements)
Diagram 19 Fire resisting ceiling below a concealed space
Diagram 20 Corridor subdivision (alternative arrangements)
Maximum Dimensions of Concealed Spaces
3.3.3 The dimensions of cavities (i.e. the maximum un-divided concealed space) should not exceed those specified in Table 3.3.
Table 3.3 Maximum dimensions of cavities
The provisions of Table 3.3 do not apply to any cavity described at (1) to (8) below:
1) Where any room under a ceiling cavity exceeds the dimensions given, cavity barriers need only be provided on the line of the enclosing walls/partitions of that room - subject to cavity barriers not being more than 40 m apart in any direction and the surface of the material/product exposed in the cavity being C lass C – s3, d2 or better (European class) or C lass 0 or C lass 1 (National classes);
2) Where the cavity is over an undivided area and is used as a plenum and exceeds 40 m (in both directions on plan), there is no limit to the size of the cavity if:
the room and the cavity together are compartmented from the rest of the building;
an automatic fire detection and alarm system is fitted in the building with smoke detectors in the cavity and in the return air ducting, and which stops circulation of the ventilation system and switches it to extract;
The surface of the material or product used in the construction of the ceiling and which is exposed in the cavity is C lass B-s3, d2 or better (European) or C lass O (National) and the supports and fixings of the suspended ceiling in the cavity are of non-combustible construction;
the flame spread rating of any pipe insulation system is C lass C - s3, d2 (European) or C lass 1 (National);
any electrical wiring in the void is laid in metal trays or in metal conduit; and
any other materials in the cavity are of limited combustibility.
3) in a wall which should be fire-resisting only because it is loadbearing;
4) in a masonry or concrete external cavity wall as shown in Diagram 17;
5) in a floor or a roof space where the cavity is enclosed on the lower side by a fire-resisting ceiling (as shown in Diagram 19) which extends throughout the building, compartment or separated part, subject to a limit of 30 m on the extent of any such cavity;
6) below a floor next to the ground or oversite concrete, if the cavity is less than 1 m in height or if the cavity is not normally accessible by persons, unless there are openings in the floor such that it is possible for litter to accumulate in the cavity (in which case cavity barriers should be provided, and access should be provided to the cavity for cleaning);
7)formed by over-cladding an existing masonry (or concrete) external wall, or an existing concrete roof, provided that the cavity does not contain combustible insulation; and
8) between double-skinned corrugated or profiled insulated roof and wall sheeting if the sheeting is a material of limited combustibility and both surfaces of the insulating layer have a surface spread of flame of at least C lass C - s3, d2 European or at least C lass 1 (National) (see Appendix A) and make contact with the inner and outer skins of cladding (see Diagram 21).
Diagram 21 Double-skinned insulated roof and wall sheeting excluded from provisions in Table 3.3
Construction and Fixings for Cavity Barriers
3.3.4 Every cavity barrier should be constructed to provide at least 30 minutes fire resistance (see Appendix A, Table A1, item 16).
Notes:
a. any cavity barrier required in a stud wall or partition may, however, be formed of
steel at least 0.5 mm thick, or
timber at least 38 mm thick, or
polythene sleeved mineral wool, or mineral wool slab, in either case under compression when installed in the cavity.
b. a cavity barrier may be formed by any construction provided for another purpose if it meets the provisions for cavity barriers.
Cavity barriers should be tightly fitted to rigid construction and mechanically fixed in position wherever possible. W here this is not possible (for example, in the case of a junction with slates, tiles, corrugated sheeting or similar materials) the junction should be fire-stopped.
Cavity barriers should also be fixed so that their performance is unlikely to be made ineffective by:
a. movement of the building due to subsidence, shrinkage or thermal change;
b. collapse in a fire of any services penetrating them;
c. failure in a fire of their fixings; or
d. failure in a fire of any material or construction which they abut.
For example, if a suspended ceiling is continued over the top of a fire-resisting wall or partition and direct connection is made between the ceiling and the cavity barrier above the line of the wall or partition, premature failure of the cavity barrier can occur when the ceiling collapses. However, this does not arise if the ceiling is designed to provide fire protection of 30 minutes or more.
Openings in Cavity Barriers
3.3.5 Any openings in a cavity barrier should be limited to those for:
a. doors which have at least 20 minutes fire resistance (see Appendix B, Table B1, item 8(a)) and are fitted in accordance with the provisions of Appendix B; and
b. the passage of pipes which meet the provisions in sub-section 3.4; and
c. the passage of cables or conduits containing one or more cables; and
d. openings fitted with a suitably mounted automatic fire shutter; and
e. ducts which (unless they are fire-resisting) are fitted with a suitably mounted automatic fire shutter where they pass through the cavity barrier.
3.4. Protection of Openings and Fire-Stopping
Introduction
3.4.1 Earlier sections of this Document describe the provision of barriers to fire, and the circumstances in which there may be openings in them. This Section deals with the protection of openings in such barriers.
If an element that is intended to provide fire separation (i.e. it has requirements for fire resistance in terms of integrity and insulation) is to be effective, then every joint, or imperfection of fit, or opening to allow services to pass through the element, should be adequately protected by sealing or fire-stopping so that the fire resistance of the element is not impaired. Building service installations should be designed in accordance with BS 8313: 1989 (section 12). The provisions of this Section also apply to openings in protected corridors which are provided for the protection of the means of escape (see B1).
Provisions for door openings and fire doors are given in Appendix B because they are relevant to B1 and B5, as well as B3.
The measures are intended to delay the passage of fire. They generally have the additional benefit of retarding smoke spread but the test specified in Appendix A for integrity does not stipulate criteria for the passage of smoke as such.
Openings for Pipes
3.4.2 Pipes which pass through a compartment wall or compartment floor (unless the pipe is in a protected shaft), or cavity barrier, should meet the appropriate provisions in alternatives A, B or C below.
Alternative A: Proprietary Seals (any pipe diameter)
Provide a proprietary sealing system which has been shown by test to maintain the fire resistance of the wall, floor or cavity barrier.
Alternative B: Pipes with a restricted diameter
Where a proprietary sealing system is not used, fire stopping may be used around the pipe, keeping the opening as small as possible. The nominal internal diameter of the pipe should not be more than the relevant dimension given in Table 3.4.
T he diameters given in T able 3.4 for pipes of specification (b) used in situation (2) assume that the pipes are part of an above ground drainage system and are enclosed as shown in Diagram 22. If they are not, the smaller diameter given in situation (3) should be used instead.
Alternative C: Sleeving
A pipe of lead, aluminium, aluminium alloy, asbestos cement or uPVC , with a maximum nominal internal diameter of 160 mm, may be used with a sleeving of non-combustible pipe as shown in Diagram 23. The specification for non-combustible and uPVC pipes is given in the notes to Table 3.4.
Ventilating Ducts
3.4.3 Ventilation and air conditioning ducts which pass from one compartment to another should be protected in accordance with the recommendations contained in BS 5588: Part 9 C ode of practice for ventilation and air conditioning ductwork.
Flues, etc.
3.4.4 If a flue, or duct containing flues or appliance ventilation duct(s), passes through a compartment wall or compartment floor, or is built into a compartment wall, the walls of the flue or duct should have a fire resistance of at least half that of the wall or floor in order to prevent the by-passing of the compartmentation (see Diagram 24). The walls enclosing the flue or duct should be of solid non-combustible construction.
Fire-Stopping
3.4.5 In addition to any other provisions in this document for fire-stopping:
a. joints between elements which serve as a barrier to the passage of fire should be fire-stopped; and
b. all openings for pipes, ducts, conduits or cables to pass through any part of an element which serves as a barrier to the passage of fire should be -
kept as few in number as possible, and
kept as small as practicable, and
fire-stopped (which in the case of a pipe or ducts, should allow thermal movement).
To prevent displacement, materials used for fire stopping should be reinforced with (or supported by) materials of limited combustibility in the following circumstances:
a. in all cases where the unsupported span is greater than 100 mm, and
b. in any other case where non-rigid materials are used (unless they have been shown to be satisfactory by test).
Proprietary sealing systems (including those designed for service penetrations) which have been shown by test to maintain the fire resistance of the wall or other element are available.
Other suitable fire-stopping materials include:
cement or lime mortar,
gypsum based plaster,
cement or gypsum based vermiculite/perlite mixes,
glass fibre, mineral wood, crushed rock, blast furnace slag or ceramic based products (with or without resin binders),
intumescent mastics.
These may be used only in situations which are suitable for the particular fire-stopping materials.
Table 3.4 Maximum nominal internal diameter of pipes
Diagram 22 Enclosures for drainage or water supply pipes
Diagram 23 Pipes penetrating a compartment wall or floor (sleeving)
Diagram 24 Flues and compartment walls or floors
3.5 Special Provisions
Introduction
3.5.1 T his Section describes additional considerations which apply to the design and construction of Car Parks, Shopping Centres, Residential (Institutional) buildings and buildings containing an atrium.
Car Parks
3.5.2 Buildings or parts of buildings used as parking for cars and other light vehicles are unlike other buildings in certain respects, and merit some departures from the usual provisions for the restriction of fire spread within buildings because:
a. The fire load is well defined and not particularly high;
b. Where the car park is well ventilated, there is a low probability of fire spread from one storey to another. Ventilation is the important factor, and as heat and smoke cannot be dissipated so readily from a car park that is not open-sided fewer concessions are made. The guidance in paragraphs 3.5.2.2 to 3.5.2.5 is concerned with three ventilation methods; open-sided (high level of natural ventilation), natural ventilation and mechanical ventilation.
Note: Because of the above, car parks are not normally expected to be fitted with sprinklers.
3.5.2.1 Provisions common to all car park buildings All materials used in the construction of the building, compartment or separated part should be non-combustible, except for:
a. any surface finish applied:
to a floor or roof of the car park, or
within any adjoining building, compartment or separated part to the structure enclosing the car park,
if the finish meets any relevant provisions in B2 and B4;
b. any fire door; and
c. any attendants kiosk not exceeding 15 m2 in area.
3.5.2.2 Open-sided car parks If the building, or separated part containing the car park, complies with the following provisions (in addition to those in par. 3.5.2.1) it may be regarded as an open-sided car park for the purposes of fire resistance assessment in Appendix A, Table A2, and for the purposes of space separation in B4, Table 4.3, it may be regarded as a small building or compartment.
a. There should not be any basement storeys.
b. Each storey should be naturally ventilated by permanent openings at each level having an aggregate area not less than 5% of the floor area at that level, of which at least half should be in two opposing walls.
c. If the building is also used for any other purpose, the part forming the car park is a separated part (see Appendix D for definition).
3.5.2.3 Car parks which are not open-sided Where it is not possible to provide the degree of natural ventilation set out in par. 3.5.2.2 above, the building is not classified as open-sided and a different standard of fire resistance is required (refer to Table A2, Appendix A). Such car parks require ventilation, which may be by natural or mechanical means. Provisions are set out in pars. 3.5.2.4 and 3.5.2.5 below. The provisions in par. 3.5.2.1 above apply in all cases.
3.5.2.4 Natural ventilation Where car parks are not open sided, each storey should be naturally ventilated by permanent openings at each level having an aggregate area not less than 2.5% of the floor area at that level, of which at least half should be in two opposing walls. The number and disposition of smoke outlets should be such as to maximise the effectiveness of the ventilation.
Smoke vents at ceiling level may be used as an alternative to the provision of permanent openings in the walls. They should have an aggregate area of permanent opening totalling not less than 2.5% of the floor area and be arranged to provide a through draught.
3.5.2.5 Mechanical ventilation In some basement car parks, and enclosed car parks, it may not be possible to obtain the minimum standard of natural ventilation openings set out in par. 3.5.2.4 above. In such cases a system of mechanical ventilation should be provided as follows:
a. the system should be independent of any other ventilating system and be designed to operate at 6 air changes per hour for normal petrol vapour extraction, and at 10 air changes per hour in a fire condition;
b. the system should be designed to run in two parts, each part capable of extracting 50% of the rates set out in (a) above, and designed so that each part may operate singly or simultaneously;
c. each part of the system should have an independent power supply which would operate in the event of failure of the main supply;
d. outlets for exhaust air should be arranged so that 50% of the outlets are at high level, and 50% at low level; and
e. the fans should be rated to run at 300°C for a minimum of 60 mins, and the ductwork and fixings should be constructed of materials having a melting point not less than 800°C .
For further information on equipment for removing hot smoke refer to BS 7346: Part 2 Components for smoke and heat control systems, Specification for powered smoke and heat exhaust ventilators.
An alternative method of providing smoke ventilation from enclosed car parks is given in the BRE Report Design methodologies for smoke and heat exhaust ventilation (BR 368, 1999).
Shopping Centres (Purpose Group 4(b))
3.5.3 Whilst the provisions in this Document about shops should generally be capable of application in cases where a shop is contained in a single separate building, complications may arise where a shop forms part of a shopping centre. A shopping centre may include covered malls providing access to a number of shops and common servicing areas. In particular, the provisions about fire resistance, separating walls, surfaces and boundary distances may pose problems.
To ensure a satisfactory standard of fire safety in shopping complexes, alternative measures and additional compensatory features to those set out in this document would be appropriate (see 0.2). While this section is concerned with matters related to internal fire spread by way of the structure, these measures will involve consideration of all aspects of fire safety and their inter-action is particularly critical in these buildings.
The measures that are relevant to B3 include:
a. sprinkler protection of all shop units, storage and service areas, and any parts of malls used for a purpose that might introduce a fire load into the mall;
b. construction consisting generally of materials of limited combustibility except for limited decorative features and limited amounts of materials in shop fascias having a lesser standard of surface spread of flame characteristics than those for walls in circulation areas (see also B2);
c. compartmentation requirements (see 3.2.4.7) including the following:
walls and floors between shop units constructed as compartment walls and compartment floors;
floors in any shop unit exceeding 2000 m2 plan area of largest floor constructed as compartment floors;
floors in any shop unit opening onto a mall at more than one level constructed as compartment floors; and
compartmentation also provided between a large shop unit (over 3700 m2) and a mall, or between opposing large shop units (each over 2000 m2) and a mall. This compartmentation could be provided by fire shutters (smaller shop units would normally not be compartmented from a mall);
d. fire resistance periods for loadbearing elements of structure which are appropriate to the nature of the occupancies and the overall size of the building.
The above items are not exhaustive but draw attention to the need to consider proposals for shopping complexes as a comprehensive fire safety package. Guidance on these matters is set out in BS 5588: Part 10: 1991 Code of practice for shopping complexes.
Guidance on smoke control measures in enclosed shopping centres is contained in the Building Research Establishment Report (BR 186) Design principles for smoke ventilation in enclosed shopping centres.
Residential (Institutional) Buildings (Purpose Group 2(a))
3.5.4 In the case of Residential (Institutional) buildings, consideration is required to the potential for fire spread between compartments by way of the external elevations. This is on account of the nature of the occupancy and the need to ensure the safety of occupants who have been evacuated to an adjoining compartment (see 1.2.7).
The potential for fire spread by way of unprotected areas on each side of a compartment wall or protected stairway should be considered, where such walls abut an external elevation or are in close proximity to a re-entrant angle, such as occurs in the corner of a courtyard or the internal angle where two elevations meet.
In the case of courtyards, lightwells or wings of the same building, where opposite elevations are wholly or partially in different compartments, separation distances between such opposing elevations should be considered.
Guidance on internal fire spread (structure) in hospitals, including the considerations referred to above, is contained in Firecode Health Technical Memorandum 81, Fire Precautions in New Hospitals, 1996 (HTM81: 1996).
Hospitals also contain a number of high hazard rooms and areas which should be regarded as places of special fire risk (see 1.0.9). These include physiotherapy rooms, occupational therapy rooms, laundries, large kitchens and laboratories. Such areas should be fully separated by means of compartmentation (see 3.2.4.1).
Buildings Containing an Atrium
3.5.5 Many large modern buildings contain an atrium, which is a large undivided space within a building (see 1.0.9 for definition). Storeys in the building may be open to the atrium or they may be separated by fire-resisting construction. T he provision of an atrium will therefore impact on compartmentation with the possibility of fire spread between storeys and as such may need special fire safety measures. The accumulation of smoke which enters the atrium will also impact on the means of escape provisions (see Section B1).
Guidance on suitable fire safety measures in these circumstances is contained in BS 5588 Part 7 “C ode of practice for the incorporation of atria in buildings”.
Like a shopping centre, the design of a building containing an atrium may require alternative approaches to fire safety and the use of fire safety engineering (see 0.2). The measures that may be required will depend on the particular circumstances and will include consideration of all aspects of fire safety. The measures to be provided are likely to include:
sprinkler protection to the building;
smoke control measures within the atrium; and
facilities to assist fire-fighting within the building;
Further guidance on smoke control measures in buildings containing an atrium is contained in the Building Research Establishment Report (BR 258) Design approaches for smoke control in atrium buildings.
