Practical fire safety guidance for existing non-residential premises

Chapter 6: Restricting The Spread Of Fire And Smoke

182. To reduce the risk to persons from fire, it is necessary to consider how to restrict the spread of fire and smoke. The majority of people who die in fires are overcome by smoke. To evaluate the risk requires a basic appreciation of the way fires grow and how smoke can spread through a building. A fire in a building can generate smoke that is thick and black, obscures vision, causes difficulty in breathing, and can prevent persons from using escape routes. Smoke is a serious threat to life which should not be underestimated.

183. Fire is spread by convection, conduction and radiation. Convection causes the major proportion of injuries and deaths. When fire starts in a building, the smoke rising from the fire becomes trapped by the ceiling and then spreads in all directions to form an ever-deepening layer over the entire room space. The smoke will pass through any holes or gaps in the walls, ceiling and floor into other parts of the building. The heat from the fire gets trapped in the building and the temperature rises. Some materials, such as metal beams can absorb heat and transmit it to other rooms by conduction, where it can set fire to combustible items that are in contact with the heated material. Radiation transfers heat in the air in the same way that an electric bar heater heats a room. Combustible material close to a fire will absorb the heat until the item starts to smoulder and then burn.

Layout of Transport Premises

184. In some transport premises, the open design gives the potential for rapid heat, smoke or fire spread and exposure to occupants. Also openings in floors may allow smoke and hot gases to move from the fire source to areas occupied by people who may not be aware of the fire. Lack of containment potentially increases the number of people at risk from a fire.

185. Consideration of materials' fire properties and smoke production is important for tunnels, stations and transport premises that are underground. The use of combustible materials should be minimised. Some materials can be replaced by materials that are more difficult to ignite, less capable of spreading flame or do not emit toxic fumes or smoke when subject to high temperatures.

Auditoria Design

186. In some traditional theatres, the stage area may be equipped with a high level outlet over the stage to allow the escape of smoke and hot gases in the event of a fire on the stage, and a fire-resisting wall and a safety curtain in the proscenium opening to provide separation between the fire and the audience.

187. In the event of fire, the direction of air movement by the ventilation system is from the auditorium towards the stage.

188. Escape from the stage area behind the curtain is independent of that from the auditorium and escape provision from elevated areas such as grids and fly galleries is direct to open air, or to another part of the premises (other than to the auditorium) via a door providing at least 60 minutes fire-resistance leading to a separate fire compartment or protected area.

189. An under stage area used in conjunction with a stage presentation may require two exits, one of which is independent of the stage. An under stage area used for storage should be separated from the remainder of the building by at least 60 minutes fire-resistance.

Stadia Design

190. The fire safety design of sports stadia may include:

• viewing accommodation separated from other parts of the building by fire-resisting construction - as far as possible no storage areas form part of the stand building; where this is unavoidable, storerooms are provided with fire suppression system appropriate to the risk and are not accessible from public areas;
• low flammability and fire risk potential in fixture and fittings, and contents; and surface finishes which have low surface spread of flame characteristics;
• contents which are not capable of being easily dismantled or moved to block exit routes;
• voids sealed so that litter cannot collect;
• a fire warning system including, where appropriate, automatic fire detection;
• a roof geometry to restrict smoke and flame travel along the underside of stand roofs;
• in some cases, the provision of smoke ventilation within structures; and
• escape routes which allow free flow of people.

191. For sports stadia, reference should be made to the guidance and benchmarks in the Green Guide. (See paragraph 8).

Fire Separation and Compartmentation

192. The purpose of fire separation is to provide a physical fire-resisting barrier to restrict fire spread between different occupancies and between single occupancy parts and communal areas. Where premises adjoin or are part of a larger building, the potential for an outbreak of fire to spread to or from the neighbouring building or another occupancy should be considered.

193. A fire compartment is part of a building constructed to provide a physical fire-resisting barrier to prevent the spread of fire and smoke to or from another part of the building. The life safety objectives of fire compartmentation may be to:

• reduce the number of occupants who may be immediately at risk;
• reduce the travel distance for persons;
• restrict the size and growth of fire; and/or
• protect occupants where there may be delayed evacuation of premises.

194. For the purposes of smoke control, corridors which are not protected corridors, and which have at least two directions of escape, and with more than 12 m in length between the exits, may be divided in the middle third of the corridor with a wall or screen with at least 30 minutes fire-resistance (for integrity) and the door in the wall or screen at least an FD30S self-closing fire door.

195. A lift well can be a route for vertical fire spread. A lift well which is enclosed by walls with fire-resistance will be a barrier to fire spread. A lift well which is totally within a protected area such as an enclosed stair, is already within a fire-resisting enclosure. Where a lift well is not the full height of the building, the fire-resistance of the floor and/or ceiling needs also to be considered.

196. Where services pass through any fire-resisting structure, gaps should be sealed or fire stopped to maintain the fire-resistance of the structure and prevent the passage of fire or smoke. Pipes should be fitted with a proprietary sealing system capable of maintaining the fire-resistance. A similar consideration exists for penetration by ventilation ducts.

197. Boiler rooms and plant rooms are a possible source of fire. To contain a fire, at least in its early stages, a room may be enclosed by walls with fire-resistance where it contains an appliance (solid fuel, oil or gas fired, or fuel oil tanks). Where the appliance or equipment uses liquid fuel, the room should be able to contain all the liquid plus 10%.

Smoke Control

198. In some premises such as atrium buildings and enclosed shopping complexes, fire safety measures may include an automatic smoke and heat exhaust ventilation system ( SHEVS). SHEVS are often used in conjunction with automatic fire suppression systems; suppression limits the size of a fire therefore controlling the amount of smoke produced. Smoke control in this context is a specialist subject.

199. In the malls and some large units of shopping centres, smoke reservoirs at roof level allow heat and smoke to vent to the outside. The design should maintain the smoke layer above head height to allow persons to use the mall as an escape route.

200. The fans or ventilators in a reservoir operate:

• on activation of any automatic fire suppression system; or
• on activation of smoke detection within the reservoir; or
• on activation of more than one smoke detector anywhere in the shopping centre; or
• following a delay (of perhaps 4 minutes) from the first fire alarm activation.

201. A smoke control system needs replacement air to function. This is provided automatically on the operation of the system with the air entering below the smoke layer level.

202. A manual override may be provided at access points and in the centre control room.

Doors

Fire doors

203. A 'fire door' is a fire-resisting door which is rated by performance to fire under test conditions. Fire doors are used to prevent fire spread and for the protection of means of escape. A self-closing device is a normal feature of a fire door, though there are some exceptions, such as doors to small cupboards which are kept locked shut.

204. A fire door rated to 30 minutes is described as FD30 ^[5] or E30 ^[6] . A suffix is added to denote that the door has a smoke control function giving FD30S and E30Sa respectively. A 60 minutes fire door with smoke control is designated FD60S or E60Sa. The door rating is an indication of test performance and is not necessarily how a door will perform in a real fire.

205. The level of protection provided by a fire door is determined by the time taken for a fire to breach the integrity of the door assembly, together with its resistance to the passage of smoke, hot gases and flame. The gap between the door leaf and the frame is normally fitted with intumescent strips, in either the door or the frame (but not at the bottom of the door). The strips expand in response to heat from a fire, to seal the gap between the door leaf and the frame.

206. Smoke seals fitted to the door leaf gap prevent the spread of smoke at ambient temperatures, before an intumescent strip expands.

207. In determining the performance of a door in fire, it is necessary to consider the whole door assembly including the frame, glazing, side-panels, transoms and ironmongery. In the case of a new door assembly, the manufacturer's installation instructions should be followed.

208. Some existing non-fire-resisting doors may have the potential to be upgraded to nominal 30 minutes standard, but replacement of existing doors and frames is often preferable.

Self-closing function

209. A fire door will only fulfil its function to provide a barrier to fire and smoke if it is closed at the time a fire occurs. A controlled self-closing device, complying with BS EN 1154, will be fitted to each fire door (other than to certain cupboard doors). The closing pressure of the self-closing device needs to be sufficient to overcome any latch mechanism. It is inappropriate to rely on a procedure whereby staff will attend and close doors as an alternative to fitting self-closers.

Hold-open and door release devices

210. There are devices which hold self-closing fire doors in the open position until a fire detection system operates. It follows that these are only appropriate in premises provided with an automatic smoke detection system. They should not be used for a door to a room in which the type of automatic fire detector is solely a heat detector.

211. A self-closing fire door can be held open by an electromagnetic hold-open device (which complies, where appropriate, to BS EN 1155 or BS 5839: Part 3) or with electromagnetic hold-open door closers (to BS EN 1155). Electrically operated hold-open devices should deactivate and release the door on operation of the fire warning system or any loss of power to the hold-open device. Doors to a stairway that forms the only means of escape from an upper floor should close automatically in the event of fault in the fire warning system.

212. An alternative release is an acoustically-activated door release mechanism complying with BS EN 1155. Acoustic devices should not be used on fire doors to a protected stair that is the only stairway serving the building or part of the building. Acoustic devices actuate in response to the sound from the fire alarm sounders so will not be appropriate where the initial fire alarm activation does not activate the fire alarm sounders (such as a staff alarm).

213. A further type of self-closing device comprises a 'swing free' arm ^[7] , allowing the door leaf to work normally and independently of the closing device in normal conditions. On the operation of the fire warning system or on power failure, the self-closer operates and closes the door.

214. Radio-linked devices are available; these reduce the need for wiring. Some acoustic systems are battery powered.

215. BS 7273: Part 4 contains detailed guidance on conditions for use of door release devices.

216. The automatic closing of doors may take persons by surprise and the force of the closing mechanism could knock someone over and be a source of injury. Consequently precautions should be taken to avoid injury, including during a scheduled test or action which will result in release of the doors.

Fire Spread Through Cavities

217. Many buildings have cavities and voids, sometimes hidden from view, which may allow smoke and fire to spread. Examples are:

• vertical shafts, lifts and dumb waiters;
• false ceilings, especially if walls do not continue above the ceiling;
• voids behind wall panelling;
• unsealed holes in walls and ceilings for pipe work, cables or other services;
• a roof space or attic; and
• a duct or any other space used to run services.

218. Potential fire spread through cavities and voids should be assessed and, where practical, examined to see if there are voids that fire and smoke could spread through.

219. Cavity barriers may be necessary to restrict the spread of fire in cavities, particularly for those cavities that could allow fire spread between compartments.

220. Certain modular construction buildings have hidden voids through which fire may spread. Modern timber frame buildings have cavities within the frame and these should have been installed with fire-resisting cavity barriers between the external cladding and the timber wall panel at the time of construction.

221. Poor work standards during building work can result in cavity barriers (or the enclosure of escape routes) being breached and/or not being reinstated. This potential needs to be considered. The control of building work is considered in Chapter 5.

222. Insulated core panels (sandwich panels) normally consist of an insulated core sandwiched between an inner and outer metal skin. They are used in buildings as exterior cladding or for internal structures and partitions. The retail sector in particular uses insulated core panels because they are easily constructed and suitable for internal alterations. Various materials have been used as a core, some of which are combustible. The existence of panels with a combustible core needs to be carefully considered since fire may spread through the combustible core. Some relevant precautions are:

• avoid siting ignition sources adjacent to panels;
• do not store highly combustible materials against the panels;
• repair damaged panels or sealed joints;
• make sure that jointing compounds or gaskets around the edges of the panels are in good order; and
• openings made for doors, windows, cables and ducts should be effectively sealed so that the inner core is not exposed.

Ventilation Systems

223. The potential for ventilation systems to allow the spread of fire and smoke should be assessed. A powered ventilation system may assist the spread of smoke unless it is designed to shut down automatically if fire is detected.

224. Ventilation ducts may provide a pathway for the spread of fire and smoke between compartments or into stairs. Where ventilation ducts penetrate the walls or floors of these enclosures, automatic dampers provided inside the ducts hold back fire and smoke. Dampers may need to be activated by smoke detection. Specialist guidance on the use of dampers is contained in BS 9999.

Fire Spread on Internal Surfaces

225. Fire can rapidly spread on certain surfaces of walls and ceilings, significantly affecting the rate of fire growth and smoke production. The potential for fire spread on surfaces in escape routes is important as this could prevent occupants from escaping. The internal surfaces may predominantly be:

• category 0 for protected stairs and escape routes, and other corridors of shops, assembly and entertainment buildings; or
• category 1 for corridors in other buildings, and for large rooms.

226. The grading system for surface spread of fire relates to performance against tests set out in certain British Standards. Examples of materials are:

• category 0 - brickwork, blockwork, concrete, ceramic tiles, plaster finishes (including rendering on wood or metal lathes), wood-wool cement slabs and mineral fibre tiles or sheets with cement or resin binding;

• category 1 - timber, hardboard, blockboard and particle board, which have been treated to achieve this category; and

• category 2 - timber, hardboard, blockboard, particle board and certain dense timber or plywood.

227. Additional finishes may be detrimental to the fire performance of the surface. Multiple layers of wallpaper or certain paints applied to the face of a wall or ceiling surface can increase surface flame spread.

228. The use of plastics for surface finishes is a complex issue and outwith the scope of this guidance document. Information on the suitability of plastic materials can be found in the Scottish Building Standards Technical Handbook for Non-Domestic Premises.

Fire Sread on External Walls

229. If there is combustible external wall cladding or construction, it will be necessary to consider the potential for an outbreak of fire within the building, or from an external source, to spread on the external walls of the building and pose a risk to occupants. Recommendations on the fire performance of external walls can be found in the Scottish Building Standards Technical Handbooks.