2. The Eurocode 1 Part 1.2 (CEN, 2002b) suggests that for calculating fire resistance, the fuel load in a sprinklered building be taken as 60% of the design fuel load. This approach could be justified by considering sprinkler failure to be such an unlikely event that the design fuel load should be the most likely fuel load rather than the 90 percentile fuel load used for design of unsprinklered buildings.
2.4.5 Repairability and Reserviceability
Repair and reserviceability may be important for some building owners. A building designed to resist a complete burnout will be severely damaged, even if the fire is contained and the structure is intact. Most performance‐based codes do not require that the structure should be undamaged following a fire. For example, Eurocode 1 Part 1.2 (CEN, 2002b) states that when designing for a required fire resistance period, the performance of the structure beyond that time need not be considered. A requirement for little or no damage to the building structure may be requested by some codes or some building owners, but this will require a greater level of passive fire protection than required to only prevent collapse.
A reserviceability requirement would limit damage so that the building could be reoccupied with no (or very little) time for repairs. Such a requirement might be imposed on buildings of social, cultural or economic importance. This is only possible with the use of active fire sup- pression systems such as sprinklers to prevent the fire from becoming large and destructive.
chemical treatment.
2.5.2 Fire Spread to Adjacent Rooms
Spread of fire and smoke to adjacent rooms is a major contributor to fire deaths. The movement of fire and smoke depends very much on the layout of the building. Open doors can provide a path for smoke and toxic combustion products to travel from the hot upper layer of the fire room into the next room or corridor. These hot gases can pre‐heat the next area leading to subsequent rapid spread of fire.
Keeping doors closed is essential to preventing fire spread from room to room. Doors through fire barriers must maintain the containment function of the barrier through which they pass, whether for smoke control or fire resistance. Door closing devices which operate automatically when a fire is detected are very effective. Other innovations to improve door performance include smoke control strips to reduce spread of smoke, and strips of intumescent material that will swell when heated to prevent fire spreading through gaps around the door.
Concealed spaces are one of the most dangerous paths for spread of fire and smoke. A haz- ardous situation occurs if there are concealed spaces which allow spread of fire and smoke to adjacent rooms, or even to rooms some distance from the fire. Figure 2.7 shows spread of smoke through a concealed ceiling cavity. Concealed cavities are a particular problem in old buildings, especially if a number of new ceilings or partitions have been added over the years.
Figure 2.7 Spread of smoke and fire through a ceiling cavity
Fire can also spread to adjacent rooms by penetrating the surrounding walls, as occurred in the building shown in Figure 2.8. Walls can be designed with sufficient fire resistance to pre- vent the spread of fully developed fires, but they must be constructed with attention to detail if fire performance is to be ensured. Fire resisting walls must extend through suspended ceil- ings to the floor or roof above so that the fire cannot spread through a concealed space above the wall. In order to prevent fire spreading over the top of a fire resisting wall at roof level, the wall can be extended above the roof line to form a parapet, or the roof can be fire‐rated for some distance either side of the top of the wall.
A severe fire will find any weakness in a separating barrier, and many such weaknesses are not visible during normal operation of the building. Care must be taken to ensure that poor quality workmanship or penetrations for services and fittings do not compromise the performance of fire resisting walls. The term ‘fire stopping’ refers to the sealing of penetra- tions and cavities through which fire might spread (O’Hara, 1994). There are many techniques for fire stopping of penetrations, construction joints and seismic gaps (Abrams and Gustaferro, 1971). Materials for fire stopping include mineral wool, wood blocks, gypsum board, metal brackets and a wide array of proprietary products such as fire resisting putty, board materials and intumescent pillows and collars (Figure 2.9).
Figure 2.8 The masonry walls of a large department store after a severe fire (Ballantynes department store, New Zealand, 1947). Reproduced by permission of The New Zealand Herald/newspix.co.nz
Air‐handling ducts which pass through fire resistant walls and floors can create paths for spread of fire. This can be prevented by the use of fire resistant insulating duct materials and internal ‘fire dampers’ which are designed to close off the opening in the event of a fire.
2.5.3 Fire Spread to Other Storeys
Fire can spread to other storeys by a variety of paths, inside and outside the building. Internal routes for fire spread include failure of the floor/ceiling assembly, and fire spread through vertical concealed spaces, service ducts, shafts or stairways. Vertical services must either be enclosed in a protected duct or have fire resistant penetration closers at each floor level, as shown in Figure 2.10. Vertical shafts and stairways must be fire‐stopped or separated from the occupied space at each level to avoid creating a path for spread of fire and smoke from floor to floor. A particularly dangerous situation can arise if there are interconnected horizontal and vertical concealed spaces, within the building or on the façade.
Another potential path for vertical fire spread is through gaps at the junction of the floor and the exterior wall, just inside the façade, as shown on the left‐hand side of Figure 2.11. This is particularly important for ‘curtain‐wall’ construction where the exterior panels are not part of the structure. A possible detail to prevent such fire spread is shown in Figure 2.12. Careful detailing and installation is necessary to ensure that the entire gap is sealed, especially at corners and junctions, to eliminate any possible path for fire spread (Gustaferro and Martin, 1988).
Gaps such as these between structural and non‐structural elements are often filled with non‐rigid fire‐stopping materials to allow for seismic or thermal movement. The filling
Figure 2.9 Fire protection to service penetrations through a fire resisting floor
material must be able to provide the necessary fire resistance both before and after the antici- pated movement (including earthquake movement in seismic areas). Filling material may be mineral or ceramic fibre batts or blankets, which must be adequately held in place. Glass fibre materials are not suitable for fire stopping because they shrink and melt at temperatures over about 300 °C. Metal brackets or angles supporting the filling material must not be made from aluminium alloys because they melt at temperatures over 500 °C. If made from steel, the brackets should be fire protected with intumescent paint or other suitable material.
Vertical fire spread can also occur outside the building envelope, via combustible cladding materials or exterior windows as shown on the right‐hand side of Figure 2.11. Combustible cladding susceptible to rapid flame spread should not be used on the exterior of tall buildings.
Vertical spread of fire from window to window is a major hazard in multi‐storey buildings.
This hazard can be partly controlled by keeping windows small and well separated, and by
Fire separation Fire stopping
Fire
Fire
Figure 2.10 Fire separation of vertical services
Fire
Fire
Figure 2.11 Fire spread from storey to storey
using horizontal aprons which project above window openings (Oleszkiewicz, 1991). Flames from small narrow windows tend to project further away from the wall of the building than flames from long wide windows, leading to lower probability of storey to storey fire spread (Drysdale, 2011).
2.5.4 Fire Spread to Other Buildings
Fire can spread from a burning building to adjacent buildings by flame contact, by radiation from windows, or by flaming brands. Fire spread can be prevented by providing a fire resisting barrier or by providing sufficient separation distances. Figure 2.13 shows a severe fire in a department store, where the entire building is fully involved in the fire and the roof framing is about to collapse, but the fire is prevented from spreading to adjacent properties by fire resist- ing boundary walls. If there are openings in the external wall, the probability of fire spread depends greatly on the distances between the buildings and the size of the openings. Exterior fire resisting walls must have sufficient structural fire resistance to remain in place for the duration of the fire. This becomes a particular problem if the structure which normally pro- vides lateral support to the walls is damaged or destroyed in the fire. Outwards collapse of exterior walls can be a major hazard for firefighters and bystanders, and can lead to further spread of fire to adjacent buildings.
Fire spread by flame contact is only possible if the buildings are quite close together, whereas fire spread by radiation can occur over many metres. Radiant heat flux from the window of a building fire can ignite combustible cladding on a nearby building, or combus- tible products inside the windows. The calculation of radiant heat flux from one building to another is described in Chapter 3. Fire can also travel large distances between buildings if combustible vegetation is present.
Fire stopping material Steel support angle
Curtain wall panel
Figure 2.12 Fire stopping between slab and curtain wall
Flaming brands carried by the wind can cause fire spread between buildings with combustible roofing materials as shown in Figure 2.14. This can be controlled by restricting the use of combustible roofing materials. Fire spread between adjacent buildings also depends on the relative heights of the buildings. A fire burning through the roof of a low
Figure 2.13 Severe fire in a department store. Reproduced from Euskonews magazine
Wind
Figure 2.14 Fire spread by flaming brands
building can spread into windows of an adjacent tall building as shown in Figure 2.15 unless adequate fire resistance is provided.