spread within the building and the overall thermal impact on the structure will be reduced.
Containment of a fire to prevent spread is a principal tool of passive fire protection. Fire resistance helps to limit fire spread from the room of origin while ensuring structural integrity of the compartment. Thus walls and floors of most buildings are provided with fire resistance primarily to contain any fire to the room of origin. Preventing fires growing to a large size is one of the most important components of a fire safety strategy. Radiant spread of fire to neighbouring buildings must also be prevented, by limiting the size of openings in exterior walls. Fire resistance of walls and floors is covered in detail elsewhere in this book.
Smoke movement can also be controlled by venting or containment. Smoke removal is an important strategy in fires whose size has been limited by automatic sprinkler systems.
Pressurization and smoke barriers can both be used to contain the spread of smoke in a building (Spearpoint, 2008; Klote et al., 2012).
(a)
(b)
(c)
Figure 2.5 Example of fire resistance in a severe warehouse fire: (a) view of the fire after roof collapse;
(b) collapsed steel beams and damaged concrete masonry wall after the fire; and (c) light timber framed wall separating the warehouse from the offices
Another example is given in Figure 2.6 which shows a fire in the 12th storey of the 62 storey Interstate Bank building in Los Angeles in 1988. This photograph vividly demonstrates the importance of providing both containment and structural stability to protect the occupants and the property in the 50 storeys above the level of the fire.
2.4.2 Objectives for Fire Resistance
The objectives for providing fire resistance need to be established before making any design, recognizing that fire resistance is only one component of the overall fire safety strategy.
Structural elements can be provided with fire resistance for controlling the spread of fire or to prevent structural collapse, or both, depending on their function. Modern performance‐based Figure 2.6 Fire on the 12th floor of a 62 storey building, illustrating the importance of providing both containment and structural stability. Reproduced by permission of Boris Yaro
codes (NKB, 1994; MBIE, 2007; ABCB, 2015) show a similar approach to the requirements for fire resistance, as outlined below:
• To prevent internal spread of fire, a building can be divided into ‘fire compartments’ or
‘firecells’ with barriers which prevent fire spread for the fire design time. The many reasons for providing compartmentation include increasing the time available for escape, limiting the area of possible loss, reducing the fire impact on the structure, separating different occupancies, isolating hazards, and protecting escape routes. The separating barriers are usually floors or walls.
• To reduce the probability of fire spread to other buildings, boundary walls must have sufficient fire resistance to remain standing and to contain a fire for the fire design time.
• To prevent structural collapse, structural elements must be provided with sufficient fire resistance to maintain stability for the fire design time. Prevention of collapse is essential for load‐bearing structural members and for load‐bearing barriers which also provide con- tainment. Structural fire resistance must be provided to the main load‐bearing structural elements, and to secondary elements which support or provide stability to barriers or main members.
• Prevention of collapse is also essential if there are people or property to be protected elsewhere in the building, and for a building which is to be repaired after a fire.
2.4.3 Fire Design Time
The term fire design time is not precisely defined. Depending on the importance of the building, the requirements of the owner, and the consequences of a structural collapse or spread of fire, the fire design time will be selected by the designer as one or more of the following:
1. The time required for occupants to escape from the building.
2. The time for firefighters to carry out rescue activities.
3. The time for firefighters to surround and contain the fire.
4. The duration of a burnout of the fire compartment with no intervention.
Codes in various countries use these times in different ways for different occupancies.
Many small single storey buildings may be designed to protect the escape routes and to remain standing only long enough for the occupants to escape (Time 1) after which the fire will destroy the building. Alternatively, very tall buildings, or buildings where people cannot easily escape, should be designed to prevent major spread of fire and structural collapse for a complete burnout of one or more fire compartments (Time 4). Times 2 and 3 are intermediate times which may be applied to medium sized buildings, to provide life safety or property protection, respectively.
It can be seen that the provision of structural fire resistance may be essential, or unimpor- tant, or somewhere between these two extremes (Almand, 1989). On one hand there may be a major role for the structure so that collapse is unacceptable even in the largest foreseeable fire.
This may occur where evacuation is likely to be slow or impossible, or where great value is placed on the building or its contents. On the other hand, there may be virtually no role for the
from 1.0 for small, single storey buildings to 2.5 for large, multi‐storey buildings.
2.4.4 Trade‐offs
One of the difficulties in assessment of fire safety is the extent to which some fire protection measures can be ‘traded off’ against others. For example, some prescriptive codes allow fire resistance ratings or fire compartment areas to be reduced if an automatic sprinkler system is installed, or they allow travel distances to be increased when smoke or heat detectors or sprin- klers are installed. Trade‐offs do not apply in a totally performance‐based environment, because the designer will produce a total package of fire protection features contributing to the required level of safety. However, in practice, most designs are based on prescriptive codes, so it is often useful to make trade‐offs.
It is often difficult to justify trade‐offs, especially reductions of fire resistance if automatic sprinkler systems are installed, for the following reasons. If an automatic suppression system can be relied on to control a fire with total certainty, no fire resistance or passive fire protection is necessary. However, no system is 100% effective, so the question is how much fire resis- tance should be provided for the remote probability that the suppression system fails to operate or fails to control the fire. As an example, it could be argued that if the suppression system fails when street water supplies are destroyed by an earthquake or explosion, the resulting fire will have the same severity as if there had been no suppression system at all, so there should be no trade‐off for sprinklers.
No codes allow a total trade‐off for sprinklers, but many national codes allow a partial trade‐off, assuming that in a sprinklered building, the probability of an uncontrolled fire is much less likely than the probability of a sprinkler‐controlled fire. Quantitative justification for partial trade‐offs is not easy, but two possible probabilistic arguments are as follows:
1. Many national codes allow a reduction in fire resistance of structural members if the building is sprinklered. A possible justification for this approach is based on safety factors.
If, for example, the fire resistance normally specified for a burnout of a fire compartment in an unsprinklered building has an inherent safety factor of 2.0, then in the unlikely event of a fire and a sprinkler failure, that safety factor could be reduced to as low as 1.0, hence the 50% reduction. Such an argument can only be used if the method of specifying fire resistance for unsprinklered buildings is sufficiently conservative in the first instance.
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.