2.2 Process of Fire Development
2.3.3 Fire Safety Concepts Tree
One of the more durable frameworks for fire safety assessment is the Fire Safety Concepts Tree developed by the National Fire Protection Association (NFPA, 2003). Figure 2.4 shows an edited summary of the Fire Safety Concepts Tree. The following paragraphs give a brief explanation of the tree, as a guide to establishing the relative importance of the various components of a fire safety strategy.
YES NO
Determine geometry, construction and use of the building
Estimate maximum likely fuel loads
Estimate maximum likely number of occupants and their locations
Assume certain fire protection features
Establish performance requirements
Carry out fire engineering analysis
Modify fire protection features
Accept design Acceptable performance
Figure 2.3 Overview of scenario analysis
2.3.3.1 Prevention versus Management
Line 2 of the tree states the obvious; fire management is unnecessary if ignition can be prevented, but if not, the impact of the fire must be managed. In reality there will always be unplanned ignitions, but the probability of these can be reduced with fire prevention programmes. Arson is a growing cause of fires which cannot easily be controlled by building designers. Unless stated
Suppress fire
Control fuel Control by
construction
Control fire movement Control lining
materials Provide structural
stability Automatic
Contain Vent
Manual Defend
in place
Cause movement
Provide route for movement Move
4.
5.
6.
7.
8.
9.
AND
AND
Figure 2.4 Fire Safety Concepts Tree. Adapted with permission from NFPA (2010a). © 2010 National Fire Protection Association, all rights reserved
otherwise, this tree shows alternative strategies, whereby the objectives on one line can be met by any one of the items on the following line. Line 3 shows that managing the impact of a fire can be achieved either by managing the fire itself or by management of exposed persons and property.
2.3.3.2 Management of Exposed Persons and Property
Line 4 shows that exposed persons and property can be managed by moving them from the building or by defending them in place. The usual strategy is to move people from a building, unless they are incapacitated or under restraint. An intermediate position for very large build- ings is to move people to a place of refuge within the building. Most exposed property must be defended in place because it is impossible for it to be moved quickly.
In order for people to move, the fire must be detected, the people must be notified, and there must be a suitable safe path for movement (Line 5). The ‘AND’ symbol indicates that success in both boxes is required to meet the objective. Human behaviour and escape route design is beyond the scope of this book. Refer to the SFPE Handbook (SFPE, 2008) for more information.
2.3.3.3 Manage the Fire
Line 6 shows three options for managing a fire. In the first case the fuel source can be con- trolled, by limiting the quantity or geometry of the available fuel. For example, this could be a limit on the amount of combustible material stored in a space. The second option is to sup- press the fire and the third is to control the fire by construction. Fire suppression is a huge topic beyond the scope of this book, but as shown in Line 7, suppression can either be automatic or manual. In either case suppression depends on early detection of the fire and application of sufficient quantities of appropriate suppressant, usually water.
2.3.3.4 Control by Construction
Control of fire by construction is the subject of this book. Line 8 of the concepts tree shows that in order to control fire by construction it is necessary to both control the movement of the fire and provide structural stability. The left‐hand box in Line 8 indicates that fire growth and severity can be controlled by limiting the fuel in combustible room linings. This box is connected by dotted lines because, strictly speaking, it should be a subset of “control fuel”
from Line 6, but it has been placed in Line 8 because selection and installation of the linings is part of the construction process, rather than a building management issue.
2.3.3.5 Provide Structural Stability
The provision of structural stability is essential if buildings or part of buildings are to remain standing during a fire (depending on the importance of the individual building), and be easily repaired for subsequent use. Structural stability is also essential to protect people or property elsewhere in the building at the time of the fire. Some elements such as walls and floors may
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).