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How Can a Fire Risk Approach Be Applied to Develop a Balanced Fire Protection Strategy
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How Can a Fire Risk Approach Be Applied to Develop a Balanced Fire Protection Strategy

By  Man-Cheung Hui | Fire Protection Engineering

Quite commonly, the first thing the fire protection engineer may do when engaged in a development project is to consult the relevant building codes and regulations to seek guidance and to establish the legal requirements. However, in a scenario where the development consists of types of occupancies that are not explicitly defined in the building codes and regulations, how will the fire protection engineer complete this job?

In another scenario, the fire protection engineer may face a different type of problem involving various constraints to the construction of the development, for example, tight financial budget, site limitations, operational requirements of the development, and regulatory requirements that may adversely affect the functionality of the development. Under this circumstance, how can the fire protection engineer juggle various fire protection requirements?


The article discusses how a fire risk approach can be employed to assist in the formulation of a fire safety strategy.


Fire Safety Strategies and Measures
A fire safety strategy can be defined as a plan on how to use one or a combination of fire protection measures to achieve predetermined fire safety objectives. There could be one or more fire safety objectives in a single project. Typical fire safety objectives include, but are not limited to, the following:

  • Building occupants' safety.
  • Property and contents protection.
  • Business continuity.
  • Adjacent property protection.
  • Protection of firefighters.
  • Averting a catastrophic loss.
  • Environmental protection.

It is possible that when the fire safety objectives are changed, the fire safety strategy may need to be modified accordingly. As a general guideline, the Fire Safety Concepts Tree1 as described in NFPA 550 and shown in Figure 1 facilitates understanding the possible ways to achieve fire safety objectives.


The NFPA Fire Safety Concepts Tree is a qualitative approach to presenting fire safety in a manner that ensures all major items of fire prevention, protection, and administration are examined in a logical fashion. It uses a similar diagram to Fault Tree Analysis to show relationships of fire prevention and fire damage control strategies.


It can be seen from Figure 1 that, fundamentally and philosophically, fire safety can be achieved by not letting the fire happen (Prevent fire ignition branch) or by dealing with the fire after it has happened (Manage fire impact branch). The Prevent fire ignition branch suggests that there could be three ways to avoid ignition: (a) by controlling fuel (through elimination of fuel or altering fuel ignitability), (b) by controlling source fuel interactions (through separation of fuel from heat-energy sources or control of heat transfer processes), or (c) by controlling heat-energy sources ( through elimination of the heat sources or controlling the rate of energy release). Reference can be made to the Fire Protection Handbook2 for a detailed listing of fire-prevention factors.

If the fire prevention measures were 100 percent effective at all times, then nothing else would be needed, and all fire brigades could be disbanded because there would be no fires. In reality, no systems or measures are 100 percent effective, and it is impractical to completely prevent the ignition of fires in a built environment; therefore, backup measures will be necessary to cater to scenarios in which ignition is possible.


According to the Fire Safety Concepts Tree, managing the impact of fires can be achieved by either managing the "exposed," i.e., occupants, contents, building fabric, operations, environment, or heritage, depending on the fire safety objectives being considered, or by managing the fire.


A detailed illustration of the Manage exposed branch can be found in the Fire Protection Handbook 2 and in a recent article in this magazine on fire alarm systems and interior finish. 3 In short, managing the exposed can be achieved by limiting the amount exposed or by safeguarding the exposed. The former method means that the number of people or amount of contents in a space is restricted, which may be impractical. To safeguard the exposed is a much more common tactic used in building projects, and fire protection measures are often specified by prescriptive building codes and regulations to achieve this objective.


There are two primary ways to safeguard exposed: (i) to move the exposed, i.e., to relocate them away from the hazardous area to a temporary safe place (e.g., staircases or protected passageways) or to a safe place (e.g., on the street or an open area), or (ii) to defend them in place, i.e., to maintain the building space to be tenable for a sufficient period of time after the start of the fire.


The fire protection measures that may be employed to safeguard the exposed are summarized as follows:


(a) Potential measures for "move the exposed" strategy;

  • Fire detection system.
  • Fire alarm system.
  • Egress system.
  • Fire-resistant elements.
  • Fire suppression system.
  • Smoke management system.
  • Fire emergency management system to protect the escape route, set up emergency control organization, and implement emergency procedures.
  • Emergency lighting and exit signs.
  • Intercommunication system for communication between occupants and fire wardens or emergency services personnel.

(b) Potential measures for "defend in place" strategy;

  • Fire-resistant elements.
  • Fire suppression system.
  • Smoke management system.
  • Emergency lighting.
  • Intercommunication system for communication between occupants and fire wardens or emergency services personnel.

Selection of Fire Safety Strategy and Protection Measures
One of the major tasks for the fire protection engineer may be to determine which strategy works best for the building/facility and the occupants therein, and then determine what fire protection measures should be provided after the strategy has been selected.

The fire prevention strategy should be considered first, following the commonly accepted wisdom that "prevention is better than cure." Research4 shows that fire prevention programs could reduce the number of fire incidents, but not totally eliminate them, and refresher courses may need to be held regularly to maintain the same performance of the programs over the years.


Currently, there is much debate onto whether the "move the exposed" strategy or the "defend in place" strategy should be adopted. There is similar deliberation about the supremacy of active fire protection systems versus passive fire protection systems. For the latter, it has to be realized that different fire protection systems work at different phases of fire development; for example, smoke detection and sprinklers typically actuate in the early phase of the fire, nonrated building elements or barriers that have inherent fire resistance may contribute in delaying the spread of fire and smoke in the growth phase of the fire, and fire-rated barriers serve to contain the fire in the fully developed phase of the fire.


At the strategy level, qualitatively, it may be quite straightforward to judge that moving the exposed is not appropriate for certain occupancies, such as the intensive care units in hospitals, nursing homes, detention and correction facilities, and the like. However, there are some occupancies and some circumstances where either the "move the exposed" strategy or the "defend in place" strategy may be desirable, and there may be a combination of fire safety measures that can be employed in each strategy. In this case, an assessment more robust than a qualitative judgment may need to be employed to assist the decision making process. Quantitative fire risk assessment is one of the assessment methods that are suitable for evaluating the options within each strategy.


Quantitative Fire Risk Assessment
There are many definitions and views of risk, depending on which part of society or which application is considered. There is even a positive side to risk in the business context, because innovation and development involve risk. In the fire safety context, fire risk is typically associated with fire hazard, and often mistakenly solely associated with fire hazard. The following example shows how risk and hazard are not necessarily directly proportional.


Imagine there is a five-kilogram hammer resting loosely on the top rung of a two-meter-high ladder. If the ladder-hammer combination is locked in a storeroom, the hazard has not changed but the risk of someone getting injured is significantly reduced. If the same ladder-hammer combination is placed on a busy sidewalk, the hazard remains the same but the risk of someone getting hit by the hammer is considerably increased.


Where it is required to determine whether the "move the exposed" strategy or the "defend in place" strategy would be more appropriate for a particular occupancy, one could utilize the aforementioned "fire risk" concept and conduct a fire risk assessment to provide guidance.


Fire risk assessment usually forms part of a fire risk management program; however, the assessment exercise is often carried out separately without referring to other parts of the management program, which may run the risk of missing the big picture. A typical fire risk management process is illustrated in Figure 2.




It can be seen in Figure 2 that the risk assessment exercise consists of three major parts conducted in sequence, i.e., fire hazard identification, then fire risk analysis, and finally fire risk evaluation. A full description of the hazard identification, risk analysis, and risk evaluation process is beyond the scope of this paper; instead, a simplified version is presented here for practical purposes.


Fire hazard identification can be carried out in several ways, e.g., by using a checklist or a HAZOP study. The hazard identification could:

  • Identify potential fire ignition sources;
  • Identify fire load quantity and arrangement that may lead to high-severity fires; and
  • Identify potential faults in hardware (such as escape routes, fire stops, sprinkler system, smoke control system, etc.) and software (such as fire emergency management organization, policy and procedures, maintenance quality, housekeeping standards, etc.) that may generate hazardous conditions or put occupants in hazardous conditions.

The personnel who are assigned the task to identify fire hazards must at least have general knowledge of combustion, fire safety, and the characteristics of various fire protection systems, and be very familiar with the operational aspects of the built environment.


Fire risk analysis consists of determining the likelihood and consequence of each considered fire scenario. A fire scenario, for the purpose of quantitative fire risk analyses, can be considered as a timed sequence of events after an ignition. The development of these events is dependent on fuel quantity and arrangement; characteristics of the built environment, such as the location and status of fire/smoke barriers and that of openings, such as doors and windows, through which air and smoke could pass; and the performance of various fire protection measures. A simplified example fire scenario is illustrated in Figure 3 in the form of a Timed Event Tree with four branches (a) to (d), and t representing time.




Since there could be an infinite number of fire scenarios to be considered, and the resources to analyze fire scenarios are finite, it is necessary to structure the fire scenarios into a manageable number of scenario clusters for evaluation. A properly established scenario structure consists of a group of scenario clusters, each with its own representative fire scenario; the scenario clusters are non-overlapping and collectively include all relevant scenarios. The likelihood (i.e., frequency of occurrence) for a scenario cluster is the sum of the frequencies of all scenarios contained in the cluster, whereas the consequence for a scenario cluster is estimated from the representative scenario of the corresponding scenario cluster.


To estimate the frequencies of occurrence of fire scenarios, the probabilities of the initial ignition, fire spread, and the probabilities of failure of various fire protection systems have to be estimated. The frequencies of initial ignition and probabilities of fire spread are commonly derived from past fire incident statistics, if such data exist. However, if statistical data are not available or the quality of the data does not allow meaningful interpretation, it may be possible to specify the initial ignition conditions and then employ fire engineering calculation techniques to predict subsequent development of the fire.


The probabilities of failure of various fire protection systems ( also known as the reliability of these systems) could be estimated from past failure data or, more commonly, derived from fault tree analyses. In a fault tree, a logic tree of 'AND' and 'OR' gates portrays the combinations of conditions that can lead to failure of the studied fire protection system, which is defined as the top event of the tree. For more complex systems that are provided with standby components or redundancies, determination of their reliabilities may need to employ mathematical techniques in reliability theories. The reliability models of three common types of fire protection systems from O'Connor6 are summarized as follows:


Basic series model
The reliability, R, which has a numerical value from 0 (totally unreliable) to 100 (perfectly reliable), of a fire protection system that has a series of n, statistically independent components (the reliability of the ithcomponent is denoted as Ri), can be described by a series reliability model as follows:



An example of this fire protection system is a smoke management system with a smoke-extraction fan driven by a power source that is switched on by a smoke-detection device through a fire alarm panel. In this system, the components can be considered as wired in series in terms of signal or power transmission, and failure of any component will lead to failure of the smoke management system.


Active redundancy model
The reliability, R, of the simplest redundant system which consists of two statistically independent components, with reliabilities Ri and R2, where satisfactory operation occurs if either one or both parts function, can be described by an active redundancy model as follows:



An example of this fire protection system is a fire detection system with a number of fire detectors protecting the same enclosure, where actuation of any one detector or a combination of detectors will raise a fire alarm.


The general expression for an active parallel redundancy system with n, statistically independent components, is:



Standby redundancy model
The standby redundancy system refers to a system where one unit does not operate continuously but is only switched on when the primary unit fails. An example would be a fire pump system that consists of a duty pump and a standby pump, where the standby pump will be switched on only after the duty pump has failed. Another example of a standby redundancy system is a series of fire door assemblies that are used to subdivide a long corridor. The reliability, R, of this system with two statistically independent units, assuming both units have equal constant failure rates λ (with units of number of failures per unit time) and there are no dormant failures (dormant failure refers to failure of the component/system in non-operating conditions) to the sensing and switching systems, can be described as:


where t is the time of operation.

The frequency of occurrence of each fire scenario (i.e., each branch of the event tree) is the product of the frequency of ignition, probability of fire spread, and the reliability of the fire protection systems provided. Alternatively, the frequency of occurrence (denoted as λ, with units of number of occurrences per unit time) can be converted into a probability of occurrence (denoted as p) if the design life of the building or the period in which the fire risk assessment is applicable (denoted as T) is known, by the following relationship: 7


Quantification of consequences of each fire scenario is generally carried out by fire protection engineering calculations. It is in this step where the performance of fire protection systems employed in the fire safety strategy has to be evaluated. It was proposed that the performance of fire protection systems can be conceptually described by two parameters: efficacy and reliability, where efficacy is defined as the degree to which a system achieves an objective given that it operates.8


Once the frequency or probability of occurrence of each fire scenario and the consequence of the corresponding fire scenario are estimated, the risk of such fire scenario can be calculated through the following procedures.


Fire risk can be generally defined as a probability distribution function over the space of all possible fire scenarios, together with one or more severity or consequence functions, also defined over that space.9 Mathematically, the risk associated with a particular fire scenario originated from a particular fire ignition source at a particular location can be estimated from the following relationship.10


where f is the frequency of the particular fire scenario (units of time -1) and C is the consequence of the particular fire scenario (in units of number of fatalities, number of injuries, or unit of dollar loss, etc.).


where fiis the frequency of the ithfire scenario cluster (which is the sum of the frequencies of all fire scenarios contained in the cluster), Ciis the consequence of the ithfire scenario cluster, and N is the total number of fire scenario clusters considered.


The last step of the fire risk assessment is evaluation of the fire risk. In this step, the total fire risk estimated by the method described above for different distinct fire protection strategies can be compared for making risk-informed decisions. The fire risk assessment method can also be used to evaluate the total fire risk of fire safety strategies that place emphasis on different fire protection systems, for instance, one that relies heavily on active fire protection systems versus the other that relies heavily on passive fire protection systems. These computed fire risk levels also can be combined with a cost model for conducting a cost-benefit analysis.


Addressing Uncertainties in Quantitative Fire Risk Assessments
In conducting a fire risk analysis, one would need to realize that many of the parameters used in the risk estimation may have significant uncertainties. These may include errors incurred during the simplification process of the problem, the statistics on which the frequencies of occurrence or probabilities are derived, the reliabilities of fire protection systems, and the calculation methods and models used. It should be recognized that there are other types of uncertainties, such as perception uncertainties, public acceptance uncertainties, behavior uncertainties, and similar nonengineering uncertainties; however, addressing these uncertainties is beyond the scope of this paper.


A detailed discussion on the topic of uncertainty in fire safety engineering calculations has been given by Notarianni.11


Man-Cheung Hui is with Arup.



  1. NFPA 550, Guide to the Fire Safety Concepts Tree, National Fire Protection Association, Quincy, MA, 2002.
  2. Watts, J., "Fundamentals of Fire-Safe Building Design," Fire Protection Handbook, 19th edition, National Fire Protection Association, 2003.
  3. NEMA Supplement, "Fire Alarm Systems and Interior Finish A Balanced Approach," Fire Protection Engineering, Issue No. 24, Fall 2004.
  4. Gamache, S., Porth, D., and Diment, E., "The Development of an Education Program Effective in Reducing the Fire Deaths of Preschool Children," Proceedings of the 2nd International Symposium on Human Behavior in Fire, Interscience Communicaions, London. 2001.
  5. AS/NZS 4360, Risk Management, Standards Australia/Standards New Zealand, 2004.
  6. O'Connor, P., Practical Reliability Engineering, 4th edition, John Wiley & Sons, Ltd, Chichester, UK, 2002.
  7. Smith, D., Reliability, Maintainability and Risk --Practical Methods for Engineers, 6th edition, Butterworth-Heinemann, Linacre House, Jordan Hill, Oxford, 2001.
  8. Thomas, I., "Effectiveness of Fire Safety Components and Systems," J. of Fire Protection Engineering, Vol. 12, 2002, pp. 63-78.
  9. Hall, J., "Fire Risk Analysis," Fire Protection Handbook, 19th edition, National Fire Protection Association, 2003.
  10. Henley, E., and Kumamoto, H., Probabilistic Risk Assessment Reliability Engineering, Design and Analysis, IEEE Press, Piscataway, NJ, 1992.
  11. Notarianni, K., "Uncertainty" The SFPE Handbook of Fire Protection Engineering, 3rd edition, National Fire Protection Association Quincy, MA, 2002.

* Reprinted with permission from NFPA 550-2002, Fire Safety Concepts Tree, Copyright 2002, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

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