Issue 31: Application and Benefits of Performance-Based Designed Fire Detection Systems
By Christopher Marrion, P.E., FSFPE
Over the last several years, great strides have been made in the
application of fire engineering. This has been facilitated in part by
the development of codes and guidelines for the application of
performance-based analysis and design of buildings and structures.1,2,3,4
Within the performance-based analysis and design process, various fire
and life safety systems and features exist that may be incorporated into
trial design(s), including fire detection systems. Fire detection
systems serve multiple purposes, including monitoring other devices and
systems, detecting products of combustion, activating smoke management
and other systems, notifying people that there is an emergency, and
summoning emergency responders (see Figure 1).
Figure 1 - Overview of Functions of a Fire Alarm Control Unit
The main objective of most fire detection and alarm signaling systems
is detecting a fire early so as to initiate various actions. Being able
to adequately assess the time when an initiating device may activate is
therefore important, especially when undertaking a performance-based
assessment and the overall development of a fire safety strategy. NFPA
721 contains information to assist in designing a detection
system prescriptively. In addition, over the last several revision
cycles, efforts have been made to help better address detection systems
in the performance-based analysis and design process. These are
reflected in 'Annex B – Engineering Guide for Automatic Fire Detector
Spacing'. This Annex has continued to build upon the work that SFPE and
NFPA have been supporting with respect to the development of codes,
standards, handbooks, and guidelines as well as research work in both
performance-based analysis and detector response. The use of
performance-based analysis in the design of a detection system is
allowed and encouraged in NFPA 72.
The following provides a general overview of the performance-based
analysis and design process and highlights the benefits this approach
brings to the performance-based design of fire detection systems.
For many applications, designing fire detection systems in accordance
with NFPA 72 on a prescriptive basis is typically adequate. However,
there are various situations or applications when one should consider
whether there may be value in undertaking a performance-based approach
when designing fire detection systems. This may include situations in
Design objectives are different than those of a Nationally Recognized Testing Laboratory (NRTL) evaluation test;
Ambient conditions (temperature, humidity, room size/geometry, etc.) are outside the performance parameters;
Very early smoke detection is required (computer/IT rooms, clean work environments, etc.);
Challenges exist with the ventilation systems (high velocities, duct/opening placement, etc.);
Detection is needed to protect an individual piece of equipment, artwork, etc.;
Activation of suppression systems is required at a given time/fire size to meet fire safety objectives;
Complex or irregular ceiling configurations pose challenges (i.e.,
complex ceiling geometries, beams, sloped ceiling, ease of testing &
High ceilings pose the potential for stratification;
Design goals require addressing potential visual obstructions
(banners, flags, etc.) and the desire for flexibility in use of the
Even if the process and analysis is not undertaken on a quantified
basis, there is value using this approach at a qualitative level.
The overall process as outlined in Annex B of NFPA 721 has been adopted to a large extent from the SFPE Engineering Guide to Performance Based Fire Protection2 and the work of Custer and Meacham.3 This involves a number of steps:
Identify Stakeholder's Objectives
Define Design Objectives
Define Performance Criteria
Develop Fire Scenarios/Design Fires
Develop Candidate Designs
Evaluate and Select Candidate Designs
Goals and objectives can be broken into two categories: "typical" fire
protection goals and "other" goals. Under the typical fire protection
goals, the desired outcomes in the event of a fire can be life safety,
property protection, continuity of operations, and limiting fire's
impact on the environment. Heritage/preservation goals are at times
considered as well. With regards to designing a detection system,
heritage preservation goals may pertain not only to limiting damage to
the property, but also limiting impact to the historic fabric arising
from the installation of a system or device. (See Figure 2)
Figure 2 – Detection goals may impact system selection, design and installation in historic buildings
In addition to the typical fire and life safety goals noted, there
may also be other goals that a fire detection system is intended to
achieve. These may include goals related to:
Design Objectives/Performance Criteria
The next step is to define design objectives and performance criteria.
These more explicitly and quantitatively express the goals and
objectives so one can determine whether the trial design meets the goals
and objectives when exposed to the design fire scenario. An example of
design objectives and performance criteria for a detection system is
given in table 1.
Table 1 - Objectives/Goals/Performance Criteria1
Fire Protection Goal
Provide life safety
No loss of life within the room of origin
Maintain tenable conditions within the room of origin
Detect the fire and activate the smoke management system in sufficient time to maintain:
Temperatures below tenable limits
Visibility above established limits.
Design Fire Scenarios
Design fire scenarios define the specific characteristics of the
building, occupants and fire that are pertinent to the analysis/design.
Some characteristics that may be more pertinent to the analysis/design
of the operation of a detector include those in Figure 3.
Figure 3 - Development of Design Fire Scenarios
In developing design fire curves, a few things should be noted.
Depending on the objectives and type of detector(s) used in the trial
designs, it may not be necessary to define the various fire stages for
all stages of a fire. For instance, for assessing activation of a smoke
detector, it may only be necessary to look at the initial periods
including ignition and growth.
It is important also to develop a design fire curve that defines
additional characteristics and details that may not typically be
addressed in other performance-based analyses and designs. This includes
defining the "fire signatures" that may be produced by specific fires.
Fire signatures are those fire induced by-products that one is looking
to detect that may include smoke, heat, flame, CO, etc. For instance, if
one had a fire that produced limited quantities of smoke, there could
be substantial delays in detecting the fire using a smoke detector, or
if one's objective were to detect a smoldering fire, then using heat
solely as a fire signature may not be appropriate.
Once the design fire and the fire signatures are defined, then one should define the Design Objective Heat Release Rate (QDO) and the Critical Heat Release Rate (QCR) (see Figure 4). QDO
represents the heat or product release rate which produces conditions
representative of the design objective. This is not the point at which
detection is needed. QCR represents that point on the fire
curve and accounts for delays in response that may include system
activation delays, fire department delays, etc.
It is important to differentiate between these as there may be delays
associated with actions that must occur before extinguishment can
begin. For instance, if the fire is going to be extinguished manually,
and the fire department is to be notified by the detection and alarm
system, actions including those in Figure 4 would need to be considered.
Figure 4 - Development of a Design Fire Curve and QCR and QDO for Fire Department Response
Develop Candidate Designs
The candidate designs represent the detection strategies intended to detect the fire at QCR. There are several factors related to the selection of initiating devices including:
Type of detection
Location of detectors, including with respect to fire location
Number of detectors
Sensitivity to the expected fire signature(s)
Alarm threshold and duration needed at that threshold
Additional selection considerations may involve those that would also reflect various goals/objectives previously established:
In terms of determining the location/spacing of detectors for the
candidate design, the designer should consider the impact of the
following as applicable regarding the ability and timeliness of the fire
signature(s) to reach the sensing element of the detector:
Ceiling shape and surface characteristics
Configuration of contents/hazard location
Fire signatures and design fires
Ambient conditions – temperature, humidity, etc.
Analysis of the trial design(s) may fall into three broad areas to be
assessed: production of fire signature(s), their transport from the fire
to the detector, and assessment of the detector response to the fire
signature(s) to determine whether the detector activates. Some of the
more pertinent aspects of each of these components are highlighted in
Figure 5 - Analysis of Fire Detection Response
The modeling and analysis tools and data available for each of these
steps varies depending on the compartment, fire signature and fire
detector being assessed. For instance, there are more and better
validated models to estimate the temperature at a proposed detector
location and the response of a heat detector than to look at the
production and changes to smoke during its transport to a smoke detector
or the response of a smoke detector.
This further highlights the ongoing need for the development of more
accurate performance metrics for smoke detector response. Designers
should also review design methods and data available in research reports
as well as analysis methods in NFPA 721 and the SFPE Handbook of Fire Protection Engineering4 and various computer models applicable to predicting detector response.
Documentation of analyses and designs is critical. Documentation should
address details regarding the overall performance-based analysis and
Scope of project
Basis for the performance criteria
Design fire(s) and fire signatures
Modeling and computational analysis
The following additional items should also be well documented to help
ensure the design matches the analysis, as well as ensure that any
special installation or testing and maintenance requirements are noted.
This should include development of:
Design specifications and drawings
Sequence of operation(s)
Special installation, testing/inspection/maintenance/commissioning requirements
Critical design assumptions
Chris Marrion is with Arup Fire.
NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2007.
SFPE Engineering Guide to Performance-Based Fire Protection, National Fire Protection Association, Quincy, MA, 2007.
Custer, R. and Meacham, B., Introduction to Performance Based Fire Safety, National Fire Protection Association, Quincy, MA, 1997.
Custer, R., Meacham, B. & Schifiliti, R., "Design of Detection Systems," SFPE Handbook of Fire Protection Engineering, Quincy, MA 2008.
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