Issue 54: SFPE Guidelines for Substantiating a Fire Model for a Given Application
By Craig E. Hofmeister, P.E., FSFPE & Stephen M. Hill, P.E. Rolf Jensen and Associates, Inc.
The use of computer fire models in fire hazard and fire
protection analyses has gained an increasing level of acceptance in
recent years, with predictive capabilities spanning a wide variety of
applications. Additionally, individual fire models have been continually
developed and refined to provide more sophisticated tools with
impressive visual graphics and output. In broad terms, fire modeling and
fire simulation can include basic simple algebraic correlations,
lumped-parameter models (zone models) and computational fluid dynamics
models (field models), which are used to predict, or replicate, various
fire phenomena within an established set of boundary conditions.
In order to be an effective tool for a fire related
analysis, the model user and involved stakeholders must have confidence
in the model results. There has been significant work in recent years to
verify and validate individual fire models; however, there has been
limited guidance for both the model user and the reviewer/authority
having jurisdiction/consumer to assess whether the selected model was
appropriate for the particular application. Therefore, the Society of
Fire Protection Engineers (SFPE) developed the Guidelines for
Substantiating a Fire Model for a Given Application1 (Engineering Guide), which was published in early 2011.
In the past year, the Engineering Guide and the process
outlined therein has gained some acceptance in performance-based design
applications. The Engineering Guide defines a process by which all
stakeholders can establish a basic level of confidence in the modeling
approach and results.
The Engineering Guide establishes a methodology with
specific steps to review the suitability of a fire model for a specific
Define the problem of interest
Select a candidate model
Verify and validate the model
Address user effects
The methodology is summarized in the following figure taken from the Engineering Guide.1 The figure outlines the process in flow chart form.
Figure 1 - Engineering Guide Fire Model Selection Flow Chart
Define the Problem of Interest
The initial step in reviewing a fire modeling
application is to fully understand the scope of the problem, the factors
that may influence the calculation or model, and if considered
necessary, review and/or search the literature for previous work in the
subject area. The basic problem definition is important to establish the
fundamental requirements of the application regardless of the user's
familiarity with the analysis.
As an example, a "typical" atrium smoke control
application can include a variety of parameters and physics such as
smoke movement, smoke temperature, smoke concentration, smoke layer
location and depth, compartment pressure, compartment ventilation,
visibility, and flame height. The analysis may also include an
evaluation of heat release rate, heat flux, ceiling jet temperature and
velocity and sprinkler/detector response as part of an initial design
fire study. Several other parameters need to be considered based upon
the available information, including the geometry of the space or
domain, the timeline of the analysis, time-based events, impact of
materials, initial conditions and boundary conditions.
The final component in a comprehensive evaluation of the
problem of interest is to clearly define the objectives and output of
the fire model analysis.
Select a Candidate Model
The baseline intent for substantiating a fire model for
an application is to check whether a model with the required
capabilities (governing equations and assumptions) and level of accuracy
are appropriate for the problem of interest. Many models and model
types are available, often with overlapping capabilities. In order to
adequately review a candidate model, the user must establish the
available model input data and the desired outputs.
The general options for choosing a fire model include
algebraic equations/correlations, zone models (lumped parameter), or
field models (computational fluid dynamics). Model selection often can
be a balance between obtaining the necessary level of output resolution
and the level of uncertainty associated with available input and the
model's use. A more complicated solution is not always a better solution
unless the problem definition has confirmed that greater detail is
Verification and Validation
Regardless of the candidate model selected by the user,
the Engineering Guide emphasizes the importance for the user to consider
and assess the predictive capability of the model to be sure it is
appropriate for its intended use.
The process of verification is intended to check whether
the mathematics of the model are correct and that the physics will be
correctly described by appropriate equations. Because a model is often
comprised of equations that are packaged into a program developed by
others, the practicality of the user's role in verification is generally
limited. It is incumbent on the user during the verification process to
have a thorough understanding of the underlying assumptions and
limitations of the calculations performed by the model.
In the event that existing V&V studies are not
available for the application of interest, it may be necessary to either
review additional experimental data or commission specific fire
experiments in order to collect measurements against which model
predictions may be compared. The blind application of a model must be
avoided at the cost of reporting erroneous results. Ultimately, the
results obtained from the most basic of calculations to more complex
fire models are only as good as their accuracy of the physics that are
Additional levels of uncertainty are often integrated
into a model by the user in the form of assumptions, input parameters,
and the spatial definition of the modeled space. User effects often
result in additional error outside the predictive capability of the
calculations. As such, a modeler should have a good understanding of the
sensitivity of user input parameters and the impact they may have on
the model output.
The user effects can include chosen parameters,
including the spatial domain, design fire and location, suppression
effects, or output parameters. The impact of user effects often
increases with the complexity of the chosen model.
While the accuracy of the model calculations has a
direct impact on the validity of the model (V&V), the detail and
accuracy of the model inputs and assumptions can have just as large an
impact on the model results.
During the initial stages of any fire modeling
application, it is considered good practice to document the steps
outlined in the Engineering Guide. Such documentation provides one with a
template that can be used as thorough justification for qualifying a
model as appropriate for the desired applications. Depending on the
project and the requirements of the client, stakeholder or AHJ, it may
be necessary to provide varying levels of detail or types of