The Design Fire: Selecting Fire Characteristics for a CFD Model
By David Stacy and Adam Edwards
One of the most valued services fire protection engineering consultants can provide a client is the use of fire modeling as part of a performance-based design (PBD) approach. The PBD approach can save valuable project budget as well as provide a more complete design that provides an equivalent, if not an enhanced, level of safety for occupants in a building. Although fire modeling and PBD have been around for years, its potential benefits and growing acceptance among jurisdictions has resulted in a continuous expansion of its use. As a result, modeling spaces for the purpose of evaluating tenability is becoming common practice in the design industry. As with all subject matter experts, fire protection engineers serve as an asset to design teams by being able to model an atrium, mall or smoke protected seating area using probable fire scenarios and analyze tenability based on performance metrics agreed upon by the project stakeholders and the Authority Having Jurisdiction (AHJ). That being said, with the increasing application of such models, the importance of properly characterizing fire scenarios--often referred to as design fires--in these analyses has never been more critical
When determining applicable design fires, the first question of the design team should always be: what is the intended use or uses of the space and how will it be furnished? For an atrium space in an office building, the typical answers from the project stakeholders are: the space will be used for circulation and seating, with some tables and chairs. It is important to consider not only what the initial planned use for the space is, but what can be envisioned for the future. The common NIST “Kiosk” or “Christmas Tree” design fire may be applicable for the atrium space for the majority of the building’s life, but is there the possibility of a large stage every year for an awards banquet, a higher hazard exhibition event monthly, or could a large corporate tenant perhaps want to place a carbon composite race car in the atrium? As experienced professionals in fire safety, it is our role and responsibility to lead this discussion and explain the importance of being accurate. Not only do we want to ensure an adequate level of safety is provided for occupants, but we also want to verify we are providing the project stakeholders with a fire protection system that will not limit future uses of the space. This article provides an examination of the importance of properly selecting design fires, specifically in regards to the impact of fire characteristics that may be utilized in a tenability analysis.
FIRE CHARACTERSTIC ANALYSIS
An analysis was conducted through the use of Fire Dynamics Simulator (FDS) to evaluate the effect of varying fire characteristics on model results. A six-story atrium was built in FDS to evaluate how fuel load composition would affect occupant tenability throughout the space. A rendering of the atrium utilized for this analysis is provided in Figure 1.
In order to properly evaluate the impacts of specific fire characteristics, it is important to maintain the constant value of other parameters within the model. Design parameters that were used as constants for this analysis included:
- Six-story circular atrium with a clerestory
- Each floor in the modeled space was approximately 8,000 ft2 with a 700 ft2 opening in the center of the floor between levels
- Exhaust capacity of 60,000 CFM in the clerestory via four 15,000 CFM exhaust fans on the roof of the clerestory
- Natural makeup air symmetrically introduced into the ground floor via four openings, each with a free area of 100 ft2
- 5,000 kW axisymmetric design fire located within the opening at the lowest level
In a typical smoke control analysis, the performance of a smoke control system is evaluated against agreed upon tenability limits of visibility, temperature, and carbon monoxide (CO), measured six feet above any walkable surface in the smoke control zone. For the majority of analyses, visibility conditions are the criteria to first provide a failing condition in a space. For the purpose of this analysis, visibility conditions were utilized to evaluate the influence of varying fuel load compositions. Visibility conditions below 10 meters were considered failure in this analysis.
Table A.39 of the SFPE Handbook of Fire Protection Engineering, 5th Edition provides material properties of many natural and synthetic materials commonly utilized in products and furnishings that comprise fuel loads. Although common fuels are composed of numerous materials, this analysis examined wood (pine) and flexible polyurethane foam (GM23) for simplicity. The design fire input parameters within FDS that are of greatest importance for a tenability analysis are the soot yield, CO yield, and heat of combustion of a material. Table 1 demonstrates how varying the fuel load composition (by mass) of the two materials being evaluated in the models affects these input parameters.
Visibility conditions were evaluated on all levels of the atrium using the different fuel compositions. As would be expected, the most synthetically heavy of the fuel loads led to untenable conditions developing the quickest. The point of this analysis is to provide an understanding of the order of magnitude that a varying fuel load composition could have on assessing tenability in a space. For instance, in Figure 2, Run E (100% Polyurethane Foam) leads to untenable visibility conditions at 175 seconds, while Run A (100% Wood) doesn’t decrease below untenable conditions until 362 seconds, a 207% time increase. Prescribing a 100% synthetic fuel load may be very conservative for a space, but take into consideration Run B, which examines a 75% Wood/25% Polyurethane Foam ratio. This ratio, which may approximate a fuel load composition characteristic of a seating area, results in a time to tenability failure more than two minutes faster than a 100% wood fuel load.
The overall goal of this article is to demonstrate the importance of placing emphasis on properly characterizing design fires in terms of material properties. In our experience, reviews of fire modeling reports and input files have shown that the majority of design fire characteristic issues were based on the following:
- An internal preset value(s) utilized by a modeler for all fire models.
When fire protection engineers are taught modeling, some users are given “preset” values such as “a soot yield of 0.04 g/g shall be used to evaluate atria.” We strongly urge modelers to not become complacent with input parameters. A value may be appropriate for many spaces, but each project and space should be independently evaluated for possible uses and the input variables should reflect those uses. No two spaces are identical, so why should the same design fire characteristics be used?
- Application of material properties based on the rationalization that “they won’t have anything else in this space except wood chairs.”
Often, when tenability failures are encountered during an analysis, a design team may push for less demanding material properties based on limiting fuel loads in a space. This is a topic that gets brought up on many projects, but should be carefully considered. A tremendous effort on enforceability of such measures needs to occur in a building to sustain a design based on less demanding material properties for a space. Within the first few years of a building’s life cycle, it may be possible for an atrium to only contain wood benches. Three years later, cushions are added, and three years after that, there is a Christmas party with a band playing on a 300 ft2 combustible stage. All possible uses of a space, within reason, should be considered in the design phase in an effort to not limit future uses of the space while also maintaining life safety of occupants.
- No specification of material yields.
In FDS Version 5, it was possible to not consider a reaction (based on material properties) and a default of propane would be utilized. The soot yield associated with propane is 0.01 kg/kg, which is less than a 100% wood or cellulose material characteristics utilized in this analysis. This led to results which overestimated the performance of a smoke control system under more realistic fire conditions. This error was even more commonly seen through the introduction and use of graphical user interface (GUI) programs, which may have led the reaction not being modified by the user. In FDS Version 6, this possibility was eliminated by requiring a specified reaction input. In conjunction with this change in FDS, one GUI, Pyrosim by Thunderhead Engineering, has set their preset reaction to polyurethane foam, presumably with the intent that if the user does neglect to evaluate material properties, the model will reflect a more conservative reaction.
The importance of properly considering and selecting design fires for a space is the responsibility of the fire protection engineer developing the fire models. It is our responsibility when utilizing fire models to perform the analysis based on the characteristics of the space and guide the design team in making a proper assessment of the hazards and parameters to be evaluated for a given project. The focus should always be to design the smoke control system based on the representative fire scenarios, using our engineering judgment to properly identify these scenarios.
David Stacy and Adam Edwards are with JENSEN HUGHES