The Model of Success

When the term fire modeling is discussed, most people immediately assume it's referring to a computer fire-model program. However, the development of a fire model primarily includes a detailed analysis of the proposed facility configuration and potential fire scenarios, as well as research of performance criteria against which the model results will be evaluated. If utilized as part of a performance-based design, much of the analysis portion of the model will be a continuation of the overall established goals and performance criteria.

Computer models are often the calculation basis for a fire model and can be used to predict many time-based quantities such as fire growth rates, temperature, smoke movement, species (air) concentrations and heat transfer. It can also be employed for related events such as sprinkler operation, smoke detector operation, glass breakage and occupant egress. Computer models offer an easier method of performing complex calculations and storing data, although in many cases, hand calculations may be warranted to obtain a specific piece of data or to confirm model results.

The most general use of computer modeling is to predict the impact of a fire on the surrounding environment. Computer fire models are generally divided into two categories—field models and zone models—dependent upon the calculation methodology on which they are based.

Both model types are based upon equations that calculate the conservation of mass, momentum and energy. However, zone models typically apply the conservation relationships to three zones within a room including the fire plume, the heated gas upper layer and the cool gas lower layer. The results of the equations are therefore limited to average values for each zone and the model assumes a firm demarcation between the upper layer and lower layer—a rather broad presumption, albeit within the bounds of a reasonable estimation for many applications.

For example, zone models and the associated basic correlation are often used for performance-based atrium smoke management system design and analysis. The model and calculations can provide time-dependent data including the theoretical smoke layer decent, and average values for upper layer temperature, visibility and toxicity concentrations. The model results and methodology have proven reliable for installed systems for most situations.

Field models apply the conservation of mass, momentum and energy equations based upon much smaller, user-defined control volumes or cells and therefore can provide a much higher level of detail for a space. Field models are also known as computational fluid dynamics (CFD) models and typically require significant computer resources as well as extremely detailed data for input.

The good news is that the National Institute of Standards and Technology (NIST) has recently developed a CFD-based fire model—Fire Dynamics Simulator (FDS)—for use on a personal computer. The development and release of FDS has significantly advanced the accuracy, and therefore the extent of data available for use in an alternative design assessment. As an example, FDS allows the user to "test" different scenarios with a relatively high degree of accuracy, such as the impact of heat-detector placement or beam depth on detector response in a warehouse setting. However, the use of FDS is dependent upon the data available for input and still requires significant computer time and resources for a single simulation.

A common situation that arises during the design process is the desire by an owner or architect to include unique design elements that conflict with applicable code requirements. A total performance-based design approach is—or will be—an option to allow flexibility throughout the facility. More often an alternative approach can be developed specifically for the non-compliant condition.

One recent case study illustrates how modeling can be utilized to tackle unique performance-based design situations. (also see "A Unique Tunnel Operation," page 56).

In this case, a four-story, historically significant building previously used as an office building was to be renovated into a hotel. The existing structure contained two ornate, open convenience stairwells that connected all four stories of the facility, and did not meet applicable floor-to-floor separation and shaft enclosure requirements. The stairwells were not required egress components of the building and a complete automatic-sprinkler installation was scheduled as part of the renovation.

The first step was to develop assessment goals. In consideration of the historic nature of the building, the interested parties agreed that the analysis would be limited to an assessment of the hazard posed by the open stairwells on non-fire-floor exit-access corridors.

To accomplish this analysis, the designers utilized the FDS model to predict the effects of a credible design fire occurring in one of the lower level guestrooms and estimating the quantity, concentration and therefore the impact of combustion products on occupants of other levels. Because of the open stairwells, analysis of the floor of origin was not included as occupants of the fire floor were deemed at no greater hazard than if the stairwells were nonexistent or protected by shaft enclosures.

A fire occurring in the guestroom located directly across from the open stairwell on the first floor level was chosen as the design fire, as the scenario contains the highest concentration of combustible fuel packages that could credibly expose occupants of the building. A variety of fuel characteristics, including effective heat of combustion and toxicity species yields, were input specifically for cellulosic and textile materials for assessment of toxic conditions throughout the simulation. The room of origin's door to the hallway was assumed to remain open to allow smoke and hot gas movement. It should be noted that an automatic door closer would likely result in a barrier to issuing smoke into the hallway.

The design fire was allowed to grow until sprinkler operation, at which point the fire's energy release rate was assumed to remain steady, with no decay, for the remainder of the simulation. The FDS model contains the calculation algorithms to simulate sprinkler operation and the effect on the fire. The input data required to properly model the specific sprinkler operation, spray and characteristics, however, is generally not available until further sprinkler type and model testing is conducted. Regardless, assuming fire control at sprinkler operation with no subsequent decay is a generally accepted conservative assumption, and provided a sufficient level of detail for this simulation. Occupant tenability criteria was separately researched and developed with appropriate safety factors to result in simple pass/fail criteria for temperature and toxicity concentrations.

As expected, the smoke, heat and products of combustion from the fire scenario generally flowed from the fire room into the adjacent corridor and open stairwells, and began filling the upper floor corridors starting with the 4^th level. Therefore, the highest temperatures and carbon monoxide concentrations of the upper floors occurred on the 4^th floor. The analysis of the developed fire scenario and calculated data from the FDS indicated that the existing open stairwells should not transfer hazardous temperatures and quantities of carbon monoxide to adjacent floor level corridors, thereby allowing the corridors to continue to serve as exit access paths.

This analysis recognized that exiting occupants from adjacent floors may be exposed to elevated temperatures and products of combustion to some extent, but the installed automatic sprinkler system, fire-alarm system and relatively short anticipated exposure time should result in a sufficient level of safety for the occupants.

As the performance-based design options and the development of the next generation of fire model computer programs continue to evolve, the opportunity to utilize flexible design approaches will also continue to increase. These tools will likely be utilized to help evaluate and implement fire protection and life-safety strategies for all types and sizes of buildings, in addition to the current equivalency based and unique facility applications.

Additionally, the development of powerful fire modeling tools allow the fire-protection community the opportunity for further insight into the theoretical behavior of fire and smoke and its effect on the building environment. At the same time, the use of the fire modeling tools—especially the advanced models such as FDS—require a thorough understanding of the fire dynamics, fluid flow principles and calculation methods on which they are based.