Experts know what tool works best for the job at hand. A Fire Protection Engineer should know which Fire Modeling method to utilize for the specific fire protection challenge they are faced with. Since The International Building Code (IBC) Section 404.5 and NFPA 101, Life Safety Code, Section 8.6.7 both require smoke control systems to be installed in atrium spaces, Fire Protection Consultants are frequently tasked with determining required exhaust and makeup air requirements for atrium projects to ensure tenable conditions for egress are maintained during a fire scenario. When it comes to design of Smoke Control Systems for atriums, the Fire Protection Engineer has multiple tools to choose from, but one fire modeling option is typically better than the others to perform atrium smoke modeling.
There are multiple modeling options for atrium smoke control design permitted by NFPA 92, Standard for Smoke Control Systems, which is referenced by both the IBC and NFPA 101:
- Algebraic Hand Calculations
- Scale Modeling
- Compartment Fire Models, broken down further into Zone Models and CFD Models below
Let’s explore these options:
Algebraic Hand Calculations:
NFPA 92 provides several algebraic equations to determine the volume, temperature, and density of smoke from a design fire; the number of exhaust inlets to prevent plugholing (fresh air below the smoke layer being drawing through the upper smoke layer); and ultimately the required Volumetric Flow Rate of smoke exhaust required to maintain the smoke layer interface at a specified height, typically six feet above the highest occupiable floor level.
While this is a great method for early estimates or very simple buildings, it is usually conservative and results in higher exhaust capacities than necessary. Consequently, project costs are higher than necessary to accommodate the exhaust capacity and makeup air. Additionally, unique architectural features of the building aren’t easily incorporated into basic algebraic equations.
See a real-world scenario where smoke exhaust capacities were reduced from 200,000 CFM using hand calculations to 120,000 CFM using another method in an atrium (LINK to FIT Atrium: https://pbfpe.com/projects-archive/florida-institute-atrium/)
This method involves creating a scaled replica of the atrium and conducting fire tests based upon various design fire scenarios. Alternatively, and less common, saltwater scale models can also be used to represent smoke movement within an atrium model that is inverted in a tank of water. Since actual testing is being performed, the simulations are very realistic and physically observable. These simulations rely on the use of scaling and require extreme care by the Fire Protection Engineer to ensure that the simulation and results are accurately scaled to the real building. Using the appropriate scaling factors, results from scale tests can be extrapolated to predict conditions in full-scale scenarios.
While most Fire Protection Engineers (including our FPEs) would love to burn scale models and use the results to dial in good smoke exhaust rates, this method requires a lot of resources including test instruments and time. It would also be difficult to make quick adjustments when the design changes, requiring modifications to the scale model and further testing. This modeling technique is usually reserved for research purposes and not normally adopted in practice.
Zone models utilize the concept of dividing an enclosure into two layers (zones): a hot, smoke-filled upper layer and a cooler, smoke-free lower layer. Temperature and smoke density is considered uniform throughout the upper layer. Zone models can help predict detector and sprinkler activation times and can simulate the effects of smoke exhaust. A common Zone fire model that Fire Protection Engineers use is the Consolidated Mode of Fire and Smoke Transport (CFAST), developed by the National Institute of Standards and Technology (NIST).
Due to the relative simplicity of zone models, the computational demand is significantly less, and results can be processed much more quickly when compared to CFD models. Zone models may be appropriate for very basic atrium spaces or for preliminary rough order of magnitude calculations, but these models are quite simplistic, lacking in the level of detail possible through CFD modeling. Zone models fail to account for any transitional region, which could lead to misleading results. Complex geometries of atriums, such as cloud ceilings or non-uniform layouts of floors open to the atrium, cannot be modeled accurately through zone models.
Atrium spaces can be modeled in great detail using Computational Fluid Dynamics (CFD) software, such as the Fire Dynamics Simulator (FDS) developed by NIST. CFD modeling involves dividing an atrium space into cells and simulating smoke and heat movement by numerically solving Navier-Stokes equations, allowing Fire Protection Engineers to realistically simulate plumes, ceiling jets, smoke layers, detector and sprinkler activation, etc. Tenability conditions can be analyzed at a cellular level instead of assuming a uniform upper layer as with Zone models.
With great detail comes the drawback of longer calculation times; some complex atrium CFD models can take several days to run. CFD modeling requires extensive training, significant experience, and a good scientific understanding of input parameters. Despite the long computation times and engineering skills needed for a proper analysis, CFD modeling performed by an experienced Fire Protection Engineer is usually the best option for design teams when designing smoke exhaust systems for atriums.
The capability to simulate fire scenarios in virtually any geometrically shaped structure, to evaluate tenability conditions within the transition zone below the upper layer, and to analyze the effects of makeup air velocities are some of the many reasons why CFD modeling is superior to algebraic calculations or Zone modeling. In most cases, CFD modeling will ultimately save construction costs due to decreased exhaust and makeup air capacities. Additionally, CFD modeling provides a method of analyzing fire scenarios without the need for a testing lab as required for scale modeling. This method also allows for quicker implementation of design changes and an understanding of impacts to the smoke control strategy throughout the design process.
Atrium Fire Modeling Projects:
See how our atrium smoke modeling effort can assist you and your team here!
Reference code sections based on the 2018 International Building Code: