Sample design fire scenario

Fire scenarios include relevant assumptions regarding the potential fire scenario and describe the meaning of the fire in terms that can be understood by the building owner.

By William E. Koffel, PE, FSFPE, Koffel Associates, Columbia, Md. May 24, 2019

The design fire scenario was chosen based on the reasonably anticipated use of the space. The design fire scenario was identified as being reasonably expected to occur within the four-story atrium: a 1-megawatt fire creating a balcony spill plume was located in the first floor study area A. An alternative 3-megawatt axisymmetric plume design fire located in the center of the atrium was investigated and found to produce a significantly smaller hazard than the 1 megawatt balcony spill plume fire. 

The fire scenario was conducted by specifying a constant heat release rate fire in the appropriate location. This is a conservative approach because realistically a fire would take several minutes to attain its maximum heat release rate and then only sustain that high output for a matter of seconds before the heat release rate would subside to a much lower value. By specifying the fire to have a constant heat output (and therefore a constant smoke output) from the beginning of the scenario, a more conservative estimate of the conditions within the atrium is attained. 

Because this is a low-ceiling area, it is prudent to assume that sprinkler activation will control the growth of any fire in this area. Preliminary fire modeling has indicated that automatic sprinklers will maintain a total heat release rate of less than 750 Btu/second (0.79 megawatt) for a fast t-squared fire, thus a conservative 950 Btu/second (1 megawatt) design fire is assumed for this area. 

Minimum fuel package separation calculations in NFPA 92: Standard for Smoke Control Systems, assuming a 10-kilowatt critical radiant flux for ignition of fuel packages, indicates that in low-ceiling areas within the atrium, where sprinkler activation can be expected to constrain the fire to 1 megawatt, a minimum of 5 feet must be maintained between adjacent fuel packages.  

A 1megawatt fuel package might, for example, typically consist of one of the following arrangements:  

  • An individual fully cushioned and upholstered wood frame lounge chair. 
  • A loveseat with a wood side table. 
  • A set of approximately four metal-frame lightly upholstered chairs with a full-size wood table.  

In the high-ceiling portion of the atrium, where a 3megawatt design fire is more likely, a minimum fuel package separation distance of 23 feet is required. 

Author Bio: William E. Koffel is president of Koffel Associates. He is chair of the NFPA Correlating Committee on Life Safety and a member of several NFPA technical committees. He is a member of the Consulting-Specifying Engineer editorial advisory board.