Fire and smoke modeling

Fire design scenarios were tested in a new building at University of Maryland Eastern Shore, scheduled to be occupied in fall 2015.

By Tiffney A. Cates-Chen, PE, Koffel Associates, Columbia, Md. December 12, 2014

The new Engineering, Aviation, Computer & Mathematical Sciences Building (EACMS) will be located on the campus of the University of Maryland Eastern Shore (UMES) in Princess Anne, Md. Completion of construction is expected by July 2015 and the building is anticipated to be open to students for the 2015 fall semester. The building was designed to comply with the 2012 International Building Code (IBC) and the 2012 NFPA 101: Life Safety Code as adopted by the State of Maryland. The EACMS building will be three stories in height, consisting of classrooms, lecture halls, laboratories, machine shops, office space, and food services. Each story is approximately 15 ft high and the clerestory above the 3rd floor extends approximately another 10 ft.

The atrium extends from the ground level up to the clerestory above the 3rd floor, for an approximate height of 55 ft. The clear space of the atrium measures approximately 245 ft long (east to west) and 20 ft wide (north to south) on the 2nd and 3rd floors. Walkways are provided on the 2nd and 3rd floors around the outer edge of the atrium, which protrude into the clear space of the atrium. An additional walkway on those floors connects the north and south walkways through the center of the atrium. There is another opening that penetrates the 2nd and 3rd floors in the south hallway as well as an unenclosed stairway in the north hallway.

Design approach and criteria

The 3-story atrium requires a smoke control system in accordance with IBC Section 404.5 and NFPA 101 Section 8.6.7. A natural smoke exhaust system will be used to exhaust smoke from the top of the atrium such that tenable conditions are maintained from the bottom of the atrium to 6 ft above the highest walking surface. Fire Dynamics Simulator (FDS) Version 5 modeling was used to evaluate the worst reasonably anticipated conditions within the atrium and to evaluate the effects of natural smoke exhaust on tenability within the space. Building areas that were not contiguous to the atrium were not included in the model.

Design fire scenarios

Four design fire scenarios were identified as reasonably expected to occur within the 3-story atrium:

  • A spill plume on the ground level in the center of the 3-story atrium underneath the walkway
  • A spill plume on the ground level underneath the stairs in the north hallway
  • An axisymmetric plume on the ground level between the studio and meeting rooms on the east side of the atrium
  • A spill plume on the 2nd floor next to the opening in the floor in the south hallway.

Each design scenario consisted of a constant heat release rate fire. 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 was modeled. A 5 MW (4740 Btu/sec) fire was used as the design fire size for all four design scenarios.

Each fire scenario was modeled with no wind, a 30 mph wind, and a 60 mph wind blowing north, south, east, and west. A sensitivity analysis was conducted for 27 different scenarios. The mesh resolution was increased to 2.5 ft per side for the sensitivity analyses to determine if increasing the model resolution would noticeably alter the results. Compared to the models that were run using the 2 ft per side mesh resolution, both mesh resolutions performed similarly.

Visibility and air temperature were evaluated to determine occupant tenability in each of the design fire scenarios. Experience has shown that concentrations of products of combustion are not a limiting factor in atrium smoke exhaust systems. Tenable conditions were required to be maintained for a period of at least 20 minutes in accordance with IBC Section 909.4.6. A minimum visibility distance of 30 ft was cited as the threshold above which egress is impeded through the smoke layer, in accordance with Purser’s Equation 25 (see the SFPE Handbook of Fire Protection Engineering).

Two maximum temperature criteria were examined in each of the design fire scenarios. A sustained maximum temperature of 169 F during a 20-minute exposure is deemed acceptable in accordance with Purser’s Equation 31 (see the SFPE Handbook of Fire Protection Engineering). Additionally, Purser suggests that a maximum temperature of 392 F (200 C) can be tolerated for up to 1 minute in Equation 32; accordingly, exceeding this temperature for any duration counted as a failure to maintain tenable conditions.

Smoke exhaust modeling

The results of each fire scenario, including smoke exhaust and sprinkler activation times, are summarized in Table 1. The values are from the no-wind scenarios for each fire location; the interior and exterior temperatures are both 68 F (20 C).

In all of the models considered, the visibility did not drop below 30 ft at any point. In all but two fire scenarios, under the south opening on the ground level and next to the south opening on the 2nd floor, the temperature did not rise above the 169 F value acceptable for exposures up to 20 minutes. This temperature above the 20-minute limit was contained to the south hallway, and at no point did the temperature approach the 392 F upper limit. Sprinkler activation was evaluated in the two cases where the temperature exceeded the 169 F limit, and it was found that sprinklers would be expected to activate in both cases in about 1 minute. After sprinkler activation, it is assumed that the temperature in this area drops sufficiently to provide tenable conditions for the remainder of the simulation.

The exterior temperature also impacts the effectiveness of the smoke control system. Under normal stack effect conditions, where the exterior is colder than the interior, the smoke control system performance varies in comparison to equal interior and exterior temperature conditions. The minimum visibility experienced decreased for a number of the fire scenarios. This can be attributed to the smoke plume being cooled by entraining much colder air, causing the smoke to become less buoyant. This leads to greater smoke accumulation on the top floor, but the smoke is cooler, about 20 F in almost every scenario.

Under reverse stack effect conditions, where the exterior is hotter than the interior, the smoke control system performance is the opposite of the normal stack effect. Instead of entraining cold air in the smoke plume, hot air is entrained which prohibits the plume from losing as much heat and allows it to become more buoyant. This increased buoyancy enhances the natural ventilation. However, the smoke layer temperature increases significantly, about 20 F in almost every scenario.

Tiffney A. Cates-Chen is a senior fire protection engineer with Koffel Associates.