Protection of Storage Under Sloped Ceilings Larger Than 2 in 12
NFPA’s Fire Protection Research Foundation published a report on large-scale fire tests performed at FM Global for rack storage of Group A Plastics under sloped ceilings with slopes steeper than 2 in 12. An excerpt of this report is presented summarizing the results from these suppression tests on ESFR sprinklers with a K-Factor of 16.8. General guidance for sprinkler design is provided by the authors.
Introduction
NFPA 13 [6], limits protection for storage applications to ceiling inclination slopes of no more than 2-in-12. FM Global requires ceiling inclines to be less than 10 degrees (approx. a 2-in-12 slope), if in-rack sprinklers are not installed. For ceiling slopes in excess of this cutoff, FM Global currently recommends a continuous false ceiling, or supplement of the ceiling sprinklers with in-rack sprinklers. NFPA 13 also requires the use of false or dropped ceilings, if ceiling inclines exceed 2-in-12 slopes. However, NFPA 13 requires sprinkler deflectors to have parallel-to-the-ceiling mounting.
The test results for rack storage protection under sloped ceilings presented in this report [1] are based on the final Phase III of the research project, where Phase I and II served as initial starting points for a better understanding of the prevalent suppression issues and resulting in test design guidance for the subsequent Phase III large-scale fire test configurations.
Phase I [2, 3] of this project involved a review of current storage configurations and numerical simulations of the effects of ceiling slope on suppression performance using a computational fluid dynamics (CFD) model with the sprinkler deflector in both the parallel-to-ceiling and parallel-to-floor orientations.
The CFD simulations found that spray simulations [1] demonstrated that “sprinkler deflector orientation, parallel-to-ceiling or parallel-to-floor, strongly affects the water flux that reaches the fire source, and that the parallel-to-floor orientation was preferable since it maintained a consistent water-flux distribution to the fire region irrespective of the ceiling inclination.”
Phase II [4] of the project included laboratory characterization of sprinkler sprays and numerical modeling that extended the work done in Phase I. The spray simulations confirmed that the parallel-to-floor sprinkler orientation was preferable.
Analysis of the results from both phases guided the development and design of the large-scale fire tests to address suppression performance effects from variations in ceiling slope, purlin and girder arrangements, and sprinkler deflector orientation.
Tested Storage Configurations
Seven large-scale sloped ceiling tests were conducted with a 4-tier-high, open shelf, a single and double row rack storage configuration of Cartoned unexpanded plastics (CUP) under sloped ceilings. Tests were performed for a 10 ft clearance between top of storage to sprinkler deflector.
Obstructed ceiling construction configurations (six test) in the form of purlins and girders were tested, as well as unobstructed ceiling constructions (sloped and non-sloped ceiling constructions). This includes a non-sloped unobstructed ceiling fire test performed to provide a baseline comparison (best case scenario).
Test Results
FM Global’s website shows a video [5] of the actual fire testing on a sloped ceiling test with the ceiling inclined at 18 degrees and obstructed ceiling construction in the form of 12 in. purlins and 24 in. girders. The tests were performed at FM Global’s Research Campus in West Glocester, Rhode Island.
Baseline – Non-Sloped Ceiling
During the baseline test and as expected, only three (ESFR) sprinkler heads opened and the fire was “quickly suppressed and did not travel to the ends of the main array (double row racks) or jump the aisle to either of the target arrays (single row racks). The test was concluded at 25 minutes and post-fire observation of the damage showed limited spread around the ignition location”.
Sloped Ceiling
Seven sloped ceiling tests were conducted at two different ceiling slopes with various obstruction criteria.
10º Ceiling Inclination
Four fire tests were conducted for a 10º (equivalent to a 2-in-12 roof slope) ceiling inclination (changing the purlin depth, closing of the purlin channels, girder spacing, sprinkler deflector distance, and sprinkler orientation). The trends of the results “strongly indicate that a parallel to the floor orientation would be beneficial for all slopes” and that “ceiling level obstructions should not occur”, otherwise the protection will be impacted.
18º Ceiling Inclination
Three fire tests were conducted with an 18º slope (equivalent to a 4-in-12 or a 1-in-3 roof slope) with similar configuration changes as tested for the 10º configuration. The results demonstrated that the applied (FM Global) pass/fail criteria for the test were acceptable, but resulted in additional sprinkler activations above the baseline.
Summary
The research report [1] lists the following conclusions for 10º ceiling inclination (2-in-12) and the specific test configurations explored:
- The sprinkler arrangement was sufficient to protect the standard Cartoned unexpanded plastics (CUP) commodity when the spray was not obstructed. It was noted that ceiling obstructions was not to interfere with spray discharge patterns.
- The pass/fail criteria for each of the fire tests was satisfied with the number of sprinklers operating ranging between 4 and 7, and additional 1-4 activations when compared to the fire test results of the baseline test.
For 18º ceiling inclination (4-in-12):
- The sprinkler arrangement was sufficient to protect the standard Cartoned unexpanded plastics (CUP) commodity.
- The pass/fail criteria for each of the tests was satisfied with the number of sprinklers operating ranging between 7 and 13*, an additional 4-10 activations when compared to the results of the baseline test.
* Note, NFPA 13 requires ESFR sprinkler systems to suppress the fire with less than 12 heads.
In summary, ceilings slopes representing a 10º inclination resulted in a number of sprinkler activations similar to the baseline, while the 18º ceiling slope resulted in significantly more additional sprinkler activations.
The authors noted in the report “While testing conducted was limited to a selected few storage conditions (e.g., commodity type, storage and ceiling heights), it is expected that the results will be applicable to other similar arrangements and that in general if the configuration (storage height, commodity, etc.) is similar to the test conditions, then the outcomes would be similar. If the storage configuration, protection scheme, or other conditions are varied from those tested, case specific analysis is recommended to support the design.”
Guidance
The guidance presented is based on the results of Phases I to III research and fire tests. While the conclusions and guidance were discussed specifically for the test configurations explored, the results are “expected to be more broadly applicable to similar configurations” [1, 7].
- Sprinkler deflector orientation should be arranged parallel to the floor to avoid a significant reduction in the water flux available to the ignition region. This is especially true for ceiling slopes in excess of 10º (2-in-12) and should be considered for all other slopes as well.
- When sprinklers are to be installed below the bottoms of the ceiling structural support members, such as purlins, the sprinkler deflector should be oriented with the deflectors on a horizontal plane no more than 6 inches below the underside of the structural support member. For ceiling inclinations up to 10º, purlin depths should be limited to 18 inches. For ceiling inclinations between 10º (2-in-12) and 18º (4-in-12), purlin depths should be limited to 12 inches.
- For ceiling slopes up to 10º with purlin depths greater than 12 inches, or ceiling slopes over 10º and purlin channels should be closed at the girder to prevent excessive ceiling jet channeling.
- For slopes in excess of 18º, and for all purlin depths, the installation of sprinklers at their normal listed spacing on a horizontal plane below the structural support members is not recommended. For these ceiling slopes alternative options including in-rack sprinklers and/or a false-/drop-ceiling with sprinklers installed below it, will need to be explored.
- Alternative sprinkler spacing options should ensure that there is no obstruction to the discharge pattern from the obstructed ceiling construction, when present.
In addition, it is mentioned that a fire occurring under a girder, due to the development of a ceiling jet in the purlin channels on either side of the girder, “is not a concern with regard to additional sprinkler activations” [1]. This conclusion is based on results from CFD simulations.
References
[1] Jordan, S.J., Ryder, N.L., Fire & Risk Alliance 7640 Standish Place, Rockville, MD, Protection of Storage Under Sloped Ceilings Phase III: Large Scale Testing Summary and Guidance, Fire Protection Research Foundation, One Batterymarch Park, Quincy, Massachusetts, September 2020.
