System Sensor Smoke Detector Spacing on Level Ceilings with Beams and Joists manual

Smoke Detector Spacing on Level Ceilings with Beams and Joists

and Vision Systems Ltd. using Fire Dynamics Simulator and Smokeview Version 41 (developed by the National Institute of Science and Technology). The results of this research were published in a report entitled, “Smoke Detector Performance for Level Ceilings with Deep Beams and Deep Beam Pocket Configurations.”2 The report was issued March 28, 2006 and was the basis for recommended changes to NFPA 72.

Based on a variety of modeled flaming fire scenarios, the results of this study indicate:

•There is no technical basis for the NFPA 72 2002 edition requirement for placing smoke detectors in every beam pocket when ceilings are greater than 12 feet in height and/or beams are greater than 12 inches in depth.

•The smooth ceiling 30-foot spacing guideline of NFPA 72, with permitted increases for narrow space geometry, allows smoke detectors to extend to approximately 41 feet on-center and along a corridor. For ceilings up to 24 feet in height, the deep-beam configurations do not negatively affect expected performance. This means that for corridor conditions, spot smoke detectors can be effectively used with deep beams at spacings for smooth ceilings.

•Standard 900-square-foot smoke detector spacing is adequate for waffle or pan type ceilings up to 24 feet in height and for beams up to 24 inches in depth that are spaced no greater than 12 feet on center.

•Spot smoke detectors may be placed on the bottom of the beams or in the beam pocket without any significant difference in performance.

•The concern about mounting smoke detectors a minimum of 12 inches from a ceiling-beam corner, per NFPA 72, is unsubstantiated. The CFD modeling showed no stagnant areas in the beam pockets that would preclude smoke detector activation. Although the modeling results showed no stagnant zones, it does not suggest that spot detectors can be installed in close proximity or contact to the wall or ceiling surface. The research report notes that such close mounting may impact the airflow characteristics into and around the detector housing, which could have a negative impact on how smoke flows into a detector’s sensing chamber.

1The CFD software is available free of charge from the NIST web site: www.fire.nist.gov/fds.

2The full report is available on the National Fire Protection Association web site: www.nfpa.org.

A recent analysis shows no basis for many of the spacing requirements in the 2002 edition.

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