Smoke Detector Spacing on Level Ceilings with Beams and Joists
System Sensor. Advanced Ideas. Advanced Solutions.
3
and Vision Systems Ltd. using Fire Dynamics Simulator and Smokeview Version
4
1
(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.
1
The CFD software is available free of charge from the NIST web site: www.fire.nist.gov/fds.
2
The full report is available on the National Fire Protection Association web site: www.nfpa.org.
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A recent analysis shows
no basis for many of the
spacing requirements in the
2002 edition.
