System Sensor Smoke Detector Spacing on Level Ceilings with Beams and Joists - Manual (Page 2)

Smoke Detector Spacing on Level Ceilings with Beams and Joists
Executive Overview
NFPA 72, The National Fire Alarm Code, recommends using 900 square feet (30 feet
on center) as a guideline for spacing spot type smoke detectors on smooth ceilings.
While this is only a recommendation, virtually all manufacturers of smoke detectors
follow this guideline. There has always been a question of how detector spacing
needs to change when the ceiling is not smooth, but is interrupted by beams or
joists. The language in the 2002 edition of NFPA 72 code was based on the fire
dynamics research reported in 1993-1994 and was considered the best available
data at that time. However, recent advances in computational fluid dynamics (CFD)
have allowed fire protection engineers to revisit the current guidelines and make
concrete recommendations for changes in NFPA 72.
Industry Challenges
The 2002 edition of NFPA 72 requires that when beams project more than 12 inches
below the ceiling and when ceiling heights exceed 12 feet, each bay (beam pocket)
formed by the beams should/must be treated as a separate area. That is, each bay would
need one or more detectors depending upon its physical size. When this requirement
was first incorporated into the code, it was backed by the best CFD data available at
the time. Subsequent to this code change, many fire alarm system designers discovered
that following the literal code text could mean using a large number of detectors and
incurring exorbitant installation costs in many common building designs. Historically,
this part of the code has been a subject of contention in the fire protection industry
and among NFPA 72 Initiating Devices Committee members.
Current Recommendations for Smoke Detector Spacing:
NFPA 72 requires different detector spacing depending upon whether the structural
or ornamental members on the ceiling fall into the definition of a beam or a joist
and the depth of those members. The code defines solid joist construction as
members that are greater than 4 inches in depth and spaced 3 feet or less apart.
Beam construction, on the other hand, is defined as members that are greater than
4 inches in depth that are spaced more than 3 feet apart. For the purposes of spot-
type smoke detectors, the spacing rules are the same for ceilings with beams and
solid joist construction.
For level ceilings, NFPA 72 currently requires spacing smoke detectors at one half
their listed spacing, perpendicular to the run of the beams on a ceiling less than or
equal to 12 feet in height, when the beams are less than or equal to 12 inches deep.
When parallel to the beams, NFPA requires standard spacing for smoke detectors.
In this case, the detectors may be mounted on the ceiling or on the bottom of the
beams. If the beams exceed 12 inches in depth or the ceiling height of the space
exceeds 12 feet, the detectors are required in every beam pocket on the ceiling.
Research
In April 2005, the Fire Protection Research Foundation funded new research using
CFD computer modeling to reexamine smoke detector spacing on ceilings with beams
and/or joists. The CFD analysis was performed by Schirmer Engineering Corporation
System Sensor. Advanced Ideas. Advanced Solutions. 2
Following NFPA 72 2002
requirements could result
in higher installation costs.
Ceilings with beams
and joists affect detector
spacing.