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Author(s): Wayne Moore. Published on May 1, 2017.

Smoke detector spacing in beamed ceilings

BY WAYNE D. MOORE

Recently, participants on the NFPA Xchange forum posted questions regarding the spacing of spot-type smoke detectors for non-smooth ceiling configurations. Before addressing question about the installation requirements for any detection device, however, it’s important to understand the owner’s detection goals—without that knowledge, device spacing becomes moot.

We will assume that because the questions centered on spot-type smoke detector spacing, the owner had set early detection as an important goal. The questioner did not mention ceiling height, but ceiling height significantly impacts early detection. Essentially, the higher the ceiling, the larger the fire must grow before it actuates a detector. As stated in the Annex A of the 2016 edition of NFPA 72®, National Fire Alarm and Signaling Code, “in order for a smoke detector to respond, the smoke has to travel from the point of origin [of the fire] to the detector.”

Additionally, the code advises that the designer should understand fire size and smoke travel concepts to determine whether or not the design can meet the early detection goal. After determining whether or not it’s possible to meet the fire protection goals, the designer can then use the spacing requirements of NFPA 72.

Non-smooth ceilings—beams, joists, or other obstructions—present a challenge for the spacing of smoke detectors, and it’s worth highlighting two of the most common issues that affect spot-type smoke detector spacing for non-smooth ceilings. Section 17.7.3.2.4.2 provides spacing guidance based on beam depth as a percentage of ceiling height. The more difficult application occurs when the beams form a “waffle type” ceiling configuration. In such applications, depending on the ceiling height, detectors will either reside in the beam pocket or on the bottom of the beams.

A research report prepared for the technical committee by the Fire Protection Research Foundation found that “the geometry and reservoir effect is a significant factor that contributes to the development of velocity, temperature, and smoke obscuration conditions at smoke detectors located on the ceiling in beam pocket areas or at the bottom of beams as smoke collected in the reservoir volume spills into adjacent pockets.” The report found that the waffle- or pan-type ceiling created by beams or solid joists, while retarding the initial flow of smoke, can result in increased optical density, temperature rise, and gas velocities comparable to unconfined smooth ceilings. For those types of ceilings, the report advised, “an alternative smoke detector grid arrangement (such as a shifted grid), with detectors located to take advantage of the channeling effect due to the reservoirs created by the beam pockets, will improve detector response and might allow greater spacing.”

The model building codes and NFPA 101®, Life Safety Code®, often require corridor smoke detection. When beams occur in these spaces, the code offers additional guidance. Corridor spacing is based on the width of the corridor not exceeding 15 feet, with assumed ceiling heights of 10–12 feet. When the beams (or solid joists) exist perpendicular to the corridor length, smooth ceiling spacing shall be permitted, and the location of spot-type smoke detectors on ceilings, sidewalls, or the bottom of beams or solid joists shall be permitted. So, in fact, corridor spacing of spot-type smoke detectors can prove relatively easy given the prescribed conditions.

The important takeaway from this discussion is that although following the code for the device installation requirements is important, it becomes equally, if not more important, to meet the fire protection goals of the owner and to fully understand how smoke will be transported to the detector for efficient detection to occur. I encourage readers to study this entire section of the code and the related Annex A material for spot-type smoke detector placement in beamed ceiling configurations.

WAYNE D. MOORE is vice president at Jensen Hughes.