54/Plumbing Engineer January 2022

By Jack DeVine, EIT, and Robert Accosta Jr., PE

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s fire protection engineers and system design- ers, Arup regularly collaborates with architects to coordinate sprinkler layout and design with the architectural vision. A long list of project-specific factors can complicate the sprinkler system design; how- ever, one thing is always consistent - the way ceilings are designed has a significant impact on the sprinkler system. As such, accurately and efficiently designing a fire protection system hinges on knowledge of ceiling design and its effect on sprinkler layout. This article introduces different strategies and select NFPA 13 requirements for laying out sprinklers under different types of ceilings. Smooth Flat Ceilings Smooth flat ceilings are arguably the easiest configu- ration to work with from a fire protection standpoint. In most situations, designers can complete the layout based on NFPA 13-compliant sprinkler spacing for the space's occupancy hazard. However, there are a few things to keep in mind. One of the most common types of smooth horizontal ceilings is acoustic ceiling tiles. It is preferable to keep sprinklers centered in acoustic tiles for aesthetic reasons. There needs to be thorough coordination with lights, HVAC diffusers, occupancy sensors, fire alarm devices, exit signs and any other equipment that also may be vying for space in the ceiling. Placement of the sprinklers in relation to this other equipment shall ensure no obstruc- tion to the sprinkler discharge as defined by NFPA 13. Obstructed and Unobstructed Construction Often in back-of-house spaces, warehouses or spaces where a more "industrial feel" is sought after, there will be no ceiling in a space. Instead, architects opt only to have the floor deck above exposed, supported on steel beams, as shown in Figure 1. Spaces open to a steel structure and floor deck above are as common as spaces with smooth, horizontal ceil- ings but offer a further challenge. This can be attributed to the fact that a space with a steel structure above can be classified either as unobstructed ceiling construction or obstructed ceiling construction by NFPA 13. Steel struc- tures can either be unobstructed or obstructed construc- tion, depending on the structure's geometry. As implied by its name, unobstructed construction is a type of ceiling construction that does not "impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire." Conversely, obstructed construction is a type of ceiling where "beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire."

Understanding the strategies may result in a more efficient sprinkler design and layout.

The Balance Between Ceiling Design and Fire Sprinkler Layout

Figure 1. Example of a steel-constructed ceiling. Photo: Arup