56/Plumbing Engineer January 2022
In a simplistic form, unobstructed and obstructed steel construction can be differentiated by one critical dimen- sion: 7 feet 6 inches. Where a floor deck or continuous smooth bay is supported on girders or trusses spaced more than 7 feet 6 inches apart, then the ceiling is con- sidered unobstructed construction. Where trusses and girders are spaced between 3 feet and 7 feet 6 inches apart, the ceiling is considered obstructed construction. Sprinkler strategies for unobstructed and obstructed construction vary, so it is important to know the differ- ences between them. Though maximum coverage areas under unobstructed and obstructed construction do not vary, the distance of the sprinkler deflector below the ceiling does. Section 10.2.6 of NFPA 13 (2019) details the guide- lines for deflector position below ceilings for stan- dard pendant and upright sprinklers. Similar to smooth horizontal ceilings, the deflector for a sprinkler under unobstructed steel structure construction must be within 1 inch and 12 inches of the ceiling (shown in Figure 2). However, this code requirement does not stand alone. Section 10.2.7.2.1.1 explains that obstructions less than or equal to 18 inches below the sprinkler deflector shall be considered to ensure the sprinkler's spray pattern can adequately cover the floor below. The scope of avoiding obstruction to sprinkler spray is outside the scope of this article. That being said, it is heavily advised to familiar- ize oneself with those requirements to make sure there is no deficiency in sprinkler coverage. When designing under obstructed steel construction, there may be multiple sprinkler design strategies to con- sider. Sprinkler design can closely resemble the strategy used in unobstructed construction spaces: install at least one sprinkler in each bay of obstructed sprinkler con- struction with its deflector 1 inch to 12 inches below the ceiling (shown in Figure 3). However, alternate strategies can be used. Sprinklers can be installed with their deflectors 1 inch to 6 inches below the structural members as long as the deflectors are a maximum distance of 22 inches below the ceiling/ roof deck (shown in Figure 4). Using this strategy will cut down on the number of sprinklers needed, therefore reducing the hydraulic demand of the system and cost of construction. When designing under steel structure ceilings, it is important to know these requirements to coordinate with other disciplines. Not only can the spacing of struc- tural members change throughout the course of project design, but designing under steel structure ceilings offers additional challenges when coordinating with other mechanical, electrical and plumbing (MEP) equipment. Open-Grid Ceilings Open-grid ceilings offer a special challenge from a fire protection standpoint. The term open-grid ceiling refers to a design where a ceiling is installed at a certain eleva- tion, but it does not make a fully concealed space above the ceiling (see Figure 5). This can be a challenge from a fire protection perspec- tive because of the way the open grid potentially delays sprinkler activation. Sprinklers activate primarily due to the heat transfer from the smoke plume and hot upper gas layer/ceiling jet to the sprinkler's thermal element. Once the thermal element/fusible link operates, water is released from the sprinkler. When an open-grid ceiling is introduced, the perfora- tions in the ceilings may not allow heat to fully collect around a sprinkler at the same elevation. Since hot gases can seep into the space above the ceiling, the sprinkler at the open-grid ceiling level might not activate. As such, in spaces with an open-grid ceiling, sprinklers will need to be installed at the higher ceiling level. However, a second issue can arise with the use of open-grid ceilings. The activation of a sprinkler at the high ceiling level will discharge water, but the presence of the open-grid ceiling below can obstruct the spray pattern of that sprinkler. If the open-grid ceiling is not open enough to the space above, or if there is not enough space between the sprinkler deflector and the open-grid ceiling, further requirements need to be met to have an NFPA 13-compliant sprinkler system. As such, Section 9.3.10 of NFPA 13 (2019) outlines the requirements for installing open-grid ceilings below sprinklers: "9.3.10 Open Grid Ceilings. Open-grid ceilings shall
Figure 2. Unobstructed construction sprinkler design. Photo: Arup
Figure 3. Obstructed construction sprinkler design, strategy 1. Photo: Arup Figure 4. Obstructed construction sprinkler design, strategy 2. Photo: Arup
Figure 5. Example of open-grid ceiling. Photo: Arup
Fire Sprinklers
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