Mixed-Hazard Office Building: Can a Light-Hazard Design Cover a Records-Storage Room?
A 4-story office building measures 120 ft by 100 ft per floor and is protected by a wet-pipe NFPA 13 system. The ground floor contains a 2,400 ft² records-storage room with 8 ft high shelving of Class III commodity (paper in cartons) and a 12 ft ceiling. The rest of the ground floor and all upper floors are light-hazard offices with 9 ft ceilings. The sprinkler contractor proposes one light-hazard design of 0.10 gpm/ft² over 1,500 ft², arguing that the storage room is small and separated by a 1-hour wall. The water supply flow test shows 68 psi static and 52 psi residual at 1,000 gpm. The plan reviewer questions the hazard classification, the design area, and whether quick-response sprinklers can be used throughout the building.
1. Is the records-storage room correctly classified as light hazard, and can the proposed light-hazard design be extended over the entire building? Explain your reasoning.
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The 1-hour separation wall does not change the commodity hazard. Class III commodity stored 8 ft high under a 12 ft ceiling is Ordinary Hazard Group 2 (OH2), not light hazard. The 2,400 ft² room is a storage area, not a minor incidental space, so the light-hazard density cannot be extended over it simply because the room is separated.
Under NFPA 13, when different hazards are on one system, the designer either (a) designs each area separately with the remote area located in the most demanding area, or (b) designs the whole system to the most demanding hazard. A single 0.10 gpm/ft² light-hazard calculation is therefore not acceptable for this building as proposed. The reviewer should require an OH2 design for the storage room and a separate light-hazard design for the office areas.
2. If the storage room is designed as OH2, what density, area, and hose allowance apply? How should the quick-response area reduction be evaluated?
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For OH2, the density/area point is 0.20 gpm/ft² over 1,500 ft², with a 250 gpm hose allowance. Base sprinkler flow = 0.20 × 1,500 = 300 gpm; adding the hose allowance gives about 550 gpm at the base of the riser before elevation and friction adjustments.
NFPA 13 permits a 20% design-area reduction for quick-response sprinklers in ordinary hazard only where all limiting conditions are met. If valid, the area becomes 1,200 ft²: 0.20 × 1,200 = 240 gpm + 250 gpm hose = 490 gpm. Do not apply that reduction automatically to a storage room. Verify the sprinkler listing, ceiling height, and the edition adopted in Maryland. If the storage room cannot qualify, use the full 1,500 ft².
3. Does the water supply appear adequate? What additional information should the reviewer require before accepting the design?
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The flow test is only a supply indicator. The calculated system demand is roughly 550 gpm, or 490 gpm with a valid quick-response reduction, which is well below the 1,000 gpm test flow. A residual pressure of 52 psi at 1,000 gpm suggests the supply can likely deliver the needed flow, but that does not prove adequacy.
Adequacy is proven by a hydraulic calculation that includes elevation, pipe friction, fittings, backflow preventer, meter, and a safety margin. The reviewer should require the calculation to plot the actual demand point against the supply curve, not merely compare 550 gpm with 1,000 gpm. If the calculation shows the demand point at or beyond the available supply, a tank, pump, or revised design is needed.