Warehouse Beam Pockets and Duct Detection
An 18,000 sq ft single-story warehouse has a 24 ft ceiling and steel joists 30 in deep spaced 10 ft on center. The space is not sprinklered. Two rooftop air-handling units each move 10,000 cfm and serve the warehouse through ducted supply and return. The contractor installs spot smoke detectors on a 30 ft by 30 ft grid at the ceiling and one duct smoke detector on RTU-1 only. The contractor argues that the ceiling detectors provide equivalent protection and that a second duct detector is redundant. The AHJ notes that the joists create deep beam pockets and that the grid crosses several joists. The owner uses the warehouse for stored paper products and wants to avoid nuisance alarms. No engineering analysis or performance-based design has been submitted.
1. Evaluate the 30 ft by 30 ft spot detector grid in the joist bays. Is it acceptable without further analysis?
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The joists are 30 in deep, which is well beyond the 18 in threshold at which NFPA 72 treats the ceiling as beam pockets for spot-type smoke detection. In that arrangement, detectors generally must be placed in each bay and spacing must be adjusted to the bay geometry; a 30 ft grid that crosses joists does not demonstrate coverage of each pocket. With 10 ft joist spacing, the layout should be evaluated bay by bay, and detector spacing reduced accordingly. The contractor’s statement that ceiling detectors provide equivalent protection is not supported without a performance-based design. Because the warehouse stores paper products, smoke production and stratification also matter. The AHJ should require a corrected layout, not accept the grid as drawn.
2. Does the warehouse need duct smoke detection on both RTUs? Explain the role of a duct detector.
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The adopted mechanical and fire code typically requires duct smoke detection for air-handling units over 2,000 cfm serving the occupancies listed in the code; 10,000 cfm is far above that threshold. If this occupancy is within the code trigger, both RTUs need duct detection, not just RTU-1. A duct detector is not a substitute for area detection and is not made redundant by ceiling detectors. Its purpose is to detect smoke in the air distribution system and initiate shutdown or alarm functions so smoke is not spread through the building. The contractor should verify the exact occupancy trigger and the required shutdown sequence with the AHJ, but one detector on one of two identical units is incomplete on its face.
3. What design solution would you recommend to address both the beam pockets and the duct detection?
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First, prepare a bay-by-bay ceiling layout using the joist geometry and the listed detector spacing, and submit it for review. If the owner wants to avoid a full grid, a performance-based design could be proposed, but it must be engineered and accepted by the AHJ, not asserted by the contractor. Second, add duct detectors to both RTUs and coordinate shutdown with the fire alarm panel and the owner’s process. Use listed duct detectors with sampling tubes sized for the duct, and test them after installation. Third, address nuisance sources by locating detectors properly and setting sensitivity per the manufacturer. The final submittal should include the sequence of operations and the duct detector locations.