Thirty-Foot Spacing at a Thirty-Two-Foot Deck in a High-Piled Warehouse
A 240,000 sq ft distribution warehouse has a 32 ft clear height to a steel deck with deep bar joists, ESFR sprinklers, and rack storage stacked to 24 ft. Twelve rooftop air-handling units, each rated 5,000 cfm, serve the warehouse; a 4,000 sq ft build-out of offices and a break room sits in one corner. The owner's insurer is pressing for early-warning smoke detection above the storage and has asked for a price. The low bid proposes 26 spot-type photoelectric smoke detectors at the deck, spaced 30 ft on center, with the note 'NFPA 72 allows 30 ft spacing, so this covers the whole floor.' The same proposal deletes duct detectors on all twelve units, reasoning that 'the units shut down on alarm anyway' and that the area detectors cover the space. The owner asks you, as the reviewing contractor, whether the proposal will do what the insurer expects.
1. Evaluate the proposed 30 ft on-center spot-type detector layout at the 32 ft deck.
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The 30 ft figure is being misapplied and the layout will not deliver early warning.
- Where the number comes from. Thirty feet is a listed spacing for a spot smoke detector under ordinary smooth-ceiling conditions. NFPA 72's tables permit increased spacing for smooth ceilings higher than 10 ft, but that is an allowance with assumptions attached - a smooth ceiling and a plume that reaches the detector in detectable concentration. It is not a method for designing detection over rack storage.
- Stratification. In a 32 ft space with storage to 24 ft, smoke from a rack fire rises, cools, and stratifies below the deck; smoke from a smoldering carton may never reach the ceiling in detectable concentration. Deep bar joists add beam pockets and dead air at the deck. A detector that satisfies a spacing dimension can still respond after the fire has grown well past early warning.
- What actually works. A performance-based design by a qualified person: listed air-sampling detection with sampling points in rack aisles or at selected elevations, projected-beam detectors arranged at the deck and possibly at intermediate levels, plus documented coverage based on the manufacturer's listing. Respect clearance from supply diffusers and wall/beam rules in the enforced edition.
- Paper trail. Obtain the insurer's written criteria and the designer's basis of design. 'Covers the whole floor' is not a performance statement, and a spacing sketch is not a design.
2. The contractor proposes to omit duct detectors on all twelve units because the units shut down on alarm. Is that acceptable?
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No. The reasoning reverses cause and effect.
Duct smoke detection exists for two separate purposes: it stops smoke from being distributed through the duct system into other parts of the building, and it initiates unit shutdown. Saying the unit 'shuts down on alarm anyway' assumes a detector somewhere will alarm first and that shutdown will follow - that is not a design, it is a hope. Mechanical code and NFPA 90A style requirements commonly call for a listed duct smoke detector on the supply side of air-handling units above 2,000 cfm, with return-side detection where capacity exceeds 15,000 cfm. At 5,000 cfm nameplate, each of these twelve units exceeds the supply-side threshold. Confirm cfm from the nameplate and the mechanical drawings, not from the schedule summary.
Implementation points that matter:
- The detector must be listed for the duct velocity and located per its listing, typically downstream of the unit and ahead of the first branch, with access for testing and cleaning.
- Shutdown must be initiated by the detector and supervised by the fire alarm panel; a unit that restarts after a reset without a signal is a defect.
- Remote test and indicator stations are needed so annual testing does not require ladder access to every unit.
- The office build-out has its own air system and its own detection; do not let one package cover both.
Deleting twelve duct detectors is a change of scope that should be rejected in writing, with the code basis cited to the owner and insurer.
3. What notification problems should you raise for the warehouse floor itself?
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The floor is a high-ambient, high-loss space and audibility is usually the weak point.
- Measure the ambient, do not assume it. Forklift traffic, conveyors, and rack movers commonly produce 80 to 90 dBA. Public-mode notification requires the alarm to be roughly 15 dBA above the average ambient, and never below the minimum (typically 65 dBA), so an 84 dBA floor implies about 99 dBA from the notification appliances.
- Check the upper limit. Appliances must not exceed the maximum permitted level at the minimum distance a person can approach. High-output horns can violate this while still being barely audible in the far aisle, which is why spacing and mounting height matter more than device count.
- Hearing protection and extreme noise. Where ambient exceeds the level at which audible signals are unreliable, or where occupants wear hearing protection, visible notification is required in addition to audible.
- Coverage geometry. With 32 ft ceilings and racking, mount appliances to be heard in aisles, not just at the ceiling grid; verify at the worst-case point (farthest aisle, engine running, during a shift) with a meter and record the readings.
- Voice may be the better answer. If the owner wants intelligible evacuation instructions, emergency voice communication changes the power supply and battery design, so decide before the panel is ordered.