Tenant Wants to Double Storage Height in an Existing Warehouse
A 40,000 sq ft single-story warehouse has a 32 ft deck height and a steel-deck-on-bar-joist roof. The existing wet system was designed for 12 ft of palletized Class III commodity: 0.20 gpm/sq ft over a 1,500 sq ft design area, K-8.0 standard-spray pendants on a 10 ft x 10 ft grid, deflectors 8 in. below the deck. A new tenant has signed a lease that includes storing the same Class III commodity to 20 ft on pallets and wants occupancy within 90 days. The 8 in. city main produced a flow test of 70 psi static and 45 psi at 1,500 gpm. The contractor's proposal is to rerun the calculation at the same 0.20 gpm/sq ft and 1,500 sq ft, confirm the remote area still works, and issue the permit drawing. No changes to the sprinkler heads, spacing, or underground are proposed.
1. Is rerunning the existing design basis at 0.20 gpm/sq ft over 1,500 sq ft an acceptable basis for 20 ft of palletized Class III storage? Explain your reasoning.
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No. The original 0.20/1,500 basis is an ordinary-hazard Group II density/area point, valid for storage heights at or below the threshold at which the storage chapters take over. Once storage exceeds that threshold (commonly 12 ft for palletized commodities), the design must be selected from the storage occupancy density/area curves, where the controlling variables are commodity class, storage height, ceiling height, storage arrangement (palletized, solid pile, shelf, rack), and sprinkler type and K-factor.
At 20 ft of Class III palletized storage under a 32 ft ceiling, the required density and the required design area are both substantially greater than 0.20/1,500. The consequence is not a small recalculation - it is a different design point, likely a control-mode-density/area (CMDA) sprinkler with a larger K-factor, and a maximum coverage area per sprinkler that may be smaller than the current 100 sq ft. The existing 10 ft x 10 ft spacing and K-8.0 heads may simply be outside the listing for the required density.
The correct response to the tenant and the contractor is that the proposed basis is not defensible, the existing system is not adequate for the new lease, and a storage-occupancy design must be produced. Approving the rerun would create a system that is hydraulically 'calculated' but not designed.
2. Evaluate ESFR as the upgrade path: check clearance, head selection, and whether the water supply can support it.
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Clearance. ESFR requires a minimum of 36 in. between the deflector and the top of storage, and 36 in. between the deflector and the deck. With 20 ft of storage and a 32 ft deck there is roughly 12 ft available, so clearance is not the constraint. However, ESFR is restricted in obstructed construction; bar joists projecting below the deck must be checked against the listing limits, and any obstruction within the required zone around the deflector must be resolved.
Head selection. A typical K-14.0 ESFR is listed at a minimum of 35 psi with 12 sprinklers operating. Confirm the exact listing for the commodity, storage height, and ceiling height, because some arrangements require higher pressure or different head counts.
Supply check. Flow per head = 14 x sqrt(35) = 82.8 gpm. Twelve heads = 994 gpm. Add the 250 gpm hose allowance for ordinary hazard: total about 1,244 gpm.
Available pressure at 1,244 gpm: 70 - (70 - 45)(1,244/1,500)^2 = 70 - 25(0.688) = 70 - 17.2 = 52.8 psi.
The 35 psi is at the head. Add sprinkler-piping friction at roughly 1,000 gpm through the grid, riser and check-valve losses, backflow loss, and static elevation to a 31 ft deflector (about 13 psi). A rough base-of-riser demand lands near 75 to 85 psi against 52.8 psi available - a shortfall in the range of 25 psi.
Conclusion: ESFR is feasible in the building envelope but not on the existing supply. It requires a fire pump or a supply upgrade, and the decision should be made before the tenant is given a move-in date.
3. The tenant cannot accept a pump-and-permit delay. What interim measures are defensible, and what must be documented?
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The only defensible interim position is one that keeps the hazard within the existing design basis. That means a written, enforceable limit on storage height at or below the height the existing system was designed for, with the limit reflected in the lease, posted on the columns or walls in the storage area, and described on a signed letter from the building owner to the AHJ.
Items to document:
- The actual design basis of the existing system, taken from the original hydraulic calculation and permit drawings - not from memory or from a rule of thumb. State the commodity, storage height, density, design area, and head type assumed.
- The physical limit, expressed as a maximum stack height measured from the floor, and the means of enforcement (signage plus a periodic walk-through).
- A schedule for the permanent upgrade: design, submittal, permit, pump procurement (long lead item), and installation.
- AHJ concurrence in writing before the tenant occupies. A plan reviewer who finds 20 ft of storage behind an unmodified 12 ft design will treat it as an unpermitted hazard change.
What is not defensible: installing a few additional heads, raising the density on paper without changing the heads or spacing, or relying on the building's 32 ft ceiling as justification for the extra height. Each of those leaves the building without a design that matches its actual contents.