Warehouse Layout: Passing Spacing While Failing Area
A 100 ft by 150 ft Ordinary Hazard Group 2 warehouse has a 22 ft ceiling and a wet system, with 10 ft of Class III commodity stored on the floor - no racks, no high-piled storage. The draftsperson submits 7 branch lines spaced 14 ft 3 in. apart with 10 sprinklers per line at 15 ft on center, 70 sprinklers total. His note reads: 'every sprinkler is within the 15 ft maximum spacing, and the remote area is taken as three branch lines of ten sprinklers, which is 1,500 ft².' The owner has asked whether the sprinklers can simply be spread out to the 15 ft maximum in both directions to save money.
1. Evaluate the proposed layout and answer the owner's question.
Read the worked answer
The layout fails on area per sprinkler, not spacing. Branch lines at 14 ft 3 in. (14.25 ft) with heads at 15 ft gives:
14.25 × 15 = 213.75 ft² per sprinkler
The OH2 ceiling limit is 130 ft² per sprinkler no matter how neatly the heads line up at 15 ft. Spacing and area are two independent limits and the tighter one governs.
The 'spread them out' request fails the same test: 15 × 15 = 225 ft² per sprinkler, 73% over the limit. It cannot be done.
A workable arrangement. Keep 15 ft along the branch (the maximum) and derive branch spacing from the area limit:
- 130 / 15 = 8.67 ft maximum between branch lines → 100 / 8.67 = 11.5, so 12 branch lines at 8 ft 4 in.
- Along the branch: 150 ft at 15 ft on center with 7 ft 6 in. to each end wall gives 10 sprinklers per line
- Check: 8.33 × 15 = 125 ft² per sprinkler; spacings 15 ft and 8.33 ft, both within limits
- Total: 12 × 10 = 120 sprinklers versus 70 proposed
Cost framing for the owner: the cheapest compliant layout is not the one with the fewest heads. It is the one whose remote area shape and hydraulic demand the available supply can actually meet.
2. Check the draftsperson's remote area claim and define the minimum compliant remote area.
Read the worked answer
Design area for wet OH2 is 1,500 ft² with no reduction taken here.
Shape rule: the remote area must be rectangular with its dimension parallel to the branch lines at least 1.2√A:
1.2 × √1,500 = 46.5 ft minimum along the branch lines
With heads at 15 ft, 46.5 ft requires 4 sprinklers on the branch (60 ft). The width then follows from the area:
1,500 / 60 = 25 ft → 25 / 8.33 = 3 branch lines
So the minimum compliant rectangle is 4 sprinklers × 3 branch lines = 12 sprinklers = 1,500 ft², and the 60 ft branch dimension clears the 46.5 ft requirement.
The 'three branch lines of ten sprinklers = 1,500 ft²' claim is arithmetically impossible - 30 heads at his own spacing is 30 × 213.75 ≈ 6,400 ft², more than four times the design area. Remote area is counted in sprinklers and then converted to area using the actual spacings; it is not a round number of branch lines.
Judgement point: round up to whole sprinklers and re-derive the area from the actual geometry. If the room forces 4 branch lines instead of 3, the design area becomes 60 × 33.3 ≈ 2,000 ft², and the required flow changes with it.
3. State the base-of-riser demand for the minimum-area case and for the rounded-up case, and explain why the difference matters.
Read the worked answer
Minimum-area case (1,500 ft²):
- 0.20 gpm/ft² × 1,500 ft² = 300 gpm
- 250 gpm hose allowance = 550 gpm
Rounded-up case (4 branch lines × 4 sprinklers = 2,000 ft²):
- 0.20 × 2,000 = 400 gpm
- 250 = 650 gpm
650 versus 550 is an 18% flow increase, which is roughly 1.18^1.85 ≈ 1.36 - about a 36% increase on the pipe friction portion of the demand. A calculation that just passes at 550 gpm can fail at 650 gpm. That is why the remote area shape has to be settled before the pipe is sized; it is not a rounding detail.
Levers if the calculation fails:
- Re-shape the remote area to the smallest compliant rectangle (12 heads rather than 16).
- Re-verify the density and hose allowance - a misclassified commodity is a common cause of an inflated demand.
- Enlarge the most remote branch lines and the main; friction is concentrated at the remote end.
- Only then consider a pump or storage, which is a capital decision the owner needs to hear early rather than at bid time.