Rack Storage of Group A Plastics - Contractor's OH2 Proposal
A 60 m x 40 m single-storey warehouse has a 10 m ceiling and double-row racks 6.5 m high. The commodity is cartoned unexpanded Group A plastic toys. Aisles are 2.7 m wide. The contractor proposes a wet-pipe system with upright sprinklers on a 3.0 m x 3.0 m grid, a density of 8.1 mm/min over 230 m², and no in-rack sprinklers. The justification is that the building is 'ordinary hazard, like a store,' and that the 10 m ceiling gives 'plenty of clearance.' The owner wants to avoid in-rack piping because it interferes with forklift operations. The water supply flow test shows 3,800 L/min at 3.2 bar, which the contractor says 'covers the sprinkler demand.' You are reviewing the design basis before hydraulic calculations are finalized.
1. Is the contractor's OH2 design basis acceptable? Explain the commodity and storage effects.
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No. The proposal is an Ordinary Hazard Group 2, 8.1 mm/min over 230 m² design, but the building is a rack storage warehouse handling Group A plastic. That is not an OH2 occupancy.
Group A plastics are a high-challenge commodity. The design must start with the commodity classification, packaging, storage height, rack arrangement, aisle width, and ceiling height. A 6.5 m storage height in a 10 m high building is not a 'clearance' issue; it is a storage configuration that must be protected using the storage criteria in the adopted standard. The required density, operating area, and sprinkler arrangement will generally exceed the OH2 curve, and in-rack sprinklers are likely to be required depending on the rack depth, flue spaces, and commodity.
The 10 m ceiling does not make the commodity ordinary hazard. The contractor has selected a density that is convenient for the existing supply rather than one that matches the hazard.
2. What information must be obtained before a compliant design can be produced?
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At minimum, obtain and document:
- Exact commodity, including whether the plastic is expanded or unexpanded, cartoned or exposed, and whether the packaging is combustible.
- Storage height, rack type, rack depth, shelf arrangement, and whether the racks are single-row, double-row, or multiple-row.
- Aisle widths, transverse and longitudinal flue spaces, and whether the flue spaces are maintained in practice.
- Ceiling height and ceiling construction, including any obstructions or draft curtains.
- Sprinkler type, temperature rating, response type, and installation orientation.
- Water supply flow-test data at the actual test hydrant, including static, residual, and flow.
- Required hose stream allowance and duration.
Without these, the designer cannot select the correct density/area, determine whether in-rack sprinklers are needed, or produce a hydraulic calculation that the AHJ can review.
3. The contractor says the 3,800 L/min supply 'covers the sprinkler demand.' How should you evaluate that claim?
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A single supply flow number does not demonstrate adequacy. The available supply must be compared with the system demand at the required pressure, including the sprinkler flow, in-rack demand if present, and hose stream allowance.
The correct method is to plot or calculate the available supply curve from the flow test and compare it with the hydraulic demand curve. The flow test point of 3,800 L/min at 3.2 bar is only one point; the pressure at the actual required flow may be lower, and the required flow for a Group A plastic storage design may be higher, especially if in-rack sprinklers are added.
The contractor's statement is therefore premature. Require a hydraulic calculation using the correct storage criteria, then compare the demand point to the available supply. If the supply is inadequate, the options include a fire pump, a storage tank, or a revised storage arrangement, not simply accepting the OH2 density.