Sprinkler Design Density in a Mixed-Occupancy Warehouse
A single-storey warehouse of 6,000 m2 is being converted. The north 1,500 m2 becomes a retail display area with a 6 m high racked storage zone behind it; the south 4,500 m2 remains high-piled combustible storage to 9 m. The existing wet sprinkler system was designed as an ordinary hazard system at the original density. The client argues that because the building footprint and water supply are unchanged, no hydraulic recalculation is needed, and that a single design density for the whole building is simpler to maintain. The water supply test shows a static pressure of 6.5 bar and a residual of 4.2 bar at 2,000 L/min. The authority having jurisdiction has asked for a design basis statement before construction.
1. Explain why the "unchanged footprint, unchanged supply" argument fails, and what the design basis statement must address.
Read the worked answer
Sprinkler design is driven by the hazard of the contents and the storage configuration, not by the footprint. Converting part of the floor to retail introduces an ordinary-hazard area with a different density and assumed design area, while the 9 m high-piled storage is a substantially more demanding storage hazard than the original ordinary-hazard basis; high-piled combustible storage generally requires higher densities, larger design areas, and often in-rack or specific protection schemes depending on commodity class and rack arrangement. A single density applied across both would either under-protect the storage or grossly over-protect the retail area, and it would not be defensible as a design basis. The statement must record: the occupancy classification of each area; commodity classification and storage height and arrangement; the chosen design density and design area for each hazard zone, with the standard or method relied on; whether a fire barrier or draft curtain separates the zones; the hydraulic calculation showing the most demanding area is satisfied at the available supply; and the residual pressure margin at the test point. The 4.2 bar residual at 2,000 L/min is only meaningful once compared with the calculated demand, which the statement must show.
2. The client proposes a fire-rated barrier between the retail and storage zones to avoid recalculating the storage area. Assess whether this is a sound engineering solution and what conditions it would carry.
Read the worked answer
A barrier can legitimately reduce the interaction between hazards, but it does not eliminate the storage hazard, so it cannot be used to keep an ordinary-hazard density over high-piled storage. Its value is in limiting fire spread and possibly in allowing separate hydraulic designs and separate design areas, which may simplify calculations. Conditions would include: a fire resistance rating justified by the fire severity on the storage side, not chosen for convenience; the barrier must be continuous and must close at roof level with no unprotected openings; the storage-side system must still be designed for the actual commodity and height; the retail-side system must meet its own ordinary-hazard basis; and the barrier's reliability must be maintained through an inspection regime. The statement should also record that the barrier is a compensatory feature, not a substitute for adequate density, and the authority should confirm acceptance of the approach before construction.