Beam Pockets in a Parking Garage
A 3.4 m high parking garage has precast concrete beams 350 mm deep at 2.2 m centres, forming 2.2 m x 5.0 m bays. The occupancy is ordinary hazard Group 1. The contractor proposes upright standard-spray sprinklers at 4.6 m x 4.6 m spacing, with deflectors 100 mm below the beam soffits. The layout crosses beams and leaves some bays without a sprinkler. The contractor argues that the deflectors are below the beams, so the beams do not obstruct the spray, and that the 4.6 m spacing is the code maximum. The ceiling is smooth between beams. You are asked to review the layout before the hydraulic calculation is submitted. The garage stores only parked cars, with no flammable liquids.
1. Is the proposed layout acceptable? Consider beam depth, beam spacing, and deflector position.
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No. The beams are 350 mm deep and spaced at 2.2 m, forming a series of pockets. That is obstructed construction, and the sprinkler layout must account for it.
Placing sprinklers on a 4.6 m x 4.6 m grid across the beams leaves some 2.2 m x 5.0 m bays without a sprinkler. Lowering the deflectors 100 mm below the beam soffit does not solve the problem, because the spray still has to reach into adjacent bays past the beams. The beams are deep enough and closely spaced enough that the designer cannot rely on throw across them.
The 4.6 m figure is a maximum spacing for ordinary hazard, not a license to ignore obstructions. Each bay must be evaluated as a pocket, and the deflector must be positioned below the bottom of the obstruction in accordance with the listing and the standard. The layout should be revised before any hydraulic calculation is accepted.
2. Lay out a compliant spacing for the 2.2 m x 5.0 m bays and check coverage and wall distance.
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For ordinary hazard Group 1, the maximum protection area per sprinkler is about 12.1 m², the maximum spacing is 4.6 m, and the maximum distance from a sprinkler to a wall or bay end is about half the maximum spacing, or 2.3 m.
Each bay is 2.2 m x 5.0 m = 11.0 m². One sprinkler centered in the bay would satisfy the area limit, but the distance to each end would be 2.5 m, which exceeds 2.3 m. Therefore, one sprinkler per bay is not adequate.
Provide two sprinklers per bay along the 5.0 m length, spaced at 2.5 m, with 1.25 m from each end. Each sprinkler covers 2.2 m x 2.5 m = 5.5 m², well within 12.1 m². The distance to the side walls is 1.1 m, and the distance to the bay ends is 1.25 m. Position the deflectors below the beam soffit as required by the listing, with the minimum clearance below the obstruction maintained. This produces a regular grid that respects the beam pockets and the maximum spacing rules.
3. How does the corrected layout affect the hydraulic demand and the plan review?
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The corrected layout adds sprinklers compared with the contractor's 4.6 m x 4.6 m grid. More sprinklers mean more flow and a larger remote-area demand, and the calculation must be revised using the actual number and arrangement of sprinklers in the remote area.
The plan review should require a reflected ceiling plan showing the beams, bay dimensions, sprinkler locations, deflector elevations, and the remote area used in the calculation. The reviewer should check that no bay lacks coverage, that the spacing and wall distances are within the ordinary hazard limits, and that the deflector position below the beams is shown. The hydraulic calculation should match the revised layout, including the increased sprinkler count. If the supply cannot meet the revised demand, the contractor must propose a pump, tank, or other approved upgrade rather than reverting to the noncompliant layout.