Duct and Soffit Obstructions in a Light-Hazard Office
A light-hazard office fit-out has a suspended ceiling at 3.0 m. A 1.1 m wide × 400 mm deep supply duct runs parallel to a row of pendent sprinklers. The duct bottom is 500 mm below the ceiling. The sprinklers are on 3.7 m spacing, and the nearest sprinkler centerline is 1.3 m horizontally from the side of the duct. The deflector is 75 mm below the ceiling. The mechanical contractor wants to omit under-duct sprinklers because the duct is less than 1.2 m wide. A 300 mm deep × 2.4 m long soffit also projects below the ceiling within 600 mm of another sprinkler. The designer asks you to assess obstruction compliance. The sprinkler branch line is already fabricated and hung.
1. Determine whether under-duct sprinklers are required for the 1.1 m wide duct.
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First confirm the duct width. Under NFPA 13, sprinklers are required under ducts wider than 1.2 m. This duct is 1.1 m, so the width threshold alone does not require under-duct sprinklers.
Next apply the obstruction rule for narrow ducts: the horizontal distance from the sprinkler to the obstruction must be at least three times the vertical distance from the deflector to the bottom of the obstruction, or the sprinkler must be relocated or added.
Vertical distance = 3.0 m ceiling − 0.075 m deflector − (3.0 − 0.5) m duct bottom = 0.425 m. Three times that distance = 1.275 m. The nearest sprinkler is 1.3 m away, only 25 mm beyond the minimum. That is technically outside the 3× zone, but the margin is too small for field tolerance and the duct is deep. Do not omit protection based on a 25 mm margin. Verify actual installed deflector elevation and duct position, check the sprinkler spray pattern and listing, and if there is any doubt, add a sprinkler under the duct or relocate the line. Document the field measurements and obtain designer/AHJ acceptance.
2. What must be done about the 300 mm deep × 2.4 m long soffit?
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The soffit is a different obstruction because it is 2.4 m long, which exceeds 1.2 m. A long soffit can block spray over a wide area; the narrow-duct exception does not apply simply because it is shallow.
Check the horizontal distance from the nearest sprinkler to the soffit edge. If it is 600 mm and the vertical distance from the deflector to the soffit bottom is 0.225 m, three times that distance is 0.675 m. Since 0.600 m is less than 0.675 m, the sprinkler is inside the obstruction zone.
Provide a sprinkler under the soffit or relocate the sprinkler so the spray is not obstructed. Also check whether the soffit creates a concealed space that requires sprinklers. Record the final layout and obtain designer/AHJ acceptance before closing the ceiling.