Corridor Detection and Manual Station Plan Review
A design-build contractor submits fire alarm drawings for a new four-story, 96,000 ft2 office building. Each floor has a 240 ft long, 8 ft wide corridor served by exits at both ends. The building is fully sprinklered. The plans show four corridor smoke detectors per floor, located at 30 ft, 90 ft, 150 ft, and 210 ft from the west end, mounted on the ceiling centerline. The manual fire alarm boxes are shown only at the main lobby entrance; the two exit stairs at the corridor ends have no pull stations. The contractor notes that the building is sprinklered and that the corridor detectors are extra because sprinklers provide equivalent protection. The corridor provides access to the only elevator lobby, which has separate lobby smoke detection shown on the plans. The AHJ's plan reviewer must respond within 10 working days.
1. Calculate the required corridor smoke detector spacing and compare it with the submitted layout. State the corrected number and approximate locations.
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For a smooth 9 ft ceiling corridor, NFPA 72 corridor spacing is 30 ft maximum between detectors and 15 ft maximum from the end walls. The 240 ft corridor therefore needs detectors starting 15 ft from the west end and continuing at 30 ft intervals: 15, 45, 75, 105, 135, 165, 195, and 225 ft. That is 8 detectors, with the last one 15 ft from the east end.
The submitted layout has only 4 detectors at 30, 90, 150, and 210 ft. It fails on both counts: the spacing is 60 ft, twice the 30 ft maximum, and the end detectors are 30 ft from the end walls, twice the 15 ft maximum. The reviewer should require a revised layout with 8 detectors, or a listed alternative spacing supported by the manufacturer and accepted by the AHJ. The corridor width and ceiling height should be checked against the detector's listed spacing.
2. Evaluate the manual fire alarm box locations against NFPA 72. What corrections should the reviewer require?
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NFPA 72 requires manual fire alarm boxes at each required exit, with the travel distance to the nearest box not exceeding 200 ft. In this building the two exit stairs at the corridor ends are required exits, but the plans show boxes only at the main lobby entrance. The reviewer should require a box at each exit stair, plus a box at the main entrance if that is a required exit.
With boxes at both ends of a 240 ft corridor, the maximum travel distance from the corridor midpoint is about 120 ft, so the 200 ft travel limit is met by the two end boxes. A third mid-corridor box is not required by travel distance alone. Mounting height, visibility, and the adopted edition's distance-from-door requirement should also be checked.
3. The contractor argues that sprinkler protection eliminates the need for corridor smoke detectors. How should the reviewer respond?
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Sprinkler protection does not automatically delete smoke detection. NFPA 101 and local building codes may permit omission of corridor smoke detection in specific sprinklered occupancies, but the contractor must cite the exact code section and the AHJ must approve the deletion. Corridor detectors often serve functions beyond general detection: elevator lobby recall, door hold-open release, stairwell or smoke-control sequencing, or early warning for occupants. Removing them changes the sequence of operations and may invalidate other approved features.
The reviewer should reject a blanket statement that sprinklers provide equivalent protection. Require a code analysis, and if deletion is allowed, require revised drawings, a revised sequence of operations, and written AHJ acceptance.